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998/12G295 Compression Dilemma


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#1 mickymow

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Posted 13 February 2019 - 07:06 PM

I have a '76 Mk3 with a 998, HS4, dish pistons and an unskimmed 12G295 head, and has likely been running this setup since the 80s or 90s. It does 135 psi across all cylinders and a 7.8ish compression ratio.

 

I'm debating what to do to improve power (it has trouble climbing hills) that doesn't involve removing the engine just yet. I put in a Powerspark ignition which has helped, and a stage one was also on the agenda early this year. One thought was to find an original head or have this one skimmed, if it can reliably be done with dishes. Or should I even be concerned about the low compression and keep on with the stage one? Thanks!

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#2 ACDodd

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Posted 13 February 2019 - 07:26 PM

Rebuild it. Then sort the compression during the overhaul.

Ac

#3 Mr Dave

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Posted 14 February 2019 - 11:30 PM

Use the low CR to your advantage and stick a load of boosts in there!

#4 Retroman

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Posted Yesterday, 12:59 AM

If its struggling to climb hills its tired, and time for a rebuild

 

Use the money for a proper rebuild instead of sorting another 295 head 

 

It will help with the refurb and save some time / work .

 

Probably also save the disappointment of the expected increase from raising the compression

 

 



#5 mickymow

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Posted Yesterday, 11:29 PM

Well it appears the head probably was skimmed. It's just that I have a Canadian Mini 1000 that starts at 120 psi from factory, according to the workshop manual. So I suppose 135 isn't terrible. I do wonder what cam is in this, even the low compression 998 got 150.

#6 dyshipfakta

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Posted Today, 12:43 AM

Compression ratio doesn’t really mean any thing compared to measured compression you can have good compression but a crap ratio.




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