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Discussion Starter · #1 ·
If I purchase a set of #215 heads with 96cc chambers and have larger valves put in them, what happens to the chamber volume? I would like to have around 102 to 105 cc chambers. Can I machine the 96cc bowls and get 104 out of them?
Thanks,
Jim R

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My 70 Chevelle
 
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Larger valves can reduce the volume depending on how for in or out of the seats the machinist places the seating area.

I would guess you can do some grinding to unshroud the valves to open them volume up, but that might be quite a bit of grinding to gain that amount.

If you need to keep your compression ratio in check, it might be wiser to have some of the dome milled down in the pistons rather than try to hog out the head combustion chamber. Most TRW/Speed Pro pistons have plenty of meat on the dome to work with.

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Todd Geisler
79 Malibu
Malibu Muscle
http://www.MalibuMuscle.cjb.net
Naive...I was.
Skeptical...I am becoming.
Determined...I have always been.
"The only way to fail is to stop trying."
 

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That is exactly what I did.
CC'd the heads myself after rebuild.
Gave this number to the guy that did my machine work, and he milled the dome on the
pistons to get the 10.5:1 I wanted.


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Dave
69 Chevelle
454 500+ HP
M21 4spd.

"Fire all your guns at once and
Explode into space."
Steppenwolf "Born to be Wild"
 
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The bowl is in the port and not the combustion chamber, so no bowl blending won't affect cc's or compression ratio.

Do yourself a favor and pull the engine. You are going to need to check piston-to-valve clearance, verify valvetrain geometry, etc., and it will be much easier to do everything on the engine stand. Also, pulling the pistons will only set you back some time and the cost of an oil pan gasket.

Take your time and do it right and you will have a better result when done.

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Todd Geisler
79 Malibu
Malibu Muscle
http://www.MalibuMuscle.cjb.net
Naive...I was.
Skeptical...I am becoming.
Determined...I have always been.
"The only way to fail is to stop trying."
 

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I see where you're coming from Epistuff. You can do alot of un-shrouding of the valves. This means grinding the edges of the combustion chamber to blend them into the chamber itself. This aids flow when the valve is open. You can remove a good bit of chamber volume here. Once you have the valves out, take a new larger-sized valve into the guide and position the head with the chambers facing up. Get your head gasket and line it up and scribe or mark the cylinder opening. Then measure a perfect circle to represent the (your bore size) bore. The gaskets are going to have a larger opening. If you have the notched cylinder tops, add them to your marks on the face of the head. Now you know how much you can grind. Don't exceed the bore size or the notches. Smooth and polish the entire chamber at this point. This will increase the volume of the chamber, up to 7-8cc's. As mentioned above, the relationship of the valve in it's seat will have an impact on total chamber volume. If you're installing new seats, chances are the valves will sit 'higher'. If you re-surface the old seats, they will sit 'deeper' and create more chamber volume. These differences also affect the installed spring hieght. Be sure to do that correctly also. Use the thickest FEL-PRO head gasket you can get. I think 0.48" I think you'll be OK.

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Triple Black 69 SS 396
 

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Discussion Starter · #7 ·
Jimmy,
Well, that leads me to the next problem. My piston deck height is .050". (.050" down the cylinder). With my standard Felpro .038" compressed gasket, that gives my a quench of .088". From what I hear, that is too much already. If I go to a .048" compressed gasket to add volume, my quench will be .098". That would most likely diminish the effect of having any quench and I would have detonation. I pulled my heads yesterday because I was thinking of swapping from rect's to ovals. I wanted to know exactly what I had before as far as true compression ratio before I bought any parts. I didn't expect a .050" deck/piston height.


Jim R.

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My 70 Chevelle
 

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I don't personally know too much about 'quench' and it's effects. But, after asking a seasoned BBC builder, he says that your quench volume won't be as critical as overall compression ratio for a N.A. street motor. If you're going to run N2O or blow it, that's different story. He said do what you can to get your final C.R to 9.0 to 9.8:1 if you can. That's I all I can tell you, as that's all he time to tell me.

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Triple Black 69 SS 396
 
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