A friend of mine just purchased a low mileage 87 930 and had a few questions with regards to boost and boost protection. The car basically cuts boost off before you even get anywhere near 5K the previous owner had an electroinic boost controller installed.

This is a stock car and what should a stock car do when launching or going through it's paces, I drove the car and was surprised when the fuel cutoff in high rpm in second gear.

thanks in advance

Norm

 

RPM doesn't dictate when the overboost protection circuit kicks in. It's all based on the amount of boost. There is a sensor that senses the boost level and a relay that grounds out the power to the fuel pumps when boost beyond the factory pre-set is reached (I think it's anything beyond 1.0 bar...maybe 1.2, but I can't remember). So, if your friends car is boosting that high, then there's a serious problem with the wastegate most likely. Or, if the electronic boost controller is still installed, it's set too high. Or (lots of "or's" here), the overboost relay is malfunctioning. Whatever the cause, it needs to be looked into immediately. Too high of a boost will quickly meltdown the engine into a very high priced pile of boat anchor.
Does the stock boost gauge work, and what is it reading when the fuel cuts off? If the gauge is suspected of being inaccurate, then I would suggest installing an after-market high quality boost gauge to make sure you're accurately reading the true boost.

 

Maybe the wastegate isn't opening? What is the boost reading at cutoff? Does it have a good boost guage or just the in-tach OEM unit? It's possible the oB switch is bad also. It's easy to bypass but I wouldn't until you know for sure what level of boost your getting at the cutoff point.

Edit: Gee mark, you out typed me!

 

Yeah Gary, nothing better to do at work today!!!!
A thought just occured to me: Are we sure this is the overboost circuit kicking in? NormD, does the car buck like it's about to self destruct when this happens (the result of the fuel suddenly being cut off, then suddenly back on when the boost drops off), or could it be a symptom of the over-rev limiter in the distributor rotor doing it's thing prematurely (5000 rpm is way too soon for that).

 

Mark that is exactly what happens you're just starting to make good power then it dies, I'm not use to that in the 951. I told him that was not normal, there is an electronic boost controller/gauge. I will have to pay more attention to where the boost level was when it died.

Norm

 

It's tripping the OB switch, which should cut out at 1.2 Bar. It's possible it's bad. You can try bypassing it by simply grounding the wire that goes to it, but if you are actually making more than 1.2 Bar you run the risk of the engine self destructing.

 

Thanks for the advice I will give him the heads up

Norm

 

By the way that is a nice 930....

 

Can you get partial boost above 5000 rpm? You don't have to floor it all the time. If it will go above 5K on partial boost then the OB switch is tripping the fuel pumps. That would mean that either the OB switch is working and the EBC is set too high or the OB switch is defective. That little tach gage is not very accurate so it will be hard to say what your boost level really is. I have a big boost gage and when the little tach gage reads max, .8 bar, I'm only doing about .5 bar on the big gage. Try to keep the little gage somewhere in the middle and see if it will go above 5K.

 

Morning guys-

Paul here, owner of the 930 in question. First, I just joined and will intro mesself in a seperate thread. 2nd, I just got the car 1.5 weeks ago and am just familiarizing myself with all its nuance.

The PO had recently had Musante Motorsports install a GReddy Profec B Spec II overboost protector. I'm imaging my cutout issue has something to do with its settings, but I don't dare touch it until I learn more about it, the car, etc.

It's an '87, 39k, all original all stock, clean car. In tach boost gauge reads to 1.0bar and the needle buries. The GReddy unit looks to be programmed to cut out at 1.2bar.

Car builds boost fine, revs fine to redline. It's just that it cuts out violently like the fuel pumps are being cut when boost hits 1.2. Doesn't seem right to me. Yes if I modulate throttle I can rev it w/o cut out, it's bar related not RPM. Can happen as early as 4,500, once the boost needle pegs.

I appreciate any of your continued insight, and look forward to further participating with you. Gary, I'm up near UCONN, where you located? Love to meet you sometime and check out that purty '82 of yours. I'm friendly with a number of local fools with some er "interesting" things heh heh heh. We're always up to "something"...

 

Which is exactly what it should do. It sounds like your wastegate is not opening properly, if at all. The OverBoost switch is the second layer of protection for the engine in the event of a over boost condition. The first layer of protection is the wastegate. It bleeds off exhaust gases which in turn reduces the compressor output of the turbocharger.
If the stock boost gauge is pegging at overboost, then either someone has the boost controller set high which is not allowing the wastegate to open or the wastegate mechanism is failing.

 

Thanks Werk-I, exactly what I was suspecting.

How can I further diagnose the WG operation? I'm semi-mechanical (fully disfunctional).

I'm heading to Musante Motorsports at lunch who did the overboost protector install and pick their brain, for a start. The OB should be trumped by WG! Really got to get a real boost gauge in there, it's creepy not knowing for real. The GReddy unit has one built in I think, like I said just got it and haven't had 2 seconds to research it.

Thanks again!

 

WG test is easy if you have access to a compressor that can be regulated down to output 20PSI max.. Chris should be able to test it in 5 minutes either way.

Good luck.

 

Sounds like you just need to have the boost controller set to allow boost pressure to 1bar.

1.2 bar with CIS is not a good idea.

 

Back from lunch over with Chris @ Musante...

The f'n wastegate is not opening. Ice cold. They installed the controller a few months / miles before the sale to me so I'm sure it's related, he's going to take a look at it when he returns from Daytona and North Carolina track time.

Hoping he doesn't find something failed *after* his install and I get stuck with a bill a week into it 8-).

Anyone care to handhold a noob here through general WG issue shakedown? I hit the cutoff a number of times in the 1.5 weeks I've owned it (maybe 500 miles) - hoping nothing traumatic. Worst case is what, pre-detonation?

Also learned the car's got gutted cat - scwheeet!

TIA!