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-   -   Mythbuster: continuing project thread of my 1303 '75 (https://www.germanlook.net/forums/showthread.php?t=9953)

Wally May 16th 2011 14:27

Quote:

Originally Posted by Humble (Post 80773)
Damn, again? What kind of pushrods are you running and how long were you on the hardcut? I've spent a few seconds on the hard cut (500 rpm between soft @7500 and hard @8000) on longer straights so I'll have to watch that :eek:

Yeah, again indeed...
I am running Gene Berg pushrods, the good ones as the PR on the effected cylinder was still straight.
I was on the hard cut maybe only less then a second, so be aware that it isn't good for your engine's valve train.

Wally May 16th 2011 14:40

Quote:

Originally Posted by TSAF (Post 80771)
Wally maybe the Pauter ones might be the solution.

Maybe, but my cam probably is not really suited for a Pauter cam and the extra valve lift does probably not clear the piston.
Oh, and I haven't got a set anymore...so if anyone...? ;-)

judgie May 17th 2011 08:09

re the rockers watch this space as there might be some forged steel ones being produced in the not to distant future, stock ratio and with the 8mm adjuster screw. being done as we are running out of good used ones which are all at least 20 years old.
seems a odd place for them to be breaking but hard cuts do funny things to engines, not a big fan of them but know why there used.

spannermanager May 20th 2011 15:56

Hey Walter, hows it going, looks like backfire damage to me, i had unexplained valve events with my turbo wassers back in the '80s, a turbo backfire has enough energy to slam those turkeys wide open, the push rod then falls from the rocker socket and engages on the edge of it, or the follower wall, instant extra lift. a good case for more spring pressures with turbo, and lightweight valve train, maybe the stock rockers saved you more serious damage, stronger ones may be a backward step, i know what I'd rather replace, good luck with it all.:)

Humble May 20th 2011 16:36

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

70Turbobug May 21st 2011 08:48

Quote:

Originally Posted by Humble (Post 80805)
Would anti-lag systems cause this same kind of problem?

Thatīs a good question - although I donīt think so,because anti lag uses extreme late timing up to 40° after OT,so backfire back into the cylinder shouldnīt occur.

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

Wally May 22nd 2011 12:26

Quote:

Originally Posted by spannermanager (Post 80804)
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 May 24th 2011 00:36

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 May 25th 2011 15:34

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:eek::eek:.... 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:lmao:, 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 ...... ........:lmao:

Bruce. May 26th 2011 06:09

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? :D

Bruce. May 26th 2011 06:14

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 May 26th 2011 08:15

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 May 26th 2011 13:22

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 May 26th 2011 16:09

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 May 27th 2011 15:15

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


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