I am trying to match data from a chassis dyno run to engine dyno data. The engine goes into a vintage Corvette racer so the engine dyno test was performed with the headers use on the race car and no air cleaner. The chassis dyno test was done with the air cleaner on going through an M 22 transmission, 3.70 rear end and tires with approx 82 inch diameter. What kind of losses should I expect through the drivetrain and tires. The numbers I'm getting look a little goofy. Any of you old engine guys have expirience with this? What kind of losses do you get through a transmission and rear end and tires?
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Re: Chassis Dyno vs engine dyno Data
Lab dyno data is usually corrected to standard atmospheric conditions, 29.92", 60F, dry air. Chassis dyno data is usually corrected to "SAE conditions", 990 millibars, 25C. This difference is about 4.5 percent.
As an overall average estimate, drivetrain losses are about 15 percent for manual transmission cars. The two together mean that your RW numbers should be about 80 percent of what was measured on the lab dyno.
For a street car additional losses in the car versus on the lab dyno include accessories, exhaust system, and air cleaner. As a general rule the difference between SAE gross on a lab dyno and SAE net in the chassis is usually estimated at about 20 percent, but since your in car configuration has the same exhaust system as on the lab dyno and no front end accessories other than the alternator, the difference would be slight leaving only the difference in air density correction.
You will find further discussion if you page down and read my posts on the thread titled "L-88 horsepower" 2 Oct, 11:08AM.
How about if you post some of the data - at least the peak power and torque numbers of the entire graph if you can.
Duke -
Those 82 inch tires
are problem causing you a bit of grief on the dyno. They won't rev up too fast, but should provide one heck of a flywheel effect.
Seriously, since you are on a race car, the wheels will affect things a bit. It is like throwing on a light or heavy flywheel. It doesn't alter the end results but will affect how fast or slow everything gets there.
If you are using steel wheels and what size, street aluminum wheels, or lightweight racing wheels, they all make a difference, just like in braking distances.
Most I have heard who have had the time and money to have both engine dyno and chassis dyno talk about 20% to 25% diferences in the two. While the losses are probably closer to what Duke says with 15%, there is the tendency for chassis runs to be with alternators, water pumps, air cleaners, and any exhaust baffling installed while none is on the engine dyno. After all, the car is in running condition when dilevered to the chassis dyno and many tracks are running sound restrictions now, so there are baffles/mufflers installed to make those numbers.Comment
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If memory serves....
there was a gross vs. net HP 'translation' table in one GM publication I thumbed through (fuzzy recollection says it was the '72 Chassis Service Manual). The numbers varied for different cars (even with the 'same' engine) in the Chevy passenger car line-up based on transmission & rear end specifics with the range being losses ranging from 13-22%. Duke's thumbnail of 15% wasn't far from the group ensemble average, though.Comment
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Re: If memory serves....
The "15 percent" number is the usual ROM estimate for drivetrain loss on manual transmisson cars. Twenty percent is the ROM estimate for the loss of SAE gross to yield SAE net. I gave actuals for 1971 in the thread I referenced in my previous post and all are near 20 percent
Actual losses would vary for each configuration, and automatic transmissions usually have greater drivetrain loss than manuals. On passenger cars some engines may have had single exhaust with dual exhaust optional. In such cases the single exhaust configuration would have more loss of gross power that the dual exhaust configuration.
It's important to understand that these percent losses and/or ratios are just ESTIMATES for a wide variety of configuratations, and not numbers that are cast in stone.
As they say, your mileage may vary.
DukeComment
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