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Combustion chamber designs/modifications?

 
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Paulyy  



Joined: 05 May 2014
Posts: 87
Location: Melbourne

PostPosted: Mon Sep 11, 2017 1:53 pm    Post subject: Combustion chamber designs/modifications? Reply with quote

Anyone attempt to put a combustion chamber into a 924 n/a cylinder head? or modify the 931 cylinder head combustion chamber?
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Paulyy  



Joined: 05 May 2014
Posts: 87
Location: Melbourne

PostPosted: Mon Sep 11, 2017 10:55 pm    Post subject: Reply with quote

I was thinking of doing something like this. Similar to the 931 head combustion chamber, and similar tot he LS1 heart shape.

This one is 5mm deep but i think that may be to deep for the n/a head.
This is going off a 87mm bore and 44mm intake and 37mm exhaust valve sizes.

Thoughts appreciated.


combustion chamber 1 by paul ilieski, on Flickr

combustion chamber 2 by paul ilieski, on Flickr

combustion chamber 3 by paul ilieski, on Flickr
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MikeJinCO  



Joined: 08 Jun 2010
Posts: 724
Location: Maysville, Colorado

PostPosted: Tue Sep 12, 2017 12:00 am    Post subject: Reply with quote

I noticed when that race car was for sale a couple of years ago that the 931 head on it had some shallow half moon shaped dishes cut into the sides of the intake and exhaust valves. When I get back to testing working on my next NA head I am going to try that. One of the problems with the valve seat is that the valve is slightly depressed into the head and the angle of the depression is maintained at 45 degrees rather than flare out. I failed to find a 120 degree cutter when I did the last one, then saw those pictures . I won't bother with the side near the cylinder wall as it is shrouded anyway. In David Vizard's book on porting he wrote that he did the same on an old Heron head Chrysler in the UK with great results. Of course it did slightly lower the compression ratio. I'll be working on an old 38mm intake head so I can ruin it with no great loss. I'm also going tc fill the bowl in the intake port as that is what David Mitchell did with some sucess.

My NA motor with 931S2 Pistons and zero deck clearance has a CR of somewhere around 11.3 so a little loss of CR won't hurt anything if it helps. Until proven otherwise it will get AV gas

Unfortunately with one car I that just got a motor in the hole yesterday and the track car very slowly getting a new wiring and brake systems it will be quite a while to get there.
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Paulyy  



Joined: 05 May 2014
Posts: 87
Location: Melbourne

PostPosted: Tue Sep 12, 2017 12:22 am    Post subject: Reply with quote

MikeJinCO wrote:
I noticed when that race car was for sale a couple of years ago that the 931 head on it had some shallow half moon shaped dishes cut into the sides of the intake and exhaust valves. When I get back to testing working on my next NA head I am going to try that. One of the problems with the valve seat is that the valve is slightly depressed into the head and the angle of the depression is maintained at 45 degrees rather than flare out. I failed to find a 120 degree cutter when I did the last one, then saw those pictures . I won't bother with the side near the cylinder wall as it is shrouded anyway. In David Vizard's book on porting he wrote that he did the same on an old Heron head Chrysler in the UK with great results. Of course it did slightly lower the compression ratio. I'll be working on an old 38mm intake head so I can ruin it with no great loss. I'm also going tc fill the bowl in the intake port as that is what David Mitchell did with some sucess.

My NA motor with 931S2 Pistons and zero deck clearance has a CR of somewhere around 11.3 so a little loss of CR won't hurt anything if it helps. Until proven otherwise it will get AV gas

Unfortunately with one car I that just got a motor in the hole yesterday and the track car very slowly getting a new wiring and brake systems it will be quite a while to get there.


I never saw it, Got any photos or links?

Interesting though.

Unfortunately i have nothing to compare to. I have a bone stock 924, and one i am building. If i slowly build on the stock engine, i could see HP gains. But i rather just build right on the motor i have on the stand.

But i think having a combustion chamber would be beneficial if done right. In my image is 5mm deep, but i think its to deep for the design and space. 2-3mm might be better.
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Mike9311  



Joined: 14 Dec 2004
Posts: 661
Location: Chicago-ish

PostPosted: Tue Sep 12, 2017 4:46 am    Post subject: Reply with quote

I am doing this very thing right now on a set of stock 931 heads. I will be welding up the chamber and reshaping along with new valves and seats.

This is the step I wanted to take before going full billet and I hope to have updates after the Chicago gathering when I switch gears.

I have seen a few chamber mods but none like the one you show.

Put some paint on the head surface and mount to a block. Reach through a piston hole and scribe the cylinder diameter into the paint on the head. Carry the bowl only out to the scribed line..no more
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Paulyy  



Joined: 05 May 2014
Posts: 87
Location: Melbourne

PostPosted: Tue Sep 12, 2017 5:17 am    Post subject: Reply with quote

Mike9311 wrote:
I am doing this very thing right now on a set of stock 931 heads. I will be welding up the chamber and reshaping along with new valves and seats.

This is the step I wanted to take before going full billet and I hope to have updates after the Chicago gathering when I switch gears.

I have seen a few chamber mods but none like the one you show.

Put some paint on the head surface and mount to a block. Reach through a piston hole and scribe the cylinder diameter into the paint on the head. Carry the bowl only out to the scribed line..no more


My design is based on the LS1 heart shape. I have a bit more work to do on the design, but I'll use the cnc to cut out the bowl shape on some MDF to see how it is overall and make changes.

I thought about using the piston to get the shape, But am going aftermarket pistons. And you can get them custom made. I could go more oval bowls like a 944 turbo as they seem to work well. or half oval and the other half with valve reliefs similar to the combustion chamber design.
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MikeJinCO  



Joined: 08 Jun 2010
Posts: 724
Location: Maysville, Colorado

PostPosted: Tue Sep 12, 2017 5:24 am    Post subject: Reply with quote

I don't know how to get a link done. Google "Porsche 923/933 factory race car#2/16" it is Don Mwluzio's old car. It is a 931 also shows a piston which looks basically flat topped with a cutout for the exhaust valve. Other pics also. The link goes thru Pelican board.

My thoughts were taking this idea to the NA head relieving the area adjacent to the valves which are slightly into the head.
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Paulyy  



Joined: 05 May 2014
Posts: 87
Location: Melbourne

PostPosted: Tue Sep 12, 2017 5:46 am    Post subject: Reply with quote

MikeJinCO wrote:
I don't know how to get a link done. Google "Porsche 923/933 factory race car#2/16" it is Don Mwluzio's old car. It is a 931 also shows a piston which looks basically flat topped with a cutout for the exhaust valve. Other pics also. The link goes thru Pelican board.

My thoughts were taking this idea to the NA head relieving the area adjacent to the valves which are slightly into the head.




his combustion chamber isn't to far off where i'm trying to go with mine.
Not sure about those pistons with the exhaust valve relief but flat top is for the NA engine.

My design isn't complete, just trying to get the ball rolling as we have already. I'll revise the drawing and cut it on mdf to see how it looks in person.
I'll probably widen it and go more of an angle on the edges. current design would probably choke the airflow around the valves.
I think the 931 heads have a 4mm deep combustion chamber.
Can anyone confirm this?
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MikeJinCO  



Joined: 08 Jun 2010
Posts: 724
Location: Maysville, Colorado

PostPosted: Wed Sep 13, 2017 12:18 am    Post subject: Reply with quote

Just my opinion, but I think you are going to shroud the valves badly and the head with its vertical valves is already quite shrouded. I think that the larger intake valve just makes it worse.

I once measured my 931 head and it was 5 mm deep and other people have measured about 22cc in volume .

Head design has advanced by magnitudes since those 931 heads were done, but the idea of relieving the sides to gain more open area has been the key to improving the flow and volumetric efficiency. Those pictures show their attempt to do that. Having much more direct(straight) flow is probably more important, but making an angle valve is head out of the question for me.

As far as the valve cut out and piston is concerned, back then very little was understood about quench so it appears they had bit of deck clearance with a flat top piston . Today the accepted ideal quench is about .040" . That is commonly achieve with a head gasket only and sometimes the piston even protrudes above the block. It will be interesting to see how my motor adapts to having modern quench numbers, I don't know if that ever has been done before. At overlap the piston is chasing the exhaust valve which grows due to heat. With a higher overlap cam it is probably necessary to relieve the exhaust side to ensure that the piston doesn't get to the cam. On my 931S2 pistons and a NA head we had to valve pockets that were 5mm deep. We checked and confirmed that by a pre assembly and clay on top of the piston. With a higher overlap cam it would have needed to be deeper as with a stock cam the piston got quite close to the valve. Our valve pockets were on the better safe than sorry idea.
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Paulyy  



Joined: 05 May 2014
Posts: 87
Location: Melbourne

PostPosted: Wed Sep 13, 2017 4:46 am    Post subject: Reply with quote

Got any photos of what you're doing. Would like to see?


I'm going for something like these between the 2.

Note how the bottom folds between the valves (bottom of a heart)
The top one would probably be something similar what im going for.
The second one shows you how big the walls around the valves are in the combustion chamber, but thats a more angled head vs the first one.




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