Quote:
The +1mm vlaves are gonna kill any off boost power anyways, so might as well make use of them when on power and rock 280's, ya know?
That might be true on many engines, but these 1st gen 4g63's are a bit of a unique case. The heads on these cars have simply huge ports as delivered, and the only place they really need any love for optimal performance is in the valve bowls. (The ports are so large, in fact, that many people looking for drivability/modest power levels and higher efficiency are moving towards the 2g/later small port heads.)
When I was still in the machine shop a few years ago, I played with throating the valve seat, matching the valve bowls, and blending the transition. I found significantly better drivability (off boost) from this mod, even on my modest set-ups.
The gains from having less random turbulence, and a more even cone more than make up for any teeny loss of velocity.
This is because the machine work (from mitsubishi) leaves a lot to be desired in the transition between the valve seat and the bowl.
The bowl is *generally* larger than the seat. Add in core shift, and all bets are off as to the actual match of the seat to the bowl. (I have yet to see one that was matched well enough for me to run out of the box.)
With a large bowl/small seat, and a bit of core shift, you end up with a situation where a "ledge" is formed in the airflow path.
The effects of that mismatch are a turbulent, non linear flow right in the area you want everything to be orderly. Getting the airflow distributed as evenly around the valve as possible is by far the most efficient technique for getting air into and out of the chamber with minimal pumping losses. It also aids in cylinder to cylinder distribution. This brings the weakest cylinder much closer to the others, and allows you to get a more predictable tune that's closer to optimal with less chance of the lean hole getting pissed and showing it's displeasure with knock. (remember, that fancy a/f gauge is reading an average of all four cylinders a/f ratio. You can and often do have one hole weaker than the rest, and if you're ballparking the a/f and then tuning to knock, well, you just gave up a buncha power/efficiency tuning to the weakest cylinder)
This cylinder to cylinder disparity is especially true on the intake side, and is really noticeable in the lower airlow numbers. Optimising distribution results in a much more responsive motor when you're operating in the lower airflow region, because you can dial in more aggressive timing without the weakest cylinder knocking.
In addition, for higher output set-ups, it has been my experience that going to 1mm oversize valves, opening the seat up to the bowl diameter and blending the transition makes a significantly larger difference than just a 1mm change in diameter would suggest on paper. Reducing the turbulence/restriction at the valve seat/bowl area will result in higher airflow at the same (or lower) boost levels, and that really pays off accross the entire dynamic range of the engine.
/ubbthreads/images/graemlins/cliffs.gif The devil is in the details on these motors, and a noticeable increase in performance can be obtained with a little work in the valve bowls.
While it's true that just chucking in 1mm oversize valves will not give much benefit...because the seat/bowl mismatch has not been addressed... 1mm larger valves, combined with an optimal seat/bowl transition is one of the easist paths to good power combined with excellent efficiency.
Combine that with a well matched cam set and an efficient turbo, and you're good to go.
Now, back to the original subject, cams. /ubbthreads/images/graemlins/smile.gif