731 head for NA eta?

[jeffnhiscars]

The real problem with the 731 manifolds is the tiny throttle body they run. You can put a B25 throttle body on them but you can't bore the hole big enough to make a smooth transition

I was intending to use a b25 throttle body (which I bought) and afm (that I dont yet have..hint hint) but based on this comment the gain is limited without going to an 885 head or would you just bore the 323 intake as much as possible (leaving a step I assume) and call it a day ?

He doesnt enlarge the port opening or remove much metal at all, its already large enough for pretty much all applications. a backcut on the stock valves is worth quite a bit.

its the subtle porting with the bigger valve is where the gains come, simply adding a bigger valve alone doesnt do much, the short side needs to be reprofiled when you add a larger valve thats where the gains will come along with streamlining the guide

Im onboard for bigger valves and I like "subtle". Do you have something that illustrates "reprofiling the short side" ?

I take it this is all done by hand with a die grinder (my CNC is out for repairs too). Do you keep the cylinders the same by eye ?

[digger]

The real problem with the 731 manifolds is the tiny throttle body they run. You can put a B25 throttle body on them but you can't bore the hole big enough to make a smooth transition.



Í thought there would be too but I mocked it up on a bare head and it's pretty much spot on. I was surprised. I can try to take some pictures through the port.

The head was probably gasket matched. As far as I'm aware, most of the gains in a 731 head come from increasing valve size, blending in the seats and cleaning up the bowl and area around the valve guides. Not so much from enlarging the port itself.

Hopefully Rama can bless this thread with his infinite wisdom on the topic of 731 heads. Here is some of his handy work, I hope he doesn't mind me posting it.

He doesnt enlarge the port opening or remove much metal at all, its already large enough for pretty much all applications. a backcut on the stock valves is worth quite a bit.

its the subtle porting with the bigger valve is where the gains come, simply adding a bigger valve alone doesnt do much, the short side needs to be reprofiled when you add a larger valve thats where the gains will come along with streamlining the guide

[5Toes]

I am watching this thread very closely, as I have a 731 head with a euro early model eta engine (11:1 comp.) block is real healthy, used head looks clean. Going for a budget build so will likely just be grinding the valves and cleaning up mating surface and of course new valve stem seals.

[digger]

If a stock 885 intake is port matched to a 731 then it would seem that the 323 intake which has smaller ports, would be stepped to the 731.

If that's the case, does it make sense to port match the 323 intake to the 731 or just leave it alone ?

personally i wouldn't bother

[Hooffenstein HD]

I would run the 323 manifold, looks like it may have a touch shorter runner and may shift your power band up slightly and not have to worry about the port shape or alter a nice head.

The real problem with the 731 manifolds is the tiny throttle body they run. You can put a B25 throttle body on them but you can't bore the hole big enough to make a smooth transition.


A picture from my build thread showing how much needs to be ground out but if you ground enough you wouldn't have any meat to seal against at the bottom.



Found in a thread on another forum. The throttle body area has been redesigned to accommodate the larger throttle body.

325i on the bottom

Still thinking that there will be a step, especially beside the injector notch where the 885 is squared off.

I had a 731 head that someone "port matched" to an 885 manifold and it was hogged out a quite a bit

Í thought there would be too but I mocked it up on a bare head and it's pretty much spot on. I was surprised. I can try to take some pictures through the port.

The head was probably gasket matched. As far as I'm aware, most of the gains in a 731 head come from increasing valve size, blending in the seats and cleaning up the bowl and area around the valve guides. Not so much from enlarging the port itself.

Hopefully Rama can bless this thread with his infinite wisdom on the topic of 731 heads. Here is some of his handy work, I hope he doesn't mind me posting it.

[whodwho]

Sorry, I thought it was posted in reference to - "Nando is correct. 885 inlet is port matched to a 731 head straight from BMW."

Still thinking that there will be a step, especially beside the injector notch where the 885 is squared off. I had a 731 head that someone "port matched" to an 885 manifold and it was hogged out a quite a bit. I would run the 323 manifold, looks like it may have a touch shorter runner and may shift your power band up slightly and not have to worry about the port shape or alter a nice head.

[Hooffenstein HD]

From the runner shape I don't believe that to be a 323 manifold and is a 325.

The manifold I posted is an 885 manifold. I was posting to show what I was referring to by the "steps and bumps". All the 731 manifolds I have seen have been comparatively smooth with greater consistency across the runners.

That second manifold you posted is for a 731, more specifically a late model Motronic M20B20. M20B23 has a heap of vacuum fittings for L-Jet.

[whodwho]

From the runner shape I don't believe that to be a 323 manifold and is a 325. Unless they had a rounded runner version the 323 has a flattened runner shape

[jeffnhiscars]

Multi post delete. My bad !!

[jeffnhiscars]

If a stock 885 intake is port matched to a 731 then it would seem that the 323 intake which has smaller ports, would be stepped to the 731.

If that's the case, does it make sense to port match the 323 intake to the 731 or just leave it alone ?

[digger]

That explains the bump. Are the steps meant for anti reversion?

steps? the port missmatch is to account for production variation so that no matter what you do the port transitions to a step up rather than step down. it probably helps with antireversion but id suspect the the effect is a secondary benefit and is relatively small

[whodwho]

Curious, can you take a pic of looking down from the top on that one?

[Hooffenstein HD]

there is a bump on the inside that corresponds to the mounting nut where they tried to get a uniform casting wall thickness to avoid casting issues.

That explains the bump. Are the steps meant for anti reversion?

[digger]

there is a bump on the inside that corresponds to the mounting nut where they tried to get a uniform casting wall thickness to avoid casting issues. people usually grind this out but on a mass production environment this would add cost so bmw never did it.

the 885 flows enough for 250-260bhp potential if you use the rule of thumb superflow calculations, the the issue with the OEM manifold is that when you bolt it to the head, the flow takes a dive. this is one limiting factor with the stock oem castings. The castings look good from the outside but if you cut it into little piece the internal looks a bit funky, some of the bends get pretty damn tight and the cross section isnt uniform

[Hooffenstein HD]

Nando is correct. 885 inlet is port matched to a 731 head straight from BMW.

I am not really sure what the bumps and rough spots on the head/port end of the 885 inlet manifolds are though. Anti-reversion? Maybe BMW put them there for a reason but each runner was completely different so I smoothed them out with a sanding roll on a dremel.


You can see what I mean by the steps and bumps in the runners here. Steps are most noticeable on runner 4/5.