Let's see how much m20 heads suck.

[ForcedFirebird]

A little. Problem is, if you want to make real power with an m20, you ditch the stock intake (where most of that would benefit). With the understanding that I do this for a living, I don't always take the time to prefect stock situations. I am often at my shop before 7am, and rarely get to leave much before 9pm - many a nights I work all night and take cat naps. Too many walk-ins and interruptions during the day to get much done. I have two very part time subs, but the majority of the hands-on work that goes on is done personally. I was in front of the bench for 12hrs straight yesterday. :/

[So Live]

John, you guys ever play with plastic work on the floor of the intake ports? In my opinion it looks like there is some room for improvement in the top of the port but volume is already borderline too large for a 2.5 from what I gather. So why not clean up the top of the port, fill the floor slightly to maintain volume, and also benefit from increasing the ssr at the same time?

Cheers,

[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...

[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]

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. :/

[KrautnotRice]

Very cool!

[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.

[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.

[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

[KrautnotRice]

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

[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.

[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

[xxlbimmer]

m20b25 with a 1 mm overbore at most!

[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

[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