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  1. #1

    Anyone seen porting like this on a GPR

    Click image for larger version. 

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    I found this picture on a old for sale thread entitled - GPR 800/1200/1300 Triple exhaust ports/Big bore Cylinders

    I was wondering if anyone ports the cylinders like this with the aux exhaust. I have a Yamaha 125cc GP road bike that has a powervalve and the aux exhaust ports and was wondering what the porting masters out there thought.
    Maybe Lowell and others could chime in on the benefits or drawbacks they can see

  2. #2
    Happily Self-Employed WFO's Avatar
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    Nov 2005
    Houston, Texas
    That doesnt look like anything I have ever seen on a GPR

    Contact Lowell for info on those ports

  3. #3

    Join Date
    Jul 2005
    Central North Carolina

    Cool Trick porting

    I noticed that is a cylinder with a liner in it and not a nickasil plated cylinder. Does that cylinder have a power valve in it? Usualy you see this type of exhaust port when the engine builder wanted to have even a bigger exhaust port but can't make it any wider due to the fact that they were afraid the piston ring would hang in the exhaust port due to the extream width of the exhaust port. Can you please post some more pictures of this porting from the bottom of the cylinder and the exhaust port looking in from the exhaust manifold side. This well may be some cylinders done by Tim Judge when he worked for Riva years ago, Thanks Tommy Jordan

  4. #4
    If you search for the thread you will see a few more and here they are as well. they look like 800 cylinders maybe.Click image for larger version. 

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  5. #5
    Happily Self-Employed WFO's Avatar
    Join Date
    Nov 2005
    Houston, Texas
    those are GPR cylinders, but not the sleeves

    the sleeves are something very different

    thats why I said get with Lowell as he deals with a lot of exotic stuff

    he rarely posts on here, so shoot him an email

  6. #6
    Yea I sent him an email to see if he has any insight.


  7. #7
    Hi Guys, just thought I'd post Lowell's reply for anyone that is interested. Here it is:

    No I have not seen this on a GPR. The cylinder is sleeved – I don’t know if it is a 66E / 800 or something else Yami made. I know both Polaris and SeaDoo /SkiDoo used their snowmobile cylinders – which are triple exh. port – on the watercraft. Most of these were low runs for racing / closed course years ago. Some sled / snowmobile owners bought the watercraft bottom ends to put sled cylinders & pipes on to make open class sleds.

    It is not feasible for me to put the aux. exh. ports in. I have done it on sled engines – one (3 cyl. unibloc) was the last.

    For the triple exh. port to be effective you need the triple pipe, or possible a twin engine to use the compression a triple exh. will run. RPM is the other consideration. All your small bikes run the triple exh. for both compression and RPM.

    I think you will find what others have found. Triple exh. will survive with lots of maintenance for Closed Course racing. Rec riding where you can hold WOT for minutes will cause heating and jetting problems to keep it running right.



  8. #8
    I must be a motor dork. I really get everything he is saying. Exhaust port duration is text book Calcalus !

  9. #9
    88kawi5fiddy's Avatar
    Join Date
    Feb 2015
    S. Utah
    just increases the blowdown duration. yz250's and most other modern bikes use triple ports with 3 powervalves. same principle as regular port timing. for that picture above to be effective, it would need cylindrical valves in the left and right small exhaust ports and a main powervalve. the transfers would also probably be raised up 1 - 3mm over stock to get good top end power. or without powervalves the motor would be a biatch worthless pile anywhere except top end, depending on porting.

    it just increases flow volume for pushing more air thru the motor. needs to get more air into the motor to get more out of it though.

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