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931 Pistons and a NA head?
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MikeJinCO  



Joined: 08 Jun 2010
Posts: 704
Location: Maysville, Colorado

PostPosted: Tue Oct 04, 2016 12:39 am    Post subject: 931 Pistons and a NA head? Reply with quote

Has anyone ever put together a motor using 931 pistons and a NA head to make sure that the valves don't hit the pistons during normal operation?

The 931 head has about .200 or so of valve movement before it gets down to the level of the block the, the NA head essentially zero.
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Fifty50Plus  



Joined: 28 Feb 2008
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Location: Washington DC area

PostPosted: Tue Oct 04, 2016 2:39 am    Post subject: Reply with quote

With that combination, you create an interference engine but it is doable - just make sure to use a new timing belt. There are no valve pockets in the turbo pistons so you could bend exhaust valves if the cam timing was off or a broken belt. You will wind up with a lot higher compression ratio than the stock NA so you can reduce that somewhat with a thicker head gasket (see Ideola's Cometic head gasket).
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ideola  



Joined: 01 Oct 2004
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Location: Woodstock IL

PostPosted: Tue Oct 04, 2016 3:54 am    Post subject: Reply with quote

I have never built one, but it is a cheap way to get a higher compression motor. I have all of the configurations pre-loaded into my compression ratio calculator:
http://garage.ideola.com/top-DL-CompressionRatioCalc.html?13

I would be very careful about testing this setup with clay before running it. Since you don't have the valve reliefs in the 931 slugs, there is a chance that at full lift the valves could interfere with the quench rings on the 931 slugs.
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morghen  



Joined: 21 Jan 2005
Posts: 7807
Location: Romania

PostPosted: Tue Oct 04, 2016 5:12 am    Post subject: Reply with quote

I have a set of ROW S2 pistons....i need a NA to rebuild with those pistons.
11.6:1 CR sounds like a fun NA engine.
Perhaps a lighter flywheel and and headers would make things even nicer.
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Lau Christophersen  



Joined: 17 May 2013
Posts: 97
Location: Denmark/Copenhagen

PostPosted: Tue Oct 04, 2016 7:10 am    Post subject: Reply with quote

Could be fun - I have both S1 and S2 pistons laying around - need to find a 924 NA and try it
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MikeJinCO  



Joined: 08 Jun 2010
Posts: 704
Location: Maysville, Colorado

PostPosted: Tue Oct 04, 2016 7:48 am    Post subject: Reply with quote

What I'm going to do. I have a block in the engine stand, I'm going to install the crankshaft with a couple of used main bearing to locate it, then install a S2 piston without rings in hole #1. Then I'll put a little clay(very little)on the quench area of the piston. Then I will put on a head gasket and head and set up the cam timing with a belt, turn it over and see if I can turn a full cycle of the cam, My fear is that the valves will interfere with the piston as the crank comes up on the off stroke ie at 360 degrees of the 720 degrees of a full cycle.

As far as an interference motor goes, even my 2000 Chevy Tracker(Isuzu Geo) which except for the motor is as sophisticated as a 1950 Jeep has an interference motor. The machinist near me does 50 percent of his work on Subaru's and VW heads due to either broken belts or unchanged belt rollers.

Our problem is that pistons are more expensive and all built to order,
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chris79  



Joined: 29 Jan 2014
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Location: milan

PostPosted: Wed Oct 05, 2016 1:30 am    Post subject: Reply with quote

and for reach 2200 cc, the work is too complex and expensive ?
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ideola  



Joined: 01 Oct 2004
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Location: Woodstock IL

PostPosted: Wed Oct 05, 2016 2:06 am    Post subject: Reply with quote

chris79 wrote:
and for reach 2200 cc, the work is too complex and expensive ?

Yes. The only way to do that is with bigger (custom) pistons, a stroker crank, and custom rods. At least $2000-$3000.
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morghen  



Joined: 21 Jan 2005
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PostPosted: Wed Oct 05, 2016 2:53 am    Post subject: Reply with quote

Coming to think about it, a nice recipe might be:

-S2 turbo ROW pistons in NA engine
-light flywheel
-Mittelmotor NA head
-headers and properly sized exhaust

All this for a couple of grand give or take would sure make for a fun 924 to drive.
Cant wait to get my hands on that automatic NA i've been dreaming about lately.
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chris79  



Joined: 29 Jan 2014
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Location: milan

PostPosted: Wed Oct 05, 2016 3:05 am    Post subject: Reply with quote

morghen wrote:
Coming to think about it, a nice recipe might be:

-S2 turbo ROW pistons in NA engine
-light flywheel
-Mittelmotor NA head
-headers and properly sized exhaust

All this for a couple of grand give or take would sure make for a fun 924 to drive.
Cant wait to get my hands on that automatic NA i've been dreaming about lately.


In addition to this even 2 Weber 40dcoe

Recently I would like change my 924 for buy bmw m3 e36 or similar car, but if I can found a perfect tuning by increasing cr , not turbo, I think 924 is the best choice.
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CÚdric  



Joined: 27 Aug 2004
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PostPosted: Wed Oct 05, 2016 3:57 am    Post subject: Reply with quote

The m3 is like driving a bus, better keep the 924:)
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Paulyy  



Joined: 05 May 2014
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PostPosted: Wed Oct 05, 2016 5:46 am    Post subject: Reply with quote

You may as well get a set of new pistons to the CR you're after. + at least you'll have a more fresh block instead of using old rings which i wouldn't do anyway.
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Fasteddie313  



Joined: 29 Sep 2013
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Location: Not Detriot - NMI

PostPosted: Wed Oct 05, 2016 6:01 am    Post subject: Reply with quote

Paulyy wrote:
You may as well get a set of new pistons to the CR you're after. + at least you'll have a more fresh block instead of using old rings which i wouldn't do anyway.


+1

Especially if you were to spring for a mittelmotor head I wouldn't think the cost of proper pistons would be that big of a deal. Especially to stay non-interference and protect that head..

Also, I can't recall reading anything about how much HP a slushbox can handle..
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chris79  



Joined: 29 Jan 2014
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Location: milan

PostPosted: Wed Oct 05, 2016 6:06 am    Post subject: Reply with quote

Really? M3 handling is not good?

The only problem of na tuning is that I have to use only 98oct fuel.

This is not cheap, in Italy is more expensive and no many distributors have this.
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morghen  



Joined: 21 Jan 2005
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PostPosted: Wed Oct 05, 2016 7:13 am    Post subject: Reply with quote

11.X:1 CR and non interFEAReance? nice dream..

Turbo pistons would go in with new rings obviously...cmon..and unless they are square old turbo pistons would do just as well as new custom ones in a NA street application as far as my/our knowledge of piston top design goes.

Why would you expect the slushbox to dezintegrade because of some extra bit of torque? Even the NA 5 spd manual can handle decent numbers.

If its like that, i crunched a 944 gearbox to bits with my red turbo so nobody is safe
It squealed like a little angle grinder to the anger of the 2L (K27 helped)


Chris: use 95 and buy octane booster...on a NA you'll be fine.
e36 is a shit box, structurally weak, death trap.
Handling is great if you want to do donuts...other than that...i havent seen any e36 do anything else...but no doubt one could setup one that can handle well...you probably couldnt though.
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