Here's a simple timing question. I have a '69 350/350 and the owner's manual says the initial timing should be 4 degrees BTDC. I've read threads from Lars, BarryK, and others saying all 350 SB engines like to run at around 36* total advance w/o vacuum. Then why is my 350/350 recommended to be set at 4* advance at idle, and the LT-1 is 14* at idle? Does that mean the 35/350 should ideally have 30* of mechanical advance, and the LT-1 should ideally have around 20*?

Or do most distributors have about 20* mechanical advance, and my idle timing should really be around 14-16* advance, not 4*?

So for starters should I just set my engine for 36 total, let the idle fall where it may, and then do a test drive? And if the 11:1 compression starts to ping on my 93 octane gas-- start backing it off 2 degrees at a time?

Or should I tweak my distributor weights to set my advance at 30*, so my idle can remain at the recommended 4*?

I'm just trying to get a feel for how important it is to keep my idle at 4* (will my engine idle or start rough at 14*?) and why different 350 SB engines have different recommended idle settings.

Thanks for anyone that can clarify this for me-
Andy

 

 

"I've read threads from Lars, BarryK, and others "

Listen to them!!!

You want 36 degrees (all in by by around 2500) with the vacuum disconnected. You SHOULD end up with a total (with Vacuum connected) around 50-52 degrees. What you end up with for an initial advance will vary.

 

With the vacuum line to the dist removed and plugged, set the timing such that you get 36* at 2,500-3,000 RPM.

You might need to change the dist springs to achieve this. If you can't, check to ensure your weights are moving freely by removing the springs entirely to see if you can get 36* at 2,500-3,000RPM.

If you can acheive the 36* at 2,500-3,000RPM, then reattach the vacuum line to the dist and let the idle timing fall where it may.

Lastly, with the vacuum line attached, rev the engine up to 2,500RPM and check to ensure your timing is within 50-52* max. You may have to get an adjustable vac advance to achieve this.

 

I believe I am fully listening to them-- I based my whole question on the idea of starting with 36* full advance.

My question is the implications of the recommended idle setting. Is it better to "let the idle fall where it may" or actually modify the amount of mechanical advance to keep the idle near the recommended setting? And since the 350/350 had a 4* and the LT-1 had a 14* BTDC setting-- does that imply that the stock distributors had different amounts of mechanical advance to get both engines up to 36* total?

I'm assuming there is a reason different 350 SB engines have different recommended idle settings (different cams, etc.). So it makes sense to me that you might want to keep the idle setting and full advance setting as the two constants in the equation, and treat the amount of mechanical advance as the variable by adjusting the weights. And then tweak the springs to somewhat shape your timing curve.

I apologize, but I thought these were valid technical questions for this forum.

 

Its been my impression that basically initial timing means squat. As long as it isn't too far advanced to start, it's fine.

 

I guess that's what I'm getting at-- what is "too advanced"? I was just curious if there is an ideal or recommended range for the idle, so you know if you need to increase the amount of mechanical advance to be able to retard the initial timing and bring the idle advance back down to an acceptable range.

It just seems like idle advance would affect emissions, ease in starting, etc. so there should be a rough value you want to shoot for.

Based on the good info marshrat99 supplied-- it sounds like you are trying to approximate a timing curve, with the initial advance and full advance as your two setpoints. The springs will basically shape the curve between the two setpoints. And the vacuum advance canister works in conjunction with the supplied manifold (or carb port) vacuum as a non-linear compensation for engine loading. So if GM had published an ideal timing curve for a specific stock engine setup (with compensation factors for loading) in my owner's manual, I'm thinking I could tweak everything to find a best fit approximation to the graph.

I don't know anything about cars, and I thank everyone for their patience and understanding with these newbie questions.

 

Andy

It's always been my understanding and as I was (and still am) learning about the basics of timimg, total timing is more important to worry about but you should still try to keep an eye on what the initial timing is doing. If the initial timing is way off compared to the stock setting you could have harder starting issues, or other problems. When I'm setting timing on either of my cars - the '65 Vette or the '78 Vette I always set them so that the initial is at the factory setting and total w/o vacuum advance is at 36* and than w/ vacuum advance is at 50*-52*.
Many, many people say to just set the total and let the initial fall where it may. This will work most of the time without issues BUT I prefer to do it as I described and make sure both are set. This now also lets me see and confirm that the mechanical advance amount is correct and the advance is correct.
Again, it's how I learned so it what I stick with.

To answer one of your questions......... the distributors were set up differently between different cars depending on the motor/trans in the car. Some had higher, some had lower initial settings, some had more mechanical advance, some had less, etc.

For example, a '67 327/300hp car w/ 4-sp tranny had a factory spec of initial timing of 6* with a total timing of 36* w/o vacuum advance so the distributor was giving 30* centrifigal advance. On the other hand, my '65 327/365hp motor has factory specs of 12* initial, 36* total, so the distributor is supplying 24* centrifigal advance.
Both are 327 motors, both cars have a 4-sp tranny, but the different in heads, cam design etc radiacally made a difference in the timing specs between the two cars.

Over the years our cars have had countless people working on them, some knew what they were doing and some I'm sure were clueless to what they were doing and distributor settings, curves, etc had gotten changed. some to try to hot rod the car and get more performance or a smoother running car, or whatever other reasons someone had in the past.
That's another reason I like to keep my initial where the specs call for - that way I know if EVERYTHING is correct than the car should run fine.
Believe me, the GM engineers at the time didn't pull these numbers out of a hat randomly.

Now, one thing that many people change is the curve. They will use lighter springs to bring in the advance sooner. For example, with stock springs some distributors won't let the advance come all the way in until 5000rpms or so.
Most people will change this to bring the timing in sooner so it's all in by around 2500 or so. This is great for performance BUT you really need to watch out and test drive the car before finalizing your settings this way because while bringing in the timing sooner helps performance it also can lead to detonation sooner. If you set the timing with a total of 36* and you hear detonation you MUST retard it back 2* at a time until it stops - detonation will destroy pistons fast.

Now, on YOUR motor, chances are if you set it for a total of 36* the initial; most likely will be above the spec of 4* initial because of past work and changes done on your car. You CAN just let the initial fall where it may and even if it's above 4* as long as the car starts and runs fine, no detonation, etc you can just leave it if you are happy with the way the car runs - just realize that your advance curve is not at factory spec that way. You can get the distributor recurved properly to get the correct settings for initial and total timings and the right curve but the best way to get this set up is to use a Sun distributor maching and they are not always easy to find at local shops anymore........ I had to send mine out to someone to get it done because no onre around me had one.

My '65 is set up with an initial timing of 12*, total timing of 36* and it's all in by 3500-4000 rpm.
Yes, I could have had it set to be all in by 2,50 rpm but the 3,500-4,000rpm setting is safter, especially for a 40yo motor that is the correct, numbers matching, original motor for my car. Based on the value associated with it being the original motor in my car I chose to take the more safe alternative rather than trying to get every last ounce of performance out of the car.

sorry, I realize a lot of what I just typed jumped around from one thing to another - I hope it made at least a little sense and helped answer your questions


EDIT: one thing I forgot to mention concerning your low factory spec initial timing - with a initial timing spec that low I'd guess that your vacuum advance was also originally hooked up to your carb to a ported vacuum source. For emisions purposesto reduce hydrocarbons they switched to a ported vacuum source so there was no vacuum advance at idle and they ran retarde spark timing with very low initial timing.
This was good for emissions but not performance.

You should switch your vacuum source to a full manifold vacuum source and you can let your initial timing come up to a somewhat higher amount of you wish.

 

Driftwood - I've been wondering the same thing as you, though I've come to the conclusion it is the same if you use metered vacuum advance rather than straight manifold advance.

If you start at the 'reference' of 8 degrees at idle and add a large advance due to vacuum at idle you can end up at the same place as if you start in the 14-16 degree advance at idle (after setting to 36 degree total w/o vacuum) with nothing contributed from the the closed butterflies with metered vacuum.

I've seen a lot of discussion as to the advantage/disadvantage of metered vs. direct manifold vacuum but the main difference seems to be emmissions at idle - lower w/metered vacuum. Once the butterflies open, metered vacuum rapidly approaches the same value as straight manifold vacuum so no difference at higher rpm.

I just retimed last weekend after replacing the intake manifold gaskets and seals and switched to the metered vacuum at the same time. I use my 76 as my daily driver (40 miles every day) and notice much better idle now than last week. I set to 32 degrees total w/o vacuum (some minor pinging at 36, but I use mid-grade fuel due to current cost) so my initial advance is in the 14-16 range with metered vacuum. It was actually quite a bit higher when I used manifold vacuum at idle.

 

BarryK-- Wow-- thank you very much!!!

That is exactly what I wanted to know. It just didn't make sense to me that the idle setting doesn't matter at all. And you confirmed what I was guessing-- that different cars and various levels of initial and mechanical advance to best approximate that engine's particular timing curve.

I just like to fully understand the why's and hows of something before I tinker with it, rather than just following a procedure. My experience is that if you fully understand a particular system, you then don't need a procedure on how to test it. In this case-- I thought if I understood what ideal timing curve I'm shooting for, then I can figure out how to set my timing to best match the curve.

I definitely agree with what you are saying-- if an engine has been modified over the years-- it is probably hard to know exactly what the timing curve should be. It's too bad there isn't some software or equations that let you enter various engine specs, cam specs, etc. to plot out a rough timing curve.

Sounds like that Sun machine somewhat simplifies the tuning of the distributor. How did they know how to tune it? Did you tell them how much mechanical advance you wanted and gave them some desired advance at various RPMs, and they tweaked the weights and springs accordingly? I guess I'll start with tweaking my weights in the driveway to see if I can keep my initial and full advance at 4* and 36*, and then tweak the springs to pull in the full advance at the right RPM.

Thanks again-- that really made my day!!

 

Andy
I just added an edit to my post you may wish to go back and read.

When I sent my distributor out to the person with a Sun machine, I told him what the factory specs were for initial and total timing and let him take it from there/ I trusted him as he is a VERY well respected person on the NCRS discussion board, has been involved with vettres and doing his own work on them for over 30 years, knows the old motors VERY well, and knows how they should be set up to not only run properly for good performance but also to have enough "safety margin" built in so the performance doesn't bring the motor to the edge of blowing itself up!

 

Great information all. I would only add that I think that most of the factory settings (certainly those of post 72 models) are designed to minimize emissions. As I recall, Lars' paper has this qualifier. His tuning method is for performance, not to pass smog. They are actually supposed to check timing here when the smog your car.

 

Just

The reason for the ported vacuum ports rather than full manifold vacuum came about solely for emission purposes, period!
almost always if you switch back to a full manifold vacuum source you will get a better running and better performing car.

The reason for ported vacuum was so that at idle the spark advance was retarded to lower hydrocarbon emissions.
The downside was motors tended to run more poorly than those running off full vacuum, they run hotter, and fuel economy decreased.
by moving your vacuum advance hose off full maifold vacuum to ported vacuum you are not running your motor in the most effiencent manner. Anyone with a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy.

If you are expeiencing better idle characteristics now using ported vacuum rather than full manifold vacuum I'd guess there was an unresolved issue that was causing your car to not perform it's best previouslythat is actually unrelated to the vacuum source. Almost everyone finds the opposite of your case true - that full manifold vacuum has the car running better.

 

very true!!

emissions requirements did a bad number on the performance of these cars in that time period and there is lots of room for improvement

 

I'm not sure how the '69s were originally setup. I recently moved to the manifold vacuum based on forum recommendations. I guess I don't quite understand why that let's me increase my initial vacuum.

Let's say my car had an initial factory setting of 4* with a ported vacuum. Now I've moved to the manifold vacuum. This has increased the vacuum at the advance canister from almost nothing (ported) to around 15 in-Hg (my idle vacuum). So now the distributor is seeing more vacuum and has advanced the timing at idle. So shouldn't I then need to retard my initial timing to offset the advance in timing from the increased vacuum?

Just trying to get it all straight in my head... thanks.

 

No, under wide open throttle your vac drops to zero and your eng only sees the int + mech adv. The extra adv at idle helps smooth out your idle, eng might run a bit cooler at idle and gives a little better throttle response under light load conditions

 

Hi Barry - agree on the other issues causing poor idle initially! Just got the car a few months ago, and this was first real opportunity to tune it (pulling distributor was excuse to wife to get timing light.

In the end though, I can come close to the book numbers AND Lar's numbers and not get any hassle from roadside sniffers that we have here.

Just like a tune up, it's all about hitting that sweet spot.

 

I have done a hundred variations on my timing curve.

MY engine runs best with 8 degrees initial and 34 total. two more on either end and I get detonation (running 94 octane). I also have the curve set to come in a bit slower than recommended by Lars because I get a bit of detonation around 2-3000, if I don't. My vac advance pulls in another 15 degrees and it's running off of manifold vacuum. I tried both ported and manifold. Manifold was better for ME. It allowed a smaller throttle opening at idle and cured a run-on condition.

I don't expect those numbers to work for everyone (or even ANYone) because my engine is kind of a bastardized mess (partially my fault, partially not). I have new one in the shop...waiting.

 

Fevre- thanks for that info-- that makes sense. So that sounds like maybe if my idle timing ends up increasing from 4* to 8*, the engine might run just as well or better anyway.

CGGorman- if detonation is a limiting factor that would be a good and quick approach. Just set your initial and full advance timing a little before detonation, and then tweak the distributor springs and vacuum advance canister as necessary.

 

Andy

yes, since you are going to be using the full manifold vacuum source and we aren't concerned about emisions you can easily have your initial timing fall at or around 8* or even a bit higher if that's where the mechanical limits are having it fall with no problems