Forced Induction

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Introduction

Converting from N/A (Naturally Aspirated) to F/I (Forced Induction) is a popular choice for gaining a fair amount of power at a fair price. There are many important factors to take into consideration when going this route that may or may not work out for your goals.

Going F/I does not mean you will automatically gain 300+ Hp, and many people are upset when their $800 eBay turbocharger can only produce 80-120 extra Hp on their completely stock vehicle. Therefore it's important to temper your nerves and do research on how to best achieve your power goals. Keep reading for more information on this fun modification!

Fueling

Injectors

It is recommended to use injectors that will flow enough for your power level, but are not excessive as to decrease low-load efficiency.

If you are looking for a tool to estimate injector flow required, please check HERE.

Fuel Pressure Regulators

If you are unable to size injectors correctly for one reason or another, or you are limited on your intake metering options, you may want to consider using an RRFPR (Raising Rate Fuel Pressure Regulator). These devices are placed in-line after your stock FPR and have a dynamic diaphragm that is linked to your intake manifold pressure. This diaphragm will adjust fuel pressure depending on your intake manifold's pressure, adding a certain pound of pressure (X) for every pound of positive pressure above atmosphere (Y) in your intake manifold. These devices are rated as "X:Y", generally ranging from 4:1 to 14:1, although there are other options.

This is considered a lesser option than using correct injectors, as the increased fuel pressure puts additional strain on injectors and fuel pumps leading to premature failure.

Power

In general, you can expect a gain of 3-5 Hp per pound of positive pressure added for every 1.0 litre of engine displacement. E.G. on a 2.5 litre engine, 10 PSI will generally add between 75 and 125 Hp ---- 3 Hp * 2.5 L * 10 PSI = 75 Hp; 5 Hp * 2.5 L * 10 PSI = 125 Hp.

In actuality, the power you gain will depend on quite a number of factors, such as: turbo trim, A/R (area to radius ratio), charge piping size and length, intercooler efficiency and mounting, elevation, tuning, and more. But speaking in general terms, the above estimations are applicable for most setups.

Intake Airflow Sensors

Any intake metering device should be within the intake track of the turbo (know as "draw thru" or "pull thru"). It is not recommended to place your intake metering device (VAM, MAF, HFM, etc) in the charge-pipe (known as "blow thru") unless it is a specifically designed piece of hardware that can properly read a pressurized system. This is because these systems are designed to meter airflow at atmospheric levels (corrected for air temperature). Therefore, when pressures are above atmospheric, reported airflow can become different than actual airflow. A graph (fig. 1) illustrates this effect in terms of positive pressure.

Some MAF sensors are less affected by pressure differences, but unless it specifically advertises that it is designed to be used in charge piping, it is best practice to install your sensor before the turbocharger or supercharger.

fig. 1 - A comparison of reported airflow by a MAF under different positive pressures.

While it is possible to use a MAF as a blow-thru style, it will usually give you incorrect readings.

  • Why is this important?
    • Because every aspect of your engine's operation is determined by intake flow rate, most notably: ignition timing, fueling, knock correction, and idle control.
  • Why does it matter if it reads differently at +8 PSI than at +0 PSI? Can't you just tune for that?
    • +8 PSI can have the same actual airflow as +0 PSI depending on a number of factors such as throttle position and RPM.
    • As a generic example, 250 kg/hr airflow at atmos (+0 PSI) reports 250 kg/hr to the ECU. However, 250 kg/hr at +8 PSI reports about 500 kg/hr to the ECU.
      • This means any tune would be created for one circumstance and not the other.

It is worth noting that no retail "blow-thru" MAF we've tested have worked as advertised, and all have been standard MAFs, sometimes with a resistor added. Yes, this includes the HPX "turbo" MAFs that were supposedly designed to be used as a blow-thru MAF.

Manifold Air Pressure Sensors

Almost every factory-equipped F/I vehicle is equipped with a MAP (Manifold Air Pressure) sensor. This sensor, in conjunction with an intake-tract airflow sensor, provide an exact intake rate and manifold pressure to an ECU, which allows for far greater control over tuning. Manifold pressure is indicative of volumetric efficiency, which drastically affects tuning choices.

When converting from N/A to F/I, it is highly recommended to use an ECU that can read manifold pressures (such as MegaSquirt). It's not always a necessity, and lower boost pressures are usually perfectly acceptable on a tuned stock ECU. However, it is possible to add a MAP sensor to most any Motronic ECU using our SSSMCK (SSS MAF/MAP CONVERSION KIT). Read more here: SSSMCK WIKI

Alpha-N

Alpha-N is a tuning solution utilizing the TPS (throttle position sensor) and vehicle RPM. This is not a good tuning solution for turbocharged vehicles, and is also a poor solution, but to a lesser degree, for supercharged vehicles. It is not recommended to use Alpha-N tuning with forced induction vehicles.