Getting my 731 head to breathe!

[chrisbmwe30]

Getting my 731 head to breathe!
im in australia where e30 parts are expensive etc
an 885 head is about a 1000 bucks second hand
and custom piston about 800-1000
any advice on what i can do to get my 731 head to breath so far i have a 325i inlet manifold and throttle body and exhaust manifold
this is for my 2.7i stroker build
any advice would be great

cheers

[KillaCam]

grind the valve seats....port polish..... Double valve springs

[clarkson]

Where are you in Aus? I am in Sydney, and for my next build I will prob be using a 731 head, and looking to do some porting aswell. I know of one place here in Syd.

The trick with porting is to increase flow but not to slow down the velocity too much. To start with you could fit the 885 valves in the 731 head, or even get the +1mm oversize 885 valves from Top end performance, see their ebay store. Then the exhaust side can be opened right up, and clean up the intake. Choose a cam to match the flow, and your compression ratio. Stock 885 double valve springs are pretty good.

[Gruelius]

if i get pissed off and give up on M20 i will have a AMC 855 with 45k miles fs for around 600 bones ;)

look up madhatters 731 jazzing up, he spent a fair bit of cash on it.

[jlevie]

There isn't much you can economically do to that head. The valves are one restriction, but the intake & exhaust passages are another. By the time you do all you can to that head, I suspect that an 885 head would be a bargain.

[clarkson]

The advantage of teh 731 head is you can use flat top pistons. Much easier to get high compression with a good squish. That's why I will be using one for my next build.

[chrisbmwe30]

Yeah i have thought about that jlevie,
so a port on the intake side leaving the surface alittle rough and polish out the exhaust side will help. just dont want it flatlining (no power) after 5500 rpm.
any advice on what size valves to go out too?

[Wanganstyle]

here are some photos of my worked #731 head. I don't have any comparisons to 885, I've never had a 885 in my own car.

[deutschman]

I would not port of polish the intake runners. You want them to be a bit ruff to help with fuel atomization. What you do want to do is cut back and smooth out any sharp angles that the air needs to flow over on the way into the combustion chamber. You will want to get a 3 angle valve job. Intake 30/45/62 and exhaust 30/45/60 works well. Go ahead and polish your exhaust runners. It might not help the exhaust flow better (then again it might) but it will help them stay cleaners because there is not much for the burnt exhaust particles to hold on to (like a slip and slide ;) ). Make sure you get a really nice set of valves. 1mm over sized valves might actually hurt you performance wise unless you are doing a stroker or turbo build where you really do need to be sucking in a lot more air. Use a cam and some 19lb injectors along with doing everything you can to open up the motor on the intake side. Run a 2.5" exhaust, anything larger and you will loose exhaust flow unless again you are running a turbo. Maybe use a m30 AFM.
Thats my 2 cents.