Street OR Street/strip/race -Does it matter how large the exhaust port gets?

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greymouser7

greymouser7

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273? 318? 340? 360? 408? 4++?

We talk about velocity in the port for intake (at atmospheric pressure), BUT

the exhaust is pressurized.. ? -Is the only concern a choke point of exhaust flow out to headers?

THANK YOU!!
 
Not totally following you here but just like the intake, there is a point of good to bad returns on n the port shape. Important word there, shape. AKA, bigger isn’t better in port size but flow is the OK word.

So, can a port be Ruined because it is to big, yes. That would be a badly shaped port in my view.
Can a port be to big for a given size engine? Only if it doesn’t flow air well.
Can a port flow to well? Kind of yes.... you would adjust the cam duration for this. Sometimes builders will use less duration on the exhaust because the port is super efficient.
 
With the exhaust, as well as intake, it's not about the size, but the shape. It's all about making the air path "more straight" if you will in and out. .
 
Most exhaust ports are too damn big. It's about senseless to try and develop an exhaust port on a flow bench because the tendency is to make it bigger so it flows more. That's just wrong.

Then you have heads with an exhaust port that is so worthless you essentially can't hurt it, like the SBM. You can hurt it, but not much. It's just bad. You put the best valve job on it you can, roll that edge called the short turn and run it because that's all there is. You may make the flow numbers go up with more work, but it won't make any more power.

Most of the small block based engines would be better off with a 1.550 or even a 1.500 exhaust valve if you can get a valve job on there. On my next set of W-2's I'm going to try my level best to get a 1.500 exhaust valve in there and literally clean up the short turn and the guide and walk away. I may have to go 1.550 so I don't need to use a 55* seat. But it damn sure won't be a 1.600 valve. That port is already on the big side and a bigger valve just makes it worse.


BTW, as an interesting note, when TF developed the new small block head, they actually FIXED the exhaust port and it is damn small but looks efficient. I'd love to flow test one. And it uses a 1.570 valve with a 45* seat. That makes me think I can squeeze a 1.500 in the W-2 with a 50* seat.
 
I've never lost any performance by opening up the bowls, and raising the roof some on a small block mopar exhaust port. Kind of like headers, can't think of any small block that doesn't benefit from better breathing exhaust. 2 points:
  1. bowls MUST be kept in a "tulip" shape
  2. exhaust port needs to keep the "C" shape - if you will
 
In theory, you would want the port area to gradually increase from valve seat to primary tube diameter in as straight a line as possible. But you wouldn't want the port to be larger than the primary or manifold. Small to big = good, big to small = bad. Large valve LA heads are pretty big enough as it is for 1 5/8" headers. The "fast side" or high velocity area on exhaust ports tends to be across the roof. A basic bowl blend, guide boss narrowing, and general clean up would probably be worth the effort. Only gasket match the top and sides of the port face. Nothing gained by lowering the floor, and some feel a small step helps prevent reversion. Another reason to not increase port size at the face is gasket sealing area. Exhaust ports on factory heads pretty much are what they are, and you'll hit water before you make them much better.
 
greymouser7 said:
273? 318? 340? 360? 408? 4++?

We talk about velocity in the port for intake (at atmospheric pressure), BUT

the exhaust is pressurized.. ? -Is the only concern a choke point of exhaust flow out to headers?

THANK YOU!!
Click to expand...
Pretty much the same question Calvin Elston has raised. LOL.
Join www.Speedtalk.com forum and you can find a fair number of discussions about exhaust port volume and velcoities.
PRH, who posts here, says to start with the valves.
my first real exhaust port job - Page 2 - Speed Talk
Elston's blog Exhausting 101
 
I wont even comment on porting threads since I joined SpeedTalk.. I just sit back and try to visualize what these guys that really do the work are talking about. Great forum !
 
"volume" doesnt equal "scavenge". There is "flow" involved which needs to be optimized.
so, to answer your ? OP, yes there is a point when too large or incorrect changes in design, will hinder or degrade the systems ability to scavenge the exh gases.
 
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