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Anybody have any idea how to figure out HP at the flywheel from a chassis dyno HP number? It is something to do with percentage drivetrain loss....Someone suggested it is about 15% - 20% loss from the flywheel to the rear wheels...
I am sure there is a formula out there somewhere.
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G. Brooke Carter
Wheat Buckley Racing
# 10 Challenge Car
2004 Class Champion
Calgary, Alberta
http://www.wheatbuckley.com/tags/brooke_rx7.jpg
There is no set formula and anyone that claims there is doesn't deserve your ear. Flywheel HP is just that. Wheel HP not only takes into account the power the engine is making but also what the rest of the car is sapping out of it. Every gear, bearing, u-joint, CV joint, tire, bla, bla ,bla is sapping power. I've read that tire pressure adjustments alone has shown 6WHP gains/loses in extreme cases. Oil viscosity in the gearbox and diff along with the temperature of those items effects their efficiency and how much of that flywheel HP is making it to the rollers. There is no set formula for cars in general or even identical makes and models. There can't be.
HOWEVER, it is generally accepted that the loses through the powertrain will fall somewhere in the 10-25% range depending on alot of factors as described above. FWD is generally more efficient than RWD. AWD generally has more moving parts and thus least efficient. Auto boxes generally sap more power than manuals.
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Chris Ludwig
08 ITS RX7 CenDiv
Chris makes some good points and I feel is compeltely correct in what he says - basically it boils down to no set formula will work because there are many variables.
Just to make a point, automatics are generally considered much more of a parasitic draw than manuals. My Lightning has an automatic and when we dyno it the operator always says "well, it is an automatic and the driveline loss is high, figure 25%" which would have us well over 500 flywheel hp - not true.
Ford produced some SAE papers on the automatic that is in the Lightning because it has very high efficieny for an automatic - their figures? 11% loss inthe driveline. Tell that to a horsepower junkie or a dyno operator and they don't believe it, but I tend to believe SAE engineers who developed the thing.
Best thing to do is not worry about it. Use the dyno as a tuning tool and look for changes. If you've go increases from perfoming modification, then good. Remember, the fellow that just passed you doesn't care what the absolute hp figure is that is on the dyno sheet!
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Ron Earp
NC Region
Ford Lightning
RF GT40 Replica
White Jensen-Healey ITS
Silver "Skull" 260Z ITS
[This message has been edited by rlearp (edited May 26, 2005).]
Thanks guys. I knew that there was a big drain on the HP, I just wondered if anyone had ever figured out a number more specifically for a race prepped RX-7 1st gen.
As pointed out, knowing the number doesn't mean beans as the other guy blows by, or vice versa, I just did a dyno tuning on the car, and wondered how we were doing over the stock 12A Flywheel HP, which Mazda had at about 101 hp.
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G. Brooke Carter
Wheat Buckley Racing
# 10 Challenge Car
2004 Class Champion
Calgary, Alberta
http://www.wheatbuckley.com/tags/brooke_rx7.jpg
[This message has been edited by bcarter (edited May 26, 2005).]
So how did it do?
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Ron Earp
NC Region
Ford Lightning
RF GT40 Replica
White Jensen-Healey ITS
Silver "Skull" 260Z ITS
I was always confused when we dyno'd the race car. The operator was not able to satisfy my science background with the explaination of how the conversion of acceleration time for the weighted roller to Hp went. My problem? I being from the '50's generation, saw Hp as a mathamatical consequence of the torque vs RPM formula.
The operator was trying to make a connection between Hp and the 'time it takes to accelerate a given roller mass to a prescribed roller RPM', and I'm unable to equate that with the formula for Horsepower.
In my mind, reduction of rotating masses -wheels, flywheels (illegal, I know)gear weights, etc- should have no effect on the torque value of an engine at a specific RPM (using a 'brake' style dyno for example). I realize that it has an effect on the track, but trying to mentally plug into the concept of the Dyno-Jet has me at the edge of understanding, and not getting closer.
As said above...just go with the numbers, and their relative values, and you will realize the improvements with the "butt dyno".
Good racing.
Bill
I'm no scientist or engineer, but the numbers I have heard for a 12a range range from 110 to 130 hp at the wheel, or 130 to 150 at the crank. They are corked up pretty bad from the factory and exhaust gives you major gains on a rotary.
I've dynoed my car three times and each time the dyno or technique were different.
First, a roller chassis dyno had me drive up through the gears. Also did a pull in 4th from about 3K or so. RWHP was about 105 or so. He did a "coastdown" which is supposed to indicate the driveline losses, which were 18%.
Second experience was a chassis dyno that bolted t the axle. All runs in 4th, and the number was....about 105! uggg.
Then the engine blew, and the rebuilt version dynoed on another roller style, a Clayton IIRC. But the operator (A well respected SCCA GT racer) did the pulls from the cockpit and he started at 9K, wide open throttle, and "pulled" the engine down with a control until it stalled at about 3K.
This number was around 130. Truthfully, I am suspect that I would ever see that number on a different dyno, which brings up the point that all dynos are not equal, even same manufacturer and same model.
Then I raced in the ARRCs, and most of the IT-7 cars were able to walk me on the srtaights, even when I was able to get a run and an overlap.
I think that a legal 12A should be making not much more than 130 on most dynos. IF you take the 130 RWHP as a reasonable max, and accept that a 1st gen has approximately 18% driveline loss, your flywheel number (SAE) could be ABOUT 153.
YMMV!
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Jake Gulick
CarriageHouse Motorsports
ITA 57 RX-7
New England Region
[email protected]