924Board.org

MikeJinCO · Posted: Wed May 10, 2023 3:54 am Post subject:

I have no idea if that applies to any other motor. He gave price dimensions for that motor, I believe many times thevalve seat is too wide.
_________________
Mike

'67 MG Midget Dp
'71 Ocelot Dsr Kawasaki 1000(under rebuild)

Mike9311 · Joined: 14 Dec 2004 Posts: 1678 Location: Chicago-ish

Mike9311 · Joined: 14 Dec 2004 Posts: 1678 Location: Chicago-ish

Sometimes being oblivious might be a good thing. Or as I tell my kids... 'the more you know, the more you will realize the less you really know'

I keep scratching itches and it only generates questions

increased 45 deg seat which takes the top cut away. Resulted in lower off the seat flow through mid-range lift but had an interesting side effect

It was a turbulence reducer. Any residual turbulence at the high end disappeared. Would just go up

Interesting test though! Thanks MikeJinCo
_________________
1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car

Mike9311 · Joined: 14 Dec 2004 Posts: 1678 Location: Chicago-ish

Ok, I am moving on with this head. I feel like it isn't exactly were I think it could be but more material removal isn't answer.

The next 931 head will likely be a 40mm since the biased flow efficiency pivot would happen at lower lift keep the flow efficiency higher throughout. With a 40mm the flow efficiency would start improving, after going down, at .394" lift rather than .413"

Same with a NA especially since its already offset toward cylinder center by 1.2mm vs 931 intake valve. The NA just might be the monster in the closet

If I do another 42mm offset head, I would change some things slightly with all that was learned. Its hard to compare to all the other heads out there with our lift levels being so low. Our lift window is all but ignored as 'low lift flow' by the general community. The ideas are the same but its important that you ignore comparing numbers.

I have a decent idea, or two, whats happening at .1" lift. I mean I literally know (slight loss in velocity even though over all its higher everywhere else). Will apply what was learned moving on

Finally I owe some testing for Cedric and Fifty ....and me of course

because I want to know too
_________________
1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car

Mike9311 · Joined: 14 Dec 2004 Posts: 1678 Location: Chicago-ish

I am glad I did all this so far but this is getting frustrating. Yesterday I made yet one more mold and while that was curing I ran the stock head again to get finer values at every 0.50" lift point (1.25mm) ...then everything went sideways

The stock head had better numbers yesterday, so to compare an apple to an apple, I pulled the mold on the ported head (it was now cured) and tested the ported head. This is minutes apart

Ported head performed as it had in the last test except one or two points down low it was actually worse, oddly worse. The stock head was beating it down low and the stock head had higher flow numbers than it had way back when first tested. This is the exact head? Never had discrepancies I couldn't explain until now. The data has been backing itself up nicely every retest of ported until now. Even installed a non-ported intake on stock head and values where within 1/2 cfm at each point.

This has been the itch I have had now in the depth of my mind... that I went too far porting wise back 10 pages or so on this thread.

Software shows what I am seeing. efficiency down on ported head until .2-.250" lift, or halfway through the curve. I lose up to 20-25% only to gain it back up top. Not at all the goal

Bottom line is I am really going to switch gears hard now. Stock head with 40mm and a mild clean up until I can figure this out with more time.
_________________
1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car

Beartooth · Joined: 05 Apr 2022 Posts: 212 Location: Roberts, MT

I've gotta say, I've learned a lot following you through this process. There's definitely not a pile of CFM on the table with these heads like there is with old American iron, but I'd think with all the work and research you've put in that you'd be seeing better numbers across the board. You definitely hit it right on the "RV cam," but of course, it's not so much an RV cam given the valves sizes. A small V8 like Ford's 302 has 1.78" intake and 1.46" exhaust valves (and it's undervalved if you want to rev it), compared to 1.57/1.29. That's 25% more intake valve area, and stepping up to a fairly typical 2.02" valve jumps you to 50% more area. Not opening a 2" valve at least .5 inch leaves a lot of flow on the table, where a 1.6ish" valve has probably stopped gaining flow at .5"

Anyway, I wonder if there's some "Ah-ha!" discovery that'd pull the gains you'd expect out of the work you've done. Also, I wonder what the dyno would say; sometimes the flow bench doesn't tell all... I'm curious how you'd build a head for a typical performance street 924 based on what you've discovered. If it were me and I were going so far as custom valves and all, I don't think I could resist going a bit bigger on valve head diameter and smaller in the stem. Of course, the 924 engine architecture is facing multiple bottlenecks, and at some point you're doing more work for less gain than sorting out an engine swap. That said, I think most of us are old and wise enough to know that engine swaps are easier said than done; we'd just like to be able to put some money into the head with reasonable assurance of another 10-20 HP (and maybe more, depending on supporting mods). I think you've done a great job exploring the possibilities here; I really appreciate your effort to share it, even if it's proving to be kind of frustrating. That's how most of my projects go!
_________________
1980 931 diamond in the rough

Mike9311 · Joined: 14 Dec 2004 Posts: 1678 Location: Chicago-ish

Mike9311 · Joined: 14 Dec 2004 Posts: 1678 Location: Chicago-ish

Couple of thoughts as I continue my progress.

My last test was the first time my graph resembled gegge's with the hump. In particular the stock blue line but in my case the ported version. This is a clear indicator of turbulence. In my case the port went from quiet to loud and any lift after that point had a slightly lower flow number. ...The hump

Also the stock head with intake only had issues down low in the first 3-4 lift points when flow was steady. Once it went turbulent the flow numbers matched without intake

I take this to mean my ported head is at steady flow with intake higher in the lift curve. The flow numbers with and without intake only match when it finally goes turbulent

still working on things plus I received my 9mm pilot for testing seat work on other heads. I am suspecting my seat work will make it worse but let us see what happens
_________________
1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car

Mike9311 · Joined: 14 Dec 2004 Posts: 1678 Location: Chicago-ish

Update:

I have been trying several port modifications with clay to see if there are dead flow areas, direct flow, and reduce unnecessary port volume with hopes that it might affect very low lift flow (right off the seat as valve opens). All of these have not really done much of anything. In a way surprising

For example, one test was to revisit the adding of a valve guide contour.

All this has caused an immediate shift in my process. This head will not be abandoned but it requires more time to work this all out properly.

I have already pulled another head I have that I can hopefully do just a basic run through. It needs to be skimmed due to its previous rough life. Almost looks like the PO had a head gasket go.

If this then gets up and running I will have something to compare it to and will allow me to sort the other fuel system mods etc. Might even dyno the car if it runs well enough. Then, ideally, if this offset valve head is ready, do a swap and test only a head change
_________________
1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car

Mike9311 · Joined: 14 Dec 2004 Posts: 1678 Location: Chicago-ish

I seriously have issues. Stuck valve in my tractor had me straightening push rods until they broke just to keep going. Couldn't take it anymore so I ordered a new head cheap

...And you know what happened next

Seriously though. Just a quick job to clean it up. There were casting burrs everywhere. No testing. Too small as well so it was kind of hard to get in there

_________________
1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car

Cedric · Joined: 27 Aug 2004 Posts: 2615 Location: Sweden

Haha, it do look a bit ugly though, a quick clean up surely help a little:) At least mentally it will feel better

_________________
1980 924 Turbo
www.instagram.com/garagecedric/

Mike9311 · Joined: 14 Dec 2004 Posts: 1678 Location: Chicago-ish

Mike9311 · Joined: 14 Dec 2004 Posts: 1678 Location: Chicago-ish

Comment on above post: tractor ended up having more power

It no longer bogged while doing the same or harder work. Glad I took the time to amuse myself

Anyway, things have been going on in the background. Originally I was machining the 6mm valve stem groove using a smaller radius tool on a CNC and never on a real stem up to this point. One issue that really played over and over in my head was surface finish. I never was happy and was frustrated to think I had to come up with a post process to improve the groove finish. The easy way is to polish while turning but that places polishing grooves (small scratches really) in the wrong orientation to the stem. Its like placing "please break here starter grooves". I thought of things like micro shot peening and/or polishing radially with a small tool so lines would be on same axis as stem. I disliked all these ideas just because it is extra work

Then I made the smartest move by researching and then purchasing other tooling which took forever. First, figuring out what I wanted followed by what I could actually get which had me buying from all over and then waiting. Still smart because surface finish is no longer an issue. I also believe the groove surface will be burnished in compression similar to peening a few seconds after the motor starts for the first time as long as the finish is decent to begin with and it really is...

Oh...the other problem was work holding a valve since you want to have the stem held close to where you cut the groove so its nice and rigid. The issue is the valve head. Your fixture has to hold the valve so the head can be "inside" the chuck with accuracy and repeatability. This was finally figured out so I could do it on a real valve and it was moved to a manual process away from the cnc. This picture shows the groove in a test valve that was already grooved so I could compare fit and finish. Worked so nicely and certainly worth all the trouble. I geek out on stuff like this so I had to share plus you all have to know I am still going to finish this!

_________________
1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car