Fuel Surge Or Fuel/boost Cut

[daisu]

Hey people,

I went out to Qld raceway on the weekend and I noticed an occasional cut of power. I've had misfires before but this was not like a misfire, it was more like a complete stall for about half a second.

It seemed to happen as I'd come around to the end of the last corner (long right hander). Also, I started the day with a full tank and didn't experience the cut in power until towards the end of the day when I had half tank, so this was making me think fuel surge.

But the other thing that changed between the start and the end of the day was my driving style. At the start I was putting around the corner a bit slower in 3rd gear, but at the end of the day (when I experienced the cut) I was sliding the corner full throttle at the top of 2nd gear (not bouncing off the limiter but was high in the revs as you'd imagine sliding a corner).

So the other thing I'm thinking was I may have hit a boost cut (or fuel cut)? I don't have a boost controller, and I'm pretty sure I have a standard turbo and actuator, but for some reason I'm getting 0.9Bar (over 13psi) at the inlet manifold. Should I expect some sort of boost cut at this boost level?

This is on a standard RB20, standard turbo, standard ECU, walbro fuel pump, R34SMIC. I do have a completely straight through turbo back, so I would expect a few PSI more than standard, but I think 0.9bar is a bit higher than normal.

If it is likely to be fuel surge then I have no problems getting a surge tank, I just don't want to fork out the money if I need to be spending it on getting a retune or something.

Thanks, Luke

Edited March 31, 2008 by daisu

[GeeTR]

Good Q!

Fuel surge, or a ECU initiated cut...

If your car never cut before (too much AFM voltage output) while loaded on the street (3rd gear, up hills = boost = AFM output) Then i doubt it would see that same load, wheel spinning in 2nd out a apex.

I reckon it be fuel surge.

Cant you run the car to the same fuel level and generate some lateral G forces to test?

[daisu]

Good Q!

Fuel surge, or a ECU initiated cut...

If your car never cut before (too much AFM voltage output) while loaded on the street (3rd gear, up hills = boost = AFM output) Then i doubt it would see that same load, wheel spinning in 2nd out a apex.

I reckon it be fuel surge.

Cant you run the car to the same fuel level and generate some lateral G forces to test?

Yeah it hasn't cut out before, but my reasoning is that say going up a hill in 3rd at full load, the RPM would probably only be getting up to about 5krpm, whereas on the track at full load in 2nd I was probably close to the limiter, so there would have been more air flowing into the engine.

I could hook a multimeter up to the AFM output and get a mate to monitor it next time I'm on track. Or are there commonly available methods of monitoring fuel pressure remotely? (like an electronic oil pressure gauge, but obviously I'd need something with a different pressure range).

[GeeTR]

Yeah it hasn't cut out before, but my reasoning is that say going up a hill in 3rd at full load, the RPM would probably only be getting up to about 5krpm, whereas on the track at full load in 2nd I was probably close to the limiter, so there would have been more air flowing into the engine.

Yah, g.point, I assumed you woulda caned it up a hill once or thrice, making your setup feel max load before.

I could hook a multimeter up to the AFM output and get a mate to monitor it next time I'm on track. Or are there commonly available methods of monitoring fuel pressure remotely? (like an electronic oil pressure gauge, but obviously I'd need something with a different pressure range).

Ummm, I think you getting confused lol.

AFM measures air into into the motor and spits out a 0-5v signal (which a fast acting MM could read)

Im not OG with RB20 cuts so cant help you there.

Fuel pressure will need to be measured with a mech gauge, T'd into your rail feed, b4 FPR obviously. Run the line dodgy styles out the grill, through the window as a temp setup. Your shotgun will have to keep a keen eye on it.

Of course a lecky gauge with a f.pressure sender, recorded by a logging instrument (Innovative, Datalogit etc) would be best.

Your correct; many manufactures use the same sender for both fuel and pressure (like Defi)

Start with the easy stuff first,

1) Find out when RB20's with your mods ECU cut

2) Fill car up, find a quiet, sticky corner, or private road. Let car run down to the fuel level it had before... and try again. Should pretty apparent I would think.

GL

Edited April 1, 2008 by GeeTR

[daisu]

Yah, g.point, I assumed you woulda caned it up a hill once or thrice, making your setup feel max load before.

Ummm, I think you getting confused lol.

AFM measures air into into the motor and spits out a 0-5v signal (which a fast acting MM could read)

Im not OG with RB20 cuts so cant help you there.

Fuel pressure will need to be measured with a mech gauge, T'd into your rail feed, b4 FPR obviously. Run the line dodgy styles out the grill, through the window as a temp setup. Your shotgun will have to keep a keen eye on it.

Of course a lecky gauge with a f.pressure sender, recorded by a logging instrument (Innovative, Datalogit etc) would be best.

Your correct; many manufactures use the same sender for both fuel and pressure (like Defi)

Start with the easy stuff first,

1) Find out when RB20's with your mods ECU cut

2) Fill car up, find a quiet, sticky corner, or private road. Let car run down to the fuel level it had before... and try again. Should pretty apparent I would think.

GL

Lol Not getting confused, probably the way I worded it.

I was saying I could either monitor the AFM to see if I'm reaching it's limits, Or I could monitor the fuel rail pressure to see if its fuel surge. Basically just what you explained.

I wasn't saying that measuring the AFM would tell me if I was getting fuel surge, I was more getting at the point that unless I used a tee piece and mounted a gauge outside the bonnet I'd need some sort of electronic gauge and sender suitable for this pressure range.

I suppose It wouldn't be that hard to tee the fuel line and sit a gauge at the bottom of the windscreen on the passenger side(outside obviously).

But now I'm think measuring the AFM voltage will be easier, safer and more accurate.

Reason is, if I measure the fuel pressure and hit an ECU cut (caused by AFM limits) the engine will stop producing power momentarily, which will also drop the boost pressure. Since the FPR is also controlled by the boost pressure I would also see a drop in fuel pressure, which could be misleading.

If I measure AFM voltage and it is reaching it's limits, I would see it increasing to a certain point at which the ECU would initiate a cut (close to 5V). Or if it was a fuel surge I'd see the AFM voltage holding steady then dropping slightly when the engine cuts out (hopefully it doesn't happen at close to 5V otherwise it would be a bit inconclusive).

Next time on track is this Friday night, so hopefully I'll be able to find the problem then.

Other thing I could do is repeat the driving style with a full tank (as opposed to a half tank). There should be no fuel surge with the full tank. So if it does cut out then I could safely assuming I've reached the AFM limits, and it's time to upgrade turbo/injectors/AFM and retune.

If it doesn't cut out then I would assume it's the fuel surge and I'll get the surge tank in there.

Also, going full throttle up a hill would be full load on the engine, but the engine would not be using the same amount of air unless the RPM was the same. Kind of hard to replicate something like 7krpm full throttle on the streets as 1st gear would just be wheel spin and hit limiter too fast to measure anything, and 2nd gear would be going too fast for public roads.

Edited April 1, 2008 by daisu