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Old 05-02-2010, 12:50 PM
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Acceptable tolerance for "cc'ing" chambers

Hey you guys,
I've been trying to equalize the chambers on my sbc heads for some time, and about to give up on getting them all exactly the same. I grind, think I took out too much, clean em up, measure again, and find I didn't make but a tenth of a cc difference when I thought it was about .5 or .6 cc's.
I don't want to have one cylinder detonating and the rest just right and have to back off timing to just to keep one cylinder from knocking. I know there is going to be a little different compression between cylinders because not all of the rods are exactly the same length, pistons not exactly the same height and volume, carbon build-up, or even a slight stroke variation.
But I'm also concerned that I may slip and damage or ruin a head.
What do you think would be close enough for an aluminum head 385 stroker at 10:1 compression?
1 = 72.4cc
2 = 71.4cc
3 = 71.4cc
4 = 72.2cc
5 = 72.2cc
6 = 71.8cc
7 = 71.8cc
8 = 72.0cc
Thanks for any and all replies!
sorry if I'm just too anal.
ssmonty

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Old 05-02-2010, 01:14 PM
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You are closer than most race shops will even worry about right now except for maybe the two at 71.4cc. Open those to match the 71.8-72cc chambers and don't worry about the one at 72.4cc.

1cc is a 1 centimeter square cube, about the size of a common sugar cube worth of material. 5+ times that amount could come out of the chamber without hurting anything as long as it isn't in just a 1 cm square spot
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Old 05-02-2010, 02:16 PM
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Well, I'd just finished grinding, polishing, cleaning, and re-measuring #3 that was previously 71.3cc instead of the posted 71.4cc. I only got .1cc after all that.#%*@&^!
As far as material already removed, the heads were advertised at 70cc, I did allot of work to smooth and polish them, no measuring done before hand, had em shaved enough to "clean em up" at the machine shop, but they didn't say how much it took? Then I measured for the 1st time.
I did what seemed like allot of grinding on the smaller trying to get them to 72.2cc(the largest). I'd grind on all that needed it a bit, and re-measure all again. Did it a few times. Still needed more when I ran out of time.
Decided to put some "singh" grooves in. Increased em all by about .2cc.
I don't know how much material I've removed altogether, but I pinched my fingers between the waterjacket and the chamber wall on the exhaust side and it was considerably thinner than the intake side.
I had been grinding on the exhaust side because I'd already ground on the intake side enough to get to the line I scribed with the head gasket at the 3-6 o'clock. I feel I'm running out of places to grind!
Thanks for your reply!!!
ssmonty
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Old 05-03-2010, 09:16 AM
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Quote:
Originally Posted by ssmonty
I feel I'm running out of places to grind!
Put a head on the block (keep the heads on the same side of the engine if you do this) and run a few bolts in to hold it securely, then scribe a line onto the head from underneath, up through the cylinder.

Use this scribe mark as a reference to remove material from the chamber that will unshroud the valve.
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Old 05-03-2010, 02:35 PM
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Even though they were advertised as 70 cc it is rare that they will all be close to dead on, sand casting is somewhat imprecise so there would have been an allowable tolerance, I don't know what the Edelbrock figure is but would expect it to be plus or minus 1.5cc allowable.
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Old 05-03-2010, 06:45 PM
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Thanks for the replies you guys!
Cobalt, I gave allot of thought about doing it the way you described, and got to thinking that I would create a small pocket between the chamber and block that might invite detonation. I'm probably wrong, but I thought that if I opened the chamber up to the edge of the gasket, that at least the flame front would have more of a direct approach to that area. I plan on running what I believe is a high dynamic comp. and figured I need all the help I can get. Most of the chambers are going to have the little pocket anyways, so in retrospect I should have done it your way. Oh well, its too late for #3, but thanks for your input. I value it very much!
Eric, I found that changing the chamber volume by 1.0 cc makes for just over 1/10 of a point of compression using a comp. calculator for my application(from 10.14:1 to 10.25:1). I'm not really happy with that. What the hey! I bought the beret for a reason, did't I ? May as get my moneys worth. I'm going to try to do like you said and get the smaller ones up to at least 71.8cc. After reflecting on it, I kind of feel guilty for asking now. I was a bit frustrated at the time.
Thanks fellas!
ssmonty
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Old 05-03-2010, 08:36 PM
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Quote:
Originally Posted by ssmonty
Cobalt, I gave allot of thought about doing it the way you described, and got to thinking that I would create a small pocket between the chamber and block that might invite detonation.
If this is done w/o regard for the scribed line that shows where the bore is in relation to the chamber, there CAN be problems. The idea is to stay inside the bore- that's why the scribed line is made from the cylinder itself and not by using the head gasket, for instance.

But there ARE cases where the chambers are taken out further. Ever notice some BBC bores that have "scallops"? The bores are relieved at the top deck surface to unshroud big valves. You might even see some HP OEM 350 Pontiac bores done this way- exhaust alone or intakes and exhausts, depending.

In any event- you should be a lot closer to having equalized chambers than some guys get.
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Old 05-04-2010, 06:15 PM
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I do remember seeing pics of big blocks with the scallops in the deck.
I got a couple more Qs. I already took the chamber out to the scribed line on the intake from 3 o'clock to 5 o'clock on the intake side, from the 8:30 o'clock to the 7 o'clock on the exhaust(you can see a new little divit), ground on the sparkplug pocket or whatever you call it, and wondering where to go next? The 1-2 o'clock on the intake, 10-11 o'clock on the exhaust, or the little raised island between the seat inserts?
I also screwed up pretty bad say some by using silicon carbide( I think) polishing wheels. They say that the stuff will embed into the aluminum and you can't clean it all out. That later it will dislodge and ruin the rings seal and whatnot. Any ideas besides a wire brush that I could use to clean it off?
Thanks for any and all replies!
ssmonty
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Old 05-04-2010, 07:35 PM
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Looking at the second pic just the way it appears in the post, remove material at the 1-2 o'clock potition on the intake valve to chamber side wall, and the 7-8-9.10 o'clock area on the exhaust side as long as it doesn't put you outside the gasket line.

Never heard the silicon carbide thing, only thing I think it would cause problems with is if you had to weld on them.

A speed secret to help airflow with big valves is to notch the bore around the intake valve head to bore clearance area just like a BBC is done.
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