Danno

tommye, yes, you are hitting the overboost-protection in the chips. This is a multi-part function that occurs in both the KLR and DME boxes. The KLR will cut back on the CV-cycling valve's duty-cycle and lower boost when it detects overboost. The DME will also cut the fuel, thus the sudden stumble you get. If you had the LBE cranked up a little more, such that you actually hit 2.0-bar on the gauge, the fuel-cut will be so sudden that it'll send you through the windshield. Make sure you have your seat-belt on!

The overboost-protection level is actually a programmable 2D table with RPM references. In the stock chips, the limit is lower in the lower-RPMs. This is the area where the LBE makes the most difference, in the range from 2000-3000rpm where the stock programming rolls on the boost-smoothly. The LBE however, will cut off all pressure to the CV, and the wastegate as a result, so you can build boost as fast as possible in the lower-RPMs. However, this will send you over the overboost-limit at low-RPMs. It's actually set to some really low number, like 1.3-1.5 bar or so. Then as RPMs rise, the overboost-threshold goes up, to about 1.8-1.9 bar, which is just a tad higher than the stock 1.75 bar of max-boost.

I wouldn't be as worried about hitting the overboost-limit as much as I would be worried about running lean air-fuel ratios. Since the stock boost-curve drops boost to about 9-10psi by redline, the stock chips are mapped for that level of flow. If you've got the LBE in-line with the CV, you'll end up with a much greater boost-level and flow in the upper-RPMs than the chips were designed for. You may be seeing 12-13psi at redline, which is about 10% more air than stock, and your air-fuel ratios will most likely be 10% too lean... be careful!

tommye

Alexands,
Oh, I see your point, yes it's confusing. I guess you can't say that you have boost at 1 bar manifold pressure since you don't have any pressure increase across the turbo. At the same time, the turbo is actually pumping air into the manifold.

NA cars always operate with manifold pressure less than atmospheric pressure - otherwise, no air would enter. (In resonant induction systems, you might see pulses above atmospheric pressure though.)


Jake and Hosrom,
I checked the workshop manual, it says that the 951S should keep 1.75bar up to 5800rpm and then drop. I think mine drops before that - I will check again more carefully.

Cheers,
Tommy

tommye

Danno,
thanks for your reply! Did I miss something with the LBE? - I thought I would get faster spool-up, but no significant peak boost increase.........
I will remove it and thoroughly test the car - if the LBE turns out to be the culprit, I guess my questions change......What can I do to improve response and spool-up ?
Rebuild the waste-gate ? New Turbo ? other?
I want to keep car basically original, at least as far as appearance.
Cheers,
Tommy

Danno

You can use the LBE with the stock chips IF you adjust it so that max-boost doesn't go over the stock 1.75-bar (12 psi or so). What you'll notice is that turbo-spool up and response in the low-end will be improved BEFORE max-boost hit. That is, if you were getting 5psi @ 3000rpm before under full-throttle, you'll see say.. 8psi now. This is because the LBE is overriding the stock KLR programming that rolls on boost slowly by slowly closing the wastegate. The LBE forces it to remain closed. However, you need to be sure that the max-boost level does not exceed stock boost.

UK952

When the overboost protection triggers for a short period the boost is limited and IIRC the ignition is retarded. Switching the ignition off/on resets it and it will also reset itself after a short period. WIth an LBE and standard chips my car would hit the protection in 4th and 5th around this time of year especially on cold wet days. Backing the BE off another turn for the winter should cure it. It is also possible the extra pressure pushed a line off, check the small vacuum lines especially the one that goes to under the throttle body.
Tony

tommye

Dear listers,
At last we've had some dry conditions over here - just went out with the Turbocab for a quick test. In the meantime, I had unscrewed the LBE as far as possible - 3 1/4 turns. At 3 1/2 turns, it is possible to blow air through it with my mouth.
Now boost is back to stock levels maxing out at 1.75 to 1.80 bar. I can do short sprints without any problems, but twice while doing longer runs on 4:th or 5:th, I got the violent cut-outs again......the second time, the engine died while letting off instead of idling - this never happens normally. It really seems like engine is not getting enough fuel-flow, but how could this trigger the "over-boost" protection (or whatever this cut-outs could be)? Is there any detection + cut-out for lean fuel conditions ? If yes, how does this work, by monitoring the O2 sensor signal?
Or could it be short spikes not visible on the boost gauge that trigger the overboost cut-out? I imagine this would be possible if I had a leak in the WG "vacuum" line that suddenly opens up only at higher pressures.....this could also explain bad/no idling afterwards....??????


Tommy
As always, grateful for advice from the boost-experts out there....

fast951

Tommy, if you are not already using an aftermarket boost gauge, it's time to get one. Are you getting any knock? Change gas stations and see if it makes any difference (not likely).

The reason you see the problem in higher gears now is because you stay at the higher boost level longer. The Overboost protection is based on RPM and has a timer. A quick overboost spike will not cause it.. A prolonged overboost triggers the protection.

The overboost protection is a very involved algorithm in the DME. Some people raise the values (or set them to MAX), still get overboost protection. There is much more to it than just a value in an array, indexed by RPM.

tommye

Sorry - no aftermarket boost gauge........
and I get no audible knock.
But the boost level is definitely back to somewhere near stock level - my SOP dyno also tells me performance is around stock
Thanks for the info about the overboost timer - I will keep that in mind...
Something is acting up intermittently or is degrading slowly - not so long ago I could do long sprints with excellent performance with the LBE set slightly higher than now - around 1.9 bar on the (not aftermarket) gauge.

Tommy

fast951

But it's a bit colder now, isn't it?

special tool

Does it get cold In Genev?

tommye

Yes, it's definitely ca bit older now - but the boost level is only 1.8 bar - just like stock .....

tommye

Its been around +6 degrees celsius (42 F) today - slightly warmer than normal.
It usually dips down to -10 (15 F) once or twice each winter.
Tommy