Horsepower ratings

[Walter F.]

I believe net horsepower ratings were introduced in the early 1970s. What was the reason for such ratings and just how much horsepower was lost say on a 427 with gross horsepower rating 435.Are the new cars of today giving us net or gross horsepower?

[Duke W.]

Re: Horsepower ratings

Both SAE gross and net were advertised in 1971, net only from '72 to the present day.

SAE gross is totally unrealistic after you add the vehicle exhaust system, induction system and all front end accessories. In addition, SAE net air density correction reduces the ratings by about 4.5 percent relative to SAE gross which uses standard sea level density.

According to data I have the net/gross ratio of a 327 with 2.5" under-the car-exhaust is about 0.89, but that does not account for the fact that 327 gross ratings were overstated by 10-15 percent.

So if you want to know what the net rating is of an L-71 you first have to know how honest the SAE gross rating is. The net/gross ratio will be less than a 327 due to higher exhaust backpressure from the higher flow.

About the best I've seen on a chassis dyno for an otherwise OE SHP big block with massaged heads is about 350 SAE corrected, which would be about 410 net at the flywheel using a 0.85 driveline/tire efficiency, so that would be something around 450 gross, but remember that is with massaged heads which increase power about 10 percent, so an honest Tonawanda-built SHP big block gross rating would likely be something a little over 400.

Duke

[Alan S.]

Re: Horsepower ratings

Hi Walter,
Here're 2 pictures of a chart that appeared in the Corvette News 1971 Corvette introduction article.
It'll give you a pretty good idea of the comparison of gross and net hp ratings for the engines offered for 1971.
Regards,
Alan

[Terry D.]

Re: Horsepower ratings

Duke

If 410hp is net at the flywheel, how much would it be at the rear wheels? I'm assuming it would depend on automatic vs manual but just looking for ballpark figure.

[Duke W.]

Re: Horsepower ratings

In the last paragraph of my previous post I mentioned driveline/tire efficiency of 0.85. This is a ballpark figure for front engine rear drive vehicles with manual transmissions in direct drive, and that's how I came up with the 410 net at the flywheel given 350 SAE corrected at the rear wheels: 350/0.85 = 411.7.

For automatics without torque converter lock-up 0.80 is generally accepted.

It's easy and inexpensive to put a car on a chassis dyno to obtain SAE corrected RWHP. Then, using reasonable driveline efficiency and net gross ratio factors you can get an honest ballpark estimate of SAE net and gross at the flywheel.

Most guys are surprised at how low measured and calculated peak torque and power are for vintage Corvettes compared to the advertised ratings. Modern Corvette net ratings are honest, which means that the current base LT-1 is a lot stronger than any OE vintage big block including the L-88/ZL-1 as delivered with manifolds and mufflers.

Duke

[Don W.]

Re: Horsepower ratings

I had my 69 427/390HP THM400 dyno tuned last week. Results were 260 HP @ 4200 rpm and 328 ft-lb torque at 3600 rpm, all at the rear wheels...Don

[Mark E.]

Re: Horsepower ratings

Hi Walter,
Here're 2 pictures of a chart that appeared in the Corvette News 1971 Corvette introduction article.
It'll give you a pretty good idea of the comparison of gross and net hp ratings for the engines offered for 1971.
Regards,
Alan

I thought '71 and '72 engines were virtually identical, and therefore the 330hp gross and 255hp net for the '71 and '72 LT-1 were equivalent.

But this chart says 330hp gross = 275hp net, suggesting the LT-1 suffered a 20hp drop for '72.

Really?

[Don W.]

Re: Horsepower ratings

I think they also dropped the 72 LT1 compression...Don

[William F.]

Re: Horsepower ratings

Yes, all went south after 71, with 71's not as strong as 70 models. Stayed that way till variable valve timing, computer engine controls, etc came along .

[Mark E.]

Re: Horsepower ratings

I thought LT-1 heads, pistons, cam and CR (9:1) for '71 and '72 were the same. No?

From Lt-1.com (can't vouch for its authority; Google found it):

"For 1971, as part of an effort to meet new incoming government emission requirements, compression ratio’s were lowered to 9:1, resulting in a reduced 330 gross horsepower. Also, 1971 was the first year that the LT-1 could run on un-leaded gasoline. In 1972 engine ratings were reported as an SAE net figure of 270 horsepower, but otherwise the car remained relatively unchanged."

[Darryl D.]

Re: Horsepower ratings

Both SAE gross and net were advertised in 1971, net only from '72 to the present day.

SAE gross is totally unrealistic after you add the vehicle exhaust system, induction system and all front end accessories. In addition, SAE net air density correction reduces the ratings by about 4.5 percent relative to SAE gross which uses standard sea level density.

According to data I have the net/gross ratio of a 327 with 2.5" under-the car-exhaust is about 0.89, but that does not account for the fact that 327 gross ratings were overstated by 10-15 percent.

So if you want to know what the net rating is of an L-71 you first have to know how honest the SAE gross rating is. The net/gross ratio will be less than a 327 due to higher exhaust backpressure from the higher flow.

About the best I've seen on a chassis dyno for an otherwise OE SHP big block with massaged heads is about 350 SAE corrected, which would be about 410 net at the flywheel using a 0.85 driveline/tire efficiency, so that would be something around 450 gross, but remember that is with massaged heads which increase power about 10 percent, so an honest Tonawanda-built SHP big block gross rating would likely be something a little over 400.

Duke

Regarding your statements I have bolded above:
No induction? pretty hard to dyno at the factory without it.

Overstated? No. However the factory played with the rating to get what they wanted to report by rating them at different RPMs on the dyno. It was common back in the day to UNDER RATE some of the hipo engine ratings for insurance and customer sales reasons. As an example the L-88 was rated 5 h.p. lower than the 435 h.p. tri-power model as one of the detourants of having some kid come in to the dealership and buying the car with the highest rated h.p. that the L-88 truly was and the kid ending up with a real race car that would overheat under 45 mph.

[Darryl D.]

Re: Horsepower ratings

I had my 69 427/390HP THM400 dyno tuned last week. Results were 260 HP @ 4200 rpm and 328 ft-lb torque at 3600 rpm, all at the rear wheels...Don

If you would show the h.p. at the cars red line it would be higher and likely about 83% of the 390 factory rating which is what the rating sheet posted here in this thread shows it to be reported by the factory for the difference between gross and net--net being basically what you see on a dyno at the rear wheels vs. at the flywheel.

[Duke W.]

Re: Horsepower ratings

Regarding your statements I have bolded above:
No induction? pretty hard to dyno at the factory without it.

Overstated? No. However the factory played with the rating to get what they wanted to report by rating them at different RPMs on the dyno. It was common back in the day to UNDER RATE some of the hipo engine ratings for insurance and customer sales reasons. As an example the L-88 was rated 5 h.p. lower than the 435 h.p. tri-power model as one of the detourants of having some kid come in to the dealership and buying the car with the highest rated h.p. that the L-88 truly was and the kid ending up with a real race car that would overheat under 45 mph.

By "vehicle exhaust system, induction system..." I am referring to theproduction exhaust system, and air cleaner, snorkel(s), and thermostatically heater inlet air.

The L-88 was probably about the only engine of that era that was "underrated", so it was hardly "common", but given the very high overlap camshaft, exhaust system back pressure would have absolutely killed power output with the production exhaust system.

Duke

[Duke W.]

Re: Horsepower ratings

I think they also dropped the 72 LT1 compression...Don

The advertised CR dropped from 11:1 to 9:1 for '71 and remained unchanged for '72. Internally the '71 and '72 LT-1s are identical, so the 20 HP drop in net '72 rating had to be due to "external factors" like the vehicle induction and exhaust systems and, in particular, changes to the spark advance map to meet emission requirements.

According to the AMA specs spark advance data, total WOT advance for the '71 LT-1 was only 32 degrees compared to the 38-40 required for peak power with the large chamber heads. For '72 it dropped to only 26! No wonder it lost 20 net HP and felt like a slug compared to the '70 LT-1. I'd bet that the reduction in total WOT advance cost more than the two point compression drop.

Duke

[Dan D.]

Re: Horsepower ratings

As I'm reading this very informative thread...I can't help chuckling a little. I have a '72 and while it's a really nice car...Let's face it...It ain't no road burner. I once toyed with the idea of having the original engine rebuilt to 1970 specs...but, I already have way more invested in this car than my estate will ever get for it...and besides, I way too old to worry about "burning up the road."