[250x 300ex oversize exhaust valves] - ATV Riders Forums - Honda, Yamaha, Suzuki, Kawasaki, Polaris, CanAm, Arctic Cat, Kymco ATVs

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250x 300ex oversize exhaust valves
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Posted by: ridinfool

Just to let everyone know,oversize exhaust valves are now available from kibblewhite!!!!!!!!!!!!!!!!!!

Posted by: ZeroLogic

Haven't they always be available?

Posted by: chapmanmd

The +2 exhaust valves havent always been. I was running +1 exhaust valves until I started this newest build and got the +2's. Have no idea how it runs yet...

Good post.

Posted by: hondapro300EX

Will exhaust valves help it breath better or take in beter or?
Sorry if sound so stupid just always been working on trucks and my jeeps and know notheing about exhaust valves.

Posted by: bradley300

bigger intake valves let more in, bigger exhaust lets more out

Posted by: 2muchquad

i would get the intakes if thats all you can afford,the pistons exhaust stroke will force the spent gases out,so the exhaust ones arent a necessity.regardless of what you might read

Posted by: chapmanmd

quote:
Originally posted by 2muchquad
i would get the intakes if thats all you can afford,the pistons exhaust stroke will force the spent gases out,so the exhaust ones arent a necessity.regardless of what you might read

Gonna have to disagree with ya on that one, depending on your cam and carb.

Posted by: 300exOH

quote:
Originally posted by 2muchquad
i would get the intakes if thats all you can afford,the pistons exhaust stroke will force the spent gases out,so the exhaust ones arent a necessity.regardless of what you might read

I was thinking it was the opposite. If you keep the stock intake valves and put oversized exhaust valves it will create a ventury(sp) effect causing the spent gasses to be "sucked" out of the combustion chamber.

Posted by: chapmanmd

The valves in a 250x/300ex head werent big enough for a 250, much less some of the big bores people are running. I KNOW that the big valves in mine made a HUGE difference.

Posted by: bwamos

quote:
Originally posted by 300exOH
I was thinking it was the opposite. If you keep the stock intake valves and put oversized exhaust valves it will create a ventury(sp) effect causing the spent gasses to be "sucked" out of the combustion chamber.

Actually, you're both correct, to a degree.

Stock intake, and oversized exhaust will get you more low end torque, due to incresed intake velocity at low rpms, and less resistance on the exhaust stroke, but you're intake gets somewhat choked off above 5k rpm's. (need a matching cam)

If you want the bigger HP up top, then you need oversized intake valves and a higher rev cam, but it will move your torque curve up hill and down slightly.

I'm almost always in the top 30% of the RPM's so I opted for the high rev setup.

Note: this applies only to the 300ex. Every model is different. Some have intake too large in stock form. The 300ex however is ported small.

Posted by: 300exOH

quote:
Originally posted by bwamos
Actually, you're both correct, to a degree.

Stock intake, and oversized exhaust will get you more low end torque, due to incresed intake velocity at low rpms, and less resistance on the exhaust stroke, but you're intake gets somewhat choked off above 5k rpm's. (need a matching cam)

If you want the bigger HP up top, then you need oversized intake valves and a higher rev cam, but it will move your torque curve up hill and down slightly.

I'm almost always in the top 30% of the RPM's so I opted for the high rev setup.

Note: this applies only to the 300ex. Every model is different. Some have intake too large in stock form. The 300ex however is ported small.

Good info.

Posted by: 2muchquad

well im sure every motor could be different but after about 10 years or so of tinkering with the 250x/300ex i know how to make them run pretty good for me.im not saying their gonna be 4 fiddy beaters but mine haul ***