I have just torn down my L-88 spec 427 because of a noise. I posted details in a different thread. This engine has the 074 open chamber aluminum heads from the late 69 L88. Upon tear down, I found that it has the closed chamber pistons. They are 30 over, so I assume they were put in during the last rebuild to reduce the compression ratio.

Is there a problem with this combination? Will they flow ok or have any other problems? The machine shop suggested that something will need to be ground to make them work right. Does anyone have experience here.

I like the idea of the lower compression, but I am concerned they will impact flow or overall HP. I don't necessarily like the idea of purchasing new pistons.

Any assistance / guidance here is appreciated.

Thought I would bump this looking for assistance.

I am rebuilding my 427 with 074 open chamber heads. It looks like I will be replacing the pistons (as well as the rods) and was hoping for some guidance from the experts here.

I found some affordable Keith Black forged Pistons that have an open chamber configuration with -37cc. Will the open chamber configuration help performance with the open chamber head or does it not matter what piston type I use?

My calculation of the compression ratio with this combo was 10.6:1. Will this be low enough to get good performance on pump gas?

Thanks for your assistance here.

As long as there is no physical contact from the dome to the head, it will work. Some closed chamber pistons will hit in an open chamber head, even though the open chamber is bigger than the closed type.

I am running a 427 with around a .500 dome (closed chamber piston ) and the heads are canfield open chamber (113cc) . I have no clearance issues !
I guess you should just clay your pistons on the corners and see what clearance you come up with. THE ONLY SURE WAY TO FIND OUT.

The clearance issues with open chamber heads and closed chamber TRW 11-1's is on the stock small valve head where the combustion chamber isn't unshrouded. The rectangle ports are unshrouded for the larger valves, which is what the 11-1's are designed for. Should be a L2268F piston and should be a no issue.

Thanks for the info.

Will I get better combustion and ultimately more power from the open chamber piston? I was told that matching the piston type to head type improves flow as well as flame travel. Is this true?

Thanks for the info.

Will I get better combustion and ultimately more power from the open chamber piston? I was told that matching the piston type to head type improves flow as well as flame travel. Is this true?

On a pump gas street engine, you will never see a difference. But if your buying pistons anyway, i would get open chamber pistions assuming you can get the compression ratio you want. JMHO.

That engine had 12.5-1 compression so stock pistons wouldn't work unless you were running racing fuel. If it ran well as is, why change it now.

All things considered, a proper open chamber piston for a given compression ratio will be a shorter "mountain" for the air/fuel mixture to combust around, and will have less chance of shrouding the spark plug. I prefer the shortest dome to achieve the compression I want.

All things considered, a proper open chamber piston for a given compression ratio will be a shorter "mountain" for the air/fuel mixture to combust around, and will have less chance of shrouding the spark plug. I prefer the shortest dome to achieve the compression I want.

That's definitely the theory. But can you measure the difference on a pump gas 427? I would like to see the data if you can.

I have just torn down my L-88 spec 427 because of a noise. I posted details in a different thread. This engine has the 074 open chamber aluminum heads from the late 69 L88. Upon tear down, I found that it has the closed chamber pistons. They are 30 over, so I assume they were put in during the last rebuild to reduce the compression ratio.

Is there a problem with this combination? Will they flow ok or have any other problems? The machine shop suggested that something will need to be ground to make them work right. Does anyone have experience here.

I like the idea of the lower compression, but I am concerned they will impact flow or overall HP. I don't necessarily like the idea of purchasing new pistons.

Any assistance / guidance here is appreciated.

Just a couple "tips" here:

Those heads will work with your combo AND you can use the original 11:1 427 (closed-chamber) slugs in that unit to get you around the 10.25:1 C.R.

I recognize those are the aluminum "square-ports" but they do have smaller intake runners at 290/295 cc's. The real "drawback" is the exhaust ports. G.M. eventually dropped that "round" deal, "raised the floors", and turned them into C/D port deals. These became very "workable" on both the 396's AND the 427's. The smaller intake runners produced some nice torque numbers with the short (3.766") stroke!

They will deliver in the neighborhood of 550 HP on straight 93 (with some VERY mild porting on the 427" platform), we refer to it as "massaging" not really "hogging" them out! The HP numbers I show here depends largely on the cam specs AND the overall combo.

The other area of concern with those castings is the exhaust rocker stud holes, they really do need some longer (bottom-side) studs to avoid any
"breakage" in the event a piston "kisses" a valve down the road!

(Add) ARP still has the studs listed for these castings. If you do use their studs make certain to change the intakes also, as the tops will have different lengths. They keep them even when you get the set of intakes and exhausts! Part numbers are (Int) 235-7207 / (Exh) 235-7206

A word of caution here, make certain whoever does the installation, there will be 2 studs (exh) over the "heat-crossover" area that MUST be shortened to work. These castings WILL be ruined if your not familiar with this procedure!

Thanks, Gary

P.S. Personally, if they were here I would "off" them and use the money as a "stepping stone" to better heads!! Just my own opinion here!

That's definitely the theory. But can you measure the difference on a pump gas 427? I would like to see the data if you can.

Not exactly a pump gas comparison but equally valid, only on a smaller level. We built 2 565's with identical specs other than cylinder heads, which required 2 different custom pistons. One had Dart 355 cnc's and had maybe .500 dome rise to achieve 14:1 compression, the other was AFR 357's and required mountains to achieve 14:1. Once assembled, the Dart engine you could see the top of the dome through the spark plug hole, the AFR engine you could only see the face of the dome. The Dart engine made best power on the dyno at 32 degrees total timing, the AFR engine needed 40 degrees. Any time you can back off timing you are reducing the negative torque created by firing the plug as the piston approaches TDC.

On a street BBC with a dome piston, lowering the dome height should result in less timing required. Would I replace pistons just to change from a closed to an open chamber dome, not likely. Is it worth buying the OC piston if you need to replace them anyways? I think so.

Not exactly a pump gas comparison but equally valid, only on a smaller level. We built 2 565's with identical specs other than cylinder heads, which required 2 different custom pistons. One had Dart 355 cnc's and had maybe .500 dome rise to achieve 14:1 compression, the other was AFR 357's and required mountains to achieve 14:1. Once assembled, the Dart engine you could see the top of the dome through the spark plug hole, the AFR engine you could only see the face of the dome. The Dart engine made best power on the dyno at 32 degrees total timing, the AFR engine needed 40 degrees. Any time you can back off timing you are reducing the negative torque created by firing the plug as the piston approaches TDC.

On a street BBC with a dome piston, lowering the dome height should result in less timing required. Would I replace pistons just to change from a closed to an open chamber dome, not likely. Is it worth buying the OC piston if you need to replace them anyways? I think so.

But how much power did each one make? That is the data I would like to see. To get that you would have to run two different dome configurations with the exact same head/engine combo. Obviously, less dome, smaller chambers tends to require less advance. But less advance doesn't necessarily add up to more power, or a noticable difference in performance. JMHO.