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Livefast123
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PostPosted: 08:46 - 07 Jun 2010    Post subject: Compression Test Question Reply with quote
After getting my NSR 125 RR running after 5 years stuck in the shed I need to do a compression test

I've stuck my finger down the plug hole and it kinda blows my finger out.

My question is that should the test be done with the engine hot or cold and can anyone recommend a reasonably priced tester

Cheers
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temeluchus
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PostPosted: 09:11 - 07 Jun 2010    Post subject: Reply with quote
cold.

you can pick up a tester for 20 quid. Make sure it has the right sized adaptor for your engine (2 strokes tend to have 14mm plugs).
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Raffles
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PostPosted: 09:50 - 07 Jun 2010    Post subject: Re: Compression Test Question Reply with quote
Livefast123 wrote:
After getting my NSR 125 RR running after 5 years stuck in the shed I need to do a compression test

I've stuck my finger down the plug hole and it kinda blows my finger out.

My question is that should the test be done with the engine hot or cold and can anyone recommend a reasonably priced tester

Cheers


If you're using your finger as a compression tester then I would definitely recommend that the test be carried out with a cold engine Laughing
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arry
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PostPosted: 11:33 - 07 Jun 2010    Post subject: Reply with quote
Temeluchus wrote:
cold.


I'd have gone for hot - any reason you're going for cold?
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The Artist
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PostPosted: 11:59 - 07 Jun 2010    Post subject: Reply with quote
arry wrote:
Temeluchus wrote:
cold.


I'd have gone for hot - any reason you're going for cold?


Because that is what most manufacturers say to do. It is how they get the spec for the manual.

If you loose compression when it gets hot then it has got to be something major anyway.
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Raffles
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PostPosted: 12:00 - 07 Jun 2010    Post subject: Reply with quote
arry wrote:
Temeluchus wrote:
cold.


I'd have gone for hot - any reason you're going for cold?


The compression test should be carried out on a cold engine primarily to establish whether or not the piston rings are providing a good seal between the piston and the cylinder.
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Alexio
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PostPosted: 13:24 - 07 Jun 2010    Post subject: Reply with quote
Theoretically compression should increase with the temperature of the engine too (I think)??? (Warmer oil means more viscose so there is even less engine resistance from what the lubrication is doing)

If that were the case then a problem is more likely to be found with the compression while cold as well. And an engine is always going to be cold when you're trying to start it, which is when you definitely need compression!

I could be wrong on that, I'd like to think I'm not.
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The Artist
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PostPosted: 13:35 - 07 Jun 2010    Post subject: Reply with quote
Alexio wrote:
Theoretically compression should increase with the temperature of the engine too (I think)??? (Warmer oil means more viscose so there is even less engine resistance from what the lubrication is doing)

If that were the case then a problem is more likely to be found with the compression while cold as well. And an engine is always going to be cold when you're trying to start it, which is when you definitely need compression!

I could be wrong on that, I'd like to think I'm not.


Working eh!

The heat of the oil is nothing to do with it.
The fact that there will be oil around the top end and around the rings is more important.
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Alexio
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PostPosted: 16:14 - 07 Jun 2010    Post subject: Reply with quote
allymoss wrote:
Working eh!


Oh come on, we all know that you're most able to multi-task effectively while working Wink

Hmm, so compression doesn't increase with temperature at all, just good oil circulation? Hot gases and expanding to taking up more room / create more pressure and all of that doesn't have much of an effect either I suppose? It's not as if the air/fuel mixture is heated up before being pumped in....

but the up-stroke is designed to compress the mixture to make it easier to ignite, and it does this more effectively when the engine is hot, which might mean as it heats up before ignited does fight the compression some-what?

I dunno, I'm just in one of those annoying curios / question asking moods and I'm totally procrastinating as you can probably tell. Shifty
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will never give up his CG. I look at my fuel gauge more as a progress bar than a fuel gauge.
G: With my GSXR I do often effectively use it as a scooter with a clutch in town.
ms51ves3: why does it need 500 miles? Are you teaching it how to be a piston?
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steven_191
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PostPosted: 16:28 - 07 Jun 2010    Post subject: Reply with quote
Alexio wrote:
allymoss wrote:
Working eh!


Oh come on, we all know that you're most able to multi-task effectively while working Wink

Hmm, so compression doesn't increase with temperature at all, just good oil circulation? Hot gases and expanding to taking up more room / create more pressure and all of that doesn't have much of an effect either I suppose? It's not as if the air/fuel mixture is heated up before being pumped in....

but the up-stroke is designed to compress the mixture to make it easier to ignite, and it does this more effectively when the engine is hot, which might mean as it heats up before ignited does fight the compression some-what?

I dunno, I'm just in one of those annoying curios / question asking moods and I'm totally procrastinating as you can probably tell. Shifty


the air heats up BECAUSE its compressed. its a natural reaction. basically friction as the air molecules are forced against each other. heance why turbo'd engines have an intercooler. the turbo pressurises the air before going into the cylinder which heats it up, the cooler brings the temperature down to get rid of detonantion, or pre-ignition. the air/fuel could ignite before the spark fires because its so hot already, the heat at the tip of the plug would be enough to do it.
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Alexio
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PostPosted: 16:32 - 07 Jun 2010    Post subject: Reply with quote
steven_191 wrote:
the air heats up BECAUSE its compressed. its a natural reaction. basically friction as the air molecules are forced against each other. heance why turbo'd engines have an intercooler. the turbo pressurises the air before going into the cylinder which heats it up, the cooler brings the temperature down to get rid of detonantion, or pre-ignition. the air/fuel could ignite before the spark fires because its so hot already, the heat at the tip of the plug would be enough to do it.


Seems so obvious when you put it like that Doh!

Why do engines needed to be warmed up at all then? Wink

I mean, I'd assume it's because metal expands when hot and under normal operating conditions the engine is going to be warmer and so it was engineered to those tolerances and to work most efficiently when warmer.

I could be wrong though? Laughing
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will never give up his CG. I look at my fuel gauge more as a progress bar than a fuel gauge.
G: With my GSXR I do often effectively use it as a scooter with a clutch in town.
ms51ves3: why does it need 500 miles? Are you teaching it how to be a piston?
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temeluchus
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PostPosted: 20:48 - 07 Jun 2010    Post subject: Reply with quote
Alexio wrote:


I mean, I'd assume it's because metal expands when hot and under normal operating conditions the engine is going to be warmer and so it was engineered to those tolerances and to work most efficiently when warmer.


Thats correct...... well we know never to buy a bike from steve 191
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Wafer_Thin_Ham
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PostPosted: 07:45 - 08 Jun 2010    Post subject: Reply with quote
steven_191 wrote:
the turbo pressurises the air before going into the cylinder which heats it up, the cooler brings the temperature down to get rid of detonantion, or pre-ignition. the air/fuel could ignite before the spark fires because its so hot already, the heat at the tip of the plug would be enough to do it.


Turbos are also sat next to an extremely hot exhaust which also may have some bearing on it. Wink
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Gazdaman
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PostPosted: 09:17 - 08 Jun 2010    Post subject: Reply with quote
If you're testing compression with your thumb, it's going to make bollocks all difference if the engine is hot or not.

It's hardly precision testing is it.

And the heat of an engine should make very little difference to compression, although it can do if your valve clearances are mega over-tight because as it expands it holds a valve ever so slightly open and you lose pretty much all compression.

Gaz
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Walloper
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PostPosted: 12:27 - 09 Jun 2010    Post subject: Reply with quote
1st)

Compression is tested on a cold engine. End of.

2nd)

Intercoolers/Aftercoolers only cool the charge air to make it more dense. (Though not quite as dense as some on bcf)
It has nothing to do with detonation. That may only be a minimal advantage.

The air is made more dense to maximise the amount/concentration of Oxygen. Nothing more.
More oxygen permits more fuel to be burned.
More fuel and oxygen in then burned means more oomph out at the flywheel/tyres.
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Alexio
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PostPosted: 14:07 - 09 Jun 2010    Post subject: Reply with quote
Walloper wrote:
1st)

Compression is tested on a cold engine. End of.

2nd)

Intercoolers/Aftercoolers only cool the charge air to make it more dense. (Though not quite as dense as some on bcf)
It has nothing to do with detonation. That may only be a minimal advantage.

The air is made more dense to maximise the amount/concentration of Oxygen. Nothing more.
More oxygen permits more fuel to be burned.
More fuel and oxygen in then burned means more oomph out at the flywheel/tyres.


I would have thought though that the more dense the air, the more engine compression it would need to get it to a good point of being easily combustible. As the dense air starts to compress it will heat up and expand and hence require more energy because there's a higher volume of it. I assume the engine would have been engineered with this in mind. Also, if the engine is spinning with a lot of momentum I'm sure a slightly more dense mixture isn't going to pose to much of a challenge to being compressed.

But would the jetting need to be changed otherwise though? You've got a higher concentration of oxygen but is it pulling in more fuel or is it then going to run more "lean" once the turbo really starts doing it's thing?
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will never give up his CG. I look at my fuel gauge more as a progress bar than a fuel gauge.
G: With my GSXR I do often effectively use it as a scooter with a clutch in town.
ms51ves3: why does it need 500 miles? Are you teaching it how to be a piston?
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Walloper
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PostPosted: 14:55 - 09 Jun 2010    Post subject: Reply with quote
Supercharged Blower/turbo/et al have different compression ratios (Lower) to those of naturally asperated engines to compensate.

Fuel injection is the most common method of introducing the correct amount of fuel into the air in 'supercharged' engines though there may be pre-charger carburetted systems. The main problem with carburretors is that they rely solely on pressure differentials between the cylinder and the outside ambient atmospheric air pressure. Compressing the charge air makes fuel metering difficult by this method.
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steven_191
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PostPosted: 16:39 - 09 Jun 2010    Post subject: Reply with quote
Temeluchus wrote:
Alexio wrote:


I mean, I'd assume it's because metal expands when hot and under normal operating conditions the engine is going to be warmer and so it was engineered to those tolerances and to work most efficiently when warmer.


Thats correct...... well we know never to buy a bike from steve 191


eh? i say the air heats up when compressed and therefore you wouldnt buy a bike off me? cheers.
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steven_191
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PostPosted: 16:44 - 09 Jun 2010    Post subject: Reply with quote
also to your orignal question. i bought one of these recently and it works fine.

https://cgi.ebay.co.uk/COMPRESSION-TESTER-KIT-AUTOMOTIVE-PETROL-ENGINE-VALVE-/180459863037?cmd=ViewItem&pt=UK_Diagnostic_Tools_Equipment&hash=item2a043efbfd

for £10 i wasnt expecting pinpoint accuracy but if thats what you want you'll have to pay a fair bit more.

as for the test at hot or cold. i would say cold if you know know the condition of your engine, but if its something you use day to day then id do it hot or at least close to temperature for the reason that it will be at operating temperature. you dont ride your bike around at 20degrees. it runs at temp, whatever that may be.

or even better. do a cold test then a hot test then it'll be £10 well spent
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Walloper
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PostPosted: 18:24 - 09 Jun 2010    Post subject: Reply with quote
steven_191 wrote:
also to your orignal question. i bought one of these recently and it works fine.

https://cgi.ebay.co.uk/COMPRESSION-TESTER-KIT-AUTOMOTIVE-PETROL-ENGINE-VALVE-/180459863037?cmd=ViewItem&pt=UK_Diagnostic_Tools_Equipment&hash=item2a043efbfd

for £10 i wasnt expecting pinpoint accuracy but if thats what you want you'll have to pay a fair bit more.

as for the test at hot or cold. i would say cold if you know know the condition of your engine, but if its something you use day to day then id do it hot or at least close to temperature for the reason that it will be at operating temperature. you dont ride your bike around at 20degrees. it runs at temp, whatever that may be.

or even better. do a cold test then a hot test then it'll be £10 well spent


It's nothing to do wih running temperatures.
You check compression cold. You are looking for the condition of valves/ports and rings.

I have spent years doing compression tests on all sorts of engines and never carried out a test on a HOT engine.

And I wouldn't buy a bike off you either.... Razz Laughing
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Last edited by Walloper on 19:24 - 09 Jun 2010; edited 1 time in total
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Gazdaman
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PostPosted: 18:27 - 09 Jun 2010    Post subject: Reply with quote
Walloper wrote:
1st)

Compression is tested on a cold engine. End of.



Walloper wrote:
1st)

I have spent years doing compression tests on all sorts of engines and never carried out a test on a cold engine.



Hmm! :S

It's all academic anyway! The guys putting his thumb over the plug hole ffs.

Gaz
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Alexio
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PostPosted: 18:49 - 09 Jun 2010    Post subject: Reply with quote
I'm sure it was just a typo Laughing
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will never give up his CG. I look at my fuel gauge more as a progress bar than a fuel gauge.
G: With my GSXR I do often effectively use it as a scooter with a clutch in town.
ms51ves3: why does it need 500 miles? Are you teaching it how to be a piston?
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Walloper
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PostPosted: 19:23 - 09 Jun 2010    Post subject: Reply with quote
Gazdaman wrote:
Walloper wrote:
1st)

Compression is tested on a cold engine. End of.



Walloper wrote:
1st)

I have spent years doing compression tests on all sorts of engines and never carried out a test on a cold engine.



Hmm! :S

It's all academic anyway! The guys putting his thumb over the plug hole ffs.

Gaz


Bastart keynoard jammed you cunts.... Embarassed Smile
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