Grant B

I'm always surprised to hear how little timing the LS6 heads need to make good power. Even my stock (-ish, a crappy aftermarket hot-air intake caused 150F IATs and 4 degrees of retard) FRC ran well on only 24 degrees, and from what I understand the LS6 heads often don't pick up anything past 24 degrees.

I haven't heard anything on the AFR 205s, though. What sort of timing do they typically like? AFR's CC design looks similar to the LS6's.

I know it will vary a bit with the CR, quench, cam, etc, but CR is another thing I'm wondering... If these heads are happy with only 24 degrees, why aren't people running more static CR? This is the first time I've ever actually built any naturally aspirated car, but I just can't see a 12.5:1 CR at 24 degrees advance knocking on 93 octane. My last 8.5:1 car would hit 21-23 degrees peak timing at 19 psi of boost and 93 (although it was a 4-valve motor, but it didn't have near the quench of the AFR heads with flattop pistons) . I'm sending my heads off to get the valve tip hardness checked, and I'm wondering if I should have AFR mill the heads a bit more...

Thanks for any assistance.

DaddySS

Excellent topic. We know a good number of AFR heads have gone on with milling to create SCRs of about 11:1 and DCRs of 8.3 to 8.8, it would be great to see some sample timing tables so that we can share information on what has worked.

Black02SS

iTrader: (36)

I had a set milled to 59cc. I was able to make peak power around 26*. My compression was around 11.8 with a DCR of 8.7

DaddySS

I'm working my timing out now - it's the lower RPM and increasing DMA areas that are getting tricky, then I'll tackle peak. Mine is 11.08 to 1, 8.8 dcr.

Grant B

Thats for the responses. Bump for more info?

I'm looking for info like:
"My CR is X, I run Y octane with Z quench, and as I increased timing, my engine knocked before power output leveled off."
Or,
"My CR is X, I run Y octane with Z quench, and as I increased timing, power output leveled off before my engine knocked."

I've never put much faith in DCR meaning anything. At high RPM, inertial and acoustic effects should play a very large role in cylinder pressures. After all, the whole reason why the intake valve is closed ABDC (aside from limitations in ramp rates) is so that the inertia of the air entering the cylinder is used even while the piston starts moving up the bore.

I'm at 0.030 quench with 61cc chambers (11.6:1), and considering milling the heads a bit more. But I need to decide tonight, 'cause the heads should be getting shipped back to me from AFR tomorrow.