As you can see in my avatar - my 928 is not stock.

I'm having a small issue I figured you guys could help with. Since the 16V 928 and 951 share very similar heads & ignition.

Under WOT it cuts out, no, more like stops accelerating, bogs a bit. If I lift the pedal just a touch and re-mash it down, it takes off like nothing was ever wrong. It will then pull to redline or do it again. Only happens above 4,500 or so RPM. At anything less than WOT, everything is fine. So if I feather the throttle when it happens, I can get the car to redline flat out, just need a little TLC and playing with the pedal.

Some specification:
1. 7psi at 6,000rpm
2. Stock plugs are WR8DC - I'm using WR7DC (BOSCH)
3. Stock timing is 23 degrees at 3,000 rpm - this is where mine is set
4. Stock plug gap is .028 - .032. I split the difference, set mine at .030
5. New plugs (BOSCH) cap, rotor etc.....every wear item on this motor was replaced before the blower went on.

I thought maybe my timing was too advanced - so I turned it down a tad. It got worse. So, I advanced it a hair - got better. Just a bit more, not it's almost totally gone. I'm afraid to advance it anymore since this car does not have knock sensors.

So, my overall question, for modified 951's - what do you normally set your gap to, how cold of a plug do you guys all use? What is the timing set to on 951's?

Once this is figured out, a 10psi pulley is going on.

So far there are no signs on detonation on any of the plugs.

Thanks!!

-Erik

 

It certainly sounds like an engine management problem. What kind are you using? Do you have a wideband and what numbers are you seeing? I think 23 degrees is high, but apparently that is not the biggest problem, if one at all.

 

I've had the same symptoms three times on two different turbo cars (rx7tt and 951) -- runs fine all day long except under highest load levels when it hesitates and sputters -- let off a smidge and it's fine again. Everytime, it was the spark plug wires in my case -- when the pressures go up in the cylinders, the wires find an easier place to arc. You said you replaced everything -- does that include the plug wires? Are they quality wires -- all the boots fully seated? If that's ok, I'd check out the rest of the igition closely -- coil new? Might want to try closing the spark plug gap a little.

 

What computer are you running? Get your car on a steady state dyno and figure out which area your having the problem at. With a steady state dyno you should be able to pinpoint the load area that your having problems with. Most steady state dyno's are $100-$150 an hour.

-Destin

 

Stock computer - using a Corky Bell adjustable fuel pressure regulator - air / fuel is perfect.

Wires are BOSCH - brand new, fully seated 100% OEM. Non OEM wires on 928's have been known to cause iffy results on even stock engines.

Others have boosted 928's with this stock system with perfect results so something is amiss on mine. I cannot find (nor have they responded) to report back what plugs / gap they used so this is the only setting I am not sure of.

I am going to pull the plugs to re-gap them closer.

So, anyone know the timing setting on the 951's?

Dyno appointment is Wednesday.

 

Are you running colder plugs than stock?

 

#2 The 7 indicates the temp - so hopefully I read the diagram correctly and a 7 plug is colder than an 8. If not, I may have found the problem.

 

For bosch 7 is colder so you are good there. Stock 951 timing is ~17 degrees below 4000 rpm, this is for stock boost levels. A little more timing gets added from then on. Less at 3000. Does the 928 have overload protection like the 951 does? It sounds like you may be hitting that, but by restricting airflow with the throttle slightly you are able to avoid it. I would experiment with the plug gap and see if any changes happen, at least to verify weather it is an ignition or engine management problem.

 

And how are you adjusting timing?

 

How does the "overload" protection work on the 951? How is it triggered? Are you talking about knock sensors?

 

The old fashioned way - loosen a 13mm net - turn the distributor. Now with the supercharger bracket installed I cannot see the timing mark anymore - it was set to stock before putting it on. The amount I moved the distributor was very slight to correct this problem. Cannot be more than 1-2 degrees at the most. I plan on verifying this with my friends identical car later to day to see how much I advanced it to "fix" this problem.

Here is another question. If everything is working well, no cutting out even with the additional advance. If there are no signs of detonation - then I have nothing to worry about, right?

 

No, overload protection cuts the engine out when an airflow meter voltage/rpm is over a certain threshold for a certain time. It is commonly miscalled overboost protection around here. A search on this forum would explain it more, I dont totally understand it, but Danno and TT have talked about it in the archives, if I remember. I have only seen it myself on the 951 but I dont have any 928 experience at all.

 

In my opinion you are running too much advance. I would turn it down and see if power changes. I am guessing it will go up. You want to tune it conservatively because on a hot day at the track it will be way easier to detonate than on a single cold tuning run.

 

The 951 does not have an adjustable disctributor, so we don't get to play with the timing like that. I think you are right about the timing -- run as much as you can without detonation -- but it wouldn't hurt to tune it on the dyno and someday get an exhaust gas temp sensor. Fairly confident the 928 does not have overboost protection -- or whatever Marc would like to call it -- and the symptons of that are a more violent power cut-off anyway, at least on the 951. If you A/F is dead on when this happens (right?), I'm still guessing something is amiss with your ignition. Smaller spark plug gaps can compensate a bit -- so I'd try that for sure. If that helps, I'd try to find the weak part. Is the coil new?

 

True -- a safety margin is a good idea.