intake vs exhaust flow

[MostMint]

I want to understand the relationship between intake and exhaust flow in the cylinder head. The exhaust ports in the Saturn look fairly small compared to the intakes. I've found some flow bench info on these heads and I think I want to open up the exhaust. I know this is not as simple as making the port bigger, so I am wondering what is the optimal ratio of exhaust port volume vs. intake port volume.

Of course the exhaust has the advantage of pressure already being built up on the way out so I am fairly certain the ratio is not 1:1.

I figure the factory may have some compromises in head design to accommodate speed of assembly, emissions and who knows what else. If I can understand these I can make the head flow better.

Any thoughts on this?

[Maverick]

If you haven't already, check out http://headporting.com/

Especially, http://speedtalk.com/shows/020_harold_bettes.html

[TireSmoker]

I've read in several places, relating to Chevy stuff, that intake-to-exhaust ratios of 75% or better were desirable. (exhaust flowing 75% as much as the intake) With that said, I've never seen anyone talk about shooting for more than 80-85% either. Those figures are referencing flow (CFM), not just basic volume.

-Dave

[Racin'Jacin]

Not 1:1 for the reason that: Once burned there is LESS left over. The "spent gas" has less mass not only because the mixture was burned but it also is much hotter and therefore less dense. That's the same reasons the intake valve is almost always bigger than the exhaust - a couple nitro motors excepted - and even there it might be more about keeping similar parts rather than purely flow requirements.


Jacin