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Unread 11-11-2013, 03:12 PM   #1
TheBlueDevil
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1997 XJ Cherokee 
 
Join Date: Mar 2011
Location: Lovettsville, VA
Posts: 192
4.0 Intake Head Port Size

So sometime back I did a 99+ manifold swap and I really got a serious gain from it. However, when I put it all together, I did not measure the intake port size on my 97 head vs the runner opening size on the manifold. Word on the street is that the opening on the older head is smaller than that of runner opening. I have a decent amount of experience with machining and such so I am wandering if it would it be beneficial at all to shave a slight curve into the head port to match the size of the runner? It would be kind of like a mini velocity stack but just at the mouth of the port funneling down. Also, any input on polishing the exhaust ports?


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Unread 11-12-2013, 07:55 AM   #2
drew27racing
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1995 YJ Wrangler 
 
Join Date: Sep 2012
Location: kansas city, kansas
Posts: 19
I ported the exhaust side and the headers and all I really noticed was that it sounded a little bit smoother, not really all that different
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Unread 11-12-2013, 09:34 AM   #3
5-90
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1988 XJ Cherokee 
 
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Location: San Jose, CA, Hammerspace
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Quote:
Originally Posted by TheBlueDevil View Post
So sometime back I did a 99+ manifold swap and I really got a serious gain from it. However, when I put it all together, I did not measure the intake port size on my 97 head vs the runner opening size on the manifold. Word on the street is that the opening on the older head is smaller than that of runner opening. I have a decent amount of experience with machining and such so I am wandering if it would it be beneficial at all to shave a slight curve into the head port to match the size of the runner? It would be kind of like a mini velocity stack but just at the mouth of the port funneling down. Also, any input on polishing the exhaust ports?
Don't curve - ramp. About a taper of 3" run to 1" rise, but go with a smooth taper into the port and blend the resulting sharp edge inside. Don't otherwise change the shape of the port.

For a more accurate idea of where to start, use a thin layer of modelling clay on the head face, put some studs in the intake screw holes to line up the manifold, oil the manifold face (light machine will do,) and gently lower & press it into the clay. Remove the manifold, you now have a reverse imprint of the manifold ports. Use this for your pattern.

Then, after you clean up the ports, strip the clay, put the studs back, and drop on your gasket. See if it wants trimming at the intake ports.

As far as polishing - don't change the basic shape of the port - too easy to screw up without a flow bench! Instead, chuck in an abrasive roll (180-240 grit) and just smooth the surface. It's not necessary to get a mirror finish - but smoothing out the sand-cast surface will help with laminar airflow. Just do not change the shape of the port! It is very, disgustingly easy to wreck a head if you can't constantly evaluate what you're doing! (Do NOT ask how I know this. Just know that it's cost me a few thousand over the years...)

The gains will be nominal at best, port work is typically done to complement other mods, not as a power-adder in and of itself (it does often have a beneficial effect - but not as much as you'd think, for the work you're doing.)

Nota Bene - changing the shape of the EXHAUST ports can have a deleterious effect on tailpipe emissions! "Exhaust Port Reversion" is when some exhaust gas gets bounced back into the cylinders - and a redesign of the port to allow a bit of reversion is how they deleted the EGR valve for 1991-up engines. Tread carefully here...
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Unread 11-12-2013, 11:59 AM   #4
TheBlueDevil
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1997 XJ Cherokee 
 
Join Date: Mar 2011
Location: Lovettsville, VA
Posts: 192
Awesome info. I'll probably only lightly sand the exhaust ports and with the intake runners I may only lightly port those. It seems like its a little risky so I may just shave off a little bit.
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