Compression test

[raz]

When I bought my strobe lamp to check scatter, I also got another toy, a compression tester. Now the only problem is I don't know what it should read

 

I guess doing this the right way involves checking/setting valve clearance first, right? Or would a little difference there not do much on the compression readings?

 

Then, what are good readings on a Guzzi 1100 in absolute terms? And what are rules-of-thumb for relative difference?

 

I already knew my heads are due for an overhaul, this is just for fun and curiosity. My current readings (without setting valve clearance first):

Left 7 kg/cm² (100 psi)

Right 8.3 km/cm² (120 psi)

 

So, doctor, how bad is it?

[Rusty Bucket]

Did you have the throttles open? These results are low for a four-stroke engine at operating temp. Your valve clearances won't make a measurable difference unless the valve isn't seating. The 20% difference side to side is a bit of a worry, but I would re-do the test on open throttles, with a warm engine, fully charged battery (faster cranking means higher readings, an unwelcome variable here) and see how it comes out. A leakdown tester is only slightly harder to use, and provides steadier, less varaible and more specific information, so if you are inclined to add tools, that's a good one.

[raz]

Did you have the throttles open? These results are low for a four-stroke engine at operating temp. Your valve clearances won't make a measurable difference unless the valve isn't seating. The 20% difference side to side is a bit of a worry, but I would re-do the test on open throttles, with a warm engine, fully charged battery (faster cranking means higher readings, an unwelcome variable here) and see how it comes out. A leakdown tester is only slightly harder to use, and provides steadier, less varaible and more specific information, so if you are inclined to add tools, that's a good one.

Thank you, throttles were closed (I did figure that one out, but only later) and engine was not quite in temperature. I'll redo the test some day after riding. I'll also test a friends similar engine, a curious comparison.

I feel like a kid with a new toy

[Dan M]

Thank you, throttles were closed (I did figure that one out, but only later) and engine was not quite in temperature. I'll redo the test some day after riding. I'll also test a friends similar engine, a curious comparison.

I feel like a kid with a new toy

 

 

Just another note - 4 compression strokes when testing compression.

20% difference is a lot

[raz]

 

 

Just another note - 4 compression strokes when testing compression.

20% difference is a lot

What, should I measure exactly 4 comp. strokes? Or at least 4? This is a cheap gauge with rubber cone inlet that I push to the spark plug hole. It remembers the max value so I just ran the starter until the needle didn't move any more, and then some. At least 5-10 compression strokes.

[Guest ratchethack]

Raz, RustyBucket is on the beam here. I've got one o' those rubber-cone testers. They're good for finding really bad stuff like bent valves, broken rings, and holed pistions, but I wouldn't rely on mine for accuracy. A leakdown test is the only sure way to do it for accuracy, and you should be getting readings of around 200 lbs., and IMHO no more variance right-to-left than 5%. You could knock your expectations back to 180 lbs. or so for low-comp pistons and heads (which I don't think you have), and go up to no more than 10% right-to-left variance for high mileage. Make sure you have both spark plugs out with kill switch on and hold 'er at WOT as mentioned.

[badmotogoozer]

you should be getting readings of around 200 lbs., and IMHO no more variance right-to-left than 5%. You could knock your expectations back to 180 lbs. or so for low-comp pistons and heads

 

200psi?? Did I read that right? That would equate to 13.6:1 compression ratio (assuming sea level). No way the Guzzi chugger is making that kind of compression. We'd have to be running on race gas.

 

Is this another number you've pulled out of thin air?

 

I'd bet the stock V11 isn't even making 10:1 which is 147psi. My money's on it being in the 8.5-9:1 range.

 

Rj

[mike wilson]

You are assuming constant temperature. In real situations gas gets hot when compressed, which increases the readings from the theoretical. Same reason you measure your tyre temp when it is cold.....

[raz]

Tried again today with engine really warmed up and WOT.

 

Left 9,25 (just above 130 psi)

Right 10.5 (150 psi)

 

Thats 13,5% or 11.9% depending on which way you calculate it. I'm pretty sure my heads are to blame and not the rings and barrels.

 

Ah well, it will be interesting doing a comparison after the upcoming head overhaul...

[GuzziMoto]

From my experience with a more accurate type compression tester that screws into the plug hole, 140-160 would be good for a stock motor. Yes heat makes air expand increasing pressure. But think about it, tyres only increase 10-15 percent pressure wise with a 100+ percent increase in temp( not trying to start a big debate). Anyway, back to my experience. I've tested my wifes V11 sport and my Griso and got readings in the 140-160 range. Have yet to test my wifes bike with milled heads and shaved cylinders, though.

Also, I had a Buell X-1 that blew 170-180 psi. It shows you what higher compression(11:1 or thereabouts) means.

Keep in mind that just like tyre pressure gauges, each one will have it's own accuracy tolerance. Some are more accurate then others so comparing two different readings from two different gauges must be taken with a grain of salt. It's a very relative test, most useful to someone to compare two different cylinders or two different motors tested by the same guy with the same gauge.

[Guest ratchethack]

Tried again today with engine really warmed up and WOT.

 

Left 9,25 (just above 130 psi)

Right 10.5 (150 psi)

 

Thats 13,5% or 11.9% depending on which way you calculate it. I'm pretty sure my heads are to blame and not the rings and barrels.

 

Ah well, it will be interesting doing a comparison after the upcoming head overhaul...

Raz, IMHO that's probably perfectly acceptable, especially considering the tester you're using. In my post above, 200 lbs (and above, see many posts on GuzziTech over the years) is what higher-comp Guzzi motors are putting out, but after thinking about it a little more, for a '97 Sporti, yours is probably pretty close to right on target.

[Dan M]

What, should I measure exactly 4 comp. strokes? Or at least 4? This is a cheap gauge with rubber cone inlet that I push to the spark plug hole. It remembers the max value so I just ran the starter until the needle didn't move any more, and then some. At least 5-10 compression strokes.

 

 

The rubber tipped style is hard to seal against the head but not impossible. I'd wonder about the quality of the gauge and accuracy of the readings so don't let your results justify a tear down!

That said, it can be used for comparing side to side.

Start by disabling your ignition or grounding your plug wires.

Open your throttle and press the gauge into the plug hole with sufficient force that you hear no seeping of pressure on the compression strokes. Watch the needle while cranking the engine. After it bounces 4 times it will probably not move much further. Keep the count the same from cylinder to cylinder.

Compere the results to the other side.

I doubt you have 20% difference.

[4corsa]

Did a compression test today (warm engine, wot) on my '03 Le Mans 15.5k miles. Looks like I'm a lucky guy: 158 and 160 psi.

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[Lucky Phil]

Got to be happy with that.

Ciao