Look at the two sets of dyno graphs I've attached.
Same car, same tune, same everything - except torque converters.
The first graph has two different converters I was testing. The second graph is another converter, but with a lock-up feature and the test was ran with the converter locked.
The locked converter test is more similar to testing a car with a manual transmission.
I had another instance to test a motor on an engine dyno, and it made ~550hp at the flywheel. In the car, different headers, full exhaust, air cleaner in place, it put 426 to the wheels on a DynoJet.
Recently, I tested my friend's TOTALLY stock '79 Trans Am, 403 2.73 gears, 27,xxx miles. It was rated at 180 or 185hp, which I always assumed to be flywheel (since chassis dynos didn't really exist at the time). On our DynoJet, it put 178 to the wheels. That was through the crappy old pellet catalytic converter.
SOOOO many variables. Like Stan stated, a Wild @$$ Guess is about as good as you can do.
About as accurate as I can state is the RWHP will be about 20% lower than flywheel - but with a +/- range of up to 18% (2% to 38% of flywheel).
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'73 T/A (sold). Low budget stock headed 8.3:1 455, 222/242 116lsa .443/.435 cam. FAST Sportsman EFI, 317rwhp/385rwtq on 87 octane. 13.12 @103.2, 1.91 60'.
'67 Firebird [sold], ; 11.27 @ 119.61, 7.167 @ 96.07, with UD 280/280 (108LSA/ 109 ICL)solid cam. [1.537, 7.233 @93.61, 11.46 @ 115.4 w/ old UD 288/296 108 hydraulic cam] Feb '05 HPP, home-ported "16" D-ports, dished pistons (pump gas only), 3.42 gears, 275/60 DR's, 750DP, T2, full exhaust
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