924Board.org

BillA · Joined: 27 Dec 2002 Posts: 170 Location: Syracuse, NY

Well, here is my problem I purchased these pistons from PAECO a few years back, my intent was to use an NA head with this setup. I was told they were rated for 11:1 CR It is a 931 block bored out .040 over, crank and rods are 931 as well. After my machinist finished boring, balancing and a couple of other things he called me and gave me bad news. With this current setup I'd be lucky to get 5 possibly 6:1 CR. Don't know if shaving the deck would help at this point any suggestions would help. Oh, I called PAECO and did not get any satisfaction. I also called VENOLIA, they made these pistons, gave him the part number off the piston and all they could tell me was they stopped carrying that piston back in 91. Any ideas would help. Thanks for the help and advice for loading pictures on the forum!

Khal · Posted: Wed Jan 18, 2006 12:42 pm Post subject:

The 931 block, crank and rods are the same as the 924, as I unsderstand it. So that will make no difference.

The 931 pistons are different, they are dished and sit flush with the deck of the block at TDC. The 931 head has combustion chamber recesses in it.

This is different from the 924 head, which is basically flat with a small recess for the valves (if you've got a 924 head, you've probably noticed that).

The 924 pistons are recessed in the bore, forming the combustion chamber in the bore.

So, shaving the deck of the block will raise the compression. But you can only shave it a certain amount (about 0.5mm or 0.020 in., I've read), otherwise valves meet pistons...

Though the compression gain is only 9.6:1 over standard 9.3:1, apparently.

BTW, those pistons look all but identical to the original ones to me...

EDIT: No, sorry. Yours look like they have a much larger/deeper bowl in them

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Peter_in_AU · Joined: 29 Jul 2001 Posts: 2743 Location: Sydney, Australia

was the compression ratio calculated using a 924 or a 931 head?
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My924gtc · Joined: 14 Aug 2004 Posts: 1362 Location: 248

Gee our first real PAECO feedback and low and behold there is a problem, a major problem. Maybe now people on this board will think twice before they recommend them to our members without having personal experience with them. Interesting that they use Venolia for their pistons considering that they are on the opposite side of the country. What nobody forges pistons in Alabama? Probably not. Jacks of all trades, masters of none.

Anyway, yea the dish in those pistons is way off. Here are Euro 9.3:1 and US 8.5:1 stock slugs

As you can see the dish is the only thing that changes and that the Euro's have nearly no dish. At 11:1 you would have even less.

Just out of curiosity...why the 924 head?

Khal · Posted: Wed Jan 18, 2006 2:09 pm Post subject:

moone924 · Joined: 05 Feb 2003 Posts: 868 Location: Douglas Wyoming

without mod he could use the turbo intake manifold.
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Ryan Moone

Always shopping for a nice s2 931 to baby.
New philosophy : one car project at a time.

Khal · Posted: Wed Jan 18, 2006 2:54 pm Post subject:

moone924 · Joined: 05 Feb 2003 Posts: 868 Location: Douglas Wyoming

the injectors are in the head on the NA head, and without walking to my shop I can't tell you where the injectors are on the turbo head, but wherever they are I'm sure someone could use the NA injectors in the turbo injectors place.

edit..... in the head or in the intake manifold. Also know it's possible to use the turbo intake manifold on a NA head with some grinding and such, so it should be the same the other way around.
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Ryan Moone

Always shopping for a nice s2 931 to baby.
New philosophy : one car project at a time.

Kenodog · Joined: 25 Jun 2003 Posts: 2669 Location: Vancouver,B.C.

Just out of curiosity what is the measurement between the wristpin and the top of the new slug ? Also measure the stock na slug.Then all you have to do is cc the dishes in both to figure out the cr exactly.I assume you're using stock 924 rods.With these engines I find it hard to believe you could even get an 11:1 cr unless you recessed the valves somehow.Someone here did the math once and figured that you would have to shave 0.050" from the block to raise cr by .5 .This means adding almost .170" to the top of an already tall Eurospec piston! Way too close for my comfort.
That being said you may find taller rods.There seems to be a bit of mix and match between Vdub rods and ours,you may be able to get GTI or Rabbit rods that will do the trick for you.You should be able to find a thread in the archives here.There is also the possibility you can use the 87mm Vdub flat tops instead?Shaving any substatial amount from either the block or head is definitly not an option.
I've dealt with Paeco on a few things and have never had a problem,in fact I found them to be really good to deal with.And hey let's be realistic,I don't know of ANY company selling ANYTHING that would do an exchange over the phone on a product bought a couple of YEARS ago.

caveat emptor
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1979 Euro 931, Olive
1981 931, Sabine
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Khal · Posted: Wed Jan 18, 2006 4:23 pm Post subject:

morghen · Joined: 21 Jan 2005 Posts: 9095 Location: Romania

if you have a machined NA head:

1. get a hudge turbo and make 300hp+ with that CR(maby get it to 7:1)

or

2. get new pistons...to raise your CR, those are turbo pistons as i see it...they were designed for low CR...
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Min · Posted: Wed Jan 18, 2006 9:06 pm Post subject:

http://kb-silvolite.com/calc.php?action=comp

Get the data, plug it in, play with the stuff you can change(like deck height and gasket thickness) until you get a acceptable cr.

Min

-nick · Joined: 16 Nov 2002 Posts: 2699 Location: Cambridge, MA

How did your machinist determine the compression ratio? Dish volume doesn't mean anything without knowing the compression height of the piston (distance of the wrist pin to the crown). I think this is what Kenodog is alluding to. It's difficult to tell, but it appears that your pistons sit up a little higher in the block than stock. Hard to say without measuring though.

My924gtc- have you had bad experiences with Paeco? Sounds like some sort of grudge? I believe that they use only Wiseco pistons now, not sure about a decade ago. Either way, Venolia has a pretty solid reputation as far as I know. I'm not sure what proximity has to do with picking a piston manufacturer?

I called up Paeco a couple years ago and got a hold of a random tech guy who was surprisingly knowledgeable. Anyone who can quote piston-to-cylinder wall clearances off the top of his head for a car that isn't necessarily their specialty makes me listen. Of course, he tried to push one of their prepared long blocks a bit, but not down my throat. My impresion is that they test their builds pretty well before marketing them. Something few "tuners" seem to do.

This is getting off-topic. Let us know how the CR numbers were calculated.

nick

My924gtc · Joined: 14 Aug 2004 Posts: 1362 Location: 248