Multiple Carbs: advantage?

[Dave8338]

Short runners are better suited for the horsepower end of the equation. Long runners give you a better balanced torque curve. In my opinion...it all depends on what you're trying to do. I chose to drop the twin carbs (v- twin bike) in favor of longer intake runners and a single carb set-up. I want the torque as the bike has more than enough horsepower.

The main thing to keep in mind (and I think that someone already mentioned it here) is that you need to have enough volume and velocity within the intake runners, to keep the fuel from dropping out of suspension and therefore, creating droplets on the runners. This causes nasty tuning problems and is one of the many reasons that "highly tuned engines", run like crap around town, at slower RPM's - there are too many other factors like lift, duration, jetting, ignition advance/retard, and a whole handfull of other reasons, but you get the idea.

[drhach]

Runners

If you think about the mixture traveling through you intake tract, there are pressure variations that occur because of the opening and closing of the valves. These varations are essentially waves (sinusoidal for you math types). What happens when you vary the length of the intake tube is that the peak of the wave hits the intake opening at different RPMs. When the peak hits, it is just like what happens when the waves come in on the beach, they slamm forward with force.

What this means for the bike is a ram-air effect and better cylinder filling. Basically, you can tune your intake manifold to give the most punch for whatever RPM you want; for instance, mayber right before a shift point, or the hot spot on a cam. Ideally, all intake runners should be the same because all cylinders have the same need volumetrically. Ther are tons of formulas out there for this and it really isn't a big deal to design. BMW actually has a design where the intake length varies according to RPM so that it is optimized over the whole RPM range.

So, there's the theory. What about reality? How much difference does it make? Well, the difference will show up on a dyno. Will you feel it in the seat of your pants? Probably not. If you are racing, could it be the differnce between a win and a loss? Maybe. Honestly, I think the system that BMW has is really about novelty. BMW owners are snobs (no offense, but that's how they market their cars). They want the latest and the greatest, even if it really doesen't matter much overall. Many older cars had different intake tract lengths. This is especially true for the inline cylinder cars. Look at a straight six. There are three different intake lengths because the intake hole is in the middle of the manifold. Those run just fine and it used to be very common to race (and win) running those mills.

People regularly post here asking about making an intake manifold to run a single carb. Nobody ever comes back saying that they have tried it. Give it a shot. Unless you're racing, you probably won't care much about the losses that you encounter and it will make for a simpler system.

[XL-erate]

Thanks drhach!

I've researched ram tuning/tuned length intake & exhaust & have some good formulas. I'm familiar with sinusoidal wave harmonics and wave reversions etc. What I've found agrees with what you suggest and also, yes, probably not a real noticeable difference if done right.

Kind of funny when a guy finally decides his bike is running lousy and time to take a look, only to find carbs a mess, ignition hanging by a thread, clogged filters etc. Seems the old beasties are more user friendly than we usually give them credit for.

[chuckycheese]

Carbs

I'll admit, upfront, I haven't read any of the posts. But....having multiple carbs is kind of like having multiple wives; it allows you a great deal of fun, at times, but is often more trouble than you'd like.:-D

[XL-erate]

Hah! No doubt a good point there, brother, but may I add: like an only wife, one single carb can be plenty of trouble to deal with all by itself too....

[drhach]

Thanks drhach!

I've researched ram tuning/tuned length intake & exhaust & have some good formulas. I'm familiar with sinusoidal wave harmonics and wave reversions etc. What I've found agrees with what you suggest and also, yes, probably not a real noticeable difference if done right.

.

I can point you to a really good book. "Advanced Engine Technology", by Heinz Heisler. It has tons of good stuff on every aspect of engine theory. It's published by The Society of Automotive Engineers. Amazon has it for $70 http://www.amazon.com/Advanced-Engin.../dp/0340568224 . It's alittle pricey but it goes deeper than most stuff you can find out there.

When I was in college, I had the oppurtunity to design an intake system for my capstone project. It was a lot of fun. I worked closely with an engineer at Ford who did intakes for a living. I did tons of computer simulations with different intake lengths to compensate for low spots on the Torque/power curve. We were modelling a Honda CB600. We decicided on an appropraiate length based on our model and we built the intake. It was fuel injected with a plenum and one throttle body. After we built it, we put it on the engine and dyno'd the engine. I was surprised how close the computer simulation came to the actual perfomance. I have to say though, that the gain was really trivial. For the street, most motorcycles are way overpowered anyway. You'd do much better putting all of your efforts into suspension tuning. Even for our case, where we were building a race application, there was way more power on thap than what we could get to the ground. It was good lesson in practicality and priority setting.