technical head thread
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Elder User
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From: Eastern Pa
technical head thread
I've read with great interest the thread on flat top pistons that turned into a head discussion. So Bill, Danlee, Brian, and any others can chime in here. After all that discussion the question then remains what would be the best bang for buck head for a 400. I know we have different applications, but probably most in this forum are running lightly to moderately modified engines, and there is probably a difference even in those two. Let's say we have a moderately modified engine that we run on the street with street gears, or at least a mid 3 range when tires are taken into account, as a part time fun truck. We want to run pump gas and generate more power. Do we go the Aussie quench route, or use the open 2v heads with careful porting? If going 2v do we install the larger valves? With an open design, is the dynamic cr the only thing we have to worry about as far as detonation goes?
Laughing Gas
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From: Fairmont
My opinion is:
The aussie head is the cheapest head that has a quench, AND usuable port size. This eliminates the 4V heads. Unshroud the valves, and leave the size alone. And of course use a "zero" deck height piston. The hypers will be here soon!
The aussie head is the cheapest head that has a quench, AND usuable port size. This eliminates the 4V heads. Unshroud the valves, and leave the size alone. And of course use a "zero" deck height piston. The hypers will be here soon!
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Elder User
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From: Eastern Pa
Ok, so what is the pro and con of a closed or open chamber head? Is it the burn rate and pattern? I noticed Danlee mentioned that an Aussie head needs to be opened up to flow better.
Laughing Gas
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From: Fairmont
As Dan has said, "opening up" or unshroudig the valves help the air flow. As stated before, a quench head help in the effiency of the burn. Somtimes it comes at the cost of poor air flow. You can open up the chamber and still have the quench needed.
Postmaster
Joined: Aug 2001
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From: Delaware
You can get sufficient compression from an open chamber 2V head, and flattop pistons, and with 4V valves installed get great flow.
With closed chamber heads you can run ~0.5 points higher CR, if you have zero deck pistons. You should also increase the valve size and unshroud the valves to improve the flow as Tim mentioned. Without zero deck pistons, the closed chamber heads offer no advantage, and maybe a disadvantage.
With closed chamber heads you can run ~0.5 points higher CR, if you have zero deck pistons. You should also increase the valve size and unshroud the valves to improve the flow as Tim mentioned. Without zero deck pistons, the closed chamber heads offer no advantage, and maybe a disadvantage.
Posting Guru
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Well, piston design and combustion chambers tend to go together. The best bang for the buck is kind of subjective since we're talking about used parts and prices vary. I wouldn't eliminate the 4Vs so easily. There was a very good 383C build in my File section using 4V heads and a stock intake. It made more torque at a lower rpm than the 379C with Edelbrock 2V heads. 408tq vs 383tq @ 3000 rpm.
I think the key to this combo was the moderate duration, high lift hydraulic roller cam. It's amazing what new cam technology can do for some old iron. If it had been 408cid, the torque would have been even more impressive. Most articles I read about the 4V heads should be labled how not to build a Cleveland engine. This one was excellent, it made 523HP @ 6200 with the tunnel ram intake.
I think the key to this combo was the moderate duration, high lift hydraulic roller cam. It's amazing what new cam technology can do for some old iron. If it had been 408cid, the torque would have been even more impressive. Most articles I read about the 4V heads should be labled how not to build a Cleveland engine. This one was excellent, it made 523HP @ 6200 with the tunnel ram intake.
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Elder User
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From: Eastern Pa
So if I were to take either the closed or open chamber 2v heads and do the same valve work and port work, used flat tops with the open and dished pistons with the closed with quench, the only advantage to the closed heads would be the .5 higher cr? Or is the burn rate of the closed head also superior to the open head?
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Posting Guru
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I've never seen a test with a 335 series engine using quench and no quench as the only variables. David Vizard, the writer of this article http://popularhotrodding.com/enginem...power_squeeze/ and other publications, stated that quench is worth 15-20hp over an open chamber. Unfortunately he never qualifies this by saying this is from compression increases, burn rate, or a combination of both.
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Joe Kucinski
Senior User
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From: Wisconsin
the closed chamber/ zero deck height piston setup allows higher CR (while still using pump gas) and prevents detonation. i've read a few posts on this site from guys using flat top pistons with open chamber heads and detonation seems to be a common problem. look into this before you spend alot of $$$. and i'll take that extra 15-20 HP without complaining...
Posting Guru
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From: PacNW
A small chamber head is going to have a faster burn rate than a larger chamber head. It's simple physics. A faster burn rate translates to a reduced chance of detonation simply because there's less time for the A/F mixture to detonate. The downside of the small chamber head is that you have to pay more attention to the valve shrouding issues than you do with a large chamber head.
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From: Eastern Pa
So when looking to unshroud the valves of a closed chamber head is there some kind of guideline to stay within? When I did the 2v open heads I currently am using, I ended up adding about 4ccs to the chamber size thus dropping the compression a little. (I didn't realize at the time that I was removing that much material.) As it is now I do run the flat tops with a static cr of about 8.7. I forget what the dynamic is but I have issues with 87 octane but 89 is fine. If I go for a change to aussies and the appropriate pistons to match I would want to do the head work before I ordered pistons so I could get the cc numbers and order accordingly. Right?
Posting Guru
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From: PacNW
When you check for shrouding, install the valves (without the springs) in the guide and run it through the range of travel. Measure where it is closest to the sides of the combustion chamber and then remove the material necessary to get the proper clearance.