Porting & Polishing For proformance? - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 7 (permalink) Old 10-30-2009, 05:45 PM Thread Starter
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Thumbs down Porting & Polishing For proformance?

Several posts address increasing HP and proformance but I have not seen addressing the basic fuel & exhast flow inprovement. Ever since my teen days, no money and junck bikes we played with porting and polishing intake & exhausts ports, opening them up & polishing them smooth for better flow, more gas & air in & out to get more power.! My first project was a Mo-Ped"Puch" Got it to do 50+ mph, No kidding with some other mods & avgas.
Wonder if any of you goros have tryed doing this & what were the results?
We know Air flow "ear shaves, jets and exhaust work! think this would work.
a good time to do it when engine is being rebuild, done that a few times!!!!
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post #2 of 7 (permalink) Old 10-30-2009, 07:31 PM
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Were you doing this to 2 strokes or 4 strokes?

I did quite a bit of "polishing and porting" on two stroke motors, with good results. Our work on 4 strokes seemed to show that enlarging the exhaust port did help, but any work on the intake ports was hit and miss.

The basic reason here is on a carbed bike, the size of the exit port on the carb itself is in fact the limiting issue. Even if you widen the intake port on the cylnder, the amount of fuel/air the carb lets in is restricted to the size of the opening of the carb itself. You can open the intake to match the opening area of the carb,..but most 4 strokes built in the last 30 years have intakes that are in fact larger than the carbs venturi. so opening them up does nothing, and can if fact "Slow" the mix to the chamber. Remember also that the area of the opening of the intake valve itself is a limiting factor. Intakes that might have smaller diameters than the carb still sometimes work better than opening them up...as they can sometimes increase the "venturi effect" , and can aid in the speed in which the cumbustion chamber fills

It was also found tha polishing the intake many times lowered power, the reason behind this was that the rough surface of the cast intake caused the mixture to "swirl" more, providing a much better atomization of the fuel..making it burn more evenly and faster. Smoothing out the intake removed or lessened this swirling effect , and power went down.

Now, increasing the area of the exhaust port can be helpfull, as it can make sure you are getting the full potential out of the intakes size. But again remember that the exhaust valve is again a limiting factor.

The best key here is to leave the intake port alone, unless you are installing a larger mouthed carb. Increasing the air flow by removing restirictive air boxes and jetting up to compensate does help alot.

But after doing this, the only way to see siginificant gains in power would be to install larger valves, and/or perhaps a less restrictive exhaust....along with opeing the exhaust ports if needed. And hotter cams, perhaps a supercharger or NO2 tank, rockets...yeah, rockets would work....LOL


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post #3 of 7 (permalink) Old 10-30-2009, 10:18 PM Thread Starter
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Did on 2strokes and 4strokes with 4s bigger carbs valves cam etc. raced abit but that was along time ago. Now just a cruiser that likes to tinker. I agree with you! thanks for the input. I'm sure you agree as soon as you start making porformaces mods you have to go all the way, heads valves cams cranks on and on$$$$$$$$. If its not right it goes BOOM, bin there done that. Vn750 is just fine and a lot of fun!
PS did 20 watt halogin driving lights, had to really think that one out no h3 holders My mods worked for $3. Interested how you did them.
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post #4 of 7 (permalink) Old 10-31-2009, 08:28 AM Thread Starter
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After writing the original post my memory bank kicked in. did a grauge project with my son in the early 90s he picked up a juck Susuki, i believe it was a 450. we ported and polished the intake & exhaust ports they have a lot of casting burrs, shaved the head to raise compression Polished the combustion chamber, then put it back together. there was not proformance difference, seemed to be starving for fuel this all trial and error! then desided to increase carb size for 30mm to 40mm not an easy fab. exhaust were straight pipes. did some sproket changes (can.t do this on Vn 750) the bike Screemed with lots of power, untill it when KBOOM! lower end let go. Moral is need to beef every thing up! harded cranks rods, pistons etc. Son went on the crouch rockets thats when I gave up! I'm sure with todays computer programs Proformance modifications can be tested in theory, flow,timing, compression etc. thats beyond my pay grade!
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post #5 of 7 (permalink) Old 11-01-2009, 12:58 AM
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I agree with not enlarging the volume if the intake runners. The best thing to do is just a good de-burr, smoth out ( not polish ) any casting flaws.
Nothing more to add to this post about the ex. side.
With all this said. I have'nt had the heads off of these bikes yet to know how the castings look, but all of the other valve- jobs I have done on other atv/bike engines the castings could use cleaning up. The only thing is that the power gains will be next to not noticeable with minor de-burring. It's more of a peace of mind thing knowing it has been done.

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post #6 of 7 (permalink) Old 11-01-2009, 01:23 AM
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I have built and repaired several of these engines, as for the heads just open the exhaust port and polish. there is nothing much to do to the intake side. These heads have the largest valves that will fit, so no improvement there.

The bottom end has room for improvement if you have the bucks. Offset grind the crank, longer custom rods, and custom pistons. A friend of mine was building drag bikes at the time and did mine free of charge. $ = H.P.
The weak link is the hydraulic lash adjusters, they can't handle 10,000-11,000 rpm but the bottom end can.


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post #7 of 7 (permalink) Old 11-01-2009, 03:32 AM
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Quote:
Originally Posted by Knifemaker View Post
Were you doing this to 2 strokes or 4 strokes?

I did quite a bit of "polishing and porting" on two stroke motors, with good results. Our work on 4 strokes seemed to show that enlarging the exhaust port did help, but any work on the intake ports was hit and miss.

The basic reason here is on a carbed bike, the size of the exit port on the carb itself is in fact the limiting issue. Even if you widen the intake port on the cylnder, the amount of fuel/air the carb lets in is restricted to the size of the opening of the carb itself. You can open the intake to match the opening area of the carb,..but most 4 strokes built in the last 30 years have intakes that are in fact larger than the carbs venturi. so opening them up does nothing, and can if fact "Slow" the mix to the chamber. Remember also that the area of the opening of the intake valve itself is a limiting factor. Intakes that might have smaller diameters than the carb still sometimes work better than opening them up...as they can sometimes increase the "venturi effect" , and can aid in the speed in which the cumbustion chamber fills

It was also found tha polishing the intake many times lowered power, the reason behind this was that the rough surface of the cast intake caused the mixture to "swirl" more, providing a much better atomization of the fuel..making it burn more evenly and faster. Smoothing out the intake removed or lessened this swirling effect , and power went down.

Now, increasing the area of the exhaust port can be helpfull, as it can make sure you are getting the full potential out of the intakes size. But again remember that the exhaust valve is again a limiting factor.

The best key here is to leave the intake port alone, unless you are installing a larger mouthed carb. Increasing the air flow by removing restirictive air boxes and jetting up to compensate does help alot.

But after doing this, the only way to see siginificant gains in power would be to install larger valves, and/or perhaps a less restrictive exhaust....along with opeing the exhaust ports if needed. And hotter cams, perhaps a supercharger or NO2 tank, rockets...yeah, rockets would work....LOL


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