Exhaust Ports?

[358T]

With all of this talk about intake port size and velocites and a recent comment by Cboggs, it got me to wondering about the exhaust port's role. Would some of the head porters on this site care to elaborate on some of the basics (feel free to go into depth though) of how a exhaust port's size effects and engines power curve and output?? Maybe some general rules of thumb convcerning SBC's? How about "big" sbc's?

Scott

[ou812]

From my experience and what has worked well for me is getting the exhaust port to flow good at higher lifts, not so much at lower lifts because I believe that if the port is left small and flows good, higher velocities will keep the burned gasses flowing out instead of creating a pressure drop by being too big. Also, the reason I like the port to flow less at low lifts is because I dont want any of the fresh intake charge short-circuiting out the exhaust. This works for me and how I port my heads and cam my engines, so maybe this is off from what others do...guess we'll see!
Brian

[cboggs]

a recent comment by Cboggs,
Scott

Oh sure, .. blame it on me !!

Kidding, .. what comment are you referring too?

I think by far the best discussion on exhaust ports is on the speed talk
interview CD with Darin Morgan. I think he really sums it up.

I do several things with an exhaust port depending on the application.
Drag race with good open headers, .. small, fast ports, ..
street car\ with full exhaust, .. bigger port with higher cfm #'s, ..
road race car with headers & long exhaust, .. bigger port with more flow, ..
Restricted class cars with a stock cam rule, .. I'm not tellin, . but there's
a bunch there.

This is VERY general, .. I'd like to hear others thoughts on this too.

Curtis

[358T]

Curtis,

It was in the "little chief -vs- SB2.2" thread". You stated that you thought the exhaust port was too big in some of the NASCAR takeoff SB2.2 heads.

There is always so much attention given to the intake port in most internet threads. I'm just wanting to learn a little more about the exhaust ports' role in the whole scheme of things.

Scott

[Cheapstreetduster]

ive been told numbers any where from 60 to 80 percent of what the intake flows..
right now iam at 72 percent with full exhaust..
some porting on my latest combo might reward me with more?
i run full exhaust drag race.. if it goes more on the exhaust and the car pics up ..it will confirm thngs more clearly for my application

[cboggs]

Curtis,

It was in the "little chief -vs- SB2.2" thread". You stated that you thought the exhaust port was too big in some of the NASCAR takeoff SB2.2 heads.

Scott

Scott,

yes that's true. The NASCAR port is designed around the full exhaust
of a cup car, it wants a better flowing, bigger exhaust port.
But in the case of a, say bracket engine in a dragster with well
designed high flow headers, .. it wants a smaller exhaust port
with more velocity. While the take off heads are a great deal
for a drag / bracket engine I was reffering to the exhaust port
as not being "perfict" for that application.

Prime example is a comp style engine I'm working on now, ...
a big, high flowing exhaust port will hurt power.

I've always felt a flow bench doesn't tell you how to design an exhaust
port, .. and Darin confirmed my fellings on his interview CD.
You should get it if you don't have it, .. TONS of good stuff.

I look at the entire package to design the exhaust port, .. the
engine's rpm, exhuast system, .. cam, .. etc.

As an example, .. if you have an engine with a stock cam rule, ..
low lift. If the head will let you do it, .. playing with the flow
curve on the exhaust port can almost be like adding exhaust
cam duration, .. and pick up some power.
It's very application specific.

Curtis