Exhaust port 65%

[rowo62]

If we say that an exhaust port should measure 65% of the diameter of the cylinder and we know that the diameter is 69 mm, so that 65% would be 44.85 mm. How do we measure these 44.85 mm? Directly in the cylinder in a straight line from side to side of the port? Or we draw the port on a paper and then curve the paper into the cylinder? There is a big difference

I believe that it applies to the curved measurement not the cord (straight line), you need to roll up a sheet of paper and tape insid the bore and then do a 'rubbing' with a pencil to get the outline to the port then roll it out flat and measure with a ruler.

I've been doing a lot of this lately

i agree with this... you should do cylinder maping on it...

[jonny snatchsniffer]

my parma kit came with an exhaust port width of 70% of bore as a chord which means around the bore its more than 70%, is this ok then ? ive always done to 70% chord and never had ring fucks

[J1MS]

Across the chord, that's port edge to Port edge.

70% is the safe max for wire rings on an open port (across the chord) without bridges or dog teeth to aid the rings on the ports leading edge.

Normally when talking about port widths using the circumference of the barrel then it's done by degrees.

[coaster]

Across the chord, that's port edge to Port edge.

70% is the safe max for wire rings on an open port (across the chord) without bridges or dog teeth to aid the rings on the ports leading edge.

Normally when talking about port widths using the circumference of the barrel then it's done by degrees.

That being the case J1MS, there would seem to be a bit of confused information and advice floating around, Bells book shows ports measured using the port map method and he talks of 65% being the safe max for road use, so given his method of measuring I assumed he meant measred rounf the circumference . On a 225 lambretta barrel that would equate to 45mm round the circumference. Using 45mm across the chord gives a port 55mm round the circumference which is huge....I know, I have one and it snapped the bottom ring and buggered the piston within 700 miles. Admittedly it was only a 2 ring Asso, not wire rings. I've been given authoritive advice on the one hand that it is fine at 55mm and also that it is way too big and should only be a max of 45mm circumference...what's a chap to do

I really would like a definitive position on this subject

[J1MS]

Port mapping is not what I'm saying but port mapping will give the same width if it's done correctly as measuring across the chord (port edge to port edge)
If you do a port map you basically do a rubbing of the barrel from inside the bore.
To transfer these measurements you wrap the rubbing around tge piston to give the same or similar curve these points (the rubbings or port tracings) are then transferred to graph paper to give actual port widths.
This is the same measurement that you will get with internal calipers or a pin gauge when measuring the port directly port edge to port edge.

The cord measurement will still be the same, but spreading the measurement by unfolding the previously rolled up paper then measuring the curve as a flat surface is not mentioned or refered to in my post.

Sorry for any confusion.

If my post is read as intended then it will Make some sense.

The reference to degrees is separate from this and as a 360 degree measurement looking down the barrel then the open ports take up x amount of degrees of the 360 degrees of the barrel surface.
This x degrees is a known maximum value and some tuners in the past have refered to this type of measuring of port widths.

[coaster]

So, if I understand it correctly, an exhaust port on a 225 (70mm) barrel which is 45mm across the chord would be concidered as 65% of the bore width and therefore acceptable for a reliable road going set up without resporting to exotic wire ringed pistons?

Sorry to labour the point J1MS but your opinion is valued

[J1MS]

So, if I understand it correctly, an exhaust port on a 225 (70mm) barrel which is 45mm across the chord would be concidered as 65% of the bore width and therefore acceptable for a reliable road going set up without resporting to exotic wire ringed pistons?

Sorry to labour the point J1MS but your opinion is valued

Yes 45mm is approx 65%. That's the definitive answer...
There a few variants even in the composition of high quality wire rings and also ring profile and surface coatings which allow some wire rings to span a gap of 70% with greater or lesser risk of the ring snagging.
But cast iron rings also have variants, there are plain cast iron rings as fitted to lambretta pistons of old. Then there are high quality surface coated, ductile cast rings with beveled edges.
A plain cast iron ring will normally only take upto 62%. But the higher quality coated and profiled ductile cast iron piston ring will still snap if a port is over sized but less likely to snag or snap than a plain cast ring.
I am usually very conservative in any porting as reliability is what I'm trying to attain not performance.

[Diablo]

Mark is has explained it how I have understood it. If you read bells book he does say about producing portmap rubbings but then transfering them onto the piston to work out things like ring peg movement and how far you can open the ports if not.
If you have Robinsons book he explains it in degrees. I think from memory he quotes 90 degrees as being the safe maximum.

[santacama]

My interest in this matter comes from the exhaust port proposed in the following link:

http://i234.photobucket.com/albums/ee97 ... 1_road.jpg

51.25 mm represents 74% of 69 mm. (Malossi bore)
51.25 mm measured in a straight line is 8.25 mm longer than the original 43 mm (62,3% of 69) measured in a straight line at the Malossi cylinder. But if they are considered (the 51.25 mm) in a circumference, the difference is not so great.

[coaster]

J1MS and Al, thanks for your input so far but I'm still not fully getting this I get the notion of transfering the 'rubbing onto the piston but that was surely just to check that ring ends will clear the port not for measuring the port width and I can only imangine that tracing the port shape onto graph paper would be in order to calculate port area. Sorry if I'm being dense

I've not read Robinsons book but 90 degrees, that's a quarter of the circumference