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  1. #1

    Smile Need Help choosing Impeller For a 2021 GP1800 SVHO 13/18 or 13/20

    Hey guys this is my first post .

    I have a 2021 GP SVHO, love the machine just want to make it a little faster.

    I currently have the Macboost Stage 1 (breather , intake , bov) [it’s awesome] .

    I plan on doing maptunerX 8000-8500 tune ,Riva ride plate , Riva waterbox, Riva free flow and Impeller .

    I need help choosing the best impeller with my mods solas 13/18 or 13/20 . Looking for a faster ski but also something I can cruise and it be smooth. I’m not looking to go the fastest but I want to go just a bit faster . Whichever is better on gas is a plus+, whichever is more reliable with the rpm’s is better too.

    ive received one recommendation for the 13/18 and one for the 13/20.
    So I’m confused , please help so I can order all these parts 😂
    Last edited by K447; 10-07-2021 at 10:30 AM.

  2. #2
    Tucker1's Avatar
    Join Date
    Jul 2019
    Lake Cumberland
    I have a 2021 GP1800 SVHO also and I'm running a 13/20. Mods are all Riva; intake, exhaust "factory outlet", engine breather, ribbon delete, Maptuner X tune. I am hitting 80mph and 8300ish rpm then after heat fade 77-78mph at 8000rpm. I chose the the 13/20 because I have an intercooler waiting to go on which will get me in the 8500 range I'm hoping. I just want to stay under 8600 because I won't be doing the valve retainers. If you don't plan on doing any more mods I would go with the 13/18 to get RMPs up. When I switched from OEM to the 13/20 I didn't really notice much difference because my rpm stayed the same but just waiting Riva the ship my cooling kit Hopefully someone will chime in with a 13/18 with some numbers. I also just installed a Riva Gen 2 plate which I have not water tested yet.

    My numbers are all GPS with a full tank of 93, 180lbs of rider and gear, water 80*, air temp 80* and high humidity.

  3. #3
    Click avatar for tech links/info K447's Avatar
    Join Date
    Jul 2007
    near Toronto, Canada
    Quote Originally Posted by Hleon1425 View Post
    … 2021 GP SVHO …

    I currently have the Macboost Stage 1 (breather , intake , bov)

    I plan on doing maptunerX 8000-8500 tune … Impeller .

    … can cruise and it be smooth. … just a bit faster. Whichever is better on gas is a plus+, whichever is more reliable with the rpm’s is better too.

    … I’m confused ..l order all these parts …
    Be careful with using ‘Stage’ numbering.
    What Stage 1 means varies somewhat depending on the vendor and the engine/model.

    Each tune has a list of required engine modifications, and also a list of optional or recommended engine upgrades.

    Often for a Stage 1 tune there are ZERO required changes to the engine. The tune is programmed to increase performance with an completely unmodified machine.

    For GP1800 SVHO MaptunerX tunes from Riva (and you should check this yourself) the Stage 1 Plus tune is intended for SVHO engines that have mild changes, including an aftermarket cold air intake and/or flame arrestor ribbon disc delete. Those physical modifications have increased the amount of air that can flow into the engine so the tune is programmed to match that added air with added fuel and thereby create more engine power.

    Here is the caveat. If you change the engine (add a cold air intake, for example) and do not install a tune that properly matches the modifications then the engine may run lean and risk engine damage.

    Find the correct tune that requires (or allows) the physical changes you have made, and use that tune. If it is labeled Stage 1 Plus to match a cold air intake, use Stage 1 Plus. Do not use a Stage 1 tune if that tune is incompatible with increased air flow.

    Not that most changes to the exhaust system do not influence the tune selection, as typical exhaust modifications do not affect engine performance. They just change the exhaust sound.

    Regarding impellers, as the pitch is reduced the engine RPM at wide open throttle will increase. The faster spinning engine will make more power and jet pump thrust, and therefore you go faster. A faster turning engine will generally consume more fuel per mile.

    If you select an impeller with a ‘steeper’ pitch then the engine will turn less RPM and therefore fuel economy ‘improves’ (by how much can vary). The engine will not be able to produce as much power since the steeper impeller is reducing the RPM and that impeller may be ‘slower’ at WOT.

    All impellers should be ‘smooth’ while riding. Some impellers will have a tendency to cavitate during very hard acceleration from low speeds. Cavitation reduces thrust and creates noise as the engine revs without producing as much thrust. As forward hull speed increases cavitation effects subside.

    An impeller that cavitates less is considered ’better’ but there are multiple factors in play when choosing an impeller.

    The primary factor when choosing an impeller is what target RPM you want the engine to run at when going wide open throttle. The tune (which is influenced by the engine modifications) and the impeller determine the actual RPM the engine will run at.

    The more RPM the engine turns the faster you can go. Higher RPM burns more fuel, generally speaking.

    If you wish to reduce risk of warranty claim denial then choose an impeller that will hold maximum RPM to just under 8000. You can also choose a tune with 8000 limiter but the impeller must also be selected for this.

    The next level is tunes and impeller that allow the engine to spin more than 8000 but not more than 8600. More RPM = more speed, also more fuel burn.

    Beyond 8600 means upgrading the engine internal mechanicals, including lightened valve train hardware. I expect you are not interested in doing that sort of changes.

  4. +1 by:

  5. #4
    Reaper9789's Avatar
    Join Date
    Jul 2014
    Canyon Lake, TX
    Stick with the 13/20

  6. #5
    Tucker1's Avatar
    Join Date
    Jul 2019
    Lake Cumberland

  7. +1 by:

  8. #6
    Saum700p's Avatar
    Join Date
    Sep 2017
    New Orleans
    Under 85 13/20 over 85 13/18 ...ish Its well worth the money to order it from someone who pitches them and then sends them to you.

  9. #7

    Join Date
    Jul 2020
    K447 has summarized this perfectly here.

    I would say, if you want to run the 13/18 make sure you have someone who knows what they are doing with regards to pitching and RPM. It makes a big difference. I am running a solas 13/18 and it is a significant upgrade to the stock impeller.

    The pics show the difference…stock impeller on left, 13/18 (pitched) in middle and 13/20 on right.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  10. #8
    mittens's Avatar
    Join Date
    Mar 2018
    Huntsville, AL
    All that is correct but saying the lower the pitch the faster is nor correct. it can be faster, but if you have the power to spin the bigger prop to same RPM its not faster.

  11. #9
    Impulses's Avatar
    Join Date
    Dec 2011
    My prop is totally stuck to the sacrificial hoop of my SVHO (too much time of inactivity) without any kind of modification, do you recommend this combination?

  12. #10

    Join Date
    Aug 2015
    West Unity, OH
    I have a 21' gp with and 13/18, riva power filter, fizzle bov, ribbon delete, riva gen 2 grate, riva 93 octane aggressive tune 8650 rpm and run 8460 rpm at 900 elevation. Best ive seen was 80 on gps and 84 on dash. No gain for me on oem grate vs. Gen 2.

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