Phlebmaster Posted December 30, 2009 Share Posted December 30, 2009 I just rebuilt the top end of my 350 sbc and need a little help by those who are more familiar than I am with using those fancy compression calculators. I know a few of the measurements, but not all of them to complete the calculations on my own. Thanks in advance for your help. Here is what I have: Standard sbc 350 Block: 3970010 date code of something 74' (can't remember) but it is not the stock engine for my 1972 C20 3/4 ton. Heads: 14101081 305 58cc 1.84 intake / 1.50 exhaust Pistons: The standard stock dished with double valve reliefs. Bores: Standard size Block has not been milled or decked. Head gaskets: Standard thickness Felpro I can't think of what else you would need to know. Here are some pictures for reference..... Completed engine I don't know why this head says 55.4..but it does. I don't know if that is the milled CCs or just some jibberish. You can see that the pistons are just the standard dished ones. Quote Link to comment Share on other sites More sharing options...
dr_hunt Posted December 30, 2009 Share Posted December 30, 2009 Actually those heads are so bad that I won't comment as to why you using them. Flow wise and cheap lightweight castings all spell disaster for any motor combination built using them. My humble opinion and that of the entire WWW. Quote Link to comment Share on other sites More sharing options...
Phlebmaster Posted December 30, 2009 Author Share Posted December 30, 2009 (edited) Actually those heads are so bad that I won't comment as to why you using them. Flow wise and cheap lightweight castings all spell disaster for any motor combination built using them. My humble opinion and that of the entire WWW. lol...ok, but notice that I didn't ask about that. I know I need to get Vortecs and I will in time, but this is my first round with the sbc and I am just getting to know her. So now that that the lashing is out of the way, can I get help with the comp? Forget about the head casting #...the 58cc head is the important thing here. I have heard many positive things about the 081 heads, they are the non-swirl port TBI heads. 081 heads flow reference 081 iron head (305 cylinder #14014416 airflow #'s): 195 intake; 110 exh (ME Leigh)Flow chart for stock #14101081 305 head 87 91 Intake lift (in) Flow (cfm) Exhaust lift (in) Flow (cfm) Ratio E/I flow (%) 0.050 31 0.050 15 49 0.100 63 0.100 37 58 0.150 92 0.150 61 67 0.200 116 0.200 78 68 0.250 134 0.250 92 69 0.300 157 0.300 101 64 0.350 170 0.350 105 62 0.400 182 0.400 107 59 0.450 189 0.450 109 58 0.500 195 0.500 110 56 They have a decent flow with a little port work and the heads I was using were the crack prone 624 heads with 1.72 intakes so this is an improvement.....that is my simple explanation. I used them because I had them. I found this thread with close to the information I need to run the calculator, but it is for a 400 not a 350. http://forums.hybridz.org/showthread.php?t=66574 Bore - 4.155 inches Stroke - 3.75 inches Cyl head volume - 64 cc Deck height - 0.010 inches Head gasket bore - 4.2 inches Head gasket thickness - .039 inches Piston dish volume - 12.5 cc Edited December 30, 2009 by Phlebmaster Quote Link to comment Share on other sites More sharing options...
jt1 Posted December 30, 2009 Share Posted December 30, 2009 Two things you need to know to accurately calculate the compression are the deck height, which is the distance from the top of the block to the top of the piston, and the volume of the dish, valve reliefs, etc. Those pistons have a fairly large dish, a big chamfer around the outside, and double valve reliefs, so that volume is fairly large, maybe 12 or 14 cc's. If you can measure or find info on the pistons, and measure the deck height, there is a good calculator on Keith Blacks's site. You can also calculate it manually, it's just volume at BDC divided by volume at TDC. Just guessing, I'd say 8 - 8.5 to 1. jt Quote Link to comment Share on other sites More sharing options...
Phlebmaster Posted December 30, 2009 Author Share Posted December 30, 2009 (edited) Two things you need to know to accurately calculate the compression are the deck height, which is the distance from the top of the block to the top of the piston, and the volume of the dish, valve reliefs, etc. Those pistons have a fairly large dish, a big chamfer around the outside, and double valve reliefs, so that volume is fairly large, maybe 12 or 14 cc's. If you can measure or find info on the pistons, and measure the deck height, there is a good calculator on Keith Blacks's site. You can also calculate it manually, it's just volume at BDC divided by volume at TDC. Just guessing, I'd say 8 - 8.5 to 1. jt Thank you for your help. I found a calculator that gives me a ball park figure of 10.81:1 Sample: 4.000 bore - 3.480 stroke - .040 head gasket thickness - 76 cc head volume should give you a CID of 349.781 (350 Chevrolet) with a compression ratio of 8.5 to 1. Chevrolet also made 64, 68 & 72 cc heads. Check the changes in compression ratio if you enter these numbers, or use a .020 steel shim head gasket. -------------------------------------------------------------------------------- Bore: Diameter in Inches 4.000 Stroke: Distance In Inches 3.480 Head Gasket: Thickness In Inches 0.040 Cylinders: Number of Cylinders 8 Cylinder Head Volume Number in Cubic Centimeters 58 Displacement: in Cubic Inches 349.78176 Displacement: in Cubic Liters 5.7329230464 Compression Ratio: 10.817268625498117 CUBIC INCH CALCULATOR and COMPRESSION RATIO http://www.aacncclub.com/crucemotor/compressionratio.html I am pretty sure based on what I have read about my set-up, that this is pretty close to accurate. Maybe not exact, but close at any rate. I was not able to account for 12.5 cc dished pistons on this calculator. I just was needing to know this because I will have to figure out what my octane should be. Looks like premium at minimun...92 octane. Thanks! Edited December 30, 2009 by Phlebmaster Quote Link to comment Share on other sites More sharing options...
Pyro Posted December 30, 2009 Share Posted December 30, 2009 here you go. this is how to calculate compression with out a website. 717cc for the cylinder 12cc for the piston (standard smogger gm dish with 4 relief) 4cc for the deck height (estimate -0.020" in the hole) 8cc for the head gasket (0.040" felpro) 58cc for the head (717 + 12 + 4 + 8 + 58) / (12 + 4 + 8 + 58)= (717 + 82)/82 = 9.74 9.74:1 cr need to use a cam in the 220 degree at 0.050" range with this compression (270 advertised). will also like a 2600-2800 stall and 3.55-3.73 gear. (depending on tire size). 1 Quote Link to comment Share on other sites More sharing options...
Phlebmaster Posted December 30, 2009 Author Share Posted December 30, 2009 (edited) here you go. this is how to calculate compression with out a website. 717cc for the cylinder 12cc for the piston (standard smogger gm dish with 4 relief) 4cc for the deck height (estimate -0.020" in the hole) 8cc for the head gasket (0.040" felpro) 58cc for the head (717 + 12 + 4 + 8 + 58) / (12 + 4 + 8 + 58)= (717 + 82)/82 = 9.74 9.74:1 cr need to use a cam in the 220 degree at 0.050" range with this compression (270 advertised). will also like a 2600-2800 stall and 3.55-3.73 gear. (depending on tire size). Now that is the help I was looking for. Thank you very much! I am sure this info will help others out as well. I just happen to have the Comp Cams 270h with 112 LSA 224/224 duration and a 4/7 swap. Cool! Edited December 30, 2009 by Phlebmaster Quote Link to comment Share on other sites More sharing options...
dr_hunt Posted December 30, 2009 Share Posted December 30, 2009 Honestly if you look at the outboard side of the head where the head bolt holes are and you see the scalloped outline, that is indicative of the lightweight castings, all of which are not very desireable. I'm not sure where you got the flow numbers but that is on the good side and they max out at .500 lift anyway. Typically, the price of gaskets, time involved, etc, etc, it's better to wait and get better heads and do it right the first time, rather than take it apart to do it all over again IMO. Hope it works out for you. Quote Link to comment Share on other sites More sharing options...
Phlebmaster Posted December 30, 2009 Author Share Posted December 30, 2009 Honestly if you look at the outboard side of the head where the head bolt holes are and you see the scalloped outline, that is indicative of the lightweight castings, all of which are not very desireable. I'm not sure where you got the flow numbers but that is on the good side and they max out at .500 lift anyway. Typically, the price of gaskets, time involved, etc, etc, it's better to wait and get better heads and do it right the first time, rather than take it apart to do it all over again IMO. Hope it works out for you. Good point. I have no problem pulling my motor apart when I get new heads. The gaskets are cheap and it takes about 6 hours to do the job. Not a huge investment in my eyes. The learning experience is worth more to me than a few dollars and little bit of time spent...besides this is a daddy and daughter project. My 15 yo daughter get's to learn how to tear down an sbc that way. If you ever have seen any of my other projects you would know that I typically take my engine apart and put it back together several times during the time I own them. I appreciate your oppinion and understand your point. Aaron Quote Link to comment Share on other sites More sharing options...
dr_hunt Posted December 30, 2009 Share Posted December 30, 2009 Cool, well then I'm sure you'll have a blast!!! Quote Link to comment Share on other sites More sharing options...
Phlebmaster Posted January 12, 2010 Author Share Posted January 12, 2010 (edited) This was my first time swaping a sbc cam. I left the 081 heads stock on my 350, but put new extreme push rods, 270h cam, and lifters. So, you can run a .470 lift cam on stock heads. (WOULDN'T RECOMMEND IT...but I did it.) I spent many weeks looking for that answer ans now I have the experience for my self. I also could not find a good sound clip of this special grind cam online- so now there is one....enjoy! Edited January 12, 2010 by Phlebmaster Quote Link to comment Share on other sites More sharing options...
jt1 Posted January 12, 2010 Share Posted January 12, 2010 Sounds Nice! You're lucky to have a 010/020 block. That's the primo stock block to have. Does it have a lifter clicking or is that the exhaust or something? jt Quote Link to comment Share on other sites More sharing options...
Phlebmaster Posted January 12, 2010 Author Share Posted January 12, 2010 (edited) Sounds Nice! You're lucky to have a 010/020 block. That's the primo stock block to have. Does it have a lifter clicking or is that the exhaust or something? jt Thanks!! That sound was one (or two) of the lifters, I left them on the looser side for the break in. I will adjust them using a procedure where I don't have to run the engine later. Edited January 12, 2010 by Phlebmaster Quote Link to comment Share on other sites More sharing options...
mobythevan Posted January 12, 2010 Share Posted January 12, 2010 I put together an engine similar to what you describe in the past, but I had flat top pistons with the small 57cc combustion chamber and the compression did not come out as high as I hoped. Like was pointed out, once you figure in how far down the hole the piston sets at TDC and gasket volume plus other small details you fall short. But it will still run good at 9:1 and be fun to assemble. I also understand Doc's input, because given what I know now I would not put any sbc engine together without a decent set of heads. But given what I know now I only work with gen 3 or newer chevy engines unless your talking big block. Quote Link to comment Share on other sites More sharing options...
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