porting exhaust runners

[newtypedsm]

i recently installed 1mm os valves in my cylinder head and ported the seats and bowls to smooth the flow. i decided to port the exhaust and intake openings, so i started with the exhaust side, matching it to the gasket opening. thats a helluva difference on a 2g head. i told someone that does porting for a living and he told me i dont want to open the exhaust side that much, that if i went overboard then id have to get it welded up and start over. i got to thinking...turbo cars need exhaust flow, as much as possible, right? plus if he said that taking in mind needed back pressure, doesnt the turbo itself provide more than enough back pressure? i hope i havent fubar'ed this head now. any comments, suggestions, input?

 

[all_motor_mike]

if u ported them to the 2 piece matal gasket then u are wrong sir.............. being a smart ass aside....... yeah thats over kill. not only does it slow down velocity but now ## going to have a step up when the exhaust goes from head to manifold causeing all kinds of turbulance. i would start over.


mike

 

[dsm-onster]

I always port my head before I clean it up (hot tank). Because you can see the ring around the port mouth that shows you what you need to port to match the exhaust manifold. That ring is smaller in diameter than the gasket. Below I have a picture describing what I saying. Ported exhaust runner is on the left. . .

A side note, back pressure is never good. To get good scavenging you need a certain diameter pipe/tube/runner. A tune for lower rpm scavenging requires a smaller diameter, and backpressure is a negative sideffect in the later rpms. You do not have backpressure when your at resonant rpm point of your port. At what ever rpm the runner is tuned, there is LESS backpressure (the vacuum behind the pulses are sucking out the exhaust gases at the right interval of the exhaust valve opening/closing events). If you do have more backpressure at your chosen rpm point, then you didnot properly design your port.

 

[e85_4g63]

Post up some pictures of your head...

 

[e85_4g63]

Here's a couple shots of a head I'm going for a guy right now. Just some back ground info. I have about 2 hours into the head so far. It's been disassembled, top tanked, and I only used a cutting bit so far, no sanding disc's or anything like that. The head is in very rough shape and isn't anywhere near done. No short side radius' / blending / nothing of the sort in these pics. Keep in mind also that this is a 1g head, so it has larger ports then your 2g head. Also you'll see on the exhaust ports that I haven't touched the overall shape of then yet. I haven't gotten that far. Most of the work on this head has been bowl work so far.

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[dsm-onster]

Keep in mind also that this is a 1g head, so it has larger ports then your 2g head.

2g exhaust ports are the same shape as 1g.

 

[newtypedsm]

im embarrased to put up any pics. when i do something, i tend to get carried away. and thats pretty much what i did. i went well beyond the black soot area.