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Valve reliefs on 951 pistons for 16v head (photos)

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Old 10-12-2011, 08:07 AM
  #16  
Raceboy
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I used one intake and one exhaust valve. These are stock S2 valves and I determined the relief depth by installing stock S piston to crankshaft and then tightening the fixating bolt.

Deburred the relief edges afterwards also. I§'m really not sure whether there's eny point in buying custom pistons unless cost is not an issue and you have the tools to perform needed operations as with 951 pistons there's still a quench area and they are strong. Never seen a piston failing due to too much power, it's always the detonation or pre-ignition that will cause the piston to fail.
And at stock redline, there's also little point in buying aftermarket rods as there's only compressive loads that increase but rods take this easily, though tensile loads that increase exponentially with rpm, are the ones that tear rod bolts and throuw rods through the blocks.

There is no rod/piston combo that could withstand poor tune in long term.
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Old 10-12-2011, 02:25 PM
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Yeah I'm fully confident in these pistons.

If you have it apart would you mind measuring the depth of the relief? It would save me from having to source an s piston.

I will try to use 968 valves since they are slightly larger.
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Old 10-12-2011, 04:13 PM
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Sid, i will send you a pm later today.
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Old 10-12-2011, 04:51 PM
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I have heard the stock 951 rods are quite heavy. I'm just curious if the pistons are also heavy compared to an aftermarket alternative.
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Old 10-12-2011, 05:11 PM
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Thanks Bruce.

Yes the factory pistons seem very heavy to me in comparison to a performance piston. However, I'm after durability along with performance. I like the factory durability. The rrast rods are pretty beefy as well.
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Old 10-12-2011, 05:23 PM
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Originally Posted by blown 944 View Post
Thanks Bruce.

Yes the factory pistons seem very heavy to me in comparison to a performance piston. However, I'm after durability along with performance. I like the factory durability. The rrast rods are pretty beefy as well.
Wouldn't something forged give you your strength, along with the piston being lighter reduce internal stress give longevity? Even though the factory pistons have lasted 25 years, most guys aren't running 20+ psi on E-85/104 octane and pushing over 400whp like some.
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Old 10-14-2011, 03:58 AM
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Being lighter gives you higher rpm capability. With stock redline, you will have no issues with longevity using stock rods and pistons.

As I wrote already above, it's not the boost or hp that kills pistons and rods (compressive loads), it's the rpm (tensile loads on the rods, wrist pins, pistons) and detonation/preignition.
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Old 10-15-2011, 03:29 AM
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Interesting idea. I have not seen thise thread earlier, funny how it popped up just when you needed this info.
Just discussed yesterday with my son about the possibilities to install 16V head to his project 951's engine. Since his a student with a limited budjet, he skipped 16V idea because of the pistons. At the moment his "hobby-fund" is too tight to buy aftermarket 16V pistons.
Peep, any chance we come over to south of the Gulf and you give use some piece of advice concerning this
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Old 10-15-2011, 09:17 AM
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Originally Posted by V2Rocket_aka944 View Post
anybody got a scratched up 951 piston that can meet my bandsaw to determine crown thickness?

Here is something close - a Wossner and a JE piston cut in half. Stock piston is about the same thickness.

Oh yeah, and a 2v head cut in half just for funů.
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Old 10-15-2011, 09:23 AM
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Originally Posted by Black51 View Post
I have heard the stock 951 rods are quite heavy. I'm just curious if the pistons are also heavy compared to an aftermarket alternative.
The stock pistons are ridiculously heavy. A good aftermarket piston is over 100g lighter.
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Old 10-15-2011, 09:25 AM
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Originally Posted by Raceboy View Post
There is no rod/piston combo that could withstand poor tune in long term.
Originally Posted by Raceboy View Post
Being lighter gives you higher rpm capability. With stock redline, you will have no issues with longevity using stock rods and pistons.

As I wrote already above, it's not the boost or hp that kills pistons and rods (compressive loads), it's the rpm (tensile loads on the rods, wrist pins, pistons) and detonation/preignition.
The man speaks the truth.
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Old 10-17-2011, 06:22 AM
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Originally Posted by Olli Snellman View Post
Interesting idea. I have not seen thise thread earlier, funny how it popped up just when you needed this info.
Just discussed yesterday with my son about the possibilities to install 16V head to his project 951's engine. Since his a student with a limited budjet, he skipped 16V idea because of the pistons. At the moment his "hobby-fund" is too tight to buy aftermarket 16V pistons.
Peep, any chance we come over to south of the Gulf and you give use some piece of advice concerning this
Absolutely no problem Just let me know when you are planning and that's it!
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Old 10-26-2011, 05:23 PM
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Default Valve Relief

All: See photos below. These is are the exhaust valve relief cuts on #2 and #3 done with the Lindytools.com in block cutter to .090". I plan to "clay" it tonight to see how much clearance I have, but if I need more, I was wondering if anyone had a junk Turbo piston to measure, cut and/or drill to determine how much material is in this region of the piston crown. I don't want to go too deep. Thanks, Mike 863-227-4315; [email protected]
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Old 10-27-2011, 02:46 AM
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Is that all what it takes to use to use 951 pistons with 16V ? I thought a lot more is needed to cut off in this conversion.
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Old 10-27-2011, 06:32 AM
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It needs a lot more if you want the reliefs to be the same depth as stock 16v pistons.
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