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Port Timing
hondatrx250r
#1 Posted : Wednesday, January 13, 2010 12:21:15 AM(UTC)

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So i just picked up a used 330 cylinder. My brother in law measured the port timing for me and they were like 84 degrees and 114 or something. Im not sure which one was transfers and exshaust. What port timing would be good for bottom end power or high end power? Do you just change the port timing by raising a port? Is a lower degree port timing mean more high rpm power? Im not sure how this all ties together. Any info will be appreciated!!


Thanks,
Matt
All250R
#2 Posted : Wednesday, January 13, 2010 1:26:10 AM(UTC)

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Hey Matt,
84* is likely the exhaust opening time ATDC and if accurate would equal 192* of ex duration - which is substantial. It's definitely going to favor upper rpm's for where the power is. I'm not sure what 114 is. Transfer durations for a ported cylinder are often around 122 for example.

In general, the more port duration (time open) the more the engine is intended to run at higher rpm (generally speaking). There is more to engine building than port timing however - a lot more. But if you go about attempting to change the engine raising ports, the transfers are the most tricky by far, especially if you raise them more than a couple degrees. The angles are precisely aligned with the opposing charge and being careless or not having the ability to measure the angles means lost scavenging efficiency. A solid amount of theory coupled with a strong ability to work with hand tools are the basic prerequisites for attempting to port a cylinder.

Having said that bottom end engines may run around low 180's for ex duration and maybe around 120 for transfer - top end around 194* with 130* transfer. Like I said before though, there is an almost blinding amount more you can do to an engine to make it run it's best for an intended application than port timing, but port timing is a basic that can't be overlooked either.
Any intelligent fool can make things bigger and more complex... It takes a touch of genius - and a lot of courage to move in the opposite direction - Albert Einstein

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hondatrx250r
#3 Posted : Wednesday, January 13, 2010 9:53:44 AM(UTC)

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yeah i cant remember what the exact numbers were. yeah i bought it off ebay and the guy said if was an mx style port. I wish i would have the money to buy a new cylinder but im a boke college kid :(. Ok thanks for the info :) Im just tryin to learn about these motors a little bit more so i can argue with my buddies who are all pro 4 stroke haha. So what else could I could a guy do to make it run better. compression? milling the head? dome size?

Thanks,

Matt
All250R
#4 Posted : Thursday, January 14, 2010 1:23:48 AM(UTC)

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Hey sorry for the late reply. Compression definitely increases power. Milling the head isn't so much applicable here as it would be probably on a 4stroke, but "milling" the head is a way to increase the power here too. The real power in head modifications is in combustion chamber shape and squish dimensions. The stock TRX cylinder needs to be decked about 1mm as well. Dome size will depend on bore size. The figure I use is uncorrected compression ratio taking into account the stroke and compressed space at TDC. You can run about 14:1 for a decent bump and run around 100 octane. It's a nice middle ground number, but the number you want for yourself is depend on how you ride and your fuel budget, among other things.

Tell your 4stroke loving friends that if their engines were so great, why do they have to be bigger and use more parts to be more powerful than a 250cc 2stroke if they're so advanced. haha.
Any intelligent fool can make things bigger and more complex... It takes a touch of genius - and a lot of courage to move in the opposite direction - Albert Einstein

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hondatrx250r
#5 Posted : Thursday, January 14, 2010 10:13:24 AM(UTC)

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Hahaha thats what I tell them but they dont care, they belong on 4 strokes because they dont ride them hard and they ride like 5mph haha. yeah we were having problems with my squish dimensions with my new motor, my piston was not going up all the way along the cylinder wall or something, so I have to use a different cylinder spacer plate. anyways thanks for the info Jason!!!

Thanks,
Matt
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