Gas Flowing head: much to gain?

[Morat]

Hi again,
I'm wondering how efficient or otherwise the M20b25 head is as standard. I'm thinking about stripping and rebuilding my existing head in slowtime and I was wondering if a banana fingered noob with a dremel could actually do any good (as long as he's careful!)
Cheers
Morat

[Turbo-Brown]

There's a book called "Practical Gas Flow" by a chap called John Dalton (ISBN No. 1-855-20564-5) which goes into all the whys and wherefores of porting on a budget.

Think you could smooth out the casting marks and probably gain a little bit of power, but to make significant gains (if they're to be had) you need to spend a long time developing port shapes and then testing them to see what's good and what's not.

Problem with that is that you could get through a lot of heads in finding the right shape so the book I mentioned gives techniques for making dummy ports out of plaster so you don't have to risk it on the real thing untill you've got it right

[Avner]

There are no casting marks on an m20 head.
I had the same thought when I replaced my head gasket, but the head design and execution looked too good to mess with.

[Morat]

Thanks for the answers. If its likely to be marginal improvements then I won't bother - far too much likelyhood of me getting carried away

[Turbo-Brown]

Found the odd little nobble in mine, but nothing that made me thing "that's gonna cost me power!"

Was just reading on E30tech about how well the heads flow as standard so it's probably not worth bothering.

Might be worth polishing up the exhaust ports though, no idea if you'd notice any difference there either though.

[Brianmoooore]

BMW know how to design cylinder heads! BMWs are designed by engineers, not accountants like lesser makes.
Only (marginal) gains to be had are by port matching the manifold, multi angling the valve seats and removing the part of the valve guide that sticks out ( at the expense of guide wear).

[Turbo-Brown]

Might be well designed, but no two ports on the original inlet manifold are alike!

Probably not a problem as long as they underlap the ports but it was interesting to find none the less.