hacker-pschorr

Doing some research:
#1. https://rennlist.com/forums/928-forum/228296-checking-the-timing-on-a-80-16v.html
#2. https://rennlist.com/forums/928-forum/268037-what-is-the-ignition-timing-at-idle-on-an-83-a.html
#3. https://rennlist.com/forums/928-forum/215486-16v-timing-what-should-it-be-set-on.html

Per thread #2 above - 83-84 cars should be set to 31 degrees before TDC.

79-82 cars should be set to 23 degrees before TDC.

These cars have identical ignition systems. 83-84 cars have slightly higher compression & 300cc more displacement.

So why the difference? Did Porsche change their mind? Do the 83-84 cars "require" high octain gas where as the early cars run fine on 87 octain?

All US 16V 928's seam to wake up quite well being set to over 31 degrees before TDC.

Whats a guy to do?

FlyingDog

83-84 cars have possibly slightly more compression, the wimpiest cams of any 928, and 200cc more displacement.

Tune based on sound and feel rather than spec.

fbarnhill

Hacker,

I have just been going through this with my 79 Euro. It runs much better at 31deg BTDC than 23.

mark kibort

23 is the vacuum disconnect, 3000rpm setting. Its 20 degrees for the 84, and i measured total advance, vacuum connected at near 35 degrees, going to 42 when i advanced the idle timing from about 7 degrees btdc to about 10.

playng around with timing, advancing more than described, lost me 3hp but gained 3ftlbs of peak torque. so, i put it back to almost stock at 10 degrees btdc at idle.

However, i never put in 110octane and advanced the timing 5 degrees or something . so, ill never know.

MK

hacker-pschorr

So what is Tammons talking about in link #2 where the WSM puts the 84 at 31 degrees with the line detached?

They have slightly less duration, with more lift than the 08-82 cams.



Kinda dangerous to do with a supercharged car.

hacker-pschorr

Just for fun - 944 cam specs.

toofast928

Good topic.
I advacned my 4.5L non I/C 2 degrees off base. Total of 25 degrees. No ping using 89 octane. I plan to dyno next month and see how far I can advance the timming.
BTW my cams are advanced 3 degrees. This raises the C/R a little.

hacker-pschorr

This reminds me, I have a farily new timing belt. Need to pull the covers and check the tension / cam timing again.

My friend has an 84 auto with headers - it was a real pooch (like most US 16V's) even after the headers it didn't feel much faster or next to my (then stock) 81. Bumping up the timing made a huge difference accross the entire power band on his car. Now if I could only get him to dyno the damn thing.

GlenL

As a performance mod, tweaking the timing is a great value. Doing it on the dyno is the best. Find max power and go back a degree or three for safety.

One thing to remember is that more advance makes the engine run hotter. That's what makes it knock more, for the most part. Also that the pressure is higher during the last bit of the compression stroke so there's more "ping."

I'm running at ~32 degrees.

Porsche was nice to put marks on the flywheel at the spec values. That does make it hard to figure out exactly where it's at when past the end. A good trick would be to add a mark at 30 degrees BTDC. Not hard to figure otut once the flywheel diameter is known.

For those curious about "knock" versus "ping" they're terms for the same effect of having spontaneous ignition of the mixture but knock is before the spark ignition and ping is after it.

Some engines that are running really hot or have carbon build ups suffer from straight "pre-ignition" which is a flame front being touched off by a hot spot in the engine. That's where it starts to run rough but no cracking sound is heard.

When I was in school (think Reagan) studies of engine combustion found that ping was very common in most engines. Just the last bits popping off by themselves and it was not an issue.

hacker-pschorr

I understand all of that (somewhat, enough to get me into trouble). I'm mostly curious why the factory went from 23 to 31? If every 16V 928 from 78-82 is "supposed" to be at 23 from the factory, this coveres a wide rance of cam profiles. My guess would be the octain question - if 87 octain is what they wanted to work in a safe level, timing would be the easiest way to accomplish this.

On my car is a 2|3 where I have my timing set to on the supercharged car & my 79. IIRC my friends 84 has a 1|0 & 2|0 marks. How is he supposed to hit 31 if there isn't even a mark for it?

theedge

Yes but its also dangerous to to use a N/A tune/specs on a boosted car...

hacker-pschorr

Anyway, that is what started all of this. I decided to retard my timing down a bit, start the tuning phase of my blower install. Anything less than stock timing, car bogs at WOT while trying to reach redline. When I say turned down a bit, I'm talking 1/8" - 1/4" of distributor movement. It was weird, so I started to research timing on 928's and 951's. If I run the car at anything but WOT (lets say 95% throttle) it’s ok and pulls like an SOB, but not quite as hard as WOT (yes throttle movement has been checked).

Now according to a very knowledgeable 16V boosted Rennlister, if the retard line is removed from the vacuum port on the distributor, boost from the motor will actually retard the timing under boost via the vacuum advance line. Since that little arm was never supposed to see boost from the factory – having the lower line open will push the arm backwards under boost retarding the timing. So it could be my car is pushing that little arm enough to retard it to the point of bogging. Not sure – dyno day is this Wednesday. I plan on using a timing light to see what is going on down there under boost. If in fact my timing starts to retard under boost – then cool, that is what you want and I need to play around with the base setting a bit.

So your comment about stock settings is 100% correct. If 31 – 35 degrees is ideal for a N/A 928 16V (what I’m trying to find out here). Then using the “stock” setting for an 81 at 23 degrees might be alright. Then throw in the monkey wrench of timing being retarded under boost as described above, I may be alright.

I’ll be at the track today, maybe I should dump in 100 octane unleaded racing gas and see what happens.

GlenL

A quick look at the 80-84 spec book shows advances of from 23 to 28 degrees at 3K RPM with hoses detached. That's all models for those years. I'll guess that the US models were de-tuned a bit to meet emissions specs.

To set the timing to exactly 31, measure the flywheel diameter and use this formula:

Length = diameter * 3.14 * 31 / 360

Cut a piece of tape to that length and put it on the flywheel to the right of the 0 mark. Scribe a line at the right end as the tape will likely fly off.

(For uber-dorks, use 3.1415926536. That last billionth part counts, too.)

mark kibort

if you dont want to use a calculator, you can just cut a piece of tape to the marks already on the flywheel. take the 0-10degrees distance and just add it to the final mark to find 33 or 30 degrees. want to go farther, just add the distances to the final marks based on the known ones on the flywheel.

mk

PorKen

Or get a timing light with adjustable advance.

I remember reading in a thread that plus (3) degrees is good for top end, and plus (6) for mid range.