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Introduction to ECU Chipping
by Dave B.

With the move from carburetors to fuel injection, computers have assumed an important role in the proper functioning of an engine. The level of precise control possible with a ECU (Engine Control Unit - computer for running an engine) can not only allow for greatly improved fuel economy, but can also allow for fine tuning of an engine to a degree that is arguably impossible with the best carburetors. Many ECUs, including those made by Honda, do data interpolation, allowing them to intelligently compensate for engine conditions that fall "between" known states, allowing for an almost continuously variable response to varying engine conditions. Additionally, the ECU can make small corrections to fuel/timing based on the state of various engine sensors. It is probably fairly fair to say that from the factory, most manufacturers tune for fuel economy, emissions and poor quality gasoline as opposed to power and race gas.
Honda is no exception to these rules about tuning. Basis for this can be seen very clearly by comparing European/US fuel and timing maps to Japanese timing maps for the same engine. Japanese have gasoline that is on average a minimum of about 10 octane points greater than gas sold in the US. Comparing JDM and USDM/EDM timing curves generally show the JDM maps having several degrees more spark advance, reflecting the manufacturer's expectation of better quality gasoline. I don't really have enough experience tuning or on a dyno to be able to say exactly what is possible, but you have to wonder about how much 205 HP out a 1.8L (ITR) can be improved on. However, I doubt it would surprise anyone if another 5-10 HP could be squeezed out of an ITR by HOURS on the dyno and some changes to the ECU program, particularly in the control over timing.

This does bring us to a valid point. Tuning is best something done on a dyno, or at least with something to give a very objective measure of how the car is responding. Wideband O2 meters and EGT probes can give you some idea of what is going on, but they are generally more effective at tuning air-fuel ratio than timing adjustment. The "butt dyno" is a totally inappropriate way to determine how the car is performing - most of the changes you will make to a computer program will be subtle enough that you probably will not be able to "feel" them. Beware of most aftermarket "chips": more often than not these programs just dump more fuel and increase timing greatly while doing things like raising revlimiters far beyond safe levels. There are very limited gains to be had from most "performance chips." Tuning not "chipping" generally produces usable, real world gains. That being said, changing the program of a ECU is generally not that tricky. First it would probably be wise to focus on some basics of how ECUs work so that it is more apparent where we could makes changes to have a positive effect on performance.

Most ECUs seem to operate in two modes. In one mode, the ECU is monitoring as many engine sensors as possible, trying to get the best fuel economy. This mode is normally referred to as "closed loop" because it is a closed feedback system. In the other mode, the ECU is generally trying to achieve the greatest level of performance possible. This mode is normally called "open loop" because it does not continuously monitor sensors and compute fuel correction: simpler routines are used. It's difficult to change the exact behavior of the ECU under closed loop conditions because it requires an understanding and possibly a re-write of that portion of the code running the ECU. Open loop conditions are generally stored in the form of a table where the ECU "looks up" fuel and timing values using a relatively simple algorithm. In most cases, the open loop tables also serve as values to use as a base for fuel correction routines. Modifying these tables is the most basic way to retune a car, however it bears remembering that the ECU will be making adjustments to whatever fuel/timing information it has.

You can completely change the operating characteristics of an engine by changing the fuel tables around. As an example, a PM6 (88-91 Civic/CRX Si) ECU is normally used to run a D16A6, ~100HP. By little more than changing the fuel, ignition tables and the revlimiter, a PM6 can run a DOHC ZC, ~135HP, like it came out of the factory. Additionally, little more than even more fuel/ignition map changes and revlimiter changes are all that is needed to make it run a B18A/B18B or a B20B! In another more extreme example, a friend of mine recently had success running a 2G accord engine with a GM computer! Please, be careful changing your engine's fuel/timing tables - understand that timing changes can completely change the operating characteristics of the engine! You can destroy your engine in seconds with a bad program. Please be very careful, especially when doing baseline tunes as opposed to modifying an existing tune.

With that out the way, we are now going to pretend that you now know what fuel and timing adjustments you would like to make, because an adequate explanation of how to tune a car is well beyond both the scope of this article and my expertise.

Fuel and timing tables are stored in the middle of the code running the ECU. Distinguishing them from code is generally not THAT hard because they "look" different than the pseudorandom data that runs the computer. A hex editor will let you manipulate the raw data in the ECU. WinHex is a quality, shareware hexeditor available from You might want to grab some checksum utilities from as they're generally useful.

In winhex:

So, you've found the fuel and timing tables in your ECU's program, edit away. Most ECUs have a checksum routine - the checksum gods must be appeased after any changes to the ROM are made or the car will not run properly.

Alternatively, you can use a ROM editor. These programs are generally specific to one or more ECUs, and allow a more visual representation of the data inside the ECU. Ghettodyne (available at is a free ROM editor that currently supports the PM6 (88-91 CRX/Civic Si) ECU. Fortunately PM6 ROMs will work in almost any 88-91 MULTI-POINT INJECTION CIVIC/INTEGRA ECU (PG7, PM6, PM7, PM8, PR4) so this is not as limiting as it initially sounds. To give you an idea of what ghettodyne looks like:

At this point, the assumption will be made (regardless of whether it is actually true) that you have successfully created a new ROM image and you want to put it into an ECU. The next step towards reprogramming is to take the electronic image you have created and program it into an EPROM(Erasable Programmable Read Only Memory). EPROMs for automotive use are generally 128K, 256K or 512k in size. Use the same size EPROM as the factory uses. Almost all 88-95 Hondas use a 27C256 EPROM. (A quick note about chips: 27CXXX chips are generally UV erasable - you need a special UV light; blacklights do not work. 28CXXX are generally EEPROM - Electrically Erasable... and 29CXXX are generally Flash ROMs. There are exceptions to these rules, however.)

In order to program an EPROM, you will need a ROM burner. It does not really matter which model you get as long as it can program the chips you need. A cheap, hobbyist-oriented burner is available for less than $100 ready assembled (or less in kit form) from Another unit is available from and is the programmer of choice for Hondata. Needham Electronics also make a quality programmer. The point of all this is quite simply that it doesn't matter WHICH programmer you have; being able to program 2XC256 chips is what matters.

Now, with your new EPROM ready, crack the case of your ECU. All 92-95 ECUs made by Honda were able to accept an external EPROMs, even if they did not come with one from the factory. All 92-95 ECUs use the same procedure for chipping. Beginners beware of Japanese ECUs as they use surface mount components that are marginally harder to work with than US/European ECUs. The section of the example P28 93 Civic Si ECU that is important for chipping is outlined in red in the following picture: (Honda was nice and outlined it in a dashed white line for you on the board too)

First, you must de-solder the parts silkscreened in white on the circuit board. I purchased a Xytronics desoldering/soldering station with temperature control for under $400 shipped from Howard Electronics Inc. Professional desoldering tools with integrated vacuum sources can be obtained readily from ebay or other merchants - Pace and Metcal gear is always on ebay. You can get a desoldering tool from radio shack, but it does not work terribly well and I would highly recommend borrowing or buying a true desoldering iron if possible.

Once all the pads are desoldered and you can see through holes for components in the red section above, you need to start soldering in components. No bizarre components are used - everything should be readily obtainable from your local electronics store, Radio Shack, Digikey, Newark Electronics or just about any other electronic parts distributor. Honda was nice enough to label on the board where everything goes. Place the 74HC373 chip on the board where indicated and solder it in place. R54 should be filled with a 1k to 1.2k resistor(the exact value does not matter). C51 and C52 are 0.1µf ceramic disc capacitors(12V or better will work fine). Place a 28 pin socket for the ROM where 27256 is marked. Once the socket is in place, place a the ROM you programmed earlier in it. Finally, connect J1 with a small wire to enable the external ROM. No changes other than cutting J1 are needed to restore the ECU to its original state.

Here you can see a chipped P28 ECU. The person who chipped it used a socket for the 74HC373 as well as for the ROM socket, and used a real jumper instead of just connecting J1 with a wire. Nevertheless, it will give you an idea of what it should look like when you are done:

If you have a OBD0 ECU, chipping the ECU might not be such an easy option for you. Almost ALL 90-91 PM6, PM7, PR4 and PP5(with catalytic converter) ECUs had external EPROMs, and are easy to change the program on. Almost all 88-89 PG7, PM6, PM7 and 88-91 PM8 ECUs did not have an external EPROM. It is possible to chip these ECUs, but it requires considerably more work and is beyond the scope of this article. The following ECU is a PM7 that does not have an external ROM:

If you are luckier, the ECU you have will look more like this:

If you are lucky enough to get an OBD0 ECU with an external EPROM, it is very simple to chip it. First, remove the ROM present by cutting all 28 pins with a razor blade or utility knife. BE CAREFUL YOU DO NOT SLIP AS YOU CAN PERMANENTLY DAMAGE THE CIRCUIT BOARD. It is not absolutely necessary to do this before desoldering, but removing the chip will make it much easier, particularly if you do not have a good desoldering station. Once the original ROM is desoldered and removed and you can see through the holes in the circuit board, add a socket for an EPROM and then place the new ROM you have created into the socket.

If you have a OBD2 ECU, you are just going to have to wait or buy commercial TechTom gear.

At this point, you should have a working ECU with a new program. If you get a solid red light on a OBD0 ECU or a continuous CEL on OBD1 car and the car runs terribly with a very low rev limit and a stumbling idle, you probably have a bad connection. If the same thing happens 30-45seconds after you start the car, you probably failed to pray to the checksum gods. If the car runs weird, try putting a stock program in an verifying that the fuel/timing changes you made were not detrimental.

Hopefully, at this point, you have learned a little about ECUs, how they work and break, and how to modify them. If you have further interest, the following sites have more information for you to process: - started out with a heavy slant towards domestics but has a WEALTH of good knowledge and tons of very smart individuals on the mailing list - a commercial vendor of standalone engine management systems that has a tech section with every article worth reading twice - a site specific to Honda ECUs

Brokedick Millionaire
40,422 Posts
might wanna plug Dave's website.........

Whose chip you think I'm running?????
Sadly, I think he is moving back to Chi-town and I'll have to learn to make my own chips.

1 Posts
Don't suppose this write up has the pictures lingering anywhere does it?
It has been a great read but i can't find it on pgmfi >.>
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