E30 Zone

Hey all

I have two options for the inlet side

320i manifold + 325i tb (allready using on my 323)

325i manifold + 325i tb (would require $$ porting)

Roughly how much porting is required to have a 855 port on a 731? or do you just match it to the 325i runner and shave some of the 731 gasket off in the areas milled?

if the 325i manifold only adds a few top end HP and costs like $400 for machining i may aswell sell the lot for $200 or so. been warned porting can easily lose hp too if done cheap.

wiki says it bolts right on, e30zone says it needs a bit of machining, but shit even my 320i manifold was a bit larger than the gaskets at some points.. was polished slightly though...

**SOLVED**

Just use the 2.5 manifold, it'll work fine on the 731 head. Are you sure you have a 731 or could it be 200 on a 323i?

$400 porting is a waste when you could buy a 2.5 engine for that amount.

Gruelius wrote:
Roughly how much porting is required to have a 855 port on a 731? or do you just match it to the 325i runner and shave some of the 731 gasket off in the areas milled?

The 885 and 731 ports are exactly the same... EXACTLY... The only difference is the valves are slightly larger in the 885 and the combustion chamber is different to match the 325i pistons.

The gasket is going to be a little smaller to allow it to squish


This is the difference in the inlet manifolds.

2.5 heads cost $600 motors ~$1300 here. and thats bargain prices.

and the 855 PORT is much larger than 731. but the 855 MANIFOLD port is the same size as 731 it seems.

and that is a ETA manifold you have there.

m_jermyn wrote:

Gruelius wrote:
Roughly how much porting is required to have a 855 port on a 731? or do you just match it to the 325i runner and shave some of the 731 gasket off in the areas milled?

The 885 and 731 ports are exactly the same... EXACTLY... The only difference is the valves are slightly larger in the 885 and the combustion chamber is different to match the 325i pistons.

im going to have to politely disagree with that,

if you look at the bit of the head just above the inlet valves in this photo:


then you will see that on the 885 head the casing here is raised higher and is also flatter than on the 731 head.

the 731 head has a rounded profile in that point thats not as raised, and after doing some fine drilling and measuring that bit of the head IS the roof of the port.

which leads me to thinking the bulk of the flow on the 731 head is physically lower down in the port and has a more extreme corner to turn to pass the valve.

the 885 by contrast has those two rounded ridges at the top of the port, which extend either side of the vlave guide, allowing the port to be effectively higher in the head and more of a downdraught towards the valve.