Bore Scratches
12-02-2018, 03:05 AM
#112
Er excuse me , I atmosphere is approx 15 psi or 1 bar , so 10-1 is surely 150psi or 15 bar give or take , 220 would equate to 22-1 or is it an oil burner or is doctor who waving her sonic screwdriver to magic up extra compression !
my engine is most likely crap considering the oil it burns , often thinks it is a deisal .
my engine is most likely crap considering the oil it burns , often thinks it is a deisal .
The reason one gets higher PSI readings than one might expect is that the displacement of the cylinder is being compressed into the (pent-roof in this case) combustion chamber, which is a much 'narrower' space.
Probably easier to imagine on a Heron head engine, which is cylindrical, but has a bowl-in piston crown; all the atmosphere ends in in the bowl, except for a tiny bit between the head and piston.
That could be one explanation...
The other one is simply that air doesn't compress statically - at higher pressures, all the bloody molecules (or their bonds, actually) get in the way; a blocked bike pump is easier to compress at the start of the stroke and normally breaks at the other end. It also gets hot...
12-02-2018, 11:57 PM
#113
Compression ratio and a cranking compression test are completely different things.
A healthy F20 engine should read around 16kg or 228psi, lower CR pistons might give a slightly lower number but I doubt it would make much difference.
Most importantly is that all cylinders reads equal but these lower number should at least be checked with another gauge or on another car to confirm if the readings are out (it's not that uncommon).
A healthy F20 engine should read around 16kg or 228psi, lower CR pistons might give a slightly lower number but I doubt it would make much difference.
Most importantly is that all cylinders reads equal but these lower number should at least be checked with another gauge or on another car to confirm if the readings are out (it's not that uncommon).
12-03-2018, 02:15 AM
#114
Well yes - CRs are calculated things; total volume (including CC) at BDC vs at TDC. Which is why a different HG thickness makes a significant difference.
And air is a variable-rate spring.
And air is a variable-rate spring.
12-03-2018, 10:46 AM
#116
Just checked mine on the fresh build after 20 mins running time so i can get a base reading for the 500 mile check, 3 came out at 235 and 1 at 240 (sealey gauge so not the most accurate more for checking the spread across the cylinders really) i'll check the ones on groover's old car sometime this week it's just turn 1 k so will be changing the oil
12-03-2018, 11:53 AM
#117
Thread Starter
Well I am still not convinced at all , there are so many variables here not least when the inlet valves fully close etc which is most likely when the piston is up the bore a bit esp in these engines and other high performance engines. Also there is the long volume of the gauge hose to take into account the variation in the size of the adaptor and the plug.
Having a quick whizz around net , dangerous I know ! Most petrol engines with 10:1 ish cr with generate around 150psi as I thought if they are in good condition. The most important thing to see is balance between all pots. Then in some respects the actual figure shown on the gauge is insignificant as long as in this case all four are equal give or take a bit. Then how about the long hose connecting the fitting to the gauge and the volume of air there and the hose itself giving a bit , then the temp generated by compression of a gas, ect ect
the other thing is with no fuel in the mix we don’t have any liquids in the chamber, as we know certainly in the volumes we are talking about compressibility of the petrol will be unmeasurable so squirt a drop of that in and the compression pressure will be higher.
so the pressure generated just cranking the engine might well be lowish I would think. I am sure the readings of various gauges will vary hugely as well.
or maybe there is some magic at work here !
So 165 figure recorded and in balance I would think is fine esp as the engines consumes zero oil , and runs well !
Having a quick whizz around net , dangerous I know ! Most petrol engines with 10:1 ish cr with generate around 150psi as I thought if they are in good condition. The most important thing to see is balance between all pots. Then in some respects the actual figure shown on the gauge is insignificant as long as in this case all four are equal give or take a bit. Then how about the long hose connecting the fitting to the gauge and the volume of air there and the hose itself giving a bit , then the temp generated by compression of a gas, ect ect
the other thing is with no fuel in the mix we don’t have any liquids in the chamber, as we know certainly in the volumes we are talking about compressibility of the petrol will be unmeasurable so squirt a drop of that in and the compression pressure will be higher.
so the pressure generated just cranking the engine might well be lowish I would think. I am sure the readings of various gauges will vary hugely as well.
or maybe there is some magic at work here !
So 165 figure recorded and in balance I would think is fine esp as the engines consumes zero oil , and runs well !
12-03-2018, 12:46 PM
#118
Well I am still not convinced at all , there are so many variables here not least when the inlet valves fully close etc which is most likely when the piston is up the bore a bit esp in these engines and other high performance engines. Also there is the long volume of the gauge hose to take into account the variation in the size of the adaptor and the plug.
Having a quick whizz around net , dangerous I know ! Most petrol engines with 10:1 ish cr with generate around 150psi as I thought if they are in good condition. The most important thing to see is balance between all pots. Then in some respects the actual figure shown on the gauge is insignificant as long as in this case all four are equal give or take a bit. Then how about the long hose connecting the fitting to the gauge and the volume of air there and the hose itself giving a bit , then the temp generated by compression of a gas, ect ect
the other thing is with no fuel in the mix we don’t have any liquids in the chamber, as we know certainly in the volumes we are talking about compressibility of the petrol will be unmeasurable so squirt a drop of that in and the compression pressure will be higher.
so the pressure generated just cranking the engine might well be lowish I would think. I am sure the readings of various gauges will vary hugely as well.
or maybe there is some magic at work here !
So 165 figure recorded and in balance I would think is fine esp as the engines consumes zero oil , and runs well !
Having a quick whizz around net , dangerous I know ! Most petrol engines with 10:1 ish cr with generate around 150psi as I thought if they are in good condition. The most important thing to see is balance between all pots. Then in some respects the actual figure shown on the gauge is insignificant as long as in this case all four are equal give or take a bit. Then how about the long hose connecting the fitting to the gauge and the volume of air there and the hose itself giving a bit , then the temp generated by compression of a gas, ect ect
the other thing is with no fuel in the mix we don’t have any liquids in the chamber, as we know certainly in the volumes we are talking about compressibility of the petrol will be unmeasurable so squirt a drop of that in and the compression pressure will be higher.
so the pressure generated just cranking the engine might well be lowish I would think. I am sure the readings of various gauges will vary hugely as well.
or maybe there is some magic at work here !
So 165 figure recorded and in balance I would think is fine esp as the engines consumes zero oil , and runs well !
battery condition and starter drag all have an effect as well, all gauges read different and it's more important to know your own gauge and checking method, i always replicate my checking sequence the same way on all engines the way i was taught many many moons ago , engine warm, all plugs out, throttle full open then 5 compression strokes then as you says i'm looking for as close to equal across the cylinders, if i get what i would call a low reading then a drop of oil in the camber and check again, compression checks are just a quick look see , leak down test's are the important ones
12-04-2018, 02:00 AM
#119
Yeah, given the consistency of the read and the LC pistons, I'd not be concerned about the 165 PSI.
A bit like different engine dynos - these things tend to be inconsistent and are best used only as their own base line.
A bit like different engine dynos - these things tend to be inconsistent and are best used only as their own base line.
12-04-2018, 05:00 AM
#120
The factory manual states a minimum pressure of 9.5kg or 135psi so I guess 165psi on a slightly lower compression ratio could be perfectly fine 
Here is a few other results from lower CR engines just for comparison https://www.s2ki.com/forums/s2000-fo...motor-1189680/
Here is a few other results from lower CR engines just for comparison https://www.s2ki.com/forums/s2000-fo...motor-1189680/