L-78 engine Birth date

[3macs1]

Here is what I got one time when I asked this question.
Insurance concerns plus General Motors Corporate policies resulted in the use of a lower RPM rating resulting in decrease from 425 HP to 375 HP as a running change for all models except corvette. The L-78 was released for sale in the 1966 SS-396 Chevelle in March 1966 with a rating of 375 HP due to this lower RPM rating and somewhat more restrictive exhaust manifolds, it was otherwise identical to the 425 HP '65 Corvette.( same cam)
The 1965 Z-16 396 Chevelle had the same engine except the Z-16 used a hydraulic camshaft which effectively limited RPM to the lower 375 HP rating.(L-37)
Also all of my 66 Impala/ Biscayne l-72 427's came with the
3880827 and 828 vet exhaust manifolds originally.

[Paul D]

My '66 Impala SS L72 also came with the Corvette cast iron exhaust manifolds.

[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.

[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

[Stefano]

I have never Dynoed the different exhaust manifolds side by side but it would be no where near a 50 hp difference.

The top dog prior to the mystery motor was the 409/425. GM could not go backwards in the public eye/marketing, at least at the initial release of their new solid lifter engine.

Some early '66 L78 Chevelles have been documented as showing the L78 option rated at 425hp.

I believe Bill Thomas' Blue printing guide/book has a dyno chart or info showing that the L78 could produce 425hp if brought to exact specs.

The First 427/L72s were factory rated at 450hp.