HoCoDave

In the 70's, I think, horsepower ratings went from "Gross" to "Net". Thus we saw a drop in reported horsepower. So, I was wondering what the C2 midyear horsepower ratings would be in today's terms? Specifically, what would be the ratings on the big blocks: 390hp (L36), 400hp (L68), 425hp (L72, L78), 435hp (L71)?

:steering:

Thanks,
Dave

lawcpi

I'm certainly no expert, but I have heard there is a 15% - 20% reduction in horsepower from flywheel measurements to rear wheel. That figure may be a bit inaccurate because I think the efficiency of the drivetrains in the sixties was not as efficient as those today.

SWCDuke

It will vary from engine to engine, depending on parasitic load (fan, alternator, power steering pump) and exhaust system efficiency, but the best overall estimate of sixties vintage net horsepower would be 80 percent of gross.

Also keep in mind that gross horsepower ratings were subject to a lot of manipulation. SHP small blocks are probably a bit overrated, but SHP big blocks may actually be underrated in terms of gross horsepower.

Observed gross horsepower is corrected to 29.92" Hg. and 59F dry air - scientific "standard sea level"conditions, but net power is corrected to "SAE conditions", which I believe are 29.38" and 77F dry air. The differences in these conditions alone is about three percent.

Duke

DansYellow66

Good question. Seems I remember in my dim memories of the early 70s that gross was a bare engine strapped to a dyno with no exhaust and net was with all accessories and exhaust. I have read a number of magizine articles in recent years where they have built up a stock 60s motor and tested it on the dyno with low restriction exhaust from the manifolds and they ran amazingly close to their stock factory rating. Pontiac RA III and IV motors with a little adjustment of timing put out right on, or slightly above their ratings (366 HP and 370 HP). A Mopar 383 Mag put out right on it's factory rating of 335 HP. A 340 Mopar put out about 300Hp versus 275HP rating, I believe. But then everybody suspected they were underrated back then. A 440 Magnum put out about 360 HP stock (versus 375 HP) I think, and I was a little disappointed in that one since I own one. I want to say someone did a stock 427/435 recently but I don't remember any details. I suspect Chevy was right in there with the others except for those motors they purposely rated at RPM below their peak power level. Unfortunately, at the rear wheels these cars are probably down a good 20% for driveline losses. Just my 2 cents. :party:

0X-AutoProducts

i know i'm in way over my head here since duke has already answered (maybe he'll provide some clarification) but i think the HP/TQ curve also plays into how powerful the engine SEEMS to the driver as far as 'seat of the pants' feel is concerned.

i've read that an engine with a higher peak TQ figure can feel more powerful (even though it may have less peak HP) due to the fact that its rate of change in acceleration is carried higher into the power band than an engine with a lower peak TQ.

as far as trying to figure out the reduction from gross to net, i can offer a couple of examples that i've read regarding the '71 & '72 LS5 engines, and the '64-'65 FI engines compared to the '92 LT1 engine?

in '71, the LS5 had a rating of 365 HP. in '72 it had a 270 HP rating. i've read that they both were built to the same specifications - both are 454 CID, both have CR 8.5:1. i don't about the manifolds, heads, or pistons, but this gross-to-net reduction in HP would support duke's comment that it depends on the engine but 80% is a general rule.

also, i've read that the '64/'65 375 HP FI cars have a power curve that closely matches the '92 LT1 car rated at 300 HP net. this is also an 20% reduction.

factory ratings were affected by factors outside of engineering (ie: marketing, corporate, competition) and how hard the insurance companies were hitting the owners of these cars with massive premiums.

just food for thought.
:flag

Vetterodder

Like Duke said, there were a lot of games played. While the engineers took the power measurements, the marketing guys were in charge of what figures were advertised. Prior to `71, mfgs could advertise whatever figures they wanted. After that, it became illegal to lie. GM advertised both gross and net figures in 1971 and those figures are a good example of what Duke said.

Base-270hp gross = 210 net (-22%)
LT1- 330hp gross = 275 net (-17%)
LS5- 365hp gross = 285 net (-22%)
LS6- 425hp gross = 325 net (-24%)

I've driven both and I don't think the power curves of the 92> LT-1's are very similar to those of the solid lifter 327's. The 90's LT-1's have much more low end power but comparable top end power (and at less revs). When GM introduced the 300hp `92 LT-1, they advertised it as the most powerfull production SB they ever built.


[Modified by Vetterodder, 6:06 PM 2/20/2003]

SBR

I agree with Duke, some engines were overrated and some underrated. I remember someone posting a rearwheel number for a 435 and it put out around 325rwhp. There was also a forum member that dynoed his factory L88 and put out 471rwhp although with headers but still needed some fine tuning!

HoCoDave

Thanks for the responses. I'm always learning!

Cheers,
Dave

SWCDuke

You bring up a good point, and many may recall me ragging about TORQUE BANDWIDTH. The "horsepower" that guys live and die by represents the peak power output at high revs that is only "felt" at four specific vehicle speeds on a vintage four-speed Corvette. You spend 99+ percent of your driving below 4000 RPM so the output from off idle to 4000 is more important than peak power.

A good street high performance engine should make 80 percent peak torque no later than about 2000 RPM. If it doesn't meet this criterion, it will like feel sluggish in normal driving.

For a racing engine you want to maxmimize the average horsepower in the upper rev range from the redline to the point that the revs pick up in the next higher gear when shifted at the redline, and this high rev maximum average power bandwidth is determined by gear ratio spacing. The closer the spacing the narrower the high rev bandwidth you need to worry about.

F1 cars have about ten percent gear spacing, and that's why they only care about the power output from 16000 to 18000, but those engines aren't very flexible, and at 6000 RPM they probably make less power than a modern 3-liter passenger car engine.

Gross horsepower was determined on a laboratory dynamometer without any engine accessories other than the water pump. The exhaust manifolds were connected to generous sized piping leading to an exhaust plenum that was aided by a pump to ensure that exhaust gases were evacuated from the dyno cell. Fuel mixture and spark advance were set to optimize power at each test speed and these did not have to be the same as production. From here the marketing guys took over. It's quite a coinidence that power ratings are always a nice even number, isn't it?

Net ratings are base on "as installed" configuration with all accessories, the production vehicle exhaust and inlet system, and are corrected to different "standard" conditions as I quoted in my previous post.

Ever noticed how almost all modern cars have electric rather than engine driven fans? An engine driven fan can consume 5-20HP at peak revs. Also, modern EFI engines have generous inlets and filter area and no manifold heating, and the trend towards non-metal inlet manifolds helps keep the fresh charge cool, which leads to more power due to denser inlet mixtures.

A few months ago I received dyno sheets for a '63 L-76 - stock rebuild - tested at what was basically SAE gross conditions and it made a little over 300 HP - almost exactly the same as predicted by DD2000. The owner wanted to know what happend to the extra 36 or so horses to make the 340 rating.

I replied that they only existed is some marketing guys wet dream.

Duke




[Modified by SWCDuke, 8:05 PM 2/20/2003]