Total Advance Question

[Paulporsche]

I recently installed a Pertronix Ignitor in my car. The engine is a 98mm bore 3.2 made from an 80 SC engine, with CIS and MSD. I use a 74-77 distributor (which runs clockwise)with the vacuum retard blocked. This distributor has no vacuum advance. There is no O2 sensor, ECU, chip or lambda control. CR is 10.3:1 and I use 93 or 94 octane fuel since, although the engine is set up with twin plugs, I am only running it with single.

I have 2 questions:

1. For static timing @ idle, I have 2 adjacent notches on the pulley and neither is marked, as far as I can see. For a 1980 engine, does this mean that the one to the right is 5 BTDC and the one to the left is 0 TDC, or is the one to the right 0TDC and the one to the left 5 ATDC? I'm thinking the former is correct.

2. How much advance should I be running @ speed? My distributor has been recurved and I have been told that it reaches total mechanical advance @ about 2000 RPM. I'm thinking I should be able to go to 28 BTDC, as with the 78-79 engines, rather than the 24 BTDC for 80-83, per Bentley, since I have no emissions controls or restrictions.

Thanks.

[Walter_Middie]

Paul,

This is what I have for a 1980 3.0:

[Paulporsche]

Thanks Rex.
I've added some more info to the original post which might be important. Do you think my higher than stock CR and 93-94 octane fuel will allow for more advance?

[Walter_Middie]

Advance usually increases with piston size/area - larger piston area needs slightly longer times for the flame front to completely burn - so more piston area, more total advance at high RPM. I don't know if your increase from 3.0 to 3.2 involved bore size or stroke or both, but I bet they are close, so the amount of advance is either the same or a very small amount more.

Certainly higher octane will also allow more advance.

[911pcars]

Be careful, higher compression ratios squeeze the A/F tighter, if the A/F ratio (AFR) is too lean, if the octane level isn't sufficient, and ignition advanced too far, it may/will detonate (most likely under load conditions) and begin destroying cylinder material.

Detonation damage (extreme):

Other graphic images (https://www.google.com/search?q=engine+detonation+damage&tbm=isch&ved=2ahUKEwi7z9OhiuzqAhUNzKwKHfdBBgcQ2-cCegQIABAA&oq=detonation+damage&gs_lcp=CgNpbWcQARgCMgIIADICCAAyBggAEAUQHjIGCAAQBRA eMgYIABAIEB4yBggAEAgQHjoECCMQJzoECAAQHjoECAAQGDoFC AAQsQM6BwgAELEDEEM6BAgAEEM6BwgjEOoCECc6BggAEAoQGFC GtB5Yy-8fYOOPIGgQcAB4AIAB9gGIAe0xkgEGNjcuNS4xmAEAoAEBqgEL Z3dzLXdpei1pbWewAQrAAQE&sclient=img&ei=6xIeX_utKo2YswX3g5k4&bih=1335&biw=1671#imgrc=S8AdHDPQuDjcNM)

A friend who autocrosses thought his engine was well within specs, but during a recent run, a lot of blowby was produced. After a compression test, upon disassembly, careful inspection showed enough detonation damage (ring erosion) in one cylinder to loose compression and thus the blowby.

Either mimic what others with similar engine specs have done to mitigate or enlist the aid of a tuner that can strap on the appropriate sensors and test and adjust under load. In the meantime, use 93 octane or better fuel, dial in more conservative ignition settings and be aware of what can happen if detonation (sometimes silent) is allowed to occur. A WB sensor and/or exhaust gas temp. (EGT) gauge can keep track of the AFR and EGT, respectively.

Rich mixtures burn at a faster rate than lean mixtures. As explained to me by my automotive instructor, slower burning lean AFR exposes more of the cylinder to combustion heat and thus increases the cylinder temperature.



Sherwood

[fred cook]

If it was my car, I would go ahead and activate the dual ignition. Either install an aftermarket ignition system like the Electromotive XDi (or equivalent) or change over to a 12 plug distributor. With the twin plug ignition, you will get more power and a better fuel burn at 24-28 degrees advance than you would trying to run 35 degrees with the single plug setup. Also, with the twin plug system, the fuel will burn cooler and you will be able to drop down to 91 octane fuel. Several years ago I built a 3.3SS engine for my 1980 SC using 964 cams, 11:1 compression ratio, twin plug ignition and large port Carrera heads. I added an A/F gauge to help with the tuning. I have it set to run at about 14.5:1 at cruise/normal driving but it drops to about 13:1 at wide open throttle. With the twin plug setup, the flame front only has to travel half the distance, thus working well with lower timing advance. Runs great on 91 octane fuel and typically runs cooler now than before the performance rebuild. Sherwood's example above is correct for a single plug system.

[Paulporsche]

I hear you Fred.

After having the car like this for almost 30 years I probably won't do it now, though. The car is plenty quick and 93 and 94 are readily available to me.

I guess what I really want to know is, since I have changed to the pointless ignition, where do I set the timing? I made a note of where it was when I previously set the dwell to 35 with points. The engine was at 9 BTDC and had a smooth idle, with the vac retard line plugged.

Since the timing would retard as the point block wore down I went for 6 BTDC, which would be as if the dwell were @ 38. the idle isn't so good now so I'm thinking to go back to 9 BTDC. BTW all these figures are with the vac line plugged, which seems to mean about 10 degrees less when connected. This would seem to be in line with the 1 ATDC that an old mechanic told me he set my timing to about 15 years ago. I can't get hold of him now so I'm unable to verify, but I can't see how it could have been 1 ATDC with the line plugged. That would probably have stalled the engine when the line was connected! I wanted to check with people to see how much advance I could then get away with when @ high revs.

[Walter_Middie]

Paul,

Your first post says the vacuum port on the 74-77 dist is plugged - so there should be no difference between plugged or unplugged.

If it ran good before Pertronix with 9 deg BTDC at idle - I'd set it back to that. Pertonix won't allow any more or less advance.

You don't say what you had your total timing set to. Seems like you should be able to shoot for 28 to 30 deg total.

[Paulporsche]

Rex,

Maybe I described it incorrectly. I have a plug in the takeoff port off the side of the can on the side of the diz. I also have the line that goes to it plugged. I have been doing this since reading about it on this forum years ago. My old mechanic may have done the setting with the lines connected. I haven't tested it yet but it seems that if I connect that line to the port then the retard would be actuated and I would get the 10 degree retardation amount, which would put me @ 1 ATDC.

I'm going to go back to 9 BTDC with the line and takeoff plugged and then see what I get @ high rev. I never knew what the total advance was when I had the points, which is why I posed this question in the first place!

When I get a chance I'll try for a total advance figure and see what response I get from people.

Just so I make sure I'm getting this--if I start @ 9 BTDC and I want to have 30 total advance, does that mean I wind up@ 30+9=39? Again, if the line were connected, this would mean 29.

Thanks for staying on this with me.

[911pcars]

The static, idle ignition timing adds to the overall ignition timing curve. If the basic timing at idle is to spec and the distributor mechanical advance is to spec, both should add up to factory spec at 6000 rpm. Yeah, that’s pretty high and intimidating holding a timing light inches away from the crank pulley.

But unless the timing has been modified to your rebuild specs (doesn’t sound that way), there’s too much spark advance which can lead to destructive detonation. Your engine should now need either less advance and or higher octane fuel. In lieu of adding octane additives or $10/gal. race fuel, reduce the spark advance by adjusting the distribution to retard the timing.

Yes. That will also reduce the idle speed. To compensate, increase the idle speed. You will notice some low speed power loss, but that’s necessary to reduce onset of detonation. Do some research and get some idea of the overal timing with similar engine specs. The correct mod is to limit the total advance by modifying the travel of the distributor advance weights while restoring the basic idle timing specs. For this you need someone who has a distributor machine and the knowhow to perform this. To be most accurate, test on a chassis dyno.

Hope this helps.

Sherwood

[Paulporsche]

Sherwood,

The distributor has been recurved by the original builder (who I can't contact). When I get time, I'll see what amount of advance I get @ speed and verify that my amount of static retard is in fact 10 degrees.
I understand and agree with what you are saying, of course. I'm not intending on using avgas or racing fuels or octane boosters, just sticking to the 93 or 94 I've been using for over 20 years.

Before I installed the ignitor I checked my static timing with points @ 35 dwell, which is the large end limit of the spec and where I usually set them to start. This worked very well, but, knowing that the timing retarded over time as the block wore down, I'm not sure if there was ever any engine damage, since the timing never stayed @ 9 indefinitely. My only concern now is that since timing will never change it it too advanced?

LOL! Your suggestion to check with other similar engines included this post! I'm hoping others here have some similar experience.

Looks like I have to go back and check timing @ both points, with and without the vac line attached.

[Walter_Middie]

Just to add to what Sherwood said:

if I start @ 9 BTDC and I want to have 30 total advance, does that mean I wind up@ 30+9=39?

No - the total at high RPM is the combination of initial timing (9 deg) and centrifugal advance with RPM (maybe about 21 additional deg due to centrifugal advance) for a total of 9+21=30 deg.

As Sherwood points out, your distributor could be modified to add more or less centrifugal timing due to the advance weights and how far they are allowed to rotate.

[Paulporsche]

I think I have it worked out!

I finally tracked down the mechanic who set my timing 15 years ago. As I suspected, he set the static timing @ 1 ATDC WITH THE VAC RETARD LINE ATTACHED. With a 10 degree retardation, this yields a 9 degree BTDC WITH THE LINE DISCONNECTED AND PLUGGED. This corresponds perfectly with my findings with the lines plugged and the old points in place @ 35 dwell.

He also stated that this would give me 30 total advance @ high end, which corresponds to what others have said.

When I get a chance I'm going to verify everything, both with and without the vac retard connected. For the meantime, I'm confident 9 BTDC w/o retard will be what I want. I can check the static timing with the vac line connected just to see if the retardation is in fact 10 degrees.

Thanks very much to Rex, Sherwood and Fred for your help.

[911pcars]

“ Just so I make sure I'm getting this--if I start @ 9 BTDC and I want to have 30 total advance, does that mean I wind up@ 30+9=39? Again, if the line were connected, this would mean 29.”

If you want 30 deg. total advance and your distributor advance is 30 deg., your initial base timing must be at 0 deg. (TDC). TDC is the reference point between + and - ignition timing.

Thus, 30 deg overall (Total) = static timing (BTDC) + mechanical advance (5+25, 4+26, 3+27, 15+15, etc.). However, if static timing is set ATDC, add that amount to the overall measured timing. E.g. 5 deg. ATDC + 30 dist. advance = 35 deg. advance.

As Rex mentioned and you confirmed, the vacuum retard mechanism is plugged, thus non-op, thus it plays no effect on the ignition timing. But if it was, it only works at small throttle openings (i.e. idle). As the throttle opens, vacuum decreases and no timing adjustment (diminishes to 0 deg.). Retarded base ign. timing is a method of decreasing exhaust emissions at idle.

Using electronic “points” has no effect on timing, nor will it change as with breaker points.

[Paulporsche]

Thank you 911 pcars.

[911pcars]

Not apples to apples, but early S and RS engine total ign. timing was around 32 deg. BTDC, and they only had a compression ratio of around 8:1. Your CR is at 10-10.5:1 with some compression loss due to a more aggressive cam (?). Be careful in hot weather, high altitudes and questionable gas and engine lugging.

Sherwood

[Paulporsche]

Thanks Sherwood. I hear you.

I've been using 30 total for over 20 years and probably 60,000+ miles with this engine, so I feel pretty confident as long as I make sure I achieve that and stick with the 93/94 octane.

[robertmark]

Originally Posted by Walter_Middie

Paul,

This is what I have for a 1980 3.0:

Hey Walter,
Great illustration you uploaded. Any chance you have this for a 2.0 for a 69E?

[Walter_Middie]

Any chance you have this for a 2.0 for a 69E?

I made a bunch of these for various years, but they only go back to 1970.



I'll have to look and see if there is difference between 1969 and 1970.

[Walter_Middie]

Here's for a 1969: