Its been awhile since I have done a "tech article" and since it seemed the piston and rod ones were good discussions I thought it was about time to do one on cylinder heads.
As some of you may or may not know we have been offering cylinder head porting on a limited basis on our bigger engine builds since we already have the engine apart. Some things we have learned about the Evo head is that they are subject to core shift much as the earlier DSM heads were and the VIII seems to be really the most violently affected.
Here you can see the edge of the bowl where it was plunge cut prior to having the valve seat pressed in at the factory:
You see its a fairly uniform machine line showing that #4 on this bowl is about where it should be. By the time we get to #1, the mold has been shifting enough during casting that the bowl is now D shaped and definitely not flowing as much. This is really bad news since its the last port on the intake and has some really restricted ports.
The VIII also has for want of a better word mold/casting marks on the intake side of the head that further disturb flow:
Oddly enough the IX really doesnt seem to have core shift problems nor does it have those mold/casting marks on the intake side. They have them, but they are on the exhaust side of the head. I think alot has been made of MIVEC making power when it has more likely been substandard bowls and ports on the VIII robbing the horsepower. Here is a pic of the same off center bowl after some initial smoothing:
Exhaust side pic-
Now off to the machine shop to have the guides pressed out, a tanking, and then finish what you cant reach if they are in there. Its interesting to note that this head went on a 3582HTA car that at 23.5psi had made 428whp. After the head port and bottom end build (no other changes in the motor, compression was 9:1 so as close as we can get it) the car redyno'd at 478. It allowed for more timing which is where some of the power came from, but with better flow the car will naturally thrive.
I dont happen to have intake port pics of this particular head, but typically nothing more than bowl work and casting line smoothing is needed. The bowls of these heads have some pretty nasty ledges typically which really hurt balanced airflow. The exhaust is generally also not very good, but since the flow laps the ledges on the way out its not quite as critical. We dont leave them like the factory does though for sure. I have seen some heads that are so smooth stock they almost seem ported in comparison to the Evo heads. BMW for instance does not have flaws in the bowls and ports but since they are trying extract every bit of all motor power it makes perfect sense.
Hopefully this can be as interesting as the last article and get some good tech and pics flowing.
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