port polishing

[bike_rider250]

I know this topic has been discused lots but i did a search and now i'm a little confused, when porting and polishing is it polishing the piston, cylinder and exhaust port? I thought it was just the piston but after doing a search i read some things and i'm not sure could some one just explain what is done when people say they have there bikes port and polished

 

[mattb348]

I know this topic has been discused lots but i did a search and now i'm a little confused, when porting and polishing is it polishing the piston, cylinder and exhaust port? I thought it was just the piston but after doing a search i read some things and i'm not sure could some one just explain what is done when people say they have there bikes port and polished

you take a sander to the inside of your cylinder, your piston, and your head.

Just kidding :)

I don't know what the heck it is either, sorry :)

 

[oldfrt613]

Polishing would apply to the piston crown, combustion chamber and exhaust port - primarily to reduce the rate of carbon build-up. Porting refers to modifying existing cylinder passages for intake charge and exhaust gasses to improve flow characteristics for given application. Intake passages should not be polished as a slightly textured surface helps keep the intake charge mixed at boundaries - at least I think that's what you are refering to !

 

[robwbright]

From Eric Gorr's Motocross and Off Road Performance Handbook:

Porting is a metal machining operation that includes general smoothing and matching of ports with port timing changes made at the intersection of the port windows and the cylinder bore.

Cylinder ports are designed to produce a certain power characteristic over a fairly narrow rpm band. Porting or tuning is a metal machining process performed to the cylinder ports (exhaust and transfers) that alters the timing, area size, and angles of the ports to adjust the powerband to better suit the rider's demands. . .

The most critical measurement is the port-time area, the calculation of a port opening's area and timing in relation to the displacement of the engine and the rpm. . .

In general, if a tuner wants to adjust the engine's powerband for more low to midrange, he will do the following:

1. Turn down the cylinder base on a lathe to increase the effective stroke (Distance from TDC to exhaust port opening). This also retards the exhaust port timing, shortens the exhaust port duration, and increases the compression ratio.

2. Narrow the transfer ports and reangle them with epoxy to reduce the porttime area for an rpm peak of 7,000. The rear transfer ports need to be re-angled to oppose each other rather than pointing forward to the exhaust port. This changes the flow pattern of the transfer ports to improve scavenging efficiency from 2000 to 5000 rpm.

For both of these types of cylinder porting changes to be effective, other engine components need to be changed as well.

 

[bike_rider250]

ok well i'm understanding it a lot better now, but i got another question. so if you get it port and polished when you do a top end do you have to re-do the hole port and polishing or just the piston crown? thanks for the quick posts also.

 

[robwbright]

The port/polish should last until you have ti have the cylinder replated . . . then I suppose the guy doing it would rework the ports after plating.

I was not personally aware that the crown of the piston was involved in the porting process, but maybe that's the case . . . never heard Eric talk about it tho.

Of course, there are different piston crowns manufactured by Wiseco or whoever (flat, normal).

 

[Rich Rohrich]

I was not personally aware that the crown of the piston was involved in the porting process, but maybe that's the case . . .

Other than choosing a flat or curved top piston to match the combustion chamber requirements the piston isn't involved in the porting process. Guys who are "polishing" things in engines are just wasting time that could be better spent reviewing their old physics and chemistry books from school. ;)

 

[robwbright]

Other than choosing a flat or curved top piston to match the combustion chamber requirements the piston isn't involved in the porting process. Guys who are "polishing" things in engines are just wasting time that could be better spent reviewing their old physics and chemistry books from school. ;)

That's what I suspected . . . Thanks Rich

 

[bike_rider250]

ok thanks for the posts