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BBC heads: open vs closed chamber

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BBC heads: open vs closed chamber

Postby chevy454 » Fri Jan 09, 2009 3:33 pm

What are the pros/cons of each design, and is there a preference for drag racing? Specific to big block Chevrolets, is an aluminum, closed chamber 842 head better/worse than an aluminum, open chamber 074?
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Postby srv601 » Fri Jan 09, 2009 6:02 pm

funnty thing is when you look at some of the current race head chambers they look more and more like the old closed chambers of the late 60's. But every thing has told us that those old heads didn't flow like the open chamber ones so we threw the closed chambers in the corner i have a set of the first 840 heads made in that pile myself just went down and looked there is a set of 077 alum heads that were deemed to small of intake port also, some flow bench guy along the wrote it to some mag and then i read it well it's in the pile also

as for the alum heads you are listing here both have big weak spots #1 most of the old castings are soft aluminum . The rocker stand bosses break when you get high spring pressures from race cams and the valve seats fall out.

there are a ton of alum heads for cheap these days,just look on ebay
dont ruin those heads just put them back in storage they are bringing real dollars, so do your homework your heads are worth twice what a new set is going for. just my 2 cents or you can send them to me i will put them in the pile for a rainy day.
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Postby Alan Roehrich » Fri Jan 09, 2009 8:16 pm

Wanna sell those 840 heads? :twisted:
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Postby chevy454 » Fri Jan 09, 2009 9:50 pm

LOL...I currently don't have a need for a set of aluminum heads (yet!), but my class requires factory original parts (I run in Pure Stock)...currently, our Camaro runs the 427/425hp L72 which used the 840 heads (closed chamber)...840 heads, 163 intake, 780 Holley, 6223 crank, etc, etc...no aftermarket stuff.

Anyway, like you I've noticed the trend seems to be towards smaller chambers, albeit they're often times twisted or on a different plane than the older heads...but still a far cry from the 118cc mammoths that the old school open chamber heads featured...but as I mentioned before, I've always heard that the closed chamber heads (840s, 842s, 392s, etc) shrouded the valves too much, while the open chambers (074s, 077s, etc) helped with flow.

While I'm not a head porter, and have basically lived in a Factory Stock world for several years, I don't see the major hang ups guys speak of...either of the old heads (open or closed) could stand improvement on the short side, but I just don't see the smaller chamber being a deal killer? Am I all wet here??
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Postby bigjoe1 » Fri Jan 09, 2009 10:01 pm

When I was a kid ( way back then) I remember that when I changed from closed to open chambered heads, The engine would rev up a lot higher with the open-- The same cam that went to 6800 with the closed would go right up 7500 with the open- This was in around 1970-1972 era.

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Postby srv601 » Fri Jan 09, 2009 10:28 pm

pure stock? I have seen bbc pistons that were supposed to run 5/64 ring pkg, these guys shim the ring land to run .043 rings the list goes on and on.

If you could run alum heads you should at least for the weight savings alone.

hey i found something in a old mag that will give you some insight to your engine question. undercover 396 the head work is on the same 840 head
your working on. the flow numbers are 353@700 lift intake and 234@700 exhaust the chamber ended up shaped like a peanut
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Postby Alan Roehrich » Fri Jan 09, 2009 10:31 pm

chevy454 wrote:LOL...I currently don't have a need for a set of aluminum heads (yet!), but my class requires factory original parts (I run in Pure Stock)...currently, our Camaro runs the 427/425hp L72 which used the 840 heads (closed chamber)...840 heads, 163 intake, 780 Holley, 6223 crank, etc, etc...no aftermarket stuff.

Anyway, like you I've noticed the trend seems to be towards smaller chambers, albeit they're often times twisted or on a different plane than the older heads...but still a far cry from the 118cc mammoths that the old school open chamber heads featured...but as I mentioned before, I've always heard that the closed chamber heads (840s, 842s, 392s, etc) shrouded the valves too much, while the open chambers (074s, 077s, etc) helped with flow.

While I'm not a head porter, and have basically lived in a Factory Stock world for several years, I don't see the major hang ups guys speak of...either of the old heads (open or closed) could stand improvement on the short side, but I just don't see the smaller chamber being a deal killer? Am I all wet here??


We run a 69 Camaro 427/425 Stock Eliminator car, and of course use all the same parts as you. We have a decent engine program and make 580, the big dogs make probably 20HP more, they've been doing it 20 years longer. Closed chamber heads can make some pretty decent HP.
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Postby chevy454 » Sat Jan 10, 2009 4:05 pm

Ahhh, another L72 Camaro guy! I'm gonna guess we're around 100hp *down* on you, though!! Our stock exhaust manifolds & stock GM "blueprint" camshaft kind of hang us up inthe HP department...but our Camaro still manages to trap 117-118mph at a race weight of 3700+ pounds (on Polyglass F70-15 tires).

Aluminum heads were never offered on the 427/425hp engines so we can't legally run them...the reason for my questions, though, basically center around bench racing about which combo would be best in our particular class: the L88 or the L71 (both in a Corvette). I seem to be the lone supporter of the L71 (427/435hp tri-power) Corvette, and the argument always gets back to the open vs. closed chamber heads...
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Postby Alan Roehrich » Sat Jan 10, 2009 4:16 pm

chevy454 wrote:Ahhh, another L72 Camaro guy! I'm gonna guess we're around 100hp *down* on you, though!! Our stock exhaust manifolds & stock GM "blueprint" camshaft kind of hang us up in the HP department...but our Camaro still manages to trap 117-118mph at a race weight of 3700+ pounds (on Polyglass F70-15 tires).

Aluminum heads were never offered on the 427/425hp engines so we can't legally run them...the reason for my questions, though, basically center around bench racing about which combo would be best in our particular class: the L88 or the L71 (both in a Corvette). I seem to be the lone supporter of the L71 (427/435hp tri-power) Corvette, and the argument always gets back to the open vs. closed chamber heads...


Why not the ZL-1? It's pretty much just an all aluminum L-88. The intake on the 68 and later 427/435 stinks, and the carburetors are hard to manage (although I have a friend who does my carburetors that can make them work). The 67 intake is much better.
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Postby Stinger » Sat Jan 10, 2009 4:17 pm

I have a customer that has a 454 open chamber engine wth a flat tappet cam built by a very reputable shop. I did a set of closed chamber heads for him and machined a block for him to build an identcal duplicate of the open chamber engine he has. He ran bolth for quite some time. The closed chamber set up runs so good that he is keeping it and selling the open chamber engine.
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Postby chevy454 » Sat Jan 10, 2009 10:05 pm

Alan Roehrich wrote:
chevy454 wrote:Ahhh, another L72 Camaro guy! I'm gonna guess we're around 100hp *down* on you, though!! Our stock exhaust manifolds & stock GM "blueprint" camshaft kind of hang us up in the HP department...but our Camaro still manages to trap 117-118mph at a race weight of 3700+ pounds (on Polyglass F70-15 tires).

Aluminum heads were never offered on the 427/425hp engines so we can't legally run them...the reason for my questions, though, basically center around bench racing about which combo would be best in our particular class: the L88 or the L71 (both in a Corvette). I seem to be the lone supporter of the L71 (427/435hp tri-power) Corvette, and the argument always gets back to the open vs. closed chamber heads...


Why not the ZL-1? It's pretty much just an all aluminum L-88. The intake on the 68 and later 427/435 stinks, and the carburetors are hard to manage (although I have a friend who does my carburetors that can make them work). The 67 intake is much better.


Well, the Mopar guys complained enough that they outlawed the ZL-1 Corvette before anyone got to build one (one of the top cars currently is an original L72 COPO Camaro with a ZL-1 engine in it), so the L88 is about as extreme as us Chevrolet guys can get. As for the L71 tri-power, part of my intrigue is based on the fact that the NHRA rates it at 495hp vs. a 480hp rating on the L88...and like you said, the taller '67 intake is the way to go and the '67 'vette is lighter than it's later brethren as well...only downside is no automatic until '69, which makes any combo a tough sell since we have to run the skinny Polyglass tires. I've also heard the 3x2 carbs are a pain to work with, but the '69.5 SuperBees on our class are a darn tough combo and they all claim the tri-power setup (virtually a twin to the 'Vette setup) is a breeze to dial in...but you've got a lot more cfm potential in the 3 carbs.

I just don't see a major hindrence with a closed chamber...seems to me what little *might* be given up with a closed chamber could be taken back with the ability to run a smaller dome (thus lighter piston/crank balance)?

But this is all a pipe dream for now, as we've got a couple projects on the list already that we need to get done first...

BTW, Alan: I'm curious, what kind of compression do you run, and what is your total ignition timing?
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Postby Alan Roehrich » Sat Jan 10, 2009 10:22 pm

You need to be looking at the Stock ratings, what you're looking at are the Super Stock ratings. The 435 is rated at 435, the L-88 430 horse is rated at 440, and the ZL-1 430 horse is rated at 445 (we get the bigger service cam in the ZL-1 because they refuse to rate the ZL-1 the same as the L-88).

The ZL-1 is a Camaro engine, only a couple of Corvettes were ever built with one, NHRA does not allow the ZL-1 in the Corvette. But an L-88 Corvette should run the same as the ZL-1 did, the only difference is the block, and the set back of the Corvette should make up for a lot of that weight.

With the super adjustable clutch set ups they have these days, you'd be amazed how much you can work with them. The 67 and earlier Corvette floorpan wouldn't take the TH400, so the high HP engines only got the 4 speed.

Another interesting combination is the oval port 427 3-2 engine with a PowerGlide in the 67. It did pretty well in Stock a few years ago. It has a good intake.

Yes, the right carburetor man can make the 3-2 set up run.

I'd have to go back and calculate the compression, we have a 34cc dome, a 106cc chamber, and a 0.042" head gasket. We run at 0.060" over. We still have to run 0.520" lift too, we just don't have duration rules, and we can run any 1.550" valve spring.

We run as much as 42 degrees, at least on the starting line, depending on conditions. I take out as much as 8 degrees above 6800 in high gear.
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Postby chevy454 » Sun Jan 11, 2009 5:40 pm

Alan Roehrich wrote:With the super adjustable clutch set ups they have these days, you'd be amazed how much you can work with them. The 67 and earlier Corvette floorpan wouldn't take the TH400, so the high HP engines only got the 4 speed.

That wasn't just a Corvette thing, it was a Chevrolet thing...no solid lifter bbc got the 400thm until 1969, except the *50* Gibb COPO L78/400thm Novas in 1968.

Another interesting combination is the oval port 427 3-2 engine with a PowerGlide in the 67. It did pretty well in Stock a few years ago. It has a good intake.

LOL...some buddies & I were just talking about that very combo a few weeks ago, because of the Glide. So, what makes the L71 more of a killer in S/S than the L88? You'd think it'd be the other way around, because of the racier setup of the L88??

I'd have to go back and calculate the compression, we have a 34cc dome, a 106cc chamber, and a 0.042" head gasket. We run at 0.060" over. We still have to run 0.520" lift too, we just don't have duration rules, and we can run any 1.550" valve spring.

We run as much as 42 degrees, at least on the starting line, depending on conditions. I take out as much as 8 degrees above 6800 in high gear.

That all sounds almost identical to our engine, except iirc we're running hte .039 (.038?) Fel Pro gasket...34cc CP pistons, 106 chambers, and the 143 Crane/GM blueprint cam...our valve spring ain't anything special, though.

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...
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Postby Alan Roehrich » Sun Jan 11, 2009 6:12 pm

The FelPro gasket, once compressed, at least on my calipers, measures around 0.044" or thicker, and the gasket bore is bigger than I like as well.

We run VP C12. We're actually running only 34-36 in high gear.

Our cam is likely much bigger than yours at 0.050 and at 0.200".

The 3-2 combo can be a killer in Super Stock because in Super Stock there's no cam limit, and no intake rule, other than it and the carburetors must fit under the hood. Many cars, especially those with multiple carburetors, use sheet metal intakes. So Super Stock negates the intake problem and the lack of lift.
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Postby VMC » Mon Jan 12, 2009 6:09 pm

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?
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