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Engine Rebuild Tricks...(I need some)
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goturbo  



Joined: 15 Jun 2004
Posts: 87
Location: Massillon Ohio, USA

PostPosted: Sat Aug 21, 2004 2:04 pm    Post subject: Engine Rebuild Tricks...(I need some) Reply with quote

OK so this site is absolutely boiling over with good information regarding 924/931 'stuff'. Here's the bottom line-I am planning a 'mini' restoration this winter on the 81 931 and I have been searching for solid information on what to do with the engine rebuild. There is SO much stuff out there that I am finding it difficult to get what I need to know. There is some conflicting stuff out there as well that adds a bit of confusion in the mix. I have a line on a cheap, late 70's 924 NA engine and I need to know what is really interchangable with mine, and if it is even worth messing with. I have read the tech section in this site but it is not real definative. I know the bolck is the same but I hear conflicting stories on the rods. Crank, I think the same?? I think that the heads have different shaped combustion chambers so they won't work, but what about the valves? What about pistons, are there any other (oversized) pistons that will work on this engine that are reasonable priced or is it just better to get the stock repalcements? For the record, I am planning to run an intercooler and about 10psi boost. So HC pistons are not a good idea?

If I could get the engine and have the machine work done on it now it would save time this winter. I am not trying to do this 'on the cheap' but I want to spend the money in the right places to get some more power and still keep it a solid daily driver. Building 300hp is not my goal here and I have read many good and logical discussions (+/-) about power and engine swaps and where this engine/head sucks...etc but I will be happy if this car has about 215hp or so. I've read that an IC and boost controller might make those numbers but I want to do what I can to the engine while I have it torn down. So if it ends up being more HP, great but I don't really want to start tearing up other components. Honestly, I would be OK with just putting the engine back together stock and making minor changes to the head and induction systems, but if there are gains to be made with limited additional cost (over a std. rebuild) while the engine is already torn down, I would entertain those changes.

I have tried to find this stuff and I am sorry if some of this is redundant, but I have not really figured out how to do a good job af finding info in these pages, I am still working on it I've found little tidbits hither and dither but hopefully this post will become a one stop shop for anyones engine rebuild questions.

Also if anyone out there has a 931 engine that they want to sell for a reasonable price I might be interrested as well.
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Lizard  



Joined: 03 Nov 2002
Posts: 9364
Location: Abbotsford BC. Canada

PostPosted: Sat Aug 21, 2004 4:18 pm    Post subject: Reply with quote

ok block is the same, however the 931 blocks seemed to have a slightly better casting mold imo they are alittle smoother, the cam, tappets, crank and rods are the same the pistons, head, valves, flywheel etc are not.

imo for engine rebuilding UNLESS you have alot of time to waste (ie no time line WHATSOEVER) then have the pistons measured for size and then go to the dealer for piston rings, the aftermarket ones Deves total seal etc are all going to take awhile and it will take a few different tries to get the right rings, pluss if the factory rings have held up this long they cant be that bad now then can they.
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numbbers  



Joined: 05 Nov 2002
Posts: 1910
Location: Highlands Ranch, Colorado

PostPosted: Sun Aug 22, 2004 11:57 am    Post subject: Reply with quote

IMHO, don't use "high performance" after market rings such as perfect circle. They are made from harder metal, so they will last longer. But, they cost too much, and they take for ever to seat, so you end up using oil for three or four thousand miles. The stock rings will last 200 thousand miles. By then, everything else in the engine is worn out, so you are going to have to rebuild anyway. Buy a set of 10 over pistons, with rings to match. Take the block and the pistons to the machinest, and have it bored to fit.
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goturbo  



Joined: 15 Jun 2004
Posts: 87
Location: Massillon Ohio, USA

PostPosted: Sun Aug 22, 2004 2:49 pm    Post subject: Reply with quote

Great stuff! Thanks Guys. What about those flat top pistons that keep the valves from crashing? I have seen some people using them and while I am not really worried about breaking a belt, but it seems like they are available and cheaper(?) than stock. I agree about the stock rings, although I like the total seal rings. I use them on my other turbo car and they are great. If I am going to bore the engine I would like to go as big as possible so long as rings don't get to be too much of an issue-which it seems it could be.
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Lizard  



Joined: 03 Nov 2002
Posts: 9364
Location: Abbotsford BC. Canada

PostPosted: Sun Aug 22, 2004 4:17 pm    Post subject: Reply with quote

if you are going to replace the pistons go with forged aluminum pistons and then you can crank up the boost
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wdb  



Joined: 02 Nov 2002
Posts: 2024

PostPosted: Sun Aug 22, 2004 10:42 pm    Post subject: Reply with quote

I'm sorry if this muddies the water further . you can get a bottom end (block ) rebiuld kit for a 77-79 AMC concorde , gremlin , spirit , for about $350 , that includes pistons , rings , con rods , main and rod bearings and all the gaskets ,seals needed from a machine shop , parts stores wont carry these old parts . using the N/A pistons in a 931 will drop your compression to around 6.7 :1 . some say that will lower performance off boost alot , but I'm not so sure it would be that much of a problem or even noticable ,but I never owned a turbo so I could be wrong . 931 pistons cost more than the whole amc rebiuld kit ,but you could use the 931 pistons with the na rods bearings etc to save a little money to spend elsewhere . be aware several members have had spun rod bearings after a rebuild , I dont remeber if any of them posted back what was the cause , but I think what they all had in the common was they assembled thier engine themselves . so I think there is something that they had overlooked , on another site, VW/AUDI forums I,ve read that its VERY important to replace the rod bolts because they stretch when torqued , and cannot be reused . but I cant tell you if that is what happened with thier rod bearing failures . I would say in for a penny in for a pound . if your gonna go through the time and money to rebuild, make sure you do it right , or you might have to do it again . if in doubt , take your parts to a machine shop , have them check the specs of everything and assemble your engine , I dont think it will hurt if you tell them its an amc motor , it could save you some money , the word porsche seems to effect some peoples math skills .
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My924gtc  



Joined: 14 Aug 2004
Posts: 1362
Location: 248

PostPosted: Sun Aug 22, 2004 10:53 pm    Post subject: Reply with quote

wdb I agree about the P word causing people to lick their chops a bit.

As for the bolts, basically none of the OE bolts can be re-used according to what I have read, and been told by my engine builder. Head bolts, rod bolts, and cam cap bolts are all stretch type and will never be as good as new. You can get away with the cam caps, but no-no on the others. It is the common factor in spun bearings after a recent rebuild (as demonstrated by me on my first and only attempt at complete rebuild) That will NEVER happen again thanks to those fine Detroit (Troy really but Detroit sounds so much better when you are speaking about cars) boys at Raceware.
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Lizard  



Joined: 03 Nov 2002
Posts: 9364
Location: Abbotsford BC. Canada

PostPosted: Mon Aug 23, 2004 4:34 pm    Post subject: Reply with quote

on the AMC gremlin, the main bearings should work, i dont know however that the rod journals are the same size, anyone want to confirm? also the pistons are we sure that they are the same size as the 924 in diameter?

man someone should get one of these amc blocks just to check
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goturbo  



Joined: 15 Jun 2004
Posts: 87
Location: Massillon Ohio, USA

PostPosted: Tue Aug 24, 2004 1:19 pm    Post subject: Reply with quote

I read somewhere that the fasteners were all torque-to-yeild, so I was looking into some ARP stuff. They get about $200 for a set of rod bolts, and about $400 for the head studs.

Are the AMC pistons forged? I want to use forged pistons for piece of mind, although I might use cast pistons with a coated top. It depends on the cost of the forged pistons vs. cast ones. If I can get the cast AMC pistons, and they fit, then coating would be the answer. But I don't want to do too much damage to the squish inside the CC, so I would have to do some homework there. I like to think that Porsche Engineers had a good idea of what they were doing in that regard. I think that a CR of 6.7:1 instead of 8.5:1 would really kill the preformance, although one might be able to run 25 pounds of boost!? .

So flat top pistons, any takers? Also I have all winter to get this one done so taking some time to scrounge some parts won't be a problem.
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Lizard  



Joined: 03 Nov 2002
Posts: 9364
Location: Abbotsford BC. Canada

PostPosted: Tue Aug 24, 2004 4:52 pm    Post subject: Reply with quote

no idea what the AMC used but I would be willing to be dollars to donuts that they are cast.

and as per using cast and getting a coating on top dont. the coating actually retains heat and you dont want that either run factory cast, or buy forged aluminum custom pistons.
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Racing  



Joined: 27 Apr 2004
Posts: 374

PostPosted: Tue Aug 24, 2004 6:51 pm    Post subject: Reply with quote

Basically goturbo the thing is that the 931 engine is becoming of age.
To us that means that a lot of the pts are starting to become scarse in the parts bin.
Pistons are one of those components.
Not only that,but the OEM slugs are like most stock NA Porsche pistons of poor quality.(After a while you start to believe that they are made out of marang)
Dunno if you´ve used the search function of this forum,or if you´ve been around while the debate has raged from time to time,but the thing is that some of us have pondered "different" solutions to the piston dilemma.

First of all the mere idea of a heron piston crown design has proved itself to be a fairly poor one to start with(Heron means that the piston crowns holds all or some of the combustion chamber space).
Engine makers like for instance Volvo and VW apart from Porsche has tried it at one time or another.
To my knowledge the only engine developer ever to muster a heron and conventional CC space in the same engine...is Porsche with the 931 engine in stock form.

Here´s the point..
As knowledge steps forward it has shown that from a practical standpoint squish areas for a more heavily boosted engine is a drawback as it makes the engine more detonation prone.
This has been proved more times that you can imagine by now.
Granted,running an engine sans squish-as long as we´re talking designs of age(like the 931)-will make for a less efficient engine in NA or low boost form,and then only minutely so.But..
As soon as you start up the boost much past 15 psi the table is turned and the amount of squish "needed" deminishes by the minute.

So.
What´s the bottom line here?
Basically that you need to clamp down on a level of performance expected/wanted and ad the amount of expense as well as trouble of obtaining the pts needed for your rebuild.

I´ve opted to mod the small end of the rod bushings to 22mm contra the stock 24 right off the bat as this gave me more leeway as far as piston choice.
Also please have in mind that the stock GTS aso sports the same 22mm dimension.
What i at least did was to bore the block out to 87mm and install what can be considered cheap generic forged Mahle flat tops really intended for use in the even older air cooled VW dub.
The ones of interest to us is the "minor" big bore 87mm ones(Stock is what...85,5 for those i believe)

For vastly LESS greens than stock cast slugs you get a truly forged counterpart to abuse as you see fit.As far as i´m concerned that´s a win/win situation,and as we all know there´s not many of those around in life..

Granted.First time out there´s that added cost of rebushing the stock rods.
Another thingy to have in mind is that as far as i know SOME of the Mk2 engines sport longer CC rods.At least so when table compression heights for different series engines is compared.

With stock head gasket,and in my case a fairly much larger compressor on the turbo,we´ve successfully boosted the stock head gasket inxs of 1,3 bars with added fuel,intercooler-the works.
At those 1,3 an educated guesstemate would be that the engine is producing in the neighbourhood of 300 honest ones on said current compressor housing aso.

After that level tho the head gasket goes AWOL at some point,no matter what you do to it-even so if you go out and threaten it..

Apart from said culprits-the slugs and the head gasket-the engine as a whole is built like a damn tank.

Word of precaution tho.
IF you foresee to,like i did,scrap the OEM headgasket all together to push the envelope(or at least be able to) make sure that you get the engine blocked decked while at the machinist.
This is absolutely essential to make a copper head gasket seal when the need of it arises.
Frankly speaking not doing so when at the machinist to have the block bored anyways is really bad engine building practice on an engine as old as he 931 if you ask me
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wdb  



Joined: 02 Nov 2002
Posts: 2024

PostPosted: Wed Aug 25, 2004 1:27 am    Post subject: Reply with quote

goturbo ,I believe the 931 CR is 8.1:1 and the na is 8.5:1 if the piston were flat top, at 8.1:1 the space between the pistonand the head would be 10.5 mm , at 6.7: it would be 12.6 . from a 1/3 of an inch to a half an inch , do you really think that small change will have a dramatic effet on performance ? you can calculate the change here

http://www.bgsoflex.com/cr.html

http://www.bgsoflex.com/auto.html

Cylinder head Volume (CC) = 0.00
Piston Top Volume (CC) = 30.5 CC's dont know wut in inches 1.2 ? maybe
Cylinder Bore (Inches) = 3.4
Cylinder Stroke (Inches) = 3.96
Deck Height (Inches) = 0.390
Head Gasket Thickness (Inches) = 0.020
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goturbo  



Joined: 15 Jun 2004
Posts: 87
Location: Massillon Ohio, USA

PostPosted: Wed Aug 25, 2004 3:01 pm    Post subject: Reply with quote

Racing-That is some good stuff. I have searched through the mounds of posts on this site and found all kinds of converstaions about pistons. I did not see much follow up, but I like to assume that no news is good news on that stuff.
First off are the 'generic' flat top pistons that you are refering to the same ones that wdb et. al. have mentioned that give a CR of roughly 6.7:1? That is the road I would like to go down if I don't loose too much compresion with them. If you have the Part number, that would be great!
Good point with the 'squish' info. I could not argree with you more and I guess that I should have said that I only plan to run 10-12 pounds (IC of course) of boost. At that point I think that it woud be line-to-line as far as the benifits from keeping the 'stock' CC shape. I wanted to throw that out there to see if anyone threw up a flag saying that it killed preformance on this engine/head design at that HP/boost range.
To me this engine is very modern compaired to my other turbo car, so copper head gaskets, custom wrist pins/bushings, mix and mach pistons, and so forth are pretty much par. Its good to see that someone else has already done the 'expensive' part of the research this time.
You did not mention if you were using o-rings with your copper head gasket. Seems like that would be a given, but I guess that not all engines need that. I guess that I have a good idea of which direction to go with the pistons, provided the CR isn't too low and the part numbers...

wdb-That web link is pretty cool Thank you I will bookmark that for future use. I did run the numbers and it looks like with what you gave, that there is roughly a 5% decrease in power. That does not take into consideration the effects of forced induction. When you run the numbers on a 'true' engine simulation that accounts for FI, it looks more like 10-12 percent which might be fair ammount to some people-myself included. I know people that would give away critical reproductive portions of thier body for said HP-I'm not one of them though!
Anyhow, if I can get those pistons for fairly less that any others, I might change my plans to accomidate them.
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Racing  



Joined: 27 Apr 2004
Posts: 374

PostPosted: Wed Aug 25, 2004 8:04 pm    Post subject: Reply with quote

Goturbo.
Nope.
The slugs i have in mind will end up TDC 5,4mm down the hole.
Pt nr isn´t on hand,but wouldn´t be to much of a hazzle to dig up.

As far as compression ratio don´t fret.
What we ended up with was just over 8:1,and the motor runs as happy as ever on that.
Might be to the point to adress that apart from said mods...
We tossed the innards of the stock dist and replaced with a Hall unit from a late 80;s rabbit/golf.
Hookup is simple.
Reason was that the stock reluctor unit had serious probs because the OEM magnet within it had vast amounts of hairline cracks in it.
As we control ad on fuelling and spark retard over a Perfect power SMT6 we needed to have a pulse that was much clearer than the OEM reluctor could provide,and a Hall from a golf is readily available at basically any junkyard for nickle and dime money.

Anyways.
Please have in mind that we run a fairly "large" compressor at a regulated boost of 1,8-2,0 bars as of current.
At those levels at least the compressor is pushing approx 450 hp worth of air.
As for how much the engine is developing...your guess is as good as mine.
This much is however clear.
We are running against a WB device based on a Bosch LSU4 with loging capabilitys as well as an exhaust temp(pyrometer).
We basically run a home brew intake(which can be seen in other posts of the forum) based on a 70mm Alfa throttle housing and Bosch 036 injectors for ad on fuelling and a slightly ported stock exhaust manifold.
Downpipe from turbo is a simple 3" as is the whole exhaust.
p1 vs p2 is within parameters and exhaust temp at full throttle stabilizes at 910 degrees celsius as of current at set boost limit.
In other words the whole system seems to be doing its job.

Also hooked up an ALS over the SMT unit,and that means all in all that without caution we´re off to smokesville through the first three gears if we go WOT no matter what we do.

Timing at full throttle and boost is set to 18 deg BTDC total.

I hear ya on the development issue.
We really didn´t have all that much people to ask around for replys either,and as due course we blew stuff up in the process too
That is not to say that what we did her is gospel,just to say that it´s ONE way of doing it that at least works.

Next natural step would be the install of a stand alone EFI if you ask me.
I spent countless hrs on trying to get the CIS to perform to my liking and everytime i got close sure enough something else within the sys F:ed up.
Point is moot tho as i sold the car approx 2 weeks ago leaving us back to scratch as one of my employees bought a 931 as well after a couple of runs behind the wheel in mine.
Or as Sven said....
"S*it broh!
If this is the performance to expect from this obviously vastly under rated car for nickel and dime money sign me up to get one!"
Said and done...
So at the time being we´re going through the stages of building his up to meet par.
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Joes924Racer  



Joined: 03 Nov 2002
Posts: 11960
Location: Oregon, Denver Colorado native!

PostPosted: Thu Aug 26, 2004 3:17 pm    Post subject: Reply with quote

So racing you said you sold yer car is that right..
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