j.kenzie@sbcglobal.net

Hi guys,

[I edited my title so maybe people with 16V SC's can find the post on the last few pages where Hacker shares his method of using a dual vacuum distributor from an 82 to retard the timing under boost. This allow the car to maintain factory advance when running off boost, then retard the timing when on boost. I think it's a heck of a great idea. No mine, but I really like it.]

I checked my timing due to an off idle stumble, and it was 4-5 degrees retarded from stock. I put it at 20 degrees BTDC at 3000 RPM with vacuum advance line disconnected and plugged, and most of that stumble went away. I wonder if I can push is a little further for even more benefit. Any ideas how much advance is safe?
Thanks,
Dave

j.kenzie@sbcglobal.net

BTW the 84 US distributor has a vacuum advance actuator only, no vacuum retard, plus centrifugal advance too.
Dave

dr bob

Intercooled? Inlet pressure and temperature along with fuel quality will define how much advance the engine will tolerate. Got knock sensors and EGT probes tied into your active pressure, fuel and timing controls yet?

I'm sure that Carl offers some guidance. What does he recommend for your car?

j.kenzie@sbcglobal.net

It is intercooled, yes.
I was just going to turn the distributor a little more. How much is OK?
Thanks,
Dave

dr bob

Stop somewhere short of the point where detonation occurs? Advance is a lot like boost -- it can be addictive.


FWIW, I sold my last Saab Turbo to a neighbor who -really- wanted it. Instructions: Change the oil regularly, and don't take it to any "mechanics" who aren't dialed in to the cars. So he took it to his "mechanic" the next day because he was disappointed with the launch torque. Mechanic advanced the timing some which seemed to help. Engine was melted by the end of that first day. He said everything felt great as he was blasting up a long freeway incline under full boost. Then the power just fell off. So he pushed harder on the skinny pedal. Car continued to slow down. Head for the shoulder. Engine stops. It spins really fast on the starter, but won't start.

I heard about it six month later from his now-ex. She was mad at me for selling him the car, thought I should give her the money back. $600. I'd driven the car back and forth to Mammoth Lakes regularly at, um, lightspeed and beyond. Easy 100+ cruiser. Gone in Sixty Seconds thanks to a twist of the distributor.

j.kenzie@sbcglobal.net

I'm kidding you, Bob. My hope was that somebody with experience would say for instance that the timing specs are very conservative and 4-5 degrees beyond stock should be no problem. I am intercooled, running 93 octane gas, and the percentage of time I spend on boost is relatively small.
I'm just asking for shared wisdom. (Yours was appreciated too, BTW)
Thanks,
Dave

Speedtoys

"Inlet pressure and temperature along with fuel quality will define how much advace the engine will tolerate."

Those are all unique to your car.

Instrument it, measure it.

hacker-pschorr

Stock timing + boost = blown engine.

You want to advance it?????

Stick with stock or LESS not more. There is a way to retard timing under boost but iirc that will require an 80-82 distributor.

Once I'm back home (about a week) I can dig out my notes & help you out. For now....for God's sake....don't advance the timing.

John Speake

The whole distributor would need remapping for a safe engine. You may be able to add some advance just off idle but as soon as the boost comes in the usual starting point is to retard the ignition by 1 degree for every pound of boost expected at various rpm and load points.

Just advancing the distributor by turning it around idle will likely give the engine far too advance when under boost and result in problems.

FredR

Bob,

I sincerely hope you advised said ex-wife to simultaneously partake in sex and travel! Generally I do not approve of using bad language in front of ladies but there are clear and obvious exceptions to any rule.

Naturally aspirated motors are reasonably tolerant of too much advance but boosted motors can and do burn holes in pistons rapidly. John's advice to retard 1 degree per psi of boost seems to be a good marker but simply tossing around with the distributor orientation would appear to be asking for trouble but then I am also sure Joe fully understands that.

Ideally anyone modifying their engines like this should have access to a WBO2 device to ensure an AFR of about 12:1 or richer is maintained when on full boost but whether or not there is any simple modification to retro fit a knock detection system I do not know but working without such must be very tricky indeed- it is a bit akin to flying blind in fog and hoping that you are not about to hit another plane- not wise.

With sophisticated systems like that on the S4+ and tuning with ST2 one can sail much closer to the wind- without such one has to be very conservative to ensure nothing untoward occurs. A very slippery slope on the earlier model systems I suspect.

j.kenzie@sbcglobal.net

OK, thanks guys.
My thinking is this:
I have vacuum advance only on my 84 distributor. This gives 14-17 advance under vacuum. Under boost there is no vacuum so I lose that advance anyway and am back to max mechanical advance, the same as the 3000 RPM spec. Currently, that is 20 degrees. Relatively speaking that's almost like retard under boost. If I added timing of say 4 degrees the most I'd see at boost would be 24 degrees, but I'd get a little more across the board while running without boost. I do have a WB O2 sensor and maintain AFR's of 13 or less off idle.
I may just let it go, but that was my thinking.
Thanks for all the advice,
Dave

j.kenzie@sbcglobal.net

Alright, I went after my off idle stumble another way.
I have been trying to run closed loop, and I get a nice idle AFR of 14 or so. The problem is that when I give throttle off idle, I get a momentary lean swing until I get up to 1500 RPM or so.
If I disconnect my O2 sensor signal to the L-jet ECU, my idle AFR goes to 12, and smells like it. On the other hand I get no lean swings, and under heavy throttle, I see AFR's of 12-13 from idle on up. The cars acceleration is strong and immediate from idle on up, with no hesitation. I guess that's a win. The only problem is the rich idle. (BTW I've adjusted the mixture screw on the AFM so it's almost all the way out).
I'll leave the timing at stock for now and drive it open loop for a while.
Thanks again for all the help,
Dave

FredR

Joe,

A stock motor like the S4 will run about 27 degrees of advance at max rpms and from memory it drops to about 20 degrees by 3000 rpm and lower down to idle. Using John's rule of thumb and if you are running 7 psi of boost then your timing at full rpm's should be in the region of 20 degrees and down to 13 degrees at 3000 rpm if I have the numbers correct.

If I understand correctly what you are saying you currently have, you must be very close to or spot on the max operating point. Is this some coincidence or does it reflect how Carl has recomended your system to be setup?

Have you had the car on the dyno and compared the performance relative to Carl's expectations for your kit? At the end of the day when it comes to supercharging the earlier I suspect Carl has way more experience than anyone else. Appreciate that chasing power is a never ending preocccuption but at the end of the day just how strong [or comparitively weak?] is your motor running?

Carl Fausett

I just got back in the office - was at a HDPE event with my son at Mid-Ohio.

Advancing your ignition timing on a boosted 928 is a BIG risk. As he maker of your kit, I cannot recommend it. We do leave your ignition advance at the stock setting and "get away with it" largely because of the lower compression ratio your L-Jet motor has. We also instruct you to run premium fuel (91 OCT or better) since the day it became supercharged.

But I certainly would not advance the timing. No. Don' do it. The first symptom we see as a sign of detonation is the blown head gaskets. However, frequently undiagnosed is the fact that the pistons in those cylinders will have broken ring lands.

Unless you have a fresh engine in your garage that you are itching to install, leave the ignition timing on your 16v L-Jet alone.

V2Rocket

FWIW this is the stock ignition timing for a 944 turbo (similar combustion chamber to the 928, 8:1 compression, running 0.75 bar/10.8 psi boost stock)

in the part-throttle (middle table) the top row % are throttle position - over 60% the DME defaults to WOT timing/fuel.