RNShaw_951

I am going to get my car DYNO'd very soon. Does anyone know what % of HP get's absorbed by the time it gets to the wheels? The last car I had it was 17% ( FF Transversly mounted ) I want to get a baseline HP so that when I have fun with my other mods I can see the difference.

Thanks!

Ryan
( 951 is finally back in action with the SFR 3" exhuast system, Tial 35mm WG )

wjk_glynn

Hi Ryan,

This particular subject usually gets a lot of discussion and if you search throught the archives, you'll see lots of threads with little agreement on this.

From what I've read and heard from other folks, 15% is the (bare) majority view for the 944 series and thats the number I use.

However, there is an argument that it doesn't really matter what the loss factor is. Its the actual-at-the-wheels that counts.

By all means get a baseline. Make sure you get actual and SAE (or DIN) corrected numbers.

Other things to note are:

(1) Tire pressure (higher pressure = lower rolling resistance = less loss)
(2) Rear alignment (scrub, toe, etc) as it affects losses
(3) Operating temperature (warmer oil reduces losses), i.e. test the car after you've driven it for 20 minutes to allow everything to warm up.

And another thing... The number of times I've heard of modified 951s going *BANG* while on the dyno is scarry. Keep a very carefull eye out knocking, etc.

I had my own car (an S2) dynoed recently and just for heck of it, you might find this thread moderately interesting:

<a href="http://forums.rennlist.com/cgi-bin/rennforums/ultimatebb.cgi?ubb=get_topic&f=15&t=011134&p=" target="_blank">http://forums.rennlist.com/cgi-bin/rennforums/ultimatebb.cgi?ubb=get_topic&f=15&t=011134&p=</a>

Karl.

MCNETT

Agree with Karl. All of the info that I have heard puts the loss somewhere between 12-15%
-Mike

John Welch

What is the compulsion everyone has to report hp in term of hp at the crank. Dynos typically measure hp at the wheels. Although even that number has had some 'corrections' applied to it, it is still the ONLY number that should be used in hp comparisons.

If I say I have 400hp at the crank, but don't tell you the fudge factor I added the the measured hp, you don't have any useable data.

If I say I have 350hp at the wheels, now you have a 'real' number, and can compare that number to other cars' wheel hp.

Sorry about the rant, but I don't understand why people feel the need to 'hide' things. If you measure a certain number, don't add x% to it and then tell me the number, tell me what you measured.

RNShaw_951

I agree with the comment that the only number that matters is at the wheels, the only reason I ask is I have friends that like to compare their cars HP with mine, well since they are not gear heads and have no idea what WHP stands for, it is just easier to give them an approximate Crank HP for them to compare with the number they get from their owners manual ( usually in Crank HP ) Even though I very well know that HP is not everything and that Weight, Gearing, Suspension, Torque and Torque over time are much more important numbers than just WHP or BHP. Your comment John is not a rant, its a valid point and thank you for your input. Thank you for your data also Karl and it was a very interesting thread.

Happy Driving All,

Ryan Shaw
951

RPG951S

Frankly, given some of the wild variations in dyno readings on the SAME type of dyno in two different locations, I'd say that anything short of comparining two cars on the SAME dyno, on the SAME day/time is pointless.
There are WAY to many dyno-operator dependant variables . Comparing a mustang dyno to a dynojet dyno, you'll see a huge difference based on how they measure the results.
Even testing two installations of the EXACT same dyno, in different locations, is bound to produce some variations in the results. Then, don't forget variations in altitude, temperature, pressure... etc..

In my humble opinion, the only way to compare two cars is to measure them both on the same dyno. The only other usefull thing is to compare two different runs of the same car (with different configurations (more boost...etc))... to see what the DIFFERENCE is.

Forget saying my car makes 300hp. Saying it makes 70hp MORE than stock is probably more telling...

944Fest (aka Dan P)

I'd say flywheel HP does have it's uses. For starters, all manufacturers and magazines refer to flywheel numbers. If you want to compare your car to new cars coming out, you can do a #/hp calculation, or see how other cars with different HP do timewise..etc. Also, It's kind of funny to know that GM may get 300 some HP flywheel from an LT1 that the heavy duty driveline soaks up 25% of it, (the king of all parasites-per my dyno guy)..Thats why a 300 HP flywheel Porsche can dust a 300 HP flywheel Chevy with similar weight.

Sure, there are variables, but I like to compare my 340HP (289 @ wheels) with new stuff coming out to see who I'll stand a chance with down the backstraight.

"At the wheels" IS a great measurement, but it's not always available..

<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

John Welch

Good points guys...

I guess flywheel hp IS good for something after all!

Danno

Check out this thread: <a href="http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic&f=18&t=000474" target="_blank">Topic: transmission losses on dyno run </a>

CamB

Craaaap, why didn't I find all this when I last looked. I was trying to figure out what a 911 loses (my tuner used 9%, and the consensus was this was a little light - maybe 12% was more accurate).

Stands to reason a 911 would be less than a 944 transaxle design - less bits???