Operation and Performance

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Electric supercharger

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Electric superchargers behave differently to conventional superchargers. Output from a mechanical or turbo driven supercharger is directly dependent on engine speed. This leads toward producing high top end performance but has far less or even no effect at low engine speeds.

Electric superchargers will switch on as soon as the throttle plate is opened wide enough and can quickly reach maximum output regardless of engine speed. Boost at lower revs yields more power and torque low down making engines much more flexible and responsive.

We ask that you also read our ‘suitability’ web page (next article) in conjunction with the following performance report.

The performance description that follows is divided into three broad sections; definitions, overview and data.

Definitions

There are a number of methods used to indicate a superchargers output. Some will provide more flattering figures than others while a few are positively misleading. Naturally marketers are not prone to letting facts get in the way of good hype so we will try and make clear the situation before describing our supercharger.

Let’s begin by defining boost. A superchargers job is to increase the amount of air being inducted into an engine and does so by boosting its pressure. Boost is a measure of that increase in pressure. Although engineers normally measure gas pressure in kPa (Kilo Pascal) we have used the more universally understood psi (pounds per square inch). There are two basic means of measuring boost, relative and absolute.

Absolute boost is the measure of pressure above or below mean (normal) atmospheric pressure whereas relative boost denotes the difference in pressure caused by supercharging. Note that this definition refers to absolute boost not absolute pressure. When a normally aspirated engine is running vast amounts of air is being drawn in. As the throttle assembly forms a barrier to this airflow (even when wide open) there is always a low pressure region (partial vacuum) down stream from the throttle body. Lets say that this part of the inlet manifold has a pressure of 1.5 psi below atmospheric at full throttle. If a supercharger is fitted and supplies 4 psi of boost then its relative boost is 4 psi whereas its absolute boost is 2.5 psi above atmospheric pressure (4 psi minus 1.5 psi lost in the throttle body). Real world fluid dynamics are a lot more complex than this but the model is a good approximation of what actually happens. Conventionally engineers only measure absolute boost whereas product marketeers prefer the more flattering relative boost. It is relative boost however that relates to real performance gains most closely particularly when dealing with small capacity superchargers.

Overview

One of the challenges in describing performance of a cross platform supercharger is that it behaves differently on different engines and fuel systems. Our supercharger was tested on a wide range of vehicles generating a mountain of data though there are some generalisations that can be made representing its performance quite well.

While all engines tested demonstrated performance gains there were some noticeable patterns in the data collected. Most fuel injected engines with air mass calibration (flow meters) generated similar results whereas MAP (manifold absolute pressure) sensed fuel injection systems were notably divergent. Where MAP sensors were involved we found power and torque gains tended to level off a bit lower down in the rev range. This is because MAP sensors are only designed to operate at or below mean atmospheric pressure so boost pressure can confuse them. Generally these EFI systems run rich enough at full throttle to accommodate our superchargers and we outline a method in our construction manual to help overcome this problem.

Carbureted engines responded well to our supercharger though some had to be re-jetted to gain maximum output (it was mostly Mitsubishi carburetors that required re-jetting).  A few mechanical fuel pumps could not overcome increased float chamber pressure. We have been informed of one instance where under maximum boost fuel was actually forced backwards down the fuel lines. Generic low pressure electric fuel pumps overcame all these problems but we ask that you power such pumps via a tachometric relay (contact us at support@gaprojects.com if you are unsure how to do this).

Engine capacity made a big difference with smaller displacements performing the best. It should also be pointed out that a number of various fan assemblies are available each with their own output characteristics, we have used a Nissan R30 unit run at 36 volts (approximately 1800 Watts) as a standard for the following performance analysis.

Data

As our supercharger fits a wide range of vehicles it is not practical to present an exhaustive list of performance data. We were concerned that just providing a few sets of performance figures would not fully represent our superchargers operational characteristics. A statistician was consulted and he suggested we use a statistical mean generated from our data and apply it to two hypothetical engines that are broadly representative of those we think most likely to use our supercharger. While this method does leave some specific questions unanswered we agree it is the best way to give an accurate but at the same time broad spectrum performance report.

The first engine is a four cylinder 1600 cc twin cam 16 valve engine using a Bosch LE jetronic fuel injection system. Our supercharger is mounted between the airflow (air mass) meter and the throttle body. Peak relative boost measured equals 3.1 psi while power and torque is shown in the accompanying diagrams. Statistically this equates to an 8.65% increase in power and a 7.7% increase in torque.  The statistical mean we have used mirrors very closely results obtained from a 1992 Toyota Corolla.

The second hypothetical engine is a four cylinder 2200 cc twin cam 16 valve MAF sensed multipoint sequential injected EFI engine. Statistical interpolation provided the following results. Peak relative boost equals 2.85 psi, power increased by 7.85% and torque improved by 7.15%. A 1998 Toyota Celica was tested generating figures very close to the ones we have presented here. Note that figures have been rounded to the nearest 0.05%.

Here is an excerpt from our identifying e-supercharger scams page that looks at performance from a different angle –

An approximation of an individual electric superchargers real performance can be obtained from details of their current and voltage requirements. A supercharger is essentially a pump and has to do work to pressurise an induction manifold. They have to be a powerful pump as a huge volume of air needs to be pressurised to keep up with an engines demand. Even the most efficient electric supercharger designs need a substantial electric motor to drive them and such a motor requires an equally substantial amount of electricity to power it. Simply multiplying voltage (Volts) and current (Amps) together will tell you how much electric power (Watts) the supercharger uses. Our experiences showed that it is difficult to achieve worthwhile results with anything less than about 560W but gains down to 480W are at least conceivable. It is worth remembering that even a minimalist 480W uses 40 Amps of current at 12 Volts which is considerable. Many electric superchargers would need hundreds of Amps of current to achieve their stated power outputs. To put this into perspective typical starter current draw is between 100 and 250 Amps on most petrol engines. Batteries and charging systems have to keep up with this massive requirement which they won’t do for long and the resulting system voltage drop can become an insurmountable problem. Our electric supercharger has its operating voltage boosted from the host vehicles 12 Volt system to 24 or 36 Volts (depending on which version is used) by electrically switching slave batteries into series circuit with the drive motor. These batteries are then switched back into parallel circuitry to recharge while the supercharger is not engaged. This configuration yields power ratings of up to 720W at 24 Volts (@ 30 Amps) and 1800W at 36 Volts (@50 Amps).

You may have noticed by now that we have not mentioned gas flow figures (e.g. CFM or L/m). Gas flow is probably the most misunderstood and abused measurement of all. Describing gas flow would take another web page just to skim the subjects’ surface. The problem being that superchargers operate in a dynamic environment where by it is extremely difficult to separate flow rate attributed to the engine from that provided by the supercharger.  Even simply quantifying the flow rate difference between a supercharged and naturally aspirated engine does not give an accurate picture as the mathematical relationship between the two is not linear.  Measurements taken when the supercharger vents into an open space are meaningless, a large fan would outperform a supercharger in this test yet fans are for the most part unable to generate significant pressure.  All in all we think it is best to leave these figures out completely rather than step into a quagmire of potential controversy for the sake of very little in the way of meaningful information. Boost pressures along with power and torque gains are the benchmarks that best describe a superchargers performance.

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We were hoping for some improved fuel consumption results in certain instances but no significant improvements could be reported. In practice the supercharger is a performance enhancement that encourages spirited driving leading to increased consumption when employed frequently.

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