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sleeving down to a 400 from a 450?

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has anyone thought about sleeving down from a 450 to a 400? I know most people not in the know would think that absurd but KTM makes an exc 400 and it has very good characteristics. anyone know who does this? a buddy of mine rode an o5 400 exc and liked it better than his 450.

PS unless you ride your 450 pinned everywhere spare me your analogy's

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that ktm will have a much greater stroke than a 450r will. if you could sleeve a 450 down i think you would loose too much. you would have a smaller bore and still a short stroke. those ktms can get away with it because they have really long stroke which allows for alot more torque.

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The ports, valves, cam timing, and exhaust ID of the CRF450 are designed around a specific ratio of piston area to valve area. Reducing the bore size without re-engineering the rest of the engine would drop intake velocity and kill off low-mid range torque and push the torque peak rpm point higher up in the rev range. So you'd basically end up with a slow slightly peaky four-stroke.

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there's no replacement for displacement! But that article listed below is pretty interesting. I don't ride MX/SX so i forget that its not always about the juice.

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Rich,

What about when you go the other way, instead of sleeving it down, going to a larger bore without changing all the other items you had mentioned.

For example there are a lot of guys talking about 270 kits in the 250. Is this a bad idea without changing everything else in the top end to match as well??

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For example there are a lot of guys talking about 270 kits in the 250. Is this a bad idea without changing everything else in the top end to match as well??

Good or bad depends entirely on what you are looking for. If you are looking for more low-mid range power then increasing the bore and leaving the cam duration, and valve area (along with port/pipe sizes) unchanged is an excellent way to accomplish this. The increase in the bore has a positive effect on port velocity and tends to improve the signal that the carb sees. It will tend to move the torque peak rpm down in the rpm range (500-900rpm lower is common) and tends to increase the bulk area under the torque peak usually at the expense of torque past the HP peak, or what a lot of people call over-rev. Here's a good example of this type of engine : http://www.four-stroke.com/Dyno_runs/SFO_index.htm

Notice the engine falls off fairly hard in the 8000-9000 rpm range, but makes a ton of torque belpow that point. For the average racer this translates to an bike that is easier to ride and and engine that will to pull a taller gear out of corners.

If your goal is to increase absolute horsepower or improve the power in a very specific way then a bore increase without complimentary changes isn't likely to be the best approach.

Clearly defining your specific goals before you start making changes to the engine is the best approach in my opinion. These are really well developed engines right out of the box so old style axe and hammer approaches to improving them rarely work out. :banghead:

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If they wanted too, the could detune the CRF by using the X cam and closing the port cross section with Devcon steel epoxy putty and using a CV carburetor..but it would be a hell of a lot easier to simply just go buy a KTM 400 EXC...

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So with big bores , Cam changes and port work will give a better overall effect. I read an article on 450's not to go larger in bore than 488 or 100mm's did that have to do with it?

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we have stock displacement crf's making more power than big bores.

the 480's and 500's struggle to get 55hp.

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I read an article on 450's not to go larger in bore than 488 or 100mm's did that have to do with it?

When you get a bore larger than about 96-97mm on an high rpm engine with a narrow included valve angle like the CRF uses you run into combustion issues at high speed. The effective combustion area gets really narrow and flat as the bore is increased and the piston crown gets increasingly deeper valve reliefs cut into it if you try and keep the compression ratio up and use reasonably high valve lift. Flow on valve overlap near TDC can be compromised as a result of all this. In addition complex combustion chamber shapes with high surface to volume ratios like we end up with in these situations require lots of additional ignition advance to burn completely in part because high speed port induced tumble gets disrupted by the piston crown cutouts. The valve cutouts stall mixture motion and make it difficult to get a complete burn. Take a close look at a stock CRF450 12:1 piston and a high compression after market big bore piston and you'll see what I'm referring to. The OEM piston crown shape has been carefully optimized to aid combustion.

Even if you can sort the combustion issues out to your satisfaction you are still faced with the problem of getting enough working fluid in and out of the engine with a cylinder head and valve area designed around a much smaller displacement.

Engines are SYSTEMS and changing one thing invariably effects lots of other things that aren't always intuitively obvious. It's not an accident that there only a handful of good four-stroke tuners out there. :banghead:

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