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mnm99

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Discussion starter · #1 · (Edited)
Here is the combo before..

350 30over (4 bolt main)
3.48 stroke (Forged crank)
.039 felpro gasket
.025 deck
12.5cc dome piston
60cc head, 1.74 valve 305 iron head
280H comp cam

New combo...
same setup new heads
76cc head 1.94 valve iron head

The compression before was around 12.95:1
The new one says around 10.46:1
(per Ross Pistons )

NOW...my Quench is the same on both, just more cc's .Is this going to give me a problem? If I lower the gasket to .015 I will have a compression of 11.12:1 I didn't want to be that high, but everyone says don't have a quench over 40??? GM motors are around 60's. What do you think?
 
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#2 ·
Shoot for a total quench of .040" with whatever combination of head gasket and deck height you need. A tighter quench will raise your compression ratio a little, but the engine will more resistant to detonation that way.
 
Discussion starter · #4 ·
The new setup is the same as the old EXCEPT for the 76cc 1.94 heads.

If I go with the .015 my quench will be at .040 and a comp of 11.12:1 I

If I go with the .039 gasket my quench will be at .064 and comp of 10.46:1.

If I go with the .025 gasket I would have a total of .050 and comp of 10.8:1.

I want to run on pump gas. What to do..........
 
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#5 ·
tough choice. I came up with the same numbers you did for compression.

I ran dcr, assuming your 280H cam is the same lobe sep (110) and intake centerline (106) as my 270H...

With the .039 gasket, you get a dcr of 8.24. very pump gas friendly, but too much quench at .064.
with .025, dcr is 8.54, quench .050
with .015, dcr is 8.75, quench .040

for reference, my DCR is 8.0:1, and my motor is happy on 89 octane with iron heads.

I'd go for the .025 gasket combo.
(was gonna waffle, but the heck with it.)
 
#6 ·
Is this engine already built and all you are doing is a head swap? If not, have the block zero decked and get the right pistons to give you the compression you want to run with teh gasket and heads you want to run. Use a .040" head gasket and be done with it.
 
#7 ·
Not tough for me --- .025 gasket and be at 10.8:1. That might not be any better than the .015 gasket and 11.1:1 though.

Whatever way you cut it, you are a little past the edge of pump gas territory. You could go to the next size up cam and have no problems though :D Like the 292H
 
Discussion starter · #8 ·
I just looked in the summit book, all the gaskets are 39, 40 ,45, 50. I guess everyone decks to 0? I didn't see one 25 or 15???? The block is already built. I bought it from a friend and don't want to do a build. Theres nothing wrong with it. I just wanted to lower the comp to make it more street friendly. Also with the 305 heads it ran out of breath around 4200rpm.

I found this on another thread.

" These are GM crate engines from the factory. For instance... the ZZ4 has a .025 deck height and a .051 gasket. Thats .076 quench....from the factory. Why does this work so well, if the desired quench is .040?? "

Anyone explain?
 
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#9 ·
The .015" FelPro is summit # FPP-1094... these a 1 to a package, you need 2.
Summit # NAL-10105117 is a GM .028"
MRG-1130 is a Mr Gasket .020"

Did not see an .025"

Also, did you actually check the deck ht?
When I put my 383 together, got a heck of a surprise... my undecked #'s block was at ZERO deck!

Quench is one of those things that gets discussed a lot...
 
Discussion starter · #10 ·
No I did not check it with a gauge, but if you look at the gasket I took off at .039 and I try to bring the piston a high as I can get it to the deck it doesn't look like the gasket thickness. I would say a little more than half of one. The piston is not flush with the deck. I can also still read the casting numbers on it. I would assume It was never decked.

What about the GM motor question?
 
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#11 ·
ain't touching it with a 10 foot pole!
Guys on here I trust say it makes a difference... guys like sean and eric. Eric runs 10's on pump gas... Sean is faster. I built my motor to .040 quench...

and I checked my deck ht. Never assume anything.
 
#12 ·
mnm99 said:
" These are GM crate engines from the factory. For instance... the ZZ4 has a .025 deck height and a .051 gasket. Thats .076 quench....from the factory. Why does this work so well, if the desired quench is .040?? "

Anyone explain?
Click to expand...
Just a guess without knowing the specs........Lower compression ratio.
 
#13 ·
Thanks jim :)

The ZZ motors come with a plastic timing chain cover too, but I don't plan on putting one on my race motor either :D Just remember that GM built their motors to run 87 octane and GM didn't push the limits. That's how they can run a lousy quench height and get away with it.

Actually, GM is better than some other manufacturers . . . look at the quench area on a set of Mopar 340 "X" heads some time :eek:

When you look at motors like my old 383 and realized that I was running a cam with only 286* advertised duration and 11.3:1 compression on pump 93 (sometimes worse) and you realize that the details DO make a difference. (and BTW I ran my quench at .033", same as my new 410" motor)
 
#14 ·
I think we all might have missed a very important detail. The only 76cc heads I know of are all iron heads. You are not going to get away with compression that high (11:1) no matter what quench, with iron heads and pump gas. Are these in fact iron heads? If so you need to shoot for closer to 10:1 compression (IMO) 9.5 - 9.8:1 would be about right. If you have aluminum heads then you can get away with ~11:1 but, the cam, timing etc... has to be on the money (along with your cooling system to keep temps down). Having a little higher (numerically) gear and a loose stall also help get away with it. Long Island is not high elevation, correct? When you are at higher elevations you can or need to bump up the compression sometimes to compensate.

I am with Eric on the quench. I have pushed the envelope a bit with compression and a good quench and it worked fine everytime. Of course the cam, timing, tune-up all come into play.
 
Discussion starter · #15 ·
Ok. Next idea. If I were to change the pistons (arggg) can I just replace them with new rings? The piston has a 030 on the top of the dome and the cylinder walls don't have a scratch on them. If I bought a 30 over flat top would it go right in? Or do I have to send out the block and have it all gone through?
 
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#16 ·
Victor makes a .026 (some on here have referred to it as an .028)
 
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#17 ·
mnm99 said:
Ok. Next idea. If I were to change the pistons (arggg) can I just replace them with new rings? The piston has a 030 on the top of the dome and the cylinder walls don't have a scratch on them. If I bought a 30 over flat top would it go right in? Or do I have to send out the block and have it all gone through?
Click to expand...
That would be your best idea. The new pistons will work but you have to get the hone job right. If you use a molly top ring, you have to use a fine ball type hone. You have to hone the block to a cross pattern. You can make the current set up work, but you need more cam to bleed off cylinder pressure. I run 11:1 on an Iron Eagle 383 and I run 11:1 on an Edelbrock RPM 383. The only thing that scares me with your combo is the open chamber heads and dome pistons due to flame travel. I never tried that setup.
 
#18 · (Edited)
ditto.
It's more work, but not much money. You got a nice engine cheap, with the wrong pistons in it.

Next thing is tho, not sure on the old heads, tell us about them? do i remember 305 heads? You sure don't wanna run 76cc heads with flat tops... maybe 1/8" dome pistons with the 76's? or search out a pair of affordable 64cc heads and run flat tops.

And yes, as long as you can't feel a ridge at the top of the cylinder, you can get away with a dingleball hone job.

Time to start over...hmmm.... interesting, srp makes a 4.030 forged piston with a 1cc dome. with the 76cc heads, with the .015 gasket, static would be 9.7:1, dcr 8.0, that kinda works.
 
#19 ·
His old heads are 60cc 305's. Flat tops with 5cc valve relief will give him 10.9 and with current cam 8.6 He may have to go inverted dome or something or get a bigger cam. Money wise, the inverted dome would be best. Anyone know of a piston that would work with the 60cc head with a little lower compression. Soup bowls are as bad as "loose" quench.
 
#20 ·
Fever,
I agree he is going to be better of with "D" dish pistons and not dog or soup bowls. When I was looking into building a larger cube small block for my blower, the problem I kept running into is getting the compression down. When you start getting the larger bore and are stuck with the chamber size of 72cc (usually the largest offered in aftermarket SBC heads), it becomes a challenge to get the compression down.

In this case depending on the heads he uses a flat top or a dish/inverted dome will be the only option.
 
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