Posted September 3, 200420 yr comment_93722 Hello all, my 1983 f54 block is all set. What I want to know is with my P90 head. I had the head milled .050, with the stock block (non-turbo), what would be my compression ratio? Is an easy answer possible? Please let me know. Thank you, Rick. Link to comment https://www.classiczcars.com/forums/topic/13109-compression-ratio/ Share on other sites More sharing options...
September 3, 200420 yr comment_93725 If you can take your head to a race engine shop and get it CC'd, I can tell you what your Compression will be. I will also need to know if you are using a stock head gasket, and what the deck height is, and if those are flat top pistons.Phred Link to comment https://www.classiczcars.com/forums/topic/13109-compression-ratio/#findComment-93725 Share on other sites More sharing options...
September 4, 200420 yr comment_93793 Phred, I want to take you up on this offer of figuring out the compression ratio. I just finished cc'ing my head and these are the #'s I came up with:#1- 38.8, #2- 38.6, #3- 39.2, #4- 39.4, #5- 38.6, #6- 38.2The block is a '81 F54 with the stock flat top pistons. And I'm using a Felpro head gasket. Also I have a .020 head gasket shim by Felpro but I'm not sure if I'm going to use it. I guess it all depends on what my compression ratio comes out to.Thanks Link to comment https://www.classiczcars.com/forums/topic/13109-compression-ratio/#findComment-93793 Share on other sites More sharing options...
September 5, 200420 yr comment_93914 Hi Ed,You're only missing one critical measurement which I will make an educated guess on. If it varies much from my assumtion, I can easily refigure it into the formula.First, figure the known actual figures:Bore/Stroke:3.39 X 3.11 = 2759.95 cc'sHead Gasket:3.49 X .047 = 7.37 cc'sHead Gasket Shim:3.49 X .020 = 3.14 cc'sCombustion Chamber volume, (Ave.) 38.8 cc'sNow, the unknown guess:Deck height: -.010 (area above the piston @ TDC) =1.48 cc'sNext we just add them up to find total compressed volume:7.37 + 3.14 + 38.8 + 1.48 = 50.79 cc'sNow divide the displacement of one cly. by compressed volume, Then add 1 (a math thing to arrive at a ratio) and you have 9.06:1 compressionIf you delete the shim vol. of 3.14 cc's you come up with a total vol. of 47.65 and a compression of 10.65:1Class is over for today, remember to play nice with the other Z kids.Professor Phred Link to comment https://www.classiczcars.com/forums/topic/13109-compression-ratio/#findComment-93914 Share on other sites More sharing options...
September 5, 200420 yr Author comment_93925 The head was at the machine shop for 2 months, I would rather not take it back. To find out the CC's might take another month. The block is a flat top piston F54 and the head gasket is from felpro. Is it possible to find out a ballpark figure? Let me know, thanks, Rick. Link to comment https://www.classiczcars.com/forums/topic/13109-compression-ratio/#findComment-93925 Share on other sites More sharing options...
September 5, 200420 yr comment_93953 Thanks Professor. Link to comment https://www.classiczcars.com/forums/topic/13109-compression-ratio/#findComment-93953 Share on other sites More sharing options...
September 6, 200420 yr comment_93970 Rick,As you can see in the figures for Ed, a small change can make a big difference in compression. There are so many variables with you head, a guess here could be downright dangerous. .050 off you head? How much before? sunk valves? new Valves? You can't guess here, as a wrong guess could mean a blown engine. Phred Link to comment https://www.classiczcars.com/forums/topic/13109-compression-ratio/#findComment-93970 Share on other sites More sharing options...
September 6, 200420 yr Author comment_94037 Thank you. I knew my question was a little vague. With all the reading and research, I am going to assume (I know) a 9.5 CR. I hope I am right. Thanks again. Rick. Link to comment https://www.classiczcars.com/forums/topic/13109-compression-ratio/#findComment-94037 Share on other sites More sharing options...
September 6, 200420 yr comment_94059 Rick, PM your address and I will send you the piece of plexiglass and the syringe that I used to calculate the cc's on my head. It's easy to do and will take you less than an hour. Link to comment https://www.classiczcars.com/forums/topic/13109-compression-ratio/#findComment-94059 Share on other sites More sharing options...
Create an account or sign in to comment