Tom400CFI

This had been touched upon in numerous threads previously and in those threads it's become obvious that some people haven't had the chance to understand the difference between GROSS hp ratings that were used prior to 1972, and NET hp ratings that have been used since. Adding to the confusion, I'd guess, is the proliferation of rear wheel horse power figures, yet another method of measuring power. So I thought I'd try to break it down as simply as possible and provide some sources to support my points.

Going chronologically, we'll start with GROSS hp. GROSS hp is the method used by all domestic OEM's up until 1972. Gross hp is measured at the flywheel. When measuring GROSS hp, OEM's were looking to get the best/highest/most appealing numbers they could get from the basic engine, so it would be set up on an engine dyno, as favorably as possible. You would see things such as: high octane fuel, optimized ignition timing, optimized jetting, velocity stack on the carb, no accessories, open long tube headers...even blue printed, and port matched parts. Here is what an engine looks like on an engine dyno, being measured using the GROSS hp rating method. Note the drag (or lack thereof) on the crank pulley...and note the similarity of the exhaust system to what comes in cars, from the factory. Also note the air filter and housing. This is what makes it a GROSS rating; everything has been optimized to produce the best numbers possible.



Even though OEM's abandoned this method decades ago, still today, any "engine dyno" test that we see, (Engine Masters, Nick's Garage, or power ratings in a Summit Racing catalogue) are still done ^^THIS^^ same way. GROSS hp ratings on a dyno, totally optimized for high numbers. Engine Masters even runs their engines at ~130*F which increases power numbers compared to "normal" operating temps.


Let's move on to NET hp. NET hp ratings were used from ~1972 onward and are still the standard method used today...by OEM's. Any hp figure that you see, given by an OEM is a NET hp rating. NET hp rating is measured at the flywheel. When measuring NET hp, OEM's are looking to get the most accurate, representative number for that engine, as installed in a car. As installed in the car.
Than means, factory manifolds, factory exhaust (for that CAR), back to the tail pipes, all accessories in place, and factory tuning, on regular 91 octane fuel. In fact, I've read a quote that SAE NET hp is supposed to represent the "power available to move the car" -so that certainly is after all losses from install and operating set up. Here is what an engine looks like on an engine dyno, being measured using the NET hp rating method. Note the drag on the crank pulley, note the air intake; open carb w/a velocity stack? No...and note the similarity of the exhaust system to what comes in cars, from the factory. This is what makes it a NET rating; every effort has been made to make the rating identical to that which you would experience, as installed in a particular CAR.










Look at ^That^ engine as compared to the one in the first pic. See a difference? They're both "engine dyno's"....but they're not measured the same way at all. "As installed in the car" is key to NET hp ratings, and why we see different numbers between an L98/LT1 in a Corvette, vs L98/LT1 in a F-body -different exhaust systems, primarily. Using the GROSS rating, both applications would have been rated the same (and with a much higher number). But they aren't the same, in service.
HERE IS AN ARTICLE for your reading pleasure, if you question my text above.



Finally, there is "Rear wheel horse power" ratings which have become much more popular in the past 20 years due to the affordability and proliferation of chassis dynos. RWHP data is not used by OEM's. These numbers are generated after sale, by the end user -you and I. The numbers are taken from the tire and reflect all of the loads on the engine, including losses through the drivetrain and tire. There is wide(r) variability here b/c of less strict management of standards...and the hardware used in the testing. It's a tuning tool...not an "end all" hp number. Here is how we test RWHP in a car.



Now, these various rating methods -especially the GROSS vs. NET have really tripped up some forum members. Especially when trying to compare old GROSS rated engines to newer NET rated engines; "But the LT-1 SAYS 370 hp on the air cleaner!". Yeah...when totally optimized. As installed, it makes about 300 NET hp -same as a '92 LT1.

And, here is some reading that supports those claims, including actual dyne testes of a '70 LT-1 clone and a '90's LT1. Read on and be surprised...

SURPRISING LS6 454

LT-1 vs. LT1

BIG BAD MUSCLE CAR NUMBERS...PUT IN PERSPECTIVE


Even after explaining these differing methods of measuring, some folks STILL trip over their own dicks, trying to explain how the old car is really more powerful than the new car. So how can we really confirm one way or the other? There is one, simple end-all method, and that is TRAP SPEEDS and weight. If we have two 3300 lb cars that run the same trap speed at the drag track...we've got two engines that are producing about the same hp, regardless of how or what "hp number" was tacked on the air cleaner at the factory. In the case of the LT-1 vs. LT1, one was rated at 370 hp (GROSS) and one at 300hp (NET)...yet we've got two cars that trap in the low 100's...and they both weigh about 3300 lbs. What does that mean? They both make the same hp.




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FASTAZU

It would be interesting to see a chart to rate what the gross hp would be based on RWHP. Say 420 rwhp to gross hp

dclafleur

If you want the car to feel faster you can also put some old skinny bias ply tires on them. That way they'll spin and smoke longer.

Tom400CFI

Yeah. One of the links I posted showed a real world spread from RWHP to Gross with the '70 Chevelle sporting a 383...

"For a baseline vehicle, we used Project Homewrecker, a '72 Corvette with a 383. We thought it perfect for the job since we had engine dyno numbers similar to gross measurements, but had yet to test it on a chassis dyno. The stroker is backed by an M22 four-speed and the IRS contains 3.70:1 gears. On an engine dyno with open headers, no accessories, and the same 750 cfm double pumper carb, it put out 425.8 hp and 473.8 lb-ft of torque. On Super Chevy's brand-new, state-of-the-art Dynojet (www.dynojet.com), it made 283 hp at 5,000 rpm and 341 lb-ft of torque at 3,900 (SAE net correction). That's with everything hooked up-alternator, power steering pump and Vintage Air A/C compressor. Eye opening, isn't it?"


So that is a ~34% loss from Gross to wheel, in THAT car, or turned around, it would be a 50% gain in "hp number" from wheel to Gross. So there is a percentage you could use, but keep in mind that will never be rock solid accurate; many factors come into play, such as:
*How much is done to maximize hp in the "gross" dyno pulls
*drive train weight
*gearing
*transmission type
*Exhaust system....

...and then there were a plethora of other cars in the same article that compared Gross to rear wheel.



HERE IS ANOTHER ARTICLE where Super Chevy took an LG4 from a 3rd gen. LG4 was rated at 145hp NET and on SuperChevy's engine dyno, it did 224hp, GROSS.

"Since it was missing the factory induction system, we simply installed a suitable cast-iron Q-jet intake and carburetor (from the Carb Shop) along with a set of 13/4-inch long-tube headers.... Run with an electric water pump (no accessories), long-tube headers and 37 degrees of total timing, the LG4 produced 224 hp and 308 lb-ft of torque."
-No mention as to how much tuning Super Chev did to it on the dyno...I'd guess that if one were motivated, they could get that number up over 240 with the right fuel and tuning, etc. So now we're looking at over a 36% loss from Gross to NET....and since an LG4 should do about 130 to the wheel...yikes, now we're looking at a whopping 43% loss in "numbers" from GROSS to wheel.






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dizwiz24

cv67

Had plent of them they were dogs, a BBC only felt decent with a cam recurve and headers .
that was decent not impressive even L88s ran slower than they sounded. Dont belive the myths...some of us owned them and know better.

GREGGPENN

It also means you have to cover HP/Weight. After that, the next logical step would be to cover how you have the fastest (unbodied/Kart) Corvette of anyone that ever lived!

Hey...If I were you, I'd "trip" on that!

Tom400CFI

LOL...I need to get the thing finished first. And when I do...Cletus McFarland has me beat...by miles!

bjankuski

Nice write-up. One thing to keep in mind is what correction factor the dyno operator is using? If the dyno operator is using J607 that is considered STD correction factor and it will read about 4% higher then the correction factor used by the OEM's. The OEM's use J1349 which is considered SAE correction factor.

Not a big deal but if the 300 HP lt1 engine was tested and rated with SAE procedures it would be 300 HP net as rated by GM, if you switch the correction factor to be STD then it would be 312 HP net. The point to keep in mind is to understand when you are comparing dyno numbers is are they shown as net or gross numbers and what correction factor are they using STD or SAE?

This information also holds true for chassis dyno's are they displaying the information as STD net or SAE net? Was the engine dyno that you saw STD gross/net or SAE gross/net? As long as you know you can adjust all results to provide you apples to apples results.

84 4+3

Adding a couple turbos is easy.... The trans might not like you after though.

As for the rating, yea. Don't get me wrong in that they were quick back in the day and they aren't slow by today's standards still. (About average.) The lower option ones are...

I've been told stories of a friend of dad's who had either a zl-1 or l88 equipped car. I can never remember which but I'd guess it's an l88 70 c3. He's built some pretty insane vehicles in his life but he said that car always scared him. It wasn't so much the power as it was the fact that he was running down the highway at around 140 and he hit an expansion joint and ended up two lanes over... it was supposedly one of the stiffest cars he ever had the pleasure of driving... At any rate technology marches on.

QCVette

Good explanation of hp measuring methods.

I liked the linked articles too.

I had an LS6 Chevelle, an LT1 and LT-1, and the Firebirds in the last link.

Today's cars run very well and make most of the older ones seem slow in comparison.

I was surprised that the LS6 only had that little hp. The 400 Firebirds ran in the upper 14's and the '79 400 was a 15.3. I had several 400 Firebirds/Trans Ams and those figures are close for a stock car.

What really puts the newer to older car comparison in perspective is that I have an Impala with the 3.6 motor that runs in the high 14's. That current 6 cylinder automatic full size grocery getter runs as good or better than the 400 Firebirds from the "muscle car" era.

pacoW

http://driving.ca/bmw/m5/auto-news/n...eir-true-power

I'll just leave this.

MatthewMiller

Of the older engines listed (L88, 426 Hemi, Cobra Jet), how many actual regular customers ever got to buy one? Very, very few. One example: for all three years of L88 Corvette production (76-69) Chevy built 218 of them. 218. And we can go ahead and assume that only a small fraction of those ever got sold to regular customers, vs going to race teams and corporate insiders. You might as well compare to a current Ford GT...except that Ford is building and selling way more GTs (at $400k or more, each) than that. GM made many, many times more ZR1 C4s than they ever did L88 Corvettes. As another data point, the LS7 in the C6 Z06 probably would make just as much or more power as a "stock" L88, and Chevy made around 25,000 of those. The Hemi was better, but not great, with less than 11k being installed in all street cars over six production years.

So yeah, there were a very few engines back in the day that made serious power. But in terms of being obtainable by real customers, they mostly weren't. Hell, a stock C7 LT1 (not the supercharged LT4) is probably very, very close to the power of an L88. The LT4 (which anyone can buy - it isn't limited, just expensive)? Fuggedaboudit. We live in unbelievably good times for motorheads.

Tom400CFI

.....aaaand this is where the bunk/lore/BS comes from. What did that link actually show us? Nothing! Engines making noise on a dyno. The text was someones opinion/guess as to "real" hp numbers....but any/all of those numbers would be GROSS, anyway and in no way representative of what the engine would do, AS INSTALLED...they way that we relate to power in cars today. I looked at the image in that first vid and just laughed. Yes, that is how engines are configured when we actually use them, in a car.

How about some real data? That "557hp, L88"? -The one where we saw it make noise on a dyno...never saw a result... anyway, the article claims a 47hp loss for "as installed" ("true output of an L88 at the flywheel when fully dressed is around 510 horsepower.") which is way off the mark....you'll lose about that with just the fan. You can always find an "article written by some whimsical, dreamy author who wants to glamorize the old days...and that is where the LORE comes from. Drilling down in to fact takes a little more effort and scrutinizing. Let's look at something based in fact, shall we? Here, someone ran a regimented test of the legendary ZL1....the top of the top, right? 508 GROSS hp That's optimized everything, even water temps at 150*! Open headers, no water pump, no accessories, etc. etc....they even have the velocity stack on the carb! That 508 GROSS hp would be dumbed down to ~400 hp as installed, and ~350 RWHP on a chassis dyno. <That is the whole point of this thread. Dreamy, GROSS hp numbers vs. realistic, NET hp numbers.








Here is another, 509 GROSS hp;



Finally (regarding that linked article), I feel that in the later examples (FGT and M5), the author goes awry with his 15% number. I don't feel that 15% is an accurate figure these days.


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383vett

Can you explain how to trip over one's dick?

856SPEED

dclafleur

Short legs I'd imagine.

This is why I've been getting a kick out of the Engine Masters series that Motortrend puts out. They compare all sorts of things, different fan types, different intake configurations to show how/if they affect power output. Adding a velocity stack and pulling off a mechanical fan is worth a pretty tangible change in output.

856SPEED

pacoW

Right, so a manufacturer would never sandbag their numbers or adjust tune to get cheaper insurance rates, pass fuel mileage standards, or pass emissions standards?
Cough...VW.....Cough...Cough.

Tom400CFI

I'm sure that they would, could and have.

That is different than Gross/net/wheel, which seems to confuse some, but yeah, it definitely happens for sure.