Paging Slipdog , Derek Harris , and engine builder.

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4/30/2018 9:07 PM

Hey guys....on the Husky / /KTM's , I know quite a few guys milled the heads down to about 0.0040 - 0.0045 on the TC and SX bikes. Stock squish seems to be about 0.0085 on the 250's , and I think the 125's are real close to that. I have to check mine before I order my base gasket I guess.

But , what was the reasoning for milling the head , instead of just using a thinner base gasket? If a guy used a 0.0040 thinner base gasket , brought the squish to 0.0045......will that be a noticeable change in port timing , and possibly change the characteristics of the motor?

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And there goes Jeffro. One of God's own prototypes. A super high-powered mutant of some kind never even considered for mass production. Too weird to live, and too rare to die.

Pimpin' Ho's , Rollin' fatty's......drinkin' beers , beers , beers!! ~ Ja

5/1/2018 3:28 AM

Gotta watch the decimal points! You are talking in thousanths like .004" instead of .040" HUGE DIFF lol.

By taking off material from the head and lessening the cc amount, the secondary comp ratio rises. This increases the engine potential of making more power, (as well as heat), effeciency, and you are basically "exciting" the engine into producing more of what is has. Current engine designs made by the mfg are set and tested to make a certain amount of power on a spec'd octane, stated in the manual. If you increase the CR, your octane will need to go up to keep something called detonation at bay.

Deto can easily described in laymans terms as "pipe bang", or motor knock. What is happening there is the uncontrolled combustion of gases at the wrong "time" as the engine fires off and combusts fuel that is underrated for the motor. It is never good to have signs of deto, the top of the piston gets a sandblasted look, pitting occurs, and the engine can be severly damaged due to low octane with a higher spec engine requirement.

So, if you lower your cyl height by changing the base gasket thickenss, you are retarding the port timing and basically "dropping" all of the ports in relation to the stroke. This tricks the engine tune and will increase the effective stroke and yeild less of a "hit" and add torq to the power curve, with a slight loss of top end over-rev.
Keep in mind, the same rules apply, by lowering the cyl, you just took away volume from the head area so the octane must follow to keep deto away.

The squish clearance is a slightly diff animal in that a tighter squish is typically desirebale bc the bikes usually come through with excessive squish. Performance gains can easily be had with a better running engine simply by tightening the squish AND removing cc's from the bowl so your CR stays the same and the octane can stay whatever is spec'd.

Thats why you see some tuners, (yes, shameless self plug), advertise a "pump" gas head mod in addition to the race head mod. What is usually happening there is the cc's are checked before the head mod, (for ex lets use 15cc), and documented. The gasket surface(s) are cut to reduce the squish clearance,(lets say to 1mm) but no material has been taken away, (so now your cc is 14cc), this means your CR just went up, your octane needs to go up. Conversely, if you take the squish down to 1mm and then REMOVE material from the bowl to get back the 15cc, your octane can stay the same.
The benefit mostly with the head mod and cc reduction is effeciency. This is due to the tighter squish, the engine will run smoother, cooler, you will see a mileage increase as well as increased power. I do this mod for a lot of guys who ride single track and need to stretch out the tank, most see in increase, some more than others, but that depends on conditions, elevation, etc.
If you straight up mod the head for more hit, keep in mind that this will slightly stress out the trans, clutch, engine parts, make more heat, and start to move the line away from reliable to less reliable.

The best answer to most of this is to tailor your needs to your budget and stick with it and enjoy your riding. If you can afford to have a full on race motor that requires race gas, frequent rebuilds, etc then go for it. If you just want a little more and keep the relaibility of the motor as best as you can, it is always a better choice to lower the octane requirements and get what you can out of it in the long run.

Jbub




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5/1/2018 7:41 AM

.010" is a noticeable change in port timing.

You can use the base gasket to move squish around only a few thousandths.

Just measure your current base gasket thickness and stay with that for future use. Let your motor builder adjust your head and port timings.

You don't need a VHM head if your having your motor modified. It's just bling

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5/1/2018 9:17 AM

Hey Jessie

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5/1/2018 10:02 AM

That was great info Josh. I'm riding a basically stock YZ, just an exhaust and v force. But I was thinking about getting the head done as I know they're pretty sloppy from the factory. After reading this though, I think I need to find out where to get a thinner base gasket first and set my squish to that, as I tend to like more low to mid and rarely rev a bike out. What do you think? Do you know where to find different base gaskets, I know KTM offers them for their bikes but I've never seen them for the Yamahas.

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5/1/2018 11:03 AM

I have personally assembled JBONE modified engines. His two stroke work is absolute perfection. +1 for Josh.

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Tony Alessi, "I think it's time to have more kids so I can have hopes of actually raising a champion."

5/1/2018 12:21 PM

Hi Jbone wink

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5/1/2018 12:44 PM

Some really awesome replies in here you guys. So thanks for all that info.

My goal was to squeeze out some more power across the board on this thing. Keeping up with top ends about every 15 hours or so. I can run strait VP 110 or C12 as well.

Would possibly milling the head slightly , running race gas and keeping up on fresh top ends....could I squeeze some more power out of this thing? I'm not looking for a 4 hour race motor.....but am willing to compromise between that and my stock engine.

I did get my hands on a new PWK 38mm carb and have the STIC metering block coming as well.

Hell....I would even change my exhaust system if I thought I could get more mid - top power.

Kind of lost on where to go from here.

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And there goes Jeffro. One of God's own prototypes. A super high-powered mutant of some kind never even considered for mass production. Too weird to live, and too rare to die.

Pimpin' Ho's , Rollin' fatty's......drinkin' beers , beers , beers!! ~ Ja

5/1/2018 12:53 PM

Jeffro, sorry for asking a question when you were waiting for an answer to your own. Interesting info here for sure!

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5/1/2018 1:44 PM

I would just narrow the squish with a slightly thinner gasket.
You might loose a bit of port timing, but the narrower squish and slight compression raise will add across the board.
It's simple, cheap and reversible.

Without dyno runs, you don't really know how much better things are getting. Plenty of after market pipes and silencers actually loose power!

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5/1/2018 2:43 PM

@hellion- Lee, the YZ250 you have could use about .020" off the cyl base, this will retard the port timing a hair and get you some torq. Also, you can use some epoxy in the back rear transfers to have them short circuit across the piston crown. That will add a little bump down low. If only you knew someone who could do this?? devil

@681- thx for the kind words!

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5/1/2018 7:53 PM

hellion wrote:

Jeffro, sorry for asking a question when you were waiting for an answer to your own. Interesting info here for sure!

No worries at all brother! We're all in here to learn , so the more knowledge the better.

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And there goes Jeffro. One of God's own prototypes. A super high-powered mutant of some kind never even considered for mass production. Too weird to live, and too rare to die.

Pimpin' Ho's , Rollin' fatty's......drinkin' beers , beers , beers!! ~ Ja

5/1/2018 7:57 PM

Btw.....for anyone that has been in here to help , thank you!

I'm riding Saturday , then tearing the top end down on Sunday. I'll measure my squish then , and maybe after I get that measurement I can give you guys an idea of where I stand and if I can fudge a little bit on my base gasket.

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And there goes Jeffro. One of God's own prototypes. A super high-powered mutant of some kind never even considered for mass production. Too weird to live, and too rare to die.

Pimpin' Ho's , Rollin' fatty's......drinkin' beers , beers , beers!! ~ Ja

5/1/2018 8:22 PM

Don't really see where there's much for me to add here, carry on!

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5/1/2018 9:03 PM

slipdog wrote:

Don't really see where there's much for me to add here, carry on!

Your input is always something I look forward to Brian. Any advice that hasn't been said , please feel free.

I got this 125 thing happening now , but I'm hoping here real soon to finally get my TC 250 too. And I know you have some good experience with those.

Frustrating as I have my RM at a stand still because of reasons I don't want to get into. And a worn 125 , where I wanted to mod a bit with a fresh top end....but hit a couple speed bumps. 2 bikes and nothing to ride really sucks.

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And there goes Jeffro. One of God's own prototypes. A super high-powered mutant of some kind never even considered for mass production. Too weird to live, and too rare to die.

Pimpin' Ho's , Rollin' fatty's......drinkin' beers , beers , beers!! ~ Ja

5/2/2018 12:01 AM

hellion wrote:

That was great info Josh. I'm riding a basically stock YZ, just an exhaust and v force. But I was thinking about getting the ...more

I used Athena for different thickness base gaskets. They were so cheap that I bought a load so I could experiment.

http://www.athenaparts.com/eng/models/view/4840-section-33/yamaha-yz-250-lc

Stock will likely be 0.5mm on your YZ. Check your squish and then buy the gasket required to achieve your desired squish measurement.

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@russ_69

5/2/2018 3:40 AM

JBoneMotorworks wrote:

@hellion- Lee, the YZ250 you have could use about .020" off the cyl base, this will retard the port timing a hair and get you ...more

Eric Gorr did this on cylinders he did for me.

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Take it to the limit, one more time!

5/2/2018 5:07 AM

Thank you JBone! I'll give you a call to catch up and schedule in said mods with my next top end job, that sounds exactly like what I'd want.

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5/2/2018 7:07 AM

cool
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5/2/2018 2:45 PM

Every two stroke I own gets squish checked. Cometic is another good outlet for custom base gaskets for bikes. One nice thing about the KTM 65/85 is OEM offers a good selection of gaskets to set deck height and squish. I love threads like this because I always seem to learn something more about setup..........

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5/2/2018 7:40 PM

Cometic will make base gaskets to your needs.

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5/2/2018 11:07 PM

I'm picking up the full top end kit tomorrow , along with all 5 gaskets that KTM / Husky supplies. Bike shop said.....use the one I want and take the other 4 back. Which is why I love Motosports!! Making my life easier.

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And there goes Jeffro. One of God's own prototypes. A super high-powered mutant of some kind never even considered for mass production. Too weird to live, and too rare to die.

Pimpin' Ho's , Rollin' fatty's......drinkin' beers , beers , beers!! ~ Ja