rwhp for shelby and roush - Ford Mustang Forum
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post #1 of 19 (permalink) Old 06-05-2014 Thread Starter
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Question rwhp for shelby and roush

Im curious on how much rwhp and torque do 13-14 shelby GT500s make on dyno, i know the motor makes around 660hp, but what do they actualy make on the dyno to the wheel?
Also the 13-14 roush stage 3, i know they say 575hp, but is that really what they make to the wheels?

anyone has info about those?

thanks
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post #2 of 19 (permalink) Old 06-05-2014
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All cars are usually advertised as flywheel Hp. I'd say roughly 25% drive train loss (correct me if I'm wrong) so take that minus the total Hp and that will tell you what's at the wheels. For example a car with 100 Hp thrown about 75 to the wheels

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post #3 of 19 (permalink) Old 06-05-2014 Thread Starter
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Originally Posted by Crak88lx View Post
All cars are usually advertised as flywheel Hp. I'd say roughly 25% drive train loss (correct me if I'm wrong) so take that minus the total Hp and that will tell you what's at the wheels. For example a car with 100 Hp thrown about 75 to the wheels

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Thats what i thought. But i wanted to see if people have exact dyno number for stock 13-14 shelby gt500s and 13-14 RS3.
I know a guy on instagram has a 1080hp shelby and says it makes 918whp. Could that be true?
When u take 25% of the 1080hp its not 918whp its 810.
So is there any way that ppl can find the rwhp of theyr factory car by calculation?

Thanks buddy

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post #4 of 19 (permalink) Old 06-06-2014
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25% HP loss is a very high over-estimation. It varies by car. By that math, the 305hp 3.7 would put less than 230hp to the wheels, when in stock form they put down anywhere from 260 to 270rwhp.
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post #5 of 19 (permalink) Old 06-06-2014 Thread Starter
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Quote:
Originally Posted by Radeyo View Post
25% HP loss is a very high over-estimation. It varies by car. By that math, the 305hp 3.7 would put less than 230hp to the wheels, when in stock form they put down anywhere from 260 to 270rwhp.
Did a couple of searches and i think i figured it out! Whatever a mustang makes on rwhp, it makes 15% more of that power on motor hp.
Eg. The shelby gt500, ford say theyr 662hp. But multiply that by 0.85 which is 562rwhp. And thats what it should make on the dyno i guess

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post #6 of 19 (permalink) Old 06-06-2014
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Thanks for the correction guys I thought it was a little hhigh. Glad you got your answers

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post #7 of 19 (permalink) Old 06-06-2014
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15% is the standard rule of thumb for parasitic loss. It would be a little bit more for an auto. But a dyno is a dyno, there are many factors that go into producing a final RWHP. There is weather, temperature, mechanically variation on the dyno, mechanical tolerances in the particular engine.

But 15% is a decent guesstimate for what crank HP will translate to at the rear.

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post #8 of 19 (permalink) Old 06-06-2014
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Glad I got you guys here to teach: )

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post #9 of 19 (permalink) Old 06-06-2014
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Quote:
Originally Posted by ydashti01 View Post
Im curious on how much rwhp and torque do 13-14 shelby GT500s make on dyno, i know the motor makes around 660hp, but what do they actualy make on the dyno to the wheel?
Also the 13-14 roush stage 3, i know they say 575hp, but is that really what they make to the wheels?

anyone has info about those?

thanks
Yousef
Can't vouch for the accuracy, but I found this HP calculator online that allows you to plug in a few basic variables to convert from rear wheel to crank and vice versa...

Wheel Horsepower to Crankshaft Horsepower Guestimator

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Originally Posted by Fatopotomus View Post
15% is the standard rule of thumb for parasitic loss. It would be a little bit more for an auto. But a dyno is a dyno, there are many factors that go into producing a final RWHP. There is weather, temperature, mechanically variation on the dyno, mechanical tolerances in the particular engine.

But 15% is a decent guesstimate for what crank HP will translate to at the rear.

I don't think a percentage is an accurate rule of thumb for parasitic loss. Say you have an engine with 500 hp in a car and take that 15% parasitic loss, you have 425 to the wheels (75 hp loss). Now say you have a 1,000hp engine in the same car; taking that same 15%, you would have 850 to the wheels (150 hp loss). How did the same drivetrain just take 75 more hp to spin?


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I don't know a whole lot about 1000+ hp cars, but I would think you would want a heavier duty transmission if you planned on driving a 1000 hp car down the strip. Do transmissions that are rated for 700+ hp take more hp to spin than the stock transmission (in this case, the MT82)? If so, maybe 15% is the generic rule of thumb for an "appropriate" transmission, given a certain amount of power. Forgive me if this is a stupid question.
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I don't know a whole lot about 1000+ hp cars, but I would think you would want a heavier duty transmission if you planned on driving a 1000 hp car down the strip. Do transmissions that are rated for 700+ hp take more hp to spin than the stock transmission (in this case, the MT82)? If so, maybe 15% is the generic rule of thumb for an "appropriate" transmission, given a certain amount of power. Forgive me if this is a stupid question.

All I'm saying is that all things being the same, a percentage is not a good rule of thumb for parasitic loss.


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The 15% is for stock cars, generally on average 15% loss to the wheels. We know it's a huge variable but it's a ball park.

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Quote:
Originally Posted by daustin View Post
I don't think a percentage is an accurate rule of thumb for parasitic loss. Say you have an engine with 500 hp in a car and take that 15% parasitic loss, you have 425 to the wheels (75 hp loss). Now say you have a 1,000hp engine in the same car; taking that same 15%, you would have 850 to the wheels (150 hp loss). How did the same drivetrain just take 75 more hp to spin?


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Quote:
Originally Posted by daustin View Post
All I'm saying is that all things being the same, a percentage is not a good rule of thumb for parasitic loss.


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This is a good question and I dont think I have a solid answer. But what I can say is that a 1000 hp car and a 500 hp car wont have the same clutch an driveline (at least not for long). So as the other poster here said, perhaps a heavier duty trans needed for a 1000 hp car eats more power to turn than one fitted to a 500 hp car.

But in general is you multiply a car's advertised HP by .85, you will come pretty close to what it lays down on the dyno, rwhp wise.

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Most of the 13/14 GT500's I've seen dyno average around 590-600 RWHP. I've also seen some put 570 RWHP down, so, when it all comes down to it, it depends on how generous the dyno is and how the dyno is setup.

I'd say 580-590 RWHP is a good realistic number for Stock 13/14 GT500's.

Example I'd use is 03/04 Cobras. They were rated at 390/390, however, Ford came back a short while after stating they were underrated and were actually rated at 425/425. A LOT of 03/04 Cobra's were making 360-370 RWHP, which would account for a 15% drivetrain loss, which is somewhat accurate. 2011+ GT's have seen 370-380 RWHP and they are only rated at 412HP for early model years and 420 for 13/14 GT's. That would account for an even less of a loss, more around the 10% mark on a dyno.


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