731 head for NA eta?

[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

[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 ?

[2mAn]

On the subject of the intake manifolds, I believe Forcedfirebird did a whole bunch of dyno runs comparing the different manifolds. Might want to search for that

[nando]

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.

they probably gasket matched it. the gasket is a lot bigger than the port on the manifold, so that would explain why they would have to hog it out.

[digger]

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 ?



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 ?

google laying back short side radius or look for books on porting. either electric or air die grinder dont waste time with a dremel and similar toys. you can measure each runner with varius tools to see if they are the same, use templates etc some areas you can judge by look and feel

[jeffnhiscars]

google laying back short side radius or look for books on porting. either electric or air die grinder dont waste time with a dremel and similar toys. you can measure each runner with varius tools to see if they are the same, use templates etc some areas you can judge by look and feel

I used to build wooden boats so have a pretty good eye for contours. I've looked at bits and seems there are lots of options. Any recommendations for the basics and how many would you likely go through on a job like this ?

[Hooffenstein HD]

I was intending to use a b25 throttle body and afm 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 ?

You throw the 731 manifold in the aluminium scrap bin and pick up an 885 manifold.

I have a 2.7i with 731. At first I installed it with 731 manifold ground out as much as possible for an 885 throttle body but it always felt like it was strangled the higher the revs climbed. Swapped to 885 manifold and it pulls clean through the rev range. Still feels a little choked above 5,500RPM but I guess that's normal for a stock M20 head especially with the smaller 731 valves and stock cam.

[digger]

I used to build wooden boats so have a pretty good eye for contours. I've looked at bits and seems there are lots of options. Any recommendations for the basics and how many would you likely go through on a job like this ?

i don't port my own heads, but did play around years ago and toy with the idea of building a flow bench but time and space limitations quashed that. if you mean cutting bits carbide burrs will do a whole head easily. i have a set of different shaped burrs that i used for grinding the odd manifold and other bits and pieces.

if you only have 1 head id be hesitant in doing it yourself as if your screw it up well its screwed and i dont like your chances of finding any scrapped 731 to practice on. most of the gains come from the seats and areas within an inch from there so without a means to cut different seats, valve profiles and measure a positive change in airflow you may not accomplish what you set out to.

[Hooffenstein HD]

if you only have 1 head id be hesitant in doing it yourself as if your screw it up well its screwed and i dont like your chances of finding any scrapped 731 to practice on

I'd have to agree. About the most I'd do on my own is clean up/smooth out the rough machined area directly behind the valve guide. Not sure that it would actually achieve much but it looks nicer.

[jeffnhiscars]

the machine shop I work with was where the local BMW dealer (owned by Rick Hendrick) sent all their heads til they started just replacing them so he will do guides, seats, faces, decking, enlarge valves etc...basically all the machining.

The porting I'll have to think about. If it's just a bit of clean up I'd much rather buy the tools and do it myself since I have no idea who to trust or if they really know an M20'from a big block (remember I'm in nascar country)

What I'm hearing though is that a 731 with 885 valves and TB Just begs for an 885 intake.

You throw the 731 manifold in the aluminium scrap bin and pick up an 885 manifold.

I have a 2.7i with 731. At first I installed it with 731 manifold ground out as much as possible for an 885 throttle body but it always felt like it was strangled the higher the revs climbed. Swapped to 885 manifold and it pulls clean through the rev range. Still feels a little choked above 5,500RPM but I guess that's normal for a stock M20 head especially with the smaller 731 valves and stock cam.

i don't port my own heads, but did play around years ago and toy with the idea of building a flow bench but time and space limitations quashed that. if you mean cutting bits carbide burrs will do a whole head easily. i have a set of different shaped burrs that i used for grinding the odd manifold and other bits and pieces.

if you only have 1 head id be hesitant in doing it yourself as if your screw it up well its screwed and i dont like your chances of finding any scrapped 731 to practice on. most of the gains come from the seats and areas within an inch from there so without a means to cut different seats, valve profiles and measure a positive change in airflow you may not accomplish what you set out to.

I'd have to agree. About the most I'd do on my own is clean up/smooth out the rough machined area directly behind the valve guide. Not sure that it would actually achieve much but it looks nicer.

[jeffnhiscars]

I've been quiet here after the seller of the 731 went dark as soon as unpaid him (funny how that works). The good news is that the head did arrive and aside from a couple of bent studs it looks to be in fine shape



The rockers slid out nicely and there's enough meat on the valves that I'd say the head has never been worked on. In light of mad hatters comment about the 885 TB on the 731 intake here's a pic of t
The difference. It's 8mm total and the only place there isn't 4 to remove is at the very bottom. I'm guessing that's a 95%+ match which, given what a hybrid this thing will be, would seem to be close enough. This is especially true given that I may keep the rest of the stock ETA externals til I can acquire all the b25 parts.

[jeffnhiscars]

I broke down my 731 today and was surprised to find the exhaust valves were only 34mm. It's listed at 36 in some of the material I've read. Clearly not the case



it looks to even my untrained eye that there is room for a quite a bit of cleanup in the ports. Sure doesn't look like the one Rama did.

[jeffnhiscars]

Id appreciate some input as to whether I can put in larger valves without changing the seats.

[Hooffenstein HD]

The exhaust ports are much worse than the inlets for requiring clean up but yeah, that rough area behind the valve guide is what I was referring to when I said there's some clean up you can do yourself.

I think you can do the 42mm valves but you also need to unshroud them to make it worthwhile and blend the seats in.

More gains will be had by laying over the short side radius but without flow bench and proper tools I wouldn't attempt it.

[jeffnhiscars]

There's a very pronounced lip on the short side on at least 2 cylinders but I'd need to get a mirror to take a pic of it. I'm sure you know the spot I'm referring to. There looks to be enough room to unshroud the valves and I agree that in the end a flow bench is a must.

I may do some of the light stuff then take it to someone like Kormans to fine tune it on the flow bench. they are an hour from here

Would I use the b27 HG ?