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
Posts
