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931 Pistons
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Peter  
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PostPosted: Mon Aug 06, 2001 11:42 pm    Post subject: Reply with quote

Hi all,
I just weighed the pistons from my 1980 931 project car. The block has been overebored to accept 86.95-mm Mahle pistons. The gram weights for each piston, respectively from cylinder number one to four, are 486g, 485g, 485g, and 485g. The pistons were weighted on a new digital scale that claims to be accurate down to one gram.
My question is has anybody lightened his or her pistons? The performance engine books that I have read pertaining to lightening pistons deal primarily with Chevy & Fords. The Mahle pistons have a raised notch on their undersized situated over one of the two openings where the wrist pin slides through. Stated another way, when you turn the pistons onto their underside, the round whole cut through the piston to allow the wrist pin to slide through is built up on the inner side of the piston for support. Both inner sides of the piston have this same built up area except that only one side has a long raised notch (since the pistons are directional, the side closest to the rear of the engine has this notch). It does not appear to offer any extra strength to the piston and I believe it is casting flash. Removing it would reduce the weight by a couple of grams.
So basically my question is has anyone ever done this? Any thoughts on lightening pistons?
Thanks in advance.
-Peter A. Holiat
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924RACR  



Joined: 29 Jul 2001
Posts: 8794
Location: Royal Oak, MI, USA

PostPosted: Tue Aug 07, 2001 12:40 am    Post subject: Reply with quote

Since this is a turbo car, I wouldn't do anything to the pistons beyond balancing, unless you were to hard anodize the crowns for durability. This is the point - build the bottom end for durability, tweak the head and induction (turbo and intercooler) for power...

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'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
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Zuffen  



Joined: 31 Jul 2001
Posts: 1426
Location: Owasso, Oklahoma 74055

PostPosted: Thu Aug 09, 2001 2:45 am    Post subject: Reply with quote

Keep in mind that if you don't have enough metal on a piston you won't be able to transfer heat. On a turbo you can add a lot of heat. A longer skirt allows for heat dispersion. Racing pistons are replaced with regularity because they get hot and gall up. I balanced mine (which were pretty good already)and easily reved to redline , with no harmonic problems. Sure it would help to lighten the assembly but spend your time on the rods and leave the pistons alone.

By the way when you balance, and you probably already know this, measure all you r parts and try to match for assembled units.

The rods, pins, clips and pistons need to be weighed assmebled and averaged as close for all four. And this after any special coatings have been applied. I would teflon coat the pistons top and sides. Unless you can do a cermic coat on the top and teflon the sides.

bob
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Peter  
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PostPosted: Fri Aug 10, 2001 1:32 am    Post subject: Reply with quote

For the pistons, I am just going to take some 400 grit paper and polish the casting/forging mark on the underside of the wrist pin whole to get one gram of weight off the one overweight piston.

For the connecting rods, I will just remove the forging marks with a carbide bit, which are very visible and run along the rod --being careful to only remove the marks and not into the rod itself. Then polishing the rods, radiusing the oil holes and any all sharp edges. I have noticed that sharp edges exist where the bolts go through the rods and that some little cuts have started to form. The deburring and polishing will lighten the rods, but its main goal is to significantly strengthen the rods.

I will have the machine shop balance the entire assembly: pistons, rod, crank, clutch, flywheel and pressure plate. Just out of curiosity, for those who have rebuilt their engines, is balancing a must if you are just replacing bearings and rings? I am not complaining about the $200 cost of balancing, but I have been told by some that Porsche balances all their engines already. I also know that new bearings and rings and the stresses that the block underwent to become “seasoned” all contribute to push it out of balance ever so slightly, and since you have it apart already, might as well balance. Also Jim Pasha recommended balancing the 931 engine. Just wondering what everyone else does.

-Peter A. Holiat
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larso  
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PostPosted: Fri Aug 10, 2001 7:13 am    Post subject: Reply with quote

u can get 1000 grit sand paper u know
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Zuffen  



Joined: 31 Jul 2001
Posts: 1426
Location: Owasso, Oklahoma 74055

PostPosted: Sat Aug 11, 2001 12:41 am    Post subject: Reply with quote

Peter I'll send you a pic of a 931 rod modified for mysting the underside of the piston if your interested. I think Porsche does a nice job balancing. but it wouldn't hurt to try it once and to see if the assembly stayed in tune after "Seasoning"

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Bob Dodd - 924turbo@cox.net
931 1982, 944 1982 euro, 924S 1988SE, 93 968 tip 06 Silver Cayenne S, 06 Black Cayenne S

I have Way too many cars, parts for the 931,944 and 951
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wdb  
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PostPosted: Tue Aug 21, 2001 9:02 am    Post subject: Reply with quote

if you know someone with a gun powder scale ,they are more accurate 3 grains to a gram,and a triple beam scale can go to 1/10 a gram. maybe your local gunshop will allow you to weigh your parts.just a thought.
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Fast924Turbo  



Joined: 02 Nov 2002
Posts: 122

PostPosted: Wed Nov 06, 2002 7:23 am    Post subject: Reply with quote

Mine are MAHLE and they're 500 g for all four pistons, they're thermal coated on the crowns, and coated on the skirts for slickness. Apparently these particular coatings don't affect required cylinder bore.

I'd offer up "yes" to balancing the system as a whole. It's of no use to have all pistons balanced if a rod is out of balance. I've done it to one of my cars and liked it, and I'm doing it this time too. Yes I'd guess Porsche would balance their engines, but I'd say no, the system won't be in balance if you take any casting marks off the rods so it's a bit of insurance that can't hurt.

You're wise to do it including the clutch.
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simsport  



Joined: 06 Nov 2002
Posts: 573
Location: UK Warrington

PostPosted: Sat Nov 09, 2002 8:45 am    Post subject: Pistons Reply with quote

Is there a good suply of the original pistons in the US Then?
I cant get them in the UK anymore. JE are about $600 and I suppose cheaper than the Mahle items????

Simon
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Fast924Turbo  



Joined: 02 Nov 2002
Posts: 122

PostPosted: Sun Nov 10, 2002 2:43 am    Post subject: Reply with quote

I dunno if the MAHLE pistons are in good supply in the USA or not. When I phoned EBS a few weeks back about getting original 2nd overbore 8.5:1 compression pistons with rings they said they'd have to get them from Germany.

If I recall the threads here right, the MAHLE are heavier than the JE. I provided the weights of my MAHLE ones in an earlier post. So many people here speak highly of the JE pistons that I'm hesitant to disagree with them - so many liking them, they can't be far wrong. And if the weight is significantly lighter, then they're less strain on the engine. I don't know the JE pistons well. I think Vaughan has some data on those. But I'm pretty sure EBS in Reno carries them.

If you want MAHLE, you might have good luck in trying to find a good UK supplier that deals directly with Germany. I got killed on duty, taxes and purchase price when I went through the USA. Not that the American prices were so bad, but I got nickled and dimed after the things arrived at my door.

My MAHLE were $750 USD plus taxes (15%), shipping ($), and duty ($300 CAD) from EBS.

I would only suggest this: If you go JE, check the weight of the pistons without wrist pins and circlips before you buy. And check ring end gap when you get the set. Heresay has it that US suppliers buy these pistons as 'end of lot' in large numbers and seldom if ever actually check the mating of rings and pistons. And think about thermal and teflon coatings. I did mine for $300 CAD crowns and skirts.
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924RACR  



Joined: 29 Jul 2001
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Location: Royal Oak, MI, USA

PostPosted: Tue Nov 12, 2002 12:48 am    Post subject: Reply with quote

I'm still awaiting my JE's from EBS... they're custom. However they are also for my IT car, not one of the turbos.

JE has a very highly respected name in engine building. I would hesitate to second-guess their work. They're also not batch-bought, but rather they are custom-made to order. Wait is 2-4 weeks - at least for the ones I ordered.

There's also other things you can consider in ordering the pistons, talk w/ John at EBS (tell him I told you to talk to him - sometimes weight can be reduced in the wrist pin. I don't know if this is adviseable for a turbo, ask John. Yes, he does know his 924's. They can be made shorter, and use teflon buttons to hold them in place. I've also heard of people (this coming from V8 engine building) using smaller bearings - like grinding the crank in a 350 down from about 2.25" to 2" and using Acura bearings with special custom rods made to use the small bearings. This would reduce weight and frictional losses, freeing up power.

Get creative... and talk to the expert that know the engines. They don't hang out here, really, but they run the shops.

BTW, the $600 or so for JE's includes new wristpins, circlips, and rings.
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'79 924 #77 SCCA H Prod racecar
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Peter_in_AU  



Joined: 29 Jul 2001
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PostPosted: Tue Nov 12, 2002 10:16 am    Post subject: Reply with quote

Vaughan, what sort of info do EBS need to order a set of custom pistons? If I'm wanting a set of 10.5:1 pistons for a NA what measurements do they need?
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1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

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Peter  



Joined: 04 Nov 2002
Posts: 379
Location: New York, NY

PostPosted: Tue Nov 12, 2002 12:53 pm    Post subject: Reply with quote

During the summer I contacted EBS racing (John I believe, but could have been Don) and JE can make any size 931 piston. They just need bore size & compression. JE pistons require a special bushing, which EBS also sells. 931 pistons include the piston rings, bushings are an extra $10 US each. They are forged and lighter; two great tastes that taste great together Total for 931 piston set is about $800 US
As for piston coatings, I looked long and hard into this subject. Some professionals swear by them, others swear at them. It comes down to a personal choice as you can speak to pro engine racecar builders and get different views. For my 931 rebuild I decided against them for two reasons. The first being that in Corkey Bell's book, Maximum Boost, he does not believe they work and does not recommend them. Because Corky knows more about turbo engines than me, I followed his advice.
Second reason is cost. I did not want to spend my money on something that was not proven.
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Fast924Turbo  



Joined: 02 Nov 2002
Posts: 122

PostPosted: Wed Nov 13, 2002 12:30 am    Post subject: Reply with quote

What does Bell say is the risk of using coatings?
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924RACR  



Joined: 29 Jul 2001
Posts: 8794
Location: Royal Oak, MI, USA

PostPosted: Wed Nov 13, 2002 12:43 am    Post subject: Reply with quote

I do agree with not wasting money on things unproven.

However, I also believe there's a huge reservoir of engineering talent out there that doesn't use new technology because they never did before. IOW, old stuck-in-the-mud types who can't deal with new concepts. Sorry, but I think that, in this case, Corky falls into that category.

As mentioned before, they even went so far as to use the teflon coating in the Saturn (SC2) stock piston design. Very cool, and those motors are slick. I wouldn't put it on the tops, though, only the sides; the top should get either anodized or ceramic-coated. I'd probably go with the ceramic. There's some 951 guys getting into this now, go hang over on rennlist, check 'em out. They have a much better knowledge base than we do when it comes to the latest tech, go figure! Might also pick Derrek Huntley's brain on coatings.

One last note on engineers and technology... a local friend and 924 owner from the track used to work @ GM way back when. He's told me some really amusing stories involving technologies which are long since proven now, and in widespread use, but were not even considered back then because of technological inbreeding. Wait to see how common coating etc. are in 10-20 years...
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Vaughan Scott
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'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype
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