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Overhaul Porsche 931 S1 engine
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MOTANUL  



Joined: 19 Nov 2018
Posts: 157
Location: ROMANIA BUCHAREST

PostPosted: Mon Jun 22, 2020 6:34 pm    Post subject: Overhaul Porsche 931 S1 engine Reply with quote

I plan to disassemble the engine in the fall to replace the following:
1-Piston rings
2-Piston pin
3-Connecting-rod bearing
4-Crankshaft bearing
-gaskets

If necessary, I will also replace the pistons.

The gaskets are already bought. The rest of the spare parts are more or less NLA.

exception: Connecting-rod bearing - rose pasion

Where to find the rest of the parts mentioned above?
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 2194
Location: Somerset, UK

PostPosted: Mon Jun 22, 2020 6:43 pm    Post subject: Reply with quote

1-Piston rings - Total Seal
2-Piston pin - why?
3-Connecting-rod bearing - Mittelmotor, 'colonel' on 924OC in UK
4-Crankshaft bearing - Mittelmotor, 'colonel' on 924OC in UK, there is a guy in France on here and 924OC remaking some

You will need to do rod bolts too. From Porsche or there was some ARP for sale on here. Similar price either way.

There is a thread on 924OC regarding forged pistons if required. But there are some cast OEMs now available from Porsche at circa €250 per piston. If you do pistons then the pin will be supplied
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1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
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Last edited by Carrera RSR on Tue Jun 23, 2020 5:59 pm; edited 1 time in total
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MOTANUL  



Joined: 19 Nov 2018
Posts: 157
Location: ROMANIA BUCHAREST

PostPosted: Mon Jun 22, 2020 7:50 pm    Post subject: Reply with quote

1. Total Seal® Piston Rings Company Sold
This company? http://www.totalseal.com/

2-Piston pin - why? - If it is not necessary, I do not change it.

3-4 Mittelmotor

Pistons - Porsche If I am lucky

Thnak you for detail info.
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morghen  



Joined: 21 Jan 2005
Posts: 8337
Location: Romania

PostPosted: Mon Jun 22, 2020 8:13 pm    Post subject: Reply with quote

If you change the pistons you should also machine/hone/match the cylinders.
Also, if you do this overhaul, dont forget about the head, new guides and valve stem seals, new springs, new tappets and obviously new valve seats, make sure you do a 3 angle job on the seats as i think whats on the car now is a 3 angle job, so if you change to new things make sure you match or improve the quality of whats on the car now, dont just assume new is better.
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MOTANUL  



Joined: 19 Nov 2018
Posts: 157
Location: ROMANIA BUCHAREST

PostPosted: Thu Nov 19, 2020 1:47 am    Post subject: Reply with quote

I took the 924 turbo engine compression: 8.2 8.4 8.6 8.2 bar

I did not find how much an ideal compression should be for 924 turbo engine.

At 924 NA I had found 12 bar.

I was unable to unscrew the exhaust manifold bolts. Some of them are rounded and blocked.
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MOTANUL  



Joined: 19 Nov 2018
Posts: 157
Location: ROMANIA BUCHAREST

PostPosted: Mon Nov 30, 2020 8:53 pm    Post subject: Reply with quote

MOTANUL wrote:

I did not find how much an ideal compression should be for 924 turbo engine.


I noticed that several 924 turbo owners on the forum made a major engine repair. Did no one measure the compression after the repair? I am very curious how many (bar/PCI) got on average on each cylinder after the repair.

I disassembled the engine. The cylinder head is removed. The strangest thing discovered would be the upper segments (compression) made of 2 pieces. All 4 are composed of 2 pieces 1/3 + 2/3.

Piston size 86,75 mm (Mahle)

The exhaust gallery has a rather long crack.
The piston shirts look pretty good.
I will measure in the near future:
Cylinder block, Pistons, Connecting rod bush, Crankshaft bearings



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924RACR  



Joined: 29 Jul 2001
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PostPosted: Mon Nov 30, 2020 10:06 pm    Post subject: Reply with quote

Sadly, though I did rebuild, i didn't get the rings broken in properly, my fault/problem, so I don't have any usable compression numbers.

Will be fixing that this spring.

Note that the compression in a Turbo should be lower than an NA due to a lower CR.
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Carrera RSR  



Joined: 08 Jan 2010
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Location: Somerset, UK

PostPosted: Mon Nov 30, 2020 11:00 pm    Post subject: Reply with quote

Its not good practice to use others compression test results as an absolute number. Too many variables...... CR ratio, uncalibrated gauges, differing temperatures.... etc. And to be honest, I never checked in the 6yrs since rebuild
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1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
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Cédric  



Joined: 27 Aug 2004
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Location: Sweden

PostPosted: Mon Nov 30, 2020 11:00 pm    Post subject: Reply with quote

The cracked top rings is something ive seen on several s1 engines, including the one my original engine. Ive not encountered the same on the s2 engines, so some changes might have been made in production.

Regarding compression, i think my s2 engine was around 10,5 something bar. But it was made along time ago so i cant remember it in detail.
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MOTANUL  



Joined: 19 Nov 2018
Posts: 157
Location: ROMANIA BUCHAREST

PostPosted: Tue Dec 01, 2020 12:23 am    Post subject: Reply with quote

C�dric wrote:
The cracked top rings is something ive seen on several s1 engines, including the one my original engine. Ive not encountered the same on the s2 engines, so some changes might have been made in production.

Regarding compression, i think my s2 engine was around 10,5 something bar. But it was made along time ago so i cant remember it in detail.


I thought I was the only one in the world who discovered something like that. No friend of mine ever encountered craked top rings in other engines (of course not Porsche 931- other brands / model).

I expect S1 to have less than 10.5 bar.


The Porsche 924 turbo (931) is a total lack of information. In golf 2 for example there is a program in which you enter:
- engine code
- thickness of the head gasket
- Piston protrusion
- Inner diameter of head gasket
- Capacity of piston (cm3)
- Capacity of head (cm3)
- CAPACITY OF COMBUSTION CAMBER (Stroke: / Bore:)

And you receive results.
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 2194
Location: Somerset, UK

PostPosted: Tue Dec 01, 2020 6:39 am    Post subject: Reply with quote

MOTANUL wrote:
C�dric wrote:
The cracked top rings is something ive seen on several s1 engines, including the one my original engine. Ive not encountered the same on the s2 engines, so some changes might have been made in production.

Regarding compression, i think my s2 engine was around 10,5 something bar. But it was made along time ago so i cant remember it in detail.


I thought I was the only one in the world who discovered something like that. No friend of mine ever encountered craked top rings in other engines (of course not Porsche 931- other brands / model).

I expect S1 to have less than 10.5 bar.


The Porsche 924 turbo (931) is a total lack of information. In golf 2 for example there is a program in which you enter:
- engine code
- thickness of the head gasket
- Piston protrusion
- Inner diameter of head gasket
- Capacity of piston (cm3)
- Capacity of head (cm3)
- CAPACITY OF COMBUSTION CAMBER (Stroke: / Bore:)

And you receive results.



We used to have that supplied by Ideola's Garage. It calculates the compression ratio. Unfortunately its gone unless someone can host it instead of Dan?
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1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
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jacobroufa  



Joined: 18 Nov 2016
Posts: 332
Location: Rockford, IL

PostPosted: Tue Dec 01, 2020 12:42 pm    Post subject: Reply with quote

https://web.archive.org/web/20171012111944/http://garage.ideola.com/top-DL-CompressionRatioCalc.html

or my hosted version below. just copied the code did not update it..

http://924.terribly.fun/compression/
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MOTANUL  



Joined: 19 Nov 2018
Posts: 157
Location: ROMANIA BUCHAREST

PostPosted: Wed Dec 02, 2020 4:42 am    Post subject: Reply with quote

Thank you @jacobroufa

I studied the pistons and segments a bit yesterday. From what I managed to understand, the S2 pistons are different from the S1 ones. But the segments seem to have the same dimensions.
Is it a good idea to use S2 segments instead of S1?

S2 Withstands better compression ratio, higher power, etc.
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Carrera RSR  



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Location: Somerset, UK

PostPosted: Wed Dec 02, 2020 6:41 am    Post subject: Reply with quote

MOTANUL wrote:
Thank you @jacobroufa

I studied the pistons and segments a bit yesterday. From what I managed to understand, the S2 pistons are different from the S1 ones. But the segments seem to have the same dimensions.
Is it a good idea to use S2 segments instead of S1?

S2 Withstands better compression ratio, higher power, etc.


S1 = offset bowl ~45cc & 7.5:1 CR (+ any skim of head)
S2 = central bowl ~35cc & 8.5:1CR (+ any skim of head)

S2 piston would be my preferred design, but with a slightly thicker Cometic MLS headgasket to bring CR below 8.5:1 for sub 1bar boost. Closer to 8 - 8.2:1CR if wanting over 1bar boost. Big boost would need to be closer to 7.5:1
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1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
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MikeDanger  



Joined: 21 Nov 2002
Posts: 636
Location: Denver

PostPosted: Wed Dec 02, 2020 4:57 pm    Post subject: Reply with quote

are you sure s2 is 8.5:1?? thought it was 8.0

944 turbo is 8.5
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