Let's see how much m20 heads suck.

[Scallywagon]

With respect to the numbers shown, what rpm range( normally aspirated) would the 885 head flow to. Before it became the limiting factor.
Looking at building a 12:1, 13mm lift M20 for racing.

I would say you need to look for a 731 head and do your research on how to work the bowl, and the combustion chamber for larger valves, or hire someone who gets it.

I believe the 885 will absolutely be a limiting factor (literally a bottle neck) for your desired range.

[xxlbimmer]

Cool info. Where would I find a 731 head casting?

[Scallywagon]

Cool info. Where would I find a 731 head casting?

323i m20's in the states and e21 320i-e30 323i abroad I believe

[digger]

With respect to the numbers shown, what rpm range( normally aspirated) would the 885 head flow to. Before it became the limiting factor.
Looking at building a 12:1, 13mm lift M20 for racing.

rpm range will depend on what size engine you are talking

[xxlbimmer]

m20b25 with a 1 mm overbore at most!

[Manu329is]

Any news about the eta manifold? Im actually changing my enlarged manifold because of your post :D and because my ported head (cnc machined) cracked and is replaced by a stock one with some minor changes on the bowl and "combustion chambers" (is this the right word?).



Nice but cracked (twice, even after welding), and maybe to much ported

[ForcedFirebird]

Back from the dead. Thousands of flow tests later and we have learned quite a bit.

First off, the heads have a huge range from the factory and the valve job make or break them. We see a range from 165cmf to 181cfm from the wort to best flowed to date (max peak). Weather or not they have ever been rebuilt or had valve jobs is difficult to tell on these old machines.

We have tested the Ireland, Super Tech and stock valve comparisons, as well as purchased several Serdi multi-angle single insert seat cutters, cut the seats fresh with differing angles (qty and degrees), then flow tested, then removed material to suit, re-flowed etc etc. There is so much data, probably just easier to mention highlights, most of the tests have been very small differences.

Big bore throttle body (measured 64.5mm) showed zero increase in flow, in fact the numbers matched verbatim to the stock 60mm (~59mm). But, when putting a 48mm eta TB in place, the flow topped out at 90% if an "i" TB, so we know the stock TB really isn't a choke point on stock components. Only thing one would gain is a very slight increase with tip in throttle response, and maybe low RPM HP gain, but would have to be measured on a dyno. We are not going to waste research/time on a dyno for the m20 bbtb, as we tried it on a GM car and it only showed a 2hp difference, but that can happen in back to back runs, not consistently measurable.

The factory manifold drops a bare head flow about 5% average over the lift range, but as much as 8% in areas, obviously, in the higher lifts.

Most areas in the chamber seem to hurt flow when material is removed, so bottom line is, you really need to flow these heads if you plan on modifying them. Best thing to do otherwise is a good stock 3 angle valve job and rebuild.

[KrautnotRice]

Super interesting thread, thanks for sharing your research.
M20 POWAAAAAAH :)

[digger]

a nice backcut on stock chunky valves also produces good results. aftermarket valve results might be slightly different dependant on shape as some are more nail head than tulip shape

[ForcedFirebird]

Slow on presenting data, but here's a comparison valve job only on a stock port with a stock valve. This is using the 5 different cutters, we tested both intake and exhaust with these cutters to determine what ones work best with what heads/valves. We now have to change all our advertised numbers as they were increased. Forget that it says Chevy, this is an m20 head, just always forget to change it in the software.

[ForcedFirebird]

This is an example on a v6 head, but we get similar results from an m20.

Valve job alone with selected cutters based on flow numbers.



and modified with a second cutter, then by hand.



and the results.

[KrautnotRice]

Very cool!

[ForcedFirebird]

So, I'm flowing a stack of stock heads to hand-pick Spec30 cores. Yesterday and AMC casting came across the bench. Didn't go beyond the second port, it was as bad as the first, and don't think the others would be far off. Test 1 is port #5 on the previous head for comparison sake (accidentally deleted #6 but it wasn't far off). Basically this shows the flow in the AMC is 73% as good as stock at .4" of lift. :/

[TobyB]

WOW! Thanks, that's good to know. Now I don't feel so bad about leaving
a very new- looking one at the junkyard...

t

[ForcedFirebird]

WOW! Thanks, that's good to know. Now I don't feel so bad about leaving
a very new- looking one at the junkyard...

t

At JY prices, you should have grabbed it honestly. They have thicker castings, so are less likely to crack in the typical 885 fashion, and can actually be modified to flow more than a stock head - just they aren't as good out of the box as far as flow is concerned...