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  1. #1

    Advent v. compression

    Hey guys, just 2 questions to get your thoughts and recommendations:

    1. For my basically stock xlt 66v motor (w/d-plate/modified FF/grate/R&D plate/solas 12/1, would you recommend a higher compression head or coughin up the bucks for an Advent/advance timing? I'm guessin one advantage of the timing option is that I could run 87 octane(?)

    2. For my stage 2 gp1300, would you recomend Advent over the keyway?

    I'm wanting bottom/mid for both ski's.

    (I'd also even consider a stinger for the xlt for more bottom.)
    Last edited by jim mcgreehan; 02-07-2012 at 05:20 PM.


  2. #2

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    I would say just a keyway on your 1300. Don't think an advent was made for the EFI ski's.

    On your 1200...either one you do...you will have to run higher octane fuel. Add timing or compression will increase the need for higher octane. In my opinion an advent would give you more rpms.

  3. #3
    Moderator OsideBill's Avatar
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    I agree with Ron, and would add if you go with the higher compression head your bottom end punch will improve. I would highly recommend adding fuel by upgrading the jetting whichever direction you go.

  4. #4

    octane & rejetting follow-up question

    Thanks guys. I really appreciate the quick replies. I started to think of two follow-up questions about my decision. I didn't realize you had to run higher octane for the advance timing cause I was thinking that an earlier burn BTDC is what lower octane does anyway...so they'd be in sink with each other(?) It would seem that alternatively if you retard the timing you'd want a slower burning/higher octane fuel so it wouldn't burn too early/deto....I certainly could be wrong with that theory/idea...no?

    And this thought came to mind Bill, I know we've got to run higher octane for a higher compression head for fear of deto of the quicker burning lower octane fuel, but I read here that R&D said you didn''t have to rejet/go richer with their 91 octane rec. heads. I too assumed you had to rejet but am starting to think only for mods to intake/exhaust... . Any thoughts there?

    (Good example there was when I did my piecemealed stage 2 and did the FF I ended up with a lean mid range.)

  5. #5
    Moderator OsideBill's Avatar
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    Great example. The stock carbs on these are jetted to meet emission requirments, as we start modifying it is necessary to adjust for the changes. Removing the cat and raising compression would warrant an adjustment for sure. You still will not get past the midrange issue unless you do the standard upgrade mod to these. http://www.greenhulk.net/forums/yama...p-removal.html
    Personally I would never run a non modified stock set of carbs on these skis, it is too easy to upgrade and the performance gains make it a no brainer.

  6. #6
    slydogge's Avatar
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    Higher oct fuel burns slower but has more energy. Therefor, if you advance the timing (make it spark x amount before the piston gets to TDC" you need a fuel that burns slower. If you advance the timing without running a higher oct. fuel the fuel will burn to quickly and try to push the piston down before it reaches TDC (pre-ignition) This applies to about all gasoline piston driven engines.

  7. #7

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    I just realized you are still running stock carbs. I'm with Bill- Go with Flame arrestors and rejet the carbs first. It will help a lot with the midrange lean condition plus give a great performance gain. Probably more gain then advent or a head will do.

  8. #8
    Thanks dudes! i appreciate ya'lls input for sure! The only remaining thing I'm not quite getting is the octane advance timing issue. So if you advance timing spark x amount before TDC, and lower octane generally wants to ignite before TDC around that same time, then why is there still a need for slower burning fuel?
    Last edited by jim mcgreehan; 02-01-2012 at 04:46 PM.

  9. #9
    just an update...being on a budget right now, I opted for higher compression with the XLT stock head. I had .022 thou cut and ran just one gasket. That gave me .045 squish clearance. And so I went from 115 psi to a little over 130. Ok for a mild upgrade stage 1, + riva stinger mod.

  10. #10
    jlfx car audio's Avatar
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    So have u ran it since the uped compresson ? Mine is at 125 stock .and im filling the pump shoe and but a 04 gpr pump on it , stock pump had bad water burn on screw and stator.

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