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.

S4 RX-7 Air Flow Meter and Adaptor Part 1.

First of all, please remember. Do the projects in my blog at your own risk. If something happens to you, it is your fault. Don't try this unless you agree with the disclaimer at the top of my blog! 

Oct 30th is my next track day. The weather is improving and it is a good chance to shoot for my 60 second goal. Unfortunately, my tires are wearing down also. I still need to do everyhing I can to bring down my laptimes. This time, I'm going to take some more drastic measures. I only have a couple of days. Need more power!

 
Check out my new intake setup. My RX-7 AFM is finally installed. Stock 1.6L airflow meter flows at about 165 CFM. At 7000 rpm, the stock 1.6L motor requires about 178 CFM.* The stock AFM is only good to about 6000 rpms. This is probably the biggest restriction in the intake system. The most cheapest and simplest way to remedy this situation is to go with an RX-7 AFM. If you got the money though, buy a good ECU and go with a MAP setup so you can do away with an AFM altogether. One thing to note: this will work for a stock 1.6L AFM also.

Which Air-Flow Meter?
The AFM out of the Series 4 ("s4") FC RX-7 (1986-1988) is the one you want. This AFM comes off of 13b rotory engines. This unit is compatible with the miata AFM. It is basically the same AFM with different dimensions. All you have to do is calibrate it a bit.

As you can see from the picture, the intake to the AFM is rectangular. It is also bigger than the hole in the stock AFM. Therefore, you cannot use the stock airbox, nor can you simply mount an aftermarket air filter on it. There are many adaptors out there that will work.

Choosing an adaptor
Many web-based vendors well cheap, simple mounting-plate type adaptors as pictured here. You can find these for less than 10 dollars on E-bay, and sometimes on Amazon. The problem with these is that the abrupt round-to-square transition is not very good for airflow. You want air to enter the AFM as laminar and fast as possible. You want the flow to stay attached to the surfaces of the intake tract as long as possible.

These are cast or machined units. Most of them are made from aluminum, but I think a few of them are made of steel or some sort of plastic. HKS, Bonez, Weapon-R are some brand-names that make this and I believe a few users on a Rx-7 forum make and sell this adaptor. Some of these companies used to sell theirs for under 30 dollars in the early 2000's. They are around 50-60 dollars now. Wow, 50 bucks for a mass produced adaptor of this sort? That disqualifies these units from our consideration. Lets make a Hi-Kick version of it. The adaptor I made has the tapered surface and should be plenty strong enough. Furthermore, the adaptor I chose is longer, and because of this the tapering is more gradual. It might even flow better!
 

Instructions

Okay, we're splitting peas here. The gains you get from having a tapered surface probably is not super-significant. And we're not going racing, where every gram of advantage counts. So why care? Because taper features apparently cost 40-50 more dollars, so we're going to come up with a solution and kick the problem in the head.
 
There are many ways to do this, but here are the specific parts I used.

1. Spectre Air Sensor Adaptor, Part 8141 or 81413   ($10-13)
2. Sheet of rubber, RTV sealant, or a usable OEM gasket on the AFM. ($5-8)
3. AFM off of a 86-88 RX-7 ($20-50)
4. JB-Stik or other putty-type expoxy that bonds to metal as well as plastic ($5-7)
5.Air Filter - From an S2000 or Honda Prelude. Prelude Part numbers: ($9-20, more in the $10 rage for the Prelude part)

  Fram CA6543
  Purolator AF4486
  Honda 17220-PK2-661

  STP CA6543
6. 3" Rubber or silicone coupler (PN: Spectre 8771) ($3-5)

Tools and other stuff you need

1. Dremel with a grinding wheel 
2. Sandpaper
3. Drill and 1/4" bit

The Spectre kit can be found on autozone or advance auto parts in the rice section. Everything else is available at your local parts store. The Prelude air filter is half the price of the S2000 filter.


Step 1. Line up the AFM, plastic plate, and the aluminum adaptor. Get the edges of the adaptor to line up with the opening in the inlet side of the AFM. Clamp it down. The holes on the adaptor plate of this particular kit line up perfectly with the AFM's holes. Mark the holes using a permanent marker, and then drill the holes using a 1/4" bit. This way, you have some room to move the items around since the holes are bigger than the holes in the AFM.

Step 2. Take the plastic plate that comes with the Spectre kit, and dremel or file down the opening so it will more closely match the opening on the AFM. Have the plate bolted in, along with the adaptor tube when you do this so it won't shift when you dremel the plastic plate. Resulting shape should be a rectangle with rounded edges.

Step 3. Sand down the plastic surface of the plate and the inner surfaces of the aluminum adaptor, on the areas that you will apply putty to. Wash it off with water and mild soap, rinse, and make sure it is completely dry before going to the next step. 
Optional step: Cut off the rubber plug that the Sepctre kit somes with.

Step 4. You can skip the sealant part of this step, but I took RTV and made a ring around the adaptor tubing, taking care to not to use excessive sealant lest it seeped into the surfaces where we are about to put epoxy on.

Optional step: before you start epoxying the entire thing, plug the hole made by the removal of the rubber plug.
Take the adaptor tube and mount it on the plastic plate, and bolt it in as how it would be when the kit is installed. Care to line it up properly. Don't mix up up and down, since the dremelwork will not be perfect. Mix up your epoxy and start filling the area left on the edges of the circle. Do one edge at a time, since you only have 5 minutes or so before the epoxy gets too hard. Be sure to press it in well so it will fill all the crevices and rough surfaces from the sanding.

Follow this diagram that I've attached, but it should be more elongated and gradual. You want a both convex and concave shape for it (I think there is a word for such shapes). Basically, you want the air to follow it up, over, and into the AFM without seperation. Try to get the surface as smooth as possible and the curvature as symmetrical as possible. Try to get the surface of the epoxy as smooth as possible with minimum bumps. Here is how to make sure the surface leading up to the AFM is flat: gob in excessive epoxy, then use a straight edge and press down, using the AFM inlet itself as a brace. Try to get the putty as smooth as possible with the least amount of bumps. It is kind of hard to do with your fingers. Matthew Wilson gave me this tip. Wet your fingertips with water, and it will allow your finger to move over the surface without gripping at it too much. It is also useful when making the edges of the epoxy more filleted. Remember: the smoother it is, the less sanding you will have to do.

Example of bad putty-work ;-)

Step 5. Let it cure for a few hours. Sand the surface of the epoxy with fine grit sandpaper until you are satisfied with the finish.

Step 6. If the gasket in your AFM is toast, use Silicone RTV to seal it properly by tracing the edges on the AFM where it is supposed to seal.

Step 7. Assemble. Self-explanatory. In order to use the Honda OEM filters, you have to use a flexible coupler since both the filter and adaptor opening is solid and inflexible.

Tada! Your adaptor is complete. J-B stik putty is pretty tough and resists vibration, so it should be good for this application. This should flow pretty well, though it might be a bit heavy side (maybe an ounce or two). Now that the adaptor is complete, you can mount any type of filter on it that has a 3" opening. Looks kinda cool doesn't it?

Enjoy!

Dog doesn't care about this new AFM set-up

Next up is an article on how to tweak the RX-7 AFM to run more optimally on your 1.6L engine, and how to mount it on the car.

* Flow data from www.solomiata.com, maintained by www.flyinmiata.com.  

Related links: Tuning the RX-7 AFM