YZ250 Squish with Rinaldi head

[ezmoto123]

So i just changed piston in my 250yz to a Vertex. The Vertex base gasket is 0.5 mm and i dont know what the OEM was. But i measured the squish to be at 1.14 mm (0.0045 inches) and im worried that this is to tight with the Rinaldi head i have on. It pulls like CRAZY, but should i go with a thicker base gasket? 

[morrison250r]

    .045" is a acceptable squish cleance for a 250cc. Before I started playing with base gaskets, I would check the uncorrected compression ratio. 

      Also, squish velocity is very important too. The company that built your head should have done that. Maybe you could ask them. Changing your squish clearance will change your squish velocity. As a general number, shoot for around 18m/s at peak power (about 8700 rpm, approximately) and 25m/s at peak speed. I dont see a yz250 reving much past 9500 rpm.

     My rb mod oem head came out to 13.14:1 uccr and a squish clearance of .0425" I would love to put the parameters into software and check my msv. Someday...

Edited July 8, 2020 by morrison250r

[ezmoto123]

What is the uncorrected compression ratio? ? Im gonna check the compression tommorow and write it!

[Matthias V.]

OEM base gasket(with the metal inside) is 0.6mm

[morrison250r]

    You take the swept volume of the cylinder (your displacement)+the combustion chamber volume/combustion chamber volume. 

     Pull the head, bring the piston to tdc, coat the sides with just enough grease to hole the fluid(like MMO) , put the head back on and fill to the bottom of the spark plug hole. 

 

[ezmoto123]

1 hour ago, morrison250r said:

    You take the swept volume of the cylinder (your displacement)+the combustion chamber volume/combustion chamber volume. 

     Pull the head, bring the piston to tdc, coat the sides with just enough grease to hole the fluid(like MMO) , put the head back on and fill to the bottom of the spark plug hole. 

 

Damn i have no equipment for that, and im going of for a whole week of racing tommorow :// Measured the squish today to about 0.0465 instead (first time i measured the gauge was not calibrated). I run 100 octane pump gas (we got that in Denmark), but should the squish be a problem u think? ?

[morrison250r]

       The only equipment you need is a oral syringe from the pharmacy, grease, oil and maybe a set of o rings for the head.

    No, I do not feel that the squish being .0465" is a problem. It is about ideal for a 250cc engine that is not rebuilt every race. 

    You have 100 octane at the pump? Is that ron, mon or ron+mon/2? With my 

13.14:1 head I am running 90 octane pump gas and vp c12 mixed at a ratio of about 3:1 for a ron+mon/ rating of 97. It may be over kill but it is cheap insurance again dentenation on a long uphill pull or getting into the over rev on a sand track.

 

[ezmoto123]

On 7/10/2020 at 9:13 AM, morrison250r said:

       The only equipment you need is a oral syringe from the pharmacy, grease, oil and maybe a set of o rings for the head.

    No, I do not feel that the squish being .0465" is a problem. It is about ideal for a 250cc engine that is not rebuilt every race. 

    You have 100 octane at the pump? Is that ron, mon or ron+mon/2? With my 

13.14:1 head I am running 90 octane pump gas and vp c12 mixed at a ratio of about 3:1 for a ron+mon/ rating of 97. It may be over kill but it is cheap insurance again dentenation on a long uphill pull or getting into the over rev on a sand track.

 

In Denmark it says Octane rating is 100. MON is 89, and is has an AKI (which apparently is something called "anti-knock index") at 94. 

[morrison250r]

8 hours ago, ezmoto123 said:

In Denmark it says Octane rating is 100. MON is 89, and is has an AKI (which apparently is something called "anti-knock index") at 94. 

     That is similar to us premium fuel which is 93 octane anti knock index. I use 90 octane (ethinol free) mixed with vp c12 at 3:1  ratio which comes out to 96.4 octane although if I need to refuel during a ride, i do not hesitate to run straight pump fuel.I just limit the long pulls at wot.