Firstly, ported heads are available in a variety of configurations, see the "other services" tab.  

F​low bench testing data from my SF-60 flow bench.   What I'm demonstrating here is the differences in flow on a JT-07 & JT-181 head.  I've purposely set test conditions where all other variables remained unchanged.   I've tested the 3 most common plates: green, purple, and blue.   The 3 test conditions I've chosen to demonstrate are:

The head as cast, no modifications.

As cast with a 3 angle valve job

3 angle valve job and port work

As you can see in the photo below, testing is done with a clone cylinder adapted to the flow bench onto which a cylinder head can be bolted down, complete with gasket.  The phenolic spacer, restrictor plate, carburator, and air filter adapter are all considered in this flow test.  What I've done here is not just flow the head, but rather flow the entire system as it would be raced, which is what really counts. 

The take away here is, it's clear that porting will improve flow, even on the green plate, but before you grab your dremel, there are a few points I'd like to make.  You should see that a professionally done 3 angle valve job is the foundation to a good flowing head.  Not only does it show on the flow bench and the dyno,  but experience shows it moves peak HP up in RPM.  It should also be noted that a good head can be quickly ruined by an inexperienced or over-zealous dremel operator. 

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