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crank HP versus RWHP
is there a general rule of them how much HP makes it to the real wheel??...so if youre getting 450 at the crank, would the car dyno at 350hp???...also, how were older cars rated..like a 65 corvetter that was 365hp...was that at the rear wheel or the crank??..
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Tim,
Usually about 15%-20% loss. Depends on transmission, exhaust, etc. Also, engine dynos vary in their numbers. I believe that in the 1960's manufacturers rated their cars on engine dynos with nothing hooked up that would take HP away to get higher numbers to sell more cars by having bragging rights to "more is better". Some engines like the Chrysler Hemi and Ford Cammer were rated far lower than what they actually produced to allow their truly outrageous HP numbers to get by things like insurance companies. Basically what your car does on a road or track is the only thing that REALLY matters. 0-60 or quarter mile times are best to see what your car ACTUALLY does powerwise. Even chassis dynos do not accurately reflect what your car will do in "real life", since your vehicle is strapped in straight and traction is not as big an issue as on the road. Hope this helps. Cheers! Dave |
On our Mustang chassis dyno, the average loss is 17%.
The old days were rated at the crank. |
From what I've seen around here, no! The engine builders are not running thru air cleaners, or sidepipes, and some w/o water pump and and alternators. On production vehicles the 15%-20% /manual-auto rule is usually close, but around here it seams to be 25-30%, all on manuals, is the norm.
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I think the last time I check mine the fall off was around 17% which is well within tolerance. When you starting getting out of the 15-20% range you need to start looking to see if everything is tuned correctly & breathing correctly. A little adjustment can have a significant effect on the reading.
J.A. |
Mine dynoed at Kieth's shop 612 horsepower 615 torque. In the car dynoed at Kieth Craft Motorsports, chassis dyno at 443 horsepower 463 torque at the rear wheels, that my friends is about 27 percent and believe me it hurts to see that kind of loss numbers. But I can live with it.
DON |
You can see in my signature line my engine dyno and chassis dyno results, which equate to about 15.2% loss on HP and about 14% on TQ. Same builder and dynos as DCCobra. There are many factors that contribute to the loss beside parasitic loss in the engine, i.e., restrictive exhaust.
:D :D :D |
Mine was about the same as DC on HP and not quite as much on torque, if it is a loss somewhere it has to be with pipes or fuel, because he ran mine with my carb with no change and only showed a 5 hp loss with my filter versus no filter. When it cools some I am going to do some pulls with and without my baffles.
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DC...Horsepower loss from engine power at the crank to rear wheels can vary greatly depending on how the car's engine is set up in the chassis vs. what the builder used on the engine dyno. Real world numbers often go far beyond the generally accepted 17-18%. Consider inlet temp, barometric pressure, tire slippage and things like alternators, power steering, air conditionong, water pump, and headers and exhaust sytems. Also ..are you driving through an independant rear suspension or live axle? 27% is probably the most honest number you'll get and nothing to be ashamed of. Badger
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Guys, I am not ashamed of my numbers at all, on the contrary my car is a rocketship. Those numbers in a BDR with an aluminum smallblock and 3.73 gears, just fantasize and you will get the picture. I am just greedy always wanting more. Mine has all the goodies and my numbers are fairly honest and probably consistent with reality. Buy the way I run race fuel (112 octane) and there is no mufflers just some vortech cones, independent rear, tk600 tranny and the oval cobra filter from finishline. I also run an electric water pump with a 130 amp alternator, you wouldn't think there would be that much parasitic drag, but it is there.
DON |
As I said in an earlier post, the ONLY thing that matters is how the car performs in the REAL world.
Don's (DC COBRA)Cobra does great there, and he is happy with the performance and not hooked on the dyno numbers. He looks at it the proper way, in my opinion. There are just too many variables to look at from engine dyno to chassis dyno to actual street performance. Add to that the fact that the results from various dynos of both types vary so much from machine to machine. Dyno results are good for bragging rights, but how the car actually runs is what really matters. Don, after looking at your pics, and reading our post, all I can say is that I'm sure that you enjoy your great running, great looking car. It's gorgeous! Cheers! Dave |
It is my belief, for these cars anyway, that the only important measure of performance is pounds per horsepower. This measurement gives you direct real world comparisons to other vehicles and does not get you trapped in the "I need 500 horse power" game.
If your replica weighs 2400 pounds and produces 375 horse power then you have a lbs/hp of 6.4 6.4 lbs/hp is the ratio of the new Z06 vette (500 h.p.) which weighs in at 3170 lbs. I have been telling customers who want very fast Cobras that handle well to consider this fact. Trying to control 500 - 600 horse power in a very light car is tedious. You will also have a more usable ride, which gets good mileage, with less mechanical trouble at lower horse power. Just my .02 :3DSMILE: --Mike / Turn Ten Racing, Inc. |
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