Gross Horsepower LT-5

[Hib Halverson]

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.

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.

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...

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?

[scottfab]

Originally Posted by Hib Halverson

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.

Sounds like a line of hair splitting to me.
ETs can be the best way to measure overall performance there is.
Sure some exceptions can be brought to bear but there's many more exceptions that can be added.
Why even the effect of the moons gravity has an effect and the
imperfections on the asphalt under the right rear wheel. There
could even be a mosquito hit the windshield and slow it down.
Same driver running two different cars on the same night could even have
different wind conditions, a huge fly hit the windshield plus the major effect of 0.01oz of more fuel in one car vs the other. gezzzzzz
Seems like an exercise in futility to me.
A huge amount of meaningless exceptions can be added to dyno derived HP numbers too but I won't go there right now.

HP numbers are a fine metric but so is how many dimes you have in
the change holder. Being able to wave bye bye puts the "P" in performance. It's all together a better metric whether it's SAE or standard or pure air guitar corrected numbers.

Originally Posted by Hib Halverson

So...who's got the best motor?
But...who's got the best ET?

Whose got the shiniest dimes in the change holder?
Depends on the quality of the eye looking at it and the ambient air quality that the light goes through. But it should be SAE corrected air quality else
it's all for naught. And one should use proper wide spectrum light to test with and of course the angle of reflection should be at 90deg +/- 0.0000001 deg or the error will be so great that all bets are off.
Which is the better car?
1. 10,000 rpm V8
2. 502ci V8
3. 1800lb with driver
4. first over the 1/4 line

Answer: depends on which one I'm sitting in and Voltage measured at the spark plug of course.
Any hair that is split can always be split again until it looses meaning.

[XfireZ51]

Pretty much why you judge power on basis of trap speed not ET. Trap speeds can vary based on track setup.

[Paul Workman]

The trouble with standards, are there's so many to chose from:

J1349 circa 1972, June 1990, 2004 (crank hp)
J1995 (gross hp)
J607
J2723 certification spec
VCA (UK certification spec)

And, NONE of these address AVERAGE torque or hp at the wheel!

Bottom line: You can measure at the crank, or you can measure at the wheels, or somewhere in between. But, trying to predict one from the perspective of the other is full of variables as to make an absolute prediction either way almost folly.

I believe the trick is to pick the one that best fits the application, make the measurements as a base case for comparison to future improvements or modifications.

Examples: Engine builders with clients paying for results aren't interested in driveline losses. Drag racers aren't interested in peak hp at the crank, they're interested in trap speed (and ET to a lesser degree).

To get J1995 (crank net hp) I have to pull the motor. But, considering inertia of drive line hardware (including rotating mass), the hp number derived is significantly removed from how the car is going to actually perform, at least where differences are subtle.

For seat of the pants fun and a more accurate indicator of drag race result prediction, the inertia dyno measuring output at the wheels is a much better fit. However, quarter mile trap speed is a very good indicator for performance comparision! It isn't perfect either, but it does take into consideration "power under the curve" which is largely overlooked when in a pissin match over gross vs. net vs. shaft vs. wheel horsepower ratings.

Where the rubber meets the road is where real performance is demonstrated...I think is the main point ... at least it is for me. I'm not saying either isn't important in their own right - engine builders have their point of view and race drivers have a different perspective on output. But, trying to predict either from opposite ends of the drive line (read: is it SAE 15% standard loss or some other 18% standard) is where things get fuzzy really fast.

Hib...You said Barney measured 413 at the wheels. You used 18% drive line loss to predict you have about 504 net hp at the crank. My 90 Z measured 432 hp at the wheels, using SAE/Marc's 15% drive line loss, my net crank calcs to be 508 hp. Now, going on net crank hp and nothing else, I imagine it would be a near toss-up in the quarter mile, all things being equal. Care to race and compare??

P.

[Fully Vetted]

[Dynomite]

Originally Posted by Paul Workman

Care to race and compare??

P.

I doubt you would want to race with me

I did the MATH

Cliff

The 500 hp LT5 Rebuild
1991 ZR-1 (LT5) Performance

[Hib Halverson]

Originally Posted by Paul Workman

(snip)
Hib...You said Barney measured 413 at the wheels. You used 18% drive line loss to predict you have about 504 net hp at the crank.

That's correct. Actually, that session was three passes on the Dynojet at K&N Engineering near the end of the period when the car was a "project vehicle" for "Vette Magazine." It was two runs at 410 and one at 413. There were also a few in the high 400s, but that was with a different calibration.

My 90 Z measured 432 hp at the wheels, using SAE/Marc's 15% drive line loss, my net crank calcs to be 508 hp. Now, going on net crank hp and nothing else, I imagine it would be a near toss-up in the quarter mile, all things being equal. Care to race and compare??

P.

Well, uh...(choke)...

I don't know how this thread ended up being a wee-wee length contest. I thought we were talking about how GM has rated engines, the differences between the old "gross" power rating system and the current "SAE net" power rating system and the ins-and-outs of comparing power output using dynos.

But...I will conceded, Paul, that your weenie is bigger.

Likely you'd wipe my sorry butt all over the track seeing as you have headers, shorter gears, more power to the ground and significant drag racing talent revered by most here on the ZR1 Net Forum.

[Paul Workman]

Originally Posted by Hib Halverson

Well, uh...(choke)...

I don't know how this thread ended up being a wee-wee length contest. I thought we were talking about how GM has rated engines, the differences between the old "gross" power rating system and the current "SAE net" power rating system and the ins-and-outs of comparing power output using dynos.

Ins-and-outs of comparing power output via dynos...WAS exactly my point!

Engine output and how it is measured has been a delema for real engineers for some time, hence references to the evolvement of various SAE standards for measurement, and their revisions over time. And, I was agreeing (somewhat, perhaps) with you with regard to the ambeguity ("grain of salt", you said) that results between direct vs. indirect measurements.

You misunderstood my piont; no dick-measuring at all. Take "yours and mine" out of the discussion and say A = 504 net chp via 18% correction between the tire and the flywheel, and B = 508 net chp via 15%. That amounts to a 1% difference in net chp - well within the margin of error to the point of being essentially the same.

However, measured at the tire, B = 4% more than A. We can agrue the differnces between dynos too, but all things being equal, which horse are you going to bet on in a race? Again...reinforcing your agruement regarding (essentially) the pitfalls of extrapolating measurements via indirect methods, and my point that measurements at the tire are much more indicative of motor output, but terminal velocity resulting from WOT accelleration testing is probably superior to anything else we have to determine real performance, i.e., where rubber meets the road...maybe.

Interesting discovery... In boning up on SAE standards re this topic, I ran across "taxible horsepower". Essentially a British standard where a horsepower tax was levied on manufactures where the number of cylinders and bore diameter were the key ingredients in the calculation. As result, Jaguar (and others) motors in the 50s were built with fewer cylinders and strokes significantly longer than bore diameter. Eventially, the standard was relaxed in order to compete effectively with other world manufactures that were trending toward over-square bore/stroke via bore diameter. Another example of govenment and unintended consequences. Makes ya wonder what sports cars would be like w/o any restrictions, huh? Supercharged V12s or V16s maybe?

Anyway...I agree w/ others that this topic has probably been beat to death...long ago.

P.

P.

[Blue Flame Restorations]

Beaten like a drumb.

You can lead a horse to water..............but if the horse thinks he knows everything there is to know about water.....................

[scottfab]

Originally Posted by Hib Halverson

... snip...
But...I will conceded, Paul, that your weenie is bigger.
...snip...

Definitely time to close this one.
But that Hib quote would be a nice addition to Paul's signature