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931 Pistons and a NA head?
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gegge  



Joined: 27 Jul 2007
Posts: 1006
Location: Sweden

PostPosted: Wed Oct 05, 2016 7:33 am    Post subject: Reply with quote

Please note that the exhaust valve close well past TDC. What happens is that the 931 piston chase the exhaust valve and overtakes the valve before TDC. The inlet valve opens before TDC as well and is open at TDC when the piston is level with the block deck. You need reliefpockets. Especially with a hot cam.

The stock cam has a duration of 228 degrees at 0.039". That is more or less equal to the height of the head gasket. With a LSA of 108/112 degrees the exhaust open 46 BBDC and close 2 ATDC. Inlet open 6 BTDC and close 43 ABDC. The concept of interference becomes obvious.
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Last edited by gegge on Wed Oct 05, 2016 7:48 am; edited 1 time in total
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Rasta Monsta  



Joined: 12 Jul 2006
Posts: 11334
Location: PacNW

PostPosted: Wed Oct 05, 2016 7:38 am    Post subject: Reply with quote

Cédric wrote:
The m3 is like driving a bus



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MikeJinCO  



Joined: 08 Jun 2010
Posts: 670
Location: Maysville, Colorado

PostPosted: Wed Oct 05, 2016 8:19 am    Post subject: Reply with quote

I can't get mine together today as the roll pin that locates the timing belt pulley broke. I got it out and hopefully the HW in town has one it is 4x12mm. Spoke to the fellow who worked on my pistons- new Goetze NA rings about the possible interference and he said that at the overlap period at 360 degrees(TDC) there needs to be about clearance of about 3mm from the head to the piston which obviously isn't there. There also needs to be at least 2mm clearance on the exhaust valve to compensate for thermal expansion(local machinist) and that doesn't include the overlap period.

So some valve reliefs in the pistons will necessary. I will also lose a bit of compression. With the valve pockets cut I'm going to be about $500 for pistons(, rings and machining. That is not much of a savings over the cost of new forged lighter weight JE or Diamond ones, but I'm too close now to want to change paths. I checked my 931 head and it is roughly about .228 deep(5.5mm?) to the floor or 5-6mm, so the pocket sizes jive.

This motor will get broken in in my street car and then transferred to the track car. Later I'll work on trying to come up with a replacement street motor. The goal here is to get good quench as Jon Mitchell remarked one time that it really wakes them up, local machinist told me yesterday he worked over an older Chrysler with zero quench like the NA and put in new pistons with quench and the difference was amazing.
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chris79  



Joined: 29 Jan 2014
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Location: milan

PostPosted: Wed Oct 05, 2016 4:42 pm    Post subject: Reply with quote

But there are not in this forum some complete examples of tuning na engine with high cr pistons?
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MikeJinCO  



Joined: 08 Jun 2010
Posts: 670
Location: Maysville, Colorado

PostPosted: Mon Oct 17, 2016 5:03 am    Post subject: Reply with quote

To answer Chris, I think the few that have been done have normally had aftermarket pistons built to be non-interference.

My progress. I put the motor together out of a collection of different parts. I first measured the to be used block height and using my non precision(Harbor Freight) measuring equipment got a height of 231.3 mm. I put a crank in using 3 bearings with a connecting rod that was laying around that had a measured+calculated C-C of 143.61mm which my data show should be 144.02mm, measured with two different digital calipers and they agreed. My piston to deck clearance as .013" or .330mm with a straight edge and feeler gage.

I used a new Reinz head gasket uncompressed and it measured 1.95mm or .075 at the crush rings. I then put clay in the valve locations and put a standard head on, not tightening it down to crush the head gasket.

I then rolled it over and surprisingly it did not jam. The clay showed that on the exhaust it was compressed to maybe half a mm with the intake somewhat thicker. Rather than try to measure it accurately I left it on there when I sent them off to the machinist.

I then also measured a used head gasket that I got off Poco(the car) and found it to have an crush ring thickness varying from 1.67 to 1.93mm with a surprising amount of variation even on the same cylinder, no wonder there was so much water corrosion in that block. Thickness appeared to average about 1.75mm for a total compression of 0.2mm

I haven't bothered to calculate what my compression ratio will be, but hopefully won't be buying gas from the local airport.

Pistons are down at the machinist and I will have pictures when I get them back. I expect them to have valve pockets of 4-5 mm, still very much an interference motor.

I also put a rod/piston with a 8.0 CR NA piston in my block and between the deck clearance of the piston of about 6.5mm and the valve pocket the bottom of the valve pocket came out to be about 10mm below the deck.
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nickthompson  



Joined: 26 Mar 2013
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Location: Central Georgia

PostPosted: Mon Oct 17, 2016 9:58 am    Post subject: Reply with quote

If you do hit an extra high compression please don't run aviation fuel. It's not designed for automotive applications and I've heard it can damage your valve train.
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MikeJinCO  



Joined: 08 Jun 2010
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Location: Maysville, Colorado

PostPosted: Mon Oct 17, 2016 1:16 pm    Post subject: Reply with quote

Actually we are planning to do a preliminary assembley without rings and find out what we have both for valve clearance and compression and then adjust from there. A thicker Cometic head gasket may well be in order .
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chris79  



Joined: 29 Jan 2014
Posts: 549
Location: milan

PostPosted: Wed Oct 19, 2016 4:09 am    Post subject: Reply with quote

I'm waiting for updates, very interesting work.
This work is in area of research, but there are not some documented works of tuning with high cr pistons?
I woul like starting from an already tested project.
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MikeJinCO  



Joined: 08 Jun 2010
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Location: Maysville, Colorado

PostPosted: Wed Oct 19, 2016 11:24 am    Post subject: Reply with quote

So would I, but---
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daniel  



Joined: 18 Jun 2009
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Location: Sydney, Australia

PostPosted: Mon Oct 31, 2016 8:55 am    Post subject: Reply with quote

Quote:
If you do hit an extra high compression please don't run aviation fuel. It's not designed for automotive applications and I've heard it can damage your valve train.


Which fuel are you referring to? Kerosene? AvGas? Most people are referring to avgas when talking about aviation fuel in a car so lets run with that. AvGas is essentially petrol with lead in it which provides two main benefits: Higher octane rating and longer valve life due to the lubricity of lead. The reason people say do not use Avgas in a car is the lead deposits clog up O2 sensors which causes EFI to lean out and punch holes in aluminium, but in cars like ours it is completely safe to use
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MikeJinCO  



Joined: 08 Jun 2010
Posts: 670
Location: Maysville, Colorado

PostPosted: Mon Nov 14, 2016 12:19 am    Post subject: Reply with quote

Got my pistons back, hopefully the pictures will get posted as I'm not good at that. I measured the volume of the piston dish using using a plastic cover with a tapered hole as I think RC worked out and came out to 42.5cc. My preliminary assembly showed the piston deck height of .013" and then we have had to take .005" off the block to clean it up. I also measured my compressed gasket in 4 places for each cylinder, threw out the high and low and then averaged the rest for a compressed thickness of .069" or 1.75mm. The whole affair is going off to the balance man this week and then we are traveling for a couple of weeks. We'll make another temp assembly when it all gets back together as the rods i used the first time are not the ones that will be in the motor. We will then remeasure and figure out which head gasket to use to get the appropriate compression. My goal was to get to a modern quench number of about .040 but I don't think I'll get there.

As I said before, I wouldn't do this again. A set of new, lighter, better pistons is barely moe than I have spent. I also converted back to using the heavier 931 rods originally in the motor as these S2 pistons weighed out at 604 grams where a NA piston was under 460gm. There is not much point in using light weight rods and then having a heavy slug swinging around out at the end.[img]http://i1180.photobucket.com/albums/x413/MikeJinCO/931pistons2.jpg

[/img]
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MikeJinCO  



Joined: 08 Jun 2010
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PostPosted: Mon Nov 14, 2016 12:20 am    Post subject: Reply with quote

Well the Pic didn't show up.
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Fasteddie313  



Joined: 29 Sep 2013
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Location: Not Detriot - NMI

PostPosted: Mon Nov 14, 2016 12:28 am    Post subject: Reply with quote



I think it may be a good idea to take a file and knock down all the sharp edges left by machining on the tops of your pistons to help avoid detonation problems..
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