Page 1 of 2 1 2 LastLast
Results 1 to 10 of 14
  1. #1
    bowsniper's Avatar
    Join Date
    Aug 2008
    Location
    macomb,michigan
    Posts
    5,896
    +1
    6

    Octane and Compression Numbers

    If a stock 785 pro has a 7:1 comp ratio and thats at about 130 psi right? Then whats the comp level at 140 ,150, 160 ,170 ,180, 190,200 psi? 14:1 or higher?

    Anyone have any insight on this?

    I heard that at 150psi you would need to run 93 correct? if thats true, you would need to be running at least 10.5 or higher for the need to be running premium no? In my old camaro, I had 10:5/1 pistons and ran 93 with it.ran great. old school cars that had a 11:1 ratio demanded premium. i know 9.5/1 is the cut off for 87 octane. Is there a chart for this stuff?

    why use 93 octane when its only a 7:1 ratio? its not needed. other than to get maybe 1 horse or hope it staves off pre-ignition? Or am I missing something here.


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
    Join Date
    Jul 2007
    Location
    near Toronto, Canada
    Posts
    36,601
    +1
    1,277
    There is a lot more than compression involved when figuring out what octane is 'enough'.

    Since compression isn't even measured when the engine is running, let alone running at high RPM, it is simply a short cut number to be used as a starting point.

    That means compression results don't say anything about the dynamic effects at high RPM, like the cylinder pressurizing effect of tuned exhaust pipes, or the effects of cylinder porting, intake porting.

    Not to mention ignition timing, spark plug heat range, and so on.

    In general, if your engine is not suffering from detonation or uncontrolled combustion then you have 'enough' octane. If you make changes to the dynamic situation inside the combustion chamber, then you may or may not need 'more' octane. When will you need more, and how much more - well those are the harder questions.

  3. #3
    You probably could run 87 or 89 no problem on a stock Pro 785, its just not that great of a idea. It can be hard to tell whats going on under the seat at WOT... Its cheap insurance against detonation, to just run 93. Plus, lots of 93 octane fuels are ethanol free.

    And technically you loose power with higher octane fuels. You only would gain power if the engine was to realize you have a higher octane fuel, and advance the timing. This may be true in some cars, but not a older PWC.

  4. #4
    bowsniper's Avatar
    Join Date
    Aug 2008
    Location
    macomb,michigan
    Posts
    5,896
    +1
    6
    From what I understand about it is, the higher the compression,the easier it is for low octane fuel to ignite before TDC. if thats the case, you wouldnt be at TDC to get the best power from that cylinder. higher octane is needed to prevent the fuel from igniting before the piston hits TDC. Higher octane allow you to increase the timing also. Higher octane doesnt make more power, it allows you to configure the engine in such a way that it creates more power by timing, compression ratio and other variables like keith said.

    I like the idea of "ethanol free" at 93 octane too! all the gas around here is crap. Hard to find true non ethanol free 87 octane

  5. #5
    Watcon's Avatar
    Join Date
    Oct 2005
    Location
    Seattle WA
    Posts
    872
    Everybody re-read the post from K-447... He is spot on the $$ with his response..
    The actual compression ratio is only one of the factors for determining Octane.

    Regards,
    Randy

  6. #6

    Join Date
    Aug 2006
    Location
    Homer Glen Il.
    Posts
    8,704
    +1
    109
    Perfect example is Hyoctane's 1010 motor he is only running around 130-145 lb compression but since his pipes where played with he must run 110 octane or better .

  7. #7
    I don't know if ANY 87 octane fuels are ethanol free...

  8. #8
    PolarisNut's Avatar
    Join Date
    Dec 2006
    Location
    Lower Michigan
    Posts
    1,809
    Yup, K447 is right. Cranking compression doesn't really mean much. If all you do is raise the exhaust port, you will lose cranking compression...but that certainly doesn't mean you can run less octane. If anything, the octane requirements will go up. Some engines require premium when they only have 120psi cranking, some don't require it at 150. Porting, head design, timing, pipe design, rpm, temp, load, etc. all come into play.

  9. #9
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
    Join Date
    Oct 2006
    Location
    Cleveland OH
    Posts
    17,330
    +1
    31
    Quote Originally Posted by bowsniper View Post
    If a stock 785 pro has a 7:1 comp ratio and thats at about 130 psi right? Then whats the comp level at 140 ,150, 160 ,170 ,180, 190,200 psi? 14:1 or higher?
    If your PRO only has 7:1 compression, it's no wonder it doesn't run right.......

    Everyone else's PRO comes with 11.8:1 compression.

    So go ahead and run 87 octane in yours!!!!!!

  10. #10
    PolarisNut's Avatar
    Join Date
    Dec 2006
    Location
    Lower Michigan
    Posts
    1,809
    Quote Originally Posted by xlint89 View Post
    If your PRO only has 7:1 compression, it's no wonder it doesn't run right.......

    Everyone else's PRO comes with 11.8:1 compression.

    So go ahead and run 87 octane in yours!!!!!!
    It's probably 7:1 "corrected" compression...That's the compression ratio once the piston closes the exhaust port, since a 2 stroke can't actually build compression until then. The Japanese like rating 2 stroke compression ratios like this...not seen as much over here.

Page 1 of 2 1 2 LastLast

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. 96 xp water in the hull and compression numbers
    By snoopy'sbitch in forum 2-Stroke Performance
    Replies: 10
    Last Post: 08-16-2009, 08:43 PM
  2. Womack old Virage, Octane, and Pro 785
    By xlint89 in forum Polaris Classifieds
    Replies: 9
    Last Post: 07-04-2008, 12:58 PM
  3. Second ride of season with new upgrades and new numbers
    By HYNES in forum Kawasaki PWC Performance (4-stroke)
    Replies: 21
    Last Post: 04-16-2008, 10:31 AM
  4. chk out these compression numbers for the kawi
    By dav_dman in forum Honda Open Discussion
    Replies: 0
    Last Post: 07-02-2007, 08:47 AM
  5. GPS and Tach numbers for analysis
    By MPower in forum Yamaha PWC Performance (2-stroke)
    Replies: 10
    Last Post: 04-16-2006, 08:26 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •