Increasing compression, need new heads?
Thread Starter
Burning Brakes
Joined: Oct 2000
Posts: 779
Likes: 1
From: Minneapolis MN
Increasing compression, need new heads?
I'm trying to get my engine pieced together. So far I've got this:
454 .30 over
Stock crank and rods
ARP Rod bolts
Forged pistons (thats all I've decided for now)
Comp Cams 294S Solid lifter
Stock 781 heads ported and 2.19/1.88 valves
Torker II Intake
Carb etc etc
My problem seems to be that I want at least 9.5:1 comp, even 10:1. As far as I know those heads have a 118cc combustion chamber. I'd need to reduce that to 100cc to even get 9.5:1. I've been told that milling heads is a bad idea because it can weaken them, and it causes problems with the intake not matching up. I don't think I could get a piston domed enough without bashing the valves to non-existence.
Other than getting new closed-chamber heads, do I have any options?
454 .30 over
Stock crank and rods
ARP Rod bolts
Forged pistons (thats all I've decided for now)
Comp Cams 294S Solid lifter
Stock 781 heads ported and 2.19/1.88 valves
Torker II Intake
Carb etc etc
My problem seems to be that I want at least 9.5:1 comp, even 10:1. As far as I know those heads have a 118cc combustion chamber. I'd need to reduce that to 100cc to even get 9.5:1. I've been told that milling heads is a bad idea because it can weaken them, and it causes problems with the intake not matching up. I don't think I could get a piston domed enough without bashing the valves to non-existence.
Other than getting new closed-chamber heads, do I have any options?
Thread Starter
Burning Brakes
Joined: Oct 2000
Posts: 779
Likes: 1
From: Minneapolis MN
Re: Increasing compression, need new heads? (1bad69)
This is probably a good point, but I see a couple problems.
1. From what it looks like it's already pretty close to 0. They are DAMN close to the top at TDC.
2. The increase seems to be minimal.
3. Wouldn't this effectively be the same as milling the heads in that it would mess up intake manifold alignment?
Please correct me if I'm wrong, I have SO much to learn.
1. From what it looks like it's already pretty close to 0. They are DAMN close to the top at TDC.
2. The increase seems to be minimal.
3. Wouldn't this effectively be the same as milling the heads in that it would mess up intake manifold alignment?
Please correct me if I'm wrong, I have SO much to learn.
Melting Slicks
Joined: Dec 2000
Posts: 2,041
Likes: 0
From: Wichita, KS
Re: Increasing compression, need new heads? (ddn)
Any GOOD machine shop will be able to calculate the correct cut for the intake manifold to maintain alignment. They will also tell you what it is in case you decide to change manifolds later. Decking the block does the same thing with the same requirement to cut the manifold.
Le Mans Master
Joined: Jul 2001
Posts: 9,622
Likes: 16
From: absecon nj
Re: Increasing compression, need new heads? (1979toy)
if they cut the heads tell them to do the intake at the same time and then you dont have an alignment problem.its no big deal machining the heads to raise compression also are calculating it with a felpro gasket or a stock gasket at 15 to 18 thousandths or 39-41 thousandths?
Le Mans Master
Joined: Oct 2000
Posts: 7,831
Likes: 5
From: Austin TX
Re: Increasing compression, need new heads? (ddn)
You should have no problem getting domed pistons that work in
your engine. Pistons are designed for specific applications and the
intake and exhaust valve relief will be cut correctly. Remember late
L-88s and ZL-1s run about 12 to 1 with open chamber heads
from the factory.
your engine. Pistons are designed for specific applications and the
intake and exhaust valve relief will be cut correctly. Remember late
L-88s and ZL-1s run about 12 to 1 with open chamber heads
from the factory.
Le Mans Master
Joined: Mar 2001
Posts: 6,238
Likes: 1,010
From: Watkinsville, GA and Glen Cove, NY
Re: Increasing compression, need new heads? (ddn)
DDN I don't think you have a problem because the 502 with iron heads has 8.75 to 1 compression with 118 cc chambers and the ZZ502 has 9.6 to 1 compression with 110 cc chambers so to mill off 8 cc's that would only be .040 of an inch which is entirely doable and they will know how much to mill off the intake to keep the ports lined up.
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Thread Starter
Burning Brakes
Joined: Oct 2000
Posts: 779
Likes: 1
From: Minneapolis MN
Re: Increasing compression, need new heads? (69ttop502)
Maybe I was a little too nervous before. Looks like milling it, if necessary, wouldn't be a problem.
Speed Pro lists a piston (the TRW-2465F30) that has a .270 dome, .025 deck clearance, lists 9.78 with a 116.9cc chamber.
I think the chambers on the 781's might actually be 113 too, I saw it on a site that listed BBC heads, so I'm not 100% sure on the stock volume.
Maybe these pistons would solve my problem?
Speed Pro lists a piston (the TRW-2465F30) that has a .270 dome, .025 deck clearance, lists 9.78 with a 116.9cc chamber.
I think the chambers on the 781's might actually be 113 too, I saw it on a site that listed BBC heads, so I'm not 100% sure on the stock volume.
Maybe these pistons would solve my problem?
Le Mans Master
Joined: Oct 2000
Posts: 7,831
Likes: 5
From: Austin TX
Re: Increasing compression, need new heads? (ddn)
Might be worth checking out your deck height and the actual
chamber volume. One clue that your block has been decked is
if the numbers on the pad aren't as deep as normal.
chamber volume. One clue that your block has been decked is
if the numbers on the pad aren't as deep as normal.
