350 timing how much

[Mertz]

I have a 350 in my 72 3/4 ton all stock except ceramic headers, edelbrock eps intake with edelbrock 1406 carb, MSD ignition and roller tip rockers.

I have had a problem with pinging pulling a 5000lb trailer in 95 degree heat. It also doesn't have the power to pull it up a 7% grade without some downshifting. I had the initial set at 12 and got 36 cent at 3000 and 56 with vacuum at 3000. I backed off the initial to 8 and am now at 32 cent and 50 but still have more that can come in above 3000. It is from 4 to 6 more at 3500. I am trying to tune the truck for the best gas mileage I can get but still have power to pull the trailer. I didn't have any pinging under normal conditions at 12. I have a blue and silver spring in the dizzy (lightest and one step up).

I think the ideal setup should be 12 initial, 36 with mechanical and 50 TT. Does anyone have any recommendation on the timing setup? I am on port vacuum but am considering going to manifold.

[Goin4aPutt]

What octane gas are you running?

[Mertz]

83 With the price of gas I am not going any higher. I did get the ping to stop when I ran 92 pulling the trailer. Got better gas mileage too when not pulling the trailer. I need to tune for 83

[Billla]

I suspect the problem is more with vacuum advance than with mechanical - is your vacuum advance adjustable, and if so have you tried backing it off a bit? Do you have a vacuum gauge that you could provide vacuum readings under those heavy load conditions?

Changing from ported to manifold vacuum only affects idle quality, nothing else.

[Mertz]

The vacuum reading was zero under heavy load. The vacuum advance is not adjustable.

The truck idle fine.

[steelhorse]

50 degrees total timing sounds like a lot to me.I believe total timing shoud be around 36 degrees +or- 2 degrees

[Billla]

Originally Posted by Mertz

The vacuum reading was zero under heavy load. The vacuum advance is not adjustable.

The truck idle fine.

With no idle issues, then there's no reason to change the vacuum port for the vacuum advance.

ZERO vacuum? This seems a bit wacky unless you're basically at full throttle - where is the gauge attached and what vacuum do you see at idle.

Do you have access to a Mytivac or other calibrated vacuum source, or do you know the vacuum advance details or P/N?

Sorry to ask so many questions - I'm confident the vacuum advance is the challenge here.

[Bus Ted Knuckle]

Mertz, you might have to break down and buy the more expensive stuff, of the cost of a tank full you are looking at about a $5 difference. I believe you will make that up in fuel economy, and not detonating your pistons.

[Billla]

Originally Posted by steelhorse

50 degrees total timing sounds like a lot to me.I believe total timing shoud be around 36 degrees +or- 2 degrees

Around 50 is normal. 36 max for mechanical (= initial + centrifugal). Most GM GEN I vacuum advance cans provide another 15 degrees of advance at around 15 Hg. 36 + 15 = 51.

[67ChevyRedneck]

83 octane? Never seen below 87 here in SC.

[Highsteel]

Originally Posted by Bus Ted Knuckle

Mertz, you might have to break down and buy the more expensive stuff, of the cost of a tank full you are looking at about a $5 difference. I believe you will make that up in fuel economy, and not detonating your pistons.

It seems way easier to leave well enough alone, burn the cheap stuff in town and use mid or even high grade to get the performance your need when pulling a big load.
Is your 'stock 350' a 1972? What is its stock HP, any idea? The powers in the octane.
Just my .02.

[WorkinLonghorn]

I have had a problem with pinging pulling a 5000lb trailer in 95 degree heat.

Just wait for cooler weather!
Seriously from your original statement I'm not sure if you mean that it knocks during a full throttle pull,or during part throttle driving. We need to know this. -Brian

[Mertz]

I ment 87 or what ever the lowest is here in WA. I had a slight ping at accelloration but very noticable pulling up the hill. I think that 36 and 51 are good. I checked the MSD info and I have the red bushing with the blue and light silver springs so I should be all in at a little over 3000 but I am getting about 4 more at 3500. I am going to double check all my numbers tonight to confirm.

[Billla]

Originally Posted by Mertz

but I am getting about 4 more at 3500.

If this is the case, then you need to go to a larger limit bushing for sure.

[Goin4aPutt]

Since I don't know quite as much about it as you guys, I have to keep it simple. Mine pings with 87, but runs great with 89. I could never get rid of the ping totally when I was running the 87, so I just run the 89.

[Boog]

Originally Posted by Billla

I suspect the problem is more with vacuum advance than with mechanical - is your vacuum advance adjustable, and if so have you tried backing it off a bit? Do you have a vacuum gauge that you could provide vacuum readings under those heavy load conditions?

Changing from ported to manifold vacuum only affects idle quality, nothing else.

Well that's not completely accurate according to this article written by a GM engineer. I forget... Aren't you the one who posted this link?
http://www.hotrodders.com/forum/dist...1-a-59033.html
Anyway here is some good reading on distributors, centrifugal and vacumn advance, ported and manifold sources. How each affect performance and emissions, engine cooling and more. It's worth the read.

[Billla]

Originally Posted by Boog

Well that's not completely accurate according to this article written by a GM engineer.

Where's the inaccuracy? There's an absolute statement at the end of the post, but this is targeted at modified engines. The tuning of the vacuum advance is critical - but just moving a stock vacuum advance unit from ported to manifold isn't going to do a durn thing other than increase advance at idle.

The GM Engineer is Lars Grimsrud, very well-known and respected in Corvette circles.

[El Jay]

Originally Posted by Bus Ted Knuckle

Mertz, you might have to break down and buy the more expensive stuff, of the cost of a tank full you are looking at about a $5 difference. I believe you will make that up in fuel economy, and not detonating your pistons.

Gasoline is cheap. Motors cost money.

I used to run 91 octane all the time.
But now, with prices being what they are, when I'm just running errands around town, I can squeak by on 89 octane.
If I'm going to take a road trip, or hauling a load; I'll go up to 91 octane, and sometimes 91, with an octane booster.

Just fill your tank according to your projected needs.

[Boog]

It's right there in the article.

"Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it."

The vacumn advance does more than just affect idle quality. The vacumn advance helps the driveability and fuel economy. Moving it from ported to manifold does make a difference in idle quality and driveability. Modified or stock.
Mertz has driveability problems not idle probs.

It's a really good article. Give it a look over.

[Billla]

Originally Posted by Boog

It's right there in the article.

The vacumn advance does more than just affect idle quality. The vacumn advance helps the driveability and fuel economy. Moving it from ported to manifold does make a difference in idle quality and driveability. Modified or stock.
Mertz has driveability problems not idle probs.

It's a really good article. Give it a look over.

We'll have to agree to disagree; you're taking a few pieces out of context from my perspective and experience and comparing apples, oranges and pears Note all the data - very low initial timing, etc. for emissions engines. Lars simply summarizes that a vacuum advance IS required (although a lot of folks disconnect it or put on racing distributors without it), and for a performance engine it should be attached to manifold vacuum to provide a better idle. On a stock engine, you'll see no difference at all between the two ports - note that once the butterflies are open a bit there is virtually no difference between the ported and manifold vacuum - go pick up a carb (or use a vacuum gauge like I did) and see. If you just make a move with the stock can, you may have problems as you need a unit that pulls advance at 2 Hg below idle vacuum. In any case, making this change won't fix the problem.

Look a little further down where the comment is made:

"Are you using “ported” or “manifold” vacuum to the distributor? “Ported” vacuum allows little or no vacuum to the distributor at idle. “Manifold” vacuum allows actual manifold vacuum to the distributor at all times."

and

"An appropriately selected vacuum advance unit, plugged into manifold vacuum, can provide the needed extra timing at idle to allow a fair idle"

I suspect what's happening with Mertz is in high load he's getting too much advance. He's noted that he's getting 40 mechanical (too much) and I suspect the vacuum advance is adding too much as well. Once we know more about the vacuum advance can "curve" and the vacuum he's seeing it'll be easy to debug and fix.

I don't want to get into a back-and-forth (really! ) - but I just suggest you read the entire thread - carefully - and then go experiment with your car/truck. I spent a week toying with these ideas on a few vehicles.

[hotrod 80]

[Billla]

Originally Posted by hotrod 80

Do you have something of value to add to the discussion? Another one for the <ignore> list...

I completely respect Boog, we just have two different perspectives on an article and we're talking through our understanding.

[Mertz]

Here are the results of this evenings test.

First, one should never rely on memory at my advanced age. I checked the springs I thought were blue and silver and they were both blue so I changed one to silver. Now I am on the E chart with a red bushing.

Idle was at 650 so I bumped it to 800. Manifold vacuum was at 18. I reset the initial to 10. No adjustments to the carb.

1000 rpms
no vacuum from the port
no advance mechanical or vacuum
manifold vacuum at 18

1500 rpms
port vacuum at 4
manifold vacuum at 19
timing 21 without vacuum
timing 21 with vacuum

2000 rpms
port vacuum 13
manifold vacuum 19
timing without vacuum 30
timing with vacuum 40

2500 rpms
port vacuum 20
manifold vacuum 20
timing without vacuum 35
timing with vacuum 54

3000 rpms
ported vacuum 23.5
manifold vacuum 20
timing without vacuum 40
timing with vacuum 60

I was all in at 3000 with the new springs. The 60TT seems pretty high. Manifold vacuum was up a little. WOT nets me about 2hg. Sounds like I might need an adjustable vacuum advance or change the bushing to silver.

Seems like the truck ran worse when I took it for a drive. I'll check the spark plugs again tomorrow.

[Billla]

Man, this is some AWESOME detective work!! I have to say - very few people are willing to put in the work to experiment and find the problem; major props to you

I take it you're up around 2500-3000 RPM when you're towing correct?

Definitely need to step up to a larger limit bushing to limit mechanical to no more than 36 degrees - as little as 32 is OK, but 36 is ideal. The additional vacuum advance is also an issue - it should be no more than 15 degrees. I see a couple of options - Lars has a list of Echlin (NAPA) advance units you could replace the existing unit with, or you can buy an aftermarket adjustable. The adjustable ones ain't cheap, but they do come with a limit mechanism so you could ensure you're getting the right max timing. I've been using the Crane unit on a number of builds and have been very happy with it.

The fact that the ported vacuum doesn't match manifold vacuum until 2500 RPM is interesting - I found on my truck and a few other vehicles that they were equal at about 1500 RPM. Also interesting that the ported vacuum was higher after 3000 RPM...I need to think about that for a bit...

[WorkinLonghorn]

I suspect what's happening with Mertz is in high load he's getting too much advance. He's noted that he's getting 40 mechanical (too much) and I suspect the vacuum advance is adding too much as well.

Looks like Billa called it.
I'm a big fan of the adjustable vac-advance.Get one of those ($20+) and limit you tt (initial+mechanical) to 36 or slightly less if necessary.
BTW. A 1972 350 pulling a 5000 lb trailer (plus contents?) up 7% grades in 95 deg.ambient temperature will be down shifting especially at higher elevations.
I think those motors were rated at about 175 Net HP. The last time I checked,a modern 4.3 V-6 produces 195 HP. So the fact that it downshifts under those conditions is not indicative of a problem by it's self. But of course you will get more power with the spark timing set correctly.

OOps,useless post,I see Billa already posted.