Miller Fuel Injection

Gary Miller, Proprietor
email: gmiller@pacifier.com

Miller F.I. Educational Page

Note from Gary: If you ask "Why?" questions, this is for you. If you don't like engineering reasoning, this may bore you. Be forewarned!
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Mission Statement: Miller Fuel Injection (MFI) is in business to put Bosch CIS fuel injection on air cooled VW motors in on-road and off-road vehicles.

Why? Bosch CIS is the only mass produced self-tuning non-computerized multi-port fuel injection shipped into the US. Flat 4 VW's are modified and used in a wide variety of vehicles, with carburation a weak link. Most VW tweakers are mechanically inclined, and the rigors of tuning and troubleshooting Electronic Fuel Injection may be outside their expertise.

Bosch CIS systems are mechanical, with some electric heating elements for warm-up functions. Being a mechanical engineer educated in the late 70's, I appreciate the ease of troubleshooting mechanical systems. The later Lambda and CIS-E generation systems were computerized, and I am not using these intact. Some components are being adapted.

Most European (except Italian) 4, 5, 6 and 8 cyl engines in the late 70's-early 80's used Bosch CIS, and exhibited good running characteristics, including:
  • easy starting (cold and hot)
  • good torque at all rpm
  • competitive horsepower
  • competitive fuel economy
  • exhaust emission cleanliness
  • good ability to ignore accelerations (brakes, steering, forward, bumps)
  • altitude compensation (some installations)
  • robust resistance to dust, moisture, other contamination
Being a buggy (and sports car) driver, I became aware of CIS performance and the potential benefits for high acceleration flexibly-driven engines. When I speak of high acceleration, I mean bounce - turn - stop - climb - tilt, as well as "Light speed, captain".
Flexibly driven means lug-scream-lug-scream, repeat.

The airflow measuring components of MFI-CIS tune the fuel flow for real motor air consumption, making set-up on new installations very easy. Idle mixture and speed are the only adjustments to be made. Care has to be taken to test for adequate maximum fuel flow matching the motor/boost requirements.

From a marketing standpoint, the supply of usable CIS systems in used vehicles is nearly infinite, and the dedication of air cooled VW owners is legendary. Carburetors are fine for stationary engines that run at one rpm and load continuously. Computers have their place, but not in my buggy or _____ (insert your vehicle description).


System Overview

CIS FI systems have air and fuel handling systems. An airflow meter / fuel distributor proportions fuel delivery based on airflow.

Air Path: Air cleaner to airflow meter to throttle(s) to plenum(s) to runners to valves. Automatic warm-up throttle bypasses air for fast idle, if installed.

Fuel Path: Tank to prefilter to pump to filter to fuel distributor to injectors(1 or 2 per cylinder). Pump is operated by 12v. Some fuel is returned to the tank from the pressure regulator, and mixture control devices add to this return flow. Fuel is supplied to an electrically controlled cold start injector if installed. Mixture control devices are controlled by electric heat, motor (oil) temperature and altitude, depending on installation.

Why inject fuel? Think about carb basics, and some shortcomings.

Carburetors are integrated fuel and air metering systems, utilizing local apparent gravity, a pool of fuel and float to maintain supply pressure. Several effective venturis (even in a 1 barrel carb.) are formed depending on air throttle position, and fuel is metered and released in some or all simultaneously, depending on forward and/or reversed airflow to and from the intake valve(s). Accelerator pumps are present to counteract the condensation effects of fuel on cold manifold walls that occur during throttle opening. Chokes are secondary throttles to change cold metering characteristics by changing pressure drops across main and idle jets. Heat must be supplied to counteract venturi icing tendencies near freezing.

Local apparent gravity - hang fuzzy dice from the rearview mirror to check how constant this is. Most carbs stall when tilted (or accelerated) one direction, slosh out of the float bowl is the probable explination. In contrast, MFI-CIS uses pressure regulators to control supply and is not affected by 2-4 G's acceleration. Fuel tanks need to be baffled or sumped to deal with this, but it can be done. CIS FI air meters are sensitive to rotation about one axis, but are damped with a built in shock absorber (Bosch engineers are studs). Off-road users are reporting improved mileage, with excellent engine performance over rough bits. Lack of slosh out of the float bowl is the probable explanation.

Carbs have several effective venturis ("Is it the main jet, or is that the emulsion tube?"). Very complex to really understand or troubleshoot without a dyno. CIS has one air flap in one conical venturi moving one plunger valve with 4 (or 8) output streams. It measures intake airflow, and meters fuel directly. The air flap can be moved manually, and fuel delivered into individual containers for field verification of the system. Carb venturis can be sized for low or high rpm usage, but the usble rpm band of carbs tends to be quite narrow compared to MFI-CIS.
In the MFI Kits Page, quite a few dyno test comparisons are made between carbs and MFI CIS. In the real world, a 1.7 914 (Ed V. (Evill914@cs.com)) is approx.. 2 sec faster over similar autocross courses when dual Webers were replaced with MFI CIS.

Forward and/or reversed airflow occurs within each intake port / runner at low revs with radical cams. The individual cylinder carb solutions measure false momentary 2x to 3x airflow and deliver fuel accordingly. CIS FI uses an upstream common airflow measurement that is unaffected by reversion, since the effect averages over the whole engine.

In carbs, accelerator pumps counteract the condensation effects of cold air and cold manifolds, needed for good hp. Basic thermodynamics. Carbs have liquid fuel, and must be thermally isolated from the cylinder heads to some extent. MFI-CIS places the injectors so that fuel touches hot cylinder head and valve, not cold manifolds. Very bad transient effects occur when injectors are misplaced, unless accelerator pump effect can be included in FI design. Throttle body EFI must have this effect programmed for each application. CIS intake plenums, manifolds and air ducts from airflow meter to throttle must be correctly sized for good mixture during transients.

Venturi icing tendencies near freezing occur because of fuel heat of vaporization, moisture in the air, low temperatures and low pressures in the venturi. MFI-CIS is not sensitive to icing, due to high intake port temperatures and port fuel injection.

In short, measuring airflow upstream with a sensitive and wide-range meter, distributing the fuel at each cylinder, and maintaining supply pressure with regulators define FI and good engine performance. Carburetors are fine for stationary engines that run at one rpm continuously. However, for wide-RPM-range, high vibration conditions, the MFI-CIS kit for your combination will deliver solid, stumble free, stump-pulling power from idle to redline.