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Miata Air Flow Meter Replacement

by Randy Stocker

Question: I read that for your 1.8 motor upgrade you modified your 1.6 fuel injection with a larger air flow meter from an '86-88 RX7 because the OEM 1.6 flow meter was too restrictive. I have modified my 1.6 with a muffler and K&N cone air filter. Will the RX7 flowmeter help me make more power?

Answer: If you never turn the motor past 6000 rpm then you'd be wasting your time. The factory flowmeter CFM threshold before limiting HP is about 6000 rpm with the 1.6 motor (the stock engine will flow 178 CFM at 7200 rpm while the meter is rated only at 165CFM). If your 1.6 motor is relatively unmodified (maybe a performance muffler and intake plumbing) but you regularly go past 6000 rpm then putting the RX-7 unit in will produce about a 5 HP gain, you'll probably only notice it after 6000 rpm though.
Dyno Proof

Flow measurements Proof.

If you have done more than just a muffler and aircleaner upgrade then you'll definatley see some gains. A muffler and custom intake plumbing with a K&N cone air filter alone do not increase the flow requirements enough to really take advantage of the RX-7 unit but it does offer a great foundation for future mods. Air flow is basically a function of displacement times RPM. When I say modified I mean thing that greatly affect air flow requirements like 2mm overbore pistons, larger valves in the head, more cam lift/duration, head port enlarging, etc. See Miq Millman's page for an idea of what mods I am talking about.

Question: If I replace my tired 1.6 with a 1.8 like you did, will I have to add the RX7 flow meter?

Answer: The flowmeter change is not a requirement. You'll only want it if you need to make HP above 6000 RPM, below 6000 rpm it offers no gains. The 1.8 motor will still go to the 7300 cutoff with the smaller OEM meter, it just doen't make any HP up there.

Question: Were there any electrical modifications necessary, or was this just a plug-and-play replacement?

Answer: The RX7 meter was plug-in. All the pins in the connector were the same and all the sensor resistance values were equal. The flowmeter only does 3 things, A potentiometer measures how far the flapper door is open (hense air flow), An air tempurature sensor measures ambiant air temp (the white plastic thingie that protrudes into the air stream of the meter), and a switch for the fuel pump to shut it off if there is no airflow (that is why people with bad airflow meters will have their cars die immediately after it starts). I did need to buy a universal K&N canister filter to replace the OEM airbox as the larger diameter of the RX7 meter doesn't fit the 1.6 box. This required getting a square to round adapter to get the filter to fit on the meter. I used an adapter that I found used on a mid '80's BMW 5 series, they are available new on the aftermarket too. These are available from Flyin' Miata.

RX7-Flowmeter1 (58 KB)RX7-Flowmeter2 (65 KB)

Question: Did the RX7 flow meter need to be recalibrated for the 1.8 motor?

Answer: No, however I did slightly adjust the RX7 flow meter to get the mid-throttle fuel/air ratio correct to eliminate a dip in the torque curve at 4500 rpm. It wasn't required but I wanted the fuel ratio to be perfect. I installed a fuel ratio gauge and an exhaust tempurature gauge prior to the 1.8 motor replacement so I could get baseline O2 measurements. Additionally I checked the plugs prior to and after the conversion. I had to adjust the new meter about 8 clicks softer (richer) to get mid throttle mixture back to where it was. This is accomplished by opening the big plastic cover and adjusting the spring tension (Never unscrew the connector as you will damage the soldered connections). After removing the the plastic door, remove the spring holddown screw. Keep your finger on the spring to keep it from uncoiling (mark a reference point with a marker just in case). Then just move the spring X number of backing plate clicks (see also links page to procedure for MR2 flowmeter).

If your throttle tip-in has a flatsport or hesitation you can also adjust the air bypas screw of the flowmeter. It is covered by a metal plug.

af_mtr1 (14 KB)

One thing that was snipped from the GRM article is that I used an adjustable fuel pressure regulator to get the WOT fuel curve a little richer too. After I installed the 1.8, my fuel ratio guage showed the fuel ratio was going to lean under WOT. I checked the plugs and they were chalk white with a little tip melting! I now have the regualtor set at 50 psi (from 43.5 stock) under WOT and the ratio is perfect. I used the Downing/Atlanta Sebring supercharger unit.

Question: Can the flowmeter be repostioned vertically or flipped on it's side or does it have to stay horizontal?

Answer: The position doesn't matter that much. As you can see from the DIY intake page (on Recipe for HP) I have had mine in both sideways and upside down. Gravity will affect the flapper door slightly so at worst you may have to adjust the spring tension of the door to compensate. Over time, AFMs that have been mounted in a vertical position have shown signs of weakening springs.

Question: I think that the 1.8 OEM meter is also a bit restrictive for the 1.8 as well. Is it?

Answer: The 1839 cc Miata motor only needs about 220cfm airflow to be able to turn 7500 RPM without dropping off HP. I spent $40 to have my OEM 1.6 flow meter checked, it flowed 165 CFM. That is only enough flow to support a 1839cc motor to less than 6000 RPM. I'll bet the 1.8 hot wire mass air flow meter is in the neighborhood of 200 cfm. (?) The calculation is (CI * max RPM * efficiency)/3464. The efficiency factor I used was .9.

112.3*7500*.9/3464 = 219 CFM

Even if the 1.8 meter is a little small it probably is not worth the effort unless you have done other things to your motor too. Increasing the size of your air flow meter will do nothing for torque or low RPM HP, it only helps high RPM horsepower. The `87 RX-7 flow meter has made a huge difference for me after 6000 rpm. No difference below that.

Question: Can I still add a larger air flow meter from another car to my 1.8 fuel injection system?

Answer: The 1.6 cars uses a traditional flapper door style air flow meter. The 86-88 RX7 meter is a flapper door style too. The 94-up Miata's use a Bosch licensed (Nipsendo?) hot wire mass air flow sensor. These two are totally different systems and are not plug-in compatible.

Since I do not have the 1.8 computer I haven't spent much time looking into them. I suggest doing what I did and find the specs for the OEM and then finding a larger unit with the same specs. Most likely you'll have to change the calibration to compensate for the larger diameter. That is easy to do on the flapper door meters, I don't know what is involved for the hot wire systems.

I found the specs for my 1.6 flow meter in the 1992 Miata workshop manual. I've read a friends 1994 workshop manual and I remember seeing the specs for the Bosch airflow meter in there. (look under the computer diagnostics section). Once you find a larger unit, you'll have to read it's shop manual to see if the specs and cable/pin arrangements are the same.

Unfortunately, nobody has really bothered to do this yet (5/1/97) so you'll have to blaze new ground yourself. From my basic knowledge of how the systems work it is probably a good bet that other Mazda (and most other makes) will work fine. Bosch has the license for almost all electronic fuel injection systems and they are almost all at least similar. I would stick with a 89-91 RX7 unit for your first try. The 89-91 RX7 was rated at 164 HP and the 13B rotary had a piston equivalent displacement (airflow requirements) of about 2.6 liters. The air intake system components from that RX7 should be easily up to the task of supporting a hopped-up 1.8 liter and you have a very good change that they are compatable.

Question: Will the RX-7 flowmeter add performance to by 6psi Sebring supercharger or Greddy turbo?

Answer: I don't know. I don't own a boosted car. Only a Dyno can tell you for sure but I suspect that a 6psi system will not see much gains from the flowmeter interchange. When I had the flowmeter bench tested it flowed 165 CFM at 10 inches water and 300 CFM at 28 inches water. The 10 and 28 inches are flowbench *standards* for NA and boosted respectivly. (The standards are generalizations so YMMV). A 6 psi boosted 1.6 will suck 300 CFM at the flowmeter at 7200 rpm (source = Norm Garrett at 2/1999 tech day) so the flowmeter is sized adequately for small boost levels. More boost, displacement, or rpm will definatly require something larger.

UPDATE 4/4/99 See Chris Erber's installion of a RX-7 flowmeter on his 8 psi Sebring equipped Miata.

Question: You stated before that it is important to keep the resonance chamber in the intake system for good torque. Do you know how/why the resonance chamber works??

Answer: (This answer needs to be updated) Not really. All I know is that if a chamber is not there then you'll get a dip in the torque output, usually around 4000-4500 rpm. I think it's something called a Helmholtz (?) theory. I understand it helps time the intake pulses to when the intake value is open. Like opening a door just when an ocean wave approaches so it doesn't slam up against the door (poor attempt at analogy).

It has to do with the pressure wave created by the intake air that had been flowing smoothly into a cylinder suddenly coming up against a closed intake valve. That pressure wave then runs back through the intake tract and can interfere with the normal flow of air. At a certain RPM, based on the engine displacement and the length and volume of the intake tract, a presure waves will resonate with enough violence to restricting airflow. The (anti)resonance chamber in the stock intake tract absorbs the pressure waves before they cause problems, i.e. at a certain rpm the frequency just vibrates back and forth in the chamber without leaving.

The chamber is basically sized by the volume of one cylinder, the length and volume of the intake runners, and the target rpm to cancel out the worst resonance. Manifold design plays a big part in the effects of and rpm of the resonance but generally speaking, the larger the resonance chamber the lower the rpm responance it will cancel and the smaller the chamber the higher the rpm it will cancel.

It has been dyno proven to provide 4 ft lbs of torque from 3000-4000 rpm over not using one. If you cannot use the OEM piece then use the stand-alone unit from the 90-93 Protege/91-93 Ford Escort GT. Protege resonance chamber (42 KB)

The Bosch fuel injection bible explains it better. Also Try Philip H Smith's book "Scientific Design of Intake and Exhaust Systems". There is also a book by David Vizard on tuning intake manifolds

Question: I'd love to be able to completely replace the flow meter with a map sensor with the same specs.

Answer: I looked into MAP sensors conversion kits too. HKS and many others sell a MAP sensor conversion, but they are about $900 (ouch), for that kind of money you can buy a completely programmable ECU and be far better off.

The 1.6 OEM flapper door flow meter potentiometer measures 3.5 volts at idle and graduates linearly to 0 volts at WOT with a 5 volt reference signal. (I found this in the shop manual). If you can mimic that signal with another sensor than you can remove the airflow meter (see the carb replacement article from the summer '92 MCA mag). A linear throttle position sensor (hard to fabricate linkage) or a vacuum sensor will work. Most MAP sensors give 0 volts at idle and 4.5 volts at WOT with a 5 volt reference signal and vacuum sensors (common on early 80's computer controlled carburated GM cars) give just the opposite at 4.5 volts at idle and 0 volts at WOT.

Be forewarned though, since the vacuum signal is not tied to engine RPM, as the flow meter is, it goes to 0 volts as soon as you floor it, so the engine runs rich at midrange RPM while accelerating. I couldn't figure that last one out so I opted for the larger RX7 flow meter as the easiest solution.

Question: Where can I find information about the fuel injection system and general cross reference information of MAP sensor?

Answer: I only know of the OEM shop manuals and the Bosch fuel injection bible. Also check the Coollinks page for more information on how to adjust the 1.6 fuel ratio.

Question: I want to get my throttlebody bored out. Will it give me more power?

Answer: Probably not for a stock 1.6. See answer to #1 above. The factory TB is 55mm which flows nearly 300 CFM. That is enough flow to support a stock cam'd 1.6 to very high rpm levels. The flowmeter and the intake manifold are more restrictive than the TB so replacing them first is better. The 1.8 could probably benefit from a resized TB for a little more airflow.

UPDATE 7/1/98 There has been some confusiuon on what an 86-88 RX7 flowmeter looks like. Seems that many junk yards simply put all the flowmeters in a large box for you to sort through. The RX7 flowmeters external dimentions are the same as the miata except for height. When laying flat with the plastic cover facing up, the length and width are the same. The RX7's internal dimentions are larger. The Miata measures 1 3/4" square, the RX7 meter measures 2" wide by 2.5" tall. Hope this helps.

flowmetercomp1 (42 KB)flowmetercomp2 (48 KB)

UPDATE 10/26/98 Seems that many people are having their 1.6 airflow meters going bad on them. I have one bit of advise if you are trying to diagnose your airflow meter: the shop manual is somewhat misleading about the testing process since it lists the potentiometer testing ranges in ohms (resistance). The resistance checks are really there to see if the contacts are good and not check the function of the potentiometer. Since the potentiometer uses a sliding contact the ohms readings will bounce around a lot and this concerns many people, this is normal for a sliding contact. The potentiometer is really checked with volts. Get an analog voltmeter and, as stated above, check to see if the potentiometer registers 3.5 - 0 volts with a 5 volt signal .

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