Engine dyno vs. rear tire dyno percent loss
 

This may not be directly C4 releated but it is interesting never the less. I dynoed my 555 CID BBC that I run in my 1973 vette drag car, on an engine dyno last night in the exact same configuration that I race it. On the engine dyno the engine made 805 HP at 6300 RPM and 739 FT/TQ at 5100 RPM. On a Mustang Chassis dyno the car put down 635 RWHP at the same 6300 RPM. That works out to a 21% power loss through the drivetrain.

The drag car is running a non lock-up convertor, powerglide trans, 4.63 rear gear in a 9 inch rear, and a 5200 RPM stall. I know that the Mustang dyno reads lower then most other dyno's and each drivetrain configuration is different but I found it interesting that the 21% power loss was in-line with Mustang's published numbers and other reported power loss numbers.

I think the percent loss thing is only good for estimating, nothing more.

Switch the gears in the dragster to 3.08s and you'll measure more power on the chassis dyno.

Is that right? That you've got a converter stalling above the peak torque number - everything I've done & read that it is normally bad for drag ET's and you're leaving some acceleration "on the table" with that.

I know loose converters tend to throw off chassis dyno numbers up to about the stall speed or even a couple hundred RPM above it's actual stall.

What trans are you running - that loss seems high for a good roller bearing powerglide but might be about right for a stock-ish TH400

thats why dyno sheets are as good as flow sheets.. they are a bit rough but i can still wipe my butt with them. :thumbs: :cheers:

just messing with you.. its cool to actually see both numbers and actually have a number.

Either way, that's some healthy horses. What does it run in the quarter?

QFT! you aint lying!

-The car is running a powerglide with 1.76 first gear
-The converter stall speed needs to be in the range where you keep the maximum HP under the curve. I shift at 6700 RPM where the HP peak is down to 780 HP, after the shift the RPM fall to around 5800 RPM where the HP is also 780 HP. If the converter stalled lower it would drag the engine farther down on the power band and the car may slow up. I am not 100% sure because I do not have not tried a lower stall conveter to see what it does. The converter was designed as a 4500 stall behind my old 427 BBC but it now stalls at 5200 behind the 555 BBC.
-The best run was 8.78 @ 152 MPH with a 1.200 60 ft., it has no shortage of low end power. It usually runs 8.80's at 150 MPH.

I agree with your statement, it may pick up a few more HP and only loss 19% to the tires. My point in posting this was that the rough 15% to 20% drivetrain power losses appear to be correct to give a rough ball park of how much HP your engine is making when you compare them to your chassis dyno numbers.

x2

really efficient FWD setups with the flywheel facing the transmission and axles right out the side are still roughly 15% add a couple % for rwd and the driveline and awd is the worst with all 4 going. if i remember right a lot of those are around 20%

what makes it even harder is different dynos reading differently. you could take a stock c5 z06 that reads 342 on one dyno and 365 on another. both numbers correct to 405hp but they dyno makes it impossible to tell percentage

I'm impressed. :yesnod:

You should see it go STRAIGHT down the track:rock:

Every time I see it run, it runs very clean consistent passes.

:iagree: Maximizing power "under the curve" is the right thing to do...your shift points are perfect!!

Good post regarding engine vs rear wheel dyno test :thumbs:

:cheers:

So higher stall converters makes the Dynos read less horsepower ?
i've read you have to adjust the dyno program guided by your current stall
Please read this article is pretty interesting :

http://converter.com/article2.htm

The article is correct but if the dyno operator knows what they are doing the errors of this article can be eliminated. To make a long story short you need to make sure you start the test on the chassis dyno higher then the flash point of the converter. In other words if you see a very high peak torque number and a steady downward sloping torque curve the dyno operator started the chassis dyno test at too low of an RPM and the peak torque numbers are inflated by the flash of the converter. If you see a nice bell shaped curve the test was performed correctly.