Yes, well, I just bought a copper head gasket on ebay, and was just wondering what the comp ratio will be afterwards? It is .080", stock is .042" uncrushed, .040 crushed. I have milled .009" off of the head, for a total decrease of .029". How can I find out what my new comp ratio will be? I am thinking in the 9.3:1 range, but I have also been drinking (for what it's worth). Anyway, I have tried the calculation myself, but with a deck of .000 (provided by Arias, seems close from personal observation), it works out to be over 11:1 (on stock pistons), which we all know is crap. Can someone please help me?
Compression formula?
cr = (displacement + gasket vol + deck clearance + piston vol + head volume) / (gasket vol + deck clearance + piston vol + head vol)
stock numbers - head volume ~45cc, piston volume ~-15cc, deck clearance ~0.3cc
If you've got a buret, nows the time to whip it out and find out the specs for your engine. It's something you might want to keep a log of when you're doing a build on your engine.
stock numbers - head volume ~45cc, piston volume ~-15cc, deck clearance ~0.3cc
If you've got a buret, nows the time to whip it out and find out the specs for your engine. It's something you might want to keep a log of when you're doing a build on your engine.
Re: (180fan)
okay, I have 596.9cc's per cyl by method - .785*89^2*96. The head is 46cc in the combustion chamber per Arias Pistons. The gasket at 2mm thickness should be 12.4cc. Arias also says deck clearance is .000" (no need to convert). So:
cr = (596.9 + 12.4 + 0.0 + 15 + 46) / (12.4 + 0.0 + 15 + 46)
or
CR = (670.3)/(73.4)
or
9.13:1, does that sound right? Since milling the head decreases combustion chamber volume, then can we simply remove it from from the the formula like we would remove head gasket volume, for sake of ease. I know that the head will taper, but maybe I can make an 'educated guess'?
If so, that puts the combustion chamber volume at about 43cc with a .014" (.7mm) cut. Which makes the compression ratio 9.47:1. I am going to kill that machinist! I am right back to stock with a 2mm head gasket!
cr = (596.9 + 12.4 + 0.0 + 15 + 46) / (12.4 + 0.0 + 15 + 46)
or
CR = (670.3)/(73.4)
or
9.13:1, does that sound right? Since milling the head decreases combustion chamber volume, then can we simply remove it from from the the formula like we would remove head gasket volume, for sake of ease. I know that the head will taper, but maybe I can make an 'educated guess'?
If so, that puts the combustion chamber volume at about 43cc with a .014" (.7mm) cut. Which makes the compression ratio 9.47:1. I am going to kill that machinist! I am right back to stock with a 2mm head gasket!
well a couple of things there. first shaving .009 off of the head isnt near enough to effectively raise the compression anyhow that is basically a cut that they make to get a clean and flat sealing surface. Ususally when you are shaving heads for compression you will take off .020 or more. That is of course if you were wanting to raise compression. If you wanna lower compression then just have some material cut off of the top of the pistons. Most aftermarket forged pistons have enough material to allow for the removal of some to achieve the desired compression ratio as it doesnt take removing much to desrease compression. Also, sorry again to ask, but is 46cc the volume for a sohc engine or dohc ? I have yet to find that info anywhere and I will soon be rebuilding my sohc engine and adding a turbo and knowing the stock volume would be helpful in calculating compression.
Re: (pmkls2)
It is 46cc for the DOHC and 62cc for the SOHC. This is why you get a huge compression increase from using SOHC pistons in DOHC. But I am talking about a .014" cut. So that is a pretty big deal. I have a friend who is going to put valve reliefs in the stock pistons. Say about .020" or .030", which will hopefully take up the difference in the head. I was shooting for something in the 9:1 range.
Thanks, I knew the dohc and sohc were different but I didnt know the specs and couldnt find them ( I really didnt look that hard ). But anyhow cutting the valve reliefs will be good since it will also give you that extra clearance if you were ever to float the valves. I figured you were needing to lower the compression since the motor is probably turboed but I wasnt sure. Also polishing the combustion chamber will slightly enlarge them and reduce compression slightly but it is good to do anyhow. Thanks again for the info
well i just went to the arias site and i found out 2 things. first was close as they show it to be about 8.7:cr. second they do actually list a part # for a sohc engine and so i searched google for that number and i found a few places that stock them at about the same cost as the wiseco dohc ones so im pretty stoked. thanks for telling me to visit the arias site or i would have never seen that...........