90-93 1.6 intake upgrades
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This page documents my experience with the intake system on the 1.6l Miata. I no longer own this car (or any other 1.6 Miata) so this represents the extent of my knowledge on the subject.


Here's shot of the engine compartment. That's an RX-7 air flow meter adapted to the Racing Beat High Flow Intake. Yes, the stock Miata intake tube can be made to fit the RX-7 air flow meter, just dip it in rapidly boiling water first.
 

Close up of the Racing Beat intake and the RX-7 air flow meter. Do NOT bother trying this because, though it fits, the flange is too short and lets air in. Using this thick cork gasket is just a temporary fix.
 

I calibrated the RX-7 air flow meter using a variable speed Miele vacuum with the filters and dust bag removed to generate the suction. Connected to the air flow meter was a +5VDC power source and a digital volt ohm meter with averaging circuit (Fluke 83). Anybody considering this mod should read RandyS' page on the subject. Though there is a lot of good information there, I used Chris Erber's instructions to tune the RX-7 meter to match the original Miata meter.

There are NO short cuts to tuning an RX-7 air flow meter. Forget the advice about just loosening the clock spring 3-4 clicks. I purchased two air flow meters and that advice did not work for either one. At 3-4 clicks, the car felt very dull and flat. Furthermore, even if you do take the time to carefully calibrate the RX-7 air flow meter, it is still not an exact match to the Miata meter. The trick is to take the two closest settings and mark the clock spring wheel. Drive the car around for a week on each and see which one you like better. Personally, I chose the setting that erred on the rich side. Anyway, even calibrated as perfectly to the Miata as possible, the spring characteristic is not an exact match. To see this illustrated, check out this graph where I compared the two RX-7 air flow meters to the stock Miata air flow meter. Each point was measured using the Fluke meter over a period of one minute. The meter was set to average the readings (they do fluctuate slightly). Here is a new graph I made after calibrating two more air flow meters. Looking at my two graphs, it's easy to see that loosening the spring only 3-4 clicks is not even close. If you had to make the adjustment blindly (without actually measuring the flow), then a better setting would be to loosen the spring 8 clicks and adjust the fuel pump shut off rod if necessary. I has been my experience that when adjusted properly, some AFMs will need to have the fuel shut off arm adjusted and some will not.

After a few weeks, I removed the Racing Beat High Flow intake and replaced it with a custom intake using the cast aluminum HKS air flow meter adapter (part number: 17106-002188). The Adapter is about $20 from your local HKS dealer. It's listed on their web site under intake accessories as "13B" (as in 13B rotary engine). The air filter was just a filter that I picked up at my local parts house. I had been looking for a filter that would fit the HKS adapter's 2.75" inlet size. The filter also has an inverted cone, which is a feature that I was looking for.

Though my new intake is likely letting more hot air in, it still feels stronger in the upper RPMs than the Racing Beat intake. The portion of the Racing Beat elbow that is near the air flow meter (where the cast logo is) is restricted to the same inner size as the stock Miata air flow meter, thus somewhat defeating the purpose when used on an RX-7 AFM. The cast aluminum HKS adapter has a smoothed square to round transition that is more appropriate for the RX-7 meter.

Anyway, I came up with a special mounting bracket to make the install clean and simple. Notice in the below picture that the air filter sits low enough in relation to the diagnostic connector, valve cover and the brake master cylinder reservoir.

In the next pic, you can see that the hood rod is touching the intake tract. It only places a slight tension on the rod and shouldn't be enough to break the plastic retainer clip behind the right headlamp.
 

Here's a shot of the air filter with inverted cone (sure wish it was enclosed). The filter and diagnostic connector were re-positioned aft this picture was taken.
 

In this pic, you can see what a nice fit it is. Notice the clearance at the brake master cylinder and the diagnostic connector. Take a look at how the air big harness runs. I reused one of the harness hangers from the original Miata air box.
 

Here's a close up of the hanger I re-used from the original air box.
 

Another pic of the fitment. The flexible portion of the intake tract isn't bent very much.

 

Here is a close up of the custom bracket I made out of a single "L" bracket and some cap nuts and cap washers I picked up over at Orchard Supply Hardware. The flange bolts on the right are 6mm X 16mm 1.0 pitch. To accommodate the thickness of the bracket and the washer used, 6mm X 20mm 1.0 pitch flange bolts were used on the left side. The hole where the lower left blot is had to be slotted because the hole that was already there didn't allow for drilling a new hole. I contoured the right edge of the bracket with a die grinder. Notice the contours inside the adapter as the pattern changes from circular to rectangular.

 

I found that when mounting the HKS adapter and orienting the HKS logo on the same side as the air flow meter plug that the opening lined up precisely with the opening on the AFM. It was no quite as good of a match with the logo oriented the other way.

The bracket was made from a single "L" bracket as purchased from the bin at OSH. I cut off part of the bracket and re-used the piece to add to the bracket, thus repositioning the assembly when installed. The flat washers are not metric but they're darned close if not an exact match for the 8mm bolt I used to tie it together. The washers are 5/16 X 1-1/4 "fender washers." Notice how I used two flat washers sandwiched in between the pieces of bracket. There's also a washer on the top to make a more secure mount. By adding more washers, this keeps the assembly mounted lower. Mounting the "L" bracket *above* the cut off portion will make the air filter sit too high under the hood. The size of the flange but and bolt is 8mm X 25mm 1.25 pitch. I got the bolt a little on the long side so that I could lower the assembly (by adding more washers) if desired. I also used the large fender washers at the point where the custom bracket mounts to the car's body. The mount I use is one of two used for the stock air box (it's the long stud that is poking up near the shock tower). The bracket is sandwiched between two flat washers in order to reinforce the mount point. You can re-use the nut that held the factory air box in place or you can replace it with a silver colored flange nut to match the other pieces. Anyway, it's size is 8mm 1.25 pitch.

 

Here's a little trick I'll share with you in the name of aesthetics. I always orient the metal seam f the air filter and the screw in the hose clamp toward the bottom. This way, all you see holding the filter in place is a shiny sold strap from the top.

 

UPDATE

My intake has evolved yet again. The goals were to shorten the intake tract even further, increase flow and to use a paper air filter. I posted more information about this version on the Miata.net forums. Here are links to those discussions:

Here are some pictures of the intake in its present state.



This page was last updated on 10/24/04