LS7Vette

Does anyone know what the SAE "Gross" Horsepower is on the 73 - 74 LS4 454's? We know what the SAE "Net" Horsepower is, as this is how they rated their cars from 72 on (and why the horsepower dropped so quickly). Just curious what the LS4 would have rated on the old system? Easy for motors like the L48 350 that was around prior to the change.

thanks

Ben Lurkin

Don't know that I've ever seen a gross hp rating for the LS4's. But rest assured, by then, the numbers were indeed gross.

I would hazard a guess they would be in the 340-350 hp range.

7T1vette

The LS4 ('72-73) was very similar to the LS5 from 1971. The LS4 compression ratio was a quarter of a point lower and the cam was a little different grind, but that's a pretty small difference. The greatest difference in those two powerplants is the way the HP is rated: LS5 rated at 5400 rpm using SAE "gross" and LS4 rated at 4000 rpm [who are the GM marketing boyz kidding?] and SAE "net" method. If the LS4 were rated similarly to the LS5, it would probably be in the 350+ (SAE gross) at more than 5000 rpm.

LS7Vette

I was thinking that this was the range but wanted to hear from you guys. Yes its not as great as the years prior but still not too bad (the change really made it sound worse then it was, atleast this is my opinion). When im done with my car I would like to finish the air cleaner off with a decal and will probably go with 355HP or 360HP. Thanks for your thoughts.

1980 blue L48

I cannot speek of the 73-74 models. I had a 72 SS 454 Chevelle, LS5. It was rated @ 270 net HP. The 71 LS5 was exacally the same motor, It was rated @ 365 gross HP. I think the 73 454 was rated at 5 hp more (275 net)than the 72 Ls5. Hope this helps you, Great Question Too.

Rebelrob

Hey guys, what is the difference and or definition of Gross vs Net HP rating. I have seen this brought up a few times in the past but forget to ask.
For instance, I know the LS5 454 in the 70 Vette was rated at 390 HP. Is that Gross or Net. I assume the 390 HP rating is at the crank. Same said for the 70 SS Chevelle with the LS6 454 450 HP rating. I always here that these ratings were fudged and actually made a lot more.
Thanks,

7T1vette

SAE "gross" hp is engine horsepower measured at the crank without any parasitic losses for water pump, fan, alternator, etc, etc. SAE "net" hp is the same measurement but including all the engine parasitic losses.

Rebelrob

I see, as mentioned, was the 390 HP 70 LS5 net or gross?
Did they change from Gross to net or vice versa for certain year models?

Good info, thanks,

LS7Vette

Thanks for the information, I guess the LS4 was not too much different then the later LS5 motors. Looking at those numbers it could have even been around the 365HP mark.

thanks for all the help guys !

gbvette62

For 1970 models, that is a gross rating. All vehicles were gross rated prior to 72. Starting in 72, GM switched to the truer net rating system.

As far as "fudging" the ratings, yes, Chevrolet did that often. Mainly it was done for insurance reasons, or to get around GM corporate edicts about vehicle horsepower levels vs things like vehicle weight or size. In the case of the L-88, I think Chevrolet rated it less than the tri-power (435 vs 430), to discourage the uninitiated from buying the L-88.

Probably the most underated Chevrolet engine (besides the L-88 & ZL-1), was the 67-69 Z28 302, at a mere 290 HP.

gbvette62

I think this is a pretty safe bet. The 73-74 454's were probably somewhere in the 355-370 "gross" HP range.

LS7Vette

1972 GM cars started SAE "NET" Horsepower, prior to 1972 (1971 and earlier) were SAE "GROSS" Horsepower. Pretty much any decent sounding horsepower rating was SAE "GROSS". The 1970 LS5 454 390HP engine was GROSS horsepower.

LS7Vette

I agree what you said about the L88/ZL1 427's along with the Z28 302's. Also thanks for the input on the LS4 HP Rating.

Rebelrob

OK, I got it now. I thought I had they changed the ratings type after 1970 or so.
BTW, do you have an LS7 in your Vette? I have an original LS7 block in mine. I am using the original forged crank and rods and oil pan. Everything else is updated though.

LS7Vette

I do have an original 70's LS7 454 crate motor that had the compression lowered to run on pump gas. It has a BRAKE HP of 636 on the dyno. I dont have it in my Vette but will probably hold onto it to put into a later project.

CCrane65

Just for clarification, the LS4 was a later motor than the LS5. The LS5 was used from 1970 to 1972 and the LS4 from 1973-1974.

cc

hdpete98

I believe the heads on the 1973-74 BB's were a little lower performance also.

LS7Vette

Yes I am well aware the LS4 was the later motor, just a typo that I didnt catch. I meant to put the earlier LS5. Thanks for catching this........

CCrane65

Technically, if the 1971 and 1972 LS5 were identical, and the 1971 was rated at 365 GHP and the 1972 was rated at 270 NHP and the 1973-74 LS4 was rated at 275 NHP then it follows that the LS4 GHP had to be above 370.

If you do the math 270 / 365 = 74% therefore 74% of 372 = 275.

cc

CCrane65

Ah you're not the only one in this thread to make a such a typo...7T1vette put the LS4 as a 72-73.

cc