EC 300 cylinder pressure

[hannesd]

Originally Posted by kendunn

Even if the fuel mixture is getting pre-compressed in the crank, why does the motor not predetonate at those pressures? I thought anything over 11:1 would need race gas, and at the pressures of 190 or whatever I don't know how it would run on anything other than diesel without predetonation.

i would say that the PCR isn't that high because otherwise it would allso blow out the seals,right?
the actual compression is built up once mixture is on top of the piston.

[twowheels]

2010 XC250, 6hrs - measured 190 psi on Craftsmen gauge (3 samples). The same gauge registered only 70psi for an 05 450 FSe with a fresh rebuild (but which started and ran fine).

[lankydoug]

Originally Posted by kendunn

So if its 8:1 and atmospheric pressure is 14 psi, how do you get 185 psi? I am not saying you don't, just wonder how. Like I said, my gauge may be off, but I am getting 125 in both my bikes and 90 in my boat (which is what the shop had marked on the heads also). I know in multi cylinder motors they say if the pressures are all close you should be OK

Using certain brands of Schrader valves in your compression gauges will affect the reading. I have a Snap-On set that requires a special low tension Schrader valve. If I use a stiffer sprung valve the reading will be off about the same as you describe. I'm not sure that is your issue but it's worth checking out.

[smallpuffydog]

Originally Posted by roostafish

I thought these bikes were 14:1.
Anyway, to get a consistent reading, put a couple drops of oil down the sparking plug hole.

The 300 I had before consistently read 180-185psi. My 200 before that read similarly.

Ron @ RB Designs did the squish band mod on my 250. He requested that I perform a compression test before sending him the head. He was not surprised by the reading.

Bottom line, if you get consistent readings you're probably fine, as long as it runs fine.

I agree, I tried three different gages and got three different numbers. I finally bought a gauge from Harbor Freight and made sure that the readings stayed consistent.

[motogroove]

Originally Posted by smallpuffydog

I agree, I tried three different gages and got three different numbers. I finally bought a gauge from Harbor Freight and made sure that the readings stayed consistent.

I'm glad I read this thread first... and wish I had a baseline with my new piston. Not like she was barely broken in before (maybe?) disaster...

Actron CP7827 compression tester kit (new, no baseline of any kind)
cold engine, no air filter, throttle wide open
I'm not very happy with 140 psi out of my 2001xc300 (less than an hour on a new top end)
This is the one that suffered the lean rev at Southwick and did not run well afterwards. Jim suggested that the rings take the first damage in a lean rev. (The bore looks fine).

So, gauges may vary, but 140 is still sounding lower than any healthy reading I've seen here.
However, I put a few drops of 2stroke oil in the plug hole and the results did not change. That's supposed to bump the pressure test up if the rings are the culprit.

I hate to pull the jug if I don't need to.

[elemetal]

Altitude will have a large effect on the PSI reading of the gauge. I got 185 at sea level and 145 in New Mexico at about 3500.

If you have raised your exhaust port to allow more RPM (more hp up top) you'll naturally get a lower PSI reading (less swept volume). Also remember the charge that gets shoved back into the cylinder via the expansion chamber, this will increase operating pressure but you'll never see it with a static PSI test.

[gasgasman]

If I remember correctly, the 300s have a "compression" release port on the exhaust side of the cyclinder. This can affect compression readings also.

[smallpuffydog]

Originally Posted by kendunn

Even if the fuel mixture is getting pre-compressed in the crank, why does the motor not predetonate at those pressures? I thought anything over 11:1 would need race gas, and at the pressures of 190 or whatever I don't know how it would run on anything other than diesel without predetonation.

Compression is not the only factor, the cut of the flame cup in the cylinder head plays into it as well.

[garretts]

My bike reads 90psi and only 50 hours on the current top end. Kick starter can be pushed down easily by hand. Should I go ahead and do a top end? the cylinder walls look fine same with the head. No scoring or anything. But the weird thing is that the bike runs amazing. No bogging or loss of power. any sugestions?

[(F5)]

I think you have your answer if it starts/runs fine and you know how old the piston is. Check ring gap is OK and button it back together.

Gauges are a blunt tool with many variables. 300 has a hole above port that makes them easier to kick but is too small to dent performance. I rotate mine if I've not used it for a month by hand.