Watch This (Claimed) Stock Rebuild L78 396 Engine Make A Dyno Pull – Guess What It Made For Power?

Watch This (Claimed) Stock Rebuild L78 396 Engine Make A Dyno Pull – Guess What It Made For Power?

The L78 was Chevrolet’s big power engine for a few of the most pivotal years of the muscle car era. Between 1966 and 1969, the L78 was the engine you optioned if you wanted the baddest Corvette, Camaro, or big car on the block. Factory rated at 375hp many people suspected that Chevrolet was going light on those numbers, like everyone else was during that era. It was a way to manipulate racing sanctioning bodies, the media (who were growing concerned about the cars), and the insurance companies who were clearly running out of patience with young muscle car owners. So what are the real numbers? Chassis dynos and engine dynos while present in that era, were not as prevalent and readily accessible as today so the only real data collection was at the drags.

In the case of the engine in the video below, the builder claims that the motor is as stock as can be, all the way down to the camshaft specs. There’s a couple things that could potentially bump the power level of this one a little just in the rebuild process. Maybe a slight overbore to clean it up, maybe the heads got “trued” up or the decks of the block were milled to clean them up. The one thing that the poster does clearly mention is making some jetting changes in the carb to really get the engine happy. Oh, there’s also the heads. So all things being equal, we have headers and good jetting and an engine that should make 375hp, maybe 400 with the listed changes.

So what does it make? Watch the video and then scroll down to see the results.

Watch the video and then scroll down for the answer – Are you buying it?





The engine shop that did the motor claimed 444hp peak. Think that’s what they were all making after some tuning off the lot?

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9 thoughts on “Watch This (Claimed) Stock Rebuild L78 396 Engine Make A Dyno Pull – Guess What It Made For Power?

  1. jerry z

    I wish I had my 396 (well actually 402) dynoed when I rebuilt it. It was a CE engine that was in my 66 Chevelle. Car was restored to stock and ran a best of 13.95@101 on redlines. This was over 20 years ago too.

  2. Overhead8

    I read somewhere cars were not tuned for top performance from factory. At example valve springs were designed to float early as a kind of rev.limiter. Changes like new springs and some other little improvements could make noticable differences, probably.
    You can buy replacement cams for the legendary 143 Big Block solid lifter cam from crane or comp cams, if you take a look on the recommned springs specs are everywhere from 105-155 seat pressure and 300-400 open pressure, so how stock is this engine really??? 😉

  3. aircooled

    Highly suspect.

    “We also kept the stock L78 cam lift and duration for a 70 Chevelle”

    Stock cam lift and duration is not the same as stock cam. Especially since Chevy didn’t really publish figures for duration at 0.050″ lift.

  4. RacerRick

    I have witnessed a dyno test of a real 1970 L78/L89 that was as stock as stock gets, right down to a factory GM replacement cam, and exhaust manifolds.

    With a MSD distributor that was re-curved after a few timing loops were done, and the carb being re-jetted, it made 404hp. I remember that. I really wouldn’t be surprised to see a good set of headers be worth 40hp over manifolds, as I have seen a LS6 454 go from 460hp to over 500hp on the dyno.

  5. Brendan M

    How many inches of vacuum is that motor pulling at idle?
    That would seem to be a giveaway if they truly are using a “stock cam”.

  6. DanStokes

    Another possibility:

    Not sure about GM (even though I worked at Chevy in that era doing emissions calibrations) but I know Chrysler detuned the stock distributor curve so that they could offer a warranty on the Hemi (that was 68-70 that I know of for sure). Pull the distributor and spin it up on a distributor machine and power and max RPM went WAY up (I won’t quote numbers as it was a long time ago and I don’t recall for sure). While I was at GMPG I worked part time for our Chrysler-Plymouth dealer in town and he had been an engineer for Chrysler at the Chelsea Proving Grounds. The first tool he bought was a used Sun distributor machine and he’d run the distributors for any of his customers who raced. They usually ran WAY harder than the competition.


  7. nxpress62

    In 1965 the L78 was rated 425 horse, in ’66 they dropped it to 375 (camaro and chevelle exhaust manifolds being the only change..) and the 427 became top dog at 425 horse.. Would like to see a L72 with big car exhaust manifolds on the dyno, probably be closer to 450 horsepower.

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