Surbo As An Air Supplier

A normally-aspirated engine depends solely on its own vacuum to pull air into itself for combustion. To accelerate maximally, the driver may have to floor the accelerator, thus using up all the vacuum at one go, so that air is sucked in at one shot and air flow decelerates near to the rpm red line, necessitating a gear change. Fuel is also injected maximally when the throttle is floored, as fuel input is proportional to throttle travel, and this can cause overfuelling. Each cylinder experiences a sudden increase in noise and compression due to maximum air pressure induced, equivalent to atmospheric pressure, and more engine work is absorbed from the previous combustion cycle for this compression. The suddenly increased air mass may have difficulty exiting through the exhaust valves. A suddenly increased air mass flow can meet with significant back pressure from the catalytic converter, or exhaust muffler, and result in a louder exhaust note. In fact, an abruptly increased air intake flow rate can have a bounce back effect starting from the inlet valves. Even if the engine has variable inlet and exhaust valve control, the valve openings do not change till the rpm is rather high, and this may not have begun when the driver started flooring the accelerator. Add to this the fact that most engines have their peak power somewhere above 4000 rpm. Frequent downshifts may be required to access the engine's power.

The noticeable bounce-back effects of flooring the throttle are:
1. impending retardation of air induction due to complete use of the vacuum upon flooring the accelerator
2. inlet valves not opening widely enough to allow the air into the cylinder
3. additional piston compression force required for a sudden increase of air
4. possible extinguishing of the flame due to overfuelling
5. exhaust valves not opening wide enough for burnt air to go out
6. back pressure from the exhaust system

all of which can slow down the acceleration of the engine.

The Surbo system, which enables the engine to reach peak torque at 3300 rpm (or the Twin Surbo which brings that down to 2800 rpm), differs from this normal scenario in these ways:
1. The Surbo, being an air suction device, pulls in air significantly, and for the same power output, requires less of the throttle to be opened, thus preserving the engine's internal vacuum to help the air to be pulled in deeper, through the manifold, and the inlet valves. It's sort of like passing the baton between 2 vacuums in series.
2. Since the throttle is opened less, the relative pressure in the manifold is less, and this leads to a lower effective compression in the cylinder. This means that less compression work need to be done by the previous cycle, so more of the power output of the previous cycle can be directed to the wheels. The engine is spared from sudden surges of high compression, which can be damaging to the cylinder and pistons.
3. Since the throttle is not floored, there will not be overfuelling or any extinguishing effects. In fact, consumption and emissions are lower. Or, see it as the engine not having to use so much of the power from the previous combustion cycle, just to pull the air in with a wider throttle.
4. A lower air mass will have an easier time exiting the exhaust valves, and encounter less back pressure from the exhaust system.

As you can see, the list of backwave effects is lessened, and the engine is able to do its work with less hindrance. The air supplies and the air deliveries between the stages will be more just-in-time. At half throttle, the Surbo will achieve the original power output, and the other half capacity unused can be called upon when not changing to the next gear (either with rpm delimitation, or that the highest gear is already used). Usually, it is possible to just use up to half of the throttle for maximum Surbo performance, which normally exceeds the original performance anyway. For example, the Surbo enables the engine to reach red line on the rpm meter with just half throttle especially in the lower gears.

To minimize the abovementioned backwave effects, a Boost Gauge can be used to make the manifold relative pressure reading steady (the boost gauge vacuum signal is tapped from the manifold). When being held constant by moving the foot slightly, this also activates the Surbo with maximum sensitivity. Performance in the mid-range and in high gears is improved, and top speed too. From a gain in economy of 10% for Surbo cars without the boost gauge aid, many Surbo cars with boost gauges each have been seen to gain another 20% in fuel economy. For example, the Honda Sir 1.6 auto, and the '06 Kia Rio 1.4.

In conclusion, the Surbo is more of an air pull-through device, to make the air pass through the engine rapidly to enable rpm to be gained quickly, with an edge at very high rpm if not rpm-limited. This is in contrast to the high air pressure input devices like turbochargers or superchargers, which use air pressure buildups at the inlet side.