Estimating RWHP from FWPH?

[mike_starling]

Guys,

Have some engine dyno stand numbers from my engine build (508 hp at 6800 rpm, 460 lb-ft of torque at 3800 rpm with more than 400 lb-ft at 6200 rpm). Numbers are with full accessories including the air conditioning compressor.

Am curious what loses can be expected through the transaxle when the car is completed. Over the years I've seen lots of percentage loss numbers over quite a range. Anyone have both measurements with G50, G50/50, or G50/52 transaxles?

 

[Mark Worthington]

A good estimate will be 15% to 20% loss through the drivetrain. The size of the wheels and tires you use on a chassis dyno will have a significant effect.

 

[Bill Musarra]

Mike,
Now that you have your dyno numbers, you need to find a good chassis dyno to see real losses from the drive train. I know that there will be some variation from machine to machine, but I have found out around the Atlanta area, racers are just as interested in rear wheel numbers as they are in engine numbers. That affects how they setup their cars, especially for drags. As a result we now have a multitude of chassis dynos around town. Check with your dyno guys and I bet they can give you more than one that is close to you.
Bill

 

The numbers jump depending on who you ask. I have an e-mail from kieth Craft (after I told him I will CHASSIS DYNO his engine) indicating 30% drop, that would 150HP in your case!!!

IMHO If you REALY want to know the number, Chassis Dyno your car when the engine is installed. Another way is to drag race your car. If you are doing high 12s and 13s,,,ohhh well. If your doing low 12s and mid 11s then that 508 number is real.

Ohh, one more thing, one's ego need to be checked at the door before going for chassis Dyno. If unable to do so, DO NOT go there /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif

 

[mike_starling]

Guys,

The transaxle and hookup losses will obviously vary some, but the transaxle itself will have a significant impact on with them. I too have seen the 15-20% and more reported other places.

That's why I asked if anyone had actually done both to a car fitted with a Porsche transaxle.

I believe in this case, the builder's dyno will be pretty accurate and will at least have been a good way to initially tune the engine. He has a Master's degree in Automotive Engeering and knows that the work is likely to be checked.

So ... anyone have both numbers on a GT40 built with a Porsche transaxle?

Thanks,

 

Rick Merz has a GTD with a G50 transaxel and a stroked 351W. If I am not mistaken, when I talked to him (last month) he indicated 375 - 400 Rear wheel HP. If he can tell us the engine dyno number, that would be what you are looking for. Rick,,,,where are you?!

 

[Neal]

There are two prominent chassis dyno manufacturers, Mustang and Dynojet. I have experience tuning the same car on each. The drivetrain losses equate to 15-20% of FWHP. Interestingly enough the operator of the Mustang setup ran the car to 4000rpm in 4th (1:1) and let the car coast to a stop. I was told this "calibration" allows for better compensation of drivetrain loss.

I wouldn't worry about peak HP numbers so much. A chassis dyno is a great way to tune the HP and TQ curves allowing for a much more pleasant ride.

 

[Howard Jones]

I am having a hard time with the % loss idea. Are we saying that if I hook up a 300 hp 302 to a 2 speed AA fuel car it will use up 45 hp(15%). But if I put the AA fuel 6000hp motor back in it will now take 900 hp to drive the gearbox? By the way 900 hp coverted to power is nearing half a million watts! That is a LOT of heat even for a short time.

I think that a gearbox must take something more like a fairly contant amount of power to drive it.

Thats my guess anyway, anybody know for sure?

 

[Malcolm]

I go with Howard on this as I have posted elsewhere. I beleive in a fixed loss for the type of gearbox and then add a small % for frictional increses as you cane the car and exceed normal operating tolerances. IMHO of course! On my renault box I suggest a 40 bhp loss from flywheel to rearwheel bhp. I would not have thought the G50 to massively different, in theory at least.

when I had my car on a rolling road dyno it acheived 306 at the rear wheels. My engine spec was predicted at the 350 flywheel bhp so perhaps this is some valid evidence?

 

[James]

Malc and Howard,

I think your both on the ball.

Think about it. If you've got a 600bhp engine, how can you lose 90bhp, all to friction!! That's a lot of heat.

I'll probably stick my 40 on the rolling road and see what happens.

I'll have figures for both then engine dyno and chassis dyno.

I would like to see over 400BHP at the wheels.

Although my engine was dynoed 449BHP at 6600RPM. We've since removed the big Edlebrock water pump for an electric Davies Craig item, and used a nice crossover exhaust. Should be good more a few more BHP.

It'll all be tweaked again by Fraser on the road with the Motec etc.

J.P

 

[Paul W]

10 - 12 % is more of a approx loss figure - otherwise gearboxes just wouldn't take the strain and would all need cooling systems.
These figures usually get hyped on dyno's to exaggerate the tuning gains you have made.

 

[Ron Earp]

SAme debate from last yeat, didn't get a conclusion then either.
--

I agree with this too. If it is a fixed percentage then the horsepower lost gets very high with high horsepower motors.

For example, if my 4.6L DOHC on 24 lbs of boost makes 760 hp at the crank with a 15% driveline loss then I'm losing 114 hp in the driveline and transmission.

One horsepower is 745 watts, so, we're losing 84930 watts on the way to the rear wheels. 1 W is 1 Joule/second. 84930 joules is a lot of energy and will generate a heck of a lot of heat.

Where does this heat go? If it is from friction then some fluid temperatures are going to get really hot.

Just looking for answers in an interesting discussion.

Ron

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[James]

[ QUOTE ]
Where does this heat go?

Ron

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[/ QUOTE ]

Into heated discussion about drivetrain losses!! /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif

 

IMHO of course, that is why all these LARGE HP claims are SOOO BOGUS. Look at Malc's set up. 307 at the rear about 360 at the flywheel. Makes perfect sense.

When I was looking to buy my new engine, all the guys on the net were claiming 400 -500 hp numbers. Then when I would bring up chassis Dyno, they would say "AHHHH, WELLLLL, it could be 30% or more loss"!!! Loss due to what??!! Bull & Horse Sh...??/ubbthreads/images/graemlins/crazy.gif /ubbthreads/images/graemlins/crazy.gif

If you have 340hp at the wheel you DON"T have 550 at the flywheel, unless it is 4 X 4 1/2ton truck /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif

 

[Howard Jones]

Ya you're right Ron, its been a while since basic elect. That 900 hp loss is more like TWO million watts. That should be enough to turn the alum gearcase of our dragster to fluid VERY quickly. At the very least the rear of the car would be glowing like a afterburner as it crossed to finishline.

Even our SBF @ 400hp with a 15% loss in the gearbox would be something on the order of 40-50 thousand watts. If you allow for low power operation driving around town at 1/4 the power output its still about 10,000 watts.

It just can't be that much.

My guess is about 30-50 hp losses at full power and fractions of that much at low speed limmit driving.

 

[Dave Wharran]

Here's a link, LINK, to a bit of data from Lloyd Butfoy on HP losses on a -2 ZF. It is from the Pantera International BB where Lloyd moderates a section on ZF gearboxes.

The BB is here:
Pantera BB

 

[Rick Merz]

If you model my engine it should be making about 500hp at the flywheel but in reality I have about 390 rear wheel horsepower. I did not have my engine dynoed before I installed it so I cannot make an accurate assumption as to my transaxle losses. I believe that I am loosing some power due to tuning issues plus an additional 15% through transaxle losses. I would think that at normal highway speeds transaxle losses would be much lower, maybe 3-5% otherwise like others have said the transaxles in our cars would be cooked. Usually only racecars have coolers on manual transmissions because they are operating at peak horsepower most of the time and if the heat is not dissipated the transaxle or transmission will fail.

 

[Howard Jones]

Rick, that's just about it. The only other thing to do would be to do a back to back, engine dyno and then chassis. This will still not give you a real world gearbox loss because you would have waterpump, alternator, AC maybe, tires, etc. that would have to be subtracted from the total difference between the engine dyno and chassis dyno.
And like you said, who drives around with their foot to the floor all the time anyway. Well.. you know what I mean.

 

[Gary Gibbs]

The majority of the losses that do occur in the drive train are turned into heat.

Here is a good test. Measure the temp of your transaxle sitting in the garage. No losses now and temp = ambient. Now run the car around the block SLOWLY but run it up to max RPM but not max power (maybe 50 HP). Do this for 5 minutes. Measure transaxle temp. Now run the car full out on the track for 5 minutes and stop and measure the transaxle temp. I expect you will find the latter case produces a significantly higher temp. Thus there is more heat being generated (thus HP absorbed) during track use.

Another good example is my excursion when towing 12000 lbs (and using more HP) the rear end gets pretty warm. The same rear end when cruising without towing stays much cooler (but still above ambient). Why does it get warmer while towing? Where does the extra energy come from?

Why do Winston Cup cars use external pumps and coolers on their 9" rear ends? They are basically the same rear end that was used in passenger cars for several hundred thousands of miles with out additional cooling. Why go to the trouble to cool the rear end?

 

[James]

I bet the type of oil used in your transaxle makes a difference to the power lost. I am using the Neosynthetic 75W-90RHD.