Chamber vs compression - Chevelle Tech
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post #1 of 15 (permalink) Old Jan 17th, 20, 3:58 PM Thread Starter
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Chamber vs compression

Looking to put 64cc heads on my 327 SBC. It has flat tops with valve reliefs. Powerpack heads. From doing research it appears it makes around 10.5:1 comp. With the 59-60cc chambers. How much compression will I lose switching to 64cc chambers?
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post #2 of 15 (permalink) Old Jan 17th, 20, 4:53 PM Thread Starter
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Quote:
Originally Posted by JacobDean24 View Post
Looking to put 64cc heads on my 327 SBC. It has flat tops with valve reliefs. Powerpack heads. From doing research it appears it makes around 10.5:1 comp. With the 59-60cc chambers. How much compression will I lose switching to 64cc chambers?
Guys I did some more research on this since I made this post.... and found what seems to be the right info. I tried to delete this post but don't know how to delete it. Haha. Don't want to waste anyone's time.
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post #3 of 15 (permalink) Old Jan 17th, 20, 5:34 PM
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Re: Chamber vs compression

Never a waste of time really.
I did a flat top 327 with 62cc heads but pistons were in the hole a ways and I used a .039" gasket.
I overcammed it 280H comp magnum.. it was a turd until 4500rpm.. but still would not pull hard like a 350" with 10.8 compression and same cam.

Do not over cam the little thing.
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post #4 of 15 (permalink) Old Jan 17th, 20, 10:41 PM
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Re: Chamber vs compression

if you have flattops with a 64cc head you'll be right at 9:1 real compression. This is good. you don't want more CR unless you want trouble with pinging and finicky about gas quality.
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post #5 of 15 (permalink) Old Jan 18th, 20, 5:26 PM
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Re: Chamber vs compression

"but pistons were in the hole a ways and I used a .039" gasket"

Well, that is one way to create detonation and pinging from too much quench distance. GM set these engines up to have between .020 and .025 negative deck (piston down in the bore), and an .020 steel shim head gasket for a reason, it gives proper quench distance. Back in the olkden "I know it all" days of design engineers that did know it all, and some that didn't, some one that didn't thought pinging required a bigger chamber volume, and told everybody. Others of his kind we nt bonkers with idiotic "fixes", stuff like head gaskets that were out of control thicker than the .020. There were ones that went to as large as .080. Then, another one at Silvolite pistons, near here in Carson City, came up with the penultimate moronic modification, raise the pin center on the piston .020, "Destroking" the piston by lowering the piston deck down even further down into the bore.

This worked excellent, it made the 4engine ping far worse than just a thicker gasket. Math: stock deck: .020 to .025 down in the bore, add Destroked height: .020, and the .080 head gasket, result, a completely impossible to save quench of .125 inch, detonation city, NO ping control. All because of two things, lower fuel quality, and a few scatter brained wrong design engineers that had no business in the auto industry.

Now, stock GM flat top pistons also had 4 valve reliefs, adding to the compression volume, so, even lower c/r. All in all, I remember a flat top, 4 valve relief piston, down the stock engineered .020 in the bore, and .020 steel shim head gasket on a 4 inch bore in a 327 gave right at 10.00:1 c/r.

Time, and sensible people revised settings, came up with "zero deck" dome heights, .039 to .045 thick head gaskets, correct quench, and a gasket that was less sensitive to leakage, and NO pinging, win, win, win, win situation. It still works today, ZZ4 crate engine has 10.25:1 or so c/r, .042 head gasket, zero deck, and when the distributor is fixed past what GMPP forced on them, the engine works, doesn't ping, nor over heat, all the way win situation continues.
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post #6 of 15 (permalink) Old Jan 18th, 20, 8:34 PM
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Re: Chamber vs compression

in case the OP doesn't already know this, you cannot possibly know what static comp ratio you'll end up with, without knowing how far down in the hole the piston tops [email protected] Sure, most factory bottom ends came with the pistons about .020" to .025" in the hole, but if you have a car and engine block that's 50 years old, then chances are that they have passed through many hands before you got your hands on them, and you have no way of knowing if a previous owner had the block decks shaved down. IDK if the OP has already measured how far down in the hole the pistons are, but in case he hasn't, I thought I'd make mention of this
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post #7 of 15 (permalink) Old Jan 21st, 20, 3:54 PM
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Re: Chamber vs compression

For a SBC, you can figure 8 cc per point of compression and be pretty close.
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post #8 of 15 (permalink) Old Jan 21st, 20, 3:55 PM
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Re: Chamber vs compression

Quote:
Originally Posted by Tom Mobley View Post
if you have flattops with a 64cc head you'll be right at 9:1 real compression. This is good. you don't want more CR unless you want trouble with pinging and finicky about gas quality.

9.58:1 with .010 deck and a .041 gasket.
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post #9 of 15 (permalink) Old Jan 22nd, 20, 7:24 PM
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Re: Chamber vs compression

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Originally Posted by Tom Mobley View Post
if you have flattops with a 64cc head you'll be right at 9:1 real compression. This is good. you don't want more CR unless you want trouble with pinging and finicky about gas quality.
It really depends on the valve timing of the camshaft that's used. For instance: I had a 355cid chevy engine in an old Vette. I used .015" thick steel shim head gaskets, and had the new AFR heads cut down to a 58cc chamber size, and the pistons were .020" down in the [email protected] The flat top pistons had the four valve reliefs in them adding up to -6cc.

This added up to an 11.0:1 static comp ratio. I installed a Comp Cams Magnum 292H cam, and the engine ran great on 93 octane pump gas alone, (no octane booster, no "water injection" no nothing). And that was with a 185 degree thermostat, initial ignition timing set at 14 deg BTDC, 36 degrees total, all in by 3,000 RPM. That cam which had a 244/244 [email protected] doesn't close the intake valves until they're 30% up the bore on the compression stroke. So with valve timing like that, you're losing the entire bottom third of the compression stroke since the cylinder pressure won't begin to build up until the intake valve is closed.

Not only can you run higher static comp ratios with cams like that, but they actualy need higher compression ratios to extract their full potential. Talk to any tech guy at a camshaft manufacture, and they will confirm that. But another car I had, pinged all over the place with a 10.0:1 static compression ratio since it had a factory stock camshaft which was undoubtely a relatively short duration cam with much different valve timing events.And I had to retard the ignition timing to avoid the pinging. It wouldn't take more than 6 deg advanced intial, and 32 deg total. I also had a 185 deg thermostat on that car too.
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post #10 of 15 (permalink) Old Jan 22nd, 20, 8:40 PM
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Re: Chamber vs compression

I had some 186 heads milled to 54cc ran the 270H in a 350" with flat tops .013 down the hole and .015" gasket.
270H magnum comp cam.. 91 octane 39 total timing 22 initial and 93 octane was 41 total timing for the sweet spot.

I later went with a solid cam then a 280H cam and .039" gasket. Liked the same timing. all in way early 2400 rpm 1600 rpm all in with 2.29 gears. it was still happy
Then pulled the 186 heads off.. they were ported and i installed 041 heads ported the same way same valve size but
the chambers were 64cc. I had to pull initial timing and make it come all in about 2800rpm or it was not happy.

Small chambers less detonation prone.
Look at the new 5.0 when they brought it back.. very high compression ..long rod and very small chamber.
Not the same old short rod large chamber 5.0 at all.
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post #11 of 15 (permalink) Old Jan 22nd, 20, 8:57 PM
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http://www.superchevy.com/how-to/pro...mouse-part-two


Here’s an excellent series, Danger Mouse. Covers and records mods taking 355 up to 550 HP at only 6000 or so. Pump gas street motor.

They actually start with double hump heads 👍
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post #12 of 15 (permalink) Old Jan 22nd, 20, 9:25 PM
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Re: Chamber vs compression

Quote:
Originally Posted by JacobDean24 View Post
Looking to put 64cc heads on my 327 SBC. It has flat tops with valve reliefs. Powerpack heads. From doing research it appears it makes around 10.5:1 comp. With the 59-60cc chambers. How much compression will I lose switching to 64cc chambers?
You will loose about .5 of compression, Your engine will be closer to 10 to 1.


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post #13 of 15 (permalink) Old Jan 23rd, 20, 8:55 PM
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Re: Chamber vs compression

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Originally Posted by Reelysalty View Post
Small-Block Chevy Test Engine - Danger Mouse Part Two - Super Chevy Magazine


Here’s an excellent series, Danger Mouse. Covers and records mods taking 355 up to 550 HP at only 6000 or so. Pump gas street motor.

They actually start with double hump heads 👍
Nice link.

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post #14 of 15 (permalink) Old Jan 24th, 20, 7:24 AM
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Re: Chamber vs compression

Quote:
Originally Posted by JacobDean24 View Post
Guys I did some more research on this since I made this post.... and found what seems to be the right info. I tried to delete this post but don't know how to delete it. Haha. Don't want to waste anyone's time.
This tool will help you a lot.....

https://www.rbracing-rsr.com/compstaticcalc.html

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post #15 of 15 (permalink) Old Jan 24th, 20, 7:42 AM
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Re: Chamber vs compression

hi
acurate and alll the info in 1 place.

Calculate dynamic compression ratio

piston head diameter
how far down till the first ring
deck height [piston down the bore] eg .005--.010 is common
piston valve reliefs 5cc is common
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