The "Hacked MAF" for an S14 KA-T...

The theory:

To run bigger injectors without an S-AFC or upgraded ECU on a KA24DE turbo, you can 'hack' your stock Mass Air Flow Sensor (aka MAF) to sneak a proportionate amount of extra air into the motor to match the extra fuel being sprayed due to bigger injectors.

If you want to run bigger injectors (say the 370cc side feeds form an SR or Z32 TT motor) you need to compensate somehow, otherwise you'll run VERY rich, and have poor performance.

The easy ways to accomplish in the past was to get an Apex'i S-AFC, which is a piggy-back controller that alters the airflow meter signal before it gets to the ECU and tricks the computer into spraying less fuel as desired...

To understand how this works, you need to understand how the Nissan MAF works - It has a hot wire in it that sees a cerain voltage. As air passes over the wire, it cools it, and the ECU sends more voltage to try and keep the wire a constant temperature. Based on the amount of voltage needed to sustain a certain temp, the ECU determines the total mass of air entering the motor. But it works based on a few assumtions, one of which is the cross sectional area of the MAF tube.

So if we increase the cross sectional area by some percentage, we get that much more air entering the system unmetered...so it can compensate for the extra fuel being dumped by the larger injectors.

An example:

370cc injectors are 37% larger than stock [270 * 1.37 = 370]

So we need 37% more cross sectional area in the MAF tube to be close to being "calibrated"...

Remember, the area of a circle is [ Pi * radius^2 ]
So we need to know the stock S14 KA MAF inner diameter (ID).

By my best measurement, it's 2 29/64", or 2.453". Divide by 2 to get the radius (1.2265"). Square the radius, multiply by 'Pi' (3.1416), and you have the stock cross sectional area:

1.2265 * 1.2265 = 1.5043 * 3.1416 = 4.7259 square inches

Now, increase that by the same increase in the injector size (37% in this example), then divide by Pi, and take the square root, and multiply by 2 to get the new, recalibrated ID.

4.7259 * 1.37 = 6.4745 / 3.1416 = 2.0609 [square root] = 1.4356" * 2 = 2.8711" ID

So theoretically we need a 2.87" ID tube - Conveniently, some 3" exhaust tubing is 3" outer diameter with 1/16" wall thickness, leaving 2.875" ID...

But I have found, with more and more testing, that 2.875" ID is too lean. Apparently the hack introduces enough inaccuracy that we need to be bit smaller to keep a proper A/F - I had to play around A LOT to figure out what worked for my setup, which is ~2.75" ID.

So we have an *almost* exact tube, for relatively cheap, and easy to find. Now you just hack your MAF down to nothing but the square box on top and the hot-wire sensor, and insert it into the 3" tube in the same manner as it was installed form the factory. JB Weld works well for this...

The tape was to keep it sealed during testing...looks much better now.

Things to note:

This only addresses one vital part of tuning a turbo car - The fuel delivery. Which is EASY. What's missed is timing control (which is really the important part!!!) - Your ignition timing should be retarded a certain amount per PSI of boost desired...

A good # is .5 degrees of retard for every one pound of boost. The stock base timing of an KA24DE is 20 degrees BTDC - So with 8psi, you'd need 4 degrees of ignition removed. A good way to accomplish this is to use an MSD Igntion box with a boost sensitive chip. But for basically the limit of 370cc injectors on a KA (which is right around 250rwhp), you'll be OK with just knocking the base timing back to ~16 degress BTDC (that's where mine is set). The stock ECU is pretty conservative, and will not give much advance under heavy load (i.e. lots of airflow, as detected by the MAF).

I know of a few setups that have run a T3, 370s, and AFC, and retarded base timing and have made good, reliable power at boost from 5-9psi. With a bigger turbo (like a T3/T04E), about 8psi will be the limit from what I've found...and give you rght at 250rwhp.

BTW: Too much timing will and can blow a motor QUICK - I have rebuilt and then subsequently replaced one KA due to timing issues under boost...

Another important thing to note is that this is THEORETICAL!!! - As of this writing, there are 3 people that have tried the 'Hacked MAF' idea on an S14:

1) Me, and I blew a motor within 2 weeks (detonation in #4 broke the ring lands). Sorted out the timing issues, and it ran great for 4 months before I switched to a JWT turbo ECU...

EDIT: I have since gone back to the Hacked MAF...the JWT ECU runs timing that is so conservative it will be safe at about 1 bar of boost (14.5psi)...but at 8psi, it pulls so much timing the car is down about 10-15rwhp over a stock ECU with 370cc injectors and either an AFC or MAF hack. But for anything over ~250rwhp on a stock block KA, I reccomend the JWT setup with 50lb/hr injectors, top feed fuel rail, and a Ford Cobra MAF. That will allow 13-14psi and up to 350rwhp...

EDIT: (Feb 2005) I have since swapped to a new JWT setup (50lb/hr injectors, top feed fuel rail, Ford Cobra MAF)

2) A guy on the 240SX forums (Freshalloy, NICO, Zilvia) that goes by 'AceInHole' - He originally tried this, and had issues with the MAF size. He used pipe that was 3" ID, which ran too lean. He tried using smaller and smaller tubes to richen it up, and wound up with a tube not much bigger than stock - Something in the 2.6" range. So the theoretical size of 2.87" ID didn't work for him (it was also too lean for me -read closely above). But he also blew one motor due to timing issues...

3)And the 3rd does not have boost just yet - IIRC he installed the 370s and Hacked MAF while still N/A, waiting for th rest of his turbo parts to arrive. We'll see how he does...


Also, the stock MAF I found was maxed out at 7psi with the JWT ECU I ran for a while - With the hack, you are sneaking enough air pas the element that it doesn't max until about 9psi from a T3/T4, which is over the limit of the 370cc injectors HP maximum anyway...and UNSAFE without timing control.

Basically, this setup will get you to ~250rwhp cheaply, **relatively safely**, and without maxing out the MAF.

SO PROCEED AT YOUR OWN RISK!!! You may blow a motor, you may be fine - Depends on too many variables to tell. But it worked for me with respect to being able to run bigger injectors for next to nothing (very little $$$).

So can this be done to an S13...??? - No...at least not in the same manner.

Due to the design of the S13 MAF (for one, it's metal), it would be much different. It could be done, but it would involve putting the entire middle of the MAF tube (the guts) into and bigger tube so that air would pass around the outside of the element - You'd need dyno time to calculate the exact, proper diameter of the bypass tube, etc...

It could be done (maybe???), but not nearly as easily...

And as for the limitations or the max boost or max power that can be attained by using this???

Again, since there is no timing control involved here, I'd say limit the use to 370cc injectors. You could *theoretically* run a Y-pipe with 2 MAFS, and only connect one - Then run 555cc injectors...but for the power that could be made with ~50lb/hr injectors, you will NEED timing retard under boost. So my opinion is that this is a cheap way to get a KA-T up and running for anywhere from 4-8psi, or up to ~250rwhp. Just be sure to set the base timing back to ~16-17 BTDC (and when you set your timing, use a good gun, and you HAVE to remember to unplug the TPS to do it correctly).

One last thing to note is that (theoretically) now that the ECU sees less air entering the system,
it may not be able to determine the actual load...

With less load sensed by the ECU, you get more timing advance, and that *could* cuase more knock under certain conditions. Supra guys sometimes use daul MAFs to run bigger injectors and have issues with this from what I've read...

E-mail me with questions - orion@gru.net.

Back to: Tech Info / Installs / KA Performance Page