High Compression piston for a YZ250F

jimt_yz400

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#1
I'm doing a rebuild on my YZ250F and I'm considering installing the
Weisco high compression piston. Does anyone think that this is a worthwhile idea or would it hurt the good performance of the 250F?
 

EricGorr

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#2
Tearing down that engine just to install a high compression piston is a lot of work. Maybe you should just try a tankfull of good racing fuel, it probably makes a similar difference.
 

jimt_yz400

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#3
Well,. I was going to do the top end anyways, replacing the piston and rings, so I thought that I'd entertain the idea of installing the wiesco piston.
I have tried some 110 Octane (unocal 76) but it ran too rich. I'd have to spend some time jetting to get it crisp but I'm not sure I want to spend $5/gal.
I'm not sure if this is the best fuel to run anyways.
Eric, should I also check/replace the valve springs while I am in there? What is
the lifespan of valve springs with these heads?
 
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#4
Jim - You might want to describe what you are looking for in terms of changes to the original power characteristics.
 

jimt_yz400

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#5
Well......I'd like more off of the bottom and a healthier mid, I don't anymore
on top since this is for off-road use only, enduros mostly and playing aroung.
My thinking was that the higher compression would give me more pull/torque
off of the bottom w/o sacraficing the top too much. But, I'm concerned about
the additional heat that this may create and the pblms with jetting (I'm sure that this could be rectified - the jetting). So, that's what I was looking for but I don't know if the HCP is the way to go.
I've been able to acheive some good results with different exhaust systems
but the pblm is that they are too loud, buy I'm working on that one.
 
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#6
The stock pipe with a 13.5:1 piston and proper deck height will meet your goals. An .080" plated overbore again with the stock pipe is an even better choice assuming it's legal for the class you are running in. Blueprinting the cylinder head holds some value, but no where near as much as the compression and deck increases. For your purposes porting is worth about $150 on these heads (simple blueprint and throat blend), and a proper blended valve job is worth about the same $$. Springs shouldn't prove to be an issue unless you have a TON of hours at high rpm.

Given it's still a fairly new engine it appears thatt the best bang for the buck in terms of response coupled with top end power is oxygenated race fuel and sharp jetting on a totally stock engine.

Best torque curve for the buck if you are taking the engine apart is a 13.5:1 compression .080" over piston, in a plated barrel with stock pipe and oxygenated race fuel. The chamber is open enough to to support even higher compression but the rpm ceiling makes it tricky to keep the valves out of the head when two-stroke MX ex-patriots ride them like 125s :) Guys who don't constantly bounce them off the rev limiter can safely run a LOT of compression. A properly setup high CR engine is no different to jet then a stocker.

Heat has not proven to be an issue on these engines so far but if you spend a lot of time slugging along in mud at slow speeds then additional compression will require a commitment on your part to tune correctly for the situation. If you get the squish clearance right and tune correctly high compression won't matter.

Pipes on these engines haven't shown themselves to have a particularly good price/performance ratio but I realize that won't stop folks from buying a trick looking pipe :)

Eric and I will know lots more about the potential (and long term problems) of these engines as we test more and gather more rider feedback information.

Good luck with your project.
 

jimt_yz400

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#7
Thx for the input. I still do not know what I will eventually do but I don't really want to throw alot of money at. The engine runs fairly well now and with what I am using as an exhaust it pulls very well off of the bottom.
I have been using a DSP SX head pipe with a shortened 426 muffler (1inch) for MX and off-road. The off-road had an adapter for the WB E-Series disks.
It is very crisp off of the bottom and in to the midrange and pulls *much*
better than with the stock setup.I can ride a gear higher with this setup but it is *loud* with the disks for off-road and I barely pass the sound check at
enduros. I do not have as much top end as I did before but I don't really nee it since it will pull the next gear quite easily. I've just bought a used
Big Gun with the quiet core which is nice and quiet but I don't have the
low end like the other setup. I trying to modify the DSP header to accept the
Big Gun silencer. I initially was able to get this to work but it had a very short power bad and would rev less than the other setup. So, I thought of digging
into the motor to gain the torque and keep the Big Gun setup.
Thx again the info!!
 

smb_racing

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#8
I have a question, by increasing the compression of the engine wouldn't he have to run a higher octane fuel to keep it from pinging?
 

WWR

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#9
higher compression / higher octane

SMB,
your theory of higher compression requiring higher octane fuel is not an *absoloute*. Rich touched on this a little when he mentioned optimum squish clearance. The proper squishband and clearance can have very positive effects on the octane requirement of an engine .

There are many different things in an engine that contribute to the octane requirement. I am not an expert, but you can find more info about this on Eric Gorr's website.
 

smb_racing

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#10
wow, learn something different every day...
perhaps I'll consult my eric gorr book once again, maybe I missed that section.
 

jimt_yz400

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#11
Rich,
After re-reading your reply, you recommended the best bang for your buck is
oxgenated race fuel with "sharp" jetting. What do you recommend ffor race fuel? I've tried the Unocal 110 octane and I attempted to jet it but I didn't get
what I thought was any agains in performace. The fuel that I was using was NOT oygenated so I gather that would have an effect. I'll try to find the fuel
that you recommend if I can find it and so long as it's not terribley expensive (I'm kinda immune to $10/gal fuel!!).

Thx!!
 
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#12
Phillips B35 is a reasonably priced alternative that should prove equally effective.
 

jimt_yz400

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#13
Rich,
One more question:
Bill of Moss Machine (SFO in DRN) has suggested that I may want to cut the barrel (base) to increase the squish( compression?) then run a decent race fuel with the (new) stock piston and rings.
Does this sound reasonable and what would I gain by doing this?
 
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#14
Jim - Running the correct squish clearance is pretty critical in these engines. Yamaha has tended to set the YZ and WR 4xx VERY loose. I haven't seen enough 250F to know if they have continued the trend, but given the elevated rpm peak and the intended audience it's logical that there would be a generous safety margin in most of these engines. At the very least the squish clearance should be checked.