I've done a search on this and still can't get the right answer. I was reading the new gmhightechperformance and their 86 iroc makes approximately 410 flywheel horses which according to them is 330rwhp is this correct? How did they determine this?

Thank you

 

The general rule of thumb is a 20% loss by the time it gets to the rear wheels. Without using an engine dyno and a chassis dyno, it's just guessing. Even 2 of the same vehicles will come up with different numbers due to machining tolerances.

 

( slopokrodrigez) love the forum id

Speedy Gonzalez 4 life

i have a speedy gonzalez stuffed animal riding shotgun with me when ever i dont have any passengers in my car

 

i know a lot of people use a percentage, but i do not think that is too accurate. without an engine dyno everything is a guestimate(unless you have a PhD in Physics or something). IMO, the best thing to do would be to find about what you 'think' your parasitic driveline loss is in stock engine form, and keep that loss with your engine mods. For instance, if you think you lose 15% on a stock motor that makes 285hp, your rwhp is 242.25. This equates to a loss of 42.75hp in the driveline. Now, say you have NO driveline changes and you swap the motor or add mods and now you make 350rwhp. This would be more likely to be 350 plus the 42.75 HP you lose through the driveline for a total of 392.75 HP at the flywheel/Flexplate. If you were to use a 15% rule, you would think you are making 411.76 HP at the Fly/Flex. That is almost a 20hp difference.

do as you like, because this is just My theory. most people add 15% for a manual and 18% for an auto....

 

The general rule of thumb:

Average drive train loss (DTL):

• 15% for manual transmissions
• 18% fo automatic transmissions

RWHP from FWHP: FWHP * (1.00 - DTL) = RWHP

FWHP from RWHP: RWHP / (1.00 - DTL) = FWHP

So, to determine FWHP for a manual tranny vehicle that produces 250rwhp on the dyno....

250rwhp / (1.00 - .15) = fwhp
250rwhp / (.85) = fwhp (simplify)
294.18 = fwhp (the answer)

 

Neither "theory" is correct.

Driveline loss consists of "fixed" losses - the inertia losses which do not vary with HP transmitted (unless you alter the rpm/sec dyno rate), and "variable" losses, like friction losses, that are largely proportional to the load being transmitted.

The "fixed" component is actually fairly small. Keith at WS6.com has done some "coastdown" dyno tests. This measures the "fixed" component of driveline loss, and typically is 20HP or less. That is the inertial component... the power absorbed in the rotating masses downstream of the flywheel, to bring them up to speed.

The remainder of losses are "variable", and for a manual tranny setup are pretty much proportional (a percentage) to the torque being transmitted. This is the friction in the gears and bearings.

I did engine dyno tests with my engine before it went into the car, using the full intake and exhaust systems. Then we put the engine in the car and compared the results on the chassis dyno. With the M6, the losses ranged from 12.8% at approx 500flywheel HP to about 12.2% at approx 800flywheelHP. Solving the actual data as simultaneous equations indicated the "fixed" component of loss was about 16-18HP.

With the TH400, high stall, non locking tranny, losses ranged from 19.8% at 500flywheelHP to over 21% with 800flywheel. But that should not be compared to something like a 4L60E with a "normal" stall, locking convertor.

 

Just had mine dyno tuned today after motor install. When I had it dyno'd at the engine builders, it layed down 557 FWHP & 490 PD-TQ.

I have,
1. TH375 with lockup in 2nd & 3rd gear
2. 9'" rear with 4:30 gears
3. 28" tall MT ET slicks
4. 3" chrome molly driveshaft.

At the dyno today only put out 390 HP & 390 PD-TQ. THAT IS 25% loss. Frind of mine has same set up as mine but he is running a M6 and he only lost 16%.

Go figure.

 

Bitcypher you mean 1.00 - .15, not 100-.15....

 

Thanks "kick" I like speedy and Odie from Garfield. Anyway, with all this debating going on and numbers flying, I believe y'all have proved the law of averages that all magazines use is 20% loss. Since all vehicles of the same make model and year are not created equal, arm chair dynos have to limit the guessing to some reasonable number.

BTW "y'all" is singular and "all y'all" is plural. Keep that in mind during the next english lesson.

 

BES: Thanks, it's good to have another real data point along with Fred's. For auto trannies, I think that there is a lot of variability between different combos when it comes to comparing calculating converter "efficiency". It seems to me that it depends not just on the converter, but on how much power you are trying to put through it. IWO, test the same converter behind two different motors, and the calculated efficiency will vary consdierably.

Some things you see at the track seem to bear this out. A car "driving through" the converter would represent a situation where the converter would have very poor efficiency. I think this occurred with the first converter I tried in my car this year. Never took it to the track, but on the dyno the revs would flare to about 5,200rpm and the wheels would barely accelerate. The measured rwhp was obviously low. I sent the converter back and Coan installed a new stator. With that simple change, the car behaved more normally on the dyno and runs pretty decent at the track, though I still feel it's not optimal. There was ~75rwhp difference on the dyno between the two converters.

Rich Krause

 

9" rearend is probably responsible for some of the extra loss as well. They eat up more power than do 10/12 bolts.

 

Something is wrong with my set up. Loosing 167 HP is not normal. Don't think it is in the rearend. Beleive it all is in the torque converter.

 

If you do the math right its even worse than that:

390 / 557 = 0.700

Your losses are 30%. Seems unreal with a locking convertor.

-Were both engine and chassis dynos "SAE corrected"?

-Were both dyno's done at approx. the same rpm/sec acceleration rate?

-Was the full air intake and exhaust system (to the extent possible) installed fot the engine dyno test?

-Was the convertor in fact locked up?

-If not locked, have them plot (or give you the data table for) HP vs. engine rpm vs. MPH. With that data, you can estimate the convertor "slip". Mine is about 11% under 800 lb-ft torque. The guys who do all my work tell me that is a reasonably tight convertor - I don't know, because I know NOTHING about automatics. This is the first "racing" automatic I have ever owened.

The Ford 9" is worth an additional 3% loss, according to published (National Dragster) data. So.... 25% for your drivetrain, assuming they didn't lock the convertor, and allowing for the 28" tires.... yes, they WILL make a difference on a chassis pull..... seems not unrealistic. But then the TH350 is supposed to be more efficient than the TH400.....

 

not to mention with tall gears like that, i'd take off 2-3 or more hp out of it...

 

Ooops! My typing got the best of me and I mistyped 100% as 100.0 rather than 1.00. I fixed my post so that it doesn't screw someone up when it comes up in search results. Thanks.