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Connecting rod swap

 
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gegge  



Joined: 27 Jul 2007
Posts: 1124
Location: Sweden

PostPosted: Mon Jan 25, 2010 11:55 am    Post subject: Connecting rod swap Reply with quote

I was looking for something lighter than 927g heavy stock conrods and searched for a replacement rod. This what I found:

The 924 crank is 47,95-47,97mm in diameter, about the same as for BMW M10, M30, S14 (M3), S38 (M5), M60 and Mercedes M102, M103, M104, M111, M119 and several dieselengines. These german rod bearings should work.

BMW and Mercedes use a 22mm wrist pin as common aftermarket pistons.

The 924 big end conrod bore is 51,6mm, the same as the above Mercedes engines. The rod lenght of the 2,3-16v is 145mm, an easy swap? BMW is slightly larger at 52mm. The BMW E30 M3 conrod is 144mm long.

What some of the MB 2,3-16v and 2,5-16v guys do is using the bearing from BMW M10 and cheap aftermarket forged conrods for Volvo B230!

The big end width is not as wide as the 924, but about the same as for VW Rabbit. Not sure if that is a problem?

Well, the conclusion is that the 924 could use other aftermarket conrods with lenghts from 135mm to 165mm
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daniel  



Joined: 18 Jun 2009
Posts: 669
Location: Sydney, Australia

PostPosted: Mon Jan 25, 2010 3:18 pm    Post subject: Reply with quote

I wonder if there was a shorter than stock rod that would allow the use of VW flat top pistons and give a reasonable compreesion ratio, say 11:1??

Daniel
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ideola  



Joined: 01 Oct 2004
Posts: 15548
Location: Spring Lake MI

PostPosted: Mon Jan 25, 2010 9:03 pm    Post subject: Reply with quote

@daniel
Yes. In fact European Motorworks used VW rod for their stroker kits.

Be careful of anything longer, depending on the profile of the big end, you may end up having to take off material at the bottom of the cylinder walls for clearance.

But nice work, gegge, sounds promising...
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gegge  



Joined: 27 Jul 2007
Posts: 1124
Location: Sweden

PostPosted: Mon Jan 25, 2010 10:28 pm    Post subject: Reply with quote

I donīt know why I am persuing this since Iīve got balanced and polished crank&rods already...

Well, this is going to be my new set up:

MB 2,3-16V forged Parner H-beam rods with ARP, a weight of 520g
Stock crank, knife-edging the counterweight by minimum 1000g.
Keep the forged VW type 1 pistons, at least 150g lighter than stock

And since the counterweights are machined to be round and smooth, a new crankscraper!

This bottom-end will take tons of boost AND rpms

By the way, AMG has got an intake valve for the 931. It is from a straight six from the mid 80ī with diameter 43mm, stem 8mm and lenght 133mm. Not cheap
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Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs
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jazz guy  



Joined: 26 Nov 2002
Posts: 434
Location: Colorado

PostPosted: Tue Jan 26, 2010 6:06 am    Post subject: Reply with quote

gegge wrote:
Well, this is going to be my new set up:

MB 2,3-16V forged Parner H-beam rods with ARP, a weight of 520g
Stock crank, knife-edging the counterweight by minimum 1000g.
Keep the forged VW type 1 pistons, at least 150g lighter than stock


Great research! I am very interested in your progress so keep us posted on how this works out for you. I am looking to do an engine rebuild/upgrade with the VW pistons after I get my suspension project completed and sorted.

What rpm range are you looking at with this configuration?

Cheers, Brian
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Martijnus  



Joined: 29 Dec 2006
Posts: 2019
Location: Netherlands

PostPosted: Tue Jan 26, 2010 11:16 am    Post subject: Reply with quote

gegge wrote:
I donīt know why I am persuing this since Iīve got balanced and polished crank&rods already...

Well, this is going to be my new set up:

MB 2,3-16V forged Parner H-beam rods with ARP, a weight of 520g
Stock crank, knife-edging the counterweight by minimum 1000g.
Keep the forged VW type 1 pistons, at least 150g lighter than stock

And since the counterweights are machined to be round and smooth, a new crankscraper!

This bottom-end will take tons of boost AND rpms

By the way, AMG has got an intake valve for the 931. It is from a straight six from the mid 80ī with diameter 43mm, stem 8mm and lenght 133mm. Not cheap


aren't 931 valve stems 9mm?
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daniel  



Joined: 18 Jun 2009
Posts: 669
Location: Sydney, Australia

PostPosted: Tue Jan 26, 2010 8:05 pm    Post subject: Reply with quote

@dan
So would the rod that EMW used yeild a lower deck height with a stock crank???

Daniel
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Over the top of skyline, total brake failure.... hit the wall at over 200 kp/h at the dipper, so anyone who has to brake for the esses is a pussy.
1977.5 Race Car, CAMS Group S Spec
1989 944 Cabriolet
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gegge  



Joined: 27 Jul 2007
Posts: 1124
Location: Sweden

PostPosted: Tue Jan 26, 2010 8:56 pm    Post subject: Reply with quote

Yes 924 stems are 9mm. The problem is to find a valva that is 133 long. The valve guide is easy to exchange.

Connecting rods Big end Small end Lenght Weight
M-B 2,3-16V 51,6mm 22mm 145mm 520g
BMW M3 2.3 16V 52mm 22mm 144mm 598g 550-570g
BMW M3 2.5Ltr. 16V EVO Sport ( S14 ) 52mm 22mm 144mm 550-570g
BMW M3 2.0L Italy 52mm 22mm 149,7mm 560g
BMW M10 52mm 22mm 135mm 525g 540-560g
BMW M30 52mm 22mm 135mm 525g 540-560g
BMW E34 M5 3.6L S38 52mm 22mm 144mm 550-570g
BMW E34 M5 3.8L S38 52mm 22mm 142.5mm 537g
BMW M88 52mm 22mm 146mm
BMW M60/62 V8 52mm 22mm 143mm 545g
Volvo B230 740 turbo 52mm 23mm 152mm 598g 731g
Volvo B230 52mm 23mm 165mm 623g
Volvo B230 52mm 23mm 158mm 638g
Volvo B230 52mm 24mm 145mm 615g
Volvo B230 52mm 24mm 142mm 610g
Volvo B230 52mm 23mm 149mm 626g
Use M-B M102 std, BMW M10 or BMW S14 1st oversize bearings
All connecting rods are thinner than stock, user washers on the wrist pin to control
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Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs
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gegge  



Joined: 27 Jul 2007
Posts: 1124
Location: Sweden

PostPosted: Tue Jan 26, 2010 9:57 pm    Post subject: Reply with quote

jazz guy wrote:
gegge wrote:
Well, this is going to be my new set up:

MB 2,3-16V forged Parner H-beam rods with ARP, a weight of 520g
Stock crank, knife-edging the counterweight by minimum 1000g.
Keep the forged VW type 1 pistons, at least 150g lighter than stock


Great research! I am very interested in your progress so keep us posted on how this works out for you. I am looking to do an engine rebuild/upgrade with the VW pistons after I get my suspension project completed and sorted.

What rpm range are you looking at with this configuration?

Cheers, Brian


I think the limit for the flattops are 7000 rpm playing safe. The conrods and crank can most likely take a lot more, guessing close to 10000 rpm.
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Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs
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ideola  



Joined: 01 Oct 2004
Posts: 15548
Location: Spring Lake MI

PostPosted: Tue Jan 26, 2010 10:20 pm    Post subject: Reply with quote

@daniel
I haven't been able to get EMW to give up all of their secrets What I do know is that their stroker kit used VW flattops and VW rods, which according to them did not require clearancing the block. They claim they were getting 2.4L using max overbore of 88mm, which would result in something like a massive 98mm stroke. I don't know how they did that without grinding the block, and of course, nothing is verified or documented.
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gegge  



Joined: 27 Jul 2007
Posts: 1124
Location: Sweden

PostPosted: Sat Mar 27, 2021 6:30 pm    Post subject: Reply with quote

I was supposed to buy BMW S38B36 144mm rods for my 931 engine, but at the moment the rods were out of stock. For some reason I bought S38B38 142,5mm instead. The shortblock ended up in my tuned 924 instead, working like a charm:

https://www.youtube.com/watch?v=ZEepQkhJKV4
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Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs
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anthonimartini  



Joined: 17 Feb 2020
Posts: 240
Location: portland oregon

PostPosted: Sun Mar 28, 2021 1:24 am    Post subject: Reply with quote

lets revive this thread!

is that engine using the 931 head and vw pistons? if so do you know what piston to deck height you achieved? i am thinking of getting the 88mm vw pistons and 144 bmw m10 rods and getting my rod journals machined to match that of the bmw rod bearing. seems the cheapest and most effective way at getting available lightweight parts.
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gegge  



Joined: 27 Jul 2007
Posts: 1124
Location: Sweden

PostPosted: Sun Mar 28, 2021 4:27 am    Post subject: Reply with quote

Well, yes and no.

In the NA engine showing in the video, I have 87 mm VW Malhes and 142,5mm BMW rods. IIRC was the CR is pretty high, but since the cam is HOT there is no problem running low octan gas.

The 931 engine has 88 mm VW Malhes with 144 mm rods. Belive CR is close to 9:1. Definetly too low for a high HP NA. The 931 head requires a longer rod due to a large cumbustion chamber. If BMW M88 rods could be sourced... (146mm)
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Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs
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