How much decking? increase in compression?

[bb71challenger]

I am going to build a motor that I hope ends up just under 10:1. I am going to use speed pro 2355F per Chrycos suggestion. He also told me to have the block decked for .005 with that piston. What I am wondering is how much will have to come off the block to get that number and how will that affect the compression ratio? I am using heads with 87-88cc combustion chambers. Thanks for any help,

Brett

[matt63]

I believe that the deck height is the measurement from the crank centerline to the top deck.  It is normal to re-machine it  to ensure it is flat and parallel with the crank centerline.  I'm not sure how they measure it in a machine shop.  If the top surface of the piston is 0.005" below the deck height this has to be taken into account in the compression calculators.  These are available via a web search.  You must also consider the gasket thickness of 0.039"-0.040".  With a closed chamber head this gives you 0.045" quench which is desirable.  I measured my deck height (I was assembling the engine myself) by assembling the pistons and rods and borrowing a bridge tool and dial indicator which allowed me to measure how deep the pistons were in the hole.  I then advised the machine shop of the measurements and they decked the block accordingly.  It worked well this way.

[Chryco Psycho]

Typically the block is taller than spec so often the piston is more than .020 below deck
basically with the piston .020 below deck & using a .040 gasket & open chamber heads you will have 9.75 compression   
 if you use the steel shim head gaskets from Mopar you will go over 10:1 unless the piston is further down the bore
 the ones I build generally use the steel gasket & have the piston .050 below deck with timing optomised run just on the edge of detonation on Premium but I live at 4000 ' altitude 

[Chryco Psycho]

http://www.csgnetwork.com/compcalc.html


this may help

[moper]

Like said. "deck hieght" is the distance from the crankshaft centerline to the flat surface of the block where the head mounts. You add up 1/2 the stroke (3.75/2= 1.875), the rod length (6.768), and the compression hieght of the piston(2.061) and total it up .  that's 10.704.  Deck height on 440s is supposed to be 10.725, so they should be .021 down from the deck on a blueprinted block. Only, like CP said...they are never at 10.725 unless they were decked once already. So you may need more decking than .015. Your machine shop better be able to tell you what height the block is now...If they can't, dont let them rebuild your engine.

[bb71challenger]

Thanks for the replies. Hey Chryco, what about the 2266F piston? it is lower in compression but I dont know if it is set up to be in the hole farther or just has a dished surface. I know from your pm's that I want to end up with the piston in the hole .005 with a .039 head gasket along with the 906 heads that have been shaved .050. I just want to get the ideal quench you suggest but not go over 10:1 compression. Thanks again. As a side note I live about 6k in elevation but the air is really dry and I had a buddy tell me that alot of times the air is more like 12k elevation. What does air like that do for detonation? make it worse or better? The highest octane at the pump in this whole region is 90-91.

Brett

[Chryco Psycho]

you will not get quench with the open chamber head & the open chamber heads seem to love to enhance detonation , from what I recall you will still have approx .110 between the piston & head even milled .050 , probably the best answer is to use a 78cc alum closed chamber head & keep the piston approx .050 away from the head with the gasket & piston below deck & the heat loss to alum will compensate for the increased compression
 the 6000 ' elevation helps a ton though , at high altitude there is far less air to get into the cylinders effectivly dropping compression & reducing detonation