[AJ-]

I have an 8.5:1 (calculated) 400 with 6x heads and a 213/218 cam with .450/.467 lift (with 1.65 rockers) and a 114 lobe separation. I am trying to get the timing dialed in. I originally had it set at 31 degrees total timing just to be conservative as a baseline and had planned to bumped it up a bit at the track to see what it liked. Long story short it was a busy night and my first few passes revealed a few other issues to sort out so it wasn’t until my last pass that I got a cleanish run in. I never got a chance to play with the timing.

Anyways, my main question is about vacuum vs mechanical advance. Since then I bumped my timing up to 34 degrees total timing and I think I noticed a very slight amount of detonation while going up a steep hill at fairly low rpm. Hardly noticeable, but pretty sure it was there. My mechanical timing comes in really quick--it’s at 31 degrees already at 2000rpm. The vacuum advance at 2000rpm adds in another 13 degrees. At idle it adds in about 16 degrees.

First question: What should I do, slow down the mechanical advance curve, limit the vacuum advance, or both? I was thinking of limiting the vacuum advance either way as it sounds like around 10 degrees is ideal but I don't know if the curve is also an issue.

Second question: How much was I leaving on the table at the track with my total timing at 31 degrees? I ran a 14.34 @95mph. I was also having some cooling issues with a hot day and a long line to get into the track and long staging lines. I was running around 210-220 degrees on that pass. If I could be closer to 190 degrees would that make a noticeable difference? Rest of the combo includes quadrajet, th350 with stock converter shifting around 4600rpm in drive, 3.42 posi rear, stock log manifolds w/2.25 dual exhaust. Minor tire spin at launch on this pass. I’m pretty happy with the performance for what the motor is, but dipping into the 13’s would be awesome.

[25stevem]

Here’s a ruff guide for how the factory use to set up the timing curve with a 360 hp 4 bbl motor during the high comp good gas era.

For you I would limit the vacuum advance to 12 at first and bring in all of your centrifugal around 2500.

Whenever you make a change you must confirm that with the VA unhooked that with the motor up at 4500 you are not seeing the timing still advancing beyon the total of your in and centrifugal .

This needs to be done every time you do anything to the weights, springs or weight cam.

[amcmike]

What's your initial timing without vacuum and @ what rpm, so we better understand the curve?

[AJ-]

15 degrees initial (no vacuum) at around 800 rpm.

[amcmike]

Slow the centrifugal curve to be all in at 2700-3000rpm (you have it way too quick). Then drive it with the vacuum connected to see if you get part throttle knock. If it does, keep removing vacuum timing until it goes away.

Between the timing adjustments, cooler temps, and manual shifting @ 5200rpm you may crack the 13s. Full length headers would make a solid high 13s car.

[AJ-]

Thanks, Mike. Sounds like a plan. I will start there.

[389]

Before you do anything, try disconnecting the vacuum advance and raising the initial. I've had a couple vacuum advances also advance during wide open throttle especially if your running them on ported. Frankly those things are more trouble than there worth. Try reading tune up books by people like Smokey Yunick, Zora Duntov, Bill Jenkins, Dyno Don Nicholson, and my favorite Roger Huntington. Roger was into Pontiacs but Pontiac engines are close to the same thing as the 400 cube Chevrolet small block but the Chevy has better heads.

Try different advance weights. Get rid of the vacuum advance and turn up the initial. Thats the beauty of running a sensible compression ratio. I have a 427 that is 9:1 compression and I run 22 initial, no vacuum advance, it runs best on 87 and has tremendous torque from idle to 6500.. In the single points distributor remove the vac advance and secure the points plate to the distributors frame for better mechanical points performance. I also remove the adjustment spring and put the exact amount of washers to get it to 28-29 dwell. Then use the points to trigger a Capacitor Discharge amp. You'll have one of the best ignitions going. There isn't a inductive discharge ignition (Points, HEI, Small body, there all the same thing) that can come close to the lighting speed of a Capacitive discharge ignition. With ignitions its about how quickly the ignition can fire the next plug and not how much power it gets.

I wonder why nobody used magnetos anymore. I had a Ronco Vertex from Bule Bell, Pa. on a 64 GP with a 421 and man did that wake that engine up. It was given to me by an old man who lived down the alley from me. He used to race Pontiacs.

[ekancler]

Originally Posted by 389

I have a 427 that is 9:1 compression and I run 22 initial, no vacuum advance, it runs best on 87 and has tremendous torque from idle to 6500.

I have to ask what kind of cam you've got in that one...

[Schurkey]

Originally Posted by 389

Before you do anything, try disconnecting the vacuum advance and raising the initial. I've had a couple vacuum advances also advance during wide open throttle especially if your running them on ported. Frankly those things are more trouble than there worth. Try reading tune up books by people like Smokey Yunick, Zora Duntov, Bill Jenkins, Dyno Don Nicholson, and my favorite Roger Huntington. Roger was into Pontiacs but Pontiac engines are close to the same thing as the 400 cube Chevrolet small block but the Chevy has better heads.

Try different advance weights. Get rid of the vacuum advance and turn up the initial. Thats the beauty of running a sensible compression ratio. I have a 427 that is 9:1 compression and I run 22 initial, no vacuum advance, it runs best on 87 and has tremendous torque from idle to 6500.. In the single points distributor remove the vac advance and secure the points plate to the distributors frame for better mechanical points performance. I also remove the adjustment spring and put the exact amount of washers to get it to 28-29 dwell. Then use the points to trigger a Capacitor Discharge amp. You'll have one of the best ignitions going. There isn't a inductive discharge ignition (Points, HEI, Small body, there all the same thing) that can come close to the lighting speed of a Capacitive discharge ignition. With ignitions its about how quickly the ignition can fire the next plug and not how much power it gets.

I wonder why nobody used magnetos anymore. I had a Ronco Vertex from Bule Bell, Pa. on a 64 GP with a 421 and man did that wake that engine up. It was given to me by an old man who lived down the alley from me. He used to race Pontiacs.

Wow. I don't have words for all this.

[25stevem]

In regards to what’s posted in #7 I will debate you endlessly in your saying that the SBC of the 60s thru 70s has better heads then our Pontiacs, especially since you gave no frame of reference as to in which ways your line of thinking has then being better.

There’s no such thing as a faster version of electricity, it always travels at the same speed when the same amount of conductor resistance is applied.
Your comment should reference how fast the spark pulse is generated, as that can veri a lot as the rpms climb.

If your talking about then end result of the spark jumping a gap then there’s verifiables to that which can be discussed.

Vacuum advance has much more of a place/ need in a 9.5 or less compression motor then one, especially you may think it seem’s

Your endless harping on people when the discussion of vacuum advance comes into a post to ditch there vacuum advance is unfounded especially when you seem to be basing your opinion on a 427 BBC motor which has far different heads and a very short rod CTC then all of the factory Pontiac motors have/ had.

PS.
Maybe when you installed that Magneto into your 421 you finally got the timing set right for what the motor wanted, no so the basic function of the Magneto was the break thru to your issue!

Note that the generation of Magnetism by such a Distributor puts even more of a load on the oil pump drive gear then do points at rpms above 4500.
It’s like driving another small Alternator.

[Cliff R]

Last time I looked it was 2022 and sadly folks still don't use vacuum advance on these engines. It needs to be there, your engine will reward you nicely if it's set up to add the correct amount of timing, and no negatives from using it, only positives.

Right to start with (and we are limited here folks by the laws of physics) it is IMPOSSIBLE to effectively burn a lean mixture at light engine load without adding timing. That amount of timing is going to be well beyond the ideal initial plus total mechanical that your distributor is providing. Most of these engine will like timing at cruise in the 40-48 degree range. Really low compression engines may go even a tad higher, higher compression engines with tight quench, efficient combustion chambers and well chose camshafts a bit less. Either way ALL of these engines benefit from using a load sensing device (vacuum advance) to add some timing at light engine load. A vacuum advance applying 10 degrees, for example, to your total initial plus mechanical adding up to 32 degrees provides 42 degrees timing at light engine load. Increase the travel in the VA to apply 15 degrees and you then have 47 degrees at cruise, etc. etc.

This will allow a leaner mixture to be effectively used for "normal" driving. Without the added timing a richer mixture is REQUIRED to provide an adequate A/F ratio for complete combustion. This means that your engine will use more fuel to accomplish the same amount of work. You will also be nicely rewarded with more carbon build-up in the combustion space, tops of the pistons, exhaust system and less money in your wallet at the end of the day.

The mentally to even think that running over 20 degrees initial/base timing on these engines isn't cutting it. IF you have optimum compression and well chosen cam you'd be lucky to get much past 13-14 degrees initial timing before it would "buck" the starter hard on a restart with the engine well heat-soaked and probably bust the nose off the starter or a tooth off the flywheel if you continued to run it that way.

Of course if your not making much cranking pressure with your "low" compression ratio and "hefty" cam choice you can probably run tons of initial timing and be fine.

I tune for a living and have leave couple of weekends open a month to have poor running vehicles brought here to correct issues with them. Most of those customers have worked on these engines to the brink of extinction along with all their friends, beer drinking buddies, local "guru's", old school shops still doing this sort of thing, and even the wife's boyfriend. They are at their wits end because despite the funds and countless hours dumped into them they don't run for chit anyplace.

The FIRST thing I do when one shows up is to remove the distributor cap, the cheap Mr Gasket or Morroso spring/weight kit, and get half or most of the timing curve out of it at idle speed. Once I get the weights to pull back below 1000 rpms I proceed to the carburetor, un-hack it, completely correctly rebuild it, and custom tune it exactly for the application.

I'll then send the owner on a test drive. I stand outside the shop and listen to them doing "John Force" burn-outs and heavy full throttle runs thru the gears as they disappear into the countryside. They will return in about 10-15 minutes grinning from ear to ear and telling me the vehicle has never idled so well and ran so good since they've owned it. They even keep smiling and ranting about how good it works while I unload their wallet.

It's almost ALWAYS the same thing when one of these troubled vehicles come in here for me to work on, the distributor is the carb problem!........FWIW.........

[mgarblik]

One of many reasons I am glad Cliff is back on here giving good advice is I can back up his recommendations in a modern way. When I hook up a scan tool and drive a modern car with full electronic engine control. I watch the timing curve in real time with the factory programming. Using a typical V8 with throttle body fuel injection which is about as close to a carburetor without being one, the timing curve closely resembles what Cliff is suggesting under various loads, RPM's and conditions.

[AJ-]

Thanks for chiming in, everyone. I had no intentions of getting rid of vacuum advance. I even had someone nicely pm me just making sure I had no intention of going down the road of post #7 for my application.

Thanks, Cliff, for posting such a high detailed unpacking of timing. I've definitely read a number of your posts on the subject and am trying to get it set up using your experienced advice. My main question was whether my vacuum advance and quick timing curve were coming together too much in that small window and whether only one or both should be addressed for my application.

Interestingly the springs that were originally in this distributor when I got it were super light and lots of advance was coming in at idle. I swapped them with another distributor I had which gave the curve noted in my post. Now comparing them to a third definitely untouched stock distributor, the stock springs are thicker still. I popped one spring on and it's still too quick so later today I will swap the second stock spring on and see where it is at. I also took the vacuum advance off that stock distributor and have limited it to half the travel so I will pop that in too and see if it's around 10 degrees. Hopefully that gets everything dialed in.

[Cliff R]

In any and all cases the mechanical advance curve needs to start just above the engine idle speed with NONE of the curve in at idle. This makes idle tuning difficult if not near impossible.

Recently had a local customer come in here with a "carb" issue. He stopped by and discussed the problems, I took a quick look and this is what I found. When fully warmed up and heat soaked the timing was set to 10 degrees, idle speed 750rpm's. Vacuum advance hooked to a ported source so not in at idle.

When you increased the engine RPM's then took your foot of the throttle and returned to idle it was now idling at 1100-1200rpm's. If you backed off the speed screw to 750rpm's it was fine. Shut the engine down then did a restart it was now barely staying running around 500-550rpm's. Increase the idle speed screw to 750rpm's all was fine until you revved it up again, then the idle speed stayed around 1100-1200 again. The customer pointed out to me that the carb was causing some sort of idle issue and he kept having to re-set the speed screw.

I already knew that the distributor springs were too weak and it was NOT a carb problem but wanted to show and explain to the customer what was going on.

I removed the distributor cap (cheap off-shore HEI from Summit) and rotor and took a look at the springs. I removed one of them and went into my collection and replaced it with one that was considerably thicker wire and stronger.

Put things back together, fired it up, checked the timing it was rock solid at 10 degrees at 750rpm's. Revved up the engine watching it with a timing light, the timing started to advance about 800-900rpms. When the carb returned to idle the timing came right back to 10 degrees and the idle speed right back to 750rpm's again. Carb problem solved.......

[Cliff R]

As far as timing curves go you must follow the rules above, NONE of the curve can be in at idle speed. The curve needs to start advancing right off idle and increase smooth and steady with the RPM's.

Since the vacuum advance is a load sensing device it operates independently of the mechanical curve. It will ALWAYS be adding timing based on vacuum. When engine load is light, vacuum is high so the VA is adding timing. Most VA cans don't start real early as far as the vacuum apply is concerned so no worry ever about them adding timing at heavy/full throttle and blowing up your engine. Using ported vacuum is exactly the same as manifold vacuum at light engine load and it goes falls off with heavy load despite what a few "experts" have posted on the subject on this and other Forum's in these discussions. The ONLY difference as mentioned before is that using manifold vacuum to the advance adds timing idle and coasting. IF you engine loves that deal then set it up that way. If it skips, miss-fires, bucks or kicks in protest reduce the travel of the VA and/or try not adding it at idle speed (ported source).

They make "adjustable" vacuum advance units by going thru the hole where the hose hooks up and turning an allen screw. Most of those you are only adjusting spring tension or when it comes in and all-in. Those VA units were meant more for tuners trying to get a LOT of timing in at idle speed in heavily cammed engine making really low vacuum to help with idle quality. They really aren't needed for tuning otherwise as all of these engines are going to make a LOT of timing in the "normal" driving range when engine load is light. Tuners can still play around with that deal some but I've never found anything there trying to improve fuel economy or anything else.

When I set up distributors here I look at the engine combo to come up with some numbers based on years of doing them. CID, compression ratio, cam, heads used, etc.

For most I'll shoot for about 10-12 degrees at idle, 9-10 degrees from the mechanical curve (18-20 at the crank), and another 10-15 from the VA.

For my last engine it didn't want, like, need or respond to a lot of timing anyplace. 455cid, 11.3 to 1 compression, 236/245/112 HR cam, ICL at 109.5 degrees, 200 lbs cranking pressure, 92-93 octane fuel.

It liked 10 degrees initial timing, another 10 from the VA and 10 from the mechanical (20 at the crank). I used medium/heavy springs on it, curve started at 900rpm's and all in at 3200. I used ported to the VA. If hooked up to manifold vacuum it didn't do much beyond developing a slight "skip" or "miss" in the idle note. So I left it on ported as it sounded better out the exhaust with very slight "lope" and still making 13.5" vacuum at 750rpm's and it barely fell off at all when the trans was placed in gear.

For the combo here I doubt if it needs a lot of initial timing. Probably 12 degrees or so, another 10 from the mechanical for 32 total, all in by 2800-3200 would be soon enough. I'd add 14-15 degrees vacuum advance and play with that some for best power in the "normal" driving range and fuel economy.

I'd add here that trying to track tune with the engine well over 200 degrees, hotter than balls outside, and slipping on the starting line isn't going to tell you much. In order to effectively track tune you need to have the engine at full operating temperature but not overheating, and good traction for every run. Otherwise you are only looking at MPH and even then it's going to vary a bit as nothing else is the same run to run. I'd go back to basics, take the timing back out of it and do some full throttle pulls in 2nd or 3rd gear and listen for detonation and at the same time makes sure it's pulling hard. It's always best to sneak up on those things, starting at 30 degrees, then 2 degrees at a time to see if it's helping our hurting. When you finally get to the track and can run cool, decent air and hook it, you'll be able to see your changes/improvements run to run and nail things down quickly...........Cliff

[25stevem]

Thanks for your input on this Cliff!

[Cliff R]

You are most welcome Steve.

You may noticed that I somewhat avoided two things.

Ported vs Manifold vacuum and how "quick" the timing curve needs to be.

Here I use what works best for the job at hand when it comes to ported vs manifold vacuum. Engine fall into three categories tuning them with vacuum advance. They don't like, want, need or respond well to a lot of timing at idle, they are OK with a lot of timing at idle, or they buck, skip, and sound funny in the exhaust note with a lot of timing on them at idle. There is actually a 4th category, when you add so much timing at idle that they are so smooth and so much vacuum you can't get the idle speed below 900-1000 rpms or so no matter how hard you pull back on the throttle arm with the speed screw backed all the way out.

Here, a simple and quick method to determine or get close to the ideal timing the engine is going to want at idle is to make sure it has adequate idle fuel/bypass air FIRST. Then simply loosen up the distributor and advance the timing till it smooths out and makes highest vacuum (going further it starts to "buck and kick" in protest}. Then slowly retard the timing until the engine just starts to slow down a tad and develop a nice exhaust note (deep/heavy sound), stop, and take a gander at it with a timing light. Of course at this point if it's up near 25-30 degrees you are screwed for setting the initial that high because you will WAY too much total timing plus it will knock a tooth off the flywheel on a hot restart. This is where dropping the initial timing back to a more optimum number and adding timing via the VA hooked to manifold vacuum comes in. I do that but not often as NONE of my engines need that much timing and with many combo's we're more times than not just applying a "crutch" fix for too much cam, LSA too tight, and not enough static compression for what they were trying to do with this "street" engine.

When I do the distributor advance/retard thing to engines I've built here they almost always "settle down" nicely around 8-14 degrees initial timing. This really makes things easy for distributor tuning because most are already adding close to the right amount to get the total where I want it. So about all I have to do at this point is to make sure the curve is starting right off idle and advancing smooth and steady all in where I want it, and shortening up the slot in the VA to reduce it to about 10-14 degrees to get timing at light engine load/cruise where it needs to be.

Here, for street and street/strip engines I actually avoid a super-quick timing curve. The ones I've done for sure with higher compression, tight squish/quench and well chosen cams will not be wanting a lot of timing/fuel anyplace right to start with.

This distributor "re-curve" thing with heavy weights, modified center cams and cheap light springs is a fad that should have been forgotten decades ago. It never worked well back in the day but folks still do this to every engine they get within arms reach of. ALL of them believe that this negates the need for a VA and don't hook it up or disable it. It seems that the Olds and Buick crowd are by far and above the worst with the "all-in" by 1500rpm thing. I lost count at least a decade ago of how many pristine fully restored Olds 442's and Stage I Buick 455's (poor running BTW) I've yanked all that crap out of, re-installed original weights/springs and hooked the VA back up for a perfect end result.

Anyhow, I'll take a break here. Had a little extra time the past couple of days and hopefully some of the info helps a bit........Cliff

[AJ-]

Cliff, thank-you for the master class on all aspects of timing. Seriously...so helpful not just in the what but also the how and the why which is the difference between knowing and understanding. It helps me out on this one but also for next time and as a car hobbyist in general.

[mgarblik]

The two long posts by Cliff mirror what I have been doing for decades as well with distributor engines of all types. The Mr. Gasket kits and others with the light springs have always been a bad idea. First time I ever had a distributor on a SUN machine that was obvious. Unstable idle is nearly impossible to tune out with the timing jumping all over the place. If your timing light is jumping around at idle, weak springs are the #1 thing to look at. I also limit the vacuum travel with a stop. Had a Ford 289-271 HP engine in my shop recently that came from the factory WITHOUT a vacuum advance distributor. Owner had installed a hydraulic roller cam replacing the solid lifter stock one. Also added a Performer intake and small Holley 4160 vacuum advance carb. It ran lousy. Sluggish, poor transition, poor driveability unless on the floor. I tuned it the best I could and it was a little better. I recommended finding a vacuum advance distributor, replacing the vacuum secondary carb with a 4150 mechanical secondary carb. It's a 4-speed. 6 months later he returns with the parts. I follow Cliff's recommendations in general and it runs like a completely different car. Mechanical advance begins at 1000 RPM. 12 degrees initial, 10-12 degrees vacuum advance, 33 degrees total @ 3200 RPM's. Pulls hard for a little engine to 6500 RPM's.

Cliff has hundreds of examples just like that I am sure. Start with his solid advice and tweak a little from there as needed.

[68 Firebird]

Last year I figured it was time to inspect my
DUI HEI that had been in service for close to 10
years. I was seeing about two degrees more
Initial timing after a long drive, with the motor
fully heat soaked. Everything looked good but believe
Springs had lost some tension over the years.

After reading several benefits of using the original
OEM centerplate and weights I decided to try it.
Had collected several old HEI’s over the years
and had some parts to play with.
Installed a 364 centerplate with 139 weights using
Fairly heavy original HEI springs. Also welded up
Vacuum advance slot to reduce travel.

I have initial at 12 with 34 total at 3500. The
VA adds another 10 from ported source.
Centrifugal comes in just over 1000 rpm and
motor is definitely happier. Just seems to have
More low end grunt. With 3:08 gears the highway
Runs are comfortable and on last run got 17 mpg.


Gerry.