HP Measurement at Wheels for 2.7RS?

[73ess]

Anyone know how much HP would typically be measured at the wheels if placing a stock 2.7RS on a dyno? I believe the 210 HP rating is not what would be measured at the wheels, correct?

[Chuck Miller]

You would think that would be an easy question to answer....

It would of course depend on the the dyno and the technician's ability to adjust it for conditions....
And, if your comparison is between the flywheel and rear wheel HP, I've heard a difference in consensus of the percentage of loss through the drive train...

If I was going to take a shot from the hip I'd say a low of about 179, a high of about 189, and a comfortable guess of about 185...

I'd like to hear from the experts that have done not only RS chassis pulls, but also the comparisons they've found at the flywheel...

This doesn't help that much:
http://www.early911sregistry.org/for...RS-spec-engine

Cheers,
cm

[pu911rsr]

The generally accepted rule of thumb is 15 % HP loss through a Porsche manual gearbox drive train. I'm sure there are plenty of folks that can give you a more precise answer. The 210 HP figure for a 2.7 RS motor was crankshaft HP. You can look up the various methods employed over the years such as SAE gross, SAE net DIN etc.

Phil

[chris_seven]

I always thought that the 2.7RS was 210 PS (pferdestärke) which is equivalent to 207 horsepower expressed in Imperial units.

The power figures quoted by Porsche were measured in accordance with DIN 70020 which requires that all of the ancillaries are fitted and that the exhaust system is the same as that used in the car. The correction factors for air temperature, barometric pressure and humidity are all specified within the standard.

In the USA up to the Seventies power was generally measured according to SAE J245 or J1995 which is a 'gross' figure and test engines were not required to be fitted with any ancillaries and long tube headers could be used in the place of the OEM exhaust. These techniques generally resulted in 'optimistic' power values being quoted which probably helped to sell cars.

Current SAE Standards such as J2723 introduced in 2005 are much better although many engines developed prior to this date still use results produced using earlier standards.

The sad reality is that there is no 'correlation' between DIN and SAE standards although it is fair to say that DIN figures are generally lower than SAE figures which can seem misleading when you look at a specific cars acceleration.

You can make a good guess to allow for differences in test methods but as the parasitic losses introduced by different accessories will always be inconsistent so comparisons will always be relatively imprecise.

If we now look at chassis dynos that have either relatively crude torque measurement or just rely on inertia of a drum then results from an engineering perspective are relatively meaningless in absolute terms.

Testing is never carried out to any standard, the precision of the transducers used for measurement are rarely stated and I am not convinced that any of these systems are correctly calibrated.

Wheel to Flywheel Horsepower is generally determined using a 'Bulgarian Constant' that tries to allow for all the inaccuracies of speed and torque measurement whilst allowing for transmission losses and the bearing friction of the drum.

Using a Roll Down can eliminate some of the errors but I would imagine you would do well to be within 5% on a specific vehicle.

They are useful for tuning as differences will be measurable even if absolute values are not entirely accurate.

The current I-phone Apps using the accelerometer built onto the 'phone are almost as good and certainly worth $10.00

https://itunes.apple.com/us/app/perf...549390700?mt=8

I am always surprised that we consider a transmission is said to cause losses of 15% of the transmitted power as in the case of a RS this would be equivalent to around 24kW.

If you do some basic calculations using a 'heat to oil' technique the gearbox would simply cause the oil to burn as the black body radiation of the transmission case just isn't enough to deal with this level of heat input.

Helical gear losses are typically 0.5% of the transmitted power with a spiral bevel weighing in at around 1.5-2%. With windage and other effects I would think around 5% is more realistic. Having spent many years manufacturing 4-Square test rigs used in transmission testing and also producing test rig gearboxes I am fairly confident that my estimates are in the right area.

This means that the other 10% is due to uncertainty in the measuring chain and who really knows where this comes from or how to make the correct allowances.

Even the most advanced Engine Dyno's are subject to routine errors and are difficult to test.

The T700 Apache engines produced by GE in Boston are used all over the world and are re-built and tested in many different countries.

Every T-700 Dyno uses identical torque and speed measurement systems and are calibrated on a regular basis but every few years GE send a 'Golden' engine on a world tour to ensure that this engine produces the same power on every test bed and even at that level of effort 2% variation is not considered significant.