Part 2
Here is my basic list of pros and cons of each system. However first I wanna preach some tuning. I own a shop, and a dyno... and I tune cars. While I am far from an authority on tuning... I am not new to it. One thing I feel is WAY too far overlooked is ignition timing control... ESPECIALLY when puting a turbo on a NA car.
Too much timing melts pistons more often then too lean of fuel. The flame front from combustion travels much faster as pressure increases. For this reason you need to retard ignition timing so that flame front doesnt jack your piston while its still trying to raise on the compression stroke or while its still dwelling at top dead center. This is by far the hardest part of tuning, and I personally feel the most important as it will RAPIDLY destroy engines, or increase and decrease power quicker than anything.. and its actually difficult to get perfect. Fuel curves are pretty easy to get right when you have a wideband o2 datalogger.
Ive seen turbo hondas come in here with nasty lean fuel curves that have been running around untuned like that for a long time and still managing to hold together. And then I have seen hondas that have too much timing crack and melt off their pistons ring lands within a day or two. I am referencing hondas because I own a shop and hondas are by far the most popular car commonly converted from NA to Turbo. For this reason they go thru it all and have it actually figured out better (as a community) than anyone as these guys are not too uncommonly seen making over 400fwhp on pump gas from very small engines.
Fuel tuning on a car like the 350Z can be difficult when going from NA to Turbo. This is because it uses an airflow meter device. airflow meter cars are typically smarter and will self adjust for increases of volumetric efficiency until the point where the meter has been maxed out. This would be super awesome if the stock airflow meter could read up to 500hp worth of airflow, but i will promise that it cant.
Heres where the complication comes in: Say you have a FMU, a turbocharger, and an airflow meter that outputs 0-5v with 5 volts being max airflow. All numbers are just random here but you will see the point. Your cruising at 2000 RPM, airflow meter is readin 2.5 volts. You decide to give it a little go. You increase throttle, airflow readings increase, and the turbo systems starts to generate pressure. Airflow readings are at 3.2 volts well within the stock fuel systems limits. But wait.. you are at 2 psi of boost. Your stock fuel system is already injecting the needed fuel for the current airflow, yet your FMU is jacking your fuel pressure by an additional 24psi... hmm, your now running way too rich. Boost response is crap, gas mileage is crap, and there is a bog sensation to pull thru.
There are ways around this if you play around. Rather then having the airflow meter after the filter, you can put it after your compressor of choice. Assuming your airflow meter can handle the pressure, this can work well because the airflow meter doesnt see the pressure. I guess you could say it is seeing the "speed" of the air coming in, but not the fact that it is pressurized. Of course most airflow meters have built in air temp sensors so its also good now because the stock ECU has fuel and spark correction tables based on air temp... so letting it see that hot post compressor air is a good idea to help the sake of tuning.
So now where does that leave us? It leaves us with the idea that all we now have to tune out is the fact that the engine has a pressurized intake charge... rather then having to deal with the stock ECU making up for some of the boost some of the time... now it will never know its there and we can start with a clean slate. If the car ALSO has a MAP (Manifold Air Pressure) sensor somewhere on the intake manifold... then you have an even smarter car. But MAP sensors are easy to overcome. You can simply moniter the output voltage with the engine off, key ON, (0 vacuum, which is full load on a NA car) and then install a voltage clamp that never lets the voltage pass this point. The idea here is to protect you from check engine lights and limp modes.
Also... it wouldnt catch me off guard if the 350z had a voltage limit on the airflow meter as well, causing a check engine light and possibly a limp mode. This could be worked around as well if it just so happened to be true.
We can now use devices that alter fuel and ignition tuning based solely on boost pressure, just like a honda guy can (hondas dont have airflow meters).
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FMU Pros:
Easy to tune, if you call that tuning.
Will get you thru smog testing in most states. Lots of states dont even do a rolling test on the dyno if the OBD-II system is not storing any trouble codes. So no problems here.
FMU Cons:
No real tunability. Some adjustable ones like the aeromotive one used in the ATI kit is the one to have if you really must go down this path.
No ignition timing control!! Add an independant timing adjustor... something like the MSD BTM that can retard the ignition timing per psi of boost.
Jacked up fuel pressure... better make sure all those lines are tight and up to the job of regularly seeing over 100psi, which is over double factory fuel systems are designed to operate at.
Generally crappy boost transition. I havent seen one yet that doesnt go way way too rich just as the boost comes on. Perhaps that Aeromotive toy provided with the ATI kit doesnt have this problem. No experience with exact device.
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Piggy Back Pros:
I will focus this only on the E-Manage and Split Second unit (which I have never used, but I am under the impression its very alike with the E-Manage, correct me if im wrong). The little guys like the S-AFC and what not just arent up to this task. If you happen to be aware of the "S-AFC Hack", I dont even want to discuss putting that on a 350z.
A modern and worthwhile piggy back system can do a few things. It can modify airflow meter sensor voltage output to the ECU based on RPM. It can tap into the injector wires and extend the injector pulse width based on a correction table of RPM and MAP. It can also advance or retard your ignition timing with a RPM/MAP table.
THIS is prime. While it can be hard to get a nice flat fuel curve due to the stock ECU having SO many correction maps... you have everything you need.
Add fuel for boost? Easy, build yourself a table and do back to back tuning runs on the dyno or other means of wideband dataloging... starting rich and work your way to your ideal A/F ratio. This is great because you are just building your own correction factor. The stock ECU is already doing a great job of firing the injectors correctly... you are simply taking that signal to the injector and holding it open longer based on boost and RPM... the system is tapped into the injector wires after the ECU so the ECU has done its thing, your just adding a longer pulse when needed.
Retard ignition timing for boost? Easy, same as above... just start with a easy table of retard per PSI, then after your fuel curve is ideal on the wideband, tune your timing based on EGTs. Since your stock ECU is in there it is already got an AWESOME ignition map and tons of great correction factors... you are simply adding a boost correction table since the stock ECU is running too much timing for boost.
Need a larger injectors? ehh... you can do it but thats where it starts to get a little bumpy... and the larger you go the bumpier it gets. A small increase is no problem. Heres what goes on... if you put in a larger injectors you need the ECU to not fire the injectors as long as it does. The only way to make it fire shorter is to make the ECU think it needs less fuel. Only one way... take the airflow meter signal and drop it by a percentage. This sounds all good but it causes one little problem. Your stock ECUs main ignition table map is based on RPM vs Airflow. By dropping that airflow signal you are now putting your on a different place in the airflow map... A place that says to add even more timing. So now you need to factor this into your ignition correction map. Not so great big of a deal, but the more do hacks like this your are gonna screw with the fact that stock timing maps on new cars are super awesome and your putting yourself on the wrong part of the map. Remember if you only need to correct for the boost, your not too bad off.
Your car will still pass the SMOG test that inspects for trouble codes.
Piggy Back Cons: With a system such as or alike to the E-Manage your hard pressed to find a Con... other than that whole injector thing. If your building a RACE CAR get a standalone EMS.
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Standalone Pros
Theres only 1, complete control of everything all the time. What else could you ask for.
