Running high compression on two stroke

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crazyYammi

Member

Aug 31, 2004

89

0

• Sep 15, 2004

• #1

I have been thinking of getting a phathead and playing around with high compression on my yz250. How much benefit do I get out of high compression on a 2stroke? I will be running trick 110 it is 4$ a gallon here.

 

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Dragmeistr

Sponsoring Member

Oct 14, 2002

149

0

• Sep 15, 2004

• #2

I put the VHM head with their motocross dome on my bike, granted it is a completely different engine and bike, but all I can say is that I didn't like it. I guess it depends on what kind of riding you want to do, but it made my bike run very flat up top. The little bit of increase in the bottom was not worth how much I lost up top. The phatheads may be different, I don't know.

 

marcusgunby

LIFETIME SPONSOR

Jan 9, 2000

6,450

2

• Sep 15, 2004

• #3

High comp heads do tend to run flat on top end.

 

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Studboy

Thinks he can ride

Dec 2, 2001

1,818

0

• Sep 15, 2004

• #4

I have also had unsatisfactory experiences with cutting the head for too much compression, it does give a little bit more lowend but it really kills the overrev and makes the bike more susceptible to pinging due to poor fuel quality.

Dragmeistr, I don't know about how your stock CR head is setup, but on my YZ the VHM had a much much wider squishband than the stock head, with a little less overall volume. Squish was set at 1.5mm.
Maybe you can clarify this for me Marcus, what is the advantage of going to such a wide squishband? Why does VHM do it?

 

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youngr51

Member

Jul 20, 2004

38

0

• Sep 15, 2004

• #5

Higher compression usually brings more power but other things have to be takin into consideration like ignition timing, fuel, and mostly reliabilty. Cylinder temps will change as well. I dont think the power will outweigh the headaches. Paying for race gas sucks as well at $5 a gallon.

 

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viking20

Sponsoring Member

Aug 11, 2002

428

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• Sep 16, 2004

• #6

I pay more than that for pump fuel... :ugg:

 

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Dragmeistr

Sponsoring Member

Oct 14, 2002

149

0

• Sep 16, 2004

• #7

Dragmeistr, I don't know about how your stock CR head is setup, but on my YZ the VHM had a much much wider squishband than the stock head, with a little less overall volume. Squish was set at 1.5mm.

Click to expand...

I had Eric Gorr redo the dome in my head when he did my cylinder. I am assuming this head will cool better than the stock head is the reason for me using it.

 

marcusgunby

LIFETIME SPONSOR

Jan 9, 2000

6,450

2

• Sep 16, 2004

• #8

Studboy said:

I have also had unsatisfactory experiences with cutting the head for too much compression, it does give a little bit more lowend but it really kills the overrev and makes the bike more susceptible to pinging due to poor fuel quality.

Dragmeistr, I don't know about how your stock CR head is setup, but on my YZ the VHM had a much much wider squishband than the stock head, with a little less overall volume. Squish was set at 1.5mm.
Maybe you can clarify this for me Marcus, what is the advantage of going to such a wide squishband? Why does VHM do it?

Click to expand...

A wide squishband tends to give more bottom end but trades for top end,the best head designs are the ktm ones, they are top hat shaped, all the volume is very close to the plug-even though the rm is a copy of the ktm engine its still not as strong, 2 reasons, one the inlet isnt as straight and two-head designs.

Th enew YZ125 is as powerfull as a 03 ktm 125 mainly due to the straighter inlet/exhaust tracts.

 

bedell99

~SPONSOR~

May 3, 2000

788

0

• Sep 16, 2004

• #9

Marcus do you think the CR's can benefit from a different shape head?? I remember looking at a KTM and the head looked very different from my CR.

Erik

 

Last edited: Sep 16, 2004

steve125

LIFETIME SPONSOR

Oct 19, 2000

1,252

0

• Sep 16, 2004

• #10

I think one of the reasons, and there are many :) the cr250 has weak bottom. Is too large a reed cage, low primary compression, and the cylinder port ducts are too large.

Shall I continue??? :) Poor PV design, the wrong carby and poor jetting, improper ign. mapping, and a real heavy crankshaft.

The spark plug is dialed in though :rotfl:

 

marcusgunby

LIFETIME SPONSOR

Jan 9, 2000

6,450

2

• Sep 16, 2004

• #11

bedell99 said:

Marcus do you think the CR's can benefit from a different shape head?? I remember looking at a KTM and the head looked very different from my CR. I wonder if Honda did, what Kawasaki did and copy Yamaha's port design.

Erik

Click to expand...

LOL on steves comments,
yes i do think the cr head is a poor design-it looks the opposite to a ktm setup.

 

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WillyM

Member

May 18, 2004

84

0

• Sep 16, 2004

• #12

Head Design

If you want to redesign your head get the book from Grahm Belle 2-stroke engine design.
I read that a top hat with a 35 degree squish angle and it needs to be 40 to 60 thousanths from T.D.S.
I did the head on my retro 88 kx 250 and I did a bunch of other like 230 lbs of compression mods and it rocks.
I use vp c12
I am working on the same mods for my 03.

 

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bigguns

Member

Oct 21, 2004

8

0

• Oct 27, 2004

• #13

i have a 2003 kx 250 with a phathead on it and the dome is 2 stages above stock compression. you would not believe the power this sucker makes. i was thinking i would have just low end power, but i was wrong. i have much more power everywhere. daylight and dark differance.

 

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john3_16

Member

May 17, 2004

808

0

• Oct 28, 2004

• #14

cool

 

Last edited: Oct 28, 2004

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Duane

LIFETIME SPONSOR

Feb 3, 2002

32

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• Oct 28, 2004

• #15

I recently bought a CR 80 bored to 108, and the head has been milled. The seller recomended running race gas. Do I have to run race gas? Will I do damage if I don't?

 

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Studboy

Thinks he can ride

Dec 2, 2001

1,818

0

• Oct 28, 2004

• #16

If it is setup for race gas, then yes you need to run it or face meltdown.