Discussion
I assume an easy way to fudge BHP is by using a lower gear, but would be good to get Charlie's input.
We had a Supra on a dyno which was making huge power, more than the turbo could flow
. The operator called Australia where the dyno was made, they told him to use a different ramp rate, hey presto, a much more realistic figure appeared.
I was just trying to see if the torque was harder to fudge then I would start ignoring the BHP and look at the torque figure when looking a sheets.
We had a Supra on a dyno which was making huge power, more than the turbo could flow
. The operator called Australia where the dyno was made, they told him to use a different ramp rate, hey presto, a much more realistic figure appeared. I was just trying to see if the torque was harder to fudge then I would start ignoring the BHP and look at the torque figure when looking a sheets.
Turbo T said:
I assume an easy way to fudge BHP is by using a lower gear, but would be good to get Charlie's input.
We had a Supra on a dyno which was making huge power, more than the turbo could flow. The operator called Australia where the dyno was made, they told him to use a different ramp rate, hey presto, a much more realistic figure appeared.
I was just trying to see if the torque was harder to fudge then I would start ignoring the BHP and look at the torque figure when looking a sheets.
We had a Supra on a dyno which was making huge power, more than the turbo could flow
. The operator called Australia where the dyno was made, they told him to use a different ramp rate, hey presto, a much more realistic figure appeared. I was just trying to see if the torque was harder to fudge then I would start ignoring the BHP and look at the torque figure when looking a sheets.
BHP = (rpm x torque) / 5252
Hence bhp and torque always cross at 5252 rpm on the graph. Thus BHP is entirely a coefficient of the torque at a given rpm. The higher an engine revs the higher the potential for huge BHP (i.e. F1 engines making 800+ bhp from ~3 litres).
As BHP is a calculation the only thing the dyno reads is torque and it's mistakes or fudging of that which leads to dodgy results.
Wrong ramp rates (as mentioned above) is a common one as is using a different gear. From what I've read one should attemt to use a gear that's as close to 1:1 as possible (engine rpm:wheel rpm). Using a sensible ramp rate will remove the effect of using the wrong gear though. My earlier dyno readings were based on using incorrect temp readings and the dyno adjusted the figure for high temps thus boosting the numbers output. very complex stuff though so one needs a good dyno-persn to get reasonable resuts out.
End of the day dyno figures are pub talk and count for little in the real world.
Essential for tuning but the bhp numbers are bragging rights only. 1/4 mile and lap times count.
Right..
Ramp rates are a bit of a red herring, a ramp rate in itself will have a small effect on the bhp the car generates because you are speeding up or slowing down the engine acceleration. However, certain dynos have modes with different ramp rates, when you switch ramp rates it can also change the dyno and user inertias, this will have a big effect on how the dyno reads.
Certain dynos (Dynojets and other American dynos) just read high full stop. Usual rule on these is to take the wheel fig and use it as the fly. I saw a megane 225 totally standard make 224bhp at the wheels the other day on one of these dynos, which i think rests my case!
The dynopack dynos should give a fig exactly between the fly and wheel fig you get on a dyno dynamics (I've tested this theory several times and it works). However, crossing out 'fly' and writing 'hubs' is not the way to do it!!!
Different dynos work in different ways, they all measure the torque at the wheels, then most use the rmp of the rollers to get a BHP fig. So, technically, as long as you don't have wheelspin, this fig should be correct (certainly on our dynos). The user then either tells the rpm the car is doing, (either by a pick up off the leads, or by scaleing it) and the dyno can then draw the torque against the rpm, nows heres the scam, by putting a shorter ratio into the dyno, the dyno thinks the car is generateing more bhp at a lower rpm.. = Higher torque readings
Now, on Dyno Dynamics dynos, 90% of the time you will get the same wheel fig in third and fouth (because the dyno is using the roller rpm, lower gear = torque measured up/ rpm down = same fig. On our dynos you should use 3rd for a 5 speed box, and 4th for 6th speed box, however, it does'nt really matter since you end up with the same fig in the end. On other dynos you will end up with more bhp at the wheels useing a lower gear, so the 1:1 rule can be correct in these cases. However, the higher gear you use, the longer the run.. hotter the car gets etc...
Now, atmos correction, this is a bit fiddle! 30 degrees in december on the dyno sheet is just wrong. (i've seen it!). Also different dynos do it differently, ours corrects the wheel figs as well as the fly fig, the MAHA rollers just correct the fly fig etc.
And finally losses... Always get a wheel fig, and a fly fig so you can see the losses. 207 at the wheels on a Fiat coupe DOES NOT 295 equal at the fly, unless the bloke is running treacle in the gearbox... Its always an estimate, and for a fwd/ rwd should never be MORE than 20% ( less for a fwd car). 4WD, 23 to 25% seems to work well (Yes, that means a Subaru with 211 at the wheels does not do 238 at the fly
). Auto boxes, 22 to 25%, Auto 4WD's, go back to the dealership and change it for a different car
HTH helps guys, i can answer these things more indepth at the RR day.
Charlie.
Ramp rates are a bit of a red herring, a ramp rate in itself will have a small effect on the bhp the car generates because you are speeding up or slowing down the engine acceleration. However, certain dynos have modes with different ramp rates, when you switch ramp rates it can also change the dyno and user inertias, this will have a big effect on how the dyno reads.
Certain dynos (Dynojets and other American dynos) just read high full stop. Usual rule on these is to take the wheel fig and use it as the fly. I saw a megane 225 totally standard make 224bhp at the wheels the other day on one of these dynos, which i think rests my case!
The dynopack dynos should give a fig exactly between the fly and wheel fig you get on a dyno dynamics (I've tested this theory several times and it works). However, crossing out 'fly' and writing 'hubs' is not the way to do it!!!
Different dynos work in different ways, they all measure the torque at the wheels, then most use the rmp of the rollers to get a BHP fig. So, technically, as long as you don't have wheelspin, this fig should be correct (certainly on our dynos). The user then either tells the rpm the car is doing, (either by a pick up off the leads, or by scaleing it) and the dyno can then draw the torque against the rpm, nows heres the scam, by putting a shorter ratio into the dyno, the dyno thinks the car is generateing more bhp at a lower rpm.. = Higher torque readings
Now, on Dyno Dynamics dynos, 90% of the time you will get the same wheel fig in third and fouth (because the dyno is using the roller rpm, lower gear = torque measured up/ rpm down = same fig. On our dynos you should use 3rd for a 5 speed box, and 4th for 6th speed box, however, it does'nt really matter since you end up with the same fig in the end. On other dynos you will end up with more bhp at the wheels useing a lower gear, so the 1:1 rule can be correct in these cases. However, the higher gear you use, the longer the run.. hotter the car gets etc...
Now, atmos correction, this is a bit fiddle! 30 degrees in december on the dyno sheet is just wrong. (i've seen it!). Also different dynos do it differently, ours corrects the wheel figs as well as the fly fig, the MAHA rollers just correct the fly fig etc.
And finally losses... Always get a wheel fig, and a fly fig so you can see the losses. 207 at the wheels on a Fiat coupe DOES NOT 295 equal at the fly, unless the bloke is running treacle in the gearbox... Its always an estimate, and for a fwd/ rwd should never be MORE than 20% ( less for a fwd car). 4WD, 23 to 25% seems to work well (Yes, that means a Subaru with 211 at the wheels does not do 238 at the fly
). Auto boxes, 22 to 25%, Auto 4WD's, go back to the dealership and change it for a different car HTH helps guys, i can answer these things more indepth at the RR day.
Charlie.
Edited by vixpy1 on Wednesday 28th June 12:32
ooh crumbs, so my auto 4wd jobby is losing masses of power through the transmission!
Well, in all honestly, I knew that. What does surprise me though is the guess work apparantly involved in these things. If the particular model of car isn't known to the system, then the % of losses are guessed at. For instance, I went to a RR day (same make rollers as Vixpy's) through the one make club a few months ago and the losses for an EVO were used to produce the numbers, many members went away very unhappy and very skeptical.
I'm not trying to cause an argument, I just find it odd that important figures seem to be guessed or assumed and that one rolling road can give completely different figures to another.
Am I completely wrong, just misguided or bang on the money?
Well, in all honestly, I knew that. What does surprise me though is the guess work apparantly involved in these things. If the particular model of car isn't known to the system, then the % of losses are guessed at. For instance, I went to a RR day (same make rollers as Vixpy's) through the one make club a few months ago and the losses for an EVO were used to produce the numbers, many members went away very unhappy and very skeptical.
I'm not trying to cause an argument, I just find it odd that important figures seem to be guessed or assumed and that one rolling road can give completely different figures to another.
Am I completely wrong, just misguided or bang on the money?
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