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  1. #1

    Expected RPM drop with these impellers?

    Does anyone know roughly what RPM drop I will see if I go from a stock 2008 Speedster 215 hp impeller to a Solas 14/19 or even to a 15/20 while stock? I have some mods in progress and a little math on the horsepower differential throws me into the rev limiter. Mods are X-charger, 4 intake. When it's warm I see 7700RPM and my calc's show at least 8350RPM with my mods (warm), not to mention cold temps.

    Wouldn't it be nice if you could wear out a rev limiter.


  2. #2

    Join Date
    Jan 2007
    Lakeland, FL
    The 14/19 will be about right for you. Of course, put on your mods first and see; easy to over-estimate what mods should earn ya. Roughly, I found the 14/19 would drop your RPMs 150-200 from the stock prop, with no power changes. Don't even think about the 15/20, it's a huge jump from the 14/19, about another 400rpms.

  3. #3

    Post A test and the math you were never going to need.

    Thanks Scott. The 14/19 seems to be the most suggested impeller in my mod range. I was only seeing 50.5 mph loaded, wife, kids, cooler, full tank, and all our stuff. The RPM's were only about 7700 (warm water and air). Kind of disappointed, I pulled out the calculator when I got home and figured with half a tank, no passengers, no gear I should get 55.5 MPH at this same temperature! This morning I got out for a "light boat run" and the GPS was pretty much dead on the money at 55.5MPH: The water temp was the same, but the air temperature was marginally colder. I just ran a temperature correction for 10 degrees Celsius (from 25C) and I get a new calculated max speed of 57MPH and at least another 210 RPM for the engine. Of course I forgot to pay attention to my light boat RPM this morning... So I am satisfied my boat is performing at par with the reports I'm reading. X-charger and injector come in next Wednesday. I will post my results.

    For any one that is interested in the calculations I used:

    Speed VS Weight
    Take the square root of the ((heavy boat weight) divided by (light boat weight)). Now, multiply this number by the heavy boat speed.

    Let's say 2200lbs/1825lbs = 1.2055
    Take the square root of 1.2055 which equals 1.098
    Now multiply 1.098 times your heavy boat speed (mine was a depressing 50.5MPH)
    New speed equals 55.45 MPH!
    It's that simple. As long as you are not using massive changes in weight (causing a different part or shape of hull to contact the water)you should be OK.

    This ratio does not apply to RPM, but the next calculation does..

    Temperature vs speed OR Temperature vs RPM

    ((Warm air Temperature + 273) / (Cold air Temperature + 273)) X Warm speed or X Warm RPM

    So (25+273) / (10+273) = 1.0530
    Now take the root of 1.0530 for an answer of 1.0262
    Multiply this 1.0262 times Warm speed (mine was 55.5) for 57MPH
    or Multiply this 1.0262 times X Warm RPM (mine was 7700) for 7901 RPM

    These are ballpark calc's. Note: The water temp also affects things since we have inter coolers.

    With these formulas you can at least make some sense of a "million mile an hour" post from another fellow who has the same machine/mods that you have.


  4. #4

    Join Date
    Jan 2007
    Lakeland, FL
    Yes, back when completely stock (a long, long time ago...), with just me in the boat, it would top out at 55-ish in the heat of the summer, and 57 in the cooler Florida winter, so your findings are normal. My first prop past OEM was 13/18, which worked awesome, but I quickly outgrew it after a couple of mods.

  5. #5
    Nice top speed in your sig. Scott! How did you like the 4" intake? Did you get to test it on it's own, or was it part of a multiple upgrade? I'm considering it, but these claims I hear of up to +300 RPM just don't make any sense to me considering that an increase of 1/2 PSI of boost at WOT with the x-charger gives about 1% more speed or +80RPM (over a base RPM of 8000.) At 300 RPM that would mean almost 2 lbs more boost.... Seriously?... but I've been schooled a few times before. Haha!

  6. #6

    Join Date
    Jan 2007
    Lakeland, FL
    The air intake on the 215 is badly choked, that's why you gain so many RPMs in changing it... especially necessary to feed a more hungry supercharger like the "X". My first custom intake netted a cool 150rpms. I've been through six or seven different and more elaborate designs since then, including huge 5" pipes that draw fresh air from flowrites placed on each side of the hull. Nothing performed better than just having a big cone filter as close to the SC as possible. 4" intake on a boat is always a DIY job. Get ya some Kanaflex tubing. Here's a picture of my current configuraion, that I took today:
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  7. #7
    Looks like I will give that a whirl.

  8. #8
    The X charger and 42's are in and I'm banging of the rev limiter in 25 degree Celsius water. My limiter seems to cut out at 8120 RPM MAX??? I thought it was supposed to be higher, so I changed my mind and have ordered and installed a Solas 15/20. I have not tried it yet.

    My rev limiter seems to be around 8100 RPM; So 7700 stock RPM in warm water, plus a known easy 500RPM from my mods and lets say at least 150 from a 4" DIY intake (not yet done) plus a "safe" drop of 550 RPM when going to the 15/20... This would land me around 7800 RPM warm and allow me to run to lake temps around 14 deg. C. before hitting the limiter.

    She better not be boggin or I'll have to bust out the adjustable.

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