Can you port 31K cylinders to 1UA spec easily?

[JonW]

We all know the 1UA cylinder is different to the 31K, and most people cite that the exhaust port height is the difference, is that all it is? Or is there more to these cylinder changes?

[headcoats]

Does one have a lower inlet floor too ??

[midlifecrisisrd]

Don't quote me but I'm sure the inlets are slightly different too

I really don't understand why they fitted a higher exhaust port to a bike that made max power at 8.5k

They seemed to have a higher tune on the barrel then destroyed that with electronics and pipes

I'm sure Slinger told me the exhaust port duration on the 1ua barrel was too long and the long stroke conversion corrected that.

Come to think about it if you have the exhaust port closed later but the same squish and chamber volume surely you have reduced the compression

Also heard it said many a time that tuners prefer the 31k barrel

Steve

[JonW]

Yes, I've heard people say the 31K is the one tuners prefer, but my problem is that Ive a 'set' of cyls that have been run as a pair for a while on a bike, yet they are not a set. I was thinking it might be possible to at least get them to be closer to 'the same' with a bit of gentle porting, and I assumed it was easier to make a 31K into a 1UA than vice versa lol

[midlifecrisisrd]

May 13, 2019 8:49:09 GMT 1 JonW said:

Yes, I've heard people say the 31K is the one tuners prefer, but my problem is that Ive a 'set' of cyls that have been run as a pair for a while on a bike, yet they are not a set. I was thinking it might be possible to at least get them to be closer to 'the same' with a bit of gentle porting, and I assumed it was easier to make a 31K into a 1UA than vice versa lol

Yes a tuner would be able to match them

Just grind the markings off After

You need to dyno it first with 1 plug out each time to find out which one was better 🤣

Steve

[JonW]

Haha, now thats a good idea Steve... LOL!

I was hoping I might be able to match them myself if it was just the exhaust port height

[jessy03]

Jess

[marrcel]

Thanks Jesse
What about the piston holes are 31k and 1ua the same?

[JonW]

Thanks Jess, That does seem like most of the dimensions on the 1UA could be ground out of the 31k cylinder, there are only a few small bits that would be different then, hmm, tempting.

[reggieperrin]

Interesting picture as the only real big difference is the length of the lower transfer ports.
The exhaust height difference is only 1mm, although understandably that would make a difference.

[bare]

Have both types 'on hand'. UA cyls do have the visibly lowered inlet ports.
BUT.. what the maps don't reveal is that while the Sleeves are cut lower.. the Ali casting mold wasn't changed .
Soooo the lower port area in the sleeve faces Cast ali barrel.
One could? port (grind out) the barrel so air could flow there.. but the ali is Thin there and with small assurance of not hitting air in process.
More than a Bit stoopid to be honest.

[reggieperrin]

May 14, 2019 17:08:28 GMT 1 bare said:

Have both types 'on hand'. UA cyls do have the visibly lowered inlet ports.
BUT.. what the maps don't reveal is that while the Sleeves are cut lower.. the Ali casting mold wasn't changed .
Soooo the lower port area in the sleeve faces Cast ali barrel.
More than a Bit stoopid to be honest.

Thanks Bare, very useful info.
I would imagine that there must be even a small amount of difference though, even if the thickness of the liner is the only transfer area.
Without having both in front of me to be able to see, is there scope for removal of the aluminium casting to allow the beneficial use of the lower ports?

[bare]

I won't even try.
Curiously; competent porting work involves Raising the top of the inlet ports.. a lot.
Rarely (if ever) lowering them.. Something to think about :-)

[urbantangleweed]

There is a difference, in the same way as 34D is different to 36C, but get one next to the other, and you'd see they're both virtually the same thing, just different.