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Iluv2fly

Porting vs bigbore

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What gives better overall power, going to a 480 bigbore or having the head ported with oversized stainless steel valves installed.

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A big bore is more or less certain to give a predictable increase in power based on the simple mathematics of how much bigger the engine is than it was. It's a pretty sure thing. It will also put a bit of extra strain on the bottom end due to the increased weight of the bigger piston, but overall, they work out pretty well.

Porting, OTOH, is something that has to be done right in order to produce any benefit at all, much less a really big one, and should be left to someone who can show you that they have had some success with their approach to it. I doubt that a 450 would need oversized valves for anything other than super moto. One thing that can be said for port work is that by itself, it causes no additional strain on any other components except for the extra power it creates.

Big Gun , the pipe people, has done some impressive things with YZ250F's (40 hp with stock cams and compression) for $700.

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If you are gonna do one or the other. I allways suggest proper port work over a big bore. Though the YZ's probally have the better ports out of the bunch.

We have many times made just as much power with porting, intake mods with stock compression and stock cams as comapred to the numbers we got with a big bore alone.

From a marketing standpoint big bores are a no loss situation. I mean bigger is better right?? They bore and replate your cylinder, give you jetting suggestions, sell you a piston and how could you ever complain.

I am a big fan of making the parts you have work the best they can.

But then again I did say proper porting. Many of our RMZ250's wind up with a welded exhaust port. In the end the exhaust port is smaller in diameter and flows more. I either build a new headpipe or swage the existing one to make it smaller.This makes for awesome rideable power.

If I were getting porting work done out of house I would choose one of the smaller engine builders. One with a good rep.. Some of the bigger engine builders arent really hand on shops anymore.

Someone like Ron Hamp or WMR.

If you have have the headwork correct and working right next time you have a little cash burning a whole then get the big bore and the carb modded then whole package will make mad rideable power.

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I'm very happy with both. RPM's did the Big Bore and Engine Dynamics did the porting and head work. I have had it done for almost two years now and zero problems. With one hard season of GP racing.

Chaz

www.gpkings.com

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i got the kibblewhite (s/s valves, ti springs/retainers) kit put in my 04 450 and had the heads flowed at the same time and along with the hotcams and exhaust it makes more on the same dyno than a 05 husaberg FE650.

in getting a valve kit and port done at the same time you save a bit on labour cos its only the head that comes off not the barrel as well.

oh and in regard to the comment by RC876 about the difference in porting characteristics between the yz/wr 400/426/450, they all use the SAME casting for the head, the differences in power characteristics is due to the differences in displacement and timing between the respective models. if you look at the top front right hand corner of the 450 head you will notice that the casting is in place for the manual decomp as per the 400 and 426 models.

go the kibblewhite kit and the porting, why bore it out now? wait til u need to rebore it and do it then.

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