Mythbuster: continuing project thread of my 1303 '75

[Humble]

Would anti-lag systems cause this same kind of problem?

[70Turbobug]

Originally Posted by Humble

Would anti-lag systems cause this same kind of problem?

That´s a good question - although I don´t think so,because anti lag uses extreme late timing up to 40° after OT,so backfire back into the cylinder shouldn´t occur.

If I have another set of rockers Wally,I´ll let you know asap.

[Wally]

Originally Posted by spannermanager

Hey Walter, hows it going, looks like backfire damage to me,...

I think you are right m8. Funny thing is it seems te generate a back-fire on a fuel cut (which comes from the hard-cut rpm limiting action of the ecu).
As I already replaced blown up vacuum lines (pressure lines actually) on the manifolds AND had warpped butterflies, back-fire as a cause is right up that line of thought.
Now, how does fuel cut-off cause a back fire (on a turbo engine)??

@ Mark: thanks man, but I have already replaced the faulty rocker by another one and the engine started right up and ran smooth, so probably no bend valve(s) (again). GB pushrod is also still straight as an arrow!

[70Turbobug]

Looks like you got lucky once again! Bent Butterflies, etc. sounds like a pretty hard backfire to me.I'm surprised that can happen with a modern ecu.Perhaps the fuel cut comes after the ignition cut? Or timing isn't retarded far enough.It would be interesting to know why it backfired since it's supposed to be a safety feature...
Good to hear that everything is up and running again!

[spannermanager]

All the elements for a backfire are in place, all it takes is a nanoseconds worth of fuel vapour, its always in there, the ignition element (a heat source) is always in there,, the fuel cut may be leaning things off, it certainly kills off any inter cooling of the exhaust valves by overlap events, so up goes the temp, nanoseconds again is all it takes, then all you need is some air in the exhaust,,,,,, port flanges, c clamps, when you think they can rip blades backwards on a turbine wheel.... i think im going to look at introducing very deep valve pockets on my new wasser, so its a 'safe' engine, if there is such a thing, the energy is huge and has to go somewhere, i recon there's enough to open valves to coil bind if the springs are not up to par, i run oil jets these days to cool the springs, imagine how hot they get just from movement/friction, say on a 'cold' test rig, also remote oiling for the springs with external hard lines and a spray bar on T1 is worth looking at, the T4 Walter runs has better top end oiling, I've yet to play with these, my equip has a very good T4 guru on hand, retired now, but a very big name from super vee in the '70s, and on to the very top of motor sports engineering, he has just sorted my wasser oil and piston ring problems with a quick look and a phone call. thanks ...... ........

[Bruce.]

Walter,

How wide is the hyteresis on the fuel cut? I'm guessing the fuel restarts above the soft cut?

If so, the fuel cuts at the limit (all injectors suddenly? or does it cut progressively cut 1,2,3 then all 4) but restarts the fuel flow before the ignition is back on (ie. above soft cut). I would think this setup would allow enough time for the temps to soar in the exhaust valve/port/primary then the fuel flow restarts without good spark (bang!).

I think a "stock" set up would cut the fuel flow but not restart it until the revs had fallen below the soft cut to ensure the sparks were functioning first.

If you want a flat shift setup (that hangs nicely at the limit) you might need another setup. Anyone tried DIY throttle by wire?

[Bruce.]

Another opinion (lifted from a subie forum) that might have merit.....

"Like a lot of people I've never liked a fuel cut on aggressive turbo engines and I think the science backs me up there. From the video they found as much as a 1/3 of an injection worth of fuel just chillin' in a puddle in the intake runner. This puddle is kept liquid by the boost pressure and sheer amount of fuel. When the boost pressure drops (like when you lift the throttle) all of that fuel instantly evaporates in the low pressure air and is ingested by the engine giving natural deceleration enrichment (which is nice) but during a WOT rev limit hit where the fuel is cut I believe the fuel is also able to be evaporated into the dry air (which now is not saturated with fuel) and also ingested by the engine causing potentially lethal lean ignition events."


more detail here of the "Tau puddle"

http://www.ret-monitor.com/articles/...he-tau-factor/

[70Turbobug]

Fuel vapor would be an explanation.The question then would be does the ECU cut fuel completely or just reduces fuel by say 95% or does it restart as Bruce mentioned? The engine stays running so maybe just the amount of pulses are reduced.Some OEM cars limit there rpm in such a way that the engine simply does not rev higher instead of the usual rev limiter chattering.Maybe a sort of trick to the MAP or MAF telling it that max airflow is reached?

[Humble]

After double checking my rev-limiter it looks like mine is set to retard spark (soft limit 7750rpm) then cut spark (hard limit 8000rpm) it never touches fuel. I don't even cut fuel on overrun (engine braking) just to be safe and help cool the heads. I wonder if that could be my saving grace.

Both of these settings are terrible for mileage and emissions but you have to pick your poison here. Wally, does your ecu offer any setting like these?

I've seen the Tau wall wetting stuff mentioned before with the megasquirt setups. They've had x-Tau enrichment since MS2 came out though I've never played with it. It's worth considering for a daily driver or street car but I'd imagine it would be kind of fiddly while setting it up.

[Wally]

As described by DTA in the manual:

Normal RPM Limit.
This limit introduces a cut on each cylinder in rotation which would be enough to constrain rpm rise in a driving situation. It has a fairly soft action and does not upset the car. Normally set to 250 rpm below the ultimate limit.

Ultimate RPM Limit.
The engine will not go through this limit no matter what the conditions i.e. off load and full throttle. It is, however, fairly brutal in operation and should be set slightly higher than the normal rpm limit above.

The last rocker breakage was absolutely the ultimate rpm limit described above, but earlier back-fires I had that broke boost reference lines and bend butterflies, was definately caused by the overboost function.
AFAIK, both the overboost and ultimate limit (hard cut) actions use fuel cut-off, but again, I am not exactly sure of that as it doesn't say anywhere for sure and I have no other evidence for fuel cut action and in what way it would do so. Ditto for the ignition cut 'map' (if there is one)...

[spannermanager]

Hi Walter, was it the same cylinder/rocker as previously

[Wally]

Originally Posted by spannermanager

Hi Walter, was it the same cylinder/rocker as previously

No, same side though, but I think its unrelated. Previously both rockers failed simultaniously on cylinder 4, now it was the exhaust rocker on no. 3

Overboost is now set on 2.3 bar (3.3 PR) and ultimate rpm at 8K (soft cut at 7200), so I hope to never reach either

[judgie]

I did some work on a buggy fitted with a 2.2 t1 engine thats was turbo efi. the overboost cut was brutle and split a barrel. took a while to sort that one out as the boost ignition retard was also all over the place.
the main problem with the boost cut was down to the poor cb header design with the waste gats off one header runner and the big turbo making so much boost the waste gate could not control the boost limit, even with the waste gate open the turbo could still produce more boost than the ecu was set to cut.
with your new settings i would think your'll run out of road before the cuts come in ::

[Wally]

Finally got hold of some Magnecor KW85 bare cables so I could make my own (longer 90 degree turns into shroud) cable ends onto them (see pic).
Just installed them into the car and I am finally totally happy about my spark plug cables after all these years...
Coming monday Time Attack! Wish me luck...

[70Turbobug]

Go get 'em Wally!!