Cylinder compression check

[Gio]

Just finished head-torque and valve adjust and thought I would do a quick compression check. With plugs out, fuel pump disconnected (as tank is still off) and throttle wide open I'm getting 135 (~5 cranks) to 145 psi max on right cylinder and 135 psi max on left ( after ~ 20 cranks) ... does anyone have spec or expected values (couldn't find anything in the manual) ..? Bike is a 2000 sport with 85,000 km and runs very well.

 

Gio

 

(I did try a search but was getting all sorts of weird results to un-related posts when I searched on "compression check" ..?)

 

Edit - just found and tried Doccs suggestion (from another post) and sorted search results by relevance followed by "update" - this produced more relevant results including this one :

 

http://www.v11lemans.com/forums/index.php?showtopic=11694&st=0&p=124262&hl=compression%20check&fromsearch=1entry124262

[thumper]

Was the motor cold or had it been warmed up?

Cold looks ok, if it wat hot may be low.

Less than 10% diff between jugs

[Gio]

Motor was cold (very cold actually!) ... and yes, this is what I got from the earlier thread, a measured difference of only 7% between the pots, and also in the "expected" range of 140-160 psi (warm values) ... thanks for the feedback

[Kiwi_Roy]

Did you remember to hold the throttle wide open?

Just asking

Roy

[Gio]

Did you remember to hold the throttle wide open?

Just asking

Roy

 

Yes - wide open.

[raz]

Edit - just found and tried Doccs suggestion (from another post) and sorted search results by relevance followed by "update" - this produced more relevant results including this one :

 

http://www.v11lemans...=1entry124262

 

Here's a follow-up to that thread: http://www.v11lemans.com/forums/index.php?showtopic=15408&view=findpost&p=171867

 

raz

[Gio]

 

Here's a follow-up to that thread: http://www.v11lemans.com/forums/index.php?showtopic=15408&view=findpost&p=171867

 

raz

 

Thanks Raz - I recall reading about your adventure but missed the compression test update ... something I learned (I think?) from that old thread would be that the calculation for a stock motor (spec compression ratio of 9.3) should give expected compression of around 9.3 x 14.7 = 136 psi ... I also learned that there are many other variables here (temperature of motor, accuracy of gauge etc) but is this a theoretically correct calculation? This is purely for my own interest ... it's still the "off" season remember!

[Kiwi_Roy]

 

Here's a follow-up to that thread: http://www.v11lemans.com/forums/index.php?showtopic=15408&view=findpost&p=171867

 

raz

 

Thanks Raz - I recall reading about your adventure but missed the compression test update ... something I learned (I think?) from that old thread would be that the calculation for a stock motor (spec compression ratio of 9.3) should give expected compression of around 9.3 x 14.7 = 136 psi ... I also learned that there are many other variables here (temperature of motor, accuracy of gauge etc) but is this a theoretically correct calculation? This is purely for my own interest ... it's still the "off" season remember!

 

136 psi Absolute not Gauge

 

i.e if you take a compression ratio of 1

1 x 14.7 = 14.7 psia or zero gauge

So the pressure you see is 136 - 14.7 = 121.3 psi (gauge)

 

Of couse the calculation doesn't take into account the additional pressure due to temperature rise.

http://en.wikipedia.org/wiki/Boyle's_law

(it's too early in the morning to do this complicated math)

Roy

[luhbo]

Due to the open inlet valve you should start your calculation with a CR of 7.6, maybe 8. Heat I would neglect because the engine is a huge cold lump and will keep the temperature quite constant during this short procedure.

 

Hubert

 

Edit: if temperature was constant a Diesel engine would not start

[Gio]

Interesting feedback guys ... Diesel engines I can relate to Hubert (I have an Italian made Lombardini in my tractor - and it always starts - even in winter), but Boyles law ... er, still too early for me Roy but I think I get your drift. Thanks for the education.

[luhbo]

Landini makes quite versatile engines:

 

 

The compression ratio/pressure thing is not that difficult as you don't need to answer on how much work or energy you have to bring to achieve the given results. You can go with p*V/T=constant. Then you get the mentioned 11bar = 160psi with a temperature rise from [Edit:]20°C up to 30°C 293°K (20°C) up to 455°K (182°C).

 

Some time ago I came over this link:

 

Shell Thuet and his tools

 

It looks as if he used the thing a lot

 

Hubert

[Chuck]

 

Aircraft mechanics use a leak down tester to determine an engine's condition at every annual inspection. It's used by bringing the cylinder up on TDC on the compression stroke, and applying 80 psi to the unit by adjusting the regulator on the left. The air goes through a metered orifice, and the second gauge shows the amount of leakage.

Naturally, a perfect cylinder will show 80/80. That almost never happens. 10% leakage would be 80/72, and is fine. Investigation begins when a cylinder is under 80/70, although that is not necessarily bad. The good thing about this is this:

If you listen to the exhaust, you can tell whether an exhaust valve is leaking, at the carb for intake valve, or at the engine breather, rings.

Simple, no? And a much better way of ascertaining engine condition than a "compression gauge."

It's $50-100 at aircraft suppliers.

[Gio]

 

Aircraft mechanics use a leak down tester to determine an engine's condition at every annual inspection. It's used by bringing the cylinder up on TDC on the compression stroke, and applying 80 psi to the unit by adjusting the regulator on the left. The air goes through a metered orifice, and the second gauge shows the amount of leakage.

Naturally, a perfect cylinder will show 80/80. That almost never happens. 10% leakage would be 80/72, and is fine. Investigation begins when a cylinder is under 80/70, although that is not necessarily bad. The good thing about this is this:

If you listen to the exhaust, you can tell whether an exhaust valve is leaking, at the carb for intake valve, or at the engine breather, rings.

Simple, no? And a much better way of ascertaining engine condition than a "compression gauge."

It's $50-100 at aircraft suppliers.

 

Interesting option Chuck - and less expensive than I would have thought. Could this also double as a compression tester? Seems to me that a compression test gives more of a dynamic snap-shot (ie piston / valves moving through complete cycles) followed by a leak-down to pin-point any problems ..? I already had a screw in compression gauge on hand and (fingers crossed) so far have not had to resort to further testing.

[luhbo]

The unit has only one single downside, which is the part in the middle, the

..and applying 80 psi to the unit ..

part. From where do you get the 80 psi if you don't have a compressor at hand? Normaly the standard tools are easier to handle, less accurate maybe, but a working starter is all you need.

If I had a compressor I'd propably go for Chucks thingy.

Hubert

[Bjor]

I have used a tester similar to the one Chuck presents here, with good results. And it's easy to use: connect it, find the TDC (compression stroke), gear in, press the brake pedal (otherwise the piston will be blowed down...), apply air pressure, read the gauge and listen for leakage.

The good thing -as Chuck explains- is that it tells you in an easy way where the eventual leakage is. If there is no leakage on either of the cylinders, I don't really see the need for a compression tester...

Of course you would need a compressor in addition to the tester, but that's almost a must even for a hobby mechanic anyway, isn't it...?