924Board.org

[F3ARED]

Good morning/afternoon/evening/night - whatever it is in the part of the world you hail from!

Recently pulled the trigger on my first Porsche after years of wanting one; sadly, it wasnt the 911 that ive always lusted after but given what I paid, it was really hard to say no. Unfortunately, its an "early" 924 being built in 1977 which means all the "good" stuff like 4 stud brakes, 4 speed Audi box etc etc. Yes, it would have been nice to start with a later car, and/or a 931, but these arent exactly falling out of trees in Australia, particularly at an asking price of less than a used LS1 engine [which does fall out of trees here]. Starting with a better base just isnt an option. To put it into perspective, the next cheapest 924 on the market was 6x what I paid for mine.

My plan as it stands is to build a Carrera GT replica albeit with some modern changes [namely, EFI and proper engine management]. In an ideal world a 931 donor car would be great but in the event that one cant be located easily/cheaply, Im considering building the XJ that the car currently has.

Now to the questions; as I understand it, the only real differences between the XJ engine and the M31.5 are the following:

-Pistons
-Head, including valves and springs
-Potentially rods? Found mixed information on this one

Note im not considering the bolt ons or accessories such as the CIS unit etc. Have I missed anything in that list?

My current plan is/was to use Pauter rods with the 931 Piston in the XJ Block and head. Using the calculator provided by Ideola [???] I get the following:



0cc Combustion chamber in head
.020 OS Piston [Mahle 931 Turbo, 87mm bore]
Gasket bore: 88mm
Headgasket [compressed]: 1.17mm
Deck height 232mm
Bore 87mm
Stroke 84.4
Rod length 144mm
Piston compression height 45mm
Piston cc -45cc
Piston to deck clearance .8mm

All this results in a 9.8:1 Compression ratio which is a touch higher than id like. Trawling through the forum/board lead me to a discussion regarding modifying the NA head and machining it to have a combustion chamber in it - if possible, removing 10cc here would lower the compression to a more palatable 8.5:1.

Does anyone know if anyone has successfully managed to machine the head in this area? It would, as far as im aware, require the valve seats to be recessed deeper and as such shorter valves - i assume this is why the 931 valves are different?

No amount of searching could give me an answer on head flow cfm; does anyone have any figures for both inlet and exhaust ports [inc the lift/" water] for both or either head?

Not chasing earth shattering power here; the 350-odd HP that the factory race cars made is plenty and should be easily and reliably achievable.

Thanks in advance.

Nick
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1977 Porsche 924
1987 Range Rover Classic 3.5L V8

[MikeJinCO]

Yu have a big challenge, Stay on the look out for a wrecked 944 for the brakes and rear suspension. The 931 head looks similar on the outside, but much different on the inside, ports are quite different, exhaust valves 36mm vs 33 and length of 133mm vs 137(depth of combustion chamber. You will have to manufacturer an exhaust manifold to fit the turbo as both the manifold stud patterns are slightly different.

At that power level the stock clutch will probably be a problem as the 931 had a larger hydraulic actuated clutch vs the stock 215mm cable operated.

Someone will chime in on how to fit some VW pistons that are new, probably much better, cheaper and will give a better CR.

A fellow called Morghen here on the board has developed a supercharger kit for the existing motor which would have less power, but is already in use and a fraction of the cost you propose. JMHO
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Mike


'67 MG Midget Dp
'71 Ocelot Dsr Kawasaki 1000(under rebuild)

[Cedric]

You could have a look at Magnus "safes" so called rat rod build(it's actually very tidy under the shell) . Pretty similar to wha you want to achieve. He might drop in here and add some info. Home built ex manifold with Borg Warner turbo, and pretty much a refreshed standard engine with new rings and bearings, and of course EFI, alu radiatior, fmic, oil cooler, spec2+ clutch. I don't remember if the CR is lowered with a thick mls head gasket though. Hes about to go ot the dyno with this setup, though i think it will be set fairly conservative in the beginning. I think there's a stage 2 with forged pistons in the future.

If i where you i would add a little porting work to the head aswell, and a stage 1 cam, that will help you reach your targets.

Do you have acess to E85? That will transform everything on these boosted engines.

You could also search for "project black" here for a good read
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1980 924 Turbo
www.instagram.com/garagecedric/

[anthonimartini]

i got a cometic mls head gasket at.090" which dropped my m31 with flattop engine from 9.3;1 to around 8.8. they list them on ebay for about 100 usd. i have a really nice turbo car that i have totally rebuilt that i would trade you for mk2 cortina! man i really wish i could spend some time down there. seems like a fun place barring the scary critters.
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ive saved alot of 924's
current:
79 MochaSchwartz 931
83 944
87 924S
Instagram @anthoni.jpeg
formerly known as turbellion, lost password.

[F3ARED]

MikeJinCO; I have come across Morghens videos/webpage before and am currently going through his build thread. While i have no doubt the kit works well [its pretty obvious looking at the videos] this particular car will be going turbo given that I already have a brand new ball-bearing TD05 rated to 500hp, a few different external wastegates, a spare haltech, numerous intercoolers etc in my stash of projects past. Fabricating a manifold should be relatively straight forward, especially given that i have friends with more ability than me should I run out of talent

Interested in the port differences between the 931 head and the 924 head; i take it the 36mm valve is for the 931? Would be good to know some flow figure differences and or see photos but ive not as yet been able to find any. As far as the VW pistons go, so far ive only been able to find flat tops off the shelf. Even with the lower compression height vs the 924NA pistons, they would give way too much compression [10.4:1]. Ideally id like to run in the 8.5:1 - 9:1 range as anything over that will limit the amount of boost that can be run.

Brakes will be one of the things that inevitably need doing. Correct me if im wrong, arent the 944 rear arms mounted off the gearbox and as a result not usable unless a G31 is used or have i read things wrong?

Thanks for the info on the chambers and valve heights, will note that down.



Cedric - Ill have a look, thanks or the tip. Head porting is a given as is a different cam grind. Was interested to see what sort of figures the heads flowed and what grind the factory 931 cam used to give me a starting point. Have previously used EAPro with reasonable results and was hoping to use it again to try and optimize the whole package.

E85 is available at-the-pump however isnt exactly at every service station. Currently only United servos stock it and their network isnt as big as others; not a problem for regular usage as there is one around the corner from my house however it would be nice to have some fuel range and the ability to travel without making plans around servos. Preference is to use 98RON due to availability however this may change.



anthonimartini - was that .090 compressed thickness? If so it would give me the right drop in compression ratio im chasing according to the calculator im using. So far ive only found the Victor Reinz kits or copper [which i want to avoid like the plague] - i presume the MLS is advertised under the 931 not 924?

Its a good place to live, just the constant battle with the wild life potentially trying to kill you sucks

Thanks for the answers.

Cheers

Nick
_________________
1977 Porsche 924
1987 Range Rover Classic 3.5L V8

[MikeJinCO]

Congrats for the fabricating ability, wish I had it. The later suspension bolts right on, the difference between the Audi and Porsche gearboxes is the Porsche 924 box has hangers that are part of the torsion bar carrier, the Audi bolts up to a hanger bar between the tub rails. The second gen. car changed how the T-bar carrier bolts to the chassis using some rubber bushings rather than a direct bolt on so you are stuck with what you have. I put the early 944 rear suspension on my '77. The 77.5 came with it the change was to dampen a bit a bit of the street ride harshness.

Many people here much more knowledgeable than myself on turbos, I have never dealt with them, I believe all the ROW cams the same, the '77 US pollution control special version had a different one that changed to the same with the 77.5 version. I believe Newman Cams has a special cam for the turbo, slightly longer duration, but then change the valve timing for the turbo.

The difference between the standard brakes and the later all disc is night and day, The disc/drum stops ok particularly if the car is set up level rather than the nose down '77 US version, the all disc version has much better pedal feel giving a much higher level of confidence.
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Mike


'67 MG Midget Dp
'71 Ocelot Dsr Kawasaki 1000(under rebuild)