View Full Version : Pic and plan: Extension of MAF hack for 550cc inj?

06-05-2003, 04:08 PM
Car: S13 '92 HB, KA24DE
Plan: Turbo with $1400 budget, intercooled

Extension of the MAF Hack - what happens when you exactly double the airflow of your intake stack, but only read half of the airflow?

If this idea would work, we could possibly use it to run Nismo 550's just a little rich with the stock ECU. I imagine I'll have to run an SAFC to fine-tune (and I want to run one, anyways), but this could get us in the ballpark.

I am planning a 200-250 rwhp setup, depending on boost. Fuel system will be 550 sidefeeds, upgraded fuel line, twin parallel Z32 fuel filters, and a Walbro 255 with #2 power wire (fused and relay'd) straight off the trunkmount battery to minimize I2R losses (power losses through thin wire).

I'd probably attempt to make the intake side plenum out of aluminum flashing, caulked at all joints and held together with machine bolts/nuts. The other side (exhaust, if you will) may be a bit trickier due to the fact that our MAFs have the 3" fitting instead of a flat flange, but I figure I could fab something up that could worm-clamp down onto the MAFs.

Am I off my rocker? I would think that with proper careful construction of this, you could get the airflow within a percent or two of even between both the MAFs. Since there is equal flow, but only one of them is being read by the ECU, it will spit out half the fuel it needs. BUT with 550cc injectors, flow rate is roughly doubled, so you end up back relatively close to your original fuel flow.

I predict that the system will need possibly +/- 5-10% injector tuning in the SAFC.

This system may also be workable for those who want to run 370's - I calculate that they'll run roughly stoich at -31% on the SAFC.

This system would also enable you to run some relatively high-horsepower setups with the stock MAF, as it can handle roughly twice the airflow before maxing out.

Ideas? Rebuttals? Mockery?

06-05-2003, 04:47 PM
No reason it shouldn't work as well as the hacked MAF for 370s...As long as the airflow is even between the 2 intakes, it should balance out well.

But you still have to address timing - 550s are enough fuel for 350rwhp, and at that level, you'll blow up a KA without some boost dependant timing retard or an ECU tuned for boost (ala JWT).

Or you can retard the base timing and lose in the mid to low end...

So it would work for your 250rwhp goal, but would be overkill - Why not just 370s and an AFC? That will get you there no problem, and cost less. The 550s I've seen sell for $100 each - 370s are $100/4 and an AFC is what...$250???

My belief is that the hacked MAF idea is good 'til the limit of 370cc injectors, or 260-275rwhp. After that, you need better timing control or you'll blow things up.

ALSO: For the AFC you said you thought 31% would be good with twin MAFs...but a single stock MAF and 370s only requires -27%: (370cc - 27%) = 270cc

Later - Brian

06-05-2003, 05:02 PM
Right, but I was using 540cc as a 'stoich' injector size to keep the A/F the same with half of the airflow being metered. If we consider 540cc the 'new' injector stock size, then 370/540 = 68.5, which translates roughly to a -31% setting.

I know I wouldn't be pushing the 550's at all, but better that your fuel system be too hoss than not hoss enough, eh?

I guess what I'm concerned about is maxing out the stock MAF with 370's. Also, I believe the MAF on the S13 and S14 is different; previous MAF hacks were done on the S14 MAF. It may require a bit of engineering to translate.

This may be more viable for tuning 370cc injectors, or perhaps 450 DSM topfeeds with an aftermarket rail (could end up cheaper than 550 Nismo's).

What are my options for boost or RPM dependent timing retard controllers?

06-05-2003, 05:34 PM
instead of making the pre MAFs pipe, would you be able to just angle the "Y" pipe that the 2 MAFs flow into at a greater angle (i.e. 45* PVC "Y") and then running 2 separate cone filters? i dont know if that defeats the purpose of your dual maf or not, but it would still have 2 airflow paths with just one being read by the ECU.

also, what is the deal with the dual Z32 fuel filters? unless you are running a dual sidefeed fuel rail with a center return and you split your fuel line before the dual filters, it doesnt make any sense. especially since you stated they'll be in parallel, meaning they would be on 2 different lines? if you were to put them in series or on the same line it would just mean fuel would go through one filter then the other then to the rail. i dont see the point in that unless using the method described above. even then you could just run one filter and split the line after the filter.

i dont see the point (even though filters are cheap) of having 2, unless you live in an area with really crappy gas. ive never heard of anyone having problems with filtering before.

06-05-2003, 05:35 PM
Ahhhh...I thought you were using the -31% for the 370s - Nevermind.

And as far as maxing the stock MAF - The S14 MAF has proven to be fine for the max of 370s...but the S13 MAF looks a little small. You're correct there. And hacking it is different for sure (as compared to hacking S14 MAF)...

Keep us updated - Brian

06-05-2003, 05:37 PM
also for timing issues, money issues would possibly allow anything from jwt to standalone systems. jwt has the fuel and timing all in one. but since you are using the afc for the fuel portion, skip the jwt and get an msd box with a boost timing retard knob (and maybe a 2 step) http://forums.freshalloy.com/images/graemlins/grin.gif
i dont know which it is. i think the 6al or dis6 or 4 or something.

06-05-2003, 05:40 PM
Ahhhh...I thought you were using the -31% for the 370s - Nevermind.

And as far as maxing the stock MAF - The S14 MAF has proven to be fine for the max of 370s...but the S13 MAF looks a little small. You're correct there. And hacking it is different for sure (as compared to hacking S14 MAF)...

Keep us updated - Brian

yeah good luck cutting up a metal s13 unit. i am sure its possible, but can you swap over to an s14 plastic unit for use on an s13? IIRC, they use different number of wires or are different in the readings maybe, but an electronically inclined person could probably come up with something for that.

06-05-2003, 10:20 PM
If I can swap to an S14 MAF and have the same signal (e.g. it works with the S13 ECU/etc), I'll swap and do the 'classic' 370cc MAF hack. But I thought this would be a neat project, at least.