Big Block 396 Chevy Engines And The Mystery Engine Confusion Finally Solved

Greg Zyla
Q: Greg I enjoy your column very much and I hope you can answer my question that I’ve been thinking about for 50 years!
I recall that you once owned a 1968 Camaro with a 396 and 375 horsepower big block. I had heard about that engine as early as 1965, but when it came time to order my 1967 Chevelle SS396 in late 1966, I was told that the 325 and 350 horse 396 engines were the only ones available that year. I eventually settled on the 325 horse with a wide ratio 4-speed because I wanted the 3:07 rear gear that did surprisingly well on fuel mileage for me. I also choose a bench seat instead of the bucket seats.
Had the 375 horse engine been available, I might have considered it and am wondering what changes made that additional horsepower? Was it compression ratio? Bigger cam? Can you shed some light on these engines? Also, I put over 150,000 miles on my Chevelle SS396 and had it not been for the spike in gas prices in the early to mid 1970s, I might still have it. Keep up the great work on your always fun and interesting nostalgia car columns. Thanks again, John Amonson, Wallace, Idaho.
A: John thanks for your excellent hand written letter. I’m also incorporating in this answer a letter I received from a reader in Massachusetts who wants to go only as “Big Block Bob’ about the first ever 427 Mystery Motor that debuted in 1963 at Daytona. Those who read my column regularly know I promised I would never write about that engine again (I have not for a few years), but ‘Big Block Bob’ asked me to authenticate that the first ever Mystery Motor was a 409.
OK. Let’s start with John and the first ever 1965 396/375 horse engine, which was made available in very limited numbers and ONLY in the ’65 Chevelle SS396.
This Chevelle SS396/375 was a near duplicate of the 1965 Corvette 396 that produced 425 horsepower. The differences between the engines were the Chevelle’s hydraulic cam, two-bolt main block and milder cam. The Corvette engine had higher compression pistons, higher lift solid lifter cam and four-bolt block. Notable is that in 1965 you could also order the hydraulic lifter 396 in the full size Chevy cars like a two-door Biscayne.
In 1966 the Chevelle SS396 was offered with three different horsepower ratings, including the 325 version, a hydraulic cam 360 horse version and the solid lifter 375 engine, the latter now an exact duplicate of the 425 horse rated version that appeared in the ’65 Corvette. Chevy did not want to rate it as 425 as to not upset the corporate mandates and soon to arrive insurance concerns. The major differences of the 375 solid lifter engines versus a 325, 350 or 360 were rectangular port cylinder heads, bigger valves, high compression pistons, four bolt main blocks and larger Holley carbs. The Camaro 396/375 I owned was lousy on gas but few cars could touch it in a full throttle contest.
When you ordered your 1967 SS396, your sales associate was correct as 1967 was the only year the 396/375 was not available. Both 325 and 350 engines were 10:25 compression with hydraulic cams. However, in 1968 the 396/375 was back, and lasted as the most powerful Chevelle SS engine until the SS454 appeared in 1970. Beginning in 1970, all of the 396 badge engines were actually 402 inches as Chevy decided not to mess with the wildly popular “396” moniker that was now as popular as the 409 that The Beach Boys made famous.
Speaking of the 409 leads us to the Mystery Motor 427 I’ve written about in the past.. “Big Block Bob” said the first Mystery Motor was a 409 and, believe it or not, he is CORRECT and the first reader to uncover this reality. The reason I’ve avoided this 409 reality is because of even more confusion concerning Chevy’s biggest, most secret engine development program ever. If I said it was a 409. Had I done this, I’d be besieged with letters telling me I was wrong as they would think it was just a bored and stroked 409, like the Z11 drag race motor. (See, it gets confusing!)
So, here’s what happened.
Chevrolet developed the all-new, combustion chamber in head canted valve Mark II Mystery Motor back in 1961 through 1962 for a 1963 Daytona 500 debut. The very first Mystery engine was built in a size of 409 cubic inches but was not the 409 we grew to love from 1961 through 1965, sans some minor similarities. The engine designers decided, however, that with Ford already building a 427 and MOPAR prepping its 426 Dodge and Plymouth wedge engines, Chevy changed the Mystery build to a larger bore to arrive at the 427 size.
However, noting that “Big Block Bob” is correct, I’ll throw another trivia note in as Chevy also built a 396 Mystery Motor at the same time as the 409 in response to a potential NASCAR 400 cu. inch engine limit. So, there were three pre-Daytona Mystery Motor race developmental engines built ala 396, 409 and the final 427. Please check this link for an outstanding explanation from Bill Howell, a person who was there when this all happened and the last living engineer who worked on the project:
Thanks John and Bob for your letters. I originally received my Mystery Motor education in person from the late Smokey Yunick, via phone from Rex White, and through several emails with Bill Howell and Mike Clements, son of Louie Clements. Thanks all.

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