325i cylinder head inlet port

[driftwood]

Ever wondered what the inside of a 325i cylinder head inlet port looks like? Me neither, but I got bored so decided to find out. I don't get out much...

[reggid]

have done that to all three heads....

[driftwood]

Bloody hell! And I thought I was bored... lol. The 885 inlet port has a big divider ridge along the top of the port that I was tempted to reduce but looking at your sections, there isn't a lot of meat on the port walls. I think I'll restrict myself to a little bit of blending. I'm in the process of doing a cast of the exhaust port. Have you done the exhaust ports too?

[reggid]

the exhaust ports are all similar but the 885 is smoother and accommodates a bigger valve. so nothing like the differences on the inlet side. i think an ETA head with its steeper port angle and smaller size might have more potential for a small capacity M20 than the other two but there needs to be alot of time spent porting it

as for the inlet, a nice multi angle valve job, blending and improving the short side is all it needs. the 885 inlet is prone to flow seperation on the short side at real pressures encountered in a running engine

[e21jps]

something interesting to note:
the 200 head came standard on all e21's with mechanical injection which has a big spring loaded flap in the intake and old school transistor ignition, max power was about 140hp and torque was 151ftlb

Then on the e30 they went to the 731 head along with EFI, possibly less restrictive intake and bigger ports, max power went up 500rpm higher but only increased about 10Hp and max torque stayed exactly the same @ 151ftlb......

so this has to make one wonder why.... essentially the port size is the only difference between the two engines cam etc is exactly the same. you would assume that the EFI must have some power benifit. This at least shows that the smaller 200 ports were not costing any performance at all

[reggid]

and for the 731 port the flow per smallest port CSA is actually higher than on the 885 so the flow on the 731 is limited by the smallest CSA which necks down quite a bit near the guide and of course the smaller valve. Imagine putting a really large valve in the 731 the short turn apex would rise substantially away from the valve which is the aim when you can only lay back the radius so much.

[HairyScreech]

the exhaust ports are all similar but the 885 is smoother and accommodates a bigger valve. so nothing like the differences on the inlet side. i think an ETA head with its steeper port angle and smaller size might have more potential for a small capacity M20 than the other two but there needs to be alot of time spent porting it

as for the inlet, a nice multi angle valve job, blending and improving the short side is all it needs. the 885 inlet is prone to flow separation on the short side at real pressures encountered in a running engine

This, and the casting mismatches in the ports produce huge eddys in the flow as well, even 28" water is enough to pick up on the turbulence caused by these bits, as can be seen in the flow tests i have done, the stock port shows a noticeable dip due to turbulence. They also howl on the flow bench which is a sign of flow separation.

Valve seat size is the sticking point with the 731, if the seats were the same size as the 885 then porting would be a lot cheaper, the cost of seat etc could easily to the cost of going for a larger capacity.
However if class regs limited you to a capacity then it maybe the one to look at.

something interesting to note:
the 200 head came standard on all e21's with mechanical injection which has a big spring loaded flap in the intake and old school transistor ignition, max power was about 140hp and torque was 151ftlb

Then on the e30 they went to the 731 head along with EFI, possibly less restrictive intake and bigger ports, max power went up 500rpm higher but only increased about 10Hp and max torque stayed exactly the same @ 151ftlb......

so this has to make one wonder why.... essentially the port size is the only difference between the two engines cam etc is exactly the same. you would assume that the EFI must have some power benifit. This at least shows that the smaller 200 ports were not costing any performance at all

The 323 motor with its 40mm valves is even more choked by the valves than the 325, when they increased the port size they did nothing to address this, so the choke point remains mostly the valve.
Neither head is the ideal shape however so some of that gain comes from better utilization of the valve at the expense of velocity in the port.

None of these heads are that good it always suprises me the clumsyness of the pinch by the valves, it doesent even try to flow into the bowl.

reggid - your point about the large valve in the 731 is very valid, dramatic improvements to the short side could be made which would be great for a small capacity m20.
I must say i think more benefit could be found in both the 885 and the 731 by adding material though.
From experimentation there is a large dead area that can be filled reducing the 885 port to ~38mm diameter without losing flow.

[reggid]

I must admit to not being present during any tests so have not heard how noisy the port is on the 885 and my data has not shown a drop in flow with lift more just a plateau, but have seen other data that exhibits this. I don’t know if some of it is in test procedure or whether many don’t use a true 28”a test.

I must admit I don’t like a clay bellmouth for testing no matter how consistent it is I don’t think it is correct. I feel the clay is too close to the short side turn, and length difference between the short side and roof as a %age is artificially high unless there is a piece of tube reasonable length to ensure that the flow before the turn has become fully developed with the most representative velocity profile across the section.

As far as the 731 goes I sectioned up the silicone mould I did (shown above) and the minimum cross sectional area of the port which is near the guide. It is 750mm^2 (using graph paper) which is equivalent to about D30mm. With a stock 40mm inlet valve at 6mm lift the curtain area becomes greater than the port min CSA so the port area starts to limit the flow in high lift area. This is displayed in the flow tests of the 731 that I have seen where it plateaus a couple of mm higher at about 8mm. Fitting bigger valves and reshaping the bowl, shortside etc will improve flow but you still need enlarge the port some in the narrow region near guide to get the flow at high lifts IMO.