Will installing larger valves improve the performance of the cylinder head?

Answer: It depends on the type of cylinder head. Generally, if the cylinder head was designed for street applications larger valves will not improve air flow. Often tines, large valves will actually decrease air flow.

Let’s take a look at the Chevy 305 heads as a good example:

The normal valve size in the “performance” 305 head is 1.84″ x 1.50″.

Thinking that “bigger is better” many people install 1.90″ or 1.94″ intake valves in 305 heads. Well, the fact is, larger intake valves in a 305 head will hurt overall air flow of the head, especially under .450″ lift.  Losing air flow below .450″ lift is especially detrimental if you have a valve lift rule.

Here is a example of “Spending Money to Go Slower”

Many years ago, I had a conversation with a person representing a company that sold complete Chevy 305 heads, mostly on E-Bay. They installed 1.94″ intake valves in all their 305 heads. I asked if they were aware that installing 1.94″ valves in a 305 head reduced air flow. His reply went something like this, “Well, yes, I do know the smaller valve actually works better. But, nobody will purchase a set of heads with 1.84″ valves; they all want 1.94″ so that is what we give them”.  We call this “Spending money to go slower”.  This is a classic case of bigger is not better. Often times, bigger is slower.

 

Air flow facts on 305 heads.

Test head: ‘450 casting number Chevy 305 head with “UnderCover Porting”.

 

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

Test don’t guess.