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2016 yz250x compression and squish test result

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So I bought a new 2016 yz250x and while I was getting ready to install radiator guards I decided to do a compression and squish test on it. Now my last 2 bikes were a 2011 ktm 300xc and the one before was a 2007 300 xcw. I have had all my heads cut on my ktms and they were both right around .080 stock and I set them at .055 after the cut. I live in idaho at 4500 ft and my compression test on the new 250x yielded 165lbs full throttle with about 20 kicks. Did a second test that yielded same results. My 300s have been right around 180 after the cut. I was not to worried at that number because stock my 300 was right around 150 and still ran pretty good. What blew me away is when I did the squish test. I did one on each side in line with the wrist pin and originally started with .080 solder which did not even touch the piston. I than used approx .120 and tested both sides. I had .109 on each side of the piston. I have never seen a squish band that terrible on a modern bike. Just wondering if anyone with a yz250 has seen this much squish. Either way head is coming off and going to dicks racing in utah for a head mod. I don't really want a ton more compression as I want to run 91 everywhere I ride but that band needs tightened up to around .050. I would like to see some others test there 250x to see if they have similar results.

Edited by Iluv2fly

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holy crap thats huge.  On a new bike like that i would go .045 should really clean up that motor.

 

 

wow, just wow

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LLuv2fly, that's interesting. I helped a mate prep/rejet his brand new 250X last week and I did a comp test @ 187psi (cold, sea level). a fresh YZ250 gives 225 to 230 on my gauge. I didn't have time to do a squish test. Like your KTMs I get my heads cut and adjusted. If the squish is large on his X - which I suspect it will be - I'll suggest cutting to 1.25mm and approx 195psi (whatever volume that is)

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Did you do test by putting solder through the plug hole? Or did you take head off? I've heard the test can be somewhat inaccurate when done through the plug hole due to piston rocking...but even still don't think it would be that bad. 2.7mm is widest I've heard of.

Bringing that down to 1.2ish mm is going to remove over 5cc of head volume. That's gonna be healthy bump in comp unless they dig some back out of the dome. At your altitude you can handle a big bump though.

Edited by E36M3E

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That huge squish gap seems totally inconsistent with all the fantastic ride reports describing just how wonderful the response of this engine is.

 

I cannot believe Yamaha would go through the trouble of designing a new head specifically for that motor and knowingly leave the squish that wide unless they thought it contributed to the design somehow. It would be good to get squish and compression confirmation from some other 250x owners too.

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So...I have been riding a 250x for a number of weeks now. I have seen yz250 threads about squish and have never bothered to read anything about it. I KNOW NOTHING ABOUT SQUISH so I'm not here for a technical debate. I'm just here to say, I LOVE THIS MOTOR. I don't want to change one single thing. I don't want an exhaust that bumps the power, or shifts it around. I don't want more compression. I don't want to change ANYTHING. I want it to ride just like it does. It lugs WAY down low and then pulls strong and SMOOTH forever. It is so easy to ride. I would hate to ruin it with "higher performance." Yesterday, I started on the way outside of a line of 50 bikes, B class. I had the farthest distance to the corner, and about a third more distance than the guys at the other end of the line. It was a dead engine start. I cracked the throttle about 1/8 turn and started in 1st gear. I kicked it, beat most off the line, and came into the first corner in about 10th. I went on in the super-slippery mud race to grab traction in the mud and then pass people on the straights. I also have an '06 yz250.

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So...I have been riding a 250x for a number of weeks now. I have seen yz250 threads about squish and have never bothered to read anything about it. I KNOW NOTHING ABOUT SQUISH so I'm not here for a technical debate. I'm just here to say, I LOVE THIS MOTOR. I don't want to change one single thing. I don't want an exhaust that bumps the power, or shifts it around. I don't want more compression. I don't want to change ANYTHING. I want it to ride just like it does. It lugs WAY down low and then pulls strong and SMOOTH forever. It is so easy to ride. I would hate to ruin it with "higher performance." Yesterday, I started on the way outside of a line of 50 bikes, B class. I had the farthest distance to the corner, and about a third more distance than the guys at the other end of the line. It was a dead engine start. I cracked the throttle about 1/8 turn and started in 1st gear. I kicked it, beat most off the line, and came into the first corner in about 10th. I went on in the super-slippery mud race to grab traction in the mud and then pass people on the straights. I also have an '06 yz250.

I'm with you, finding traction is always at a premium here.  I don't need a hard hitting motor, smooth is better for me.  I would think it would be cheaper to buy the YZ head rather than modify, also I'd bet there are timing and ignition differences as well.

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Just a thought. Could the huge squish, low compression design be part of what makes this engine so smooth? Tight squish has two benefits traditionally right? It allows for higher compression without detonation, and it makes for a more efficient burn (less wasted fuel = more power). Maybe with the compression this low it's not really necessary?

 

I am eager to hear the ride report from Iluv2fly after the squish is reduced.

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I used rosin core solder and 0.080 would not even make contact. I did go through the sparkplug hole which is very accurate in my opinion. Just have to make sure your against the cylinder wall. I just got my bike and I am not sure what to do. I have heard so many stories that they run great stock so maybe I will ride it awhile and see how it does. You have to remember my 165 psi compression is at 4500ft on an engine that is not broke in. It has only ran 10min.

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 You have to remember my 165 psi compression is at 4500ft

 

Do you know of a conversion to equivalent PSI at sea level?

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Not sure but I bet it gains at least 10psi when it breaks in.

Edited by Iluv2fly

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Perhaps. Keep us posted. And be sure to report back if you do that squish mod.

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Do you know of a conversion to equivalent PSI at sea level?

Yes, 165psi at 4500 feet = 187psi at sea level. LOL. The new X I comp tested also had approx 15 min run time on it. In my experience they rise 5 or so psi after a few hours. Edited by riles

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Yes, 165psi at 4500 feet = 187psi at sea level. LOL. The new X I comp tested also had approx 15 min run time on it. In my experience they rise 5 or so psi after a few hours.

Thanks. What is the formula for conversion?

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Thought Id share this since the thread is already started

 

My mate has a new 250X and I did a squish test tonight on it. As I posted earlier on this thread the compression is 187psi (cold, sea level)

 

The squish gap is 2.61mm or 0.103 inches. That is considerably large. Larger than any KTM or YZ250 Ive come across. (Ive measured a few over the years)  

 

I know there are a few people with Xs that will say "its a great engine, don't touch it, leave it as is, etc" but I don't roll like that.   Im sending the head off to TSP engineering tomorrow to be modified. Not to increase compression but to tighten that gap to around 1.2mm and keep compression around the same psi.   

 

My mate has injured himself and is off the bike for a month and he very generously offered it to me today and I rode it for 70km, tight single track and some more open trails. Great bike, especially the suspension, steered very nicely - better than my KTMs/Husqvarna.  

 

The jetting is still slightly rich, especially up top. Ill do the head then set the jetting later. It currently has 50p, N3CW clip 3 and 178 AS 1.25  (standard from the showroom was 52, N3EW 180 AS 1.5)

 

We rode 64km (40 miles)  and it ran dry completely - I didn't expect that. Had to take fuel from another 2 stroke to get home.

Im hoping with the head mod, lowering the float height a touch and going slightly leaner on the jetting will get another 20km or so out of a tank - I hope so.

 

Ill update this thread once its done

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Very interesting. Would you mind posting a picture of the dome and measuring the squish depth? (The distance from the head deck to the squish surface)

 

I know the X has lowered the compression intentionally, and I believe I read that they reduced the CC volume by 2.5cc. I wonder if they did the entire compression reduction by simply increasing the squish gap.

 

To add 2.5 ccs to the head volume one could increase the squish by about .030" (.76mm). This would move the typical crappy 250 squish from .075 to about .105, which happens to be exactly what you measured.

 

It may be that they made no change at all to the dome. We can confirm this if you measure the depth of the band in the head.

 

EDIT: Another important piece of information would be the deck height of the cylinder. In that picture you posted of the solder squish. Is the piston at TDC? I suspect not, but if so, it is a lot lower in the cylinder than a normal 250. WOuld you be so kind as to post a pic at true TDC and if possible measure height of piston crown from deck?  We can learn a lot from this bike while you have it. Thanks :)

Edited by rjpjnk

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When I first tested mine I could not believe what the squish clearance was and I was ready to pull the head right there and have it machined. I decided that I was going to ride it some first and go from there. This bike runs so well and the power is so good and smooth that I have decided to just ride it and leave it alone. Conventional wisdom says to machine it with that large squish gap but it runs smoother and stronger than my ktm 250 xcw that had an xc head that was machined. I will be interested to see your results but for once I thing I am going to leave it alone and just ride it.

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