Can anyone tell me what the difference is between a closed chamber and an open chamber head?

 

The basic answer is the shape of the combustion chamber, which must be matched up with the appropriate pistons.

Open chamber heads make more horsepower (and are better for emissions I think).

 

I like your cars! The closed chamber looks like a peanut and the open is laid back to the the cylinder edge on the bottom. The open will make more absolute horse power but the closed makes more torque and usually is a better choice for most cars.


[Modified by oger, 12:34 AM 7/29/2002]

 

This debate has raged for years. Open Chamber! No closed! No! closed with oval ports! No Open with Rect ports and on and on it goes. My 15 year experience fooling wth BBCs is that what is "better" is VERY dependent on what you want to do with the car. I think the statements in the reply are a generalization of a very complicated set of variables not the least of which is what the vehicle is suypposed to do. Let me try to share a little info here.

First you have Rectangle versus Oval Port debate. (Asume open chamber ovals versus closed chamber rects and open chamber rects versus closed chamber chamber ovals. In either case the ovals, regardless of the chamber shape will feel more powerful at the lower RPM ranges than the rects because fuel mixture speed in the ovals is faster than in the Rectangles. The Rectangles tend to have lazy flow rates at normal street RPM levels. I have a 454 LS-6 and have considered many times the idea of dumping the heads for open chamber oval ports. My buddy had a low compression, low perf cam 454 with ovals in a station wagon a long time ago. In terms of low end grunt that car always felt stronger to me than my LS-6

Now consider rect port open chamber versus rect port closed chamber or if you want oval port open chamber versus oval poort closed chambers. Here, in eather comparison, you are talking about the combiustion efficiency of the chamber design. Hands down the open chambers are better. Why else would GM have abondoned the closed chamber in the face of emissions regulations??? They could have designed any piston they wanted to arrive at any compression ratio they wanted for the junk gas that arrived in the 1970's. Did they stay with the closed chamber???? Nope they dumped em.

I don't think anybody makes a closed chamber after market head except some Chevy service replacement heads. Wonder why? Closed chamber heads are inherently tougher to keep happy with todays gas. They are in my opinion not a good choice for most engines. They are a good choice if you just want to raise the compression and yes compression helps make more horsepower, sure then bolt em on. You need to be ready for detonation problems with the closed chambers. Detonation problems will occur for two reasons that feed upon eachother. One makes the other worse. The addition of closed chamber heads increased the compression ratio AND decreased combustion efficiency at the same time. Bad mix. Not a good decision for a street engine from my point of view.

I also want to take issue about the statement that the closed chambers make more torque. Test after test I have seen suggests that port and runner size and shape not chamber shape is the most important factor in creating torque with these engines. I dont think I ever saw a test where substitution of a closed chamber rect port for an open chamber rect port or substitution of a closed chamber oval port for an open chamber oval port made more torque. I suspect if a comprehensive test of these two combinations were made there might even be a loss of torque with the closed chambers versus the open chambers due to tuning restrictions imposed by the combustion efficiency and increased compression as mentioned above.

GM even made a "peanut" oval port which was a very very small oval port head. Where did these heads end up? On trucks! What do you want from a truck engine? Torque. The rectangle port heads were for high RPM high perf cars, Standard size oval ports were for med. perf and low perf passenger casrs and the small size oval port heads were for trucks. Again as with the performance heads Chevy dumped the closed chamber heads completely in favor of the open chamber heads even on the trucks where torque was of the essence.

Now if you start comparing say open chamber ovals with closed chamber rectangles or closed chamber rectangles with open chamber ovals or .... well you see where I am going. There are so many combinations and variables like compression ratio. valve size, runner size, vehicle weigh, vehicle use, cam profile and on and on.

Be careful of the generalized rules you can get bitten.


[Modified by TheOman, 8:43 PM 7/28/2002]


[Modified by TheOman, 8:50 PM 7/28/2002]

 

I agree with nothing the Oman says. First and most important a closed chamber head is nowhere near as detonation prone as an open. The open chamber has almost no quench area the biggest help for detonation controll. In the race cars the max amount of advance that an open chamber will tolerate is about 42 degs. I have run as much as 54 deg in a closed chamber motor that was set up correctly. The closed chamber always make more usable torque while for an all out race motor an open chamber is always prefered but who builds all out race motors with factory heads.

 

The primary motivation behind Chevrolet's switch to the "open chamber" configuration was emissions. Large quench area is good for detonation resistance, but it increases the amount of unburned HC that is ejected with the exhaust. Emission regs were tightening rapidly in the late sixties and early seventies, and virtually all of GM's engine design and development work became emissions related in that era. The days of designing for power were behind them and the goal was to get all the old engine designs to meet current and future emissions regulations. That's the reason why the tunnel port heads I saw on Pontiac dynos in the summer of '68 were never put into production.

Beyond this basic reason for the redesign, the debate over torque and power seems to be endless. From what I've read, the open chamber design tends to create less inlet valve shrouding, so they flow better, but won't handle as high a CR for a given fuel octane level. Peak torque is primarily a function of displacement and compression ratio, and a restrictive port or shrouded valve will create more turbulence, which will usually improve low end torque, but limit top end power. For a given displacment and CR, peak power is a function volumetric efficiency at high piston speed, so a less restrictive port/valve is going to make more peak power, even if you have to drop the CR a point or so.

That's what I like about small blocks - the basic design didn't change for nearly thirty years. You have medium port and "big" port heads with two valve sizes for the latter. I'm not counting the later large chamber versions since you can build a small chamber engine with a moderate CR that will live comfortably on unleaded premium. The default choice is big port small chamber heads, so all you have to decide on is valve size, and the choice is simple. You use the 1.94/1.50s if you want the heads to last and the 2.02/1.6s to make two percent more power in return for a good chance of having a head crack, so the only thing left to debate is the cam. Yup, life is a lot simpler with small blocks. ;)

Duke




[Modified by SWCDuke, 10:09 PM 7/28/2002]

 

Ok Closed chamber are just plain better for torque, always have been always will be.

 

Isn't this fun?

Gm was playing with open chamber I believe primarily like Duke said, for emissions. It just so happened that they also found airflow went up a good bit with them because the valve was unshrouded more. Also able to install larger ex. valves with the larger chambers. Think of the 2nd design L-88.

The bigblock combustion chamber is not an exercise in efficiency. It's pretty sloppy. By removing quench area you increase the tendency to detonate, but hey, they were going down to 8.0 or so anyway, so not much chance of that happening. But on race motors you run race gas anyway, so they could fit super tall pistons to the chamber with killer compression and try to build some quench along the edges ala Hemi's. The added breathing along with the same 12.0 or so compression and race gas did wonders for power.

If you look at current thinking with swirl chambers and all the work to maximize efficiency you find that the tendency is to use smaller chambers and dished pistons. Given equal compression, it usually makes more power. Of course there is a lot of chamber shaping going on to do this. Plus there is the emissions part again about having less surface area.

In reality. on the street, it ain't going to matter much. Figure out what compression you can live with and go from there. Almost all aftermarket heads are set up as open chambers, so to get closed you're going to have to go old. I've had great luck with the GM "semi-open" chambers on a 427 with the 10.25 small dome type pistons. You actually end up around 9.8 honest compression.

The other thing I can tell you is to make sure you actually CC whatever heads you get. Most old ones are much larger than the books list. I had a set of pure closed chamber rectangular ports that should have been around 100cc. Mine were at 115cc. It took .070 angle milling and .040 flat milling to get down to 95cc with new valves installed!

Compression makes HP and torque. Get all you can make live.

Also, a sign of an efficient chamber/piston design is less advance needed to make power. One that takes tons of timing is moving pretty slow. You don't want to have to start it too early to get it all burned. If you have to, you are hurting power by having the piston rising against an expanding flame front.

JIm

 

You've got mail...

 

I figured I would get wacked for that post but WTH! I should have known better. This topic has so much fire behind it and will live on forever from what I can see. It is like Ford vs GM or Yankees vs Red Socks or Cletics vs Lakers. My experience has been that statements like "X is good or bad" or "Y is always a better" or "Z is always bad" just don't hold water when subjected to the test of actual operation. Witness your statement that you have had good luck with semi open chambers. That could be extrapolated to show that things get better as you go from fully closed to fully open or that things get worse as you go from fully closed to fully open or that semi open is really the best compromise betwwen open and closed. Truth is whatever works for a particular combo of parts is what is best. But when the truth be known and all the dust clears closed chambers do produce more torque it is just an irrefutable fact ....right?

Want to start another fire? How about using Rectangle port intakes on Oval port heads? There is a Chevelle forum where this gets debated add nausium. Same I suspect with this topic of the chambers. On the manifold head mismatch you will get anwsers like: It cannot work, They will physically fit but the port match is so bad the car won't run, The port mismatch causes more turbulence at the point where the rect port manifold necks down to the oval port in the head and the car runs better, The port mismatch causes gas mixture puddling at the point where the rect port manifold downsizes to the oval port and the car stumbles and misfires and farts, You can do it but there is no dif in performance and on and on. One of my favorites is "Well I don't know about rect ports on oval heads but my buddy had a car with an oval port manifold on rect port heads". I just love that one. Each person is frevent in their answer of course and cites their own set of facts to prove their point.

One thing I am sure of however blanket absolute statements are absolutley always disproven under the searing light of actual empiracle comparison.

 

OMAN.....Like I said ...this is always fun!

I'm with you, blanket statements can almost always be proven wrong just by changing one set of conditions.

You really have to decide what you are going to do. If you will never turn over 5000 rpm on a 427, you better run oval ports. If you want to START having fun at 5500 rpm, run rectangulars. But you better have the rest of the combo ready for it... big cam...big exhaust...big carb/intake and big gear.

Now change one thing...like make it a 540" motor. Now those rectangulars start acting closer to ovals on a 427. If you want to make it run above 5500 rpm you better have BIG stuff in the rest of the areas!

Now change any one of those gearing, exhaust etc and you can mess it up again.

If you are building a motor where max airflow is critical for max high rpm power, (like any motor) the the open chamber type usually wins out. You give up a little burn efficiency for the increased cylinder filling. Unless you are getting into an area where the head is becoming a restriction around the valves it won't be an issue. Mild cams don't have much issue.

Many times custom piston/ open head combos can be put together to create very tight quench areas, but high compression usually comes with it. The Hemi is terrible in that area, but they flow air like crazy. Dick Landy makes reverse dome pistons made to create tighter chamber clearances, more similar to a closed chamber head.

But to your point, if you look at race engine technology, they are often designing the heads to have very small chambers, like 80cc or less on BIG big blocks yet developing it so the valves still flow. The advantage of starting with a clean sheet of paper! We are stuck with original designs that can fit the vast majority of OEM parts too.

I've run back to back tests on a 454 Camaro race car. Well almost back to back..one weekend to the next. But we knew the car well. Actually I know another guy too who did it twic on his race cars. Now these are 9 second cars. The closed heads were consistently .2 slower and 3 mph off the open even though they were very similar otherwise. But that's high rpm race stuff with 13.0+ compression. The large domes were down around 12.0 in the open heads and even with lower compression probably increased flow even more by getting them out of the way some.

I used the semi opens because they were available and cheap. Plus they put compression where I needed it without lots of machine work. For most folks on the street, I'll go back to say that use whatever you need to to get compression right. Since most heads are designed these days as open chambers, you deal with it. Edelbrock has a 100cc set that look like the old semi-opens! They call them the "high torque" ones!

I would love to build a bigblock with about a 80 cc chamber, 11.0 compression and dished pistons that could flow around 425 cfm! Wow!!

PS- I've done the rectangular intake on ovals but I always opened the head to match. Sure it will run the other way, but I just can't see doing it without doing the head too.
JIm

 

Interesting topic. I certainly was brought up feeling that closed chamber heads are the way to go. Sure you could make other configurations that would make closed chamber not as desirable, but lets face it for most applications closed is the best choice. Call me a throw back from the old camelback days. :yesnod:

My vote is get yourself a set of closed chamber and you have a good start.
Curt

 

Okay smart guys.... :jester ......I have a nice set of rectangular-port closed-chamber cast iron heads with lots of porting and valve work. Any idea how to convert them to aluminum? :eek: :D :lol:

 

Thanks guys. I hired a tutor to take me through all of the posts but I think I've got it!

 

"The closed heads were consistently .2 slower and 3 mph off the open even though they were very similar otherwise."

Well if torque gets the car off the line and moving and horsepower results in top end speed then I guess the old closed chamber heads fall on their nose at the compression ratios that you were working in that Camaro.

I thought I just this week learned that closed chambers were the best choice for most engines? How can that apparent discrepancy between what I learned and you empiracle results be explained????? Hmmmmmm.

 

Oman... I think we're agreeing.....

I believe closed chambers are the best IF they do not impart a restriction to flow. Whatever HP you are going to make takes "X" amount of airflow. IF the airflow you need can can be met by a set of closed chamber heads then that's all good. Look at my dream engine description....

But I believe the impact of extra airflow is bigger than the faster burning chamber. At high RPM and HP levels you gotta get lots of air in in little time. Open chambers do that well.

It's kind of the same argument about bore size. We all know that a small bore is much more efficient. It concentrates the power very well and burns much cleaner which means it uses it's fuel better to make more power. Look at the LS1, and most modern engines. Just like SWC said, emissions play a big part. The LS1 is a relatively small bore engine with fantastic heads that can feed lots of air. So you get the advantages of great airflow and tight cylinder burn characteristics.

BUT.... we also know that a huge bore unshrouds the valves more and lets in more airflow. That makes more power.

Once you start trying to move the LS1 out to the big HP numbers, the heads can't flow enough and the bore becomes a huge shroud.

Typically if you dyno a 496 with a 4.310 bore and a 4.250 stroke against an otherwise identical 502 with a 4.47 bore and a 4.0 stroke, you will find the 502 always makes more power. Like around 30 or so, much more than the 6 cubes would allow. Torque will be very similar down low, but the 496 will probably make it a little lower in the rpm band.

Think of the 502 as an open chamber and the 496 as a closed chamber. The better breathing at the top is a good tradeoff. If you need the power very low in the band and aren't concerned with the top end then the 496 would be better. Might even be just as fast with correct gearing. Of course a 496 with a huge bore to get airflow in is called a ...... 540!!!! Best of both worlds!

It all still depends on what you need. And there is ALWAYS several ways to get to the same result. High rpm small block vs low rpm big block. Turbos or nitrous. Some will swear rod length is the cure all, yet Reher Morrison have always said they could care less as long as it stayed together. They found a way to get the same result with a different combo.

Remember the mag articles a few months back where they built similar engines from everyone, Ford, Chevy Mopar etc. They all made very similar power with many large variations in basic engine design. They all worked, even though the Mopar 440 won! You just work with what you have an tailor the rest.


Jim

 

We ARE agreeing. I just got a hair across my ____ today. I was not trying to take a shot at you. Got a little flustered at binary yes no answers that were sent off earlier in the thread before you came aboard. I was using a little sarcasm that apparently did not come thru the way I wanted it to. Must be a little off my game.

No arguement with anything you said. Then if you get my drift. there are those who don't agree with anything I said. There go those absolutes again...gotta watch them gotta watch them.

Based on the times you rack up with your car it is obvious you know what you are talking about. As they say in the consulting business "Them that can..... do! Them that can't offer opinions" There are indeed so many ways to get to the same end point.

Personally I am not a "Brother of the Open Chamber" cultist. I really could care less if it is open chamer closed chamber or semi open chamber. I don't stay awake at night thinking how much torque I lost with my open chamber heads eather.

No offense meant to you. I hope none was taken.

Oman


[Modified by TheOman, 7:56 PM 7/31/2002]

 

No offense taken...I love a good healthy discussion!

And just like you, sometimes I write this stuff and even I read it later and go "Huh"? That didn't come out my fingers the way intended!

You can join the club now.... there are a few of us folks on here that get fluffed when someone presents their argument as THE ONLY way to to do it. If that was the case, you wouldn't see so many races....there are always DIFFERENT WAYS TO DO THINGS. You just gotta figure out how your piece works an dwhat you need to do to compliment it with the right parts.

Cubic dollars don't always make the fastest deal.

And GM didn't build the ONLY good parts! There have been a few improvements over the years.

I guess the last point is... it all depends on WHERE you want to make torque. My open chamber free flowing head idea is making more TQ at a higher rpm and thus makes more HP. That's good if you NEED or can use it. Otherwise you end up with a pig that gets spanked easily.

Right now I'm trying to cut some low end torque and move it up higher to help traction etc!


See ya,


Jim