L-78 engine Birth date

[WILMASBOYL78]

Wasn't the L72 rated at 450hp initially and then lowered to 425hp....??? Read an interesting article the other day about a dyno test on the original 65 L78 and then revised results following a blueprint by Bill Thomas...these motors seemed to need a little help to achieve the factory ratings...

Neat stuff..wilma

[3macs1]

Yes sir it was on the l-72. I have never heard of a 66 chevelle however even the early ones being rated at 425.
learned something new today

[Salvatore]

I think GM changed ratings at times with a stroke of a pen. A lot depended on insurance companies back in those days and the cars they wanted to promote.

[JRSully]

I recently had a 70 L78 rebuilt dead stock except for a .030 overbore (70 396'a started life as a 402) 11:1 compression (verifed) nos 143 cam, 569 intake, 4557 holley, stock distributor, stock, uncut heads (hardened seats)stock valve size. Motor was pretty much clearenced and balanced. 422 hp @ 5900 WITH headers, 400 lbs torque on the Dyno. Everything is essentially the same as the 65-69 L78 except the intake (flatter, slightly lower performance) and the I think the 291 heads (70) may have had slightly larger valves vs. the 840 and also what equates to a .060 over bore over an actual 396 inch motor WITH headers, so who knows for sure..

[@wot]

GM also had a marketing dilema. The L78 was originally listed as making 425 hp at 6400 rpm in 1965. Then the L72 came along and that made 425hp at 5600 rpm. So the L78 was dynoed and became a 375hp motor at 5600 rpm. This arrangement gave Chevy two distinct levels of hp they could offer the public.

[olredalert]

-----If you guys are talking strictly about 1965 Chevelles and 1965 Corvettes the difference is the cam, I believe. I dont have the specs in front of me, but the 201 Chevelles all came with a hydraulic cam and lifters while the 425 Corvette came with a solid cam and lifters. The 65 Chevelle 396 was the only instance in the history of rat-motors from Chevy that they put big-port heads and intake together with a hydraulic cam.
-----After 1965 alot of what you all are saying above is true.....Bill S

[WILMASBOYL78]

[ QUOTE ]
I recently had a 70 L78 rebuilt dead stock except for a .030 overbore (70 396'a started life as a 402) 11:1 compression (verifed) nos 143 cam, 569 intake, 4557 holley, stock distributor, stock, uncut heads (hardened seats)stock valve size. Motor was pretty much clearenced and balanced. 422 hp @ 5900 WITH headers, 400 lbs torque on the Dyno. Everything is essentially the same as the 65-69 L78 except the intake (flatter, slightly lower performance) and the I think the 291 heads (70) may have had slightly larger valves vs. the 840 and also what equates to a .060 over bore over an actual 396 inch motor WITH headers, so who knows for sure..

[/ QUOTE ]

Is that with the turbo on high???

wilma

[JRSully]

Headers is the key there Thomas, I expect new upholestry in the Black Beater next time around, fun time though, I must say SULLY

[nuch_ss396]

Hey guys,

This was posted over on the NCRS forum in regards to this topic:

The 1965 Corvette L-78 and the 1966-69 L-78 used for Chevelles and Camaros were, basically, the same engine. All internal parts were exactly the same or, essentially, the same. Ignition and carburetion was different but, essentially, the same (e.g. obviously the Corvette and Chevelle/Camaro applications would use a different distributor since, with just a few very minor exceptions, only Corvette used a tach drive distributor). Intake manifolds were different but, essentially, the same.

The real differences between the applications was, as has already been mentioned by others, the exhaust manifolds AND the RPM that the engines were rated at. The latter was the biggest factor in the horsepower rating difference.

By the way, the 1965 L-78 used in passenger cars was exactly the same as the L-78 used in Corvette EXCEPT for the distributor. The exhaust manifolds were the same and the engines were rated at the same RPM.