Chassis dyno - the real story
Discussion
I have been meaning to do a thread on the Do's and don'ts of putting your car on a rolling road, for some time now, but with the recent spate of new dynos popping up it is now time to post.
Preparation for the Dyno.................
1. Make sure your car has enough of the correct fuel in it for your journey and dyno runs.
2. Check all oil / water before leaving home and make sure to take some spare so you can top up at the dyno.
3. Look at your tyres and check pressures before you run your car on the dyno (Hub dynos being the exception here)
4. Find your locking wheel nut socket if you are going to a Hub dyno.
5. Arrive at least an hour early and open your bonnet to get the car as cool as possible
6. Check your air intake and filter before the run.
7. Talk to the dyno operator about what you what to achieve from the runs, as this will have a bearing on time and cost of you runs
8. Get the car loaded onto the dyno without running the car to much (as the car will heat up very very fast and under perform)
9. Cooling fans need to be positioned so they point at your cars air intake and radiator not just stuck in front of the car!!!
10. Don't run the car for more than a minute before the first pull as it will heat up and under perform (leave the bonnet up during the run)
11. Do the runs in quick succession to avoid heat build up (three runs should produce a good average)
12. Ask for a print out of the runs showing Torque and BHP in the same scale on the same sheet and taken from the wheels NOT the flywheel.....(fig 1)
13. Ask for a print out of the runs showing BHP and AFR on the same sheet. (fig.2)
Below is some of the reasoning behind the suggestions above………………
1.Fuel is a major factor in a cars performance! And more so on a tuned car, it is very important to have the correct octane rated fuel in your car as a lower spec fuel will reduce the cars performance.
As a safe guard make sure you have had at least two complete fills of your chosen fuel before the dyno runs as lower grade fuel will dilute the high octane fuel.
If the car detects knock it will reduce timing and so power and on a 400hp car one degree of timing is worth 4-5hp so a loss of 4 deg of timing is around 16-20hp loss in power.
2. This one is easy to understand, your car needs to be in its best state of tune if you want to get the best results. The dyno is a very harsh place for a car and it gets put under a lot of strain, so it is imperative to check over everything as a car engine seizing on a dyno is not a good idea........
3. To get an accurate result from a traditional rolling road the contact point of the tyres on the rollers needs to be consistent i.e. even wear and even tyre pressures. The tyres form one of the highest losses from the engine to the road, on average 37hp loss!
This is where up to date dyno's are a must as the tying down of your car is very important, too loose and the figures can be over calculated, and strapped down to tight (guys sitting in the boot!!!!!) and the figures will be to low.........
This is where the Hub dyno systems come into there own as the dyno bolts to the hubs of the car after the removal of the wheels so there is no problems with slippage or overheating......
4. Simple one again......On a HUB Dyno if you can’t get the wheels off you can’t get on the dyno and it is time to go home....
5. Easy to say hard to do sometimes.....Getting to the dyno early means more time for the car to cool down and more time for you to make sure you have completed all the other tasks on you list.
6. This is another simple one...... make sure your air intake is clear before you get on the dyno as any obstruction (leave’s/dirt/rabbits) will mean a reduced air flow and loss of power.
7. When you talk to the operator before the runs you can discuss how he wants to run your car and you can explain what you want from the data collected. A dyno has many different modes to calculate power and the "Power run" is just one of them.........
A simple power run means the car is fitted to the dyno and the operator performs a full throttle run from a low RPM in one gear to the red line, this will give you a plot showing how much Torque your car makes at all points along the curve and from that a BHP figure is calculated.
You could also ask for a run where the operator holds a fixed RPM then accelerates hard in one gear to simulate overtaking (this is a good bit of info for turbo cars to show where best to change gear to maximise boost)
Five minutes spent at the start of the day can save you time and money......................
8. Almost all modern cars suffer from heat soak into the engine and ancillaries, this is due to the amount of stuff that is packed into the engine bay and the limitations the body shape has on space.
As your car will be standing still it will heat up very fast and the engine bay will get hot along with everything else in the engine bay. Heat is the single biggest cause of a poor dyno result as hot air reduces power and sensors reading to hot will make the engine management reduce performance to save the engine.
9. I cannot stress enough the fact that a hot car will under perform but if you look at it this way a Monaro at the red line in 4th gear is pulling around 130mph and I know of no dyno that has fans that can produce that much air flow. At 130 mph it would be a wind tunnel and people would be blown over. Also a 5.7ltr V8 moves 34,200ltr of air at 6000rpm (or the size of a large petrol tanker!!!) so again a couple of fans are not going to get close to supplying enough air flow!!!!!!
The best dynos cells have a temp control environment to keep air temps low this way they don’t need to produce large amounts of air flow to keep the car cool.......
When an engine management system detects an inlet air temp in excess of its safe level it reduces ignition timing to reduce power and thus save the engine from damage. On the road this is happening all the time (after being stuck in traffic you put your foot down and you think the car feels slow, that is because it is slow!!!!) but it soon recovers, on a dyno this is not the case and you just get less and less power.
While on the engine management front, to properly dyno a modern car you need to have full control of the cars ECU/PCM this is because you must see what changes the car is making during the dyno run, is it reducing spark due to heat, is it invoking some form of torque management due to Throttle position/RPM/Torque, is the traction control system still having some effect (off is not off on most modern cars!!)
Without this level of info your results could appear poor but in fact were totally
normal given the operating conditions the car was dyno’d under.........
BMW have just about produce a UN Dynoable car in the new M5 as it has so many systems to monitor wheel speed, Throttle etc it just puts the car into a safe mode and that is the end of your fun.
When doing some dyno work with the “VXR Performance Centre”, "Triple Eight" and "Vauxhall" on their new Astra VXR we logged air temps of 77deg at the throttle body and we were pumping in air at 16deg and the car just reduced power. It was also not possible to dyno the car without removing relays to stop the traction control cutting in.......
10. As above "Heat is bad"
11.The first run will loosen up the car, second run will probably make the best power and the third will come close to the second and verify the power your car was making on that day!
12. your evidence of the day is a print out but it is only any good if you understand it.......
(fig 1)
A good dyno print out should show Torque and BHP on the same sheet and use the same scale. One thing to do when checking a dyno sheet is to look at the 5252rpm figure as it should be the same number on Torque and BHP because BHP is calculated from torque......HORSEPOWER = (TORQUE x RPM) / 5252
Also make sure your sheet is produced using a DIN standard and is taken from the wheels and is not a calculated flywheel figure.........
13. A very good starting point for more power is Air Fuel Ratio and getting a printout showing your current AFR will give you a heads up on current/future performance. (fig 2)
FIG 1
FIG 2
As with all test equipment the set up of the Dyno is very important to maintain accurate results, so far I have assumed the dyno is correctly calibrated and the operator has ensured the car is correctly mounted on the Dyno.
But there are some things that can affect your results which you need to know about.
1. The intake air temperature probe should be mounted in a position which accurately monitors the actual air entering the car. If this is not done the dyno will make a correction to the results in an attempt to allow for different air density and therefore more or less power.
The most common problem I have seen is when an operator puts the temp probe to close to the radiator or lays the probe on a surface which has a large degree of heat soak and this then makes the reading to high. The result of this over reading means the dyno adjusts the final figure in an upward direction.
On the dyno sheet you will see and ambient temp number printed and an air intake number. You should be happy that the ambient temp was accurate based on the weather on the dyno day and before you trust the results the IAT number should be within 5deg or so of the ambient temp. (fig 3)
As a rough guide a 20+deg difference in the two numbers could equate to a 20BHP gain (a cheap way to get more power is to put the probe in a sweaty arm pit !!!!)
2. The gear the car is dynoed in make a difference to your results as you gain mechanical advantage by using a lower gear and thus more Torque.
The ideal situation is to dyno the car in a gear which is as close as possible to 1:1
3. On dynos using rollers it is important to make sure the contact point is the same each time the car is tested, as when the car pulls up onto the front roller the further up the roller the wheels move the higher the torque figure can be............
4. When your car is run on the dyno the operator selects a "Ramp Rate" which in simple terms is the steepness of the hill the car is climbing during the run and the speed of that accent.
A supercharged car, a Turbo car and naturally aspirated cars should have different ramp rates based on how they produce there power and using the wrong rate will affect the final figures...........
I have glossed over many problems involved in dynoing your car in an attempt to give you an overview of the process. In conclusion if you get within 5% of the figure you expect you have probably got what you expect.....
>> Edited by wortec1 on Wednesday 19th April 22:37
Preparation for the Dyno.................
1. Make sure your car has enough of the correct fuel in it for your journey and dyno runs.
2. Check all oil / water before leaving home and make sure to take some spare so you can top up at the dyno.
3. Look at your tyres and check pressures before you run your car on the dyno (Hub dynos being the exception here)
4. Find your locking wheel nut socket if you are going to a Hub dyno.
5. Arrive at least an hour early and open your bonnet to get the car as cool as possible
6. Check your air intake and filter before the run.
7. Talk to the dyno operator about what you what to achieve from the runs, as this will have a bearing on time and cost of you runs
8. Get the car loaded onto the dyno without running the car to much (as the car will heat up very very fast and under perform)
9. Cooling fans need to be positioned so they point at your cars air intake and radiator not just stuck in front of the car!!!
10. Don't run the car for more than a minute before the first pull as it will heat up and under perform (leave the bonnet up during the run)
11. Do the runs in quick succession to avoid heat build up (three runs should produce a good average)
12. Ask for a print out of the runs showing Torque and BHP in the same scale on the same sheet and taken from the wheels NOT the flywheel.....(fig 1)
13. Ask for a print out of the runs showing BHP and AFR on the same sheet. (fig.2)
Below is some of the reasoning behind the suggestions above………………
1.Fuel is a major factor in a cars performance! And more so on a tuned car, it is very important to have the correct octane rated fuel in your car as a lower spec fuel will reduce the cars performance.
As a safe guard make sure you have had at least two complete fills of your chosen fuel before the dyno runs as lower grade fuel will dilute the high octane fuel.
If the car detects knock it will reduce timing and so power and on a 400hp car one degree of timing is worth 4-5hp so a loss of 4 deg of timing is around 16-20hp loss in power.
2. This one is easy to understand, your car needs to be in its best state of tune if you want to get the best results. The dyno is a very harsh place for a car and it gets put under a lot of strain, so it is imperative to check over everything as a car engine seizing on a dyno is not a good idea........
3. To get an accurate result from a traditional rolling road the contact point of the tyres on the rollers needs to be consistent i.e. even wear and even tyre pressures. The tyres form one of the highest losses from the engine to the road, on average 37hp loss!
This is where up to date dyno's are a must as the tying down of your car is very important, too loose and the figures can be over calculated, and strapped down to tight (guys sitting in the boot!!!!!) and the figures will be to low.........
This is where the Hub dyno systems come into there own as the dyno bolts to the hubs of the car after the removal of the wheels so there is no problems with slippage or overheating......
4. Simple one again......On a HUB Dyno if you can’t get the wheels off you can’t get on the dyno and it is time to go home....
5. Easy to say hard to do sometimes.....Getting to the dyno early means more time for the car to cool down and more time for you to make sure you have completed all the other tasks on you list.
6. This is another simple one...... make sure your air intake is clear before you get on the dyno as any obstruction (leave’s/dirt/rabbits) will mean a reduced air flow and loss of power.
7. When you talk to the operator before the runs you can discuss how he wants to run your car and you can explain what you want from the data collected. A dyno has many different modes to calculate power and the "Power run" is just one of them.........
A simple power run means the car is fitted to the dyno and the operator performs a full throttle run from a low RPM in one gear to the red line, this will give you a plot showing how much Torque your car makes at all points along the curve and from that a BHP figure is calculated.
You could also ask for a run where the operator holds a fixed RPM then accelerates hard in one gear to simulate overtaking (this is a good bit of info for turbo cars to show where best to change gear to maximise boost)
Five minutes spent at the start of the day can save you time and money......................
8. Almost all modern cars suffer from heat soak into the engine and ancillaries, this is due to the amount of stuff that is packed into the engine bay and the limitations the body shape has on space.
As your car will be standing still it will heat up very fast and the engine bay will get hot along with everything else in the engine bay. Heat is the single biggest cause of a poor dyno result as hot air reduces power and sensors reading to hot will make the engine management reduce performance to save the engine.
9. I cannot stress enough the fact that a hot car will under perform but if you look at it this way a Monaro at the red line in 4th gear is pulling around 130mph and I know of no dyno that has fans that can produce that much air flow. At 130 mph it would be a wind tunnel and people would be blown over. Also a 5.7ltr V8 moves 34,200ltr of air at 6000rpm (or the size of a large petrol tanker!!!) so again a couple of fans are not going to get close to supplying enough air flow!!!!!!
The best dynos cells have a temp control environment to keep air temps low this way they don’t need to produce large amounts of air flow to keep the car cool.......
When an engine management system detects an inlet air temp in excess of its safe level it reduces ignition timing to reduce power and thus save the engine from damage. On the road this is happening all the time (after being stuck in traffic you put your foot down and you think the car feels slow, that is because it is slow!!!!) but it soon recovers, on a dyno this is not the case and you just get less and less power.
While on the engine management front, to properly dyno a modern car you need to have full control of the cars ECU/PCM this is because you must see what changes the car is making during the dyno run, is it reducing spark due to heat, is it invoking some form of torque management due to Throttle position/RPM/Torque, is the traction control system still having some effect (off is not off on most modern cars!!)
Without this level of info your results could appear poor but in fact were totally
normal given the operating conditions the car was dyno’d under.........
BMW have just about produce a UN Dynoable car in the new M5 as it has so many systems to monitor wheel speed, Throttle etc it just puts the car into a safe mode and that is the end of your fun.
When doing some dyno work with the “VXR Performance Centre”, "Triple Eight" and "Vauxhall" on their new Astra VXR we logged air temps of 77deg at the throttle body and we were pumping in air at 16deg and the car just reduced power. It was also not possible to dyno the car without removing relays to stop the traction control cutting in.......
10. As above "Heat is bad"
11.The first run will loosen up the car, second run will probably make the best power and the third will come close to the second and verify the power your car was making on that day!
12. your evidence of the day is a print out but it is only any good if you understand it.......
(fig 1)
A good dyno print out should show Torque and BHP on the same sheet and use the same scale. One thing to do when checking a dyno sheet is to look at the 5252rpm figure as it should be the same number on Torque and BHP because BHP is calculated from torque......HORSEPOWER = (TORQUE x RPM) / 5252
Also make sure your sheet is produced using a DIN standard and is taken from the wheels and is not a calculated flywheel figure.........
13. A very good starting point for more power is Air Fuel Ratio and getting a printout showing your current AFR will give you a heads up on current/future performance. (fig 2)
FIG 1
FIG 2
As with all test equipment the set up of the Dyno is very important to maintain accurate results, so far I have assumed the dyno is correctly calibrated and the operator has ensured the car is correctly mounted on the Dyno.
But there are some things that can affect your results which you need to know about.
1. The intake air temperature probe should be mounted in a position which accurately monitors the actual air entering the car. If this is not done the dyno will make a correction to the results in an attempt to allow for different air density and therefore more or less power.
The most common problem I have seen is when an operator puts the temp probe to close to the radiator or lays the probe on a surface which has a large degree of heat soak and this then makes the reading to high. The result of this over reading means the dyno adjusts the final figure in an upward direction.
On the dyno sheet you will see and ambient temp number printed and an air intake number. You should be happy that the ambient temp was accurate based on the weather on the dyno day and before you trust the results the IAT number should be within 5deg or so of the ambient temp. (fig 3)
As a rough guide a 20+deg difference in the two numbers could equate to a 20BHP gain (a cheap way to get more power is to put the probe in a sweaty arm pit !!!!)
2. The gear the car is dynoed in make a difference to your results as you gain mechanical advantage by using a lower gear and thus more Torque.
The ideal situation is to dyno the car in a gear which is as close as possible to 1:1
3. On dynos using rollers it is important to make sure the contact point is the same each time the car is tested, as when the car pulls up onto the front roller the further up the roller the wheels move the higher the torque figure can be............
4. When your car is run on the dyno the operator selects a "Ramp Rate" which in simple terms is the steepness of the hill the car is climbing during the run and the speed of that accent.
A supercharged car, a Turbo car and naturally aspirated cars should have different ramp rates based on how they produce there power and using the wrong rate will affect the final figures...........
I have glossed over many problems involved in dynoing your car in an attempt to give you an overview of the process. In conclusion if you get within 5% of the figure you expect you have probably got what you expect.....
>> Edited by wortec1 on Wednesday 19th April 22:37
What Paul said....
Good write up, I think it summarises well the 'do's and 'dont's' of dynos and will help people stop been disappointed with the numbers in the future or at least offer an explanation.
Paul, have you taken any of your turboed cars on a RR yet? Because often turboed cars have problem building boost on rolling roads and can't replicate the boost seen on the road because not enough load is put on the dyno... something to think about anyway.
Good write up, I think it summarises well the 'do's and 'dont's' of dynos and will help people stop been disappointed with the numbers in the future or at least offer an explanation.
Paul, have you taken any of your turboed cars on a RR yet? Because often turboed cars have problem building boost on rolling roads and can't replicate the boost seen on the road because not enough load is put on the dyno... something to think about anyway.
We are still in the Road Testing stage of the Turbo tune.......(1500 miles and no problems)
Our aproach has all ways been to tune the car on the road and use the dyno to provide conformation of figures, so if we do find we can't get the full potential of the car on the dyno it is not a real problem as the car is fully logged on the road.
BUT the way this turbo gets on boost I think we won't have any problems
>> Edited by wortec1 on Thursday 20th April 10:49
Our aproach has all ways been to tune the car on the road and use the dyno to provide conformation of figures, so if we do find we can't get the full potential of the car on the dyno it is not a real problem as the car is fully logged on the road.
BUT the way this turbo gets on boost I think we won't have any problems
>> Edited by wortec1 on Thursday 20th April 10:49
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Rolling roads figures, fact or fiction?
--------------------------------------------------------------------------------
I came across this well written article over on the Renault Turbo Owners Club site and thought it would make some good reading to folks on here.
MEASURING ENGINE POWER
There is in fact no way of directly measuring power - all types of dynamometer measure torque and then power is calculated from the formula we saw in the previous articles - BHP = Torque (ft/lbs) x rpm/5252. This basic equation is the cornerstone of all engine design and development work. Two main methods of measuring power are used in the automotive industry - (1) measurement at the crankshaft of the engine or (2) measurement at the driving wheels. We'll look at both of these separately.
ENGINE DYNAMOMETERS
If we want to know the power of the engine alone then an engine dynamometer (or dyno) is used. This is how nearly all manufacturers rate the output of car engines. The engine is bolted into a cradle and connected to the dyno with a prop shaft which bolts onto the back of the crankshaft (or the flywheel). The power figures measured in this way are therefore usually called "flywheel power". The dyno is essentially a "brake" which can apply a known torque (or "load"
to the engine. When the engine is holding a steady speed under a given dyno load then the torque being applied by the dyno must be exactly equal to the torque being produced by the engine. If this were not so then the engine would either accelerate or decelerate. Let's say we want to know the engine torque at full throttle at 3,000 rpm. The throttle is gradually opened and at the same time the load applied by the dyno is increased - eventually by juggling the amount of load applied we get to the situation where the throttle is fully open and the rpm is steady at 3,000. The torque being applied is written down and then the operation would be repeated at say 4,000 rpm. Soon we get a complete chart of torque at all engine speeds. Of course we could also measure part throttle power if desired. Modern dynos are computer controlled and can generate power and torque curves very rapidly without the operator having to manually adjust throttle and load controls. They can be programmed to measure every so many rpm, say in 250 or 500 rpm steps - or they can measure a continuous torque curve while the engine accelerates at a preset rate. This can be used to simulate how the engine would actually operate in a particular gear when installed in the car.
There are various ways in which the dyno load can be applied. Older dynos use a hydraulic system with a rotor inside a water filled cavity - rather similar to the torque convertor in an automatic gearbox. Modern dynos generate the load with large electric motors. Even a simple friction disk or drum brake will work fine and this is where the name "brake" in Brake Horsepower came from. The important thing is that the load is able to be measured accurately and that there are no frictional losses in the system that escape measurement.
In order for dyno results to be comparable and universally understood there are a number of things that need to be closely controlled during the measurement process:
Operating Conditions
Air temperature, pressure and humidity affect the amount of power an engine produces. Cold dense air means a greater mass of oxygen per power cycle and thus more power is generated (provided of course that air/fuel mixture is properly calibrated for the conditions prevailing). There are formulae that can be used to calculate how much the measured power would change if the test conditions were different. This enables dyno results to be "corrected" back to standard conditions to enable comparison with anyone else's test results. Sadly however there is no one universally accepted set of "standard" conditions because different automotive bodies in different countries use different standards to calibrate to. "SAE" power standards are used in the USA and sometimes in England. "DIN" standards are used on the continent and there are a few other oddball systems just to confuse the issue. So just because your car is rated at 100 bhp and a friends at 110 bhp doesn't necessarily mean that his engine is more powerful - it depends whether both measurements were corrected to the same standard conditions.
One of the tricks I've seen used to get bigger "corrected" bhp numbers is to use a very high ambient temperature reading for the dyno test. If the operator measures the temperature close to the engine rather than well away from it then obviously he will get a reading that is much higher than ambient. When the bhp numbers are corrected back to a lower standard ambient temperature they will increase. I saw an engine dyno sheet the other day where the ambient air temperature in February, in England was supposedly 37 degrees C. Now either that test was done with the temperature probe sat right on top of the engine or it's a part of country I don't yet know about where I would very much like to live !!
Engine Ancillaries
When installed in the car, the engine has to drive a number of items like the alternator and power steering pump which sap power. Also the exhaust and air filter systems will reduce power to some extent. If the engine is tested without any of these ancillaries fitted then it will show much higher power figures. The Americans used to rate their engines like this back in the fifties and sixties and often the installed power of the engine would only be 2/3 of the claimed figure in the sales blurb. This used to be called "gross" flywheel power and if the ancillaries were fitted the power was called "net" flywheel power. Nowadays the gross system, which was very misleading, is not used and all modern published data should be "net flywheel" power. Major manufacturers abide by rigorous standards which set out how the engine should be installed on the dyno to simulate closely the "in car" conditions.
ROLLING ROAD DYNAMOMETERS
Also called chassis dynamometers, these are used to measure power at the driving wheels. This avoids the inconvenience of having to remove the engine to test it if a tuning modification has been made. However, it means that the power figures obtained will be lower than the flywheel power because of the frictional losses in the drivetrain and tyres. This leads to one of the biggest sources of confusion, error and plain misinformation in the tuning industry. You see, as discussed above, all major manufacturers quote flywheel power so it is understandable that people want to know if the hard earned cash they spent on tuning mods increased the power of their engine and by how much. To know this for certain means knowing how much the transmission losses are. There is enormous pressure on rolling road operators to be able to quote flywheel bhp rather than wheel bhp and most operators now run proprietary software systems which "supposedly" print out flywheel power.
PROBLEM !! - THESE SOFTWARE SYSTEMS DO NOT AND CANNOT WORK !!
Yes - I know - the whole chassis dyno tuning industry quotes flywheel figures and here's me saying none of it works. So I'd better explain some more and then you can make your own mind up.
First, let's look at how a chassis dyno works. The car is driven onto a rig so that the driving tyres are resting between two steel rollers. The torque is measured at different speeds in exactly the same way as an engine dyno works except that it is torque at the rollers rather than torque at the flywheel. The braking load is applied to one of the rollers by either a hydraulic (water brake) or electrical system again in just the same way as the engine dyno would apply a torque to the crankshaft of the engine. The same universal equation at the top of the page can then be used to calculate bhp at the rollers by knowing the torque and the rpm of the rollers (NOT the rpm of the engine at this stage) - but if the engine rpm is measured simultaneously then we can know roller bhp at a particular engine rpm. The BIG problem with all this is if any tyre slip is taking place. Remember these are smooth steel rollers which over time get quite polished. How much grip do you think you would get if roads were made of polished steel rather than tarmac? The effects of tyre slip are complex (i.e. I don't pretend to fully understand them myself!) but what I do know is that you can get some really strange bhp figures from highly tuned engines on narrow tyres and the readings are invariably too high not too low.
__________________
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Last edited by J333EVO : 29-10-2005 at 15:44.
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29-10-2005 #2
J333EVO
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Re: Rolling roads figures, fact or fiction?
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What is a transmission loss ? Well all mechanical systems suffer from friction and a proportion of the power fed into a system will get dissipated by friction and turn into heat and noise. Note the key phrase there - "power fed into a system". For there to be a loss there must be an input - simple and obvious yes but we'll see the relevance in a minute. When your car is parked overnight with the engine switched off, the transmission losses are obviously zero. When the car is running then some proportion of the flywheel power will be lost in the gearbox, final drive, drive shaft bearings, wheel bearings and tyres. For a given mechanical system these losses will usually stay close to a particular fixed %, let's say 10% for arguments sake, of the input power. So if the car is cruising and developing 20 bhp then 2 bhp will get absorbed as friction - under full power, say 100 bhp, then maybe 10 bhp will get absorbed. Now it is true that not every component in a transmission system absorbs a fixed % of the input power. Some components like oil seals and non driven meshed gears (as in a normal car multi speed gearbox) have frictional losses which are not affected by the input torque. These losses do increase with speed of course but at a given rpm can be taken to remain constant even if the engine is tuned to give more power. We'll look at real world transmission loss percentages later. Finally, the biggest source of loss in the entire transmission system of a car is in the tyres - they account for half or more of the total losses between the flywheel and the rollers. Each set of driven gears, i.e. the final drive gear or the particular gearbox ratio that you happen to be testing the car in, only absorbs about 1% to 2% of the engine's power.
Ok - so how do these software systems that supposedly measure transmission losses so as to "predict" back to the flywheel bhp work. The power curve at the wheels is taken in the usual way as explained above. Then, at peak rpm, the operator puts the car into neutral and lets the rollers slow down under the drag of the tyres and transmission. The software then measures this drag (or "coast down loss"
as "negative" power and adds it to the wheel power to get back to the supposed flywheel power. BUT - and hopefully you've all spotted the problem now - the engine is not feeding any power into the drivetrain while the car is in neutral - in fact it isn't even connected to the drivetrain any more!! Whatever drag this is that's being measured it has nothing at all to do with the proportion of the flywheel power that gets lost as friction when the engine is powering the car in the normal way. The engine could now be an 800 bhp F1 engine or a 30 bhp mini engine for all it matters because it isn't connected to the gearbox or feeding any power into it. Obviously this "coast down loss" is something to do with the transmission and tyres but it is not the true transmission loss - in fact this coast down loss should never be expected to change for a given car regardless of how much you tune the engine whereas a true transmission loss will increase as the engine power increases because it is dependent to a large extent on the amount of power being fed into the transmission. I've seen a car that over time was tuned from 90 bhp at the wheels to 125 bhp at the wheels and the "coast down loss" stayed the same for every power run to within a fraction of a horsepower - exactly as you would have predicted. As the engine was tuned to give more power the "true" transmission losses must have also increased to some extent but these chassis dyno systems don't, and can't, show this happening. So is there any way of really measuring the true transmission loss of a car? Yes - only one - by measuring the flywheel power on an accurate engine dyno, the wheel power on an accurate chassis dyno and taking one away from the other. There is no way on God's green earth of finding out the true transmission loss just by measuring the power at the wheels.
So hopefully that's got you all thinking a bit more now instead of just taking for granted the "flywheel" figure you were given last time you took your car to the rollers just because a nice man with some flashy equipment told you so. Even worse is the fact that some of these software systems allow the operator to just programme in the % transmission loss he wants the system to add to the wheel figures !!! So if that isn't a nice easy way to show some big fat flywheel bhp then I don't know of a better one. It's certainly a lot easier than actually doing some proper development work to make the engine perform better - just dial in a bigger transmission loss and bingo - the same wheel bhp now turns into a bigger flywheel bhp - happy customer, happy dyno man - just a shame it was all sleight of hand. See the end of this article if you doubt that this sort of thing really happens.
So what should you do when you take your car to a rolling road? Firstly, make sure you get printouts that show the wheel bhp and not just the flywheel bhp. Then at least you can see if they look sensible in comparison. If you have a desperate need to know the flywheel bhp then you will have to estimate it - there's no other way short of using an engine dyno. The corrections you need to make for cars with manual gearboxes are these:
The average front wheel drive road car with between 100 and 200 bhp loses about 15% of the engine bhp as transmission losses.
The average rear wheel drive road car with between 100 and 200 bhp loses about 17% of the engine bhp as transmission losses. The increase in % loss over front wheel drive is because the differential has to turn the drive through 90 degrees at the back axle which soaks up a bit more of the engine's power.
What your own specific car loses is anyone's guess - yours is as good as mine - but it shouldn't be far from the figures above. For sure though, no car in the world, unless it has flat tyres and a gearbox full of sand, loses anything like 30% of the engine's power in the transmission and tyres as many rolling road operators would try to have you believe. So take the wheel figure and divide by 0.85 for FWD or 0.83 for RWD and that will get you as close to the true engine bhp as you are ever going to know. In general though it is fair to say that low powered cars have higher % losses than high powered cars. For example, a 60 bhp Fiesta will have around 14 to 15 bhp total transmission and tyre loss whereas a 90 bhp XR2 will only have about 17 to 18 bhp loss - a smaller % obviously. By the time you get to cars with engines in the 300+ bhp range, losses can be down to 12% or so.
Another rule of thumb I use which is quite accurate is to treat the losses as being 10% of the flywheel power plus 10 bhp for FWD and 12% plus 10 bhp for RWD cars. This equation "loads" low powered cars more than high powered cars which is more closely like what happens in reality.
Remember, these percentages are not "gospel" - they are good realistic averages. The measured wheel bhp can change depending on tyre pressure, tyre size, suspension angles and other things which won't affect flywheel power - so the actual transmission loss % will also change. It pays to try and standardize as many of these things as possible if you intend to do a series of power runs over a period of time. Always use the same tyre pressure because this is a factor which can easily change from day to day.
Some time ago I had three almost identical race cars set up together in a group session at a rolling road. The engines were very similar except for minor differences in the camshafts fitted. One showed 118 bhp at the wheels, another showed 124 and the third showed only 98. The operator spent ages I'm told (I wasn't there) trying to find why the third car was so poor. It wasn't till the next day when that particular owner was checking things before the race that he noticed that the tyres only had 7 psi in them - the car had sat unchecked over the winter and no-one had bothered to standardize the pressures before the dyno test. In the race, that car went just as well as the other two and if anything was slightly the fastest of the three. That gives you some idea of how much power a set of flat tyres can absorb.
As you tune a particular car, the losses won't increase exactly in proportion to the power because as mentioned above, some components in the transmission have fixed losses which are not dependent on engine power. However, neither you nor the dyno operator will have any real idea of exactly how the losses have changed so you might as well just continue to apply the percentages above to give some sort of realistic guide to the new flywheel bhp.
What sort of % transmission loss do these software systems show? - well for normal road cars in the 100 to 200 bhp category, I've seen as high as 35% and as low as 10%. So take the same car with 100 bhp at the wheels to 2 different rollers and you might get anything from 110 bhp to 140 bhp being "predicted" as the flywheel figure. In reality 100 bhp at the wheels will be no more than about 120 bhp at the flywheel. If being told a bigger figure makes you happy then good for you - the car won't go any faster and you'll be no nearer to knowing whether you really got more power out of it than standard.
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"A well looked after Integrale makes most other so-called performance hatches look distinctly toothless and has an irrevocable coolness that none can match. Are you hardcore?" Car & Driving 2005
Cars; Metalescent black integrale EVO 1 & imola blue Cinquecento Abarth hi-boost turbo.
www.fiatforum.com/showthread.php?t=44413 Thread here.
If of any interest, and by coincidence I have been posting about Rolling Rip Offs on another thread on PH having first put on flame-proof overalls 
. I found the rolling road owners responses enlightening
www.pistonheads.com/gassing/topic.asp?t=256877&f=116&h=0
>> Edited by oe_cosgrove on Friday 21st April 13:04
. I found the rolling road owners responses enlightening www.pistonheads.com/gassing/topic.asp?t=256877&f=116&h=0
>> Edited by oe_cosgrove on Friday 21st April 13:04
Good write-up Paul.
The only thing I'd add is that the correction to the actual (raw) figures is still not properly understood and most operators don't seem know how to work out Density Altitude.
When it is applied correctly it shouldn't matter wether it's done on a hot or a cold day, or it's dyno'd on top of a 5000' mountain or at sea level. This is what having a standard correction is all about.
And yes I agree with you that the IAT probe should be adjusted manually if needs be to read as you mentioned, around 3-5C above ambient.
Just about all manufacturers have been using J607 which was also known as STP or Standard Atmosphere.
This was agreed & accepted as +15C (59F) 1013Millibars (29.92" Hg)and Zero Relative Humidity (It still is by ICAO and the aircraft industry).
This for some time, was felt to be too unrealistic for cars throughout the planet and J1349 was introduced. GM use it but it has not been universally accepted by all manufacturers.
J1349 corrects figures to: +25C (77F) 990Mb (29.235" Hg) and Zero RH.
For example the LS7 which was validated under J1349 @ 505hp would make around 525hp with a J607 correction.
So as you can imagine, most of the car industry are reluctant to publish figures that are going to be 4-5% lower than what they've been claiming under J607(STP).
Some good info here:-
www.csgnetwork.com/relhumhpcalc.ht
http://wahiduddin.net/calc/calc_hp_dp
>> Edited by sid447 on Tuesday 25th April 08:10
The only thing I'd add is that the correction to the actual (raw) figures is still not properly understood and most operators don't seem know how to work out Density Altitude.
When it is applied correctly it shouldn't matter wether it's done on a hot or a cold day, or it's dyno'd on top of a 5000' mountain or at sea level. This is what having a standard correction is all about.
And yes I agree with you that the IAT probe should be adjusted manually if needs be to read as you mentioned, around 3-5C above ambient.
Just about all manufacturers have been using J607 which was also known as STP or Standard Atmosphere.
This was agreed & accepted as +15C (59F) 1013Millibars (29.92" Hg)and Zero Relative Humidity (It still is by ICAO and the aircraft industry).
This for some time, was felt to be too unrealistic for cars throughout the planet and J1349 was introduced. GM use it but it has not been universally accepted by all manufacturers.
J1349 corrects figures to: +25C (77F) 990Mb (29.235" Hg) and Zero RH.
For example the LS7 which was validated under J1349 @ 505hp would make around 525hp with a J607 correction.
So as you can imagine, most of the car industry are reluctant to publish figures that are going to be 4-5% lower than what they've been claiming under J607(STP).
Some good info here:-
www.csgnetwork.com/relhumhpcalc.ht
http://wahiduddin.net/calc/calc_hp_dp
>> Edited by sid447 on Tuesday 25th April 08:10
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