Predetonation pinging and rattling

[JOJABOY]

I am working with a friend on his truck. 350sbc with HEI from an '86 Blazer. We have it timed right at idle. However, when he gets up to speed (45+ mph) and is accellerating, the valves start rattling. I think he is geting predetonation from the vacuum advance. Could I be wrong? If I'm correct....what is the best way to adjust it? Not my wheelhouse.

[Longhorn 70]

I'd drive it with the advance hose pulled and plugged. There may be another issue. I have seen the modules cause the same thing.

Also make sure the advance is hooked to a PORTED source and not manifold vacuum.

[JOJABOY]

Like I said, not my area of expertise. By pulling and plugging the vac line will it retard the timing back at high throttle? And you are saying vac advance line should be plugged into body of carb not the bottom where it pulls intake vacuum right?

Not getting the rattling when reving the engine to 3000 rpm in park. Only happening on highway.

[cleszkie]

If pinging under acceleration, the vacuum advance is not to blame. Vacuum drops under load (acceleration), therefore the vacuum advance will actually retard a bit. Sounds like you have too much total advance from your baseline advance plus mechanical advance. You could retard your baseline advance a tad, or install stiffer springs on your mechanical advance weights to push the point of maximum mechanical advance to a higher RPM.

[JOJABOY]

I've been reading several articles on the topic today. This truck seems to have a pretty big cam. He said that he put some stiffer springs in the HEI. It idles well with ported vaccuum to the vac advance. (I haven't checked) but he is saying that it is set at 36 degrees of total advance.

From what I know....the vac advance should not be advancing any whatsoever at idle being that it is ported vac from the body of the carb. As you accellerate the carb creates vaccuum within the carburator which in turn advances the timing. Then when you reach a cruise speed and rpm's come down....so does the vaccuum and in direct relation the vaccumm advance.

Is that correct? Could it be that his engine is creating too much vaccuum? I drove the truck. It definitely sounds like pre-detonation and does not occur except under heavy load.

[JOJABOY]

I am going to have him plug his vac advance line and drive over to the house. If the pre-detonation goes away then we will know. I'm guessing he will experience some power loss. If this is the case.....would installing an adjustable vacuum advance can solve the issue???

[JOJABOY]

Ok, for those of you not reading..... Pulled and plugged the vacuum line going to the vacuum advance and the engine stopped predetonating under load. Now to go buy an adjustable vacuum advance.

I was told to put a vac gauge on the carb while driving and get a reading at 2500 rpm. Then to put a pump on the vacuum advance and pump it to the same reading and adjust. Thay way I can simulate the vehicle being under load.

[geezer#99]

Originally Posted by JOJABOY

I've been reading several articles on the topic today. This truck seems to have a pretty big cam. He said that he put some stiffer springs in the HEI. It idles well with ported vaccuum to the vac advance. (I haven't checked) but he is saying that it is set at 36 degrees of total advance.

From what I know....the vac advance should not be advancing any whatsoever at idle being that it is ported vac from the body of the carb. As you accellerate the carb creates vaccuum within the carburator which in turn advances the timing. Then when you reach a cruise speed and rpm's come down....so does the vaccuum and in direct relation the vaccumm advance.

Is that correct? Could it be that his engine is creating too much vaccuum? I drove the truck. It definitely sounds like pre-detonation and does not occur except under heavy load.

THe carb doesn't create vacuum. THe action of the pistons creates it. When your motor is running it has vacuum full time. Ported vacuum is a vacuum source above the throttle plate. No vacuum at idle because the throttle plate works like a shut off valve. When the plate opens the full time vacuum is open to the ported source in the carb.
At cruise your manifold vacuum is highest. Under acceleration it drops off.
You likely have too much total time at the point where it detonates.
With a big cam you need lots of initial (16-20*) with a limited mechanical of (14-18*) and only 10-12 Degrees from the vacuum can.
Some vacuum cans have up to 28* in them.

Also likely no way to similate the vehicle under load. Just revving it and set the vac timing with a pump won't show anything. You might be able to hear it detonating though.

[JOJABOY]

Originally Posted by geezer#99

THe carb doesn't create vacuum. THe action of the pistons creates it. When your motor is running it has vacuum full time. Ported vacuum is a vacuum source above the throttle plate. No vacuum at idle because the throttle plate works like a shut off valve. When the plate opens the full time vacuum is open to the ported source in the carb.
At cruise your manifold vacuum is highest. Under acceleration it drops off.
You likely have too much total time at the point where it detonates.
With a big cam you need lots of initial (16-20*) with a limited mechanical of (14-18*) and only 10-12 Degrees from the vacuum can.
Some vacuum cans have up to 28* in them.

Also likely no way to similate the vehicle under load. Just revving it and set the vac timing with a pump won't show anything. You might be able to hear it detonating though.

By carb...I meant ported. I have that part. I am thinking that the vacuum can is pulling way more than needed. I appreaciate the comments. We called it quits for tonight. We can't do much with me being on crutches. He is going to pick up an adjustable vacuum advance can from O'Reillys tomorrow. I have a funny feeling that this is going to turn out to be a trial and error adjustment and just work it until it's right. Only butt pain is going to be having to take it on the road to get it to pull a load. Reving in the driveway isn't going to get us far.

[70blackfish]

SO what is the timing at idle? you never said

[70blackfish]

you could just set the total in the driveway, rev it to 3000 and set it to 32 or so.

[JOJABOY]

10* at idle on the balancer. Two blue springs and some aftermarket weights on HEI. So total mechanical advance with timing light is 39*

[70blackfish]

39 is to much....36 might be to much. 32-34 should be good

[JOJABOY]

It runs really good with vac unplugged now. and has always idled smooth at 500 rpm. Has one of those potato cams. Potato, Potato, Potato LOL

[Boog]

This sounds just like my experience with my stock 85 Blazer some years ago. Great running 305 on 87 octane gas then gradually began to ping under throttle. Finally got to be hard pinging. I backed the base timing down which helped some but it also made the engine sluggish. Higher octane gas didn't make that much difference. I installed stiffer advance springs that helped some but I could never get it just right for all around power and economy. I learned through my local dealer mechanics the electronic ignition control boxes were usually the cause. They disconnected the control box harness to the distributor and bypassed the box by installing a jumper on the four wire connector just out of the distributor. Readjusted the base timing and it ran much better. At least it didn't rattle as much. I did this with mine and ran it this way for years. Back then a new control box was high but later a remand box was available from the dealer for just over $100. I finally got around to replacing the box on mine. I know the truck ran more efficiently with a properly working control box.
I forgot what color wires to put the jumper on but I bet someone here will remember it. Seems like it was wire A to C or B to D.

[JOJABOY]

I have an extra HEI in my garage. I might give it to him to try a the different block and see how it does. I'm still convinced it's got too much timing advance curve. Cause when we disconnected the vac advance it ran great at idle and cruising speed (65). It didn't feel like it lost power in the acceleration so..... may be switch springs to one blue and one silver and then add adjustable vac advance can. Hmmmmmm

[cubber]

Mine was acting the same way when I first got it running, and, sure enough, I just needed a little more retardation (never thought I'd hear that). I have a "potato" cam, too, lol.

[DANTIP]

Here is a Great explanation of Ignition Timing written by a GM engineer and it perfectly explains ported versus manifold vacuum. Happy Reading.

As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

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.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.

[Longhorn 70]

Originally Posted by DANTIP

Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for.....

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.

This goes against what I was taught over 35 years ago.....WOW... I have to go try it.

THANKS for posting

[Longhorn 70]

But won't putting my vaccum advance on manifold vacuum make the advance high at idle and drop it when I stomp on it?

Is that where light advance springs come in to play?

[DANTIP]

Originally Posted by Longhorn 70

But won't putting my vaccum advance on manifold vacuum make the advance high at idle and drop it when I stomp on it?

Is that where light advance springs come in to play?

Remember, when you stomp on the pedal the vacuum advance goes away at the same time the mixture becomes Richer. The Richer mixture doesn't need the advance. Next when the RPM picks up your mechanical advance comes into play to compensate for the needed advance.

[nate_g_2003]

So would the line going from the vac advance to the carb be considered manifold vacuum?

I'm a little confused now. I have an Edelbrock 1406, with my vac advance hooked to my carb (which the edelbrock manual says is ported vac), but it seems like that would be considered full manifold vac because it is below the throttle plates...I'm so lost now haha.

Sorry for the possible threadjack. DANTIP, thanks for the post above on how things work.

[JOJABOY]

I've read alot and called my cousin who is a dealership service manager. I must say after all the reading, mr. goodwrench summed it up in simple words that I could understand. Here's what I got. sbc 350 like to have around 32-34* total advance when not running under load. Idle timing of 10* is the standard. This makes the spark 10* before the piston reaches TDC on the compression stroke. The additional 22* of mechanical timing comes from the distributor springs and weights. The total timing advance of 32* puts the spark at 32* before the piston reaches TDC. This is where the engine likes it to be when cruising down the road and the distributor spinning 2000 rpm holding a cruise speed of 55 mph. The springs in the distributor dont allow the weights to come fully out at low rpms...keeping the timing down near 10* which is what the idle timing needs to be. As the rpm's pick up speed the weights centrifigal force pulls outward and the springs can no longer holg them in and the timing advances along with the rpm's.

Now the vacuum advance... This comes into play when the weights are fully out and the timing is at 32*. Like I said above 32* is good for cruise control speed. But when you are getting on the throttle and taking the engine up to 3000 rpm's....32* isn't enough of an advance. So when you get on the throttle the intake starts to create vacuum. The vacuum is applied to vacuum advance can and the timing is then advanced another 15* or so. It's increases along with vacuum increase. This adds to throttle response as well.

Then at wide open throttle WOT the vacuum drops back off and timing comes back to total mechanical advance around 32* or so.

All of that being said... (and this just be me repeating what I was told yesterday)... this is for stock sbc setup. Having a large cam like my buddy will change things a bit and may require more advance. Insert trial and error here.

[JOJABOY]

I think with my friend's pick up having the set up that it has with big cam and all.....that he doesn't need quite as much vacuum advance. I read about different vacuum advance cans last night and I see that some don't start to apply vacuum until it reaches a certian pressure and some don't apply as much advance as others. I think he is just going to need to get an adjustable one and dial it in to where his motor likes it best.

[DANTIP]

It's really not as mysterious as it's made out to be. At idle, the mixture is lean and the engine can benefit from additional advance that manifold vacuum supplies. When you press on the gas pedal the engine vacuum is reduced and at the same time the mixture becomes richer. This richer mixture does not need as much advance. That's why the vacuum advance dwindles with manifold vacuum.

When you pull out to pass another car, you press on the gas, the engine load increases, the manifold vacuum disappears, the mixture becomes richer,the vacuum advance goes away, and the mechanical advance advances based on engine speed until it's fully MECHANICALLY advanced.

When you reach cruising speed, the load on the engine reduces, the manifold vacuum increases, and the ignition timing advances.