When I built Donnie Cornelis' `69 L78 it made 462 HP and 439 lb.ft through headers (which were too big for that engine but were all I could get my hands on in time to test with) and ~430 HP and 411 lb.ft through the manifolds. However, the peaks don't tell the whole story. The engine lost 60+ lb.ft down low with the manifolds in place, and that you will definitely notice.

When they were 425hp in 1965, that was with the more streamlined Corvette exhaust manifolds that also fit & were used on full-size L78 cars that year. They became 375hp when fitted with Chevelle, Nova, Camaro log-style manifolds. It would interesting to see an exhaust manifold comparison on the dyno.

What were the final numbers?

For comparison here is an original 1969 L/78 restoration engine I rebuilt last year. All stock except Comp/Cam replacement cam, EDM lifters,+030 TRW pistons and run with shop headers. Fifteen minute break in & one dyno pull 38 deg timing pump 92 octane. We did not jet or do any other tuning as this is the original motor from a SS 1969 Chevelle.

Yikes! Thanks for all of the input. We were able to coax a few more HP after re-jetting. Then we put the shop carb on and it woke up! 446 hp at 5700.

I built a 66 L78 all stock except for a little head work. It made 444 @ 6000 rpm with headers and a 3247 Holley carb.

L78...
 

Impressive numbers from the "baby rat'...especially some of the torque figures. So, this begs the question...'if the L78 makes those numbers...what does it's big brother, the L72 really make? I would assume well north of 450hp...

-wilma

That is a very impressive torque curve from a stock L78 IMO.

every 427/425 that I have had done developed between 490 and 505 hp through headers with stock carb & dist.

Sounds about right.

I installed a 3910 on my 496 for the initial dyno tests and the engine picked up somewhere around 20 when I swapped to a 3418 (`67 L88 855 vacuum secondary). During the last series of tests, the engine picked up another 30+ when I replaced the L88 carb with a 4781 850 double pumper, and another 20 when I switched to a 1000 HP (I built & calibrated myself). That's over 60 HP just in the carbs.

The factory carbs are good, but they do leave room for improvement. Comparing them to a newer "race" type carb really is an apples to oranges comparison.