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  1. #1

    Difference between 12/19 & 15/21 props

    Anyone know what the effect will be of changing from a 15/21 to a 12/19 prop, in terms of rpms and where it will change the power distribution bias (holeshot or top speed).

    I know I'm currently over propped at 15/21 (was planning on adding ET wheel etc, but decided to wait until I overhaul engine and check alignment etc). Therefore, thinking of running the 12/19 prop that I have kicking around. (just want to know if it will give me more rpm's before I drag all the IBR / bucket / pump off to swap - my nemesis).

    If I do change, I'll log data before and after and post for future reference for others.
    Last edited by tornado34; 04-08-2015 at 03:38 AM.


  2. #2

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    What kind of RPM are you turning now at what air temp? 15/21a does feel a bit tall for your mods. Probably 15/19R would be better. 12/19 seems a bit low on the leading edge, probably ok on the trailing. I wonder if you'll have cavitation issues with the low a pitch on the leading edge. I'm definitely not an impeller expert though. Since you added springs, your loosing RPM maybe with the IC your back even. You need some power adders, intake flame arrestor removal, spacer at least and maybe super charger and cam upgrade. Your setup for it other than maybe flywheel bolt upgrades. The easiest bang for the buck is removing the flame arrestor.

    I'm going with 14/18 skat trak swirl and an 8375 vtech tune for my AS based on some conversations with Jerry. Once I pull the deck off I'll do the flame arrestor, spacer, IC. I want the acceleration and don't care about loosing 1-2mph on the top end. I have a cam and a few big super chargers laying around, not sure how far I'm going to go with the AS being that it's so hard to work on.

    IBR gate/pump is easy after you do it a few times. I can have it off in 30min. The only really tricky part is pulling that back bolt and spacer off on the right side. Once the bolt is loose I use a long needle nose to pull the spacer out with the bolt in it. Between my 3 ski's I've probably done it at least 10 times.

  3. #3
    Quote Originally Posted by DinoMite View Post
    What kind of RPM are you turning now at what air temp? 15/21a does feel a bit tall for your mods. Probably 15/19R would be better. 12/19 seems a bit low on the leading edge, probably ok on the trailing. I wonder if you'll have cavitation issues with the low a pitch on the trailing edge. I'm definitely not an impeller expert though. Since you added springs, your loosing RPM maybe with the IC your back even. You need some power adders, intake flame arrestor removal, spacer at least and maybe super charger and cam upgrade. Your setup for it other than maybe flywheel bolt upgrades. The easiest bang for the buck is removing the flame arrestor.

    I'm going with 14/18 skat trak swirl and an 8375 vtech tune for my AS based on some conversations with Jerry. Once I pull the deck off I'll do the flame arrestor, spacer, IC. I want the acceleration and don't care about loosing 1-2mph on the top end. I have a cam and a few big super chargers laying around, not sure how far I'm going to go with the AS being that it's so hard to work on.

    IBR gate/pump is easy after you do it a few times. I can have it off in 30min. The only really tricky part is pulling that back bolt and spacer off on the right side. Once the bolt is loose I use a long needle nose to pull the spacer out with the bolt in it. Between my 3 ski's I've probably done it at least 10 times.
    Last summer, air temp 25C, water 17C at sea level I could only manage 8100/8150 (logged boost at max 11psi and WOT afr @11.9) The springs hurt me, but the GEN3 balanced that out. I will eventually put on a Rotax 137 +2 wheel, so that it's a bit kinder on the SC slip (I don't wanna keep pulling the SC to check/adjust) - was seeing GPS 72/73mph.

    The flame arrestor is out (IMUK).

    My objective is to get the rpm up to 8500/8600, keep my ski as hooked up as it is now (rock solid planted - no cavitation) and to get a solid summer 75mph.

    The 12/19 prop is actually the stock prop, I measured it this morning.

    I feel that the prop I have on at the moment, 15/21, will get me just about right with the extra 3 or 4psi boost from the Rotax wheel once fitted.

    For now, I just want to get the rpms up and take some of the stress off the flywheel bolts (will do these when I do SC, alignment etc).

    I've done the IBR on / off maybe 4 or 5 times, but each time I get it back on, I count myself lucky and that I've used up one more chance of luck!! Maybe because each time I clean all the bolts from Loctite, then re-apply without getting them greasy on the way back in, whilst wrestling the 'hand chopper' mechanism, makes it more difficult. Oh and remembering not to screw up the right hand hull inserts - oh the joys.

    As for working on an aS, it stops you testing things out as you would do on a normal ski. even simple jobs become long and fiddly. I love my aS, but boy do I swear often at the PITA.

    I am relying on you to be the first (that I've heard of) to shoehorn a turbo in an aS ski - don't wimp out now, the world is watching and waiting.....

  4. #4
    NateRXP's Avatar
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    All those mods and your doing 72/73??? Unless someone ran their ski then went back to the ramp and did a complete Valvetrain job then ran it again same day you can't really determine a loss of rpm from Valvetrain upgrade. Throw on the 14/19 and give Chris a shout for one of his Cams.

  5. #5
    Quote Originally Posted by VXRslayer21 View Post
    All those mods and your doing 72/73??? Unless someone ran their ski then went back to the ramp and did a complete Valvetrain job then ran it again same day you can't really determine a loss of rpm from Valvetrain upgrade. Throw on the 14/19 and give Chris a shout for one of his Cams.
    The issue lays with such a tall prop. I know that to drive it at the rpm that I'm looking for I need to increase power (cam, sc ect) - I'm trying to just up the rpm's for now, by changing out the prop, but didn't want to go backwards in doing so (hate pulling IBR / pump, so don't wanna have to do it twice for no gain).

    Looking for thoughts on the likely outcome of the prop switch before I do it.

    At the end of the season I'll pull the engine, rebuild the internals, fit new mounts and set alignment. At this stage I'll change the SC wheel, upgrade flywheel bolts and make the power needed to drive the 15/21 at 8500ish. plan is to try and hit a solid summer 75mph without over stressing the engine.

  6. #6
    Bob 1tommygunner1927's Avatar
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    Quote Originally Posted by tornado34 View Post
    Anyone know what the effect will be of changing from a 15/21 to a 12/19 prop, in terms of rpms and where it will change the power distribution bias (holeshot or top speed).

    I know I'm currently over propped at 15/21 (was planning on adding ET wheel etc, but decided to wait until I overhaul engine and check alignment etc). Therefore, thinking of running the 12/19 prop that I have kicking around. (just want to know if it will give me more rpm's before I drag all the IBR / bucket / pump off to swap - my nemesis).

    If I do change, I'll log data before and after and post for future reference for others.
    You should gain about 300-400+ rpm and about 3 mph!

  7. #7
    Quote Originally Posted by 1tommygunner1927 View Post
    Quote Originally Posted by tornado34 View Post
    Anyone know what the effect will be of changing from a 15/21 to a 12/19 prop, in terms of rpms and where it will change the power distribution bias (holeshot or top speed).

    I know I'm currently over propped at 15/21 (was planning on adding ET wheel etc, but decided to wait until I overhaul engine and check alignment etc). Therefore, thinking of running the 12/19 prop that I have kicking around. (just want to know if it will give me more rpm's before I drag all the IBR / bucket / pump off to swap - my nemesis).

    If I do change, I'll log data before and after and post for future reference for others.
    You should gain about 300-400+ rpm and about 3 mph!
    WOW, return to stock and win rpms and top speed, has to be the new future of modding!

  8. #8

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    Quote Originally Posted by tornado34 View Post
    Quote Originally Posted by 1tommygunner1927 View Post
    Quote Originally Posted by tornado34 View Post
    Anyone know what the effect will be of changing from a 15/21 to a 12/19 prop, in terms of rpms and where it will change the power distribution bias (holeshot or top speed).

    I know I'm currently over propped at 15/21 (was planning on adding ET wheel etc, but decided to wait until I overhaul engine and check alignment etc). Therefore, thinking of running the 12/19 prop that I have kicking around. (just want to know if it will give me more rpm's before I drag all the IBR / bucket / pump off to swap - my nemesis).

    If I do change, I'll log data before and after and post for future reference for others.
    You should gain about 300-400+ rpm and about 3 mph!
    WOW, return to stock and win rpms and top speed, has to be the new future of modding!
    stock prop sounds like the easy answer for now didn't realize it was 12/19

    im usually one for torqie to spec and loctite but not on the bolts that hold the Ibr on. I clean them and then snug them. I like them easy to come out. I check them regularly and never had an issue.

  9. #9
    Bob 1tommygunner1927's Avatar
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    Quote Originally Posted by tornado34 View Post
    Quote Originally Posted by 1tommygunner1927 View Post
    Quote Originally Posted by tornado34 View Post
    Anyone know what the effect will be of changing from a 15/21 to a 12/19 prop, in terms of rpms and where it will change the power distribution bias (holeshot or top speed).

    I know I'm currently over propped at 15/21 (was planning on adding ET wheel etc, but decided to wait until I overhaul engine and check alignment etc). Therefore, thinking of running the 12/19 prop that I have kicking around. (just want to know if it will give me more rpm's before I drag all the IBR / bucket / pump off to swap - my nemesis).

    If I do change, I'll log data before and after and post for future reference for others.
    You should gain about 300-400+ rpm and about 3 mph!
    WOW, return to stock and win rpms and top speed, has to be the new future of modding!
    More rpm will have the supercharger producing more boost, but you'll need more fuel for the increased boost. Do this and your ski will be faster. Better holeshot too!

  10. #10

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    For reference.

    My 2010 TX260, has Riva IC, regulator set at 58psi, flame arrestor removed, exhaust manifold spacer, stock intake box, riva grate, and I run 72ish at 8400 on the limiter with a 15/19R solas, full tank of gas say 70f air temp. 126 hrs never logged it, early vtech tune over 60 hours ago at least. So early my ski always comes up in sport mode. I finally have vtech logger coming, I'll be very curious to see what the afr looks like. I'm about to take the prop out my AS260 and move it to the TX260. I suspect I'll need to pitch it to get the revs back down....will be interesting.

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