Well as you know low displacement 4 stroke technology that trickles down to our cars can be sourced from either Formula One racing or Superbike racing developments. VTEC and i-VTEC are just a couple of examples where the bikes had variable valve timing and variable cam timing long before automobiles did.

A. Exhaust Throttle Valve - Shaggadelic Baby!!

In both these arenas (F1 and Superbikes), custom exhaust/header manufacturers have been employing an ECU driven throttle valve located in the collector to control the exhaust pulse energy travel at each rpm.

Let's get into the basics again:

Needing Backpressure - Myth or Reality?

The goal of any exhaust system is to efficiently remove burnt gases from the combustion chamber, prevent reversion at overlap, and by enhancing exhaust gas velocity leaving the chamber, create a vacuum to help draw or scavenge in more intake charge volume at cam overlap.

The key is maintaining exhaust gas velocity or energy as the gases leave the exhaust port when the exhaust valve opens.

As the exhaust gas leaves the exhaust port in a 4 stroke engine , it creates a series of pressure waves travelling at the speed of sound that move towards the exhaust tip (or forwards) and then some reflects back. Like the water waves coming onto the beach, forward and back, forward and back. The main overall direction is forwards but there is some reflection back to the exhaust port (reversion).

Simple enough...everyone knows this. So what's new and groovy?

The problem is at cam overlap (when both the exhaust valve and intake valve are both partially open and when the pressure in the chamber is greater than in the intake port).

If a high pressure wave is reflecting back and arrives at the exhaust port at the wrong time (i.e. when burnt gases still need to leave), it blocks the flow out. You see these instances when a high pressure wave is reflected back at the wrong time as dips in the torque curve AT REGULAR INTERVALS (usually in the midrange rpms).

If a low pressure wave is reflecting back at the correct time at the exhaust port it actually helps pull burnt gases out of the chamber and also helps pull in more intake air/fuel at overlap. You see these favourable low pressure reflected waves occurring on your torque curve as small torque increases AT REGULAR INTERVALS.

Now here's the first bone of contention and a source of debate between exhaust makers.

1. Is a reflected high pressure wave <U>always</U> bad?

Most of the experienced people I speak to and read on the various boards say YES! You never want backpressure and you want it as low as possible for as long as possible. The low backpressure assists in maintaining that high exhaust gas velocity. They then design anti-reversion chambers and/or place steps (increases in diameter at various proprietary points along the length of the header) to prevent the reflected waves from travelling back to the head.

There are also some pretty smart people who believe slightly differently ...They believe that if you have a high pressure reflected wave arriving a few milliseconds before exhaust valve closure, you prevent the loss of intake air:fuel out the exhaust valve at cam overlap. The exhaust backpressure at this crankshaft degree in the exhaust stroke prevents leaking out or bleeding out of you intake charge into the header and ensures all of it goes into the chamber for combustion.

However, these people do NOT use the exhaust diameter as a way to create this backpressure. That would be too crude or less precise, since the backpressure would exist at all times and they only want this backpressure over the few crankshaft degrees when the exhaust valve is just about to close ,when the intake valve is opening further, and the piston has reached TDC and starts downward for the intake stroke. Using an exhaust just to have backpressure then is like cutting butter with a chain saw.

The people who agree with this will often tell you that combustion chamber and intake port pressures are higher than the pressure in the exhaust just before exhaust valve closure . So some intake flow into the chamber can get pushed out the closing exhaust valve by the higher combustion chamber pressures.

So all you guys that say backpressure is a good thing...I don't believe so...not at <U>all</U> crankshaft degrees which is what you get with a restrictive diameter exhaust. You don't want to have too big a diameter (actually it's cross-sectional area) that will slow or kill velocity or energy. But no backpressure most (99%) of the time is good.

2. How do we get low pressure waves and high pressure wave to arrive at the correct time?

The conventional way to get the exhaust gas harmonic to do this dance of low pressure to pull in more intake charge and high pressure to prevent bleeding off all at the right time is by changing the tube layout on the header: using lengths, diameters, collectors with various merge angles. But these are limited to one harmonic or exhaust gas speed.

So some Japanese engineers at Yamaha (figures, it's always some genius engineer at some bike manufacturer that comes up with these wild ideas) thought: "What if you have an exhaust throttle valve (located in the header collector or at the entrance to the secondary tubes in the first merge collector) that could control the pressure wave behaviour?".

The throttle valve angle would vary as the speed of the exhaust gases changed to control the reflected waves. In an 11,000 rpm bike, the valve opens progressively as the rpms climb as the tubes are "in step" with the engine harmonics and less reflected waves occur but at around 7000 rpm, the valve is closed down to 40-60% of wide open when the harmonic is "out of step" with the engine and at 8500 rpm the exhaust throttle valve is progressively opened. How much to change the throttle angle is based on crankshaft angle input or ignition signal input to an ECU with then controls the throttle valve angle knowing the harmonics of the engine.

We see these in the Mercedes McLaren F1 car. If you think this is somebody's Frankenstein pipe dream then guess again. The new Suzuki GSXR1000, Honda Fireblade, and Yamaha R1 already have these. And those are today's street bikes! Can the new RSX and Civic Si be that far away from the next stage forward for more power? The impetus will not be performance oriented but the drive to bring this to the market place will likely be more practical, as this throttle valve (the first one was called the Exup or Exhaust Ultimate Power by Yamaha in the late 80's) gains better emissions and lower exhaust noise (less pollution is good ...admit it Kyoto is the right thing to do).

So the new toy for exhaust makers will be like variable valve timing and variable cam timing...the mating of electronics to optimise exhaust harmonics at each rpm as the harmonics change with the rpms climbing. It won't be just cut and try any longer...it will be cut try and reprogram. Welcome to the new millenium.

Here's looking at the ports of a Honda Fireblade exhaust throttle controlling flow in 2 of the 4 throttle ports. When the 2 valves are fully closed, the flow is at the tuned length (harmonically) of the 2 ports below. As the upper 2 ports opens, the total cross sectional area increases and the exhaust gases follows the new tuned length.

from August 2001 Performance Bikes



Here's the effect:

Notice with the tuned fixed header length and the throttle valve off in red, there's a huge power dip at 5400 rpm and the air:fuel ratio goes wicked rich at 6500 rpm.

What happens if you close 2 ports all the time in the exhaust throttle valve and control some low end rpm harmonics? (in green)...you get rid of that 5400 rpm dip but lose power from 6000 rpm onwards .

When you get complete variable exhaust harmonics (stock in blue) you get low end and high end grunt.



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B. New Honda 4-stroke 990 cc 75.5 degree V5 for Grand Prix Motorcycle Racing

Yes you read correctly...not inline and not V4 but 5 cylinders with oval pistons, 4 valves per cylinder making more than 200 bhp (estimated...Honda obviously won't say). The engine is designated the RCV. With a 24 L fuel tank rules limit, this engine is also fuel efficient...and more green friendly than the current V4's. This is part of Honda's " Total Balance Concept" in racing which shoots for concentrating the mass (a more compact engine with proper center of grsvity is part of this), lowering frontal area (aerodynamics), and optimising traction...chassis development is next on their list...see this in the 2002 World Grand Prix Superbike Series Hondas and maybe a V5 in a future integra...in 2008?

The engine has already been tested at Suzuka in Japan.



cheers

 

I am trying to figure out how you find the time to do this all. On the Flywheel thing I wanna get pics before I post the Solution.