Scooterotica

Not being an expert by any means on tuning (complete novice, but willing to give it a go), but is widening of the exhaust port on the cylinder simply governed by the distance between the ring pegs. I have read threads about 65-70% of bore but don't follow why this percentage. Could someone elaborate, please? And I understand that widening of inlet/exhaust ports makes for a more torquey engine, whereas heightening of ports makes it more revvy. I understand that port timings affect how much fuel/exhaust can be inducted/exhausted whilst the port is open, but widths of ports must have a similar effect, as although the port is open for a shorter time, the width of the port allows more fuel/exhaust in/out.

The maximum width of any port is limited by the material that the ring is made from,
jap pistons use rings made from steel which is very rigid and almost unsnapable (you can tie them in knots) they are less likely to pop into the ports when the engine is running, so you can go for port widths of about 70-72% of the bore with good port shaping, this will allow you to keep the port height down and help to produce torque rather than revs.

cast irorn rings are much more brittle and will deffinatly snap with ports of this size as they will pop into the ports, hence the 62% limit. but even that is only on ports that are quite oval.

cast iron rings do have one very good point that is they tend to last much longer because the iron contains plenty free graphite which acts as a lubricant

imagine say two barrels,both with say 180 degree exhaust ports ..but one is wider...
although both have the same timing ,the wider port gives more area.and so flows more.

same for inlets and transfer ports
its a time/area thing.

tzr.steve wrote:The maximum width of any port is limited by the material that the ring is made from,
jap pistons use rings made from steel which is very rigid and almost unsnapable (you can tie them in knots) they are less likely to pop into the ports when the engine is running, so you can go for port widths of about 70-72% of the bore with good port shaping, this will allow you to keep the port height down and help to produce torque rather than revs.

cast irorn rings are much more brittle and will deffinatly snap with ports of this size as they will pop into the ports, hence the 62% limit. but even that is only on ports that are quite oval.

cast iron rings do have one very good point that is they tend to last much longer because the iron contains plenty free graphite which acts as a lubricant

Whoops, I better start stuffing that steel back in the exhaust port I opened to 38mm on the LI 125 I am working on

oops 39mm is about tops for 57mm with good rings, you better off doing a 175 now

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Well, y'know, time flies, especially with them carbide burrs on a die grinder
Lovely looking port, only one owner, bueatiful plumage.
Well, he wanted to go faster, now he's going to be getting a cc diplacement increase for free as well as a port job.

So if I get a 175cc conversion piston and have my 150cc cylinder rebored out, does my reborer/tuner need to do anything to the existing ports/piston? ( I have already opened the transfers on the cylinder and matched the casings to them)
Would shaving a bit off the skirt of the piston on the induction side give a little power boost?
Should I tell him to just widen the existing inlet/exhaust/transfer ports?

Come on Guys, just looking for some simple tips/ideas for a little performance boost.

Whilst on the subject, what can be done by the home tuner to lighten pistons? What's safe and how far to go? Is it a No NO?

ere lopez i take it youve taken the port width measurement as a chord across it and not round the barrel have you ?


dapper, you will benefit from widening the exhaust to about 41mm which is about 65% you could go to about 43mm with a good wire rings
exhaust 170 degrees, inlet widened and lowered to give around 155/160 should be a nice job, ive done a couple with that sort of timings and they have been good
tell your tuner he will probably break into the inlet manifold stud hole when he enlarges the port, but its ok
30mm carb nice pipe and off ya go

jonny snatchsniffer wrote:ere lopez i take it youve taken the port width measurement as a chord across it and not round the barrel have you ?

Calipers in from the outside. I thought it was going to be on the money, but I used 72% instead of 62% for ma' figuring.

Does this formula also relate to 125's though? Because at 62% on a 125 is only 32.24mm and that my friends, is tiny. I'm sure it measured 33mm before I even touched it. i'll go and check the other 125 cylinders I have.