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  1. #1

    I want to take my already awesome 15F and mod it to be better need help

    Hi there again I'm new here. After all my researcheverything has pointed me towards Riva for making my already great factory STX15F a tad bit better without adding problems. My post may be a little long but I want to beas specific as possible so Iím giving the best advice. I noticed at 35mphcruising speed she titters at times. Also at low turning speeds sometimes thesponsons catch and can cause her to jolt suddenly. At WOT she runs great noproblems. With a full tank gps speed she runs 60 mph I weight 170. With halftank and lower she runs 63mph. Iíd like to add 3mph without losing hole shot. Theonly friend I always ride with has a Yamaha FX/SHO. My wife likes the 250 260ultras and Yams FX cruiser HO not because of performance but because of thehigher seating and comfortable ride with better stability. I would like to adda little more mph at top end to hang around skis like these. Iím not looking todo 70mph. I love how my 15F rides now. Thereís no cavitation and it has great throttleresponse and low end. The fuel economy is good so I donít mind losing a littlethere. I do not want to have to run higher octane and I do not want cavitation.
    What type of riding I do: I usually ride in rivers and lakesbut their usually choppy from all the other boat and pwc traffic. This style isusually play time wave jumping or just cruising. A lot of starting andstopping.
    Other style is riding in the bays and inlets which isusually choppy 2 to 3 waves. Sometimes for fun but mostly for fishing. Yesfishing I have a humminbird combo setup and rod holder cooler setup.
    I already know I want to add Riva Sponsons and ride plate. Iwould like to add a better airbox or cold air intake, possible fuel tuner, andtweak the exhaust. Id like to get rid of second waterbox maybe add freeflowpipe or have work done to first water box which ever is better. I donít want tofool with timing. My theory is to add a little more hp from exhaust and intakeand then an impellar. I have searched high and low to find out what the stockimpellar is. I have heard it is a 13/19. Anyhow if I added a ride plateSponsons different intake and freeflow exhaust what impellar would be recommended?Id like to keep the 13 not go to 14. In my mind a 13/21 sounds like it would beperfect for what I want but Iím no pro. So if I do your rpm kit what impellarwould be suited best for what I want main thing is no cavitation more hole shotand more top end. Thanks hoping to hear from some pros on what to buy myselffor Christmas.


  2. #2
    BigHaag's Avatar
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    I have the R&D intake and ride plate, stock sponsons, k&n filter with the stock air box removed. I have not replaced the impeller. I have seen about the same top speeds as you (all dependent on conditions)...the handling is great with the intake and ride plate combo. I have raced this setup and did not have any complaints...by the way, if your ski is newer (09-14), you don't need the exhaust mod Riva sells. I know R&D sells the fuel tuner but I do not know if the performance gains are really worth the money...

  3. #3
    BigHaag's Avatar
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    By the way, it looks like Riva has stopped selling a lot of their 15F parts. I have read here, R&D parts are harder to get as well...Good luck in your search.

  4. #4
    Thanks for the response. I was hoping to get a little more out of her but sounds like im going to have a hard time. I would like the k&n air filter. I have an 07 so exhaust is old model. I would love to find out about impellers. I dont feel like buying and trying until I get it right.

  5. #5
    I can't even find an rpm kit or performer kit for my ski. Out of stock everywhere. I really liked the rpm kit. It came with R&D ride plate, solas 14/21 impeller, K&N air intake, Riva freeflow exhaust, and Riva sponsons. The weird thing is this kit claims 5mph more while the performer kit claims the same. which is R&D ride plate, R&D intake, riva freeflow exhaust, solas 13/19 impellar, another air intake, and R&D sponsons. This one is for ruff water. Cant see how they both do the same thing. Guess I have to find things here and there and do some dyi mods.

  6. #6
    03stx12f's Avatar
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    Try eBay for the ride plate. I had one on my 08 and It gained two mph with it alone. You can also make your own intake with parts from eBay. You need a piece of aluminum L pipe a couple of silicone couplers and cone intake filter. This will mount the filter in the rear bucket and gives it a real nice sound to it. Do a search on here for intake mod 15f. Or cold air intake mod 15f. Shredmaster makes a plate for the 15f too.

  7. #7
    RLACEMAN's Avatar
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    An aftermarket intake to replace the stock airbox is your best bet, that gave me 1.8mph according to my GPS, & really increased the holeshot! I have tried the following impellers, 14/21-13/19-14/20-13/20. Best hole shot is the 13/19, best top speed is the 13/20, but only by 1/2 mph & has some cavatation on hole shot. If you are at sea level you may be able to use the 14/21, for me it hurt hole shot & was slower than with a 13/19(I am 900ft above sea level) If your stock impeller is in good condition leave it in.(its a 13/19 similar to the Solas) A Shredmaster ride plate will get you 2-3 mph, an R&D does also, but is no longer available. Removing the 2nd muffler will gain you nothing but noise, it has twice the flow capacity of the first one, I cut one apart to find out for sure. With these mods you will also gain fuel efficiency, & the nice thing about the 15F is that it does this using 87 octane crap gas.

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  9. #8
    thanks for all the advice. I decided to leave the exhaust alone. im going to stick with just changing air box to cold air intake and change ride plate. I found a really nice K&N intake I like. K&N Apollo Universal Closed Intake Systems. This way you can hook up piping from outside the hull and pull in cold air using pipe. I like it. its a very tapered cone that's enclosed. you suck air in from the bottom bigger diameter. It's the closes thing to true cold air intake and possibly forced air if done right. plus you can add a water resistant cover to protect engine from water getting in. I had a K&N cold air intake on my truck and noticed a big difference so I'm going to diy it for now and over the winter I'll add something nice. For now i'm just running a hose from air box to outside of hull for cold air. I'll get a ride plate over winter. I decided if I want speed I'm going to find a new 2013 left over 310lx for the wife but its really for me. I wont fish off of that one lol thanks again you guys saved me a lot of time and money

  10. #9
    Banned User
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    You going inside the motor?

  11. #10
    How are you routing cold air to the intake? I have been trying to figure out how to grab the colder air from the front of the ski, but there really isn't much room in the hull to route hoses along side the engine. I saw where one guy actually mounted his air filter up front, but don't know where he routed the hose.

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