hacker-pschorr

I understand 23 degrees is probalby too much - thing is, it's missing at anything less than stock.

theedge

Could get a knock sensor. www.viatrack.com has them as kits with Bosch sensors (what 951s use). Its a little box that can light up an LED when theres knock. He now tunes them to the size of the engine, so he will want to know what your bore and stroke is. I have one for my MegaSquirt, looks like a nicely made product.

mark944turbo

I re-read your first post. It doesnt sound like a fuel cut type problem you are having, like I originally thought. In that case, rule out the overload protection like tom said. If it is a bogging it is almost certainly a spark problem. Experiment with the gaps!

ibkevin

It should have a pair in the "valley" correct?

hacker-pschorr

Not until the 32 valve did they add knock sensors. The mounts are there so they were thinking about it, the L-Jet & CIS systems in these cars do not support it.

Took the gap down to .023 vs the .030 I was at before (.028 is the lowest in the factory range). It's better, much better but not perfect. Tomorrow I'll try a new coil.

mark944turbo

"Took the gap down to .023 vs the .030 I was at before (.028 is the lowest in the factory range). It's better, much better but not perfect."

Cool!

hacker-pschorr

Oh yea, the engine in my avatar is not in the track car in my sig. The blower is on my street car (next year supercharging the track car).

hacker-pschorr

Doing some more research, 31 degrees before TDC is where slightly newer 16V 928's were set to (same ignition system).

https://rennlist.com/forums/928-forum/282126-16v-ignition-timing-i-m-confused.html

I dunno, see what the new coil does tomorrow & then off to the dyno on Wednesday.

five-o

let us know what you find out. I have the exact same problem with my 951. I just haven't troubleshot it. Seems to be a common problem with boosted cars though.

hacker-pschorr

Add this to the scenario. Jean-Louis, 928 PCA racer is running 5psi non-interooled in his Euro 928. His motor is 10.4:1 compression - 4.7 liter. He is running 31 degrees advance @ 3,000rpm. So far so good, no signes on detonation - 24 sessions at Road America so far with the blower.

#1 difference between his car and mine is the compression ratio (mine is 9:1) and the cams:

My cams are the US 81. He is using the 83-85 EuroS cams

five-o

Did you replace the coil yet?

Chris White

I would think you can run more timing, well tuned 15-18psi turbo motors can run with (well) over 20 degrees (can’t give away too much here!). So 5 psi or even 10psi would run fine at up to 30 degrees – just watch the intake temps. Anything over 200 and you are inviting the knocking monster in!

Chris White

hacker-pschorr

Final update – thanks everyone for your help.

The early 928’s have a traditional distributor that allows manual adjustment just by turning it. The vacuum diaphragm is unique on these distributors, they have two ports. One on top for advance, one on the bottom for retard. These ports are on either side of the throttle body. They act sorta like an equalizer under any vacuum conditions. Once 0 inches of vacuum is reached, by balance each other out.

Here is where it got interesting. I was running with the lower retard line removed (hose was sealed off to prevent a leak) the port on the diaphragm was left open.

What did this cause? Now under boost, the arm that is affected by vacuum is now being pushed in the opposite direction, causing the ignition to retard under boost. Pretty exciting if you ask me. This was causing my bogging condition. With my car set to 23 degrees @ 3,000 rpm – my rough calculations puts me at 15 degrees of advance above 5,000 rpm at 7lbs of boost. Way to low for these cars.

With the lower hose put back on – car runs fine. A bit sluggish at 23 degrees so now I’m running at about 26 degrees at 3,000 (both hoses removed). Not sure on max boost – need a passenger to watch the gauge (not in the best location at the moment).

Going forward, I’m going to put a bleed off valve in the lower port to allow boost to be bled from the lower hose. This will allow me to dial in whatever boost retard I want and still maintain high levels of advance under vacuum / low boost. With a simple check valve this will only bleed off boost, not start sucking in vacuum.

hacker-pschorr

The REAL final update - in case anyone here really cares, I hate leaving threads unfinished.

Fuel pressure was WAY to high, I was drowning my system. How high? Lets just say I now know this system will flow enough fuel for 15+psi. I'm only at 7.

Tom M'Guinn

Thanks, I hate unanswered mysteries too...

But didn't you say your A/F ratio was good when this happened?