Shad Laws

Hello-

I've seen the plots online that show what the factory fuel cutoff is for overboosting. My question is this: does this actually work well? In other words, if the line from the WG or PCA pops off and the boost skyrockets, will the engine self-destruct or will the ECU kindly just kill fuel?

It seems like a stupid question, yes I know, but recently I had an odd experience. I was troubleshooting the turbo control system - I had full secondary boost but only 7-8psi of primary. I popped off the line to the PCS to force the PCA closed, then went for a test drive while staring at the boost gauge. I floored it and it went to 13psi very quickly around 3500-4000rpm and I released the throttle. I didn't *notice* any cutoff... but then again, I pulled out of the throttle so quickly I may have missed it.

Just curious :-).

Thanks!

Take care,
Shad

poss

iTrader: (5)

You can run lean when you hit fuel cut. I don't know why mazda didn't do an ignition cut instead of fuel (probably emissions actually).
Plus you didn't notice anything because it takes something like a half second for it to cut fuel. You must have let off wuick enough.

rynberg

Yep, I believe you have to overboost for at least one continuous second for the fuel-cut to kick in.

Mahjik

iTrader: (1)

Yep, rynberg is correct. This is why boost spikes can be dangerous as the boost doesn't spike long enough for the ECU to enable fuel cut.

If you look at the fuel cut mapping on the stock ECU, it's higher near the transition area probably to account for a little spiking.

Shad Laws

Hello-

Ignition cuts aren't good for this sort of thing. If you still give it the correct amount of fuel, even without a spark, autoignition (a.k.a. preignition) can happen and cause detonation waves.

However, if you cut fuel, you can quickly drop the mixture below the combustion threshold (i.e. so lean that sparks don't even ignite it!). If the idea of a mixture threshold doesn't make sense, consider this: put a can of gasoline on the workbench with the top open. Stand 10' away. Can you smell the gasoline? Yes - that means that there is SOME gas in the air. Now, light a match. Did it explode? No... because the mixture where you are standing is so lean that not even a flame will light it.

Ignition cutoffs were popular back in the days of carburetors when fuel cutoffs were impractical, but with fuel injection, fuel cutoffs are the way.


Aha! That makes perfect sense. I suppose that was to keep their customers from getting pissed off that their car is hesitating when they floor it :-).

Thanks!

Take care,
Shad

ZeroBanger

I can say that I probably hold the record for the most fuel cut off's in the world. You can search my screen name over the last 2 years and find 2 dozen posts about it. I have had 2 sets of turbos and two engines and in both circumstances at the track I would hit fuel cut off about 2 out of every 3 runs. It was quite annoying. I had about 85 runs at the drag strip, probably 50 of them I had hit the fuel cut. It always happened to me from shifting redline 2nd into 3rd.

I tried everything in the world to get rid of it. I went to the track saturday and for the first time ever I didnot hit the fuel cut. That was kind of exciting, I made 3 changes to my car, so im not sure what caused it.

Anyway, im really getting off topic. My point is I would not be concerned about the fuel cut off making you run lean. Like I said, I have done this many, many, many times without any problems. I'm not sure if It cuts fuel or ignition, the only thing I can say for sure is that I had to shift to the next gear to get power/boost back.