LT's, O/R h pipe, Catback??? Decision Time

George Klass from accufab(they make custom headers too) once told me there is no point in getting a bigger header primary tube than the diameter of your exhaust valve.


Now if size didnt matter, why doesnt everyone just go straight to a 3" exhaust system?


A few years back i also had lunch a couple times with Jay Allen. He explained to me how a big throttle body doesnt kill the low end or velocity.....its in the cam. That said, its also true at the other end of the motor. A bigger header primary will result in a slower exhaust gas velocity. When the exhaust is being pushed out, its pulling in the air from the intake. Slow down the exhaust...and you slow down the air coming in. Its ALL in the package!


That said, if gt40p's are your heads you will be running in the end, 1) make sure your headers you buy now will swap over. seems like there were a lot of threads about this recently. 2) There is no point in getting headers that are larger than the 1.46in exhaust valve on the gt40p's. Unless you plan on upgrading yet again, and are willing to lose power now, to gain in the future, just get the right part. 1 5/8" is more than enough on these heads.

I dont believe peak HP/TQ is affected by large headers or a 3" catback. I think the power "under the curve" is though. Power down low will be affected. We dont need "back pressure"...thats the wrong term. You need the exhaust to do its job that the cam wants it to do in order to make an efficient combination.