OK...So what is the "FBI" & what is the FBI 500????

[mike100]

I honestly believe that the C4 manual drivetrain only uses 60-70 hp (at LT5 rpm ranges). I'm not surprised by the big inch LT5 results at all.

And to comment on Paul's explanation, there exists the intangible properties of engine designs willing to change speed rapidly that is not easily shown on dyno plots, but rather track performance or reputation that a certain car is 'fun to drive'.

[BigJohn]

If you turn your LT5 90 degrees and your wheel turned the same rotational direction as your engine your hp loss would be less.

[QB93Z]

I have a really basic question about comparing Crank HP to Rear Wheel HP.

I believe that an engine dyno measures the power output of an engine on a test stand. The engine is run at a constant, optimum rpm and a machine applies load to the crank shaft and measures the power produced.

I believe that a "Dyno" is a calibrated mass that the car's rear wheels spin as the engine is accelerated for low rpm to max rpm. A computer measures the rate of acceleration of the rotating mass and calculates rear wheel torque and then does mathmatical formula to determine horse power.

Is my understanding correct? Thanks.

Jim

[Dynomite]

Originally Posted by QB93Z

I have a really basic question about comparing Crank HP to Rear Wheel HP.

I believe that an engine dyno measures the power output of an engine on a test stand. The engine is run at a constant, optimum rpm and a machine applies load to the crank shaft and measures the power produced.

I believe that a "Dyno" is a calibrated mass that the car's rear wheels spin as the engine is accelerated for low rpm to max rpm. A computer measures the rate of acceleration of the rotating mass and calculates rear wheel torque and then does mathmatical formula to determine horse power.

Is my understanding correct? Thanks.

Jim

Hmmmmm.....I think the machine applies load to the wheels (resistance to wheel rotation on the rollers). I do not think anything is measured in terms of the rotational acceleration of the rotating masses. What you get is rear wheel torque (wheel deflected diameter is determined) and wheel spin rate and maximum rotation force is measured (torque). So we end up with wheel rotation speed and torque which is converted to horsepower.

Nothing to do with how fast you accelerated the rotating masses. I am just guessing but when I see a car on a dyno run I see the rear wheels on two BIG cylinders that the rear tires sit on and spin up as the car is tied down on a fixture.

Same on engine test stand where the output shaft is connected to and drives a mechanism applying torque and rpm to that mechanism so you can measure hp at various rpms and thus tune an engine in the same way you can tune an engine on a dyno with the car tied down as explained above.

Jim......,.It is not rocket science. On the dyno measure tire thrust in lbs (sum of both tires). multiply that thrust by tire deflected radius in ft. You now have ft lbs (torque). Measure tire velocity at interface and calculate tire spin rate in radians per second. Multiply spin rate by torque and divide by 550 ft lbs per second (one horsepower). You will then see how many total horsepower you have.

That is what I am thinking today

[Hib Halverson]

Originally Posted by Paul Workman

(snip)

However,

413/.85 (15%** drivetrain loss) = aprox 486 chp, not 513

Paul is correct. I made a mistake with the 15% figure. I apologize for any confusion. I will edit that post to correct the error.

But...I suppose my error with the 15% computation is moot, at least for C4. Back in the day when I was covering ZR1 and LT5 for media, I talked to my pal Jim Minneker, who back then, was Corvette Powertrain Manager, about parasitic loss in the later C4 driveline. He told me that GM testing showed that the average loss in the manual powertrain (ZF S6-40) was 18% and the average loss in the automatic powertrain was 22%.

Since then, with LT5s, I've always used 18% when converting from SAE-net rear wheel power numbers to SAE-corrected flywheel power.

Note that this math only works if you are working from SAE-net rear wheel power figures. It won't work if the chassis dyno numbers are either "standard-corrected" (popular with the aftermarket) or raw uncorrected figures.

Also, if you want to split hairs... there are some other variables:
1) tires
2) lubricant types used in the trans and diff
3) temperatures of lubricants in trans and diff
4) mfg. and assembly variances
5) testing in gears other than fourth

An interesting post script to this discussion is in later years, once C5 was out, I asked a similar question of the folks at GM Powertrain and they told me, with respect to manual powertrains, the C5 (and, obviously, C6) drivelines are more efficient than that of C4 and that the parasitic losses decreased to about 15%. I attribute that to improvements in transmission and drive axle design and manufacturing along with the switch from Cardian type joints in the axles to CV joints and lubricant choices.

[QB93Z]

Since this thread got my thinking, I did a search and found this description of how a DynoJet dyno works.

What is a dynamometer? A dynamometer is a device that measures force and power. There are lots of different kinds of dynamometers, including the kind that test springs and shocks, but we don't care about them because I don't have any. I have an inertia-type chassis dynamometer. It measures the force and power that the spinning wheels of an automobile produce. It is not a "brake-type" dyno that measures the power that is actively absorbed by a water, oil, or eddy-current brake or by a generator. An inertia-type chassis dyno consists of two great big heavy drums hooked up to a computer. The wheels of an automobile spin the dyno drums, and the computer measures the speed.
Simple? If you remember the stuff you were supposed to learn in high school, it is. The computer calculates the acceleration of the dyno drums by continuous measurements of their speed and the time. If the surface of the drums spin from a speed of zero to a speed of 10 feet per second in one second, then their surface acceleration is 10 feet per second per second, or 10 ft/s2. Sound familiar?

Since the rear wheel dyno measures acceleration, drive line rotational mass will directly affect the measured output. So, a light-weight flywheel will "add" horsepower as measured on a DynoJet type dyno. This is because less engine output is used accellerating the mass of the flywheel.

Am I right in assuming that the mass of the flywheel has very little affect on the measured horsepower measured on an engine dyno at constant speed?

Jim

[Paul Workman]

Originally Posted by QB93Z

I have a really basic question about comparing Crank HP to Rear Wheel HP.

I believe that an engine dyno measures the power output of an engine on a test stand. The engine is run at a constant, optimum rpm and a machine applies load to the crank shaft and measures the power produced.

I believe that a "Dyno" is a calibrated mass that the car's rear wheels spin as the engine is accelerated for low rpm to max rpm. A computer measures the rate of acceleration of the rotating mass and calculates rear wheel torque and then does mathmatical formula to determine horse power.

Is my understanding correct? Thanks.

Jim

Yep! I recon the only reason to do engine or dynamic (inertia) tests would be for comparison sake, or to document performance before and after modifications (for example).

Is the loss factor a constant, or exponential (or a mixture of factors)? Comparing an LS7 Z06 that is advertised to be 505/≈450 crank to wheel hp to my LT5's 508/432 (fixed 15% factor) it suggests only about an 11% drive train loss for the LS7 vs 15% for the LT5. Apples and oranges (far as C6 vs ZF transmission go) or, as (I think) does rpm play a role? (No replacement for displacement, in other words.)

Bob G and Pete have often expressed their doubts regarding a fixed % conversion factor, regardless of engine size/output, based on real world observations. But, ya know...all of this is so much achedemia when compared to real world results: TIME SLIPS!

I agree w/ Bob G (and others) when they say the real test is speed through the traps. All the BS regarding which conversion factor to use, whether they are dynamic or static, peak or power under the curve, becomes so much .

So, I guess (for me) RWHP (under the curve) is far more important that CHP that is extrapolated via a plethora of coversion factors and constants, etc. And, even then, if one is to get an accurate comparison, it makes a lot of sense to use the same dyno on the same day (for example).

p.

[Dynomite]

Originally Posted by QB93Z

Since this thread got my thinking, I did a search and found this description of how a DynoJet dyno works.

What is a dynamometer? A dynamometer is a device that measures force and power. There are lots of different kinds of dynamometers, including the kind that test springs and shocks, but we don't care about them because I don't have any. I have an inertia-type chassis dynamometer. It measures the force and power that the spinning wheels of an automobile produce. It is not a "brake-type" dyno that measures the power that is actively absorbed by a water, oil, or eddy-current brake or by a generator. An inertia-type chassis dyno consists of two great big heavy drums hooked up to a computer. The wheels of an automobile spin the dyno drums, and the computer measures the speed.
Simple? If you remember the stuff you were supposed to learn in high school, it is. The computer calculates the acceleration of the dyno drums by continuous measurements of their speed and the time. If the surface of the drums spin from a speed of zero to a speed of 10 feet per second in one second, then their surface acceleration is 10 feet per second per second, or 10 ft/s2. Sound familiar?

Since the rear wheel dyno measures acceleration, drive line rotational mass will directly affect the measured output. So, a light-weight flywheel will "add" horsepower as measured on a DynoJet type dyno. This is because less engine output is used accellerating the mass of the flywheel.

Am I right in assuming that the mass of the flywheel has very little affect on the measured horsepower measured on an engine dyno at constant speed?

Jim

Yes Jim ......at constant speed which is really what we are interested in, rotating mass is not a variable .......not of interest. Most guys talk in terms of maximum horsepower to the pavement. You are specifically interested in having the exact gear ratios (transmission/differential) that will match the top end speed with the top end RPM. If that is not correctly matched, you cannot achieve maximum horsepower at the maximum speed.

If you are drag racing however, you ARE interested in rotating masses such as lightweight flywheels.

[QB93Z]

My conclusion then, is that the conversion factor for Rear Wheel Horsepower to Crank Horsepower is not a constant for a car that is being modified by drive train changes.

So, the 18% that Hib discussed (from GM) would not apply to a C4 ZR-1 with a lightweight flywheel, or different drive train lube, or different differential gear ratio. The effect of all these changes could be measured on a rear wheel dyno (DynoJet type), but none of those changes affect crank horsepower.

The only way to change crank horsepower is to make changes forward of the flywheel. For example porting, tuning, headers, bore and stroke, etc.

Jim

[BigJohn]

Now my headache is back!!
What I really like is having my socks blown off with big torque.