Tuning the RX-7 AFM - Resistor Trick

A bit of searching on the forums revealed an old but simple and effective to tune the RX-7 AFM. The RX-7 AFM runs too lean, even though it still performs better than the stock AFM. This is especially true for the lower RPM's. It would help to adjust add more fuel at all RPMs. This isn't optimum but a good compromise between simplicity and price. Thanks DVCN for the information. Basically, you will be soldering in a resistor which would adjust the AFM throughout the entire RPM range.

Stuff needed:
1. RX-7 AFM
2. 25ohm resistor (Actually, anything between 20-25ohms) - Less than a few bucks at Radioshack
3. Soldering equipment
4. Sealant (RTV works great)

Air to fuel ratio
I'll do a short write up about what Air-to-fuel ratios are. There is a certain optimum Air-to-fuel ratio in each engine. Theoretically, the most optimum ratio is called "Stoichiometric mixture," which is approximated as 14.7 parts air and 1 part fuel, with numbers representing mass. The AFR is generally expressed in this manner. When someone says "AFR at 15" it means 15 parts air, 1 part fuel. Maximum torque usually occurs a bit under Stoic mixture. According to my co-driver Brian, who is also a mechanical engineer, this is due to the fact that air and fuel does not optimally mix. That is partly why electronic fuel injection is better than carburation, and direct injection is better than regular injection, and why all these different injection methods have been developed. Some of these injection methods attempt to create a condition where maximum torque occurs closer to the Stoic mixture. This makes the car more fuel efficient and more powerful.

Generally, a leaner burn is good for fuel economy because less energy is lost to heat. Slightly richer mixes result in more torque and cooler burn, preventing knock. Air-fuel mixtures are usually measured with an O2 sensor. A wideband O2 sensor is crucial for this kind of tuning, not that you need one to do what I did. Anyways, a 16+ year old miata engine is far from having any sort of fancy injection technology and optimum torque occurs a bit below 13:1.

People adjust this a little bit by adjusting the spring tension of the flapper door in the AFM. Looser spring means more fuel, as the door opens more and the AFM sends a signal to the computer that more air is coming in. Of course, in reality the same amount of air comes in regardless of the flapper door angle, but it does mean that the door opens up a bit more. Usually people tune by adjusting the spring 5-7 clicks looser. I don't know how much AFR is adjusted this way, but I do know that it is less optimum than this trick. Furthermore, the spring method messes with the idle, and you have to take further steps to make the idle behave.

The Resistor Trick
DVCN, who came up with this method of tuning. He found out that adding a resistor to the circuit on the AFM adjusts the mixture. He measured the following on the wideband:

20 ohms: 15's below 2k, 13's in the midrange going to 12.7 at 7000rpms.
25 ohms: Just slightly richer everywhere, going to 12.3:1 at 7000 rpms.
100 ohms: In the 11's at 7k.

I chose to use a 22.5ohm resistor. If you have 50 ohm resistors lyring around, you can run it parallel and you'll get 25 ohms. DVCN stated that 25ohms felt the best. I picked 22.5 because I just wanted the best top end I can get and I thought something richer than 12.7 but leaner than 12.3 would be more ideal for top end. I may be completely wrong on this.

Instructions
Anyways, here are the instructions. I did this with the AFM removed, with good lighting.


First: Open up the black cover on the RX-7 AFM. I used a screwdriver to pry out the cover. Try to be careful not to nick the plastic too much.
Second: Clear the way. The connection you want to work with is the third one from the right, with the AFM's inlet to the right of you. Bend the fourth one out of the way a bit so you can get the soldering equipment in there.

Third: Bend the resistor to the shape you think you will be using. Cut the connection appropriately. Clean the area if there are any visible impurities.

Fourth: Carefully solder the area. You don't need excess lead on the contacts. Make sure it doesn't overflow and touch other contacts.

Fifth: Seal the cover. I gobbed on blue RTV onto the cover edges and then placed it back. I gobbed on some more RTV around the edges afterwards. It will dry to a nice rubbery seal. Finished! Do a test run and see if you like it. I did not have to adjust idle at all; it sits okay at 850-1200 rpms. Funky because my car has power steering and AC removed, yours could be more stable.

Results? Added 1+mph on my maximum speed on my home track. Note that the second fastest speed I recorded were done on cooler temperatures. Between my mods, I accelerated faster than a 1.8l miata with a bone-stock engine.