cam selection

[Kenth]

Are you already using full manifold vacuum to the vacuum advance?
If not try, engine will run smoother and this will often cure run-on (diesling) at shut off because the engine will run with more closed throttle blades.
It certainly wont hurt anything.

[tayto]

I am running ported. will try full vac the next or 2 and report back,I suspect my in gear vacuum will go up a few points.

[Cliff Ruggles]

Timing can be a big player with idle quality.

Some engines like a LOT of timing at idle, others not so much.

I would get the engine fully warmed up, adjust the idle mixture screws for best idle quality and highest vacuum.

Then loosen up the distributor so you can turn it. Advance the timing till it runs dead smooth and highest RPM's, then start retarding it until it just starts to slow down and the exhaust note develops a slight deep/heavy sound.

Take a look at where the timing is at.  That's going to be pretty close to the ideal setting for your engine.  If it's back around 10-14 degrees there will be no need to run manifold vacuum to the advance at idle.

Most well thought out engine combos will not like, want, need or ever respond well to a lot of timing at idle.  This assumes the tuner has the carb set up to deliver adequate fuel to the mixture screws from rich to lean, and that there is no nozzle drip from the carb so adequate bypass air.

Almost every engine I've built here in the past 25 years or so has been fine with about 10-14 degrees timing in it.

I've tuned some troubled engines brought up here that needed a lot of timing at idle.  In every single case the cam was simply poorly chosen for the total combination of parts (low vacuum at idle speed).  I consider running the timing clear off the scale at idle a "crutch" for poor parts selection and just as often poor tuning skills.......FWIW......

[Kenth]

I have found it is an undeniable fact that most 1967 and earlier engines used full manifold vacuum for the vacuum advance. 26-30° timing advance at idle was the norm. These engines were in no way a result of poor engineering, this was a way to make the engines work as efficiently as possible. In 1968, there were legal requirements for reduced exhaust gas values ​​and one way to achieve this was to delay the ignition, even to the negative side of the scale, so that the cylinder heads would be heated up at the end of the combustion cycle by the mixture still burning when the exhaust valve was opened to reduce NOX. This did nothing to improve the efficiency of combustion, quite the contrary. When the throttle blades opening became larger with later ignition, idle bypass air was introduced in the Quadrajet to cure problems that arose with the later ignition, e.g. run-on/dieseling. With a hotter camshaft than standard, however, it can be difficult to get enough vacuum to fully activate the vacuum advance and then it can be advantageous to use a ported vacuum source for the vacuum advance and help the carburetor deliver a sufficiently combustible mixture and add more idle bypass air to avoid having the dampers open more than necessary and that the dieseling at shut-off. If anything, i think the "crutches" started 1968, and in the end the level of needed "crutches" to have a proper running engine depends on the skills of the engineer/designer/tuner.

JMHO

[77cruiser]

You ever see how much timing a TBI 350 idles with? About 30 degs. till you pull the tan wire.

[Glutenfreecarbs]

I agree with Kenth. ALWAYS connect the distributor to full manifold vacuum; "ported" vacuum is just a primitive emissions aberration.

And I've seen either 23° or 30° idle timing for stock TBI 350s on various squarebody (and Camaro?) forums.

[Cliff Ruggles]

It is NOT always possible to use manifold vacuum to the advance. 

For some combinations you aren't going to have enough vacuum at idle to effectively use it.  I run into that all the time here tuning engines that don't make enough vacuum to employ the vacuum advance. 

In these scenarios you MUST locate a vacuum advance that is either adjustable for when it applies all the timing or one that has a really "low" rating on it. 

There are also engines out there that are already making enough vacuum at idle with reasonable advance in them that adding more has the throttle plates almost or completely closed and they will not idle down to the desired idle speed.  I run into that scenario on occasion as well.

The most common scenario for troubled engines brought here for custom tuning is that the engine builder or whoever chose all the parameters LOWERED the static compression ratio (for pump gas) then installed some whiz-bang modern lobe profile cam to try to bring all the power back.

Yes, companies selling short seat timing, fast ramp, tight LSA camshafts advertise that those camshafts will idle better, improved throttle response, better fuel economy, more power everyplace, walk on water, tuck you in at night and cook breakfast for you in the morning.  It's marketing genius but in the real World lowering compression hurts power, vacuum, throttle response and fuel economy all else being equal, and closing the intake valve early and adding overlap isn't going to do much beyond killing off upper mid-range and top end power and put a little "attitude" in the exhaust note.

You'll also find yourself requiring a LOT of timing at idle and fuel to the mixture screws to make those engines happy.  You will also need to speed up the mechanical advance curve as well.  Here in lies the problem that I see.  About 80-90 percent of the customer base installs some POS distributor advance weight/spring kit, unhooks the vacuum advance (most add too much timing now that we are running so much initial and bringing in all the mechanical right off idle). 

This makes for a final product that doesn't idle well, lackluster power production, and all sort of tuning issues because the owner/tuner doesn't have the skill sets to improve the carburetors idle system to supply adequate idle fuel at the much lower than stock readings and some of the mechanical spark curve is in at idle so idle tuning is difficult or impossible right to start with.

I make a nice living sorting these things out and have for decades.

The FIRST thing I do with 99 percent of the vehicles brought here for tuning is to remove the distributor cap and replace the cheap Chinese weights and springs with OEM parts.  Once I get the timing under control and NOTHING in below about 900-1000rpm's I move on to the carburetor an get the idle system up to par for the engine combo.

We put the carb back on, set the timing, hook the vacuum advance back up (in most cases it wasn't being used) and guess what?  The owner comes back from their first test drive grinning from ear to ear, well, at least until I unload their wallet!

I started a thread on some of these tuning sessions and will be adding to it, time permitting.  Right now I'm so busy I don't have time to reach around and scratch my backside........Cliff

[tayto]

switched to manifold vac over a week ago. have about 12 or 13* @ idle now, from 7*. with my latest fillup looks like i picked up 1 or 2 mpg. definitely seems to like it at idle better, vac needle isn't vibrating like it use to and with the throttle plates closes vac slightly increased too. 16" in park, 14" in gear. still would like to try and close up the lower IAB but have not been able to source small brass set screws locally. i will probably be calling you soon Cliff there's some other stuff i would like to order as well.

[Cliff Ruggles]

"switched to manifold vac over a week ago. have about 12 or 13* @ idle now, from 7*."

I'm confused?  Timing at idle went from 7 degrees to 12 or 13 degrees?

Then below that you mention that vacuum increased to 16" in park and 14" in gear......

[tayto]

Yes my timing at idle went from 7* to 13*.

My vacuum @ idle in gear went from 13" to 14". I assume this is because i was able to close the throttle plate more. Hopefully I'll have time this week to call.

[Cliff Ruggles]

Are you saying that adding manifold vacuum to the advance increased timing at idle from 7 to 13 degrees?

That's only 6 degrees if my math is correct.  Most engines will like a bit more than that at light engine load and some like it at idle speed.

The BIG myth with all of this is that all engine like, want, respond well to and run better with a lot of timing at idle speed.

In reality the better we do with compression, squish distance, cam selection, etc, the LESS timing the engine will want everyplace.  They will also require LESS fuel as they are more efficient.

Tuning for a living I get to see all sorts of troubled engines combinations.  The trend in recent years is to LOWER the compression ratio then install camshafts with advanced intake lobe positions, shorter seat timing, and tighter LSA.

The thought process there is to bring back some of the lost compression, power and efficiency from lower the static compression ratio.  Sounds good in theory but doesn't work well in actual use.  At least not if you are trying to make optimum power on pump gas with these N/A engines.

I run higher compression instead, then use cams with more duration at the seat, later intake closing and wider LSA.  The engine result is an engine that idles well without a lot of timing in it, and has a broad/flat/strong power curve (torque). 

Combine that with good flowing heads and the end result will run as good as or better than a lower compression engines with smaller a smaller cam, then reward the end user with a LOT more upper mid-range and top end power.  I'd also mention that even with more power output octane requirements will often be less than a lower compression engine doing it the other way. 

This simply happens because efforts to increase dynamic compression early in the RPM range spike cylinder pressure high even if you have a relatively low static compression ratio.  SO basically having peak VE occurring lower in the RPM range increases the chance for detonation/ping all else being equal.  For these engines I like to push peak VE up higher in the RPM range which makes detonation less likely as the events are simply happening much quicker so not so high of a "spike" in cylinder pressure.

If you don't think this works well below is a clip of my car at the track.  Notice how well the engine idles with only 10 degrees timing in it.  The static compression ratio is 11.3 to 1 and it runs fine on currently available pump fuel with zero issues anyplace..........Cliff

https://www.youtube.com/watch?v=6zVdoLR-VzM

[Too Fast]

My question is where to source the springs/weights for the HEI? The distributor I am running I don't remember where it came from, I put it in because the Mallory Unilite was burning up modules suddenly when I had no issues the previous 10 years, and it left me stranded multiple times.  The HEI has been reliable for 3 years now except the mechanical advance now needs help.

 I don't want the cheapie spring kit.  With my Kauffman 74cc heads/041 cam/406 Pontiac/4 tubes/Pypes 2.5" with X-pipe the information I have found says my timing should be around 32* by about 3000 RPM.  I found out I was only 26.  I had to advance my base timing to 14 to get my desired 32.  I have ported vacuum advance that adds another 10.

[Kenth]

I´d say you´re right on the money with your timing.
Remember, Pontiac used 15° initial for the RAIV.
Only thing i would add is a stop for the centrifugal advance so the weigths don´t override max advance.

[Cliff Ruggles]

Good advise.  Installing a positive stop for the advance in an HEI is not optional...IMHO

IF you install lighter springs the weights can and will ride out past the "flats" on the center cam and add more timing at high RPM's.    I discovered this fact on the dyno decades ago following an engine built here with a timing light to verify total timing on a dyno "pull".

There are several different methods folks recommend to provide a positive stop. I use the MIG and a round chainsaw file here.  I can also closely control the exact amount of advance added with this method......Cliff