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Gramps Unleashed
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morghen  



Joined: 21 Jan 2005
Posts: 8868
Location: Romania

PostPosted: Fri Jul 22, 2016 1:09 am    Post subject: Reply with quote

I have a set of euro pistons sitting around.
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Noahs944  



Joined: 08 Dec 2015
Posts: 782
Location: Calgary, Alberta, Canada

PostPosted: Sat Jul 23, 2016 12:01 am    Post subject: Reply with quote

ideola wrote:
Noahs944 wrote:
Question: if you wanted to up the compression to say 11.5:1 (using stock pistons), is this possible on the na 2.0l by merely shaving the head, or is shaving the block required?


DO NOT shave the head! The NA head is a heron style head, meaning it is flat, with no combustion chamber in the head itself. You will not gain a single point of compression by shaving the head, you will only affect your ability to properly tension the timing belt.

Have you seen my compression ratio calculator?
http://garage.ideola.com/top-DL-CompressionRatioCalc.html

If you start with US-spec long block from 1980-82, you have 9.0:1 CR pistons to start with. Assuming you don't want to go to the expense of finding Euro-spec pistons, or swapping in something aftermarket or custom, this is your best starting point.

You can gain another 0.4 points of compression by using a Cometic .027" head gasket.

If you take the block down to 230.0mm, you gain 2 whole points of compression, so that would put you at 11.4:1.

So that's how you do it.

If you remove that much material from the block, you should consider an adjustable cam gear to dial in cam timing.


Thank you for your insight. Where do we measure?


What's the thickness of a standard Reinz head Gasket on an NA... (or are the turbo and na the same)?
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Noahs944  



Joined: 08 Dec 2015
Posts: 782
Location: Calgary, Alberta, Canada

PostPosted: Sat Jul 23, 2016 12:19 am    Post subject: Reply with quote

I am getting excited to work on Gramps It's been a long time since that was the case... or come to think about it, I think this is the FIRST time I have been excited.

I admit to have fallen a bit in love with the car. I like lowered 924s... they remind me a alligators or salamanders with their bellies pressed to the floor.

I have some 30mm lowering springs which tempt me, but I must resist for the time being. Or should I? The car comes with 14mm and 23 mm sway bars and the Sach shocks are relatively new.

Part of my reason for resisting the lowering is because it's ALREADY a low car. I am addicted to gravel roads & well... sometimes it snows a lot here, so factory ride height is probably the best.

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Noahs944  



Joined: 08 Dec 2015
Posts: 782
Location: Calgary, Alberta, Canada

PostPosted: Sat Jul 23, 2016 12:20 am    Post subject: Reply with quote

Regarding the head's valve guides: is replacement something I can do at home with regular tools?
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Noahs944  



Joined: 08 Dec 2015
Posts: 782
Location: Calgary, Alberta, Canada

PostPosted: Sat Jul 23, 2016 12:36 am    Post subject: Reply with quote

Here's some pics of someone else's 78 924, which was at the last TSD rally.



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Noahs944  



Joined: 08 Dec 2015
Posts: 782
Location: Calgary, Alberta, Canada

PostPosted: Sat Jul 23, 2016 1:03 am    Post subject: Reply with quote

https://www.youtube.com/watch?v=dzC16ozVANs
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ideola  



Joined: 01 Oct 2004
Posts: 15548
Location: Spring Lake MI

PostPosted: Sat Jul 23, 2016 1:45 am    Post subject: Reply with quote

Noahs944 wrote:
Thank you for your insight. Where do we measure?

What's the thickness of a standard Reinz head Gasket on an NA... (or are the turbo and na the same)?


Block deck height is measured FROM the head-to-block mating surface TO the main bearing cap mating surface. I purchased a $60 12" digital caliper from Harbor Freight to measure my blocks. I have never seen an official OEM spec for this measurement, but as near as I can tell, a fresh block will typically be in the 232mm range. Most of the engines I've prepared have ended up in the 231mm range. The tech specs booklet does not indicate a "wear limit" or minimum height.

For NA motors using stock pistons, the compression height of the pistons results in the piston crown sitting approximately 6mm below deck height, with valve reliefs cut into the head to allow for the valve to protrude from the flat heron head into the combustion chamber during opening and closing. One thing to be aware of, therefore, is as you decrease the deck height, in addition to increasing compression ratio, you may also end up with an interference motor. The only way to know for certain is to assemble the long block and use clay on the piston crowns to see how your deck height, piston compression height, and cam choice (i.e. valve lift) affect valve-to-piston interference.

Frankly, I personally don't care about the motor being interference because the timing belt service on the 2.0L is so cheap and easy to perform. But you need to be aware of this, as decreasing the deck height too much could result in interference, particularly from the valve float that can occur with weakened, aging OEM valve springs, high lift cams, and high RPM. Word to the wise.

All that to say, you can probably go to 230mm without too much concern, but to be safe, you should blueprint and double check rotating assembly before firing the engine!

Noahs944 wrote:
Regarding the head's valve guides: is replacement something I can do at home with regular tools?

Highly doubtful. I have had great difficulty finding machine shops that have or can source the completely unique machining bits that are required for punching out and reboring our valve guide bores. The 9mm stems on our valves and the valve guide diameters are highly unusual, and every shop I have ever used has need to make or purchase special bits for dealing with the valve guides. YMMV.
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