Another head, gasket, and Compression Q
Thread Starter
Safety Car
Joined: Aug 2002
Posts: 4,812
Likes: 1
From: New Albany IN
St. Jude Donor '03-'04-'06-'08-'09
Another head, gasket, and Compression Q
I search for answers guys but couldn't mind my answer so please don't flame me for asking a common question I am sure...
So I have '71 heads...76cc.
I had them milled .30 to reduce the cc's somewhat. I have used the "compression calculators" that are on some of the sites but none seem to account for the milled .30 off the heads. So I guess I need to know apprx how many CC's it actually takes off the total number instead. Any ideas?
Also, I have TRW flattop pistons with 4 valve reliefs. I have heard they are +5 cc due to the reliefs and I have also heard +20cc...which is correct? and how do I enter that into the calculator? as a "+" or "-"?
Lastly, my best estimate is that my deck clearance is .025" and I plan on using a .015" shim gasket. That should be a .040" quench correct?
My goal is a 9.5:1 CR if possible. Is there anything else I need to know or look at for before I put this together?
So I have '71 heads...76cc.
I had them milled .30 to reduce the cc's somewhat. I have used the "compression calculators" that are on some of the sites but none seem to account for the milled .30 off the heads. So I guess I need to know apprx how many CC's it actually takes off the total number instead. Any ideas?
Also, I have TRW flattop pistons with 4 valve reliefs. I have heard they are +5 cc due to the reliefs and I have also heard +20cc...which is correct? and how do I enter that into the calculator? as a "+" or "-"?
Lastly, my best estimate is that my deck clearance is .025" and I plan on using a .015" shim gasket. That should be a .040" quench correct?
My goal is a 9.5:1 CR if possible. Is there anything else I need to know or look at for before I put this together?
Drifting
Joined: Dec 2002
Posts: 1,946
Likes: 28
From: Long Beach CA
My calculator says you will get to 9.6 with heads with 70cc chambers. I don't know what your budget is, but those factory 76cc heads are really low performance. You'd be better off getting a set of aftermarket heads that are 70cc, or slightly smaller if you want a little more compression.
Thread Starter
Safety Car
Joined: Aug 2002
Posts: 4,812
Likes: 1
From: New Albany IN
St. Jude Donor '03-'04-'06-'08-'09
Well yeah, I was able to trade out work to have them machined. Can't afford new heads right now. So I got new valves, valve job, and milled for nearly nothing. If I can get 9.6 that would be great.
Drifting
Joined: Dec 2002
Posts: 1,946
Likes: 28
From: Long Beach CA
The 9.6 is with 70cc heads. I doubt very much they milled enough off of your heads to make the 70cc. You can ask your machinist, but they're probably around 74cc now. That'll put your CR somewhere in the low 9s.
Racer
Joined: Jul 2000
Posts: 281
Likes: 1
From: Traverse City, Mi
St. Jude Donor '12
Best way is to just measure the heads - piece of thicker Plexiglas, some Vaseline to get it to stick to the head and seal, drill a hole and fill the void with a syringe.
Its not gonna pass NASCAR standards, but cheap and easy way to get closer than just guessing...
Its not gonna pass NASCAR standards, but cheap and easy way to get closer than just guessing...
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,113
From: Crossville TN
Since you've already sunk the money into the heads, what difference does it make. You're locked-in to using them, aren't you?? The CR will be between 9-9.5 and you will make maybe another 10-20 hp because you are still using those same heads. Just put it together and enjoy it. Oh, don't forget that by milling the heads the intake manifold fit will be off. You will need to use thinner intake manifold gaskets to keep the fits right.
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
OK, you have access to and know how to use a compression ratio calulator, right? Well, if you enter your measurements, and when it asks for head gasket thickness, you enter a figure .030" less than the actual compressed thickness of the gasket, it should get you pretty darn close, especially with those big open chamber heads, they don't have much of a quench area anyway.
Scott
Scott
Last edited by scottyp99; Apr 30, 2011 at 09:17 PM.
Race Director
Joined: May 2005
Posts: 10,674
Likes: 121
From: Millington Illinois
St. Jude Donor '08-'09
Flat milling .006 per 1 cc on a stock SBC head so you have 71 CC heads or very close to it. L48 dish pistons are 20 CC. Your flat top 4 valve relief pistons are 5 CC. It is entered as a +5 in the calculator. They are asking for piston volume. It seems backward but it isn't. Adding .030 to your gasket thickness don't work for calculation since the chamber is not covering the entire bore. Felpro 1205 intake gasket will work fine with the milled heads and thin head gasket. My heads are milled .030 and have .040 quench and used a .060 thick gasket.
Last edited by 63mako; Apr 30, 2011 at 11:21 PM.
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Le Mans Master
Joined: Sep 2001
Posts: 7,739
Likes: 630
From: Unreconstructed, South Carolina
Different calculators Require Different data entry Format.
Some require a dished piston to be entered as a Positive value ... But others require Negative value for same piston.
following is the Federal-Mogul (fka TRW) SCR Calculator ... it is very accurate ... it requires the dome volume of a dish or a flattops' valve reliefs be entered as a Negative value. I account for typical chamfer + gap between top land & wall by adding Positive about 1.7cc to chamber volume. So, stock bore & stroke. +71cc chamber (+1.7cc total 72.7), -5cc flattop, +3.2cc (1094 shim gasket), 0.025 deck clearance ... I see about 9.3:1. so, 7T1 hit it, regardless.
fyi ... while most flattops have about 5cc Neg dome volume ... not all do ... TRW forged L2417F flat with trough (as in L82) has 3.4cc (Neg 3.4)
http://www.fme-cat.com/Calculator4.aspx
Some require a dished piston to be entered as a Positive value ... But others require Negative value for same piston.
following is the Federal-Mogul (fka TRW) SCR Calculator ... it is very accurate ... it requires the dome volume of a dish or a flattops' valve reliefs be entered as a Negative value. I account for typical chamfer + gap between top land & wall by adding Positive about 1.7cc to chamber volume. So, stock bore & stroke. +71cc chamber (+1.7cc total 72.7), -5cc flattop, +3.2cc (1094 shim gasket), 0.025 deck clearance ... I see about 9.3:1. so, 7T1 hit it, regardless.
fyi ... while most flattops have about 5cc Neg dome volume ... not all do ... TRW forged L2417F flat with trough (as in L82) has 3.4cc (Neg 3.4)
http://www.fme-cat.com/Calculator4.aspx
Last edited by jackson; May 3, 2011 at 09:25 AM.