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  #1  
Old 9th June 2010
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Default Compression Measurement - Look Here

I have a few questions for all of you.

What compression readings should I be getting on my 1982 XLH Ironhead per cylinder? I have read 110. Is this true?

It has about 16,500 miles and I would like to see if I the compression is good and to see if I need new valves or seals, etc., etc.

What compression levels indicate a need to change valves or seals or rings, etc.

As always your feedback provides a wealth of knowledge and insight.

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BrownSkull
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Old 9th June 2010
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You really need to get a factory workshop manual before digging into a top end overhaul.
From memory, it specifies 120psi as the expected minimum for an Ironhead.

Make sure you hold the throttle wide open when cranking the motor over for compression test. It can affect the outcome a lot.
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Old 9th June 2010
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Late 84,125 p.s.i. per cylinder,has 46K
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Don't get involved with such satanic rituals as compression testing, save it for the 2-strokes, ride it until it either stops or won't start or burns so much oil you can't afford to ride it.

Its an Ironhead, looking too closely at the data can cause a financial crisis.....
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Old 9th June 2010
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Engine must be at fully warmed up operating temperature for a valid compression check. And as already pointed out throttle plate [and choke plate if there is one] must be fully open. See the Sticky.
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120 is okay...normal atmospheric pressure is about 15 psi so if you are running 9:1 you should get about 135. I thought the minimum was more like 110 but if it runs and doesn;t burn enough oil to drain the Gulf of BP oil you should just ride til winter
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did you have the throttle WIDE open when you did the test?
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Quote:
Originally Posted by fergerburger View Post
120 is okay...normal atmospheric pressure is about 15 psi so if you are running 9:1 you should get about 135. I thought the minimum was more like 110 but if it runs and doesn;t burn enough oil to drain the Gulf of BP oil you should just ride til winter
A newly fitted/seated set of pistons running the OEM "Q" cams will easily provide 150 to 170 psi of compression when wet tested.

In an adiabatic process of compressing air from 1 volume to 1/9.1 th of a volume you can produce well over 300 psi due to the heat of compression and the exponential gamma factor of 1.4, e.g. 15 psi X (9.1)^1.4 = 330 psi.

The actual static compression reading in a Sportster engine is much less than the ideal calculated one - due mostly to the intake valve closing at 32 degrees ABDC (after bottom dead centre)and the less than ideal volumetric efficencies at the low cranking RPMs.

The increased ABDC closing of higher performing cams (the 'Y' @47 or the 'R5' @41 degrees) is why one must use the higher compression ratio pistons - 10.5:1 - to compensate and provide better low end torque until the RPMs get above 3000 RPMs.

Cheers;
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where did you get that equation

Pressure is inversely related to volume PV =nRT it is linear with temperature

your not going to get 300 psi from a 9:1 volume change. I agree the temp goes up on compression but not by a factor of 2. you'll have to explain how the cams can actually increase the theoretical yield. it would seem the max compression is calculated from the volume change
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Quote:
Originally Posted by fergerburger View Post
where did you get that equation

Pressure is inversely related to volume PV =nRT it is linear with temperature

your not going to get 300 psi from a 9:1 volume change. I agree the temp goes up on compression but not by a factor of 2. you'll have to explain how the cams can actually increase the theoretical yield. it would seem the max compression is calculated from the volume change

The ideal gas law you have quoted is used for a situation where you have Constant Pressure and are adding or varying the other variable OR it works when you have Constan Volume and likewise vary the other variables

However. in our Sportster engines undergoing a compression stroke situation we have both situations working concomitantly.

Also we are not adding any heat to the system as the compression occurs.

This is where the combined specific heat formulas are needed as in the adiabatic scenario.

Please check out my prior reference to "adiabatic" as this is the sole reason for the exponential factor.

Adiabatic Process
An adiabatic process is one in which no heat is gained or lost by the system. The first law of thermodynamics with Q=0 shows that all the change in internal energy is in the form of work done. This puts a constraint on the heat engine process leading to the adiabatic condition. This condition can be used to derive the expression for the work done during an adiabatic process in other words the work needed to compress the air in an engine.


The ratio of the specific heats γ = CP/CV is a factor in determining the speed of sound in a gas and other adiabatic processes as well as this application to heat engines. This ratio γ = 1.66 for an ideal monoatomic gas and γ = 1.4 for air, which is predominantly a diatomic gas.

As air is 80/20 diatomic nitrogen and diatomic oxygen and the gamma factor of 1.4 is the ratio of their respective Specific Heat Capacities and as such the diatomic molecules have 5 degrees of freedom rather than 3.

The constant pressure specific heat is related to the constant volume value by CP = CV + R. The ratio of the specific heats is called Gamma and is = CP/CV. This is the factor used in an adiabatic type engine analysis.

Some researchers use a gamma factor of 1.3 instead but it still needs to be accounted for or else how do I get 160 PSI in my stock "79 Ironhead?

Even if you use the gamma of 1.3 you still get a maximum compression reading of 265 psi with the 9.1:1 pistons and 280 psi with 9.5 pistons and using the 15 psi as 1 bar.

Your ideal gas equation above is only accounting for the specifice heat at a constant volume while assuming the compressed air's temerature is the same as the uncompressed except for the decreased volume which is definitely not the case in the engine's scenario.

It takes a lot of work and some time - energy - to compress the air fast. It is this added energy that produces the gamma factor in addition to the reduced volume and which gives rise the non-linear relationship.

If you have ever placed your finger over the end of the spark plug hole as the engine is cranking you can easily get a second degree burn if you do not remove it or let some of the compressed air sqeeze by as the air is being compressed. The faster the comprssion occurs the more heat that is created.

If you had a set of cams that had closings of zero after bottom dead centre (ABDC) then you would get compression readings close to the 330 PSI level. It would be near impossible to kick over and the starter would be hard pressed to do its job.

The cams in every engine reduce the theoretical value when the correct theory is being used.

The later the intake closes the lower the effective compression one will observe it's all a matter of degree.

There are real cases where the person has miss timed the intake cam/cams and was seeing compressions of over 250 psi until they reset the marks correctly.


Cheers;
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