Chevy Impala SS Forum banner
1 - 9 of 9 Posts
G

·
Discussion Starter · #1 ·
I've always read and heard that the exhaust port on a cylinder head should flow about 70% of what the intake flows. Why is that? it seems to me that you'd want to the exhaust to flow as much as possible. This makes sense to me for a few reasons. A) the better the port flows and the larger the valve, the easier it is for the piston to push the exhaust out. B)with a better flowing port, you could empty the cylinder faster and run a smaller duration camshaft with less overlap. What am I missing? What happens if an exhaust port flow to well? Just curious.
 
G

·
Discussion Starter · #2 ·
Exhaust gasses are still expanding when the valve opens this is without the piston driving them out, the small port can work in conjunction with the high velocity of the exhaust gas rush to actually suck some fresh air charge into the cylinder as the intake valve opens. If the port is too large and lazy you are not able to take advantage of the exhaust pulse inertia like that.

Don't get too hung up on theory like that though, notice almost every cam you see any of us speak of has 6+degrees of exhaust duration more than the intake. Smallblock Chevy heads have always been weak on the exhaust side(at least till the LSX series) the cam can be used to account for some "mismatch".

If you are looking to port a set of head worry about clean flow and equal work in all ports, do not worry about the ratio, if in the end it shows the exhaust ports to me weak the usual split duration cams will help you out.
 
G

·
Discussion Starter · #3 ·
An exhaust port will flow more with a pipe attached to it (or a header primary). On the other hand, an intake port will not flow as much with an intake manifold attached. So don't get too hung up on the flowsheet numbers.
 
G

·
Discussion Starter · #4 ·
The reason the exhaust it's smaller than the intake is because there's only a finite amount of room in the cylinder to fit the valves; so you have to compromise in order to maximize your power. Turns out that the best ratio is somewhere around 70%, although the exact ratio isn't that important because you can fine tune the cam to compensate.

Remember that the exaust is under a lot more pressure than the intake, so it doesn't need as big a port in order to efficiently empty the cylinder. Vizard has an excellent chapter on this in his cylinder head book.
 
G

·
Discussion Starter · #5 ·
Bugman,

Your thinking is actually completely right.

"A) the better the port flows and the larger the valve, the easier it is for the piston to push the exhaust out."

Yep, lower pumping losses are always better. You don't want to crowd the intake valve because you always want as much there as possible for the bore size, but the larger the exhaust valve the lower the pumping losses usually are in a motor.

"B)with a better flowing port, you could empty the cylinder faster and run a smaller duration camshaft with less overlap."

Why would you want LESS overlap? Why is that such a bad thing?

The smaller duration point is good though, you can do some good things when you reduce exhaust duration.

Dwaye,

A exhaust port is like a intake port you don't want to put the port into choke with high velocity, but you do want to work with the high pressure. High port pressures in the intake or exhaust system are always a good thing.

I wouldn't characterize a 23° head as always bad on he exhaust side. I've had some 300cfm heads have a 80% E/I ratio and when you cam them right make killer TQ. In fact I've seen some heads improve just exhaust flow and pick up every time at the track time and time again.

I've found there is no ideal ratio so don't look at heads thinking that one is better than another because there is a ideal ratio. If that was the case Pro Stock heads would not be 68%, and even most Cup heads are in the 70-75% range.

Bret
 
G

·
Discussion Starter · #6 ·
There is still pressure inside the cylinder after a combustion event, even when the piston is near bottom center, so when the exhaust valve opens (EVO), the combustion gases help themselves out of the cylinder .... at least until the cylinder pressure drops.

So initially, at EVO, the engine doesn't need to push the exhaust out because a lot of it happens naturally. The lower you can reduce the "back pressure" in the exhaust system, the more the combustion product will leave the cylinder by itself. When the cylinder pressure equals the exhaust system "back pressure", that's when the piston has to do work to push the rest of the exhaust from the cylinder.

The intake side is different, because the piston has to do all the work in sucking-in the next round of air:fuel. Since the intake side requires more effort pulled from the engine to sustain itself, then there is merit in reducing the losses where they are most needed, and that means using a bigger intake valve than exhaust valve.

The above intentionally ignores the acoustic effects that help cylinder filling, or exhaust scavenging, because they are important but not relevant to explaining why the exh valve is smaller than the intake valve. HTH.
 
G

·
Discussion Starter · #7 ·
Ok. So, theoretically speaking, if I had the room in the chamber for 2 big valves(like a Hemi) and a supercharger, would a 1:1 I/E ratio make more power than a 1:.7? Because the forced air of the supercharger would overcome the need for the engine to draw in the air, and the pressurized intake air dictates a short overlap cam(so you don't loose to much out the exhaust), wouldn't I want as much exhaust flow as possible?
 
G

·
Discussion Starter · #8 ·
I'd say it would depend somewhat on how much boost you're running, but yes, forced induction would increase the exhaust need. Vizard talks about this in his chevy smallblock cylinder head book.
 
G

·
Discussion Starter · #9 ·
I was talking inside the box for the first part here, (2.125-1.94 intakes and 1.625-1.50 exhaust valves)

Going to two valves of the same size is not going to work or help. Look at a Top Fuel setup. They would gain the most out of doing that and they dont go past 88% in size (2.6 int 2.3 exh)

In fact some you would see on earlier Pro Stock heads when they worked with a standard BBC head was shrink the exhaust valve down to 1.85-1.88 and throw a 2.300 intake in the heads.

Most times what limits the valve sizes is the bore and the head casting. If you go too big the port is going to be limited because of the material in the head and where the head should be ported.

Bret
 
1 - 9 of 9 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top