WizardBlack Posted April 15, 2009 Share Posted April 15, 2009 OK, I have an N42/N47 (dished piston) setup that has been turbo'ed. I noticed a blowout on the HG and the engine had been weeping coolant out the passenger side of the head gasket as some people had noticed. I decided to use a Fel-Pro gasket to give myself an "out" for detonation and pick up the compression a bit. For this weekend car, I suspect I will use leaded race fuel blended with premium anyways. Likewise, I had the head taken to a performance machine shop and told them to deck it until straight (one thousandth at a time). It came back requiring .010" decking to get straight. How can I calculate my new compression ratio? The L-series calculator does not provide for decking the head or block. I did search but evidently haven't hit upon it. Stock indicates 8.29:1 and the Fel-Pro raises it slightly to 8.47:1. Quote Link to comment Share on other sites More sharing options...
grumpyvette Posted April 15, 2009 Share Posted April 15, 2009 (edited) OK youve decked the block .010 durring the block machining process , how did that change the compression? if you decked the block .010 youve effectively moved the piston .010 closer to the cylinder heads unless the head gasket is selected to be .010 thicker to compensate I don,t know the bore dia. but lets assume in a calculation its 4" bore cutting a deck on a block and not compensating with a thicker head gasket on that 4" bore had about a 2 cc reduction in the total area over the piston being compressed, into the combustion chamber above it, simply because the area of the bore times the depth of the cylinder results in that volume formula bore radias sq x 3.147 x .010 x 16.39(changes cubic inches to CCs) (referance you need TO COMPLETE THE CALCS) http://www.hotrod.com/techarticles/engine/how_to_calculate_compression_ratio/index.html http://www.circletrack.com/techarticles/ctrp_0505_calculating_engine_compression/index.html http://forum.grumpysperformance.com/viewtopic.php?f=86&t=1330 http://forum.grumpysperformance.com/viewtopic.php?f=52&t=727 http://forum.grumpysperformance.com/viewtopic.php?f=53&t=726 Edited April 16, 2009 by grumpyvette Quote Link to comment Share on other sites More sharing options...
WizardBlack Posted April 15, 2009 Author Share Posted April 15, 2009 OK youve decked the block .010 durring the block machining process , how did that change the compression? if you decked the block .010 youve effectively moved the piston .010 closer to the cylinder heads unless the head gasket is selected to be .010 thicker to compensate I don,t know the bore dia. but lets assume in a calculation its 4" bore cutting a deck on a block and not compensating with a thicker head gasket on that 4" bore had about a 2 cc reduction in the total area over the piston being compressed, into the combustion chamber above it, simply because the area of the bore times the depth of the cylinder results in that volume formula bore radias sq x 3.147 x .010 x 16.39(changes cubic inches to CCs) (referance you need TO COMPLETE THE CALCS) http://www.hotrod.com/techarticles/engine/how_to_calculate_compression_ratio/index.html http://www.circletrack.com/techarticles/ctrp_0505_calculating_engine_compression/index.html http://forum.grumpysperformance.com/viewtopic.php?f=86&t=1330 http://forum.grumpysperformance.com/viewtopic.php?f=52&t=727 http://forum.grumpysperformance.com/viewtopic.php?f=53&t=726 I had the head decked to clean it up. Quote Link to comment Share on other sites More sharing options...
WizardBlack Posted April 15, 2009 Author Share Posted April 15, 2009 Actually if someone can check my math, I think I have done it. Taking the info off of the L calculator, I have: Volumes at BDC: 458.83 - cylinder sweep 10.9 - Piston Dish 6.1 - Gasket (Fel-Pro) 44.6 - Chamber ________ 520.43 cc's total Volumbes at TDC: All less cylinder sweep ________ 61.60 cc's total Compression with an L28 N42/N47 with Fel-Pro gasket comes out to 8.45:1. Now, with the skim of the head, ASSUMING the removed area is the same diameter as the bore and ASSUMING I didn't get into any reduced cross section of the N47 head (I don't believe so), I have: .010 inches removed, or .0254 cm. Area is 58.09 cm^2 for stock bore L28. Volume removed: 1.48 cc. New chamber cc = 44.6-1.48 = 43.12 cc. Recalculating gives me a compression ratio of 8.63:1. I likey. Assuming no piston-to-valve and I did everything right. Does this sound reasonable? Is my cam timing gonna be totally borked? I am not sure I could convert .010" reduction in height to angular retard of the cam sprocket without getting back into some college books. As an aside, decking the block dropped compression from 8.45 to 8.42. Quote Link to comment Share on other sites More sharing options...
spawpoet Posted April 16, 2009 Share Posted April 16, 2009 Decking the block will raise, not drop compression. You are de facto reducing the volume of the combustion chamber. Quote Link to comment Share on other sites More sharing options...
WizardBlack Posted April 16, 2009 Author Share Posted April 16, 2009 Decking the block will raise, not drop compression. You are de facto reducing the volume of the combustion chamber. Oh, you are right. Hmm, I guess I should have taken it from the chamber cc's? Anyone know if ten thou off the N47 head will cause piston to chamber or piston to valve issues? Quote Link to comment Share on other sites More sharing options...
1 fast z Posted April 16, 2009 Share Posted April 16, 2009 Oh, you are right. Hmm, I guess I should have taken it from the chamber cc's? Anyone know if ten thou off the N47 head will cause piston to chamber or piston to valve issues? No it will not. Quote Link to comment Share on other sites More sharing options...
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