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Mad day at the workshop

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Hi folks.

Really like to share my story of the day, its been a bad and strange one.

Started of this morning with a BMW M20B25 with the following specs;

- Stock bottom end

- Flow ported head

- Larger turbo

- Approx 9:1 CR

- 98 pump gas

- Autronic SM2 ECU

- MS2 SDI unit and a 3x2 mercury? coil pack (strange stuff..)

The engine had been tuned before with the same setup but a built bottom end.

Owner wanted to lower boost and have necessary timing and fueling changes made for the changed setup (very small VE change, mostly only mech. power limit). Sadly i was not able to verify that the timing is correct vs ECU numbers on this engine as that would require me to strip radiator etc and clock in a proper dial on the crank as it does not have any marks (timing set-by-trigger-standard), but its set like it was before and starting and running the same trigger settings and nothing has been changed according to the owner, and nothing really indicated issues for me also, it sounded and ran fine and resonable to the timing in its map. So basically it starts and idles ok. Got it hooked up in the dyno and got it rolling.

I look at the timing map, looks ok to me, i subtract 3dgs in midrange and 4 at boost for margin before starting any work.

Hook up my Plex knock monitor etc and got to work.

Started adressing down low and cruising fueling, fueling is a just a tad of as expected from the changes thats been made.

Make some corrections and work my way in the vaacum side from 1000 to 3000rpm.

Now i notice that i get alot of electronic interference, my bluetooth keyboard and mouse that i operate the dyno with looses connection with the PC, i have rpm errors on my PLEX (rpm from SM" ECU) and the dyno PC (housed in a steel cabinet but with open doors) starts to intermediately freeze for maybe 0.5secs.

At the same time i had been observing that the car had some typical "trigger cuts" when it cuts for maybe 1-2 igniton events, i also observed that i got some sudden drops in AFR to real rich and then back to normal (jumping around the map from fake trigger and rpm readings?).

Now im concerned that there is some bad noise from the coil/igniton system that is messing with things and im planing to take a time-out when i rap-up the vaacum side fueling in steady state.

I bring the car to 3500 rpm and go to 100kpa, then push it to maybe 110kpa for a sec, and then i get alot of extreme noise/knock readouts on the plex, like dots everywhere(also on rpms way up above what i was at). And in the headphones it was just like going from normal to instant white noise, but not like typical noise, just constant white noise..

I backed of and wondered... was that really knock? at this realatively low load, that timing(retracted from what the car had been run at for a while) and that AFR.

I checked timing map and subtracted about another 4dgs in the area and carefully ramped it back up to see if it changed and as such establish if knock or not.

I went through 100-110kpa from 2000 to 3500. When i hit 3500 it reached 130kpa as i entered the spool area, AFR was at 12,7, peak recorded power 170hp and 300nm, and then within 2seconds from reaching that point in the map rod no.4 separated from its piston and the rod took the rear exit trough the sump. Ie. catastrohic failure. No mechanical or abnormal noise was heard outside prior to failure. Two guys actively listening as i alerted something strange was going on.

Thats not the end of the world or anything, this is a low budget car with a questionable build with a used, non re-built or checked bottom end.

we found that the piston crossbolt had separated in two but with its bearing surfaces ok. And the cracks looked like maybe half was new and the rest old so that might have been a latent mech. fault.

However, the story continues. After rigging down the car and mopping some oil of the floor, we brought the next car in line onto the dyno and hooked everything up. Then when i was about to start working on it i discover that i aint getting RPM readout from the dyno. I check the lead to the rpm pickup and even used my scope to check that the trigger pulses was ok and they where perfect, but my dyno electronics wasnt reading rpm.

I had to lower the trigger threshold by 1v from 4.5v to 3.5v and then i got rpm signal again, but it was still unstable and at higer rpms the reading went crazy.

Now i at least got to operate the dyno, and as im matching car rpms to the dyno i see that the torque (loadcell) readouts are WAY off, so the issue wasnt related to the rpm pickup only. Now im getting worried....

So, what im thinking is, could it be that severe ignition/coil RFI could have caused everything to go downhill?

Im thinking that the old Autronic SM2 doesnt really filter triggers in any advanced way, ie if the trigger voltage is within the envelope it will act on it and faulty trigger events could cause the engine to fire all over the place and that ofcourse would cause mayhem..

And at the same time could the RFI have jumped to the dyno electronics, as i had EGT, MAP, Lambda, RPM hooked up to the car, although the RPM signal taken from the tacho output on the ECU is the only one electrically connected. the EGT has shielding witch is earthed only at the manifold in that end and the lambda goes trough another converter and lambda was working fine at the end of the day.

Any experiences related would be of great intrest.

Right now my dyno is out of service and god knows what that may end up costing me, it will quite certainly be far more than a stock M20B25 bottom end...

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