I believe the ECU tries to protect the engine if any of the readings get towards it pre-set limits.

 

driving 881 miles in two days.... can you top that? i hope my box dont blow up

 

yes, 960 in 18 hours... and I thought my box was going to explode too!!!

 

1900 in 2.5 days...

Another time we went from Boston MA to Lafayette LA in 25 hours...(1600+ miles)And back the same holiday weekend. Another 25 hours (Thanksgiving weekend). A long rough trip, but we did ok. Drove straight through both ways

 

ouch, LA to MA and back.

 

I just came back from Roswell NM...Lets just say at 7500 rpm & 110 mph the Xa's engine sounds like no other I have ever heard!

 

^^^I didn't think the xA or xB would go much over 6000 RPM. How did you manage 7500?

And I'm SURE you are correct that it DIDN'T sound like ANY OTHER xA you have ever heard! LOL.

 

He obviously has a Roswell engine...

(Seriously, though, that is pretty high.)

Tomas

 

Especially since fuel cutoff is way before 7500 rpm.

 

not sure at all how you hit 7500 rpms...

my xB along with many xA's hit 6500 and just bounce on the rev limiter... maybe yours is special

 

Is there a rev limiter or not? What makes people think there is? I have not seen any mention of it in the xB technical and diagnostic manuals.

 

there is no speed limiter, which would stop the car at a certain speed...
for example a crown victoria has a speed limiter which stops the car from obtaining a faster speed by stoping fuel delivery..

our cars have a rev limiter, because if they didnt, then our cars would blow up if we topped the speed out...

 

Simply put, beyond "X" RPM the ECU will not tell the engine to turn faster. It's not a 'speed limiter' or 'rev limiter' but just running off the end of the programmed curve.

Tomas

 

That sure sounds like a rev limiter to me.

 

It's probably because you got rid of that vacuum (dead air space) behind the box... interesting!

 

I am speculating on the differences. In the old days, a rev-limiter was a cut-out that killed ignition at a certain rpm. When fuel injection came, it was a little gentler to just cut the fuel. Now the ECU can do it even better, creating a soft wall by retarding ignition and not increasing the injection of fuel, so the engine stops increasing the rpms and stays at its preset maximum.
If that is correct, then what is the feeling of "bouncing off the rev-limiter" that one person reported?

 

I suspect that may be the effect when the rate of change is be faster than expected, and the engine RPM actually exceeds the point the ECU thinks is safe for it and finally takes drastic measures by cutting fuel (and maybe even spark) to save it.

There was (maybe still is) a vid of a mufflerless box with forced induction running up and down the street testing in second gear, and repeatedly getting past the absolute top RPM its ECU was happy with.

The effect was sort of rrrrrrRRRRRRRAAAAAAAA...AA..AA..AA..AA...AA as the engine kept exceeding absolute ECU set redline and the ECU removing fuel until it dropped below that setpoint, then allowing it again, etc.

Effectively that acts as a 'rev limiter', but is still just the normal actions of ANY ECU in trying to keep its engine within the boundries of what it has been told is safe. No different than a similar instruction of 'don't idle below 600' or whatever.

I guess what I'm saying here is there isn't a special device of some sort used as a rev limiter, but simply a fairly sophisticated computer that has tables of allowed operating ranges for all parameters (RPM, temp, ignition timing, fuel flow, valve timing, etc.) and tries it's best to keep things within its pre-programmed limits, and that includes a total RPM range of something like 600-6500 (or whatever it is). Anything out of that range it isn't allowed to support, that's all, it doesn't have suitible instructions on how to do it, even.

In that sense, ALL IC engines have rev limiters - you won't find an old all mechanically controlled monster V8 running 10,000 RPM straight from the factory - there are physical limits to such things in ANY engine. They just can't do it.

(My old MGs had hard rev limiters - above a certain RPM limit the connecting rods would come through the side of the block and all the oil would drain out. These newer engines are a little more gentle about it.)

Tomas

 

The natural built-in rev-limiter is valve float. At a certain rpm the springs that close the valves can't do it before the cam opens the valves again. The valves then don't close all the way, but float between open and almost closed. Without fully-closed valves, the engine loses compression and power, at a natural redline.
The workaround for valve float was to eiminate the springs and close the valves with another cam - the desmodromic valves of the old Ducatis.

 

I can say for sure that the xB does 5000 rpm at 100mph (on the speedo) for long periods of time quite happily... I can also state from personal experience (as I did on another thread) that the rev limiter does kick in at 6400-6500 rpm in 2nd gear. That was 62 mph as I glanced over and frantically grabbed for third. The limiter drops spark randomly when the rpm exceeds the programmed mechanical tolerance of the engine. It is not an absolute speed limiter. (other factors control that) and the thing about valve float vintage42 said will never happen in the vehicles that 99.5% of us all drive. My point? Drive the heck out of that little car all day and don't, whatever you do, don't look back! (remember Lot's wife) These engines were designed to spin at 4K rpm or better for over 200,000 miles. Wring that thing out like a sponge!