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Tech differences between M31/04 blocks and M31/01

 
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Walter  



Joined: 17 Jul 2018
Posts: 2
Location: Berry NSW

PostPosted: Thu Jul 19, 2018 10:08 am    Post subject: Tech differences between M31/04 blocks and M31/01 Reply with quote

Hi to fellow lovers of the 931/2.
I have had 924's on and off since the early 80's and currently have a 1980 RHD 931 with the oval throttle body manifold and a Eur /ROW Vin No in Sydney Australia.

It has had continual overheating problems in normal use despite overhauling all the usual suspects and fitting an electric water pump and manual fan override switches to try to reduce heat buildup. It also hasn't the power of similar 931 Eur/ROW cars I have driven.

I think i may have found the cause of the problem and need some tech advice. My engine block is a M31/04 which was apparently sourced by an earlier owner from the US during a visit. He bought a long engine with the round throttle body but only fitted the block so I have a hybrid M31/04 block with M31/01 everything else.
What are the differences other than pistons and compression? The flywheel is an M31/01 item with timing marks in the 20 degree range and I understand timing for a M31/04 is 6-10 degrees and the later flywheel is lighter with different markings. Is there other differences in maybe the distributor mapping or engine control unit which creates the different timing.
As I have the original M31/01 block I(less pistons and rods) I could rebuild that but trying to avoid that option. Alternatively I could change just the pistons (advice on supplier of oversize Turbo pistons appreciated) to ROW items but that will not address overheating. I could fit a M31/04 manifold and distributor( Part No?) if that addresses overheating but that would give me a lower power engine.( I could live with that...just)

Looking for some solid knowledge of what is the actual differences between the two engines so I can sort my problems, make a plan and fall in love with my 931 again. Thanks in anticipation.
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peterld  



Joined: 10 Dec 2006
Posts: 694
Location: Noosa Heads QLD Australia

PostPosted: Thu Jul 19, 2018 6:40 pm    Post subject: Reply with quote

Hi Walter, welcome aBoard.

I don't believe your overheating issues are due to the mismatch of blocks per se! It may well be in the installation of them, but not due to the different years as such.

M31/01: 1979 and 1980 924T 7.5:1 comp 170bhp (ROW) oval, two chamber throttle body. Normal (lighter) flywheel, clutch and pressure plate.


M31/04: 1981 Japan and 1982 USA/Can. Round single chamber throttle body. And should have rubber damped (heavy) flywheel/clutch. The 1981+ 924T in ROW specs had 8.5:1 comp and 177bhp but USA/Can versions were less due to catalytic convertors. These later 924T, known as Series 2, also used the DITC system to manage spark, which was timed off the flywheel which has an imbedded precious metal cypher.


Most items can be mixed and matched.

EG. I have a S1 block with S2 head/induction/Ditc etc. using S2 USA flywheel with normal clutch and 951 turbo........so almost anything is possible!


As for overheating, I would systematically check:

thermostat/radiator/fan sender/overflow bottle/water outlet at back of head for blockage/head gasket/water pump gallery.
_________________
80/81 932/8 ROW
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peterld  



Joined: 10 Dec 2006
Posts: 694
Location: Noosa Heads QLD Australia

PostPosted: Thu Jul 19, 2018 6:43 pm    Post subject: Reply with quote

Hi Walter, welcome aBoard.

I don't believe your overheating issues are due to the mismatch of blocks per se! it may well be in the installation of them, but not due to the different years as such.

M31/01: 1979 and 1980 924T 7.5:1 comp 170bhp (ROW) oval, two chamber throttle body. Normal (lighter) flywheel, clutch and pressure plate.


M31/04: 1981 Japan and 1982 USA/Can. Round single chamber throttle body. And should have rubber damped (heavy) flywheel/clutch. The 1981+ 924T in ROW specs had 8.5:1 comp and 177bhp but USA/Can versions were less due to catylitic convertors. These later 924T, known as Series 2, also used the DITC system to manage spark, which was timed off the flywheel which has an imbedded precious metal cypher.


Most items can be mixed and matched.

EG. I have a S1 block with S2 head/induction/Ditc etc. using S2 flywheel with normal clutch and 951 turbo........so almost anything is possible!


As for overheating, I would systematically check:

thermostat/radiator/fan sender/overflow bottle/water outlet at back of head for blockage/head gasket/water pump gallery.
_________________
80/81 932/8 ROW
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Walter  



Joined: 17 Jul 2018
Posts: 2
Location: Berry NSW

PostPosted: Tue Aug 07, 2018 7:02 pm    Post subject: Reply with quote

Hi Peter
Thanks for your reply. One thing I can't get my head around is the difference in ignition timing between motors if they are so interchangeable.
Haynes manual for Turbo models on P124 gives 20 BTDC at 2000 for all except UK and UK 25 BTDC at 2000. What causes that difference and 5 deg is significant in timing and can lead to heat issues.
I have the low compression block with a engine management for the head suiting a high compression block. Is there something in the management that compensates for timing difference.
I have reviewed cooling except rear of head around the heater and block pipes and have soured a replacement pipe. Next job!!!!
Still not 100% though on why timing is so different..???
Any thoughts appreciated.
Thanks Walter
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 1551
Location: Somerset, UK

PostPosted: Tue Aug 07, 2018 7:29 pm    Post subject: Reply with quote

I chased a coolant issue for over 6 months after engine rebuild + performance mods. Complete overhaul and replacement of the coolant system, larger ally rad holding far more coolant than stock, more efficient fan, manual over ride, swirl pot, etc. etc. Tried various concoctions of de ionised water + water wetters and no anti freeze and still had hot running and when boosting it would spit coolant out of the expansion tank. Turned out to be a cracked/porous head leaking boost into the coolant. Replaced the head and the problem evaporated.

Stock S1/S2 ignition curves will not be the cause of increased combustion/coolant temps

The difference in ignition curves is a slow evolution of the turbo motor, turbo specs and CR changes. In the most part all are interchangeable.

Using the low compression S1 block and S2 ignition/DITC will not upset the motor, performance or coolant temps. I think there is something else amiss with the system, pump, radiator. Unless the ignition/boost is way out of kilter with stock spec.
_________________
1980 931 - forged pistons, Piper cam, CGT turbo, 951 ducted FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife LSD
Now www.924board.org/viewtopic.php?t=34690
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