I am rebuilding my 302 to a 306.With the new TRW pistons and boring supposedly my compression ratio will go up to around 9.4 from about 9.0.My heads have 64cc combustion chambers.I have heard you can mill the head deck to increase compression another .5 or so by in effect reducing the combustion chamber size.I assume that with more compression you get more power but what are the cons?Any cautions or negatives for street use?
More Compression?
- Thread starter Topdowncruise
- Start date
bubba-dough
Active Member
I run 11:1 in a daily driven 347 stroker and not a bit of problem. Like mentioned above by modular plus you might see cooling issues so you would need to upgrade that. You can always use the spray though for some extra ponies.
Michael Yount
Mustang Master
There are lots of variables that impact how much timing you can run and what octane will be required with a particular static compression ratio. Aluminum heads may allow you to run more timing and/or less octane compared to all else equal with iron heads. Also how much overlap the cam has makes a big difference. With more overlap, you bleed off some cylinder pressure at lower rpm allowing it to tolerate more static CR with less chance of detonation. Also, if the engine has a good tight quench dimension (.030"-.035") it will tend to resist detonation more than it otherwise would as CR and timing are increased or fuel octane is decreased.
As you can see it's complex and there's not a simple one size fits all answer. VERY general rule of thumb --- with a relatively mild cam, as you start approaching 10:1 on iron heads or 11:1 on aluminum heads you're getting to the point where you may not be able to manage detonation easily with reasonable timing settings or pump gas.
An aside - a number of European manufacturers are introducing direct injection gasoline engines where the fuel is injected right into the combustion chamber just before firing the cylinder off. Fuel pressures of over 3000 psig result in VERY atomized/vaporized fuel that's mixed well with the air, and the vaporization in the chamber removes heat. The result is static CR's as high as 12:1 on pump gas -- with more hp, more torque, better fuel economy and lower emissions. Just write your $30K-90K check for one of your favorite Audis and you too can take advantage of direct injection and high CR's.
As you can see it's complex and there's not a simple one size fits all answer. VERY general rule of thumb --- with a relatively mild cam, as you start approaching 10:1 on iron heads or 11:1 on aluminum heads you're getting to the point where you may not be able to manage detonation easily with reasonable timing settings or pump gas.
An aside - a number of European manufacturers are introducing direct injection gasoline engines where the fuel is injected right into the combustion chamber just before firing the cylinder off. Fuel pressures of over 3000 psig result in VERY atomized/vaporized fuel that's mixed well with the air, and the vaporization in the chamber removes heat. The result is static CR's as high as 12:1 on pump gas -- with more hp, more torque, better fuel economy and lower emissions. Just write your $30K-90K check for one of your favorite Audis and you too can take advantage of direct injection and high CR's.
Michael Yount
Mustang Master
....also keep in mind when you mill the heads you introduce other variables that may need to be managed. The pushrod length may have to change; valve train geometry may have to be adjusted; the holes on the lower intake may not line up with the holes in the re-positioned heads requiring oval-ing them out; the interface between intake ports in the heads and lower manifold may not line up; piston to valve clearance will be reduced by the amount cut off the heads; you may have to have the bottom of the intake milled as well to get it to fit properly.
As you can see -- it may not be as simple as cutting the head.
As you can see -- it may not be as simple as cutting the head.
Michael,I was following until you mentioned "quench".With my new cam I had close to .200 P/V clearance on both intake and exhaust.But in any case,I will leave well enough alone and be satisfied with the little bump to 9.4.Well actually, I am way past that like pretty much everybody on this forum.
Michael Yount
Mustang Master
Do a google search on Quench -- it's the distance from the top of the piston to the head OUTSIDE of the combustion chamber. It should be as small as it can be without letting the piston smack the head due to rod stretch at high rpm. The smaller that space is the more a/f mixture is forced into the chamber at very high speeds/turbulence as the piston comes over tdc during combustion. Makes for a more efficient burn....which means more power, economy, better emissions, etc.
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