small block heads

is there much of a difference between 305 heads and 350 heads in terms of port size ?

will putting 305 heads on a 350 boost compression ratio for a slight power increase / better combustion?

They will raise your compression ratio, but the valves are smaller and the ports are not as efficient on a 350.

im askin cause i have a good set of 305 heads with new valves and i wanted to use these heads on a 350 i have, would there be any power increase from the boosted compression ratio?

im askin cause i have a good set of 305 heads with new valves and i wanted to use these heads on a 350 i have, would there be any power increase from the boosted compression ratio?

No. You will boost the compression up to the point where you will need to run premium gas, and the small valve size and inefficient port design will negate whatever the compression gives.

so i cant use the valves or heads of my 305 on a 350 without haveing to use premium gas?

wat would happen if regular gas was used?

So, let's ask the questoin: Which 305 heads? Which 350 heads? Please give the last three digits of the casting numbers. There are 305 heads that "help" and there are 350 heads that "hurt". Before making generalizations, we must have more accurate information.

Jim

yes we need to know casting numbers here

we cant give an accurate answer without knowing what castings we are talking here

even for the 350 ones

as there are so many differetn castings GM made that you cannot say one displacement head to another

even some castings shared between 305 and 350 commonly over the years


some 305 heads were actualyl alot nicer than others

such as TPI and 4416 and so on

so you cant generally say that all 305 heads are worse than all 350 heads

thats impossible and untrue

some flow better than others, even smaller displcement ones vs. larger displacement ones


4416 heads for an 86 and older 350 would be excellent if you take the time to put in 1.94 or better valves, and port and polish them and etc

i have done this in the past the most recent on a 72 chevy van 4 bolt 3970010 350 engine bored .030 with old school TRW flat tops, stock 70s style manifolds, stock aluminum 1980-88 carb Q jet intake and stock Q jet carb, stock 3/4 length 2.75" dual pipes

ran VERY strong and pushed a 3850lb '68 chevy long bed truck around like it was a little pinto,
with a stock 350 trans and stock converter and the original 12 bolt 3.73 peg leg

the front end would come up ALOT to about the top of the travel of the coil springs

so thats VERy damn good for a heavy vehicle like this and being mostly stock

high RPM was nothing good but off the line and lower RPM was AWESOME, and it never got outrun off the line by anything i was against

camshaft was unknown but it didnt have a noticeable lope or anything and it ran fine with the stock converter so it was probably a near stock 350 specs replacement most likely

good luck

i cant tell u the 350 head castings but the 305 heads are from a 77 monte carlo if that helps, but i cant find a cating number? so do u need these numbers to tell whether or not i can use pump gas?

i cant tell u the 350 head castings but the 305 heads are from a 77 monte carlo if that helps, but i cant find a cating number? so do u need these numbers to tell whether or not i can use pump gas?

Casting # will be a 6, 7 or 8 digit number located under the rocker arm cover, usually between valves or above them. Most common casting numbers are 7-digit. The '77 should, IIRC, have 436 heads on them (the last 3 digits are commonly used in place of the full number, i.e. 492 in place of 3991492).

ok thanx . . . the numbers on the 305 heads are 354434. the manual with the car says the 305 stock has 8.5:1 compression ratio .so, are these heads any good? can i use them on a 350 without having to use premium gas?

The 434 is a large-chamber, small valve head. Yes, it will "fit" the 350. No, it won't help make much power. Chamber volume is right around the 68 CC mark, which IS a bit smaller than the same era 350 head. But it has poor runners and not the most efficient chamber shape. Generally, the "mid" 305 heads ("HO" engines like what's found in early '80s F-bodies) are the better flowing ones. 601 is a casting number that comes to mind. But it will make the 350 require a mix of 93 and "race" gas. The 434 will require 93.

Your best bet, if you REALLY want to change heads, would be to find some 993, 997, 487 or 441 350 heads. Those are good flowing, but still have a large enough chamber to keep fuel requirements reasonable.

While compression DOES "make" power, it brings with it a whole new set of problems. If not properly dealt with, these problems WILL cause engine damage in the form of detonation. We've learned, flow "trumps" compression. That is, a better flowing head with a lower static compression ratio, will make more power than a higher compression head with less efficient ports.

The chamber volume of a given head cannot be guessed. Published data regarding chamber volume is "nominal". You MUST actually measure the volume of the chamber to know with certainty. Chevys are among the most varying chambers around.

Jim