stock or 1mm oversize valves

[Paradox]

I'm about to start my journey into the world of the M20B25.
Recently bought a 325ic that had its timing belt snap.

One broken intake rocker and 5 bent exhaust valves. My past experience with rebuilding a motor was a VW VR6 - I even broke the 200whp barrier with it. Which is not an easy feat.

I am probably going to turbo this motor in the future, but my $25.50 question is:

Would buying 1mm oversized exhaust valves for Ireland Engineering be worth the $25.50 more then stock valves?

[nando]

are you doing any other mods?

My feeling is +1m on intake, stock on exhaust. the intake/exhaust ratio on a stock B25 head is already skewed towards the exhaust side.

but if it's going to be your *only* mod, I'd save the money. It would need to be coupled with a lot of other things to be worth it. Although new OEM valves aren't cheap either..

[slammin.e28]

Do it. Get some headers to compliment, and a nice intake setup. Not some dopey cone filter either. Real shit. A mild port/polish. There's not much you can do to improve the flow in the head, but you can do little things to compliment each other and work together. Get a nice cam too. While you're at it, get a super eta 2.7 block and throw you nicely rebuilt head on it. Maybe custom high compression pistons? Bore the block a bit past 2.7?

How deep down the rabbit hole do you wanna go?

[nando]

"not much you can do to imrpove the flow of the head"

nonsense really - although it's tough, there are definitely people with the skills to improve the head, especially at high lifts. :D

[jlevie]

"not much you can do to imrpove the flow of the head"

nonsense really - although it's tough, there are definitely people with the skills to improve the head, especially at high lifts. :D

I agree! To see what can be done, take a look an at MM Hi-Flow head.

[Paradox]

Cool...thanks for the input guys!
I will most likely compliment this with cams, headers and exhaust.
What do you guys recommend when it comes to cams?

[structured]

I agree! To see what can be done, take a look an at MM Hi-Flow head.

Interesting that you bring up MM as an example. Jim Rowe once told me that he did extensive testing and came to the conclusion that bigger valves does not necessarily provide any performance gain on an M20. The logic is that, although the open area is larger, the travel distance is increased. The cost negates the benefit I guess.

I am sure that there are cases where they are beneficial, though. If nothing else, it might make it easier to cut a nice radius on the ports.

[nando]

it depends on what you are doing - MM is actually pretty conservative with their engines.

[digger]

Oversized valves only on a bigger bore (86mm) to stop shrouding. I agree that MM are conservative with their engines however they do build race engines where emmisions etc wouldn't drive their builds.