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Saab NG900 & OG9-3 Performance, Mods & Tuning Covers Tuning & Performance modifications for the Saab NG900 (1994 to 1998) & OG9-3 (1999-2002) & '03 Convertible

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  #1  
Old 07-06-04
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Fast_Ed Fast_Ed is offline
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Default Need Advice Quickly: Pistons

Well all, My aftermarket warranty will not cover the damage to my engine. This is, of course, horrible news. However, it does afford me the opportunity to get Forged Pistons for my 2000 Viggen. The dealer is about to order the parts, but I would like some advice on wheter or not I should use a non-SAAB part at a SAAB dealer, and if so, which ones are worth it.. along with any other concerns I may have for this modification, as I am a bit leery of making this change myself..
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  #2  
Old 07-06-04
TurboRush91 TurboRush91 is offline
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I may be a little late here.. but what happened to your Viggen?
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  #3  
Old 07-06-04
SQR-900T SQR-900T is offline
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get after market forged, better choice than stock. Or even older pistons from a 9000 Aero. Id send one of your used pistons to JE and have them make you a Forged set.
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  #4  
Old 08-06-04
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I am glad this topic came up. I profess to be more than a little ignorant about aftermarket piston options. Any input???
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  #5  
Old 08-06-04
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Fast_Ed Fast_Ed is offline
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Turbo, I guess you must have missed that one.. Remember how you're not supposed to let friends drive your Viggen? Yeah, that's pretty good advice: My best friend put it in 1st gear on the highway doing 70 mph. On my birthday, no less. But I didnt hit him even once. So my engine 'over-revved' and the pistons struck the valves. Dont do that. It's not pleasant

Now the car is on a lift, and they're ordering parts. I need my car back, so I can't wait for JE to make pistons for me. Will all Aero 9000 pistons fit my car? Thanks for the replies, I'm really in the dark when it comes to this.. never had anything like this happen in my 200,000+ c900's. Of course, they didnt shift so nice, either.
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  #6  
Old 08-06-04
Duxthe1 Duxthe1 is offline
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Is the car modified or are you planning on modifying it in the future? If it's not going to be modified very much I'd say go for a good set of factory pistons. If on the other hand you're planning on running a lot of boost then I would use this opportunity to get a custom set of lower compression pistons installed. That way you can add boost and keep your ignition timing from retarding the spark a ridiculous amount. I'm down to 8.6:1 from a stock 9.3:1 in my SE. It can hit 20psi with no intercooler or exhaust mods without even a hint of knock, pulling like a freight train the whole time. There are 2 ways around knock, compression and timing. To add more boost you have to back down one or the other, and well timing costs a lot more hp than a drop in compression. With the engine torn down now's the time to make the call :wink:
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  #7  
Old 08-06-04
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How much did the pistons set you back Duxthe1, if you don't mind me asking? Or did you get a special mechanics-only sweetheart deal?
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  #8  
Old 08-06-04
SteveL SteveL is offline
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Quote:
Originally Posted by Duxthe1
If it's not going to be modified very much I'd say go for a good set of factory pistons. If on the other hand you're planning on running a lot of boost then I would use this opportunity to get a custom set of lower compression pistons installed....

There are 2 ways around knock, compression and timing.
First of all, DON"T GO WITH STOCK PISTONS!! Haven't you guys read any of the posts about pistons in the b235r?

Second of all, get your car away from the dealer, they will anally rape you with labor costs, get your car to a reputable indy.

Third, while I recommend going with lower compression pistons, there are more ways to fight knock besides compression and timing. How about fuel octane levels and intake charge temps, not to mention adding more fuel to the mixture. Not that these are recommended by me, just an observation.

I'm about to start rebuilding my blown Viggen motor (cracked piston) and I'm going with lower compression forged pistons like Duxthe1. In fact I'm about to start looking for forged crank too.
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  #9  
Old 08-06-04
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Isn't the crank already forged???
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  #10  
Old 09-06-04
Duxthe1 Duxthe1 is offline
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I'm actually using a set of stock pistons that I personally machined for a lower deck height. I also enlarged and polished the chambers in the cyl head which also accounts for some of the reduction in compression ratio.

And yes there are other ways around knock, but after optimizing those variable factors, you're still left with a compromise between two basics, how hard you squeeze it, and how soon you light it off. It is generally accepted that 15 degrees ATDC is optimal timing for peak cylinder pressure. Assume your engine is optimized to run where peak cyl pressure occurs 15 degrees ATDC. Assuming no other changes besides an increase of boost pressure, that engine will require a reduction of ignition timing to run without knock. Positive psi equals negative ign timing. Pretty soon peak cylinder pressure ends up 5,10 or more degrees later than optimal. Although you have more charge lit in the cylinder there is a less efficient transfer of energy due to a late push on the crank pin.
The alternative is to lower the compression ratio, thereby increasing the threshold before the onset of knock. Since our trionic ecu pretty much time the engine by reading peak cyl pressure they will time very near optimal until knock forces a timing "retreat"

Bueler..........Bueler.........
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