New to CA18DET

[tree fingers]

Hey there. I'm completely new to the CA18DET, and actually, this is my first Nissan engine. I am in the process of a CA18DET AE86 build, and I have some general questions. I will be running stock injectors and MAFS with a S15 BB turbo. I have so often read people say things like MAF sensors have a static horse power limit, but I can't wrap my head around that. The volume of air that an S13 T25 pushes at 14 PSI will surely be different than the volume of air pushed by a S15 T28 at 14 PSI? My general concern is.. after my engine is broken in, will the stock MAF be sufficient to run the S15 turbo at 14PSI?

[Sideways s12]

You shouldn't have a problem in the maf area and these motors run rich naturally but I would keep an eye on fuel.

[mdb4879]

14psi? I doubt it. 10psi would be a much safer bet. IIRC the stock fuel system (MAF included) is good for around 200whp. These motors do run rich from the factory, so maybe if you had something good to une with you could squeeze more power out of the injectors by leaning it out and making the 370cc injectors go further. But the MAF can still only read "x" amount of air going into the motor. Plus if you have something to tune with you could get a larger MAF and correct for its different reading. Just to be safe I'd stick to 10psi.

PS: A smaller turbo could run 14psi, but that's because it'd hold 14psi in the mid-range and make all the torque, but the motor wouldn't be flowing a volume of air per minute that would max out the fuel system. Then as revs climb the motor would actually choke out the turbo and your 14psi spike will taper off. Oh, and 14psi is boost cut on the stock ECU anyways.

[blownhemi]

I have so often read people say things like MAF sensors have a static horse power limit, but I can't wrap my head around that.

They do, but it's not like every Nissan engine got it's own MAF for its own specific horsepower rating. There a a couple of ranges, and there's usually some room to grow. (airflow wise in the MAF)

The volume of air that an S13 T25 pushes at 14 PSI will surely be different than the volume of air pushed by a S15 T28 at 14 PSI?

Yes, and you have to tune your ECU to suit the changes. A more efficient (newer) compressor usually puts out a slightly cooler, slightly denser charge, while still drawing in the same amount of outside air, as seen by the MAF before the turbo. So you'de have to add fuel to keep from leaning out, and of course to make good use of the extra air in the cylinders.

My general concern is.. after my engine is broken in, will the stock MAF be sufficient to run the S15 turbo at 14PSI?

You'll have to see, but probably yes, or very close to it. If you get unexplained fuel cut in high boost+RPM, then you've reached the end of your MAF. If you/your tuner is tuning via an emulator, you can maptrace your VQ table. If it reaches the last cell, your MAF is (almost) maxed out.

Oh, and 14psi is boost cut on the stock ECU anyways.

There is no boost cut on these ECUs. The ECU would have to know the *boost* for that, and it doesn't have a MAP sensor. That cut is actually "airflow" cut. When the MAF output reaches its maximum of 5V, the ECU cannot determine the amount of airflow any more, which means it can't ensure safe engine operation, so it cuts the fuel. What boost this airflow limit is reached at, depends much on the turbo+setup.

[float_6969]

As the others have explained, the restriction with the MAFS has nothing to do with boost levels. It has to do with mass air flows. Regardless of the turbo or boost level, when the MAFS gets close to it's max output ( it maxes out with an output of 5V, but I think the cut is a little under that, about 4.8V, IIRC) then the ECU shuts the engine down. Depending on the turbo, this could happen at various boost levels.

You can get around this by running larger injectors and fuel pump and some sort of piggyback. The piggyback will "lie" to the ecu and reduce the voltage of the MAFS signal to accommodate the larger injectors. This has the side effect of increasing the max boost/power the MAFS can handle before it cuts. This comes with a limit though. If the MAFS hits it's 5V max, and the piggyback drops that voltage down below the cutoff, the ECU won't know the MAFS is maxed out and will continue to run the engine. Any increase in air flow over the MAFS max of 5V, and the output flat lines, meaning no more fuel is injected and the engine can run lean.

This situation is why I never recommend piggybacks. Back in the day, they were the only option. Now you can ROM tune for injectors and MAFS or use Nistune.

[tree fingers]

Thanks for the help, guys. I have a few more questions, though. It might be easier to explain my situation. I have a stock compression rotating assembly with a .20 bore. I will be running the stock MAF sensor with the stock injectors mated to an s15 turbo. While Im waiting for my other ecu to get back from nistune, will the stock ecu work with the upgraded turbo under lower boost pressure, say 10PSI? I intend to use 440cc injectors and a bigger maf, but there doesnt seem to be much of a point to installing them without some sort of tune beforehand. Please forgive my ignorance on this one. Thanks!

[blownhemi]

Thanks for the help, guys. I have a few more questions, though. It might be easier to explain my situation. I have a stock compression rotating assembly with a .20 bore. I will be running the stock MAF sensor with the stock injectors mated to an s15 turbo. While Im waiting for my other ecu to get back from nistune, will the stock ecu work with the upgraded turbo under lower boost pressure, say 10PSI? I intend to use 440cc injectors and a bigger maf, but there doesnt seem to be much of a point to installing them without some sort of tune beforehand. Please forgive my ignorance on this one. Thanks!

If you already have a wideband O2 sensor installed, it certainly doesn't hurt to give it a try, and see what it does.
Do you have an oil restrictor in for that BB turbo? I keep forgetting if it's built into the turbo or needed separately, in any case, don't forget it, when you're switching from the stock non-BB one.

[float_6969]

As long as you run the stock MAFS and injectors, the ECU will be fine with the upgraded turbo, just keep the boost low. IIRC, the MAFS maxes out with an S15 T28 @ 10psi, so you'll probably have to run less than that, but it won't hurt to try and see if it maxes out.

[Izento]

Why not just rewire a Z32 maf when you go get rdy for tune? Z32 maf you wont reach anywhere near the cap with your goals. This is the same formula the SR guys do. Then when you get rdy for tune, just drop in injectors, wire maf, hook up Nistune and tow to dyno to get a basemap.