Type 4 2.0l Deck Height and Compression Ratio

Discussion in 'Mech Tech' started by Deefer66, Feb 29, 2016.

  1. In advance of me getting there on my build just to make sure I'm getting it right in my head....For a holidaying me and Mrs Deefer bus with no racing intentions...

    Ditching the cylinder to head shims and going straight metal to metal there.. (with a light fine valve paste to clean up the sealing face?) no other goo. ..

    If chambers confirm to 55cc and piston dishes to 5cc....

    I'm assuming I'll shim (no other gaskets just sealant?) at the base of cylinders to get a deck height of about 1.5mm giving a CR of 8 : 1.. are these good target figures to work to given that the steps of 0.25mm in available shims will give a + - 0.2 change to the CR.
  2. On my build thread there is a link to the John Maher Racing website which shows the CR calculator.

    We ditched barrel to case shims and ales head to barrel shims.
    I think our deck height was 1.52mm, slap bang in the middle of the tolerance.

    We ended up with a nice 8.2:1 compression ratio!
  3. Ta...
    made me own excell spreadsheet that will do it . . I like excell.. I'm sad that way
    I can have separate piston dish and chamber volume and make it solve deck height for given CR or CR for a given deck height...

    8ish and 1.5ish are good numbers then

    Time to start cutting up some perspex. ...While my end play shims are in the post....
    MorkC68 and zedders like this.
  4. It's impossible to lap a barrel to a cylinder head ......refacing is the only true way ......I added copper shims but I'm Forced ind.
  5. It should seal OK with a clean head and a clean cylinder . The cylinder head should not need any fiddling as it will deform naturally over the harder cast iron cylinder.

    I took out all the spacers on my T1 and have a deck height about 1.3mm. The case I am using has never been machined since new.

    And now it feels like it has the same power as the old oily leaky loose dowelled junkheap I ran before.

    I think it spent a good many years not running stored with a burnt valve gradually filling up with water.
    MorkC68 and Deefer66 like this.
  6. Well having had a quick bolt together to get the ball park.. no shim deck height is about 0.65mm..

    Wich gives me good scope to hot either end of the optimum in Tom Wilson book with a 0.75mm shim giving deck height 1.4mm and a CR of 8.07

    Or a 1mm shim giving me 1.65mm deck and a CR of 7.9.

    Problem is I don't know any better to pick one over the other.
    Any opinions on the merits of a greater deck height vs a higher compression ratio when they're that close on an battered old engine anyway?
  7. Our pistons and barrels where all new so no wear was there from the get go.

    I guess having a 1.4mm deck height just means 0.1mm less distance between valves and pistons - not a lot really!

    Ours just bolted together and gave us 1.52mm immediately - ours runs very well as it stands.
    Deefer66 likes this.
  8. If you make the deck height too big the engine will have poor efficiency. Go for a deck of 1.4mm and a CR of 8.07.
    Deefer66 likes this.
  9. You won't be able to tell the difference either way IMO.
    Faust likes this.
  10. You won’t, but there is no advantage in having a deck height greater than necessary and if there is a choice you might as well make the engine very slightly more efficient. Tiny improvements made throughout an engine build all add up.
    Majorhangover likes this.
  11. So why not , with the decent fuel we now have make your CR 9.0 or even more ? Surely you'll get more power ?
  12. You could and many do but with the bits Deefer66 has he can’t – too much combustion chamber volume, dished pistons and not enough deck height.
    Deefer66 likes this.
  13. I did that to a 1600 and it went very well.
  14. I don't know what modern cars run CR wise but I thought you could run up to 9.5 before it detonates
  15. The thing to watch for is if you have too little deck height when it warms up the piston will start kissing the head. Been there done that.
    Deefer66 likes this.
  16. But IME CR from 8 to 9 is OK. It's worth noting that higher CR will run hotter.
  17. My M3 runs 11.5:1...... but the ignition system and engine management is clearly somewhat more complex.....
  18. Thanks all ..I'll set them all up and do a proper remeasure and aim for the lower deck height optimum of Tom Wilson....
    MorkC68 likes this.
  19. After a revisit to measure with feeler guages I get an average deck height (taken at top bottom left and right) on #1 and #2 of 0.4mm, #4 has 0.47mm but #3 has 0.56mm.
    Will I notice?
    Should I dig out the 4 pistons from the other case and see if there's a good one closer?

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