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931 chamber question

 
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Mike9311  



Joined: 14 Dec 2004
Posts: 990
Location: Chicago-ish

PostPosted: Thu Jan 07, 2021 1:17 am    Post subject: 931 chamber question Reply with quote

I am pretty sure I already know the answer but i'll ask to confirm anyway....

What is the largest 931 chamber size anyone has opened their chamber to while still maintaining some sort of quench surface?

So far I have seen 6cc more than stock seems to be the limit. So starting at 20-ish, 26cc seems to be the limit?
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1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA M650 Herausnehmbares Dach
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car
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MikeJinCO  



Joined: 08 Jun 2010
Posts: 970
Location: Maysville, Colorado

PostPosted: Thu Jan 07, 2021 5:49 am    Post subject: Reply with quote

I'm taking my ported 931 head to the machinist in a couple of days. I didn't enlarge the combustion chamber at all, just polished with a cross buff. I my opinion the only valve shrouding is on the cylinder wall edges which stays shrouded all the way down the bore. When I put a head gasket on, the gasket edge was close enough to the cc that I just decided not to go there. I do wonder what that little square notch is for? Possibly part of the casting process? I hoping to pick up the air flow with 7mm stem valves and a 12 degree back cut rather than 20 on the intake.
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Mike
Poco a Poco, #033 '78 Vintage racer, SCCA Dp-81
'77 924
cricketdesigns.com
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Mike9311  



Joined: 14 Dec 2004
Posts: 990
Location: Chicago-ish

PostPosted: Thu Jan 07, 2021 6:36 am    Post subject: Reply with quote

Nice. Will be cool to see your results

From what I know the casting notches are locators for some part of the production process.

I plan on placing the head on the block with machinist dye on the face of the head so I can scribe the bore edge in the dye on the head face. I've done this before. I come from the bottom of the block with a scribe through the bore. This way I can carry the chamber edge right out to the cylinder bore edge.

I am working on a motor right now where I thought I could use off the shelf pistons with some chamber modification. Its doubtful now though. I needed a lot to bring the compression down. Every time I think I have a solution another issue arises. Will be calling the piston producer soon to check on options. It was a good option since it was a clean simple dish but only -15.5cc. A little over double that would be nice for me

Here is an example pic of my scribing on a ducati head. Took my chamber all the way out to the scribed line.




_________________
1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA M650 Herausnehmbares Dach
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car
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MikeJinCO  



Joined: 08 Jun 2010
Posts: 970
Location: Maysville, Colorado

PostPosted: Thu Jan 07, 2021 10:52 am    Post subject: Reply with quote

You're a much braver man the I.

I have read of piston experiments to find the minimizing top ring land thickness for pollution control purposes. If you could find some pistons that clear the deck than maybe the top of the piston could be cut int\7 to improve compression. My race motor with a NA head and has 931S2 pistons basically flush to the deck and using a .040 Cometic head gasket and it is running quite well. Local motor machinist says he doesn't like to go below .035" clearance for the pistons due to thermal expansion.
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Mike
Poco a Poco, #033 '78 Vintage racer, SCCA Dp-81
'77 924
cricketdesigns.com
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Mike9311  



Joined: 14 Dec 2004
Posts: 990
Location: Chicago-ish

PostPosted: Thu Jan 07, 2021 1:05 pm    Post subject: Reply with quote

MikeJinCO wrote:
You're a much braver man the I.

I have read of piston experiments to find the minimizing top ring land thickness for pollution control purposes. If you could find some pistons that clear the deck than maybe the top of the piston could be cut int\7 to improve compression. My race motor with a NA head and has 931S2 pistons basically flush to the deck and using a .040 Cometic head gasket and it is running quite well. Local motor machinist says he doesn't like to go below .035" clearance for the pistons due to thermal expansion.


It seems like .035" is the limit and RPM will be the killer. Just read a story on a 2.0L NA 300Hp Build were the first dyno attempt with 0.035" allowed the pistons to slap the head at 9000rpm. Shattered the piston. Rod stretch. Lower mass obviously helps along with good parts like a nice wrist pin for that purpose anyway

I will admit right now the idea I have works well for a very high compression NA but that's not what I need

You have a nice setup at 1mm / .040 ...It has to run nice!

I will keep at it
_________________
1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA M650 Herausnehmbares Dach
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car
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View user's profile Send private message
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