2016 yz250x compression and squish test result

[YZ109]

From my reading and experience I know that an overly large squish can contribute to detonation. It has to do with "boundary layer cooling" and other like factors. Detonation is not only caused by high compression or too tight squish.

 

Some text taken from a two stroke engineering book

 

"In a perfect head design, the fuel mixture in this very thin squish-band is greatly cooled by nearby surfaces of the piston crown and head.  If the squish clearance is too thick, the cooling of these end gases is not sufficient to keep them from exploding, and so they become the source of detonation."

 

I like and understand Yamahas logic in lowering compression on this X endure/trail/GNCC type model - compared to a YZ - but the way it went about it leaves a bit to be desired

Riles, kindly take me through. What is the indicative squish clearance of today's YZ250's? 1.8 mm to 2.0 mm?

I heard some of the members here had their squish corrected anywhere from 1.15 - 1.3 mm (or 0.045-0.055 inch).

Thanks for the heads up

[Dussucker3000]

Hey riles, you ride it with the machined head yet?  

[riles]

dussucker3000, Yes, quite a few times. I've been trying various jetting Combinations. I do like how it runs with the head modified. It has all the qualities of a tighter squish gap and it's easier to jet. As mentioned the PSI or compression is only a touch more than standard. On my other two strokes you can feel the difference that .5mm makes, well this head had just over 1mm taken off (and volume modified) so you can really notice it. Nice even idle, very crisp through the range. No spooge even with a large pilot (I tried smaller but went back to a 50) It still has that X wide spread of torque which makes it such a nice engine. I run the standard pipe and muffler.

With float height adjusted 1mm lower, a rejet and the head mod it now gets 20km more out of a tank. 85 compared to 65.

[Dussucker3000]

Very nice. Thanks for the knowledge!

[sholley1]

If the goal in machining the X head is to obtain more compression couldn't you just put the 250 head on it?

[Dussucker3000]

The goal was to bring the squish to a known optimum measurement while keeping the compression the same to maintain the X characteristics. He has gained efficiency, possibly reduced the chance of detonation and made it easier to jet.

[MANIAC998]

If the goal in machining the X head is to obtain more compression couldn't you just put the 250 head on it?

 

And the YZ head's squish is to large also, and needs machining.

[sholley1]

So how is it determined what the corrected squish should be?

[MANIAC998]

So how is it determined what the corrected squish should be?

 

It basically goes by bore size, but the general consensus is about 1.15mm to 1.3mm wide.

[sholley1]

It basically goes by bore size, but the general consensus is about 1.15mm to 1.3mm wide.[/quote

Is this a standard measurement for all 250 2 strokes or is this just for the X?

[218]

Squish clearance is calculated off stroke, not bore. Most commonly used calc. is;

 

.013 X stroke = minimum squish clearance in MM.

[Thag]

The squish on my 295 is .065. Who in Canada can do the machining on my head?

[andrzej]

I would also like to know if there are any good/experienced shops in Canada that can do squish head mods?

[Hands On]

Iluv2fly what part of Idaho? Eastern? I'm in Boise at 2800 if you're familiar with KBOI.

[JP80]

Riles, 

I have also had my head set up very similar to yours and generally run at sea level, what needle and main did you end up with? 

I tried the Suzuki needles as I had a lot of success on a couple of other 2ts I set up, but found the n3 needles to be better on the yz250x.