Porting and Polishing the Intake & Exhaust Ports

Has anyone out there ever ported and polished the intake and exhaust ports on this bike? They look so nasty!!! If so, let me know how it went. It looks as if they were at least cleaned up and polished the motor would breath much easier. Any input or suggestions would be greatly appriciated!!


I did it.

It'll be better absolutely.

But I didn't know well at that time.

So I couldn't conpare in same day.

But my bike is still very fast.



I wonder if anyone has attempted to fit a small supercharger to one of these things yet? Seems like that would be a natural mod for a high performance 4-stroke. :)

How about a streetbike "Turbo Set-up". I believe we could get about 85-100hp out of the thing.......

Yeah..... for about 15 minutes.

Remember these have about the same displacement as a lawn mower.


MX Tuner

If you look at any big block truck engine you will notice this exact same thing in the intake runner, the texture will be rough to better atomize the fuel and air to give you torque.

If you look at any engine designed to run at a consistant high RPM you will notice the smooth polished intake runners and also exhaust runner.

These bikes run at both ends of the spectrum as far as RPM but produce most of there power from 3000 to 7000 RPM, this is the reason for the textured intake and exhaust runners, as far as porting and polishing the runners.....this would be total guess work without the use of a flow bench. A flow bench is the only true way to tell if your doing any good when porting and polishing.

You have to make the fuel tumble to get it to atomize on a engine designed for torque, this is what the texture does.

With a smooth polish you run the chance of sending the fuel in wet and causing the engine to fall on its face at low RPM's.

I'm not saying you can't do it I'm just saying this has been my experience with high performance car engines, If you want something that will pull stumps uphill you want texture, if you want power to pull a stump down hill real fast at high RPM's go with the smooth finish.

Later, Jason

Yea i have my bike ported and polished, it runs smooth. i also have my cam welded so that its in between WR and YZF setting...so i get bottom and top end...and i have on a white brothers e-series shorty muffler...my bike pulls harder off the bottom than a 426 (mine is a 98 yzf/bbr400) and itll hang with it through the mid...and then it screams on top. you can DEFINATLY tell the difference. it runs sooo much smoother. contact BBR motorsports by e-mail.. www.bbrmotorsports.com and ask them how much they charge... i think pro circuit will do it too...but i would stick with BBR because they know ALLLLL about these bikes. just my two cents.


I have mildly ported and polished my head on my street bike. Take care that you don't polish the intake up too smooth. Like the previous post stated, you will cause some unwanted turbulances in the intake system if it is too smooth. On the exaust side however you would want this to be as mirror smooth as you can get it. I have also been dissapointed in production casting marks in engine flow components. Obviously the manufacturers don't think it is worth the extra money to make them any smoother. After putting about 40 hours in my street bike head, the difference felt is only in my head. 1/4 mile times did not decrease. I do feel better, that is about it.

Hope this helps


I don't think you would do very much with out changing the valves or cam at the same time you port it.



01 YZ426F #85 Vet C

Actually there are a couple misconceptions going on in this thread.... maybe I can throw a nickle into the batch.

Several years ago it was assumed (somewhat rightfully so) that bigger, smoother ports and *more* flow meant more power, HOWEVER in todays engines (specifically high performance MC engines) the tolerances are so close with respect to valve clearance and surface area that simply opening up a port will NOT guarantee higher performance (more power) and in some cases may actually hurt performance. This was correctly stated in one of the above posts. It may accurate to say that a "polish" and port match may actually help the performance of this (426) motor but anyone willing to do this for you without offering to do a baseline dyno run FIRST and then a series of dyno runs after the work is completed adjusting for temperature, barometric pressure differences, and also making the necessary jetting changes to offset or account for the higher "flow" is there simply to take your money and is blowing smoke up your "tailpipe". These machines are done high volume production line style yes...but the R&D that go's into them is significant to begin with and if *they* (Yamaha) could reliably acheive better performance by smoothing out the intake runner, they would do it in the casting process ahead of time.

YOUR best bet for power gain through MAJOR engine modification (which porting/polishing falls under) is to seek out those factory race teams that have a bunch of R&D under their belt making their race teams un-beatable, and bugging the chief mechanic for some of his 'last years' tricks....these guys are accesible...all you have to do is track em down....and ask em how they get 65 HP out of one of these motors....then take notes...but dont assume that you will get more power from a "simple" port job everytime...it go's way deeper than that.


01' WR426,YZ timing,EMP #2,#48Pilot,#100PAJ,#172 Main,BK mod,stock pipe-no baffle, open airbox,Gray-Wire Pulled,K&N Filter, throttle stop mod,CA Street Legal,Pure SuperMotard 17 in. wheels, 15/46 gears,ProTaper bars,

Smooth polished exhaust ports, cleaned up/textured intakes. Don't take off too much material, Yamaha spent lots of time designing these intake and exhaust ports. Textured intakes allow the flow to be less laminar (smooth) and more turbulent close to the wall. This results in more atomization and a better burn. I used to race YZF-750 Yamaha superbikes and this is exactly what the tuners did to the head. Also, match the valve guides so that the stem of the valve is all that is seen by the intake flow. The reason that production bikes do not generally have this done is due to the time and excessive cost associated with the process.

[This message has been edited by Scooter426 (edited September 25, 2001).]

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