Just a silly porting question

[fah_Q]

When you port an engine do you loose compression? My friend told me that if i ported it i would have to get a new head with some slightly smaller domes.

[Duninshee]

I'm not quite sure if you loose compression, but I had to do it to mine. I had 20cc domes in the cool head and once I had her ported I had to drop to some 17cc domes. You wont have to get a new head if yours is stock. Just have it milled.

[SDD]

If you raise the exhaust port you will lose "kicking" compression but when the motor is actually running if everything is setup correctly you will actually have a gain in compression. Boring out a cylinder a few overbores also will raise compression if you don't recut the head to compensate.

[Ducman]

Like SDD said. You loose static compression when the exhaust port is raised so when you measure compression you will get a lower # but dynamic pressure goes when you get "on the pipe" in the upper RPMs up if done correctly. If you get a fairly top endy port job with high exhaust ports and get real low static pressure, it does hurt the bottom end a lot and can be compensated for somewhat with smaller domes but only to a certain point before the smaller domes start to hurt the top end performance. I had this delima with my 4mm stroker when I first got it going, 125 psi and a top end shaped dome/squish aprox 19cc, I got some new domes fron NOSS, 145 psi more of a midrange dome approx 18cc and it helped the botom end a lot, top end was a little better tool.

 

To answer you question, if you do a mild port then no, if the exhaust ports are raised a lot then you might want smaller domes.

[fah_Q]

Yeah, i planned on doing just a mild port job. Thanks for your help. How do t6's perform with a port job, i hear they can be tricky.