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Ironhead Sportster Motorcycle Talk (1957-1985) For all those that wanna talk about Ironhead Sportster Motorcycles

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  #21  
Old 10th June 2010
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Quote:
Originally Posted by SilverShark96 View Post
Well, the way to get around that is with putting a tablespoon of oil down your cylinders. If the compression goes up, then it's your bottom end. If it stays the same (and you're convinced you have a problem), then it's the valves.
Uuuhhmmmm, this is not a way to test the bottom end. With the oil, if the compression goes up it is the rings that are not sealing - the oil temporarily makes them seal better. Yes, if it stays the same it is the valves.
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  #22  
Old 11th June 2010
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Sorry...meant rings
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  #23  
Old 11th June 2010
ratt2y ratt2y is offline
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you 2 guys are funny fe head and fergerburger. i think fergerburger is more right than fehead.you can,t get 170psi out of a 9 to1 motor.STOP jerken the poster.I would just say
116psi is a little low,but not that bad.if it don,t burn oil and the SUV,s ain,t passen you?
keep riding .OK one more thing check your valve adjustment.to tight and you might lose
3-5 psi.
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  #24  
Old 11th June 2010
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Quote:
Originally Posted by ratt2y View Post
you 2 guys are funny fe head and fergerburger. i think fergerburger is more right than fehead.you can,t get 170psi out of a 9 to1 motor.
That's not necessarily true. When I bought my '77 XLT with 7,700 original miles, I did a compression test and got 180 psi on both cylinders, repeated several times for comparison. This was with stock pistons and P cams.

Compression dropped to about 130 psi when I installed Y cams with much more duration/overlap.

Eric
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  #25  
Old 11th June 2010
ratt2y ratt2y is offline
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OK you 3 guys are funny.stock is 120 10:1 is 132psi 11:1 is 145psi and so on and so on
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  #26  
Old 11th June 2010
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Quote:
Originally Posted by ratt2y View Post
OK you 3 guys are funny.stock is 120 10:1 is 132psi 11:1 is 145psi and so on and so on
No way. I got 145 stock with 20K miles. With 10:1 pistons and PB+ cams, I now get 165/170
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  #27  
Old 11th June 2010
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compression numbers (really cranking pressure) has nothing to do with compression ratio......it's a function of volumetric efficiency and cam profile......... 110 PSI is a little on the low side but gauge accuracy and testing conditions can vary so as long as they're both the same (10%) then it's fine.....

there is no way to determine compression ratio by this test...... no way

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  #28  
Old 11th June 2010
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Quote:
Originally Posted by ratt2y View Post
you 2 guys are funny fe head and fergerburger. i think fergerburger is more right than fehead.you can,t get 170psi out of a 9 to1 motor.STOP jerken the poster.I would just say
116psi is a little low,but not that bad.if it don,t burn oil and the SUV,s ain,t passen you?
keep riding .OK one more thing check your valve adjustment.to tight and you might lose
3-5 psi.
I guess some of the theory and the a whole lot of the practical understanding is not being well understood here!

If you in fact do actually compress air exactly 9.1 :1 in other words reduce it down in volume to 1/9.1 th then the resultant PSI WILL BE as I stated which is

{9.1^1.35 X 14.7} = 290 PSI absolutely true at sea level and 293 degrees Kelvin!


The above is the true case IF it were possible to perform in an engine.

However. it is not possible because of the two reasons already stated - namely

1. The intake valve stays open after bottom dead centre so there is no way to keep 100% of the air inside the cylinder.

2. The slow cranking speeds do not always draw in 100% of the cylinder's volume.

Neither I nor anyone else has stated it is possible to get even close to this number even with the lowest performing "Q" cams let alone the other more aggressive ones.

It is a trivial matter to calculate what volume is actually being compressed - just do the math! I have already provided the formula.

If all you can muster up is a measly 120 PSI then you are only achieving an effective compression ratio of 4.73 : 1 while cranking!

If this is too hard to imagine then realize this - the air volume being compressed in one cylinder of a 1000 cc XLH is not the 500 cc but a mere 260.22 cc.

The other 239.78 cc is either not being drawn in or is being pushed out because the intake is staying OPEN after bottom dead centre allowing it to escape forever.

There is no jerking going on here. My statements with respect to the above numbers are valid.

Furthermore there is nothing funny about it - the compression tester can only read the remaining air that is allowed to be compressed.

I can appreciate not everyone would want such an esoteric explanation but I do believe many here are interested enough to learn and or make comments based on better, more substantive evidences than base innuendo.

If the applied theory and my analysis of it is wrong - ratt2y - please take a moment to produce the rationale behind your view as to why? I have already provided the universally accepted, refereed and published methods for calculating adiabatically compressed air when neither the volume nor the pressure is constant.

Your analysis as I see it is for a NON adiabatic system where either the volume or the pressure is constant - this is not what is happening in a non running cranking compression stroke of any internal combustion engine.


Cheers;
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  #29  
Old 11th June 2010
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The bottom line is; the OP's compression is probably in the ballpark and nothing to worry about. At least that would be the uncomplicated version..
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  #30  
Old 11th June 2010
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I need a lawyer. Thanks for all the wonderful info!



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