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CR to cylinder PSI. Relationship?


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Further to Lebro post of his cylinder pressures I pose this question.
 

Bob.

What CR did you aim at with your engine?

my old collected notes shew this relationship between cr and psi
140 psi~8:1
150 psi~8.5:1
160 psi~9:1
170 psi~9.5:1
180 psi~10:1
190 psi~10.5:1

Please feel free to rubbish this info.   

Does cam timing and tappet clearance affect the cylinder pressure?

My own engine measures 180-185 psi wot. cranking speed.

The build measurements and calculations indicated 10:1 using https://speedmaster79.com/tools/engine-compression-ratio


Peter W

Edited by BlueTR3A-5EKT
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  • BlueTR3A-5EKT changed the title to CR to cylinder PSI. Relationship?
1 hour ago, BlueTR3A-5EKT said:

Further to Lebro post of his cylinder pressures I pose this question.
 

Bob.

What CR did you aim at with your engine?

my old collected notes shew this relationship between cr and psi
140 psi~8:1
150 psi~8.5:1
160 psi~9:1
170 psi~9.5:1
180 psi~10:1
190 psi~10.5:1

Please feel free to rubbish this info.   

Does cam timing and tappet clearance affect the cylinder pressure?

My own engine measures 180-185 psi wot. cranking speed.

The build measurements and calculations indicated 10:1 using https://speedmaster79.com/tools/engine-compression-ratio


Peter W

Peter 

Your numbers are about right and 180 is the sweet spot 

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The exact numbers do indeed reflect the compression ratio.

If we assume atmospheric pressure is approx 14.7psi then there should be a reasonable correlation between the measured pressures. However the exact figure can be affected by things like engine temp and indeed the heating effect of compressing the air.

The key thing is that the figures are within 10% of each other - more than that you start to suspect uneven wear, ring damage, leaking valve seats, head gasket failure and so on. Obviously all engines wear and it is plausible that even on a totally shagged out engine they will be similarly worn but the amounts are rarely identical on every cylinder.

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Peter

It’s handy that you can multiply the Compression Ratio by 20 and then subtract 20 to get an estimate of the compression pressure in PSI. This seems to work pretty well as a starting point even though it’s not an exact science. All plugs out, good battery and wide open throttle seems to give comparative values

Rog

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I don't agree! ( No surprise there, eh?)

A compression gauge is a simple instrument, and they are never calibrated.   Plus, the technique used will affect values achieved, as will atmospheric pressure on the day.   IMHO, those values are only useful for comparing compressions across the block, to check for piston or valve seal leaks.    They are not comparable between different gauges, although the same gauge could compare different engines, but only on the same day.

Thus, predicting, or even guessing the CR from a compression is just that, a guess.    The CR can only be determined by measuring the combustion chamber volume - and that is only the static CR.   The actual, dynamic CR will be influenced by the inlet duct geometry, the cam grind and the engine speed.

John

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Theoretically with butterflies fully open. allowing for volumetric effic of  typical road engine, low rpm, ca 80%, we get:

(14.7 x 0.8 x 10 ) + 14,7 = 132 psig, for 10:1 Comp ratio.

If engines is hot psi will rise. A cam with lots of overlap will lower the figure as it hits VE at low rpm

As Andy says equality between cylinders more useful in assessing condition.

Peter

 

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3 minutes ago, Peter Cobbold said:

Theoretically with butterflies fully open. allowing for volumetric effic of  typical road engine, low rpm, ca 80%, we get:

(14.7 x 0.8 x 10 ) + 14,7 = 132 psig, for 10:1 Comp ratio.

If engines is hot psi will rise. A cam with lots of overlap will lower the figure as it hits VE at low rpm

As Andy says equality between cylinders more useful in assessing condition.

Peter

 

Brain fade:  132 psi abs not gauge

Agree with John, and the volume of the gauge's hose adds further imprecision, adding  to the combusiton chamber volume.

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To Answer Pete's question I was aiming for 10:1 with 87mm bores.

Out of interest I will check my Gunson compression gauge against some other gauges I have (& believe) on my compressed air system.

I do not get pinking even with foot down at low revs. Ign timing is 10° tick over, 28° max advance

Bob.

Edited by Lebro
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It won't be for a couple of days because I have other things to do, but when I get a moment I will do a compression test.  Certainly Peter's website aligns with my spreadsheet in calculating a CR of 9:1 for my engine.  And from memory, last time I checked the compression was somewhere around 160-165 psi.  I have 11° advance at about 800rpm.

Rgds Ian

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To help in making an informed guess of the expected compression pressure in PSI, multiply the compression ratio by 20 and subtract twenty. Making an informed guess using this method is considered preferable to making an uninformed guess. 

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Any measurement involving psi can only be approximate. I gave a figure that is for isothermic compression where all the heat of compressing the air is lost to the walls and piston. Andy correctly raised the question of the effect of compressing gas  Compressing a gas requires work and that raises its temperature, it tries to expand and that raises its pressure further. If the compression happens without loss of heat ( ie to bore walls or piston) it is termed "adiabatic". The equaiton for the pressure rise that allows for heating is

https://www.grc.nasa.gov/www/k-12/airplane/compexp.html 

bottom of page

p2 / p1 = (v1 / v2) ^ (gamma)      where gamma for air is 1.4

so if V changes 10 fold in our engine P rises 10 ^1.4 which is 25.  Hence for adiabatic compression the gauge would read 25 x 14.7 or   367 psi.  We never   see that when using a compression tester as the cranking is so slow that heat is lost to the walls and the pressure drops. The conditions are  not adiabatic. Faster cranking will deliver a higher reading, But as a way of establishing compression ratio a gauge is nbg.

( However 367psi will be roughly the pressure in a running engine before the spark, where the speed of compression leaves less time for heat to be lost,, ie it is approx adiabatic)

Peter

 

 

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I have a compression tester which is like the old Schrader tyre pressure tester.   I screw it into the plug hole and over about 4-6 compressions, it gets pumped up until it reaches the final value.

I wonder whether a geared starter motor, which turns the engine faster than the original type, gives a higher reading?

Ian Cornish

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Just looked back at previous compression checks I have done:

Standard engine, 83mm pistons after approx 64000 miles:   151   161   126  145     volume of chambers  60cc    standard  CR  8.5:

Standard engine, 83mm pistons after approx 67000 miles:   145   168   127  160     volume of chambers  60cc    standard  CR  8.5:

Standard engine, 83mm pistons after approx 72000 miles:   151   168    88  173     volume of chambers  60cc    standard  CR  8.5:1

Re-worked cylinder head Vissard style 60 thou off face:        172   179  188  188     volume of chambers  58cc    expected CR  8.7:1

Rebuilt engine, 87mm pistons head as above at     0 miles     209  200  203  206     volume of chambers  58cc    expected CR  9.13:1

Rebuilt engine, 87mm pistons head as above at 250 miles     213  200  204  210     volume of chambers  58cc    expected CR  9.13:1

1971971903_Crcalc.jpg.8c5e58d4e08471a6aa576d4c9d707a72.jpg

So CR actually should be only 9.13:1  not the 10:1 I had thought.  Will check calibration of Comp tester later today.

Bob

P.S.   gauge now checked against the one on my compressor. at 132PSI, both gauges read the same

Edited by Lebro
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4 hours ago, ianc said:

I have a compression tester which is like the old Schrader tyre pressure tester.   I screw it into the plug hole and over about 4-6 compressions, it gets pumped up until it reaches the final value.

I wonder whether a geared starter motor, which turns the engine faster than the original type, gives a higher reading?

Ian Cornish

Ian,  Yes faster cranking gives less time for loss of heat of compression, the gas temperature will be higher and psi recorded higher. To get the adiabatic pressures you could run the engine at wot (briefly) on 3 cylinders with the gauge in the fourth. Stand well clear- the gauge will explode !   Maybe wiser to do the reverse expt and slow crnaking by comparing no plugs with all plugs fitted.

The numbers are so dependent upon loss of heat of compression that they tell us nothing about the true value of the CR. 

Peter

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10 minutes ago, Peter Cobbold said:

Ian,  Yes faster cranking gives less time for loss of heat of compression, the gas temperature will be higher and psi recorded higher. To get the adiabatic pressures you could run the engine at wot (briefly) on 3 cylinders with the gauge in the fourth. Stand well clear- the gauge will explode !   Maybe wiser to do the reverse expt and slow crnaking by comparing no plugs with all plugs fitted.

The numbers are so dependent upon loss of heat of compression that they tell us nothing about the true value of the CR. 

Peter

Thank you.  
I had ignored/ forgotten heat was generated by compression which affects the pressure.  
 

Peter W

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2 hours ago, BlueTR3A-5EKT said:

Thank you.  
I had ignored/ forgotten heat was generated by compression which affects the pressure.  
 

Peter W

No problem Peter, our memory is volatile ! I had to revive and  refresh A-level physics from 60 years ago to write the adiabatic post. Peter

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Thnak you Peter!   I shall add compression heating to the list of factors that make a Compression guage unreliable!

John

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You can do the numbers how you ,will I stand that it’s a good guide to finding the approximate compression ratio seen it proved. Do the test and fit a thicker head gasket then do it again 

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2 hours ago, ntc said:

You can do the numbers how you ,will I stand that it’s a good guide to finding the approximate compression ratio seen it proved. Do the test and fit a thicker head gasket then do it again 

Neil,  On the same engine the trend towards lower psi with lower CR will be found. But the relationship will not be linear as compression heating drops non-linearly with CR, its in the equation. I use a cgauge to check compressions are similar across all 6.   Peter

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Wiki is useful; https://en.wikipedia.org/wiki/Compression_ratio

image.thumb.png.23f544af940b6309b72fce35d880be34.png

====

 To get approx  adiabatic compression the engine has to be turning at running rpm so that heat losses are minimal. When cranking the adiabatic condition is lost and the pressure drops due to the compressed gas cooling.

Measurements of cylider psi on lab engines driven at typically 3000 rpm ("motored engine") are described in textbooks.

The dynamic comp rat is interesting, a lot of overlap drops the CR a lot, whcih is why race engines stagger at low rpm.

Peter

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Does it make a significant difference if a compression test is carried out with the engine hot as opposed to cold?  And is one or the other correct?

Rgds Ian

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Compression check with engine hot gives an easier turning engine which achieves the max reading quicker and you have to be smarter on the starter if you try and limit the engine turns. It also has as accurate a piston ring seal as you should get with the dispersment (I think I just made that up) of oil throughout the engine and more especially on the top of the cylinders liner walls and piston rings. 

As for accuracy I've never considered the readings as anything other than a comparison between individual cylinders and whether you have an unfeasible reading achieved which may indicate a problem. To try and "read the runes" as to how any compressions achieved relate to a compression ratio (other than a rough guide probably within a 10 or 15% range) is I think optimistic.

The machining heads, pistons and blocks, and measurement of the static compression ratio itself is only a guide being as the other variables in the inlet/exhaust tract, different camshaft profiles and overlap and mixes of combustion means it's likely non of us are running at the true ratios we think. The upper limit for road use is achieved when your car cannot stop pinking until additive is used.

Mick Richards 

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Well for what it's worth my measurements were done following a quick blast around local roads, so were done "hot"

As the calculated CR (9.13:1) based on known data:

12 thou piston deck to top of liner, 1.3mm compressed gasket, 58cc chamber capacity, 92mm stroke

would not be expected give 200 + PSI (& I checked my gauge against a known good air pressure gauge on my compressor)

Then the variance must be down to a combination of the cam (Newman PH1,) my modified combustion chamber shape, smoothed out ports in the head, & matching the TR4A inlet manifold ports to the TR3 high port head (larger diameter).

It does not pink on 99 RON fuel (which I use due to it having zero ethanol) so I won't concern myself further with the high figures.

Bob.

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Just read this:

https://mgaguru.com/mgtech/power/pp105.htm

When the gauge pressure reading is 190psi, the absolute pressure is 204.7psi.
P1=204.7
P2=14.7
V2/V1 = effective compression ratio
V2/V1 = (204.7/14.7)^0.71 = 6.5/1

So, putting my gauge reading of 200 PSI into his formula: CR =  (214.7/14.7)^0.71 gives a "gauge" CR of 6.71:1

Bob

Edited by Lebro
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