Quote:
Originally Posted by Paul Workman
(snip)
But, back on topic, there's gross and SAE net and RWHP and RWHP "under the curve". JMO, but HP at the wheels, where the rubber meets the road is the only meaningful measurement. Everything else is just so much hot water generated that means nothing, far as performance goes, methinks.
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Quote:
Originally Posted by efnfast
I must agree with Paul, all this manipulation of numbers is meaningless. As Paul said, "where the rubber meets the road". SAE corrected or not, me and the guy next to me are breathing the same air when the right foot hits the floor.
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I'm not so sure we should rush to accept the idea that chassis dyno numbers are a true "benchmark" of engine performance.
There are many factors which skew chassis dyno numbers but have nothing to do with engine performance. In no particular order, some of them are:
1) tires...casing design, tread configuration and tread compounding. (I once did a back-to-back test of two tires just comparing tread confiuration and compounding-the tires were both on the same casing design-and the differece was 5-hp at the wheels. Same dyno, same pressure, same vehicle, some IAT and ECT, and tests 10-min. apart.)
2) tire pressure
3) dyno type, inertia or brake
4) single roller, two rollers or no rollers (direct connection to axles)
5) roller surface
6) wheel spin or lack thereof
7) trans type
8) trans lubricant and lubricant temperature.
9) rear axle ratio, lubricant and lubricant temperature
10) powertrain mountings
11) coolant temperature and temperature rise during test
12) different in IAT of more than 7°F regardless of correction
13) technique of dyno operator
14) condition of dyno
So, you see...chassis dyno numbers should be taken with a "grain of salt". They are embraced by the aftermarket performance industry and the enthusiast community because chassis dynos have been well-marketed by their manufacturers, they're loved by the automotive press and widely used by tuners. Chassis dynos are cheap and easy and perfect for bragging rights but fraught with potential inaccuracies.
I'm first to admit to using chassis dyno data all the time. Sometimes I strive to eliminate as many of the potential inaccuracies as possible. Sometimes I don't, but I know in the back of my mind that engine dynamometers are far more accurate devices for comparisons.
As for corrections...the belief that atmospheric corrections don't make a difference is just ridiculous. In fact, when I look at chassis dyno data, I'm more concerned with whether or not a correction has been applied and if so, what correction than I am with the inaccuracies of chassis dynos.
Recently, I was discussing a particular engine modification package with an aftermarket vendor. I asked for some of his confidential test data and he provided it. Same engine, same dyno, same test run...the engine made 646.0-hp "standard" corrected and 613.9-hp SAE-corrected.
See why the aftermarket and braggers like "standard" corrected data rather than SAE-corrected?
But, SAE-corrected is much more characteristic of the real world because of its use of 77°F for intake air temperature.
Lastly...
Quote:
Originally Posted by scottfab
(snip)
HP #s are great talking points but ETs are better  |
ETs are either the most inaccurate way to bench mark engines or the most accurate way to benchmark the whole "package"...driver and car.
Do drivers are bragging about their engines. The first driver who majorly sucks on the starting line and has crappy tires but has a really good 350 in a ZR1 might go 13.2 The other driver who gets really good 60-foots, has his/her tire configuration just perfect but has a so-so 350, can run a 12.8.
So...who's got the best motor?
But...who's got the best ET?