What is up with my compression... or is it these tester's ?

[Shifty]

I am still not sure of my compression. I have an '08 SJ, im sporting a limited pipe and a riva girdled head w/ 35cc domes. Before running the head I did a comp test at a local marina... I got 195 across both cylinders on a decent gauge, except he only had the short plug adapter so I would think the reading would read slightly lower. Thinking that was too high for 35cc domes I tested again after burning though roughlyl 10 gals of 93 octane (no long high RPM runs) this time I used a cheap -O autozone rental guage to test. I got 170psi on this guage with the correct adapter for the long reach plugs.

Thinking all of these gauges sucked, i bought a used snap-on gauge 0-300psi with the correct addapter. I have run about 20 gallons with this new setup now and I just tested on my new to me gauge and got 140psi on both cylinders. Whats the deal?

I have checked the plugs frequently and they show no signs of detonation and I refrain from long high speed runs. Why are my reading falling like this? I think I need to go see the guy that did the test originally to see if the reading is the same.

Note - All of these tests were done with one plug in and the throttle wide open.


Thanks for reading my novel,

 

[Sdiddy]

compression testing

A stock superjet Only has 135 psi compression. No matter what anyone tells you. I have the Yamaha Repair manual and it lists compression at 135. Every compression tester will read different. If you are above 135 your good.

 

[Mark44]

If you replaced the head what was your squish?

Mark44

 

[Shifty]

Squish; I wish I knew, I dont have a caliper to measure it with though. I think I am going to pull the head and have a look at my pistons and cylinder walls. The ski runs good, a little rich actually.

 

[Shonuff]

A stock superjet Only has 135 psi compression. No matter what anyone tells you. I have the Yamaha Repair manual and it lists compression at 135. Every compression tester will read different. If you are above 135 your good.

All my stock Superjets have been right at 150 psi on a craftsman gauge.

 

[WFO Speedracer]

Bend a piece of .050 solder into an L ,remove the plugs and insert the solder till it touches the cylinder wall,rotate the engine through by hand and remove the solder.You will need to take the solder to someone with a caliper or micrometer to measure the portion that was flattened but that will tell you what your squish is without removing any parts and it is dead on accurate.

 

[feartheride]

Start by using one of the 2 original guages you used to see if you have a problem or just guages that are all over the map

 

[Shifty]

Yeah, I was going to go check back and test with the original gauge. I called Riva and they said maybe the rings are stuck? Could this be likely?

 

[Shifty]

Ok, on my lunch break I ran a little test. I have two adapters, a short threaded and long threaded. I jacked my compressor up to 110psi and shot air into the gauge fitted with the short reach adapter... reads 110psi. Try that with the long reach adapter and I get 40-50psi. After swapping the check valves (tire valve) from the end of each adapter I am now getting 197psi on my motor,

I am happy my gauge is working well and my motor isn't loosing compression, but I am @ 200psi:shocked:

My plugs look good and I am not blasting away for 5 minute runs at top rpm, am I safe to keep this up witout race gas until I can get new domes, have them machined???

What domes would bring me down to pump gas? What other options do I have to get this think under 185? I will check squish tonight.


Thanks for the assistance,

 

[Mouthfulloflake]

thicker base gasket maybe?

 

[Shifty]

True, but I dont know if I could get one thick enough to lower the compression enough???

 

[Big Kahuna]

I was told years ago to figure 7 lbs compression per CC.

 

[Shifty]

Ok, I measured my squish with some solder and it is at .046. I am still getting about 198psi with this head though. My gauge looks to be about 10 psi on the high side because when it is unpresurized, the needle sits just at the 10psi hash mark. These are all cold #'s also, shouldn't a compression test be performed on a motor at operating temp?

 

[Big Kahuna]

your squish is good.......... sounds like you got a bad guage.

 

[Shifty]

True; this may be a dumb question, but if my squish is good and my motor is actually pushing 200psi, am I ok to run like this?

 

[Big Kahuna]

200 lbs, I would be running race gas mixture with 92 octane.