924Board.org

[Mike9311]

Arvidw

I have also done the same in the past and believe a small combustion chamber (shallow chamber) could be machined and/or formed by hand for the NA. I am about to do an Ideola big valve head on a 931 head with some major mods there and I have been welding up an NA head along the lines of what you show. The 90 deg abrupt angle in the NA head always bothered me as well as the need to raise the roof on the intake like you show.

Right now there is a mildly ported NA head on the 80 924 while I work on a better one with the welded up version. I hope to cnc a small chamber on that one and we will see how that goes but first the 931 head. The 931 head will be unshrouded especially on the intake side like your nice picture shown above.

I like you valve tool. There is always a better way to build a mousetrap
_________________
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

[Arvidw]

Do you happen to have a couple of pictures of your (mild) modifications? I couldn't find a project topic of yours with these modifications.

I'm not quite sure what shape your shallow combustion chamber has but I'll sketch my own idea in CAD and show you. I'm currious what you think about it. In the mean time i'll have the CAD software calculate increase in CC of the combustion chamber.

Edit: Did a litlle bit of CAD drawing and came up with the folowing:



A very rough calculation shows a significant loss of compression. (can also be concluded from only looking at the picture ) Tomorrow i'll improve the shape and take some more time for the determination of the compression ratio (with thinner gasket)

I don't think this is the correct shape for me, i need something much more simple because I'll have to be able to make 4 identical ones by hand. Maby enlarging the chamfer around the valves + smoothening around the spark plug is sufficient.?
_________________
Porsche 924 2.0 Kompressor 136.1wHp @ 5650rpm / 201.9wNm @ 3900rpm (dyno @ dp-engineering.nl)

[Mike9311]

[Mike9311]

I was thinking more along these lines but at the same depth you show. The idea is to unshroud the larger intake valve just as it opens and closes. Imagine its position right at those times. At 1mm of lift, for example, right as it comes off the seat. The flow is blocked but opening the chamber around it will help at those times. The exhaust side is done a bit too and you will need custom pistons to bring back compression you will lose with these mods.

From www.utzon.se (an awesome site!) Of course these are 931 based but the shape concept is correct and typical no matter what brand of automobile people modify.


The current mild mods include put a secondary angle on the stock valves. This will help again at these low lift points. I hope you can see this in this comparison picture. The left is stock and the right side shows the sharp edge broken to help transition the valve shape to the seat angle.



The only other picture I have is terrible on any real detail. Sorry about that. I wasn't too interested in these heads. They are only to get the car on the road while another set is built.



One more tip that I will be doing soon on the 80 931. You can blue or red machinist dye the head surface and put the head on the block. Then scribe the outer perimeter of the cylinder bore (don't use a head gasket!! It may have too large a diameter which will hurt instead of help). Then make the head chamber out to your scribed line.

See these motorcycle example pics I did of that process (Ducati heads):






Use old valves to protect the seats while you work. The arrows show the sharp edges from the casting process. Like I need detonation source help.




Motorcycle intake valve backcut example


There is an argument on backcutting exhaust since a sharp corner could help with reflections. Also, and this is super important, too THIN a valve at that area could burn the valve quickly so be careful and think about what you are doing especially on the exhaust valve if you choose to do something 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

[Arvidw]

I really like the modifications you have made to the cilinderheads you are showing. I think i'll try a litlle bit of all the modifications you have made. I hope it turns out as well as yours did

Recently i've bought GSXR600 K3 throttle bodies complete with injectors, wireing, secondary butterfly valves and sensors.



I chopped up an intake manifold with a hacksaw just to get rid of the plenum. The final cut will done somewhat more professionally





As you can see the bodies do not line up with the manifold just jet but after chopping the manifold again near the green line the transition should be quit okey.





I was pleasantly surprised by the inner diameter of the intake manifold. I think that after the 2th cut the diameter will be around 38mm which is exactly the diameter of the throttle bodies.


I've chosen not to space the bodies themselves because then the linkage, fuel rail and wireing must be modified. Secondly a brand new manifold has to be made which I find too much work for the marginally better results is is likely to give
_________________
Porsche 924 2.0 Kompressor 136.1wHp @ 5650rpm / 201.9wNm @ 3900rpm (dyno @ dp-engineering.nl)

[Arvidw]

[Joes924Racer]

Cause its a big job, itll take time sure.. I like it though.
_________________
1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

[Martijnus]

That looks nice!
I've been thinking about using a stock manifold too, but since I'm currently using a 931 head and I don't have a manifold, I'm building one from scratch.

Seems you've got the same tools at hand as I have Sometimes you have to be creative because you are limited in tools and that's part of the fun.
_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)

924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment

[Joes924Racer]

The CAD drawings ! Theres a nice tool. Those
are a healthy set of carbs by the way. Cheers.
_________________
1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

[Martijnus]

btw, I just realized... I'm using the exact same TB's iirc.

The secondary valves aren't controlled by a stepper motor but a solenoid/servo... it only has 3 different positions.

much easier to control than a stepper motor I've got a drawing somewhere for MS to control it with the generic outputs and a set of relays.
_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)

924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment

[Arvidw]

@Martijnus, I think that building your own manifold is better for your application. My manifold ibt combination will take a lot of space which you might need for the alternator. I hope that there will be enough space for a supercharger with my IBT solution...

The secondary throttles are indeed controled by a electromagnet. I'll ask my brother to build me an electrical circuit to control this with my megasquirt (whithout steps) He told me it was not that hard with a current limiting circuit in combination with the secondary throttle positioning sensor

I would like the secondary throttlebodies to work like the sliding valves in a Constant Vacuum carburetor. Maby do something with pressure differencence... This is ofcourse not the main priority, getting the IBT's installed and working is more important
_________________
Porsche 924 2.0 Kompressor 136.1wHp @ 5650rpm / 201.9wNm @ 3900rpm (dyno @ dp-engineering.nl)

[Martijnus]

[Arvidw]

The IBT's are in, is it normal that the engine on IBT's idles at 65kPa?

I thought engines should idle between 20-50kPa? Before I installed the IBT's my engine idled at 50-55kPa which was allready quite high. Compared to a modern 2l 4cil car it creates much more exhaust pressure, engine design or worn parts?

The engine idles great: 650-700rpm without any difficulties. (vacuum was tested at 650 and 950rpm) The throttle bodies are synchronised.

The throttle response is good however the acceleration enrichment needs some work.
_________________
Porsche 924 2.0 Kompressor 136.1wHp @ 5650rpm / 201.9wNm @ 3900rpm (dyno @ dp-engineering.nl)

[Martijnus]

[Arvidw]

At first i picked up the MAP using the original vacuum hoses and connections on the IBT's. Because the readings where so high 65-70kPa i moved the pickup point to the CIS injector holes but that didn't make any difference.

"alpha N" is the injection method? Does that influence the MAP value?

I have replaced the stock ignition for EDIS ignition. I'm keeping it at 700rpm because it idles so nicely and it reduces engine noise quit dramaticly compared to 950rpm


How dit you hook up the throttle cable to the IBT's? The IBT's need a stroke of around 20mm and the pedal in the car gives around 40.

I've shortend the lever which pulls the throttle cable. This works however it aint very neat. However I need a stronger retraction spring near the throttle bodies. (maby fabricate something with the second throttle cable that can be installed at the throttle bodies) Therefore i would like to know how you solved this problem
_________________
Porsche 924 2.0 Kompressor 136.1wHp @ 5650rpm / 201.9wNm @ 3900rpm (dyno @ dp-engineering.nl)