Well I got to thinking today about how all wheel drive cars have around a 20% drivetrain loss. Ok for the ease of understanding. the EVO comes from the factory with "275" hp. Lets go with 220awhp as our baseline dyno # . That is a loss of 55hp by the time it gets to the wheels which represents a 20% loss. Now say we mod our Evo to where it is making 360AWHP. That would equate to about 450 @ the motor. Now what I am trying to understand is originally the car lost 55 HP through the drivetrain, now with more power it is losing 90 HP to achieve the same job. When I hear people talking about power loss it is always the same percent even if you are making more power. Why would it take the extra 35 hp to do the same thing from your original baseline dyno? I don"t understand why power loss would increase, you would think it would stay the same???? Anyone care to elaborate. Sorry if it seems like I am rambling, just curious.


Shane

 

Only way i can think of explaining it would be that with more power running through the same drivetrain, more lose is bound to happen because of the restrictive nature of the drivetrain parts and such forth. Think of a hose with a hole in the piping. The more water running through the pipe, the more water gets lost through the hole.

 

Because in this particular equation power loss is shown as a PERCENT. A constant percentage (in this case 20%) will get larger as the number it's being applied to increases.

Example: Baseline 275hp x0.8 (20% loss) = 220.

Example: Modified 450hp x0.8 (still a 20% loss) = 360.

The power loss is still the same, but only because the numbers are larger does the power loss seem to have gotten bigger. So the car isn't wasting more power to do the same thing (like you said 35 more hp). It is wasting the exact same amount of power as before. (a constant 20%)

Sorry if I confused you.

 

Unfortunately... Drivetrain losses in a modern car are closer to a fixed number than a % from what I have seen. So a 300HP car with 250WHP will be about a 350HP car at 300WHP.

 

Forbidden gave you the right answer... 20% is 20 pecent no matter what. That is basically what it comes down to.....

 

Simple terms

The drivetrain loss of 20% remains constant, modded or not. 20% of 450 is more than 20% of 275. So the loss is the same percentage, but that percentage is equal to more because the HP is more.

 

Strange then, that in all the time I have messed with cars and the huge numbers I've seen on the dyno... That doesn't seem to be the case. Strange that Bosch in thier automotive handbook all but states that a modern car has fixed losses.

Think of it this way... Do you really think that the 700WHP EVOs out there are making 875HP @ the crank?? Look at the turbo sizing and think on it.

More over its often ignored, but driveline losses also are a function of RPM. Since most cars here will be making thier power in the same general rpm band its a small factor. But if you are talking about a EVO cranking out peak WHP at 9K instead of 6K, its going to have a hell of alot more losses at 9K.

 

yea i have to agree-- i dont think it would constantly hold that same percentage. im sure its close, but I would think as you mod it, the percentage is going to change as your setup is now changed...

if this is the case though.. im putting 291 to the wheels which means im making 363.75 crank hp!! yayayaya.. i guess for safety's sake though, i'd tell ppl im making 350 hp at the motor.. think this would be accurate or should i stick to our 20 percent loss thing and claim 360hp??

 

the 20% isn't a constant. To figure it out for your car you would have to dyno the motor it self and the dyno it in the car, and then do the math. There are many many factors that will play into your actual driveline loss....

 

Haven't most EVOs dyno numbers been higher than 220AWHP, more along the lines of 230-240AWHP?

 

Depends on Dyno. The dynojet usually the evo's see 230-240.

 

very informative post. Although some mixed opinions, most of what people were saying made sense and made convincing arguments.

 

Hi,

I would say that it's not a fixed percent and it's not a fixed hp "number" that is lost. It's probably something in between. As someone stated, the only "real" way to do it would be to dyno the engine alone, then dyno in the car, then calculate.

In the end, it doesn't really matter. Kind of like it doesn't matter if it is 220 or 240 stock or 180 on the heartbreaker dyno. It only matters if you are trying to tell someone, probably a non-enthusiast, how much power it makes. YOu will give them the "crank" amt, since it's too labourious to try to explain the whole drivetrain loss. I've encountered this with my own father who has a Porsche 911 C4s. He asked how much power my car makes, I said probably 280-300 at the wheels on a dynojet, he said that it was "almost" as much as his C4s (320 crank). I tried to explain that my car probably makes more than his car, but I gave up and now just tell him it makes about 320-330 hp.

You see a lot of people argue the 20% number, then get that by using the 270 crank hp in the calculation, as if the 270 crank hp is accurate. There are many examples of cars making more (Neon SRT) or less (last gen Mustang Cobra, current RX-7) "crank" hp than advertised.

Furthermore, it's not likely that the percentage would be fixed, and a car making 800 hp at the wheels is really making 1000 hp at the crank (from say 240 at the wheels vs 320 crank stock). That is just an example to get a "20%" loss... In this example, the car lost 80 hp stock through the drivetrain, and might lose 100 hp when it's making a lot more power, but probably not all the way to 200 hp.

Oh well, sorry to ramble. This is just conjecture on my part, but I think it makes sense. I don't know if there are actual objective experiments. Again, it's not really that big of a deal, kind of like the arguments between different types of dynos.

Thoughts?

FB

 

I was hoping to get a few views on this. I would say the loss would go up as power goes up, but I don't know if it would be a constant 20%. Engine versus wheel dyno would be the only true way.

Shane

 

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