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Apologize for the super late response, I advanced the ignition and curved it already, and I have some cheapo summit long tube headers into 2.5in with an X pipe. I havent found much about it but is the 76 L48 cam slightly retarded?
If you guys mean ignition timing I genuinely felt no reasonable gain after changing springs and weights, the biggest gain was the exhaust. Im grabbing lars's paper because I have a feeling im running lean and thats why im not getting the most out of these changes.
Yes opening up the factory "corked-up" exhaust has the biggest effect on these post '75 engines. 40+HP
Ignition timing goes far beyond springs and weights. You need around 12* initial, and 36*total, with another 12* from the vac can. Many posts on this. Biggest effect is 2000 rpm TQ, unless your total was low, then you will feel it up top.
Opening up the exhaust may well make it go lean, especially at cruise. Fix the exhaust and ignition timing curve first. Then do you get "surging" or misfiring stutter at cruise or throttle tip-in? That's lean mis-fire.
i run mine rich and have seen a small increase in power but i don't know if that's just me running it harder, i used the dial by the air filter housing, don't know if there is a bedder way.
Look closely at specs of Both '70 and '71 base (aka RPO L48 aka RPO ZQ3) 350ci motors.
Internally, they Both have Same camshaft; thus Same IVC angle. Both have same size valves & same port volumes.
The sole significant internal difference is static compression ratio.
Older motor has much smaller total comb chamber volume and lots more scr.
'70 has flattop pistons with head chamber about 64cc.
'71 has dished pistons with head chamber about 76cc.
* all easily fact-checked
FWIW: 487 heads can be readily milled to yield 70cc chamber volumes; T-shirt.
Most of the later 76cc castings are lighter, thinner & undesirable.
Last edited by Rebelyell; May 19, 2026 at 12:18 PM.
