How much do you really lose to the wheels??!!

[yeah_knee]

Been doing a lot of research on crank HP vs wheel HP. I've heard you lose anywhere from 15-25% to the wheels. This gets really confusing when it comes time to dyno your car. And speaking of dynos, i know dynojet reads high and mustang dynos read low. My supercharged (ess 625 kit) e92 m3 reads 530 wheel HP on a dynojet... but 493 on a mustang dyno. So can someone make sense of all of this? What exact percentage of power do you lose from crank to wheel (or does it depend on different variables) ? ... and which dyno is more "accurate" to display your realistic HP numbers.

[Mkhoid]

Personally, it's just numbers. There's no accurate readings because of the way of each dynos calibration. The most accurate is crank because that's how much the engine actually puts out without drivetrain.

[Gossypiboma]

Describing drivetrain loss as a percentage is completely wrong. A specific drivetrain has a fixed amount of power loss as it transmits power to the wheels, this does not increase as you increase the cars power.

I never understood why this myth continues.

[pbonsalb]

I’d go with 15% on a stock motor, which means the 414 crank hp give about 350 rwhp. At least on a dynojet in SAE, 350 rwhp is in the ballpark for a stock car - some make 10 less, some make 10 more. That means the drivetrain loss is about 65 crank hp on a stock car.

With a supercharger, there is additional loss due to the crank power consumed driving the supercharger. I am not sure how much that is. Not much when not in boost due to the bypass valve but still a little because there is another accessory spinning. It is significant when in boost because it takes power to compress air.

I’d say 15% loss should give about the right result. If a 625 kit is rated at 625 crank hp, it should give about 530 rwhp on a dynojet. Again, some will be a little more or less and it is also Important to note with boost that there is more heat and therefore more risk of timing pull and the dyno cannot compensate for timing pull like it can for air density.

[Mvy]

On a dynojet, I was told to add 60-70 hp to the wheel result to determine the crank hp. So if you are pulling about 600 crank hp with your kit. That sound right where it should be for a 625 kit.

[SVH]

Originally Posted by pbonsalb

With a supercharger, there is additional loss due to the crank power consumed driving the supercharger. I am not sure how much that is. Not much when not in boost due to the bypass valve but still a little because there is another accessory spinning. It is significant when in boost because it takes power to compress air.

If I recall correctly Gintani comparison between their Stage 2 SC @ 7.5 PSI and their Twin Turbo @ 7.5 PSI resulted in a 50 HP increase for the TT at redline (and pretty much across the dyno chart). One could attribute the HP loss of the SC to their parasitic power consumption.

[tdott]

Unless you can engine dyno and dyno the same engine, there is no way to tell for sure.

Everything is just a guess based on the crank numbers the manufacture gives.

[yeah_knee]

Which dyno do you guys trust the most?

[Green-Eggs]

Maha Dyno will tell you both wheel and crank horsepower. It uses a very novel approach to do this. Very accurate. OEMs use it, including BMW.

[buddalun]

Originally Posted by Green-Eggs

Maha Dyno will tell you both wheel and crank horsepower. It uses a very novel approach to do this. Very accurate. OEMs use it, including BMW.

What is the approach the Maha uses to accurately gauge both HP readings? TIA.

[Green-Eggs]

Originally Posted by buddalun

What is the approach the Maha uses to accurately gauge both HP readings? TIA.

Whp is very straight forward, same method as other roller dynos. Dt loss is calculated by applying light load during deceleration (engine braking) and using the same math as calculating whp, but in deceleration gives you DT loss. Add the dt loss to whp to get crank hp. Pretty brilliant idea.