Cheapest and easiest method of reducing engine BHP?
Discussion
Not a commom question asked on here, I know 
I have a 5.0L, Rover V8, fitted with quad 48mm Dellorto carbs delivering 380bhp at the fly. The race series that I use it in have decided to drop the class power to weight ratio which means I have to either load up with loads of weight, drop the engine bhp by 25bhp or a combination of both.
I don't really want to put any weight in the car so was considering just retarding the ignition timimg by a degree or two and restricting the air flow into the engine, a little, by installing those little mesh filters directly over the trumpets. Engine reaches peak power at 6000rpm.
Any other ideas chaps?
I have a 5.0L, Rover V8, fitted with quad 48mm Dellorto carbs delivering 380bhp at the fly. The race series that I use it in have decided to drop the class power to weight ratio which means I have to either load up with loads of weight, drop the engine bhp by 25bhp or a combination of both.
I don't really want to put any weight in the car so was considering just retarding the ignition timimg by a degree or two and restricting the air flow into the engine, a little, by installing those little mesh filters directly over the trumpets. Engine reaches peak power at 6000rpm.
Any other ideas chaps?
NTEL said:
Not a commom question asked on here, I know
I have a 5.0L, Rover V8, fitted with quad 48mm Dellorto carbs delivering 380bhp at the fly. The race series that I use it in have decided to drop the class power to weight ratio which means I have to either load up with loads of weight, drop the engine bhp by 25bhp or a combination of both.
I don't really want to put any weight in the car so was considering just retarding the ignition timimg by a degree or two and restricting the air flow into the engine, a little, by installing those little mesh filters directly over the trumpets. Engine reaches peak power at 6000rpm.
Any other ideas chaps?
And exactly how are they determining what power you have, or have reduced ?I have a 5.0L, Rover V8, fitted with quad 48mm Dellorto carbs delivering 380bhp at the fly. The race series that I use it in have decided to drop the class power to weight ratio which means I have to either load up with loads of weight, drop the engine bhp by 25bhp or a combination of both.
I don't really want to put any weight in the car so was considering just retarding the ignition timimg by a degree or two and restricting the air flow into the engine, a little, by installing those little mesh filters directly over the trumpets. Engine reaches peak power at 6000rpm.
Any other ideas chaps?
NTEL said:
stevieturbo said:
And exactly how are they determining what power you have, or have reduced ?
All the cars in class use DL1 data loggers and, to be fair, they are pretty accurate when compared to rolling road figures. Data is taken from each car after every qually and raceThe DL1 can estimate power but it does require certain data to be input to the unit, and any test should be carried out with no wind and on a totally flat straight road.
But in a race situation...havent a clue.
Sounds more like they might try and estimate/control power on acceleration ?
As others say, a simple throttle stop would probably be the easiest. But given the crude power measurement...that's going to be impossible to determine by how much, or if you even need to restrict it at all.
You'd need to find the exact parameters they use within the DL1 for the calculation in order that you can stay within this limit.
But in a race situation...havent a clue.
Sounds more like they might try and estimate/control power on acceleration ?
As others say, a simple throttle stop would probably be the easiest. But given the crude power measurement...that's going to be impossible to determine by how much, or if you even need to restrict it at all.
You'd need to find the exact parameters they use within the DL1 for the calculation in order that you can stay within this limit.
the stigs dad said:
Remap it for less peak hp and more midrange torque. Or introduce a lower rev limiter.
It would need an ECU for that I think.The easiest way by far is going to be to tweak the throttle linkage to stop it from opening fully.
But possibly putting air filters on would be a sensible thing - they'll pay dividends in engine longevity too.
There is no point in doing anything until you know how they will determine your power.
You could just tell them you now have 25hp less, and it's up to them to prove otherwise.
Then you can work with your setup to fall within their measurements.
I just dont see how using the DL1 can be consistent unless the tests are carried out under very controlled and repeatable circumstances.
ie same weather, road, tyres, fuel etc etc etc which is pretty impractical for all entrants.
You could just tell them you now have 25hp less, and it's up to them to prove otherwise.
Then you can work with your setup to fall within their measurements.
I just dont see how using the DL1 can be consistent unless the tests are carried out under very controlled and repeatable circumstances.
ie same weather, road, tyres, fuel etc etc etc which is pretty impractical for all entrants.
If you currently have 380bhp @ 6000 rpm then you also (should) have 355bhp @ 5600rpm, so as has been said you could limit the rpm to this point....
Ideally, why not have 355bhp @ 5600rpm and also gives 355bhp @ 6000rpm, as rev limiting the car could mean you change gear more times than you'd like...
So the intake throttling would be the way to do that (to the best of my knowledge).
As to the "prove it" issue raised by Stevie, imho I'd guess that any car that is noticably faster on the straights would be given extra scrutiny...
Ideally, why not have 355bhp @ 5600rpm and also gives 355bhp @ 6000rpm, as rev limiting the car could mean you change gear more times than you'd like...
So the intake throttling would be the way to do that (to the best of my knowledge).
As to the "prove it" issue raised by Stevie, imho I'd guess that any car that is noticably faster on the straights would be given extra scrutiny...
stevieturbo said:
The DL1 can estimate power but it does require certain data to be input to the unit, and any test should be carried out with no wind and on a totally flat straight road.
But in a race situation...havent a clue.
Sounds more like they might try and estimate/control power on acceleration ?
As others say, a simple throttle stop would probably be the easiest. But given the crude power measurement...that's going to be impossible to determine by how much, or if you even need to restrict it at all.
You'd need to find the exact parameters they use within the DL1 for the calculation in order that you can stay within this limit.
Unless they are going to strain gauge the driveshafts or find some other way of measuring power, they are simply deriving it. So that means there are loads of ways of getting past it.But in a race situation...havent a clue.
Sounds more like they might try and estimate/control power on acceleration ?
As others say, a simple throttle stop would probably be the easiest. But given the crude power measurement...that's going to be impossible to determine by how much, or if you even need to restrict it at all.
You'd need to find the exact parameters they use within the DL1 for the calculation in order that you can stay within this limit.
Just fit a line lock to the brakes, hold a small amount of brake pressure for accelerating then release it for coasting down.
To be honest, I wouldn't even bother doing that; I sincerely doubt they will be accurately modelling every vehicle to calculate your BHP.
This is actually a very good, and quite complex, question and one which has attracted some spectacularly incorrect suggestions. To answer it fully requires a very detailed knowledge of vehicle dynamics and ideally a computer simulation program to run through the options.
If you were just to try and reduce any part of the power curve above a certain bhp figure without affecting the rest then this is quite hard to do without fully mappable fueling and ignition. Restricting throttle butterfly opening will hurt power everywhere except maybe at rpms too low for you to be bothered about. Putting a shorter duration cam in is even worse. It will reduce the rpm limit and hurt all power at high rpm, not just the excess bit you are trying to reduce. Remember you don't want to lose any part of the power curve above peak power rpm that has already dropped below the cutoff bhp limit.
Reducing choke sizes will add low rpm power that you probably never use on a race track anyway and hurt everything else.
The real answer is actually more about aerodynamics than anything else. At low speed where aero drag is not a factor then vehicles accelerate according to their power to weight ratio. Adding 10% power but also 10% weight won't change things much at very low speed. BUT, and it's a big but, at higher speeds the main thing the engine is fighting is aero drag. The car with more power and the same percentage more weight will have a higher top speed and reach any given speed faster.
So if your weight penalty really is correctly calculated to just preserve a given notional power to weight ratio then leave the engine alone and go for the weight.
For instance if I input the details for my Ford Focus into my simulation software and then add 10% more power and 10% more weight the 0-60 mph time barely alters but the 60-120 mph time drops by almost 6 seconds and the top speed increases by 4 mph.
However pure acceleration is still not the complete answer. The extra mass might hurt cornering speeds and braking so it very much depends on how much mass we're talking about here, where you can locate it and what effect it has on the chassis. I'd suspect though that its effects can be compensated for and the best solution will still be to keep as much power as you can get.
If you were just to try and reduce any part of the power curve above a certain bhp figure without affecting the rest then this is quite hard to do without fully mappable fueling and ignition. Restricting throttle butterfly opening will hurt power everywhere except maybe at rpms too low for you to be bothered about. Putting a shorter duration cam in is even worse. It will reduce the rpm limit and hurt all power at high rpm, not just the excess bit you are trying to reduce. Remember you don't want to lose any part of the power curve above peak power rpm that has already dropped below the cutoff bhp limit.
Reducing choke sizes will add low rpm power that you probably never use on a race track anyway and hurt everything else.
The real answer is actually more about aerodynamics than anything else. At low speed where aero drag is not a factor then vehicles accelerate according to their power to weight ratio. Adding 10% power but also 10% weight won't change things much at very low speed. BUT, and it's a big but, at higher speeds the main thing the engine is fighting is aero drag. The car with more power and the same percentage more weight will have a higher top speed and reach any given speed faster.
So if your weight penalty really is correctly calculated to just preserve a given notional power to weight ratio then leave the engine alone and go for the weight.
For instance if I input the details for my Ford Focus into my simulation software and then add 10% more power and 10% more weight the 0-60 mph time barely alters but the 60-120 mph time drops by almost 6 seconds and the top speed increases by 4 mph.
However pure acceleration is still not the complete answer. The extra mass might hurt cornering speeds and braking so it very much depends on how much mass we're talking about here, where you can locate it and what effect it has on the chassis. I'd suspect though that its effects can be compensated for and the best solution will still be to keep as much power as you can get.
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