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Old 11-06-2010, 02:59 AM   #11
Spark01's Avatar
Joined: Aug 2010
Location: The Netherlands
Oddometer: 306

FI systems don't require high intake velocities through the throttle bodies...
Yes and No
A EFI engine can run fine without a high(er) velocity in the throttle bodies.
Because it automaticly adjust it's fuel to the ammount of air that is sucked in to the engine.
But to make an engine run more smooth in the lower or mid/range if you immediatly open the throttle, you will need a smooth alteration in air velocity.
That's why secondary valves are used..

here some more about the secondary's

Secondary throttle valves....There are many thoughts on just what the heck they do. Some say it's for noise abatement, others think they just get in the way and affect the true performance of the bike and some would just rather remove them.

Since the introduction of fuel injection, we no longer require a pressure drop through a ventury to pull fuel from a fuel bowl, and introduce it into the engine. We have a fuel pump that sends pressurized fuel to fuel injectors, and thanks to IC chips, and sensors, we can spray the fuel into the engine. No need for fancy needle jets, main jets, primary jets, and airbleed jets. Never mind making sure the fuel level in the carb bowls is the correct height.

There is one thing that's important with the CV carburetor and that's how it keeps the air velocity constant through the intake ports regardless of throttle position. If you ever had flat slide carbs, and immediately opened the throttle, you would notice that the engine would take a big gulp of air, and hesitate, or even stall. Accelerator pumps were introduced to keep this in check. So, in regards to keeping air velocities constant especially at lower engine rpms, one concept for a fuel injected engine is to add separately controlled throttle plates that can move independently from the main throttle plates. This way, if the throttle plates are opened quickly, air velocity will drop, but with the Secondary Throttle plates lagging just behind, can keep the air velocity up, and help with atomization of the fuel injected into the engine. of course if you are racing the bike, and it never sees rpms below 6 to 7,000 rpms, there is no sense on having the secondary throttle plates there, and can be removed for some added increase of air. Most of us don't race, and use the engine through a wide range of rpms, and with the STV's installed really doesn't detract from performance, but keeps it more tractable

On a german KTM forum they did a test with installed/removed 2nd valves on a dyno with a 990sm.
With the valves removed it did run a little bit smoother on the low rpm's but it did loose some power on top?? .
With the valves installed again, the power output on top was back to "normal".
They could reproduce it by removing/installing the 2nd valves.

My sugestion is to set the 2nd valves in the TuneEcu a little bit more open in the 2nd throttle map on the low rpm's.
But don't remove them.
'10 990SMT

Spark01 screwed with this post 11-06-2010 at 03:08 AM
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