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2.4 litre conversion on 931 engine
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simsport  
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PostPosted: Tue Oct 08, 2002 10:06 am    Post subject: Reply with quote

Anyone ever done this?

I spoke with a gent from Eurorace who was very helpful. He said they can put together very strong pistons (JE?) with a set of golf rods to give 2.4ltr when the crank is offset ground.
This should give my motor around 250bhp and looks cheap enough given that I find myself having to buy new pistons anyway after a plug failure and a holed piston.
Just wondered if this had ben done and if so was there any advice on other things to do to make it all work?
Cheers
Simon
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John H  
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PostPosted: Tue Oct 08, 2002 10:32 am    Post subject: Reply with quote

I didn't think there was enough meat on the crank to grind it offset.
To get 2.4 litre you will need a throw of around 94mm and a bore of 90mm.
If you just use standard bore and go to third oversize (87.5) you will need to have a throw of 98mm - fairly big increase.
I've heard the engine can be taken to 2.4 litres and there was a few articles in European Car in the late 80's about this. All of them required some machine of the sump and crankcase to make it all work.

Let us know how you go on this one as I'd like to do it to one of my motors if it works.
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Peter_in_AU  



Joined: 29 Jul 2001
Posts: 2742
Location: Sydney, Australia

PostPosted: Tue Oct 08, 2002 11:50 am    Post subject: Reply with quote

there's always the remote possibility that a VW 2.0/Audi 80 crank might fit. They have a 92.8 stroke which would give 2.2L.

Would be nice to know one way or the other.
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Lizard  
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PostPosted: Wed Oct 09, 2002 6:05 am    Post subject: Reply with quote

there is someone who has done this with the 924 NA engine and he has it up to 2.4l so it should be possible on this engine too
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simsport  
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PostPosted: Wed Oct 09, 2002 6:50 am    Post subject: Reply with quote

It has been done by Eurorace and they are confident it will work. My worry is piston strength as my engine is supercharged with about 12psi of boost.

A friend of mine over here in the UK is going to have the work done on his N/A engine running about 12:1 ratio. He estimates about 175bhp!

Still would like to know if anyone has tested the pistons in the turbo 924 if possible.

Cheers
Simon
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Lizard  
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PostPosted: Wed Oct 09, 2002 8:05 am    Post subject: Reply with quote

there are fairly stong pistons I believe
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Rick MacLaren  
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PostPosted: Wed Oct 09, 2002 9:59 am    Post subject: Reply with quote

12:1 compression? Jeese.

I've just removed the 7.5:1 compression pistons from my 931 and bought 8.01:1 (or 8.5, see previous threads re: EBS). The brand I've got is Mahle and the ones I've got are heavier than JE. I can't vouch for their strength except to say that they've taken 12 PSI with no skirt damage.

With the potential for detonation looming, and the boost you're running, I'd worry more about the rods and rod bearings personally. The shift from upstroke to downstroke would be tremendous. I've only ever busted a piston from collision but I suppose it's possible to do so from other causes.

When I contacted Mahle's racing division, the fellow I spoke with said that they don't use coated pistons in their race efforts because the pistons are made such that they don't require it. But he's obviously biased. Still, there is kind of a point to it, namely, if the structural integrity of the piston is in need of a coating, it can't have been engineered to withstand the kind of beating you're intending for it.

Why don't you get a set of LOW compression 7.5:1 pistons from Mahle? If you're supercharging, low compression is a great way to go. If I had to do it again, and wanted a racer and not a daily driver, I'd use the 7.5:1 over the 8.01:1 anyday. You lose in driveability but can make leaps of gain in terms of speed.

[ This Message was edited by: Rick MacLaren on 2002-10-09 10:02 ]
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simsport  
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PostPosted: Wed Oct 16, 2002 9:56 am    Post subject: Reply with quote

Rick
I take your point and in fact I will be running at 8.5:1 in the blown engine. Its my friend who will be using the higher comp pistons from the same supplier who expects about 12:1.

I am told the JE pistons are a lot lighter and hence will give the rods etc an easier time, but I would still love to hear of a 924 turbo owner who has gone to 2.4 lts with these pistons/rods....ANYONE OUT THERE!!

Cheers
Simon
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Rick MacLaren  
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PostPosted: Wed Oct 16, 2002 10:00 am    Post subject: Reply with quote

Yes the JE are waaay lighter.

So how will you adjust your timing to account for the higher compression 8.5:1 compression pistons? Or will you?

[ This Message was edited by: Rick MacLaren on 2002-10-16 10:01 ]
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Peter_in_AU  



Joined: 29 Jul 2001
Posts: 2742
Location: Sydney, Australia

PostPosted: Wed Oct 16, 2002 10:42 am    Post subject: Reply with quote

according to my calculations, if you used the maximum recommended overbore to give you a bore of 87.5mm you'd need to increase the stroke to 99.8mm to get 2400cc. That's an extra 15.4mm which would be a crank journal offset of 7.7mm. Does that sound right?

Looking at an old 924 block and crank I've got tied up out back you'd probably need to grind the block to get clearance for the rods. Depending on how much metal you had to remove you might need to add a girdle for strength.

Don't know about using GTI rods. They are the same length as the 924 rods so you'd need to move the piston pin up 7.7mm (relative to 924 pistons) in the new pistons. Wouldn't that increase your piston slap? I suppose if you got short skirt pistons then it would be less of a problem.

Another option for rods might be Honda 1.6 rods. Their C-C length is 7mm less that the 924/GTI rods, the Pauter ones are nearly 100g lighter than their GTI rods and the big end bore is 3.6mm less than the 924 rods (less metal to grind).
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John H  
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PostPosted: Wed Oct 16, 2002 3:25 pm    Post subject: Reply with quote

All the guys I have spoken to in the past when I was looking at going to a stroked motor said the same thing once you took the motor beyond 2.1/2.2 on max bore you had to carry out modification tot he sump and to the block to get clearances.
One guy even went so far as to say they recommended that I should throw the block away after two season racing as the bore would be scoured and there was no way of repairing as it was at it's maximum.
Eurorace used to do a 2.4 litre crank and when I spoke to them in the early 90's and they said they recommended you only went to the 2.2 litre, as you didn't have to do any machining to the block or sump.
Till someone’s done it or seen it done it's all conjecture
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Rick MacLaren  
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PostPosted: Wed Oct 16, 2002 3:34 pm    Post subject: Reply with quote

What kind of HP gain can you expect from 0.2 litres of displacement? I know in my case going from a 1.8 litre (or slightly above) to (nearly) 2.0 litre didn't itself result in neck-snapping performance. Is the rise from 2.2 to 2.4 litres dramatic?

[ This Message was edited by: Rick MacLaren on 2002-10-16 15:54 ]
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Peter_in_AU  



Joined: 29 Jul 2001
Posts: 2742
Location: Sydney, Australia

PostPosted: Wed Oct 16, 2002 4:13 pm    Post subject: Reply with quote

theoretically, a 10% increase in displacement from 2.0 to 2.2 would give a 10% increase in power.

Of course we all know that's wrong 'cause it doesn't take into account the engine efficiency. With the NA you might need to add some head work to be able to fill the extra capacity.

With a turbo or supercharger in Simon's case you're running at greater than a theoretical 100% so you'll be getting a better return. Either way, by itself 200 or 400cc is not going to turn a 924 or 931 into a bowel-clenching monster.

Only way to be sure is to suck it and see.
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sandgroper  
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PostPosted: Wed Oct 16, 2002 6:36 pm    Post subject: Reply with quote

personally i think dollars are better spent on things like engine management,intercooler upgrade from none,a 2.2 would be nice though

[ This Message was edited by: sandgroper on 2002-10-16 19:04 ]
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Peter_in_AU  



Joined: 29 Jul 2001
Posts: 2742
Location: Sydney, Australia

PostPosted: Wed Oct 16, 2002 8:16 pm    Post subject: Reply with quote

of course if you're in Simon's position where he has done all those things more cubes is the next logical step.
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