317 or 243 Heads?
Restricted User
Joined: Mar 2014
Posts: 7,192
Likes: 109
From: Ohio
317s give you a much better window of error for tuning. Yes, they'll make less NA power/torque. You can usually make up for the difference in boost no problem.
A damn good tuner could make use of the smaller chambers. Denmah ran the tiny-chambered 862 heads with flat tops in his 5.3 (10.5:1+) and ran 23 PSI on pump gas. The flip-side to that is that he ran like 8 degrees of timing. With 317s he could have probably made similar power thanks to more boost or more timing and been just as safe if not more-so on pump gas.
Most people here won't even notice the extra 25-30 HP from an extra half-point of compression, especially when an efficient turbo setup can make that much power with a single PSI.
A damn good tuner could make use of the smaller chambers. Denmah ran the tiny-chambered 862 heads with flat tops in his 5.3 (10.5:1+) and ran 23 PSI on pump gas. The flip-side to that is that he ran like 8 degrees of timing. With 317s he could have probably made similar power thanks to more boost or more timing and been just as safe if not more-so on pump gas.
Most people here won't even notice the extra 25-30 HP from an extra half-point of compression, especially when an efficient turbo setup can make that much power with a single PSI.
8.75 is a great target IMO. But not exactly easy to reach using factory parts. I wouldn’t’ go lower than 8.75 in most cases.
4% is a typical power gain for one FULL point of compression. GM rates the 10:1 LQ9 at 345 crank hp. So dropping to 9:1 you lose roughly 13.8 crank hp, which will double per bar of boost. Half a point of compression we are talking 6-7hp.
Basically you aren’t giving up a lot of power. You’re just trading NA response for the ability to run additional boost or less octane with a larger tuning window.
