Jump to content

- - - - -

998/12G295 Compression Dilemma

  • Please log in to reply
7 replies to this topic

#1 mickymow


    Just On Tickover

  • TMF+ Member
  • Pip
  • 5 posts
  • Location: Regina

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!

Attached Files

  • Attached File  1.jpg   96.4K   3 downloads
  • Attached File  2.jpg   73.28K   2 downloads

#2 ACDodd


    Up Into Fourth

  • Mini Docs
  • PipPipPipPipPipPipPipPipPip
  • 2,137 posts

Posted 13 February 2019 - 07:26 PM

Rebuild it. Then sort the compression during the overhaul.


#3 Mr Dave

Mr Dave

    Passed Test

  • TMF+ Member
  • PipPip
  • 32 posts
  • Location: Northampton

Posted 14 February 2019 - 11:30 PM

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

#4 Retroman


    One Carb Or Two?

  • Traders
  • PipPipPipPipPipPipPip
  • 921 posts
  • Location: Sunny Sabden
  • Local Club: CDMC

Posted 15 February 2019 - 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


    Just On Tickover

  • TMF+ Member
  • Pip
  • 5 posts
  • Location: Regina

Posted 15 February 2019 - 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


    One Carb Or Two?

  • Members
  • PipPipPipPipPipPipPip
  • 1,483 posts

Posted 16 February 2019 - 12:43 AM

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

#7 Mrpeanut


    One Carb Or Two?

  • TMF+ Member
  • PipPipPipPipPipPipPip
  • 1,018 posts
  • Location: Southampton
  • Local Club: Wessex MOC

Posted 16 February 2019 - 09:35 AM

If you don't want to pull the engine your choices, if the ratio is 7.8, is to turbo it or skim it to within an inch of it's life to get the CR to something sensible. Personally I'd do the latter.

#8 mickymow


    Just On Tickover

  • TMF+ Member
  • Pip
  • 5 posts
  • Location: Regina

Posted 16 February 2019 - 03:54 PM

I think my math was funny earlier. The CR should be around 8.5. It does seem to be a good candidate for super or turbo.

0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users

Mini Spares