Can someone that had first-hand experience with both fuel-cut AND detonation please define their differences ?

I'm hitting way too much boost right now, so I'm using 99+ fuel to prevent detonation, and taking it easy on the gas pedal, until I can install FD pump and S-AFC. However, when on the highway and in 5th, I encountered what can be described as "hickups", very light, just enough to notice them, 4 or 5 times. Thing is, these did not happen under full boost, but rather when in half-throttle or while deccelerating.

Anyone has a definite answer ?

 

you will not deonate under deaccell. I'm sure that's imposable as the injectors are 100% closed any ways.

fuel cut = hitting brick wall.

 

fuel cut = no more go

detonation = game over, hopefully you'll have a couple more quarters in your pocket

 

So you're saying that I havent hit either of those ? Maybe its some quirk in the intake system or the fuel rail ?

These "hickups" -almost all of them- happened like this: I am accelerating in 4th, and when I reach about 4K rpms, I shift to 5th. As soon as I press the gas pedal again, boost shoots off towards "1 bar", so I ease off the gas pedal trying to take it easy. At that moment (as I "ease off the pedal"), these little hickups happen. Very minor, just enough to notice them. Maybe its the secondaries closing ? Could it be this ? I dont have a problem with the "3800rpm stumble", but I think I may have the reverse one , when the secondaries close. I didnt notice if these happened at around 4K, but it could be.....

Is that possible ?

 

They're two very different things, detonation is when the air/fuel mixture explodes before it's supposed to, it's also called knock, when this happens in a rotary engine, the rotor damn near stops from what I've heard.

Fuel cut is just the ECU saying "OK we're going fast enough" and closing the injectors, it's like you slam on the brakes.

 

I know very well what both mean What I dont know (because I have no first-hand experience) is what they feel like.

From what I gathered, I dont think my "problem" is either fuel-cut or detonation, but something else, probably minor...could have been the secondaries closing, or the TPS giving me signs that its on its way to "TPS heaven" (if such a thing exists )

 

For me, when I had a case of light detonation, it seemed like the car lost power for .5 seconds and went back to normal. It did it a few times. Then it decided to make the hammer on metal noise.

My situation when that occured. Stock. AC on full, 102F 115HI, full boost, 95 MPH, and worst (ashamed to admit) 87 octane gas. I've switched to 91 and haven't had a problem since, and hopefully, what detonation I did encounter didn't damage anything.

How's your TPS? I hate to be the guy beating that drum, but when mine was on its last leg, I would get pops in the exhaust on decel and light bucking on accel (most the time it wasn't WOT). It wasn't even bucking, just like the someone let off the pedal for a split second and returned the accelerator to where it was prior.

 

detonation CAN happen under decel. that is when the car is getting less fuel. that is when MOST apex seals kick the bucket and come flying out your exhaust

Justin

 

Yep, the feeling was much like that (as if I let off the gas for a split second). I suppose that the TPS is giving me warnings...but I have no other problems whatsoever with my idle, backfiring or stumbling during acceleration...

Anyway, I dont want to make my Rex a guinea pig, so in about two hours, when I'll be traveling to my summer house for the weekend, I'll just use 99+ again , take it easy on the throttle, and try not to hit max boost (which is REAAALLLYYYY easy ).

Good things: seems like I wont need to mess with my boost sensor to disable fuel-cut (seems like I dont have one), and that even with my stock pump and injectors, I can go by without much trouble. After FD pump is in, I'll tune the S-AFC, and I wll be ready to start ripping the asphalt

Yeah, but they also dont get much air either (since plates are closed). Why would Mazda make the mixture intentionally lean when deccelerating, if this meant trouble for the engine ?

 

i know that but most apex seals DO let go under decel. this is just the best explanation i could think of.

Justin

 

Sure hope it's just a minor problem, and not a "rebuild-your-engine" problem

Cannt wait to get the pump and SAFC and get that dyno-tune....

 

I'm guessing under normal loads with a stock configuration, their wouldn't be such high levels of boost heating up the engine. Now toss in a lean mixture into a hot combustion chamber and I suppose it could happen.

 

Have you considered putting the stock airbox back on for less boost?

 

Probably will do. I'll get my g/f, then head over to my house to pick up some clothes and the stock airbox , then I'll see how it goes...

I still dont get how an engine could detonate during decceleration. When you let of the gas, first thing that goes is the air, so mixture gets richer. Then the ECU "sees" the closed throttle, and goes into "lean-mode", so then it cuts off fuel. By that time, boost has disappeared (dont forget the BOV too), so the next cycles should be lean, but also boost-free....

Now, letting go and again pressing the gas pedal continuously, now that could cause detonation....but I have heard only a couple of occurances....

 

I DID put the stock airbox in, I DID take it easy, I DID put in 99+ in order to prevent detonation.....


....all that counted for absolutely ****, because my engine is DEAD

****, ****, ****

 

Huh? The rotor most certainly does not "stop". However, detonation OPPOSES the rotation of the rotor (in a broad sense of speaking). This puts tens of thousands of times more stress on components than normal operation does, which causes the weak spot (usually apex seals) to possibly brake.

The stock ECU cuts fuel to the rear rotor if the boost goes over 8.6 PSI. This is BAD, and quite a stupid move by Mazda if you ask me. Should have cut ignition instead.

 

HEAVILY agreed.

 

Probably too late for this information, but here it is anyhow...

The bucking is either 1 of 2 things. First is compressor surge - I had some problems when rolling off the throttle with my old Greddy BOV where the car would kind of stutter on deceleration. Second guess is the knock sensor pulling back timing.

Dale

 

There is a decent answer for this...ignition cut dumps a lot of raw fuel into the exhaust system and onto the catalytic converters.&nbsp This will kill them in very short time.


-Ted

 

Well guys, I may not know what fuel-cut feels like, but I sure as hell know what detonation feels -and sounds- like

Well, actually I know what fuel-cut feels like on piston engines, but I suppose the same principle -and feel- applies here as well......


....god damn......

 

you could also still get pre ign. (the other form of detonation) if the ign was cut. with fuel cut no chance.

 

pre-ignition = detonation = auto-ignition = knock = pinging...


Theres no "forms" of it. Theyre all terms for the A/F misture igniting BEFORE the spark is fired.

Just adding that in