teazer, I was thinking of a couple of factors aside from the ones you mention.
If you use your simple formula (psi=cr x 14.7) the gauge should read 14.7 at a 1:1 compression ratio (no compression at all), but it doesn't--it reads zero. At a ratio of 2:1 your formula would suggest the gauge will read 29.4 psi, but it reads 14.7. The formula doesn't take into account the fact that the gauge subtracts atmospheric pressure. I'm thinking a more realistic formula would be: psi=(cr-1) x 14.7 At a 10:1 ratio that suggests a gauge reading of 132 psi compared to the 147 psi predicted by the simple formula.
The other thing I had in mind is the fact that when you compress air it heats up and that raises the pressure. To account for this, the formula should be more like this: psi=(cr-1)^1.4 x 14.7
At a 10:1 ratio the gauge should then read 319 psi. It doesn't (in our engines) because of valve timing, a less than perfect seal at the valves and rings and a host of other factors. If the gauge reads 147 psi it's by the coincidental result of all these factors and not because psi=cr x 14.7
That's why I figure the term 'normal measured compression' is used.
Wilf
are kenig hi comp.pistons any good ??More here:
http://en.wikipedia.org/wiki/Compression_ratio specifically discussed under the "Fault finding and diagnosis" heading about halfway down the page. 66 dream, 78 cb750k, 02fz1, 09 wing
Well last I checked my Snap-On compression gauge will when installed correctly give me the numbers I need/want and I don't need any formula.
Why don't I need the formula ya ask. Cause I'm testing it not designing it. .......lm
teazer, I beg to differ.
I admit to having minimal experience with these engines and that I am prone to challenging posts in an effort to broaden my understanding of how these machines work. What I don't understand (and please correct my thinking if it's wrong) is how you maintain that your formula (psi=cr*14.7) is a 'reasonable place to start'. Given all the variables (some of which we have spoken of) how can that be? I don't understand. I would be grateful if you could explain to me how this works. Wilf Elementary my Dear Watson. We are not trying to define the parameters pertaining to engine design. We simply looking for a rule of thumb for our expectations.
That nice formula said it should be 25bar (368psi) and the wiki article explained that the real number will be less and that there is no real way to define all the variables to convert Compression ratio to Compression pressure. It then went on to make a nice assertion that engines with 10:1 CR should probably have a pressure of 150-200psi or so. The contributor decided that a rule of thumb he was happy with, was 15-20 x CR. My experience leads me to suggest that's a little high so I decided that as a rule of thumb, I might expect to see a Compression Pressure of say 15*CR which is the lower end of the WIKI expectation. It is possible to create a multi dimensional model to predict pressures at different engine speeds. I have software that does that in a running 2 stroke simulation. It predicts pressure waves, temperatures in pipes along the wave front etc. Bottom line is that we don't need it. I was simply coming up with a crude rule of thumb expectation. At 15:1 a C77 would generate 127.5psi if CR were 8.5:1 and a CB72 at 9.5:1 would be 142.5 at 20:1 those would be 170PSI for a Dream and 190 for a CB77 (too high). They are just rules of thumb - a quick idea of what to expect. The real answer as Loud Mouse said is to measure it and see. Hope that's clearer. Richard
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