I have had a FAST xfi computer in a new 427 sbc motor in my '76 for about 15-16 months now. It has never run right and I have spent almost $6k trying to trouble shoot this system. I previously had an older edelbrock system on this car that ran flawlessly for 14 years or so.
427 with dart little M block etc.
10.4:1 cr
comp cams solid roller .670/.670 lift 250/258 dur, with stud girdles etc.
AFR elim 210 heads
1 7/8 hooker sidepipes with spiral baffles
MSD 6A with comp cams dual sync distributor

I have to say that FAST refuses to honor their warrantee, so watch out for that too. Here are the problems I had.
1. Had a bad dual sync sensor. Put an MSD crank trigger on it
2. Motor was leaning out at 5,000 rpm so bought a new TB, ported intake, adjustable fpr and injectors for roughly $2500
3. Motor still flatlined with hp at 5,000 rpm even though tq peak was roughly 4700 rpm, so spent $500 on new springs. This did nothing
4. Sequential mode never ran right and peaked at 350 rwhp.
5. alpha N mode peaked at around 390 rwhp with the above flatline issue at around 5,000 rpm
6. Have had all kinds of TPS problems with it jumping around inconsistently.
7. The computer would not retain the tps values despite multiple reflashes and resets.

Warrantee
1. After The efi tech and I both had worked on this thing for almost a year, I sent the computer and O2 sensor into FAST to test it, worried that they wouldn't honor my problem outside of the year warrantee.
-They said the computer was fine, but didn't test the thing in sequential or even send me a read out of any sort.
-they said my data logs showed the tune was fine
-they said the O2 sensor was bad but that it was a 'high wear' item and they wouldn't cover it in their warrantee, but they would give me a good deal on a new one at cost for $280.
2. So despite being pissed, I bought the new O2 sensor, put it in and this sensor showed up almost immediately as an error 16 and quit working in closed loop mode.
3. Now they are demanding to send the computer and O2 sensor in again before they will do anything. All they are going to do is claim it is again a high wear item anyway, so what is the point?

This system seems to be very unstable. Problems flashing the computer etc. I think it has a bad computer and have maintained it all along, but FAST does nothing. Remember I had efi on this car for 14 years without a single problem.

Anyway, I'm not too happy and wanted to know if anyone had any similar such experiences or any suggestions.

 

That's the first I've heard of a problem with the XFI. Unfortunately I don't have any suggestions but to sell it and get a BS3.

 

OK thanks hotrod. I don't think too many ppl on this forum have used the FAST in their C3's, but I thought I would check.

 

There is quite a bit of info at theturboforums.com in the EFI section. It's mostly turbo related, but the guys know about aftermarket EFI.

Bummer that you didn't buy my Big Stuff

 

fuel cutoff in view/system configuration/operational parameters, maybe?

I thought it was a fuelpump cutoff but it actually cuts the injectors.
ask me how I know. I think the default setting is around 5000

the XFI is a great system let me know if I can help

 

Thanks guys. I'll double check the fuel cut off, but I know I looked at this the other day and it was around 6800 rpm if I remember right. I'm hoping to mess with it over the weekend if I get my new O2 sensor.

I put my first injectors back in thinking the tuner talked me into too heavy of an injector (put the 48 lb ones back in and took the 55 out). So, I need to retune it. Where should I begin tuning to get it to idle? Is there a pattern of tables I should address in a given order? I can't afford anymore dyno tuning, so I'm on my own this time.

 

Start with the car warmed up and make any changes to the main VE table. Then start it cold and see how it idles. You can make any needed adjustments to the warm up tables at that time. I had to run the idle on my Big Stuff quite a bit richer than I would have liked just to get rid of an off-idle stumble.

I don't know if you'll have to do that or not. I don't know if you have yours set to be in closed loop mode at idle but if you do, adjust it so it's not. I'm sure your car misses quite a bit at idle like mine did. Each time it misses it sends a big shot of oxygen past the 02 sensor screwing up the reading. The computer will keep adding fuel thinking the motor is lean. I ended up setting the minimum rpm for closed loop to 1800. I think you can also set a minimum throttle opening for that too.

 

I ran 72lb injectors in a motor that makes less power than yours should without a problem. Going from a 55 to a 48 isn't going to make any difference.

 

OK Hotrod thanks for all the tips. Well, since the 55's are better, low impedance injectors, I might put them back in...This is frustrating.

Why can't I get my idel A/F ratio up to around 14:1???

 

I can't offer any help but I ran across this board dedicated to Fast etc EFI systems. Might be some help there. Careful it's for FORDs

 

I don't know why you can't, but mine had a stumble off-idle if I leaned it out too much. It might have been because I had a big throttle body and very little tps change resulted in a ton of air flow. I don't really know. I've got a 90mm on my 6.0 liter and it works fine.

Did you figure out why it won't run above 5 grand yet?

I'd consider borrowing a carb intake, carb and HEI ignition if it were me. Just to rule the EFI out or in as the problem.

You're not running out of fuel pump are you?

 

corvette_dave, I offered to help you with this, but you never answered me back.

Just for your info, when you run a large camshaft with alot of overlap you can't always get the engine to idle in the 14's. Since there is exhaust being pumped back into the intake it is less efficient at idle. That's what gives you that choppy idle that everyone likes. That's also why the LS engines all run very wide lobe separations.