I put heavier duty valve springs in my 250 OHC engine and it still bottle necks hard at 5,150rpm. Almost flat stops accelerating by 5,600rpm. When I had 4.56 gears I tried hard to push over 6,000rpm and I don't think I ever hit 6,000rpm despite trying. This was with my home 1bbl ported head, dual exhaust, EFI and tried both the "C" and "E" cam's. My sheet metal intake seemed to add about 500rpm to it, but still didn't clear 6,000rpm. It acted like valve float or airflow restriction. The springs I have in it have about 50psi on the seat over the stock 4bbl dual springs, and it's got 2.5" exhaust, and running a throttle body equivalent of a Holley 4bbl carburetor, sans ventries. So if there is an air flow issue it's in the head/cam.
Few factors could effect this. Crap exhaust scavenging? I have too large of a collector pipe on each outlet. The low compression with poor ring seal limiting the amount of air sucking into the engine? My lash adjusters still pumping up and hanging a valve open despite having a separate oil pressure regulator?
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"How I learned to stop worrying and love the OHC Pontiac L6"
The Silver Buick- '77 Skylark coupe w/455, SPX, MegaSquirt 3 & TKO-600 (Drag Week 2011, 2012 & 2015!)
1969 Firebird with a turbo'd Pontiac L6 controlled by a MegaSquirt 3 and backed with a microsquirt controlled 4L60e and 4.56 gears! (Drag Week 2018!)
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