talk about exhaust ports

[Jim Ratto]

Let's talk about exhaust porting

not much attention paid in discussions to the exhaust ports. Remember the Eliminator 2000 CB heads from the 1990's... with their "squeeze ports"? the exhaust ports from a plan view, actually made a venturi profile. I ran a set of those heads, and no, never did any dyno time with them, but at one point, I ground out the venturi shape, and created these big 1.625" tunnels.... and the motor fell on its face after that.... lost a lot of snap all over. Imagine taking a good running motor with 32 deg of timing in it, and turning it back to 17 deg.... that's what my mistake was like.

Why? I would think on "blowdown" you'd want to scavenge as much psi out of cylinder as possible, to free up breathing and make more room for fresh intake charge. Maybe it was over scavenging? It had a 1-3/4" merge on it with a Magnaflow.
If we don't want our intake manifolds to waist down and then flare open, then why would the exhaust be different?

I know the best heads have typically run "small" exhaust ports.... even on big cc stuff. Jayburn refers to those big ports as ....   well we don't need to go there....hahaha

What about D exhaust ports?

What about an exhaust port that (from plan view) makes less of a tight turn (yet retains stock flange where header bolts, but maybe move bolt holes "out"), port the outside wall out towards valve covers?

I've read leaving a step from header ID to port diameter is a good thing too... so how about moving outside wall further out, leave step @ inside wall?

thanks for any comments, thoughts....

[Shubee2 (DSK)]

I was Talking with Steve Tims About this A while Back He Told Me That he has Told Pat Downs (CB Performance) for the Longest time to Make the Exhaust port Smaller on the CNC Wedgeports  Steve has Experimented with this on the flow Bench and at the Track and Gained  a lower E.T. and a boost in H.P.  Pat Says the Ports Flow fine the way they are  you be the Judge

[K-Roc]

Yup, small high airspeed Exhaust ports make the power that's for sure, I love the Street Eliminator exhaust port. I wish CB would put it in the 044. 

[lawrence]

I was thinking about the theory behind this thread during breakfast this morning. I'm not sure if this analogy makes sense but here goes. If you exhale through a straw it takes little effort to push a large amount of air out. Then if you try exhaling through something that has the diameter of 1", for example, it takes much more effort. So an engine is working harder and using more power to move air out big exhaust ports. So, im sure there is some type of happy medium when it comes to sizing exhaust ports. And all of that relates to the size of the carbs being used, intake runner diameter, intake port size etc.