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

Sorry if its here on the list and I have missed it but............ Can anyone tell me what the clearance is between std valves and pistons at tdc on a 931?
I am looking at taking my car to 9:1 with the supercharger running slower and cooler....plan is to deck the block.
Thanks
Simon
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morghen · Joined: 21 Jan 2005 Posts: 9102 Location: Romania

The haynes has this info.

http://porsche924.tmfweb.nl/haynes_vraagbaak_online.htm

http://www.924.id.lv/Heynes%20Manual%20924/

http://www.geocities.com/eturbo924/Haynes.html

if you cant find them in any of those links i'll look in my haynes when i get home from work.

EDIT: i found this on an older topic...but i'm refereing to the NA engine there...so the 931 might be different.

Raceboy · Joined: 01 Mar 2004 Posts: 2327 Location: Estonia, Europe

Simon wants to know how much is a gap between pistons and valves, not valve lash.

Sorry Simon, don't know that.
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morghen · Joined: 21 Jan 2005 Posts: 9102 Location: Romania

ah...sorry...woke up too early in the morning and the brain was still asleep

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RC · Joined: 25 Mar 2007 Posts: 2637 Location: Australia

Sorry Simon, I don`t know either.

However IIRC saw it somewhere on a cam site or possibly in one of Leadfoots threads, since he had a 931 head, albeit with VW flattops.

If you`re running a Piper cam then you`re in luck.

http://www.pipercams.co.uk/NewPiperWeb/redesign/PiperCamFrames.html

Still a good indication though as stock would be less lift and duration at overlap. Perhaps ask Steve at Integral as he has the info available, just not on his site.

bass gt · Posted: Mon Jan 19, 2009 11:34 pm Post subject:

Simon,

At TDC your valves are fully seated, so the clearance is the depth of the head combustion chamber. Not sure what design your pistons are, as i seem to remember you using JE items. However, to increase the CR by half a point is so minimal, i really can't see there being an issue. I have a head which is over skimmed by nearly 2mm, and with my Integral stage 2 cam, i have no issues.
Why not skim the head as opposed to the block?

Regards,

Steve
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RC · Joined: 25 Mar 2007 Posts: 2637 Location: Australia

Martijnus · Joined: 29 Dec 2006 Posts: 2019 Location: Netherlands

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

I tend to agree, skimming the block is preferable to skimming the head, as most 931 heads I've seen are dangerously close to minimum height, if not already out of spec. Blocks are relatively cheap, good usable 931 heads are much more rare, and therefore expensive. Skimming a perfectly good head beyond its minimum spec seems a bit short-sighted to me.
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leadfoot · Posted: Tue Jan 20, 2009 1:57 pm Post subject:

Went through a few of my contact letters RE; protrusion of the 931 valves from the head.
"I measured mine and the sit about 5mm (.195") below the surface"
This would have been with the stock cam 12mm lift.

So depending on piston top height to deck, cam lift, and gasket thickness.

I remember measuring mine clearance when I put it all together. I did turn the engine over twice with no cam belt on to make sure. So at least the clearances were fine on a cold engine....
The piston top to deck height was 5mm, gasket was stock 931, I measured the full crush of a 924 N/A at 1mm, cam lift was 11.4mm.

Probably best to get some plasticine and stick it on the top of the piston and check for sure.
Stu
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simsport · Joined: 06 Nov 2002 Posts: 573 Location: UK Warrington

Thanks for the info, sorry about delay in reply, credit crunch has hit and so job has changed in last week.

I am in favor of decking the block , dont want to risk det isues with the head if that was machined.

I will measure the clearance on my current head and let folk know what that is at full lift

The plan this year is to run the supercharger slower, raise the comp to 9:1 , improve the airflow into the blower (its an eaton so it helps) and to have the blower body ported for cooler outward airflow. Plus a basic 4 branch manifold.

Target power is in stages, phase 1 (slower blower and ex manifold)250bhp which is just 10bhp more than now but is the groundwork for next step , phase 2 (9:1 and blower porting) 275+bhp...but depends on money being available!

cheers
simon
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