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billy2billy2 Member, OS Professional, Mentor, Developers, User Group Leader Posts: 1,507 PRO


  • billy2billy2 Member, OS Professional, Mentor, Developers, User Group Leader Posts: 1,507 PRO
    edited October 2020
    My motorcycle's name is Suzy, I have no idea who these girls are.

    The speed limit on Neptune is 20 mph and I had been following these girls for some time. They were tearing up the road.

    This post is about designing a new velocity stack:

    Some good friends of mine & myself were discussing a new design for velocity stacks which would improve the scavenger zone for a stack. You can break a stack into 2 zones: compression & scavenge. Because of the ratio between length & diameter, you can achieve an extremely efficient compression of the flow between the inlet & outlet. You don't want to create turbulence in your flow.

    The other part of the velocity stack that is overlooked is the inlet and scavenging zone which I'm going to work on in this design.

    My friend races a normally aspirated alcohol dragster and with modified stacks, won last years division. He's the division champion.

    I'm going to try and duplicate his success and give Suzy a little more breathing room.

  • matthew_stacymatthew_stacy Member Posts: 194 PRO
    @billy2, tell us more.  What is Suzy's displacement?  How many cylinders?  Maximum RPM?

    Do you have any CFD software?  Any thoughts on how you might validate the design?
  • billy2billy2 Member, OS Professional, Mentor, Developers, User Group Leader Posts: 1,507 PRO
    edited October 2020
    She's a 440cc after installing a bigbore cylinder and putting in a wiseco piston & rings. She's over square and matches the newer race engines on the market. Rev limit is 12k rpm due to the over square and lack of stroke. Her top end faded at about 40k miles when the valve seat wore past the valve hardness and stopped creating a seal. This is typical with this type of engine. 

    I took her head into the local Suzuki dealership who inserted new valve seats and faced them. They do a really good job, far better than if I had hand lapped them. I got new stage 1 hot cams for both intake & exhaust.

    The #1 thing to soap her up, was that, when the cylinder was off,  I removed the base gaskets. Remember, you can take grandma's sedan and remove .030" off the heads, fill it full of premium gas and head to the races. Suzy's compression is hard to say. Blocking the compression release on the cam, the electric starter could barely turn the engine. Even with an impact wrench attached to the crank and the electric motor, I could barely crank her over. The compression is over 13:1. I added .010" to the head gasket so I could run pump gas. Race gas is expensive & hard to find.

    The clutch is original because I don't slip clutches. The front sprocket is 2 teeth up and the rear is down 3 teeth. She's really starting in 2 gear. With the narrow ratio gearbox you end up shifting as fast as possible. She's nice in the corner's when downshifting to shed speed; you don't really need to use her brakes. You can carve up canyons using the transmission to accelerate & deccelerate.

    I re-sprung her. Stock was way too soft. I think she was designed for someone who weighs 160lbs. I stiffened the suspension to match a modern day 450 class dirt bike. I've had her over 6ft in the air on a track. She's not a crf450 because she's heavy with that steel frame.

    I've removed a lot of stuff so she's probably 250lbs and the motor could to be producing 50hp. When I holeshot her, she's hard to beat unless your on a modern day dirt bike.

    Thanks for asking about her,

  • matthew_stacymatthew_stacy Member Posts: 194 PRO
    @billy2, this sounds like a DR400 project unless I'm misreading the tea leaves.  That velocity stack looks cool as hell.  But without an air filter your days on the motocross track may be numbered.
  • billy2billy2 Member, OS Professional, Mentor, Developers, User Group Leader Posts: 1,507 PRO
    edited November 2020
    Yeah, I don't ride her in the dirt anymore and I miss going to the track.


    So here's the idea. It's a simple concept helping to stop stagnation at the inlet.

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