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#1
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Cylinder pressure/ compression ratio
So I read last night that if you take the psi reading from your compression test and divide it by 14.7 you will get a fairly accurate actual compression ratio for your application... I have been frustrated because with the flattops I have in my 360 speed pro rated them as 8.8 to one, 50 thousanths in the hole! Now I wasn't there when this engine was built, and I don't know what they may have done to the heads or the deck, but I remember one of the previous owners claimed 10:1. So a cold reading gave me 152 psi. Not even warm. That is more than 8.8:1 according to the result if divided by 14.7 (atmospheric pressure at sea level, right?) It is much closer to 10.5!!! Could someone please help me figure out the approximate cr on this thing so I can more accurately pick a new cam? Thanks!
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#2
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Differences in camshaft duration will make any fixed formula innacurate.
Here's one opinion on the subject: http://www.type2.com/library/engineg/comrat.htm |
#3
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Mike, you're practically at sea level so 14.7 would be close IF your barometeric pressure was 30.00"Hg and the tempature is 70*F. Now,for the weird part. That "formula" would only be valid if you have no fuel in the cylinder, and there was no "ram effect" from your intake manifold and heads. Poor comparison here, but my '98 5.2L Magnum stock(9.5 CR advertized) showed about 165PSI brand new, so that's within "spittin' distance" of 9:1. Static pressure is a very poor way to estimate compression ratio as I hope I've explained. There are ways to to get closer, but I don't think it's worth the effort. Best way to establish CR is to measure how far down the piston is in the cylinder, measure the cc's of any piston dishing, measure the volume of the head combustion chamber, plug the numbers into one of many formulas published or I think Comp Cam's website shows one, and there's your static CR. Comp Cam's has a fairly new computer program that might can work off of your cylinder pressure, but you really need your head flow numbers to run it. Check their web-site or call them...lots of places will help you with cams. Good luck.
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#4
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That formula is certainly not even close. I have a measured and known static compression ratio of 10.6:1 in my 360 and it has 180-190 psi of cranking pressure. Assuming an average of 185 that formula would suggest I have a 12.6:1 compression ratio.
If you are going to install a new cam just take it one step further an pull one of the heads. Measure how far the pistons are down in the cylinder and take a piece of plexi glass with a small hole and an place it over the chamber and use a cc graduated syringe to fill the chamber with alcohol. Also if the pistons have a dish or valve reliefs try and measure that two. This web site http://www.csgnetwork.com/compcalc.html is a good calculator for taking into account all the variables in calculating compression ratio. If you are already at 152 psi then going with a cam that has more overlap than the one you have is only going to lower that number and result in less bottom end torque. |
#5
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some piston manufacturer websites will also be able to tell you the CR based upon the piston part number, head cc, stroke, and bore. i have dished KB pistons and ran into the same situation not too long ago with my 360, and their website is pretty well laid out. perhaps you can check Speed Pro and see whats available on there as well.
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#6
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Quote:
I've measured a lot of motors, and formulas don't match. Look at how broad factory compression specs are. That should say something . |
#7
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cylinder pressure
so what would 210 compression be ?
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#8
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Like everyone has said you can't make a determination of what compression is from cranking psi.
210 psi is certainly pushing the limits of pump gas and would require a very well sorted out build to work with pump gas. |
#9
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Except some motorcycles have nearly 300 PSI cranking pressure and run on the cheapest stuff there is! But big aircraft engines have cranking pressures of 100lbs or less and have to have 150 octane gas, and they aren't supercharged or turboed. With a math formula 2-5 pages long, and every iota of information about combustion chamber design, intake runner length, cam duration and lift and about 1000 other things, you could relate cranking pressure to compression ratio, very approximately. Check the cranking pressure of a chain saw or Weed Eater and you'll walk away shaking your head. Now throw into the mix "static compression ratio"(measured with the engine apart) and "dynamic compression ratio" taken with the engine running at various throttle settings, and you may begin to understand why engines blow-up, when the "numbers look good", or run just fine when they "shouldn't". Trust us, there's "stuff" going on inside your engine that Ph'D's and engine guys don't completely understand after nearly 130 years. Otherwise, NASCAR and Indy cars wouldn't blow 6 to 8 figure priced engines.
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#10
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Not to hi-jack this thread, but I was have a 360 in my 1-ton 4x4 with only 44,000 miles and wanted to stick a Voodoo 60401 cam and 4 bbl on it. I pumped the cylinders when warm, but no fuel in them, and read a pretty consistent 120 psi. I know this isn't very high, but I also know that a '79 360 was pretty anemic on the compression side, so I wasn't too shocked. Do you think that using this cam will be too much for this low conpression? I didn't really see this cam as being very wild at all, so I think it should work OK.
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#11
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Walkercolt, the cycles don't have nearly as much swept volume in the chamber either. That is why they can get away with that. I have built engines with 10.5 that got 205 psi that were fine with one cam, and was 188 per with a different cam. No other changes. :-) PCRMike
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#12
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dgc333, i dont run pump gas, i run 111 track gas, its a 318 in my strockcar, i just wanted to get a idea what the compressed ration was
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#13
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Compression Ratio
StrokerMike what's the specs on the cam that you're running now? You can use this as a starting point as to what you want from the motor. If you want more bottom end then you will need to pick a cam with less duration @ .050 than the one you currently have. If you have a need for more top end then you'll need a cam with a bit more duration @ .050. Be very careful when going up on duaration cause if you go too high you'll kill the bottom end torque that you do have. Makes the motor very soggy out of the gate or off the corners. Most of when you go up on the cam you really need to up the compression to take advantage of the added duration of the larger cam. If I were you I'd go up on the cam and get the block "0" decked withe pistons you have. Are you running open or closed chamber heads? If open chamber then you may even want to go to a positive deck height of say .025" and use a .040 head gasket. This would really surprise you! Wedgehead
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#14
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the fixed formula will always work when you are talking static compression (the humber everyone talks about, but is really the least important) because it is mathematical...that is if you know all your specs:gasket volume, head cc, deck height,and piston head volume. kieth black has a great calculating tool for that. dynamic compression or compression pressure is the most important and things like camshaft spec, camshaft timing, and rod length can have an extreme affect on it. the kb site also has a calulator for that too. i used it alot when i was selling hotrod parts and designing engines for my customers, and it is really acurate.
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#15
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Thanks for this thread..........
I have a general idea about the relationships of cams and static compression but I'm still confused. Part of my confusion comes from when I was 18 and worked at a gas station. This was in the 60's and the owners son had a 63 Chevy Impala that has a 300 horse 327 with 4 speed. One day we ran a compression check on his car and it came up with 165 on one cyc. and 175 on about 6. The other cyc came out at 180. Buy the first formula presented, it had almost 13-1 compression. This was a stock motor that had a smooth idle. This only adds to my confusion.
Then I had a 318 that only had about 120 lbs comp but ran really good. I raced several 350 Chevys and beat them. The overlap of the cams have a lot to do with the numbers that we often count on but that is highly suspect in my opinion. |
#16
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Well, the 327 High-Performance Toranado Engine Plant engine was rated at 12.8:1 in everything but the Hilborn Fuel-Injected Corvettes which were 13.5:1. Cylinder pressure is such a poor indicator of compression ratio, it's not worth bothering with, IMHO. If you have a method for checking it, the pressure at the exhaust port can be used to check CR on a specific head. Racing guys use this test sometimes to choose a camshaft, but it's still pretty vague.
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#17
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I built a spec'd 9.5:1 comp 360 that showed 188 in every cylinder with a .440/.440 @260*gross duration Comp cam, installed 1* advanced. I agree no indicator will be exact. Built a nigh identical 360 with an Isky MegaCam at .435/.435 cam at 262* that came to 191 psi on every cylinder but one and it was 192. It was put in straight up. :-) PCRMike
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#18
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Quote:
Working the equation backward, a gauge reading of 152 psi is an absolute pressure of 166.7 (152+14.7). Divide the 166.7 by 14.7 = 11.34. take the 11.34 and raise it to the 0.83333 power (1/1.2) or 11.34^0.83333 = 7.5656. The 7.5656 number is the Effictive compression ratio when the compression ratio is calculated from using the piston position at the time the intake valve closes. Anyhow, I guess you have the Speed Pro H405CP pistons with a compression height of 1.637" as this would give you the piston being 0.050" down the cylinder. The chart I have just shows the pistons having 4-valve reliefs, but not the number of cc's the valve reliefs take up? The chart shows a compression ratio of 8.83:1 using 65cc heads? (and I guess a 0.040" head gasket?) After looking at the numbers, your compression would almost have to be higher than 8.83:1 or you are running a stock cam or smaller at the intake closing point would have to be about 51-degrees after BDC. This would be the same as using a stock 360 cam installed 6-degrees advanced? If your using a stock 360 cam installed @ 110 intake centerline the intake would close at 56-degrees after BDC, and to get your pressure reading of 152, the actual compressuin ratio would have to be about 9.14:1. Because you mentioned the engine may have been built to 10.0:1 compression ratio, we could estimate the size of the cam required to produce the cranking pressure? Using that info, the intake would have to close at 67-degrees ABDC. This would be simular to running a 278 advertised duration cam installed at 108 intake centerline, or the 340 cam with 268-degrees intake duration installed at 113-degree intake centerline. Note that the stock intake centerline for the 340 cam is 112 degrees? Using all this it looks as the engine probbably is around 9.5:1 to 10.0:1 depending on the cam used and it's installed position? |
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