Apeiron

Would a 396 from a '68 Chevelle have oval or rectangular port heads?

87RS402

It should've had oval ports from the factory. Doesn't mean a whole lot now, alot can be changed.

One thing you'll want to make a note of is it should have closed chamber heads. For the street that's what you want. The rectangular ports need to high of an RPM to work effectivly. With the ovals you can make your power in a usable part of the powerband, while running a 10:1 or slightly better CR, and all of this on pump gas. I've even seen 10.5:1 on 92 octane with a 312/322 cam on the street. It was drivable too.

BlackZee

My first car was a '68 SS Chevelle with a 325 hp 396, I wish I still had that car! The 325 hp engine has oval ports-great street heads, the 350 hp had rectangular ports, and the 375 hp had aluminum rectangular ports-very rare. What are your plans for the 396?

Apeiron

It's in a buddy's 69 Nova SS, its an original 396 car but somewhere over the years it inherited the Chevelle engine. He's pretty sure its the 350/396, but that could be wishful thinking. I suppose the best thing to do would be to take it apart and look.

five7kid

iTrader: (14)

Did any factory 396 have rectangular port heads? What you're saying, BlackZee, does not sound like 396's (350 horse had better cam; 375 horse 11.5 compression, solid lifter cam, Holley carb, etc.).

Forget the speculation. I have an old article on Rats hanging in the garage (I'm sure it covers these details). I'll check and repost.

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RB83L69

Seems like they did, five7, but I could be mistaken. I know some of the 427s did, I had one of those hanging out in my garage for years. It was about a 67 model crate motor that somebody had dropped into a boat.

Big blocks are a different beast. There are alot bigger differences among their heads than in small blocks, and those differences make them suitable or not for entirely different applications. The interaction of the chambers with the pistons is alot more complicated, nearly all big blocks have domed pistons, which also add a major variable to a buildup.

I would avoid 68 model heads. They will be oval port closed chamber with no squish area... very prone to detonation. They can be made to work with a very low-compression piston, but it won't be optimum.

IMHO the best junkyard street head is the typical 402 head, 820 casting IIRC. It has oval ports, small open chambers (as opposed to typical 454 heads), with squish area. It's a good street-friendly head. The open chambered heads are all easier to get to run on pump gas.

The rectangular port heads are nearly all closed chamber. Supposedly the ones that came on SS454 motors were open, but I've never seen one with my own eyes to know for sure. They are not at all appropriate for the street.

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AlkyIROC

Good "large" oval port castings are 781 and 049. The most popular rectangular port casting is 990. Very few engines need rectangular ports. Even my race engine only needs oval ports.

Even though some factory 396. 427 or 454 engines came with rectangular port heads they were strictly designed for racing and not street use. Oval port heads are good up to around 6500 rpm or a little higher while rectangular ports are usually used for up to 8000 rpm.

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jms

iTrader: (1)

325 horse and 350/360 horse 396s had cast-iron heads. 1965 through early 1970 used closed chambers. There was even a 265 horse, 2-barrel 396 in 1969!

375-horse 396s, available 1965 (and 1965 'Vette 425-horse 396s)through early 1970 (some '70s were 402s) used cast-iron rectangular port heads with closed chambers through early 1970. Like most of GMs changes over years, you could run across some variances in 1970 and '71. There were also aluminum-headed 375-horse 396s available. Very rare.

The 427s (through 1969-the last year for the car 427) with 335, 390/385, and 400 (3-2 barrel carbs) horse used the same oval port closed chamber heads as the 396s.

The 427s with 425 horse (and more) used the rectangular port heads. Some versions used aluminum heads with a couple of different versions of those.

I've run variations of big blocks with each of the above mentioned heads (including the aluminum versions). Street and race.

For the street, I found best success with the oval port-open chamber head from a 1971-1972 CAR (mostly off 402s but available on 454s). The truck heads will hurt performance. They are NOT the same as car heads.

A little trick to I.D. heads on the car.

You know how to tell an early double-hump small block head?

The I. D. for a big-block is similar. Factory "regular performance" heads use no identifying marks on the casting line that is about an inch below the valve cover on the front (or back) of the head.

The factory "high-performance" rectangular port cast-iron heads use a single "hump" on that casting line.

Factory truck heads have a small rectangle on the casting line.

Factory "high performance" heads do not use a cast-iron intake. The intake runners are not nearly as big as the ports in the "hi-perf" head. MAJOR vacuum leak.

The bolt holes (above the intake runners) on "regular" heads are farther away from the top of the runner than those on the "high-performance" heads, due to the size of the ports.

However, I've seen guys use aluminum rectangular port intakes on oval port heads. Big mistake that actually loses more power than using the old regular cast-iron intake.

Look under the valve cover. If it says "Hi-Perf", it is. However---I have seen some late model truck heads that said "Hi-perf" that were MOST definately NOT. Don't know what GM was doing there.

Also, most "hi-perf" engines used 7/16" push rods and thus corresponding push rod guide plates. Most "regular" heads used 3/8" push rods and guide plates.

There are a few other ways to I.D. the heads but with a big-block, even the early oval port head can make good power on the street.

Since the question didn't ask about 454s I'll not go there.

Sorry for going on so long but big-blocks are the apple of my eye.
jms

87RS402

I had a 68 396 for my first big block... I want to say it was originaly a RPO L36. When I got it the top end was in a fairly molested state though (the shortblock hadn't been touched). I thought those heads were the best ones I've ever had. I wish I still had them. I was running 10.2:1 CR and could get by with just about any 92 octane gas I found. The wierd thing is those heads were ported out to match a rectangluar port manifold that I belive was a factory Z28 piece. It was aluminum and had GM cast into it.

I've got those now, not a bad second choice.

Not to be disrespectful or anything...Are you sure? I think I have a set of 990 ovals in storage and I'm going to be real upset when I dig all the way back to them just for the casting #'s

jms

iTrader: (1)

The small-block intake will not fit the big-block. They do look somewhat similar, though. The factory aluminum rectangular-port intake is a pretty good piece (when used on the correct heads).

Interestingly, the Z-28, 290-horse, 302 used the same size carb (780 cfm, I believe) as the L-78, 375-horse, 396.

The porting of the oval-port heads to match the rectangular-port intake mentioned above was somewhat common. I've taken engines built that way and put on a proper oval-port intake and made them faster.
jms