Chevrolet “305” Cast Iron Small Block Cylinder Heads

‘601 casting with “UnderCover Porting”

Let’s face it: Stock Chevrolet 305 heads do not perform very well on a race engine.

When we modify 305 castings air flow on a stock head is improved from 165 cfm at .400″ lift to 195-200 to cfm with our “Undercover Porting“.  This amounts to a 35 cfm improvement at .400″ lift.

Exhaust air flow on stock 305 heads is 115 cfm at .400” lift. With our “Undercover Porting” this is increased to 140 to 150 cfm. These air flow improvements have gained over 22 horsepower on the dyno.

We use a variety of 305 heads like the ‘601, ‘450 & ‘416 casting numbers.  Our bare 305 heads start as quality, pressure tested and magnifluxed castings. They are machined for screw-in studs, pushrod holes opened, new valve guides installed and “Undercover Porting” is performed.

Will installing larger valves improve performance of a 305 cylinder head?
Thinking that “bigger is better” many people install 1.94″ intake valves in 305 heads.

Well, the fact is, a 1.94″ intake valve in a 305 head will hurt overall air flow of the head, especially under .400″ lift. Many tracks that require 305 heads have a valve lift rule.

Air flow facts on 305 heads.
Test head: ‘450 casting number Chevy 305 head with “UnderCover Porting”.

Air flow with 1.84″ valve. Ferrea # F5060.
.200   126
.300   173
.400   206
.500   212
.600   213

After the head was flow tested with the 1.84″ valve, we machined the valve seat to accept a 1.900″ valve. If all you look at the high end flow numbers, you would think that the larger valve was a improvement. However, the small improvement at .500″ and .600″ lift was more than offset by a sizeable reduction below that point.

Air Flow with a 1.900″ valve Ferrea #F5072.
.200   124   -3
.300   170   -3
.400   198   -8
.500   214   +2
.600   215   +2