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I recently purchased a 1967 GTO with a 428 engine - rebuilt 1000 miles ago. Should I add a lead additive to the gas? Any recommendations on octane? I could use some advice.
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If the engine was rebuilt with hardened valve seats no need for a lead additive. But if not you need lead additive.
 

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GM and the SAE did some pretty extensive research on the matter of unleaded fuel in the non-hardened seat engines back in the mid-70s. Here are the basic conclusions:

If valve seat spring pressures are kept under 120 pounds seated (most stock hi perf Pontiac enigines run 90 to 110 pounds spring pressure on the seat), there is no need for any lead additives, and there is no valve seat erosion, when using unleaded fuel. Valve seat erosion on non-hardened seat engines can occur when using unleaded fuel if towing a heavy trailer at high throttle settings for extended periods (extended time with elevated exhaust gas temperatures - EGT), but there is no adverse erosion under normal driving or short periods of Wide Open Throttle (WOT) application (such as when drag racing). The installation of pressed-in hardened seats poses a higher risk of engine damage than the use of unleaded fuel due to the number of shops performing the installations incorrectly. But unleaded fuel poses no risk of seat erosion under normal high performance driving conditions.

For octane, use the octane needed to avoid/suppress any detonation. Use of a higher octane than that needed to suppress detonation has no advantage. But the engine cannot be allowed to have any detonation, as damage will result. If detonation cannot be suppressed with the highest available octane, you must either lower compression, retard timing, or use an additive that is effective for increasing octane (not many of these around). The additive does not need to be lead-based, since lead is not needed for the seats unless you are doing heavy trailer pulling.

For info, all GM heads manufactured 1971-on have induction-hardened seats in them.

Lars
 
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