Big block intake strange question

[Steve Antonucci]

Hi everyone,

I have a question concerning big block engines in general. This may involve
the Corvette as well as other SHP engines using rectangular port intake designs.
It's a little off-base, but please indulge me.

I'm involved in a discussion concerning the use of rectangular port intakes
mated up against oval port heads. According to some, it is not that uncommon
to find this combination. I was cautioned early in my big block endeavors to
be very careful not to buy a so-called SHP engine based on the intake port
design alone ( rect. vs. oval ). I have also been told that the use of
rectangular port intakes with oval port heads oddly runs well. How can this
be?

One would think that the turbulance caused by the sharp transition from a
rectangular port intake ( like the 3933163 ) to the oval ports found on a
cylinder head ( like the 3931063 ) would be disasterous for performance.
I'm by no means an authority on the subject, but a certain amount of common
sense comes into play.

Due to the numerous posts I have read here over the past 5 years, I know there
are some very knowledgeable members and I has hoping to get their input on
this. It's nothing critical. It has always been a topic of passing interest
to me. I'm never amazed at what people will do in an attempt to get more
performance. By the way, have any members here ever seen an L/71 that was
actually an L/68 ( intake swap )?

Lastly, this topic concerns only stock intakes & heads. I have no interest in
aftermarket components.

Steve

[William C.]

Re: Big block intake strange question

Yes, it will work, the port mismatch turbulence issues would be of major concern on a race engine, but for street use the problem is not major, and again for street use, the oval port is so superior to the rectangular port in normal driving and performance under 4000 rpm (low and mid-range torque) that some might say it "runs better". Different than "dynos better" or "Races better"

[Michael H.]

Re: Big block intake strange question

Steve,

The stories about rect port intakes used with oval port heads are true. The combination actually does fit without leaking and it actually does make gobs of low/mid range torque. The power falls off fast at around 5000-5500 but pretty incredible at 2000-5000. It's all about port velocity at lower engine speeds. Everyone knows that the big head ports kill low/mid torque but the mismatch with small head ports and large intake ports kills higher RPM horsepower. Somewhere between is a nice balance that keeps port air velocity high. The fact that even the oval ports are large helps to off set the problem of the mismatch. There are more reasons but way too deep to cover here. Has to do with the wave bounce and speed.

In the 60's, a friend bought a new L88 short block for his 57 Chevy race car. He couldn't afford the new big port Aluminum heads for quite a while so he installed a set of oval port heads on the engine and went racing. Amazingly, the car ran times in the same range as all of the other guys that had the same L88 block assy, but with the proper big port heads. When he finally purchased the new aluminum heads, he was surprised to leard that the car ran times that were almost identical to his original times with the small ports.

In the late 60's, I was able to "make a few adjustments with the GM warranty paperwork" and help a few buddies with new 68-69 Camaros get an L88 short block under warranty to replace their 396 short block. One car was an original 375 HP with the big port heads and another was originally a 325 HP with oval port heads. For reasons I don't remember, the oval port heads were reinstalled on the 325 HP car with a new rect port 375 HP intake and L88 short block. All else was the same from car to car, right down to the header mfg and diff gearing. Both cars ran nearly identical times with the headers open but the car with the weird combo oval port heads was slightly faster on the street.

[Mark #28455]

Other "advantages"

If you get the "correct" date heads, the external casting of the oval vs. rectangular port heads should be undetectable once the intake is bolted in place in the 1965 to 1967 engines. (think of all the money somebody could save in "restoring" their 435 HP car ). By 1969, there was a small casting mark on the ends of the heads (like the SB camel hump), with the oval ports having a 1/4 x 3/4 rectangle, and the rectangle ports having just one point of the "camel hump". But then again, certain circle track shops (like Brzezinski?) were changing casting marks on heads 20 years ago!

Buyer beware!
Mark

[William C.]

Re: Big block intake strange question

Michael, as I recall, the combustion chamber size of the oval port heads is much smaller than the rect port heads, depending on year, for example in '69 the rect port heads were 103 cc and the oval ports 96 cc, so the "combo" would pick up around 7% on compression ratio also.

[Steve Antonucci]

Re: Big block intake strange question

Michael,

Thanks for the fairly indepth explaination. This topic sure is a strange
one and while I'm happy to be getting answers to my questions, I'm also
surprised that this mis-match is not all that uncommon. I sort of figured
at least some of you guys would have dabbled here.

How in the world did you get the L-88 pistons to fit in the oval port
combustion chambers? I'm thinking severe interference issues there.

Steve