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385cc Big Bore

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My DR350 SE is in need of a rebore , cant decide if i should just go to next oversize , currently stock bore, or go for the 4mm overbore 385 piston,

Dont feel like i really need any more power for the type of riding i do but a little more torque is always nice,

So any downside to the 385 kit ?

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Only thing I know about this kit is, if you go with the 385 that's as big as you can go before  having to re sleve the engine. Also there is no room for error with how thin the sleve will be. It has to be bored perfectly straight. It definitely adds low and mid range torque. Not much on the top end if an at all. I have this kit on my bike and I haven't had an problems with it yet, been running it for a couple years now. I do know it's a one and done sort of deal. Its definitely all the power I would ever need.

20180127_123308.jpg

Edited by lzam6285
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I had a 96 SE with a 385 kit, a 97 standard bore dirt model 350 and a  99 SE with a Thumper racing 435 kit. The 385 offered a bit more torque than the 350, but it required premium fuel or it pinged. The Thumper Racing 435 offered considerable more torque and ran on regular 87. Thumper now offers a 441 kit, still with standard CR. If I were going to do anything related to bore, I would certainly do the Thumper racing 441 kit.

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Thanks for the replies guys, plenty to think about there,

Found these dyno readouts online, they're from an old bike magazine , the 385 gets some impressive figures for a relatively  simple inexpensive kit

image.png.1769b137514fd8d01c33dbfb864d2b80.png

 

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the dip at 6k on the 385 graph was due to an ignition fault that couldnt be felt when riding apparently 

Edited by paint it black

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the piston for the 385 is a dome top, it would be possible to mill some material off it to reduce the compression ratio. no idea how much youd have to remove to achieve a particular cr though (or what a good target would be, although Im assuming <= 10:1). easier than messing with the cdi and won thave to worry about fuel quality

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apparently you cant edit a post, but i did some back of napkin math and I think milling it down to a flat top would only reduce the compression ratio to 10.3:1, so youd probably also need to use a slightly thicker gasket. there are other advantages to having it be a flat top though

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Thanks for your napkin math ☺️

I've ordered the 385 piston, should arrive in the U.K in another week,

I'll fit it and then see if there are any issues with detonation,

I have a big bore hi comp (10.5:1) piston in my NX650 and never had any issues with pre ignition , 

think U.K regular gas is slightly higher octane than U.S stuff , regular here is 95 and super 97-98 , different methods are used to measure the octane here than in the U.S but am sure our gas is less prone to pre ignition 

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Dont think the numbers between Aus, U.S and U.K are directly comparable due to rating being measured in a way, but am pretty sure U.K regular gas still works out with a higher octane so less prone to pre ignition issues

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On 11/8/2018 at 9:15 AM, paint it black said:

Dont think the numbers between Aus, U.S and U.K are directly comparable due to rating being measured in a way, but am pretty sure U.K regular gas still works out with a higher octane so less prone to pre ignition issues

I had to check this out, 

U.K regular gas 95 is equivalent to U.S 90/91

U.K super 97 is equivalent to U.S 92

So dont think pre ignition is going to be an issue running on U.K regular gas

Edited by paint it black
more info

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