Originally Posted by highcompression
ok so after reading all the threads about how people refuse to put 305 heads onto a 350 i decided to do it myself and see what all the fuss is about....so i took a set of 144.....416 305 heads and put the pair onto a 1979 camaro 350 engine, stock pistons...new bottom end build with all new bearings and such.edelbrock 2102 cam. with an edelbrock airgap intake, longtube hedders on em with a 600 cfm edelbrock 4barrel carb w. manual choke. 4 speed 4x4 standard into an 86 k-1500 pickup with true dual super 40s at 2.5" i.d with crossover pipe....and let me tell all you non believers when i fired this mother up it sounded like something from hell came up!!! the compression is at 190 psi per cylinder now instead of 150. and i have virtually lost no top end to this setup. it idles nicely at 600 rpm and the valve clear. is nicely done with the stock pistons, no knocking or pinging what so ever the head gaskets measured at 42 thousandths thick. the low end torque now....well it picked up quite a damn bit and now i have a harder time moving without roasting the damn tires lol...but needless to say it tops at 6800 which i lost about 150 rpm...big deal really lol...and let me tell everyone ....it runs off 87 octane gas with ZERO hesitation...and the carb is setup for higher fuel economy....i tried for power and it was pointless to burn even more rubber so i set it back. now if anyone thinks that a build with 305s on a 350 is not sensible id be thinking differently....i mean when people read they want to see how people actually got setups like this working right...not all the people who talk about it and have never actually done it....i will attempt to post pics if i can on here through out the week.
Old school, we did this back in the mid to late 1970s thru the 80s to replace SMOG heads when we didn't have the money for GMPP and aftermarket stuff was way more expensive against income than it is today. When ported and larger valves are installed these heads will do an adequate job of feeding a 350 up about 6000 RPM. If left unported even with larger valves they have a real big bark because the smaller ports create a high speed mixture flow at low RPMs. The engine likes this as it gets the cam working sooner in the rev range. But getting up over 6000 they will reach their peak flow sooner than a larger port/valve combination. So is your 6800 RPM arrived at by blipping the throttle in the garage or is that top RPM in high gear?
The small combustion chamber runs the compression ratio way up. The spark plug is off to one side resulting in a longer burn time. This is compensated for with more ignition advance. Relative to fast burn heads these will tend to burn more fuel for the power developed. For fuel quantity restricted racing classes, this can be a problem. Otherwise its just an expense.
I have a hard time with 190 psi cranking and 87 octane, unless you've discovered something tens of thousands of hot rodders and engineers haven't. But weather, altitude, humidity, and topography are big players in octane requirement against cranking pressure. A manual transmission and stiff overall gearing will also let you get away with more compression ratio than will an automatic, especially when teamed with high ratio overall gearing. That's a matter of loading on the crankshaft versus throttle position. One of the worst performance corners on the envelop is low RPM coupled with high loading and a lot of throttle opening. This is what is meant by "lugging" the engine. With a stick gear box and low gearing this situation is easy to avoid. With an automatic and high gears, not so easy. So a lot of the design work we do is around the needs of automatics. Newer automatics with more gears and lock up converters let you get away with engine designs more like what you can do with a stick but an old TH350 and 2.73 rear gears quickly demands a low compression engine or a loose converter.