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Oversized pistons


alperin

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I am currently (and slowly) building a 944 racecar and would like to stick to spec rules. Upon tear down I noticed the bores were reasonably scratched and I would like to bore the cylinders to 100.50 mm and get oversized pistons. Is this legal?

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Let me make it cystal clear.

 

NO

 

Stock pistons only. No Oversized pistons.

 

 

If you bores are worn then get a another block. The uses ones are cheap enough. If you just think the block is worn, I'd bet it is A-ok. The silicon/aluminum material of the block is very long lasting and bore do not need to be honed in most all cases.

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Alperin

sounds great that you are building a car to come race in 944-spec. I hope you can see why the rules are so tight. our racing is based on equal cars at a low cost. some of the other "spec" class cars have seen the build costs go through the roof for a race winning car We ae still trying to prevent that here. If we allow larger bores and you get 2 HP extra then someone will "need that 2 HP" and do a rebuild etc etc etc. it seems to me the most guys do is change out rod bearings with most of us just changing the timing belt and off we go to the races. i have run 2 different cars this year and I did not do a darn thing to the motors (since i got them) .... I do plan on changing the oil this winter.

 

Hope this helps

 

Chuck

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Alperin

sounds great that you are building a car to come race in 944-spec. I hope you can see why the rules are so tight. our racing is based on equal cars at a low cost. some of the other "spec" class cars have seen the build costs go through the roof for a race winning car We ae still trying to prevent that here. If we allow larger bores and you get 2 HP extra then someone will "need that 2 HP" and do a rebuild etc etc etc. it seems to me the most guys do is change out rod bearings with most of us just changing the timing belt and off we go to the races. i have run 2 different cars this year and I did not do a darn thing to the motors (since i got them) .... I do plan on changing the oil this winter.

 

Hope this helps

 

Chuck

 

Thank you for the insightful post. Looks as though I have some thinking to do.

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The upshot of this thread is that I went back over the records that I got with the car, and found out it has 100.5mm pistons . The good news is, that if the car sells I will look for a built spec 944 racer .

 

So... my car is FS:

 

I purchased this car this spring, and loved it, so much that I want to go spec 944 racing. To that end the car has been gutted, cat removed, & Lexan side windows put in. Unfortunately I've recently learned that the .5mm overbore that the car had as part of a 4K motor rebuild is illegal in spec 944. I've decided to go out and look for a built car instead of pulling the motor and finishing the build on the 924S. Therefore the 924S has to go. The car has no issues, and has run fawlessly for me through the summer. It's now down to 2,350 lbs, and is quite fun to drive in autocross or trackday events. Due the 924/944's spectacular balance/handling, it is very quick, while still being easy on tires & brakes - great budget racer. While it still has plates and a current street regitration, it's has no cat and is therefore not technically street legal. Very solid car, very tight, no rust. The car has a 3" stack of records and receipts from the prior 2 owners - Highlights:

 

$4k+ motor rebuild at 137,000 miles (chassis has 173K miles). New susp. bushing and Koni yellow struts put in around that time.

New water pump last year.

Slotted, cryo treated rotors & HP + pads this year.

 

I will sell the car for $4,000 with stock seats & steering wheel (not pictured)

The Recaro SPG seats, Momo steering wheel & Sabelt Harnesses cost me a bundle, so I would like to keep them, but would consider selling that as as package for $1750 (seats alone cost me more)

 

The car will be at the October NASA event at Road America.

I will store the car indoors over the winter for free if needed.

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Sterling,

Sorry to hear about the engine. The reason we don't allow the overbore is that it is very expensive to soruce pistons for it and because if allowed EVERYONE will feel that they NEED overbore pistons to run up front.

 

Make sure you post this on rennlist and even 944 cup. I believe they allow the overbored motors so the car has some value for them.

 

You know you could also pull the motor and just sell it. Then get another block and run that.

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Hang on, guys.......

I just got word of this thread.

I'm looking at the rules and the factory manual ( Volume 1 Engine,page 13-31).

The rules say:

 

Item #8- "All published measurements infer a tolerance of +/- one half of the last specified decimal place."

Stock standard pistons are about 100mm. There are actually 3 tolerance groups of these, from 99.980mm through 100mm.

 

First oversize pistons also have 3 tolerance groups, 100.480, 100.490 and the biggest, which is 100.5mm. Half of one millimeter.

 

The last specified decimal place of 100mm is the 1's column. Half of 1 is .5mm.

 

Further, The rules state in Item #11 that "All engines and their internal components most remain stock......and within factory specified tolerances."

These oversized pistons are a factory stock fix and clearly within factory tolerances. There are no provisions in the factory manual for going any bigger than 100.5mm pistons, so THAT would seem the correct time to buy a new block.

Further still, the factory manual states that while individual pistons and cylinder bores must use matching tolerance groups, "Different tolerance groups could be used in one engine." So you wouldn't be forced to buy a set of 4 oversized pistons and bore out all 4 cylinders.

 

I've seen some pretty noticeable scoring that turned out to be much ado about nothing when pointed out to the guys at Motor Works, our local German machine shop. So make damn sure you actually need to "bore" out a 944 block vs. just honing it.

 

I see this as both legal and within the spirit of the rules and I'd love to see both these guys racing in our class, instead of throwing away their otherwise good engines.

Anybody see anything different?

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Tim,

Stock pistons are stock pistons. Oversize pistons are well oversize #1 and although in the factory manual they are not as the car came from the factory. They are clearly stated as "oversized". If need be maybe we need to say explictly no oversized pistons, but allowing oversized pistons in 944 spec is not a good idea. How much do oversized pistons cost? That plus the cost of honing the bores will scare anyone away from allowing overbores.

 

 

I do have to agree on light scoring. My currnet block was rebuilt in Jan 2003. It had a blown headgasket and coolant mixing. #4 piston rings were fused in the piston. So I tried to put a different piston (same tolerance group) in the bore. Well it hung up and scratched the top of the bore for an inch or so. Well a little light sanding with fine grit paper took off the high spots and we put it together and hoped for the best. That motor still runs in my car and runs strong. 134.2 hp strong with a nice torque curve.

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Why would you want to shut down headers (at $650) but allow boring and over-sized pistons? Have you done this yourself? Has anyone else out there done this? I'm not going to the rule book now, too many other things going on. But, I thought we couldn't "machine" parts? Isn't boring considered "machining"? Hey, you want to try your luck with a piece of sandpaper, good luck to you. But boring out and over sized pistons! Now your really openning a can of 'expensive' worms.

 

P.D.

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I am under the impression that (slightly) oversize pistons DID come stock... It seems that when you disassemble a motor there are pistons markings that determine whcih piston group they came from....

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I am under the impression that (slightly) oversize pistons DID come stock... It seems that when you disassemble a motor there are pistons markings that determine whcih piston group they came from....

 

Jim,

Those are the 3 tolerance groups 0, 1,2 marked on the pistons. These are not considered oversize, but in the factory tolerance. Tim's post has their dimension and thier max is 100.00 mm. These are the stock pistons.

 

The pistons in the Oversize 1 tolerance group are not allowed these are close to 100.4 to 100.5 mm.

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I appreciate all the thought and support put into this. It took me all of one day to have a handshake deal on the car for what I paid for it. I'm going to keep the extra wheels & tires and go looking for a built 944 over the winter (hopfully closer than California.) I'd still like to see spec 944 get off the ground in the Midwest, and it may happen faster this way than building one from scratch. Sorry to open a can of worms here .

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Dilly, I see a big difference here between this act to keep and engine alive, and headers, which are only for added performance.

 

A new 944 could have been delivered with 99.980mm pistons in all 4 holes. No real disadvantage compared to other cars that came with 100mm pistons. They're all considered stock.

 

What we're talking about here is half a mm allowance to keep an engine alive. Think about it in these terms.

100mm = 100%

.5mm allowance is one half of one percent of the total displacement. No real advantage on the track.

Yes, in inches this fix is about .196 to .020.

How much shaving do we allow in order to save a head?

 

Sterling, No worries, mate!

It's a great point and one I've never come across. Good luck finding a 944 spec car closer than California. If you don't find one, I can build one and you can drive it back. Keep spreading the word about the class.

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I am currently (and slowly) building a 944 racecar and would like to stick to spec rules. Upon tear down I noticed the bores were reasonably scratched and I would like to bore the cylinders to 100.50 mm and get oversized pistons. Is this legal?

 

How much is boring and a set of oversized pistons? How much is a used block or for that matter, a used engine? It completely defies all your other arguments about cost containment. You have no ability to measure any gains, well, except maybe dyno results but you could not determine what gains on the dyno were from rebuild versus bigger pistons. But just as you couldn't measure these gains, you can't measure the gain from Lexan. This would become another 'must have' item, based upon the logic applied to Lexan, LSD's and Headers. So which is it? Things that can't really be measured in time are out for fear of the 'must have' syndrome? Or only 'selective' items are out for fear of the 'must have' syndrome? Have some consistency about your decisions.

 

P.D.

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Tim.

 

Oversized pistons really would fall under the "perception" problem. Once you allow them, everyone will build their motors with them...

 

Are these Porsche OEM over size pistons?? Could we then run aftermarket oversize pistons of the same size?

 

I actually thought you could not bore alusil(sp?)

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Dilly,

No need to buy a whole SET of pistons. This is a "per cylinder" fix. A call to SSF parts resulted in a quote of $1,100 for a SET plus some other stuff. But you'd likely only need ONE. That's $275.

There is consistency in my thinking here.

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Hmmm, let me think. First, the weight of 1 piston at over size versus the weight of 3 others? Yeah, balance issues. Secondly, your rules don't restrict anyone to just replacing one piston, thus, $1,100 a set of 'must have' pistons plus the rest. Sorry, but no, there is no logic to your thinking. It's cheaper to buy a replacement block or complete used motor.

 

NEXT!

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Different size pistons in a single block? Come on! I've seen different sized bearings across a crank and different heat plugs across a head but different sized pistons?!?!? Without the ability to "balance internals"?!?!? Then again, if I could time my laps so the fat cylinder fired JUUUUST when I was apexing the corner, maybe I could get a little extra pull coming out of 13.

 

I also think a necessity 'relative' cost which HAS to be factored in to an overbore situation would be the cost to re-alusil the block. If it's even possible. So your $1100 just went up.

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Guys boring the block is possible. However it takes the right tools and process. Not just any engine shop can do the bores on these blocks.

 

 

So while not impossible the cost for this just went up. $1100 for pistons, plus the cost of the boreing and you now have things that cost quite a bit and have a strong percieved advantage. Really this will create the need for a "built" motor rather than a junkyard rebuild.

 

I really think this series needs to allow junkyard motors to to be competitive. As soon as people believe they need motor built by XXXX shop with oversized pistons will be the start of the end for the class. We state in the rules at the begining that this is NOT AN ENGINE BUILDER'S CLASS. While we can't prevent folks from tearing down and rebuilding and engine we CAN force them to use STOCK parts that includes stock bore dimensions and not oversized.

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