A/F ratio for BPU supras...some facts

I am beginning to think I am running out of fuel. I though my S-AFC wasn't working, but now I think I am just asking for more fuel than the my stock system will deliver. My S-AFC seemed to work fine at lower rpms (up to about 4,500rpm), but not at all when the boost comes up. I have replaced the S-AFC with a new one and it did the same thing. At about 16.5psi (around 460rwhp) I can maintain 650c (at the top of the downpipe) with my S-AFC set at 0. I can then richen the S-AFC up 50% and the car still runs about 650c by redline in 3rd gear. If I turn the boost up to 18.5psi (about 490rwhp) my EGT's will stay at about 650c until 6,000 rpm (about 475rwhp) and then steadily climb to 680c by redline. The most I have seen is 720c after a run through 4th and partially into 5th. My car isn't too lean, but I don't seem to be able to give it any more fuel. I wonder if the fuel pulsation dampener is more restrictive than we might think. I have seen several cars make 530+ rwhp with the stock pump and injectors, but only one had the fuel pulsation dampener (Brad Mayo). I already have the FPD bypass hose, I just need to installed it. I will let everyone know if this allows me to reduce my EGT's at 18.5psi. My point is the FPD may actually start running out of fuel at around 460rwhp. A BPU car with around 430rwhp could be make around this number on a cold day with a high volume of air going through the intercooler vs. a fan on the dyno. Plus the stock twins would be pushing higher EGT's because they are heating the air considerably more than my SP63. Just a thought.

Later, Steve

Steve,
Did you try changing both the High and Low throttle setting fuel maps on the SAFC?

I'm not sure how the SAFC senses throttle position but wondered if a problem with how it senses it might be why you can change the fuel at low boost but not at high boost.

To be honest, I have no experience with this yet but I'm getting an SAFC put in over the next week with a bunch of stuff (including VPC, Fuel FJO WB O2, SP57) and I picked it because I hope the High setting can be set so it only modifies the Open Loop behavior of the ECU.

From all I've read modifying the Low Throttle setting is useless other than an "across the board" fixed delta so the ECU comes out of reset with the right correction factor for the larger injectors and VPC. Modifying the low setting aka "closed loop" seems to just result in the ECU adjusting for what the SAFC is doing. I think that just results in the ECU backing out the Low throttle SAFC settings after it comes out of reset and starts learning based on the air fuel ratio it actually sees vs what it wants to see.

All theory for me right now but with the FJO wideband O2, Sp57, 750cc injectors, VPC & SAFC going in, I'm going to have to start my real world education.

BTW, recently started lurking here & this is my first post. Been lurking on the MKIV list since I got the car in May. '93 auto w 8,158 miles. BPU at 9600 with some advice off that list. (RMM DP, HKS EX, BCC, Superdragger). Blew #2 at 11,200 a couple of weeks ago in the first planned street race of my life on the 4th run. Cold night in the 30s and at 1.42 kg/cm2. Was sequential due to the auto and sure it was shaft twist. Bad luck and maybe an actuator problem??? Before you ask, lots of reasons I'm sure about the mileage.

Chris Johnson who does all the work on SW's Supras and did my BPU is doing the work. Starting with the SP57 while waiting for the new multi-disc PI Torque Converter and seeing whats up with the recent tranny options and also to make sure I can handle 500 rwhp before I go near 600 with the SP63

Same philosophy I used by waiting a couple of months and 1500 miles before I went BPU. Sure I'll never go past an SP63 because I want the car for more than straight line drag/street racing. The winding roads are still my love. Just can't stay away from some of the adrenilin kick that rwhp brings

BTW, beautiful car!

Steve Jarvis said:

Plus the stock twins would be pushing higher EGT's because they are heating the air considerably more than my SP63. Just a thought.

Later, Steve

Click to expand...

The intake charge is almost completely cooled by a good FMIC, and the stock IC does almost as well during street bursts and drag racing (I used a multimeter thermocouple to test this).

The stock twins do produce a shitload more exhaust backpressure, because it takes so much more energy for them to move as much air as the SP63 (And because their a/r and manifold is restrictive). This raises EGTs, although it might not show up in the DP.

Why not read voltage of the stock O2 sensor? Its no wideband, but it will give you a good idea of whats going on. I'd look for no less than .92v with those kind of EGTs (narrowband O2v varies a lot with EGT). That should be about 11.5:1.

BTW, every car is different, some can run leaner than others w/o problems. I datalogged my car when it still had both turbos, and anything above 690 at any speed was too high IMO. This is with the DP completely heat soaked at high speed. Any higher than 690 ment the ECU was pulling timing.

Well I installed the fuel pulsation dampener bypass and a new fuel filter Friday night. It made no difference. I will be checking the MAF voltage into and out of the S-AFC to once and for all determine if it is at least trying to do something.

Another thing that may be a problem is the MAF. It was at 97% on my 526rwhp dyno run and I actually hit 99.2% on a 20.5psi pull this weekend. I am wondering if the MAF is really that accurate at those levels. At the top of third I see 650 degrees at 15.5psi, 680 degrees at 18.5psi and 720 degrees at 20.5psi. Regardless of what I set at the S-AFC at.

Mike_Austin,

I have tried the S-AFC in several different configurations including one that doesn't use the low and high throttle settings. I can make the car where it won't rev past 4,000 rpm, but if I lean out at 4,000 rpm and the boost comes up the car revs fast whether I have the AFC set at 0 or +50%. At the top of third I see 650 degrees at 15.5psi, 680 degrees at 18.5psi and 720 degrees at 20.5psi. Regardless of what I set at the S-AFC at.

Later, Steve

Steve,

is it hard to install the FPD bypass hose?
I just received also from Dusty and gonnna install it myself soon.

I'm having problems with my EGt also as you know, but I'm at BPU. I've been having problems with EGT and A/F ratio. my EGT raises to 740C at the end of 4th , so I can never go into 5th.
whereas my A/F reads 14:1 using 91octan at 18psi.
I replaced my fuel pump, FP regulator, filter, and MAF. nothing is making my EGT nor A/F go down. I even swapped my ECU with my friends but its not ECU cuz his car runs fine with my ECU... my HKS AFR doesnt seem to work either with 20% fuel dialed in. the next thing I'm gonna check is my injectors voltage and signals.....BTw, I have a FP gauge and it reads I have enough fuel pressure.

so my question is, when there is enough fuel presure, does it mean there is enough fuel for the engine?

thanx

Merry Christmas guys!
Jonas