my buddy is trying to convince me that my car will be able to handle approx 6 psi without buying an fmu. will it be possible to adjust fuel enough with the afc to compensate for the extra air without an fmu and only the s-afc? or would i at least need to get some bigger injectors and that would work? he showed me this article http://www.thedropshop.tv/vafc.htm
that shows a pretty good setup, considering the low cost of the parts involved and how smoothly it works. basically, if you boosted folk could let me know how i am going to be able to work out the air/fuel management such of the equation, that'd be great. i bought a dirt cheap setup of parts on ebay (in fact, here's the auction http://cgi.ebay.com/ebaymotors/ws/eB...tem=2434992965 ) and while i'm just wanting to get started by adding a setup with a small turbo and such, i don't wanna mess my engine up. any input on the s-afc would be great, thanks!!

if you are on stock injectors you will need an FMU(or some type of BRFPR) probably..thats should be fine for 6psi

I've done the AFC hack(as you posted in that article) on 2-3 different Hondas right now to run 450s...that method is decent but part throttle bucking and detonation is a *****, you HAVE to dynotune it for sure. those setting they provide are pretty much base setting to get you to the dyno and thats it. untuned and even sometimes tuned, as soon as you let off the throttle the car bucks to all hell....its a nifty little setup, but kinda buggy untuned. DSM'rs use almost the same method as well

and BTW don't use that SAAB IC...plastic endtanks = the big suck!!!

so i should stick with an FMU? i was probably going to get the cartech (or similar style) unit if i went that route...

Well, let's see. The 6 psi setup is supposed to give the Maxima an extra 80 hp, according to Stillen. So that would be 190+80=270 hp at the crank, not counting any extra gains from exhaust mods.

The amount of fuel needed to provide 270 hp is given by:

injector flow rate in lbm/hr = (hp X BSFC)/(duty cycle X no. if injectors)

Let's assume a Base Specific Fuel Consumption of .5 lbm/hp-hr, and assume the injectors are maxed out with a duty cycle of 100%, so the required amount of fuel is:

(270 hp X .5)/(1 X 6) = 22.5 lbm/hour

Applying a conversion factor to get cc/min,

22.5 lbm/hour X 10.5 cc-hour/lbm-min = 238 cc/min

What this means is that the stock 4th gen injectors (240 cc/min) are able to provide enough fuel at the rated fuel pressure of 43 psi for 270 hp at the crank, given the assumption of BSFC = .5 and assuming the ecu will allow a 100% duty cycle for the injectors. If your BSFC is greater than .5 (altogether possible since we like to run rich under boost), or if the ecu allows a maximum of 80% duty cycle (something I have been trying to find out but haven't yet), then not enough fuel will be supplied and the car will run lean at the top end.

We can overcome any ecu limitation on duty cycle with the SAFC, but there is no headroom for enriching the afr at the top end unless you increase the fuel pressure as well.

So, to sum it up, I think running 6 psi of boost at a safe afr is beyond the capability of the stock injectors at stock fuel pressure. Maybe only just a little bit, but a little lean can result in big headaches.

If you're going to spend the $$$ on a SAFC, you might as well go ahead and get the FMU with the 6:1 disk. They are not expensive, and though they result in pretty crude tuning, the afr can be refined very nicely with the SAFC.

okay, so if i installed some 300zx injectors (370cc) would that make the difference? chances are i'll just get an fmu for now. i'm just toying the tuning possibility of having injectors, s-afc, and such. what i'm worried about with just getting the fmu is if i even wanna turn the boost up just a little bit more that my stock injectors aren't going to hold up for higher boost and i'll end up having to change over anyways....

Yes, the larger injectors would take up the slack. You will have to adjust base fuel pressure downward to keep it from idling too rich. You could use the SAFC to adjust the afr while dyno-tuning, and you probably would not have to run an FMU at only 6 psi of boost. If you go to more boost you will eventually have to increase fuel supply with an FMU, though.

For a 6 psi setup, it is much less expensive to keep the stock injectors and tune with both an SAFC and 6:1 FMU. You won't be running unsafe fuel pressure with the 6:1.

okay, well, i probably will be going that route for now. i've found a really good deal on some injectors, but the guy hasn't messaged me back yet. anyways, there's one concern with the safc. i forgot to mention i don't have a local dyno. there's basically no place for me to tune it without driving for hours. shouldn't i still be able to carefully tune it going by my air/fuel ratio and egt gauges?

tunining properly and reliably(<----keywords) off gauges is for the birds IHMO, especially with bigger injector sizes...if you plan to self-tune and have no dyno access, at least buy some type of datalogging wideband O2 unit for more accuracy.

okay, if i already have an air/fuel ratio gauge... and i buy a wideband o2 sensor, will it work?

The Cartech FMU or the Vortech Super FMU I think is your best bet - it's adjustable and tunable unlike the regular Vortech FMU where you have to change discs to change FP.


I've had one for a while now, and I love it.
(IMHO)

No...you need an entire wideband unit...look to spend $3-500.

iansw, which one is it you have? also, isn't the vortech s-fmu an adjustable fpr as well?

hey jdm, if you have to dyno tune, we're having a dyno day up here in chicago fairly soon.

steve

i'm still waiting for my parts to arrive, let alone get this whole project started. thanks for the offer. if we have a dyno day closer to the end of the year, i'll be there. i'm hoping i'll have everything purchased and here by the end of the month...

okay, another scenario people... i've found injectors and an fmu. would the two provide enough fuel and such to run 6-8 psi safely? or do the injectors require the afc to make corrections?

I imagine you could run a Cartech FMU and a SARD FPR with that setup.

(The FMU controls Fuel under boost, the AFR controls it at idle)

But why would you want to? The stock Injectors take 8PSI of boost easily with a Cartech FMU only.

just because i've found a decent deal on some injectors, so i'm seeing if i should get them for either now, or for bigger boost in the future...

dyno day was changed for 11/16