PraxRX7

Quick question:

The FCD fools the ECU about the boost levels...so...would that screw up the cars A/F ratio much or does the ECU rely fully on the Oxygen Sensor for determining A/F mixture? I just don't want to get an FCD and then start running lean mixtures (I just need the FCD in case I hit 9 PSI, I hit 8.1 PSI today without WOT, so I just want to have some security that my ECU isn't going to lean out my rear rotor and cause an explosion.
-Thanks

Josepi

Yes it would throw off the A/F ratio after the boost climbs higher than the FCD allows the ECU to see. You'd run lean, that's why everyone beats the drum of piggy back fuel computers or ECUs. If you wanted to be super **** about it, you could rig up a light display to cut on after a certain voltage is reached (meaning your FCD is now busy confusing the ECU) and if you don't have any fuel mods, that would be a good thing to know so you don't grenade the engine via detonation.

vosko

iTrader: (4)

yeah what josepi said. i went the tuned ecu route much less chance of blowing the engine. my car ran between 7-14 psi depending with stock fuel system. the car never leaned out. an apex seal broke due to age engine had atleast 120k + miles on it and for the three months i had it had 2 days of road course time, 50 runs down the drag strips, burnouts, brake stands, 150+mph top speed runs etc in other words i was very easy on the car while it ran

PraxRX7

ok, now as far as a piggy back fuel computer goes, won't it's signal be fooled by the FCD as well or would that be directly tapped into the boost sensor?

Just wondering how the S-AFC could make the proper corrections to richen the mixture while the FCD is messing with the ECU. (I am going to a larger fuel pump and injectors, which with the stock ECU alone would run super rich, hence the S-AFC). I am also thinking of porting the wastegate next re-build.

I am going to be doing the setup for this soon and need to figure out all the facts before I start ripping things open.

Josepi

That's funny, I had the same thought about the FCD and S-AFCs. It got answered too, i'm gonna do a search...

PraxRX7

What ECU did you get, or where/who did your ECU-tuning...

Did you just get the ECU re-programmed, or did you go with aftermarket like Haltech?

Josepi

Well, the q I had was about elec boost gauges and FCDs...oh well. damn my memory sucks. Anyhow, if you wanted to run the S-AFC using the pressure sensor, nothing would stop you from getting another sensor and T-ing a vacuum line so you'd have a signal for the S-AFC only.

PraxRX7

I like that plan, I am running a mechanical boost gauge right now for ease and reliability. I wouldn't mind T'ing off to another sensor for the AFC...Thanks

Josepi

Read the 2nd page of this thread, plenty o neato info about the S-AFC and boost (instead of throttle position and rpm ((I think...))):

https://www.rx7club.com/forum/showth...5&pagenumber=2

vosko

iTrader: (4)

i'm running a panspeed light sport tuned ecu from YAPAN if i had $$$$$$$ to burn i'd buy another haltech E6K or F9A and put that in my TII but as of right now I LOVE THE PANSPEED! the car ran great on it. got better gas mileage better throttle response etc. i couldn't ask anything more of the ecu. the only problem is i lucked out and found one its one of the only ones i know of in the states.... you could find a used gforce ecu pretty easily those run pretty nice too.

NZConvertible

There’s debate about whether the pressure (MAP) sensor really affects mixtures much. Apparently it has more to do with controlling ignition advance. The ECU’s main load input is the airflow meter, not the MAP sensor.
Also, the S-AFC is used to fool the ECU via the AFM, not the MAP sensor. So an FCD (which is connected to the MAP sensor) has nothing to do with the S-AFC’s operation.

RETed

Our experiments conclude that the boost sensor on a FC3S turbo does NOT affect fuel delivery.&nbsp There were several people who were going to try their own experiments to prove otherwise, but I haven't heard from them yet...


-Ted

NZConvertible

So was I right about the ignition? I'm guessing the ignition is retarded according to manifold pressure, and not doing this (because of the FCD) could be as bad as leaning out.

RETed

We haven't confirmed it was retarding ignition timing either, but it's a pretty good assumption that it does...


-Ted

JarrardB

so what you are saying is that If I run a FCD my engine will lean out at high RPM/boost levels and my engine will die....am i correct?

OK...is there a good cheap alternative to the FCD?

PraxRX7

The only cheap alternative to an FCD is back pressure in your exhaust.

If you want to run more boost, you NEED an FCD and you NEED a fuel upgrade and you SHOULD buy a fuel management system of some kind. (Also ignition timing management would be a good thing.)

If you want to have your engine still running, I would recomend what I just said.

JarrardB

OK....new aftermarket programmable ECU = $$$$ i don't have..

will a FCD do more good than not having one? i mean, is it actually worth getting one...i guess thats what I'm trying to ask here...

And when you say fuel upgrade....do you mean something like a malpassi fuel regulator?

PraxRX7

If you are going to be running over the fuel cutoff of boost (8.7 psi on 86-88, 11.2 psi on 89-91 [some 89-91 are 8.7psi as well]).

If you are running over those PSI, you are going to need to know a few things.

1. Yes the FCD will stop the car from hitting fuel cut, however, the FCD tells the ECU that you are only making 7.5 PSI, so the stock gauge will become useless, hoever, the boost sensor doesn't have anything to do with fuel regulation so oyu don't really need to worry abou that.

2. If you are going to be running over those boost levels, you will REQUIRE more fuel to be pumped ot the engine, the only way to really do this is through upgrading the fuel injectors and the fuel pump.

3. If you do #2, which you MUST if running beyond the fuel cutoff of boost, you will need a fuel management system (like the APEXi S-AFC) or your stock ECU will have to try to make compensations, which it CAN do but your car will run like crap and the engine won't be healthy long.

4. Also if the ECU relies on the Boost sensor for ignition timing (assuming that the sensor is there not just for the gauge) then you are also running the risk that your ignition might be screwing up, hence why I mentioned an ignition management system.

Your only other option is to go with a fully tuned ECU like a Haltech or other such ECU's. You don't have any other options. If you can't afford this, don't aim for more horsepower or you will kill your engine, stick with the stock air-box and a stock type exhaust so that you do not over-boost.

THat is that.

NZConvertible

No, that's exactly what Ted said didn't happen. But if the pressure sensor is controlling ignition advance as boost increases, at high boost levels an FCD could cause the ignition to be too advanced, causing detonation.

NZConvertible

While I agree fuel cut is to be avoided, I have my doubts about this theory. The ECU doesn't reduce fuel to the rear rotor at 8.6psi (which would result in a lean mixture), it cuts fuel entirely, resulting in no combustion at all, just air and spark. I don't see how this can instantly kill an engine. Boost limiters like this are found in all factory turbo engines, and some modern engines actually run with some cylinders shut down to save fuel! Yet the 13BT seems to be the only one people say can be killed by it. Perhaps someone can explain?

PraxRX7

I would like to get that number confirmed...

8.6 PSI is NOT fuel cut on my car, an 89 TII. Everything is still stock and there is no FCD.
I have been told by some mechanics that on most (but not all for some reason) the actual fuel cutoff is at 11.2 PSI and not 8.6 PSI.

Can anyone confirm this? My boost sensor is correct, had that tested, and I have tried it on 2 different boost gauges. I am hitting 10 PSI-10.5 PSI running rich and hard with no fuel cut. and I am not talking about spikes or creep to those PSI, that is just what is goes to. (Used to be only 7PSI until I got my stock exhaust changed out with a custom...but I still have the stock air-box).

JarrardB

OK...I'm sorta confused...

here's the short of it...

I have a new exhaust system....I hit fuel cut at 5500 - 6000 rpm.....

should I get a FCD or are they not worth getting?

What else do i "need"...

What else would you "recommend"....

PraxRX7

its like this:

The fuel cut is designed so that you do not go past the limitations of the fuel that can be provided to the engine. The FCD allows you to go past that fuel cut, HOWEVER, if you are boosting a lot past it on a stock fuel system (11+ PSI) Then you NEED more fuel(larger injectors and fuel pump). In order for those to run properly you SHOULD have a fuel management computer, but you don't HAVE to (risk engine trouble). The FCD can also mess with ignition timing, so you SHOULD have an ignition management computer (risk engine trouble, less risk than not having fuel management).

That is about all I can say, I at least hope that you have an EGT or Air/Fuel gauge to be able to watch for symptoms of lean conditions, if not... ... ... need I say more. Aftermarket Boost gauge is also something to consider.

Moving a car from being stock is not a cheap venture.

RETed

It's is not the overboost fuel-cut itself that kills the engine; it is the constant cycling banging into the overboost fuel-cut that's bad.

Here's the death cycle:

1) Boost raises towards overboost fuel-cut...
2) Overboost fuel-cut triggers, fuel is cut to rear rotor...
3) Due to the aburpt hesitation, boost is LOWERED...
4) Since boost is momentarily back to safe levels, fuel resumes delivery to rear rotor...
(repeat back to step 1)

Now, I hope it's obvious.&nbsp Cycling into the overboost fuel-cut cycles the fuel between on and off to the rear rotor - this is how it leans out and blows up.&nbsp when you hit overboost fuel-cut, the engine just doesn't die like you lost all power; it bucks like a bronco, which means it's cycling between fuel being on and off to the rear rotor.&nbsp I've done it on purpose and killed a 60k mile engine checking to see if it was overboost fuel-cut or not...


-Ted

rx7_turbo2

I think info on the forum has proved fuel cut is different on every car. I have never seen a set pressure. Just like some car's rev limit buzzer come on earlier than others. If you plan to run higher boost with the stock ECU an FCD is needed, just put it like that. A ton of forum members use or have used FCD's they work just fine. The debate about them comes up every once and awhile but never seems to prove anything. The map sensor really does very little. The most you should really run on the stock turbo is 15psi (though I've seen higher) For this the FCD is fine and should not have a huge negative effect on the timming. However if running a much larger turbo then timming control becomes an issue.

I think this list seems to be accepted by the majority of the forum for proper fuel mods with a stock turbo.

1)FCD
2)Fuel Pump
3)Larger Injectors
4)SAFC

There really is no need to over complicate things, read the forum a little more in depth and you will quickly learn what others are doing, and finding success at.