Shaved pistons. - IDI Engine - VWDiesel.net The IDI, TDI, and mTDI source.

• Shaved pistons. by subsonic on 13 Aug, 2007 15:59
• I am going to have my block deck resurfaced to ensure that it is 100% flat.  I know that the more that  comes off, my compression rate will go up.  I am at second overbore with new pistons, so that will also raise compression.  I am going to be running more fuel and boost, so I want it to go down :?   Besides shaving the pistons, what else can I do?  I have the 1.9 head going on the 1.6, so that should help a little.   3 tab gasket is sitting on the shelf, Any additional idea's?
  I am looking for about the min compression I can run and not screw up my maine winter starts.  Guessing about 19 or 20:1
  
  Jim
• #1 by OM617 on 13 Aug, 2007 17:18
• Isn't the head deck flat? Cutting it shouldn't change compression ratio.
• #2 by Black Smokin' Diesel on 13 Aug, 2007 18:00
• Take too much off the block and your pistons will say "hello" to your valves.
• #3 by boosted_diesel_84 on 13 Aug, 2007 18:09
• how would having the block bored over raise compression? it would be a bigger area (not much .020-.040 in. isnt sh*t) in a small block 350 v8 chevy for instance, boring it .030 in. will give you 5 cubic inches over the entire block!, so it is not even enough to feel any extra power, but having a bigger bore will LOWER compression due to more area, but not by much.
• #4 by lyeinyoureye on 13 Aug, 2007 18:29
• Larger bore will increase the effective compression ratio since there is more swept area from the bigger bore being compressed into the same size head. But, it's not a whole lot. Can't we grind the cam proportionally to how much we take off the block. And having a thicker HG seems like it would help too, but if it's too big it can screw up the squish(?) and result in poor performance. I think everything can be made to work, but we really need to do our arithmetic on this one.
• #5 by subsonic on 13 Aug, 2007 21:21
• Quote from: "OM617"

  Isn't the head deck flat? Cutting it shouldn't change compression ratio.

  
  
  Correct.  Work on the head should not change the compression ratio.  If the head is surfaced, other aspects there can be brought back in line so there is no change.
  
   boosted_diesel_84
  
  

  Quote

  how would having the block bored over raise compression? it would be a bigger area (not much .020-.040 in. isnt sh*t) in a small block 350 v8 chevy for instance, boring it .030 in. will give you 5 cubic inches over the entire block!, so it is not even enough to feel any extra power, but having a bigger bore will LOWER compression due to more area, but not by much.

  
  
  But I am talking about CC not cubic inches.  I am not sure of the conversion, I am adding CC's with the bigger pistons. Bore times stroke.  Even if the bore increase is very small, it will still be an addition, so my end number will increase.  That will result in a slightly higher compression ratio.  To add on to that, When I have my pistons ceramic coated, they will be "taller".  Again, not by much, but this will bring them closer to the valves, and again, raise the compression ratio a fraction.
  
  I am trying to lower my compression ratio.  If I want to use this great Giles pump I have on my shelf, and run between 20 and 30 psi of boost( :twisted: ), I should be lowering my compression ratio.  The bigger 1.9 pre-cups will help, but will probably be offset by the other factors.  HG thickness will come into play, but then I am messing with the squish factor.
  
  Has anyone here had any block deck work done?  What did you do to offset the difference?  Did you run into any piston projection problems?
  
  Perhaps having the valve pockets on the pistons milled a few mm deeper?
  Can the rods be rebuilt to offset the difference in deck height?
  
  Open to suggestions.
  Jim
• #6 by 935racer on 14 Aug, 2007 08:57
• You shouldn't have to take more than 5-10 thou off the deck of the block, which is fine, just check the piston protrusion before the head is installed. The 1.9 head and a 3 hole 1.9 gasket are going to lower compression as much as you'll ever need. Running 20-30 psi with stock compression is no where near the danger zone especially with a stock t3 turbo.