Injuneer - I've heard that dynoing on slicks or ET streets will also scrub RWHP. Got any figures on that??? I was wondering what I'm loosing by running the 28x13.5 ET streets, as opposed to something like a stock 245 radial.

 

If stock tires will hold all your hp on the chassis dyno, yes, you should see a gain vs. slicks.

Even tire pressure will make a difference. Try 45 psi (on p-metric tires) vs. 25 psi back to back. You might be surprised.

Could an unscrupulous dyno operator make his "tune" look better by overinflating your rear tires for the "after' run? Nah, nobody would do a thing like that.

Grat post by Injuneer. Read it a couple of times, guys.

 

Excellent, and thanks for the replies (esp Injuneer). Like I said (in this thread or the one I posted in the lounge; I can't remember), I hadn't given it a thorough engineering thinking session. What Injuneer (and others) said sounds right to me. I was only accounting for the fixed rotational inertia losses, and incorrectly treating the frictional losses as a constant as well. Injuneer's explanation sounds pretty good to me, and it sounds like he's done the testing on his car to back it up.

VERY interesting to see that the inertial losses are so much less than the frictional losses.

So, I guess to do the quick and dirty conversion from rwhp to fwhp, a percentage might be pretty dang close (and in fact, either method gives similar results until you get into some seriously big hp numbers!). Excellent discussion guys, and thanks again for the feedback.

 

It's not a fixed percentage. The more powerful the car, the less proportional hp loss. A lot of the loss is inertial, which does not vary with hp. This fixed component is constant, while the other losses are proportional to hp. And of course, as mentione, the TC in an automatic will eat up hp.

For typical manual 4th gen's, figure 12-15%. An automatic with a tight converter may lose an additional 3-5%. If the converter is loose, the loss may be much higher.

Rich Krause

 

When I dynoed my car it was with my DR's @ track psi (14). I know many guys that show up with street tires @ 50 psi to get better #'s.

#'s are great here on the internet, but on the street or the track, it is another story.

You would think that the more powerful an engine gets, the less is takes to turn the drivetrain.

500 fwhp = 425 rwhp in a 15% loss = 75 fwhp loss
800 fwhp = 680 rwhp in a 15% loss = 120 fwhp

So how could a substantially more powerful motor takes 45 fwhp more to to turn the identical drivetrain than a much weaker engine?

I just think a general consenses should be on how much a tranny takes to turn...like a TH400 takes 60 fwhp in most apps, a t56 takes 30 fwhp in most apps, etc....

 

Read Injuneer's post above. You are saying basically what I said earlier in the thread, and you are partially correct. Some portions of the loss are a fixed hp number. Say, for example, it takes 30 hp to overcome the inertia (mass) of the gears, driveshaft, axles, wheels/tires, etc in the driveline. That accounts for PART of the losses (the "fixed" part). However, there are other losses: frictional losses where the drivetrain parts interact. For example, frictional losses in the ring and pinion or in the gears of the transmission (not to mention the tq convertor in an auto). These frictional losses are somewhat proportional in nature, if I'm to understand Injuneer's explanation above. So, using your example, the friction claims a bigger amount of hp on the 800 hp engine than the 500 hp engine, because there is more power going through the driveline.

Also, according to Injuneer's results on his car, the frictional (non-fixed, proportional) component contributes more (especially in high-hp cars) than the fixed hp required to overcome driveline inertia.

 

Right, that's what I said? Didn't read the whole thread however. Just trying to help based on many thousands of chassis dyno runs. Hundreds just on my own cars.

Rich Krause

 

I just made a dyno run yesterday and the dyno calculated my horsepower at the crank the same way. 478hp and 490 lb ft.

You plug in the type of tranny, converter stall, rear end, and tire psi and it calculates your crank hp. You get the same number if you divide by .79 etc.

 

Yeah, I was telling CANTONRACER, not you, to check out Injuneer's thread for an explanation. Looks like I could have pointed him to yours as well.

 

Yup division is the mathmatically correct way to do it. If you take 100 hp x .85 you get 85 hp right?? Take that same 85 HP then try and multiply by 1.15 and you get like 97 !!! But take the 85 HP and divide by .85 and you're back up to 100 HP