L78 horsepower
 

Made our first dyno run on the L78 engine for my 67 Camaro. It's bone stock carb to pan. Generated 397hp at 5,500. Was running a little lean, so re-jetting tonight and run again tomorrow. Anyone have input on what HP a stock L78 should put out?

Was the session with headers or exhaust manifolds? Ours with a little lower compression made 450 with the shops carb, headers and dist. but almost 70 less with the stock carb and distributer.

The answer is.... has Eric laid his hands on it or not!!

Mine made 420 with shop headers but, was not "dialed in" all the way

How much do you think headers add over manifolds? Eric is gonna build my 402 for me and I am going back and forth. It's in my Monte Carlo.

Eric did the carb. Ran on dyno with headers, stock dist and carb.

Seems impressive to me. Was max HP at 5500? Seems to me there would had been a little more runway left for more HP?

Should take roughly a 74 primary jet with headers on the dyno, will want the 71 with manifolds in the car. Secondary jetting should be good as is.

L78 Horsepower..
 

When the L78 was introduced in 1965 it was rated at 425hp...this was based on 6400 rpm's...almost 1,000 rpm lower than the engine being discussed. Most folks would agree that the later 375hp rating was low...headers, properly tuned distributor/carb and blueprinted assembly all make a huge difference.

Last one we did was a 70 402 version...pancake intake, Eric prepared 4492 and run thru headers...it made north of 430hp.

Again, the real power difference in these motors is in the details.

-wilma

My 69 L78 was just a basic build,trw pistons,bullet stock spec camshaft,it was balanced.heads are stock 840 castings.with headers it made 415@5500 and 400ft lbs from 2000 to 5500. My nova ran a 13.02 off the trailer at scr with headers 4.10's and street tires

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.

What's the general difference in the carbs?
It would seem to me that if they flow enough air and are jetted correctly, they should be similar. But that clearly isn't the case.

There's a lot more to it than CFM and jetting. Jetting only changes the overall fuel curve up or down. Air bleeds, PVCR's and emulsion (along with booster feed size and design, etc.) change the shape of the fuel curve. That's where a lot of the power is.

Like I mentioned previously, an 850 picked up a bunch over the 3910 on my 496, and a 1000 HP main body conversion on the 850 picked up even more.

In contrast, my trusty "mule" 3310-1 780 Holley was within single digit power levels compared to a 950 HP on my 302.

Generally speaking, any "tweaks" I perform on an OEM carb during servicing are mostly limited to cleaning up the idle and off idle transition along with a small bump in primary jetting (application specific). It would involve a lot more time--and subsequently cost--to "race" tweak every carb that comes across my bench. If the carb's going on a driver, it's not worth the extra time & cost. If it's going on a hot rod where every HP counts, then it may be worth the extra effort.

My engine builder says that vacuum carbs don't dyno well compared to mechanical carbs. Any opinions?

AND what makes HP on a dyno may not apply to race track conditions ... just sayin' ... TAZ

This is 427 Yenko engine same cam & lifters, +.030 TRW pistons & parts as the 396.
One dyno pull, ignore the figures bellow 4k RPM vac sec acted up.

Dave Beems L/78 made 450 STP HP on my dyno with my carb and headers, all factory parts but .030 over on the pistons.

If any one is interested here is a dyno sheet from today.
Original Yenko L/72 .030 11to1 speed pro pistons Comp cams replacement L/72 cam 93 pump gas shop headers.

L72 dyno numbers...
 

Impressive :biggthumpup:

68 nova ss 396
 

recently I dynoed my 396/375 hp motor for my 68 nova ss matador red car...with lots of tuning through stock manifolds and 520 lift flat tappet camshaft best hp was 465 hp and 455 torque.. Chris teed

L78 Horsepower..
 

Chris...those are very strong numbers with cast iron manifolds....nice job.

-wilma

tom, we built the motor to run purestock drags in Stanton, Michigan yearly race..but, didn't quite make it..alot of little things make a difference..it runs good behind the Muncie and 4 ;10 gear..

I'm surprised at how rich the 2nd L72 is running compared to the first.

There is more HP left on the table. Holley new replacement carb 4346, org 4346 carb not restored yet, no need to tune or beat up org engine with wrong carb. I would think if the AF/R was in 13.4 range it would pick up 20+ HP