Type 4 Camshaft selection

Discussion in 'Mech Tech' started by Gnasha, Feb 25, 2019.

  1. Yes we did measure from the step downwards, our disks we cut sat on top of the step, Ive just checked back.

    Along with the deck height, we got a consistent 1.52mm across all four cylinders which gave us a nice 8.1 CR but that was a static measurement (I think) and not dynamic
     
  2. With flat top pistons?
     
  3. yes they are flat top's
     
    77 Westy likes this.

  4. what are the advantages of skimming 0.5mm of the piston, as opposed to raising the cylinder 0.5mm with a shim?
     
  5. Machining the top of dished pistons reduces the volume of the dish because you only remove material from outboard of the dish so the dish gets shallower. If you raise the cylinder the deck obviously increases but so does the total combustion chamber volume and the problem we have with 2.0l bus dished pistons and 1800 heads is getting the CR high enough with an acceptable deck.
     
  6. Ha, yes I see now thank you.
     

  7. Thinking again, isn't it a trade off , decreasing volume of the dish but still increasing deck height? Or is this the fine tuning button for CR adjustments? Sorry if this appears obvious
     
  8. Yes, but you then readjust the deck.
     
  9. So how does your 2.0 with the dished pistons perform? Is it still better with the C25 cam than it would be with a standard cam? Even though the CR is not as high as is ideal.
     
  10. It goes well enough and pulls from not much more than idle revs, in traffic it will crawl along feet off the pedals and top end is okay but it’s definitely happier climbing hills than it is on a motorway. It bugs me that it could be more efficient with a higher CR but I can’t compare it to a standard engine because I’ve never driven one.
     
  11. Looks like i will run with it. Thinking now I should have gone for flat top pistons but another £400 on the already expensive rebuild makes it too much. Can’t return the ones i have after balancing them.
    Thanks for the info, much appreciated.
     
  12. Flat tops would have been better, shallow dish ideal. The problem with flat top is the CR is too high - circa 9.9:1 with my 96mm bore x 80mm stroke and 1800 heads.

    Your configuration will work okay, at least you have a tight deck so squish will be good and I doubt if you’ll be disappointed with the engine performance and as I said earlier according to John at aircooled.net your CR is fine. Remind me - what carbs are you using?
     
  13. Dual Weber ict’s supplied by Eurocarb. Have been fine with the 1911 i have been running.
    Will probably need a bit of jet tweeking for the 2.0.
     
    77 Westy likes this.
  14. So this will require shim removal from under the cylinder or if no shims machine cylinder landing faces?

    Regarding material removal from the piston top: this work must be done before balancing the reciprocating parts?

    I'm learning here thanks for your patience....again.
     
  15. The 34 ICTs are probably too small for a 2.0l and you’ll lose a bit of top end but they’ll work fine at lower revs. It’ll be a nice engine.:thumbsup:
     
    Gooders likes this.
  16. Yes, adjust the deck with cylinder shims.
    Do your measuring and calculations to decide if you need to machine the pistons, and only do it if you have to. And balance after any machining.
     
  17. Are these more of an ideal piston with the 7cc valve pocket? Not changing them now but interested to know.
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  18. Gooders likes this.
  19. With 1800 heads valve pockets help, they aren’t necessary for valve clearance but as you say they reduce the CR which makes using flat top pistons slightly easier.

    Note the comment from VWH – they bolt straight on a 2.0l. They do but you still need to check deck height and CR. AA quality can be a bit hit and miss and barrels are often longer that standard and not exactly the same length one to another. Cheap as chips and my guess is that their manufacturing tolerances are quite generous and you must check them carefully before use. You must check everything for that matter.
     
    Gooders likes this.

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