BBC heads: open vs closed chamber

General engine tech -- Drag Racing to Circle Track

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chevy454
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Post by chevy454 »

VMC wrote:
chevy454 wrote:...But, interesting about your timing...our car runs best at 34 degrees total...seems like every season I have 1 guy that tells me my timing is wrong, and he has me bump it up to 36 or 37...and then they watch the car drop 3mph until I pull the timing back to 34 and the mph instantly comes back. Because of the skinny tires, we try to cheat some of the power further down track, so the big MPH helps us...
What happens to the 330' and 660' numbers when you go up to 36°? (I'm guessing the short times are harder to manage @ 36° due to traction issues?)

Do you have 291's or 840's on the engine now?
It's like a totally different car, a slower car...I've probably got some slips from some timing testing somewhere...I throw all the slips in a folder when I get home, but we're probably nearing 500 passes on this car since we finished the resto in '96...

BTW: for future reference, here's a picture of our Y-Camaro, and a couple of my recent slips (so you know what the car is doing down the track)...

Image

ImageImage
'69 Yenko Camaro - 11.75 @ 118, Pure Stock
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840 vs 291

Post by StockerChevy »

Does their seem to be a difference between the 2 heads 291 vs 840 other than the size of the exhuast valve? Performance wise, which is prefered for a stock drag racing ?

Jim
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Re: 840 vs 291

Post by Alan Roehrich »

StockerChevy wrote:Does their seem to be a difference between the 2 heads 291 vs 840 other than the size of the exhuast valve? Performance wise, which is prefered for a stock drag racing ?

Jim
It varies from casting to casting. I know of guys who buy as many sets as possible, take them to their cylinder head man, pick a couple of the best sets, and sell the rest. Those are the fast guys. Some fast cars run 840's, some 391's, some 858's, and some 291's. The 291 is the most common, cast between 1970 and 2000. Most of the others were already discontinued by the time 291's were cast and used by Chevrolet.
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Post by Abbottracingheads »

I have done big block heads for thirty plus years. Give me the open chamber every time. The late model small chamber heads you elude to are much different than the old closed chamber heads. For any given amount of work the open chamber heads will make more power.
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open chamber heads

Post by raceman408 »

abbottracing heads, how about the 402 open chamber heads, with the open part around the ex valve, are they just as good as the rest of the open chamber heads.
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Post by bruce69camaro »

Not to step on this thread, but what are the opinions of the 3964290, oval port, closed chamber heads? I'm running them on my car.

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Bruce
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Post by StockerChevy »

bruce69camaro wrote:Not to step on this thread, but what are the opinions of the 3964290, oval port, closed chamber heads? I'm running them on my car.
3964290 are the 396/350 and the 454/390 LS5 heads. I know guys that like to carve really like the open chamber 049 oval port but could the experts chime in?

Jim
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Post by Alan Roehrich »

bruce69camaro wrote:Not to step on this thread, but what are the opinions of the 3964290, oval port, closed chamber heads? I'm running them on my car.

Thanks
Bruce
I think they do pretty well myself. I like a closed chamber oval port head with larger valves on a 454 with flat top or small dome pistons. The closed chamber head there seems to be less prone to detonation, especially when run with about 0.042" total deck clearance.

Those heads flow about the same as the "049" and "781" open chamber castings, and respond to work as well. Do a search here for those two casting numbers and you'll see how well the open chamber works. I think the added compression and reduced detonation from the closed chamber makes it even better.

It's hard to get good compression for the street with open chamber heads without a good sized dome, and I think the open chamber tends to aggravate the tendency to detonate you get with a large bore. I think having to put a big dome in the open chamber head makes that worse.

Since Chevrolet went to a small chamber on the Vortec 454 head, I guess they felt the same way (although the Vortec 454 head isn't really a closed chamber head). I could be way off base, I suppose, but what I'm saying has been my experience over the years.

I will say that I'm not at all interested in getting rid of any of my collection of closed chamber oval port heads (3-4 pair, and always looking to pick up another pair). If I were building a 454 or 496 for a street car, or for a heavy duty truck (dooley to tow a trailer), I'd be leaning strongly towards oval port closed chamber heads with 2.19 intake valves, and maybe even bigger exhaust valves, up to 1.84 or 1.88. Now, if I didn't have the old iron heads, I'd look at the Edelbrocks, the only reason I don't look at the Edelbrocks, is I have the castings, and I can put valve seats and big valves in myself. If that were not so, I'd look seriously at a set of Edelbrocks.

To answer another question, I do not like the "closed chamber" with the big relief behind the exhaust valve. I prefer the true closed chamber.
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Post by bruce69camaro »

The heads are on my camaro that I'm using just for the strip. I've been in debut with my engine builder about what to use. I've had these heads and a lot of people tell me to keep them and just have the builder rework them, but he would like to see me go with an aluminum set, which I would like to, but I'd like to get my money out of these heads first.

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Post by Alan Roehrich »

bruce69camaro wrote:The heads are on my camaro that I'm using just for the strip. I've been in debut with my engine builder about what to use. I've had these heads and a lot of people tell me to keep them and just have the builder rework them, but he would like to see me go with an aluminum set, which I would like to, but I'd like to get my money out of these heads first.

Thanks
Bruce
Bruce,
If you have a good set of true closed chamber oval port heads, and you have access to someone who can install the big valves, as well as properly port and polish them (again, a search here will show the results gotten from "049" and "781" heads), then I'd say using those heads is a good idea. The last set of Edelbrocks we did cost more than $1500 when they were finished. If you spent $1K on iron oval port closed chamber heads, I think you could get a lot out of them. I'd certainly prefer a set of ported and polished closed chamber oval port heads to the off brand Chinese aluminum stuff.
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Post by bruce69camaro »

Like you said, increasing the valve size has always been an issue, but what I like about my builder is, he wants to compare apples to apples.
We agreed that we would run the motor "as is" and then he would install a larger set of valves, do more testing and then he would perform his port magic.
I like the steps that we are taking, but if I keep dumping money into these heads, would it be better to just go out and get a set from Edelbrock? These are just things I'm trying to figure out before too much money gets wrapped up into my current heads.
I appreciate your advice.

Thank you
Bruce
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Post by Alan Roehrich »

bruce69camaro wrote:Like you said, increasing the valve size has always been an issue, but what I like about my builder is, he wants to compare apples to apples.
We agreed that we would run the motor "as is" and then he would install a larger set of valves, do more testing and then he would perform his port magic.
I like the steps that we are taking, but if I keep dumping money into these heads, would it be better to just go out and get a set from Edelbrock? These are just things I'm trying to figure out before too much money gets wrapped up into my current heads.
I appreciate your advice.

Thank you
Bruce
Bruce,
I'm off to work, and I'm not really familiar with what you have now, what you're getting out of it, and what your goals are. Without that information, I'd just be guessing. If you want to post that, or send it in a PM, I'll be glad to take a look, maybe this evening after work.
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Post by ashtrak »

What kind of power do you think a BBC open chamber 026 iron head has in it. Can't seem to find any info on this head?
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Post by StockerChevy »

Does anybody have some normal flow numbers on a closed chambered head 840 or 291? You just don't see this information printed as most have moved on to a more modern cylinder heads.

Jim
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