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28-11-2009, 09:50am
So. Why does the VMax have gobs of low down torque and out accelerate the Busa?
When it first came out I was intrigued by this so I read up everything I could find on the new engine. It has a number of nice features, but I think the killer here is the variable length intake manifold.
Remember the EF Falcon? It caned the EB/ED Falcon despite being almost the same engine. The difference was the variable length intake manifold. I was working for a quasi government body that was acting as a clearing house for the Aussie car manufacturers back in the mid 90's. This clearing house was established to distribute new and/or test cars and components to TAFE colleges. Ford did a lot of work on the manifolds to get it right, I know because I was able to pull apart all their prototypes! Many car manufacturers now use variable length manifolds because they're simple and effective. But you don't see them on bikes much because they take up room.
Yamaha has got it right. They actually seem to have perfected the theory in applying it to a bike. Some people on the forum here have played with 'bellmouth' lengths and have noticed a difference. Basically, a longer narrower bellmouth provides more torque because it keeps the airflow through the throttle bodies high and helps assure good cylinder stuffing. But as your engine speed rises these long bellmouths become restrictive and choke the engine. Conversely, relatively short bellmouths cause problems at low rpm because the airflow rate is low and you can't get good atomisation of fuel. A simple example of this when you are running carbies on a big-bore road bike and take the airbox and bellmouths off; you can see 'fogging' and petrol spitting back out the carby. The engine is basically 'gulping' air from the area closest to the back of the valve and not getting a good fuel air mix stuffed into the cylinder. However, as engine speed rises so does the airflow down the inlet tract and as a result so does power.
The best of both worlds is to have an inlet manifold that can change from long bellmouth to short bellmouth. Which is what Yamaha have done. The inlet manifold is much more refined that the old Falcon' and car designs (thank fcuk). It actually is a bellmouth shape, or rather two, one which fits inside the other for each throttlebody. At low RPM the upper, narrower bellmouth sits snugly inside the lower, wider bellmouth. At 6.800 rpm (thereabouts) the upper bellmouth is moved up about 2 cm to allow air to flow through the gap between the two. Actually, the design is so well done that the air flowing through the upper bellmouth creates a venturi effect which draws in extra air through the gap than would otherwise normally happen. Brilliant!
Brilliant and so easy to replicate. When I saw that the Gen II Busa had a programmable shift light I had a germ of an idea begin to fester in my noggin. If I could wire in a relay to run off the programmable shift light then I could use that relay to power a stepper motor to run a variable length inlet manifold on my Busa! Instant V Max killer!
Then I got involved in development of ECU Editor and found that there is an unused EXUP-type map already sitting in the Gen II's software......... EXUP is Yamaha's version of an exhaust valve which works similarly to the variable length manifold concept. Instead of increasing air flow to get torque it restricts exhaust flow to help stop the inlet charge coming straight out the exhaust (due to extreme valve overlap). Ideally you'd synchronise EXUP with a variable length inlet manifold to get a broader spread of power in an engine, but for the Busa I reckon the dual injector and throttle body setup could benefit further from a variable length manifold. Sure the secondary throttle plates are designed to increase airflow at lower revs, but the venturi effect mentioned above, plus the fact that bellmouths are far less restrictive than throttle plates (even when fully open) would make the variable length manifold a great power increasing proposition. That being said, I wouldn't remove the secondary throttle plates because they're essential for fuel atomisation (look at the angle the secondary injectors sit at).
There's all manner of secrets stashed away in the standard Gen II ECU.... Anyone noticed the traction control boss cast into their rear brake stay arm???
Camel
When it first came out I was intrigued by this so I read up everything I could find on the new engine. It has a number of nice features, but I think the killer here is the variable length intake manifold.
Remember the EF Falcon? It caned the EB/ED Falcon despite being almost the same engine. The difference was the variable length intake manifold. I was working for a quasi government body that was acting as a clearing house for the Aussie car manufacturers back in the mid 90's. This clearing house was established to distribute new and/or test cars and components to TAFE colleges. Ford did a lot of work on the manifolds to get it right, I know because I was able to pull apart all their prototypes! Many car manufacturers now use variable length manifolds because they're simple and effective. But you don't see them on bikes much because they take up room.
Yamaha has got it right. They actually seem to have perfected the theory in applying it to a bike. Some people on the forum here have played with 'bellmouth' lengths and have noticed a difference. Basically, a longer narrower bellmouth provides more torque because it keeps the airflow through the throttle bodies high and helps assure good cylinder stuffing. But as your engine speed rises these long bellmouths become restrictive and choke the engine. Conversely, relatively short bellmouths cause problems at low rpm because the airflow rate is low and you can't get good atomisation of fuel. A simple example of this when you are running carbies on a big-bore road bike and take the airbox and bellmouths off; you can see 'fogging' and petrol spitting back out the carby. The engine is basically 'gulping' air from the area closest to the back of the valve and not getting a good fuel air mix stuffed into the cylinder. However, as engine speed rises so does the airflow down the inlet tract and as a result so does power.
The best of both worlds is to have an inlet manifold that can change from long bellmouth to short bellmouth. Which is what Yamaha have done. The inlet manifold is much more refined that the old Falcon' and car designs (thank fcuk). It actually is a bellmouth shape, or rather two, one which fits inside the other for each throttlebody. At low RPM the upper, narrower bellmouth sits snugly inside the lower, wider bellmouth. At 6.800 rpm (thereabouts) the upper bellmouth is moved up about 2 cm to allow air to flow through the gap between the two. Actually, the design is so well done that the air flowing through the upper bellmouth creates a venturi effect which draws in extra air through the gap than would otherwise normally happen. Brilliant!
Brilliant and so easy to replicate. When I saw that the Gen II Busa had a programmable shift light I had a germ of an idea begin to fester in my noggin. If I could wire in a relay to run off the programmable shift light then I could use that relay to power a stepper motor to run a variable length inlet manifold on my Busa! Instant V Max killer!
Then I got involved in development of ECU Editor and found that there is an unused EXUP-type map already sitting in the Gen II's software......... EXUP is Yamaha's version of an exhaust valve which works similarly to the variable length manifold concept. Instead of increasing air flow to get torque it restricts exhaust flow to help stop the inlet charge coming straight out the exhaust (due to extreme valve overlap). Ideally you'd synchronise EXUP with a variable length inlet manifold to get a broader spread of power in an engine, but for the Busa I reckon the dual injector and throttle body setup could benefit further from a variable length manifold. Sure the secondary throttle plates are designed to increase airflow at lower revs, but the venturi effect mentioned above, plus the fact that bellmouths are far less restrictive than throttle plates (even when fully open) would make the variable length manifold a great power increasing proposition. That being said, I wouldn't remove the secondary throttle plates because they're essential for fuel atomisation (look at the angle the secondary injectors sit at).
There's all manner of secrets stashed away in the standard Gen II ECU.... Anyone noticed the traction control boss cast into their rear brake stay arm???
Camel