Correcting Air Fuel Ratio (AFR), introduction
(The test will be next week, take notes!)

Last update: 11/09/09 Return to home page links

Links to other articles in this series.

Purpose of article:

This series of articles is concerned with the 2002 GSXR-1000 fuel injected motorcycle engine as used in  my Locost 7 bike engined car (BEC). Although the same principles apply to an engine with a carburetter(s), the methods of correcting the air-fuel ratio (AFR) are different. That is, changing carburetter jets etc vs: changing the length of time the fuel is injected into the engine by a computer.

This series of articles assume that you are installing a wide band oxygen sensor, the controller and a new engine control unit  to an automobile or motorcycle fuel injected type engine.


Right now my car is getting less than 15 MPG on whatever it is they call "gas" now days. I recently refurbished my 1954 Sun distributor machine air-fuel ratio meter and have found that the engine is running very rich above 4,000 RPM. Several months later I bought a wide band oxygen sensor (o2) and a controller kit for it. I performed the same driving tests with the new o2 setup and the readings are very similar to the readings with the Sun meter. I guess that old stuff wasn't so bad after all.

The overly rich readings are quite perplexing since most people end up with a lean condition when they replace the stock motorcycle air cleaner with foam ones.  I think several factors are causing the rich condition, such as the increased aerodynamic load and the drastically changed rear end gear ratio. After studying the problem for quite awhile I've decided to bite the bullet and install a second electronic control unit (ECU) to map and control only the fuel injection. The original ECU will continue to handle the other tasks it originally does.

What causes the stock engine control unit (ECU) to not work properly on a BEC?:

The ECU also goes by several other names such as ECM (engine control module), the "computer" etc.  I prefer ECU and will use that from now on.

When building a Locost with a normal car engine, the loads on the engine actually go down since the Locost will in all probability, weigh much less than the donor. Even though the aerodynamic Cd of the Locost is bound to be much worse than the donor, the frontal area of the Locost will probably be less than an older model donor. Therefore the total  load on the engine tends to be less overall.

Building a BEC presents the usual conversion problems. In addition to the obvious  differences such as the engine mounting and mating a drive shaft to the normal chain drive sprocket there are some hidden problems. Several things that will directly affect the fuel mileage are,
  1. The aerodynamic loads of the Locost will be much greater than the motorcycle engine donor.
  2. The rear end ratio of the car axle will be much higher (numerically) than the motorcycle donor (3.93:1 vs 2.47).
  3. The rear tires on the car are smaller in diameter than the motorcycle tire. This also "gears" the car lower than the bike.
  4. The car will weigh much more than the bike (on the order of three or more times).
When building a BEC there are several other reasons why you most likely won't be able to just connect everything up and expect the stock ECU to deliver the optimum AFR. In my case the engine runs fine but it is always way too rich and the gas mileage suffers. My early ECU system doesn't have an oxygen sensor so the ECU isn't smart enough to correct the fuel ratio in a "closed loop" mode..

The overall gear ratio of the motorcycle and the typical BEC are very much different. My engine in the motorcycle can go 175+ MPH. There's no way the brick shaped Locost will reach that speed with a stock GSXR engine (or any other normal engine).  The stock Locost 7 Cd is ~1.  That is as bad as you can get. Surprisingly though, Formula One cars have about the same (bad) Cd depending upon the way they are set up for different race tracks.  Even the large Hummer has a better Cd than 1 !  Of course the Hummer has much more frontal area than the Locost though.

As an example, to drive at some set speed, say 60 MPH, on the bike would require a certain throttle setting. The stock ECU was factory programmed to deliver the correct AFR under those conditions. When the same engine is used in a car, the load on the engine typically increases and the total gear ratio from the engine to rear tires will be much different than the motorcycle. It seems reasonable that the throttle butterflys will be more open on the Locost than on the motorcycle. The ECM usually "corrects" this by dumping more fuel into the engine because of the increased throttle opening to maintain 60 MPH.

Another subtle difference is that the GSXR-1000 (and perhaps other makes) motorcycle has two air intake tubes that run from the sealed air cleaner box to a pair of openings in the front faring. The tubes are placed so that at high speeds air is rammed into the air cleaner. This results in a positive pressure in the air cleaner above ~75 MPH. This acts similar to low pressure supercharging. The various ECU sensors (which are very similar to a normal fuel injected car) cause the AFR to go richer to utilize this denser air. The fuel map in the stock ECU takes the rammed intake air at high speeds into account which results in more engine output power.

As most BEC builders have done, my new air cleaner did away with the rammed air affect. So on a typical BEC we don't utilize forward air ramming. Unfortunately the ECU still gathers the same sensor information (minus the increased positive pressure in the air cleaner box) which seems to result in more fuel being added anyway.

When I measure the AFR on my car, I see a change towards an overly rich condition above 4,500 RPM even when cruising at steady throttle openings. This would be about 67 MPH on the bike. On my car though I am only driving about 39 MPH at 4,500 RPM. Above 4,500 RPM the engine is always too rich, above 5,000 it is always *way* too rich!

My stock Suzuki ECU system doesn't have an oxygen sensor (o2) so there's no way for it to correct for changes in air cleaner restriction etc and runs open loop at all times. In addition, with the low geared rear end I have the ECU thinks I'm driving much faster than I am. (By a ratio of 3.93/2.47 as determined by the rear axle/sprocket ratios.)

I have a program called "Car test" that can calculate the expected fuel mileage after you input your car's parameters. As my car is set up now I should be able to get 25 to 35 miles per gallon (MPG) city/highway. Right now I'm getting less than 15 MPG. In talking to GSXR riders they usually don't get 35 MPG on the highway. Of course when you're doing a wheelie at 100+ MPH your gas mileage may vary.

OK, enough background let's get to the next step.

Some methods of correcting the AFR:

In the process of modifying a motorcycle engine there are several products (PowerCommander is one of them) that you electrically connect between the stock ECU and the fuel injectors to alter the signals to the injectors to change the AFR. These devices are somewhat limited in what you can change and the designs are proprietary. As time goes by, the units are coming out with more features.

A group of people, the "do it yourself" (DIY) group have developed kits to build homemade oxygen sensor controllers and ECUs to replace the stock units on cars, trucks, motorcycles etc. They encourage people to build and modify the hardware and software for the benefit of the group. Several people involved in the original projects have branched off and sell open source kits and components for DIY people.

"Open source" in affect means that people meet on various forums, suggest hardware & software improvements and the Gurus then post the results for everyone to use. These DIY ECUs have evolved so that they now duplicate all of the stock ECU functions and actually have features  the stock ECUs don't provide.

Another feature of the DIY route is that it is also intended to be a learning process. Like a lot of situations in designing and building your own car it won't be easy at times. But if you stick with it you will become the neighborhood expert (maybe even the whole town?) on fuel injection.

Megasquirt is one of the open source DIY supplier of ECU kits.  They are used on many road going, racing and record setting vehicles, boats etc. So the DIYs do work. You not only build the hardware, you can learn how the cars sensors and computer work and most importantly, you can tune your car while it is being driven by using a laptop computer (or run it on a dyno and tune it). You can log data from the ECU while driving the car and then make corrections to the tune when you get home and have some time.  There is now a reasonably priced program for the MicroSquirt ECU that can tune the car as you drive!

You want to add nitrous or a turbo later?  No problem, just re-tune the ECU.  Want better gas mileage? Tune the ECU with that in mind. And there are forums where other DIY people will help you through the process.

Considerations to take into account for my BEC:

GSXR-1000 engines prior to 2003 use a 16 bit micro-processor and do not have an o2 sensor as part of the ECU system. In 2003 Suzuki changed over to a newer 32 bit EC. I think that 2005 was the first year that Suzuki started using an o2 sensor and is better able to control the AFR. At that point people started hacking into the ECU and they are now able to directly modify the software code that controls all aspects of the engine operation (fuel, ignition etc).  Naturally my 2002 engine has the older 16 bit ECU and no one has hacked into those units yet.

While it is possible to convert my electrical system and ECU to the newer 2003+ ECU, it isn't the best way for me to proceed for several reasons. One important reason is that even after changing to the new system the DIY systems still have more features for about the same or less cost even if I used "pre-owned" Suzuki hardware.

Another consideration is that the DIY ECUs provide several extra functions that I would like to have on my car.  These include a way to very quickly cut the fuel OFF to the engine for my paddle shifter, spare outputs that can control the carbon canister emission purge cycle etc and other features.

Features of the stock GSXR ECU that can NOT be easily incorporated into a DIY ECU at this time;
  • The stock ECU controls a secondary set of throttles that improve the engine response etc which the DIY units don't provide for.
  • The stock ECU controls the fast idle circuit. While the DIY can do this, the GSXR circuit is somewhat different than the DIY method.
  • The stock ECU drives the motorcycle instrument panel that I am using. (This is very important to me!)  The DIY can NOT do this at all.
  • The stock ECU can display sensor error codes that the DIY units don't do at all.
  • Most importantly, the stock ECU controls the ignition which is not similar to car timing. The two outside cylinders of the Suzuki engine run at 2 more advance than the inside two cylinders. And truthfully I have no idea what the advance requirements are for a high strung motorcycle engine.
For these and other reasons, I have decided to retain the stock ECU to control the ignition and the above items and will add a DIY ECU to control the fuel injection.

What sensors are required for the ECU to operate:

Actually a fuel injected motorcycle engine requires the same information from it's sensors as a normal fuel injected car engine. The Suzuki sensors should be able to drive the stock ECU and the DIY ECU in parallel at the same time with no modification.

As is the case of many parts on a car or motorcycle, each manufacturer may use different methods to reach the same goal. One area that varies between manufacturers is the way to  measure the weight of air that is consumed by the engine at any given time. Air-fuel ratio is determined by weight, i.e. for gasoline 14.7 parts of air by weight to 1 part fuel (by weight).

Some car/truck manufacturers use a flapper valve in the intake duct work that is opened/closed by the volume of air passing by.  The weight is calculated by the ECU taking other sensor readings into consideration.  Another way to compute the weight of the air is to sense the difference in air pressure before and after the throttle valve and again use the various other sensor readings.

This is a list of the sensors used by the stock ECU on the fuel injected GSXR motorcycle engines;
  1. Camshaft position sensor (CMP) to determine the position of the reference piston for timing
  2. Crankshaft position sensor (CKP) to determine ignition and fuel injection timing
  3. Engine coolant  temperature (ECT)
  4. Intake air pressure (APS)
  5. Intake air temperature (IAT)
  6. Intake manifold air pressure (IAP)
  7. Oxygen sensor (HO2) (Used on later models)
  8. Throttle position sensor (TPS)
  9. Gear selected (position) sensor (GPS)
Sensors 1 through 8 will be used by the DIY ECU. My engine uses an eight tooth crank "wheel" and a single tooth camshaft sensor.
The motorcycle has a few more sensors not related to AFR that I did not list. For instance a car does not need a kick stand UP sensor etc.  Although the roll over switch is still wired into the car but hopefully won't be utilized in normal use.

Throttle bodies:

This topic seems out of place in an introduction correcting the AFR but it is one of the things I had to consider in order to install the new ECU. I have a separate page devoted to this subject here,  Throttle body modification

Return to top of page

Return to "Injection links" on home page

Continue to next article in this series, "Bung installation"