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  1. #1
    Senior Member EVOL Tweety Bird's Avatar
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    Got this idea, any thoughts?

    So on my day off I'm of course tinkering with the bikes. Both have modded Predators, one Hemi, one non.
    Flywheels/rods/cams/emulsion tubes/jets/ported and polished/22 pound springs/governor and low oil deletes, high flow air filters.

    Okay as to the latter As most know you have an adapter that bolts to the front of the carb so the air filter coupler can attach to it. Now looking at it I'm thinking that the adapter design can be detrimental to air flow, as you have the small hole in the center for the air to travel through, but you also have a flat approx. 3/8" around the hole. So any air traveling down from the filter hits the flat and can disrupt the air getting into the carb. Yeah I know, way over thinking it.

    But hear me out, old school racing time, when you wanted a straight shot of air getting to your carb you installed velocity stacks to direct the air straight to the carb.

    Now finding a tiny diameter velocity stack is a bit of a problem. So here's what I'm thinking and what I built. A velocity stack to fit the coupler on the air filter adapter, but with a tweek. I made up a venturi to direct all the air from the stack into the hole on the adapter. At low speeds the engine doesn't need a lot of flow so the setup does really nothing. But at higher speeds instead of a bunch of tumbled air going to the carb, you would get a higher speed of more clean air going to the carb. Higher speed, lower pressure to help pull more fuel/air to the head.

    And just to be nuttier than a jay bird, the velocity stack is a left over header collector connector, 3" long and 2 1/2" in diameter, with a trumpet end on it. The venturi is nothing more the the top of a Sea Foam can stuffed in the stack upside down. It's small opening is nearly a dead nuts match to the one on the adapter. For a filter I cut a piece of heavy screen that lays inside the stack on top of the venturi, with a piece of the foam filter from the non Hemi engine on top of that.

    Engine fires up just like normal, but I swear I hear more of a cackle sound now from the stack, I guess the harmonics from when the intake valve slams shut. Engine seems to respond normally if not a little bit more snappier to the throttle. With the hurricane that just pushed by I haven't had a chance to see how it runs on the street, but I plan on testing with the old set up and then with the new to see if it actually did make a difference.

    I'd show you photos but the site still won't let me post them, so you'll have to use your imagination.

  2. #2
    Senior Member DeadPixel's Avatar
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    I have thought about this too cuz the intake almost has no length to it for tuning. Its easy to put a long header on the bike and get a little bit of an effect for tuning but the intake is nothing. You really want the length between the carb and head but though that doing like you did might still have some ram effect besides smoothing out the air like your trying to do. I have thought lightly modding a mikuni intake with the stock carb set up to see if it has any effect on power. You got the wheels turning again so now I might have to start this project.

    I think the best way to see if you gained some power is to do a power run up a mild grade with and with out the intake and see what gives you the best average top speed up the hill.

  3. #3
    Havasu Dave's Avatar
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    I made a couple using stainless steel shot glasses I found on Ebay, hard soldered to flanges I made. They bolt on to Tecumseh, Tilly, etc anything with that bolt pattern on the air filter housing adapter. The concept is still valid all over this town, especially on top of Hillborn and the other mechanical injections systems on street rods. Also still used in vintage karting classes, where allowed. I have one on a warmed up WB as well.

    Solid technology.

  4. #4
    Senior Member EVOL Tweety Bird's Avatar
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    Hmm,never thought of the shot glass idea Dave, oh well, I kinda like the look the bigger stack has. Still raining out so no rides today before work.

    Back to the idea, does having more then one venturi actually add a bit of boost per se. No not like a turbo/supercharger, but then again maybe it does? Now instead of the tumbling air being delivered to the carb venturi, the stack with it's venturi is delivering a faster increase of clean or smooth air. Like DP said sort of a ram effect on the carb. Similar? as the ram systems don't do anything for putting around, but they can make a difference in hard all out power.

    If this contraption actually works I guess I'll have to look into jetting as I'm sure the change will affect the mixture when it's used.
    Better go buy some more sacrificial spark plugs for reading the results.

  5. #5
    Senior Member EVOL Tweety Bird's Avatar
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    Sorry for the belated response, work kept getting in the way.
    Anywho... My seat of the pants impressions.

    #1 Sound. Engine is louder with the stack/venturi. Kinda makes sense as at least my thinking, with the angled air filter, any harmonics tend to have to travel around a corner in the adapter and then can exit all over from the filter, andwith the sealed end on the filter sound would bounce back possibly cancelling some of the sound. Also the rubber adapter may help absorb/deaden some of the sound.
    With the stack, and maybe even more so with the venturi, the venturi acts as a megaphone for any sound coming back through the intake, and the stack directs it straight out the end.

    #2 Performance. Oddly the engine idled faster and smoother with the stack. Maybe because of the more direct higher speed airflow from the venturi,causing more fuel to be pulled through the carb? Rapid throttle response has improved, really noticeable on a rolling start and going full throttle. Before there was a miniscule hesitation followed by a strong acceleration. Now it's instantaneous response and power has increased. Same rolling start and then hammer the throttle has the front wheel getting very very light light.

    As a side note, prior, I had to idle the engine to the point the T/C had the bike creeping forward. Not good if the kickstand catches on something. Now it is turned down, runs smooth and no more creeping along the garage floor.

    On my little test track, I've gained over 3 mph with the stack vs the filter, over the same distance. Around a 100 yard pull, but even that has me thinking this this may be faster even over a longer distance. As it didn't feel like it had fully wound out as it did with the stock setup. Maybe it got the T/C to lock up faster with higher rpms?

    Also the spark plugs told a real difference. Air filter, with the stage 1 emulsion tube and a 36 jet had the plug a wet sooty black.
    Second run,new plug, same set up, swap the filter for the stack/venturi and the plug was clean as a whistle.
    These were the Autolite Racing plugs so reading the insulator is near impossible, I have some standard NGK's coming so I can get a reading on them. But it didn't act like it was running lean as it didn't fall off on the top end, it just kept pulling.

    #3 Looks. Come on, velocity stacks look so much cooler than a clunky air filter hanging off the side. I'm already building another one for the Hemi engine. Even invested in Edelbrock air filter material for them both.

    To me I think it was a plus. Your results may vary...

 

 

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