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  1. #1
    Island Hopper TM1's Avatar
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    Anyone done a 4" intake mod on the 155hp Sportster/S150?

    I gathered up some 4" pvc pipe and elbows from Home Depot and 3"- 4" flexible rubber reducer. Well, there is no easy way that was going to work with the proximity of the fuel tank. So I just removed the air box and attatched the stock kanaflex hose to a Spectre stainless steel filter along with ride plate mod.
    Took it out today for a 53 mile trip, air temp 79* and water temp 75*, wind 5 knots and sea state calm (pretty flat), full fuel load, and approx 400 lbs total passenger weight. Results were disappointing, max rpm = 7600 and max speed was 46.6 mph on smooth water in the intracoastal and 48.6 mph in slight chop out in the Gulf. So overall max speed was unchanged, while max speed on smooth water was slower by about 1.4 mph, iirc from last year. Rpm on smooth water was on average 7400 rpm. What's really disappointing is the fact that the air and water temp are considerably cooler than it will be in a couple more months, and the boat ran in the 48's in the dead of summer last year, totally stock. I'm not sure the current set up would even do that.

    So it looks like I need to figure out a way to get some 4" hose to the throttle body, looking for some input and maybe some pics of a setup on the n/a version. I'm not expecting miracles, but I'd like to see 50 mph or at least a little closer to it.
    Last edited by TM1; 04-04-2007 at 10:55 PM.


  2. #2
    cyoungesq's Avatar
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    Slight chop would probably help your top end b/c of less drag on the hull as compared to smooth water where there would be less interruption of the water against your hull.

    You might want to consider taking a bit of pitch out of your impeller to bring your rpms up.

  3. #3

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    My first inclination is to think that without a supercharger to feed, the naturally aspirated engine is not starving for air as much, and therefore that gains realized from this mod would be much less. The 155NA stock intake seems to be less restictive anyway, primarily because of the throttle body being on the forward part of the engine, thus requiring less tubing. One important thing to consider is a flame arrestor; the 215hp version doesn't need it, because the intercooler does that job, but your stock intake has an "intake screen" which probably acts as the flame arrestor. Without that, a backfire could be deadly

  4. #4
    Island Hopper TM1's Avatar
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    I've always realized that a little bit of chop yielded higher speeds, but losing some on flat water is obviously a concern. I'm not really keen on putting too much effort into the pursuit of 1 or 2 mph, so I don't think I'll chase impeller changes just yet, perhaps if I ever damage the stock unit I'll see about choosing one that will yield those extra rpm.

    There is no spark arrestor on the n/a model, the air box has the rectifier in the upper portion and the air is circulated through the box in a serpentine fashion. the only screen involved is from the factory recall which is only to collect debris from an airbox that might come apart and therefore end up in the throttle body or engine. Backfires on stock fuel injected cop ignition equipped engines are pretty rare given the corrective control the pcm can excercise to make up for failed components.

    Your theory that the n/a engine may not have as much to gain from an intake mod could well be true though. I'll experiment a little with it and post up any useful results for other n/a owners.

  5. #5
    Yuji's Avatar
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    An increase in air flow (volume) and a decrease in intake temp (denser) air will always yield more hp...I found better throttle response was the biggest improvement from my mod'd intake.

  6. #6
    cyoungesq's Avatar
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    Just food for thought . . . if hitting 50 would be pretty satisfying, and if you are running 46 to 48 at 7600, then just by modifying your impeller pitch, you might just get there. You have at least 400 to 500 rpm to go which roughly corresponds to 4+ mph. Get your rpms up to 8100 and you're there. Modding the intake will help, as will filling the ride-plate holes, but you need a bit less pitch to make it happen.

  7. #7
    Island Hopper TM1's Avatar
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    The increase in airflow volume and cooler intake air are under question in my scenario. There is not really an inexpensive and accurate method of verifying either of the two are taking place (access to the pcm data would be really handy for testing the effectiveness of mods). The air temp is likely to be relatively unchanged, the filter is essentially in the same place as the stock air intake was. The volume will always be limited by the throttle body's maximum air flow rate. The delivery to the throttle body might yield some improvement, but I'm not sure it will be of the same magnitude as the scic's.


    The rev limiter is set for 7650 rpm, so I only stand to gain 250 rpm on flat water (which is not too common) and 50 rpm in choppier conditions. I'll need to replace the stock pcm to get around that and the costs keep climbing for not very much speed in return. Looking at Yuji's mods and the return on investment via speed increase is not appealing to me, but I certainly don't expect others to have the same opinion, for some it's worth it. I just figured I'd do the two most common mods and see what happens since the time and financial investment is low. I've been through this type of stuff with cars and since I used to wrench on them for a living I'm not into working on my vehicles as much as I once was, but I'm always open to quick and cheap mods that might make a difference.

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