Compression Test Results

[delagem]

Should I still do a leak down test with these results?

I wouldn't bother. A Leakdown Test is great for identifying where your problem is, if you have one. (For example, you see a significant leakdown number, say 20%; then, with the air still on, listen at the oil fill, carbs, and exhaust to determine if the leak is from bad rings, intake valves, or exhaust valves).

The other thing it's good for is if you want to track engine wear over a prolonged period. (For example, track wear after every race weekend, build a database and then use that data to determine when you need to tear the motor down).

In your case, it doesn't sound like you've got a problem (based on good compression numbers), it's not a race bike, and (in my experience) the HF gauge isn't repeatable enough to use for long-term testing in any event.

You've got excellent compression readings. I'd start chasing that "no-start" problem next. If you're bored this winter, maybe play with the leakdown tester!

Good luck!

Michael

 

[grizld1]

Right, Michael, a compression tester will usually tell you what you need to know, but it can also fool you. It will tell you the highest momentary pressure the cylinder will achieve, but sometimes it won't tell you how well the pressure will hold. A few years back I had an issue with a ragged idle on an XS650D motor with high compression big bore pistons (77.5 mm). Trouble shooting the ignition and carbs brought no joy. Compression testing showed both cylinders exactly even at 180 psi. When I locked the crank at TDC at the top of the compression stroke on the right cylinder and hit the air, a strong breeze could be heard and felt from the intake bell on the carbie, and the gauge indicated 20% pressure loss (100 psi in, 80 psi retained). 10% is considered the maximum allowable loss on a street motor. Tear down showed a cupped intake valve. A fresh valve (and grind, and lap) eliminated the problem.

 

[TwoManyXS1Bs]

...Your compression ratio is 8.5:1 if anybody wants to figure out the theoretical maximum compression...

Now, there's a good thinking challenge for the members.

Assuming atmospheric pressure of 14.7psi.
Compression ratio of 8.5:1 (based on swept volume and combustion chamber volume).

Which will actually be less at cranking speed, due to intake valve late closure ABDC.
Intake valve closure of at least 67° ABDC, the piston has risen at least 18mm.
From an initial stroke of 74 mm, now down to 56mm.
1/4 of the swept volume is lost due to the late intake valve closure.
Leaving only 3/4 of the original swept volume.
From about 327cc down to 245cc per cylinder.

Which leaves a compression ratio of about 6.6:1.

So, how can you get compression readings of 140-150 psi?

 

[Srawl]

Now, there's a good thinking challenge for the members.

Assuming atmospheric pressure of 14.7psi.
Compression ratio of 8.5:1 (based on swept volume and combustion chamber volume).

Which will actually be less at cranking speed, due to intake valve late closure ABDC.
Intake valve closure of at least 67° ABDC, the piston has risen at least 18mm.
From an initial stroke of 74 mm, now down to 56mm.
1/4 of the swept volume is lost due to the late intake valve closure.
Leaving only 3/4 of the original swept volume.
From about 327cc down to 245cc per cylinder.

Which leaves a compression ratio of about 6.6:1.

So, how can you get compression readings of 140-150 psi?

I just screw the tube in the spark plug hole and jump on the kick start a few times...

 

[canuck1969]

Don't forget that the 150PSI is based on atmospheric pressure at sea level. One persons 150 PSI reading can be someone else's 120 PSI at 5000 ft on two engines in the exact same condition. Just make sure you are talking apples and apples when you start comparing compression. Just saying.........

 

[delagem]

I just screw the tube in the spark plug hole and jump on the kick start a few times...

Perfect. Don't change a thing.

Don't mind us as we chase the rabbit down this hole for a few pages...

Now, there's a good thinking challenge for the members.<<SNIP>>
So, how can you get compression readings of 140-150 psi?

Lots of people love to do that "compression ratio x 14.7". Problem is, it's wrong. While the two numbers are indeed related, they are not DIRECTLY related. It's not a simple multiplication exercise.

 

[MaxPete]

Lots of people love to do that "compression ratio x 14.7". Problem is, it's wrong. While the two numbers are indeed related, they are not DIRECTLY related. It's not a simple multiplication exercise.

Indeed - and like I said, there are some fairly gross approximations here but I didn't want to get into the ideal gas law or other thermo issues. etc. etc.

 

[Paul Sutton]

I like PV = nRT. It never gets me into trouble as long as I use Helium!

 

[TwoManyXS1Bs]

Great! I was hoping to get the thinkers cogitating.

...Lots of people love to do that "compression ratio x 14.7". Problem is, it's wrong...

I like PV = nRT...

It's a foundational principle of the Otto cycle.

Hint: Google "adiabatic compression and expansion".

Or, just say ENUFF, and we'll drop it here.

 

[TwoManyXS1Bs]

Well, I guess we'll close out this thinker.

Reducing the volume of a confined gas will raise its pressure.
That's the easy part.

The energy (work) that goes into compressing that gas is converted to heat, which raises the temperature of the confined gas. That increased temperature increases the pressure as well.
Calculating that isn't easy. Involves using thermal coefficients and *math*.

However, if you let the temperature of that confined gas cool down to ambient, then the pressure drops to the more easily understood ratio of volume reduction.

Same thing happens to the combusted fuel. Oversimplified here, If you were to ignite the fuel mix, prevent the piston from moving, and let it cool to ambient, the pressure will be the same as before ignition.

In summary, the 2 biggest influences on compression pressure readings are,
- Leakage
- Temperature loss

Now, have a second think on compression readings taken on a cold engine, versus the warm engine...

 

[Jim]

Gas laws.... I used to blow my students minds with 'em. Air enters a turbine engine compressor at ambient temp. and pressure. A fraction of a second later, it's at 150 psi and 400 deg F..... and we ain't even squirted fuel in there yet. They used to say it ain't possible. I'd invite them to get a firm hold on the hanger air compressor while it's runnin'. You could almost see the bulbs lighting up above their heads.

 

[TwoManyXS1Bs]

... You could almost see the bulbs lighting up above their heads.

Haha, and flames emanating from that firm grip...

 

[Paul Sutton]

I have been thinking about making one of those syringe type things that were used to light fires based on the heat of compression. On the topic of jet engines it always amazed me on those old ones how little the pressure increased over each compressor stage. There was one that went up by a factor of 1.07 over 17 stages which sounds like nothing until you take 1.07 to the power of 17. What are modern compressor stages managing these days?

 

[TwoManyXS1Bs]

Oh, yeah, the "Fire Piston".

Dear ol' dad used to tell me about '50s era jet engines, that the compressor blade geometries were so critical, that with as little as a 1° variance in the blade pitch, the engine wouldn't run...

 

[Jim]

What are modern compressor stages managing these days?

Most are about 1.25 to 1.

 

[MaxPete]

I recall the Dynajet - which I think was a pulse jet unit that folks used for racing on control lines. Hard to start, but once they were running....ooooooo-eeeeee!

 

[Srawl]

Don't forget that the 150PSI is based on atmospheric pressure at sea level. One persons 150 PSI reading can be someone else's 120 PSI at 5000 ft on two engines in the exact same condition. Just make sure you are talking apples and apples when you start comparing compression. Just saying.........

View attachment 123249

I’m at 300-400ft... is my 150 still 150?

 

[MaxPete]

I’m at 300-400ft... is my 150 still 150?

That altitude wouldn't make any difference that you could detect with a typical automotive compression tester Srawl - and in any event, you're really looking for an engine defect which means that you are trying to find:

• a really BIG difference between the two cylinders

and/or

• a really low compression reading.

Cheers,

Pete

 

[canuck1969]

I’m at 300-400ft... is my 150 still 150?

About 3.5% difference every 1000 feet. Nothing that I would even worry about when you get numbers that high. If you are getting 100 just make sure you are not standing at the top of Pikes Peak. And if you have ever been up there, take my word that you don't want to have a celebratory cigar when you are up there.

Your 150 reading is probably 153 at sea level......if you really want to be accurate.

 

[Paul Sutton]

After many years I finally made one based on the video you posted TM!

What the video fails to tell you is that wood, depending on type, is porous so under compression the air escapes. After many attempts to light some tinder I sealed the end of the piston with varnish. Once this dried the first attempt resulted in ignition. I bet that many years ago people would say "You can't use the the XTree, it must be wood from the YTree because it contains more heat demons".