Here's a short run down of the motor. 460, bored .060". 9.0:1 CR with D3VE heads (decked .010", minor port & polish). No EGR equipment. Strong Crane torque cam (forget the number). As the motor approaches cruising RPM, around 2500 RPM's, a distinct rattle is audible. The detonation goes away when the vacuum advance is disconnected. From what I can figure, the cause is the combination of removing the EGR (which can lower combustion temps) and increasing the compression ration. Now, I've flipped the mechanical advance plate from 21 deg. to 16 deg. I've also tried installing an adjustable vacuum advance from Crame. After following the instructions, which also tunes the mechanical advance, it still detonates. Manifold vacuum reads 21" at idle. Any suggestions?

 

What is your total mechanical advance without the vacuum hooked up and at what RPM? What is your total with the vacuum connected? Total mechanical should be no more than about 34° to 36° and the vacuum added should stay under 50° (I find that 50° is the max you can usually run sometimes only 45°).

 

I agree with Bear 45/70's diagnosis. I would recurve the distributor so that total advance is not more than 38*- 40*. Also, I think your static timing is a bit much, try for 16"-18"Hg instead(read 12* initial). Bear in mind that vacuum readings will differ since you're using an after market camshaft. A common curve modification probably should look like this:
12* static
0*@500RPM
2 1/2*@1500RPM
4*@2000RPM
8*@4000RPM
12*@6000RPM
In short, you've dialed in 24*, plus vacuum, for a total advance of not more than 40* advance. This should be suitable for recreational use. I loathe not using vacuum advance as all hopes of economy are thrown away.

As for the EGR if you still had one, they normally function at an idle only and would have no effect on your current situation, other than if the intake is full of carbon it has the potential of becoming a giant heat sink reducing air density and power, but that's a Chevy thing. The "ping" you're hearing can be fixed with slight timing mods, or with increased octane.

 

P.S. You might pull the plugs if you suspect a lean cruise mixture.
p.s.s. Did you polish the combustion chambers too? Their might be something in there causing a hot spot. Smooth chambers don't allow much of a purchase for carbon.

 

Vacuum advance is always in addition to the total mechanical advance of 38°. You do them mechanical advance curve then add the vacuum on top of the that. Vacuum advance will icrease the timing past the 38° and that's why it improves gas mileage over running a mechanical only dist. Also full mechanical advance should be in by no more than 3500 rpm and remain there until max RPM are reached. So instead of

12* static
0*@500RPM
2 1/2*@1500RPM
4*@2000RPM
8*@4000RPM
12*@6000RPM

you want

Distributor Crank
6° static 6°
0°@500RPM 6°
6°@1000RPM 18°
9°@2000RPM 22°
12°@2500RPM 30°
15°@3000RPM 36°
16°@3500RPM 38°

Then vacuum advance will advance even farther under light load conditions.
As long as it does not rattle with the vacuum advance disconnected the the mechanical advance is not the problem as long as you are not over 38° at 3500, the vacuum is.

 

Once again, I agree with you. My advance curve reflects a good, smooth advance, but neglects the fact that all mechanical advance should be in place somewhere between 3000-3500 RPM.

Anyway, what isn't known is his static timing. Even if he's in the middle of the Mojave Desert it must be up there if he's pulling 21"Hg, either that or he's got a teeny carburetor. With the equipment 460_This described, it sounds like he's on the right track -he just needs to change one thing at a time starting back with his original settings.
One thing I would do is study the new vacuum advance unit closely, and be sure to use ported vacuum, not maniflod vacuum. Somethings that work well with 16" or 17"Hg might not work too hot with 21"Hg. This would explain why he had problems in the first place -the factory can was always "full on" with 21"Hg, yet the ping went away when he disconnected it.

 

Agreed, 100%.

 

Yeah, I left a few details out. Yes, I polished the chamber domes. I didn't go all out, but I did smooth them out. I am running manifold vacuum for the advance. I was told that manifold and ported vacuum are essentially the same off of idle. The carb is a Holley 750 vacuum secondary, on top of a 1" phenolic spacer, on an Performer intake. Even though I'm not turning anything over 5000 rpm's, I figured the larger mains would give me improved gas mileage since I'm not breaking into the secondaries on a large hill. The carb alone gained me almost 1 MPG (6.5 to 7.5 can make a heck of a difference on a long haul!). No, the mixture is not lean. It's a little fat if anything. I figured I'd resolve the timing issue first, then tackle the mixture. The baseline timing is set at 6 deg, with the vacuum disconnected. I will admit, I have limited knowledge of timing curves and what should happen where. Bear, can you elaborate on the timing curve that you posted, I'm a little confused on your use of static, crank and distributor terms? Also, the detonation doesn't happen until I let off the pedal as I reach crusing speed. One other thing, my cruising speed puts the motor between 2500 & 3100 RPM's. Should the full on mechanical advance be set different? I intended on taking some advance measurements tonight, but it just didn't happen. Your responses have been very helpful, thank you!

 

First off you were told wrong. Manifold and ported vacuum are not the same off idle and you should never run manifold vacuum to the advance unit unless you can't get the engine to idle any other way. As to static. That's what the dist is set to with the engine not running and it should also be what it is at idle. However with manifold vacuum to the advance unit, you now have cranked in full vacuum advance at idle. So you probably have about 26° advance at idle (6° static plus 20° vacuum). Distributor has say, the 16° advance set in it. At 16° advance in the dist. there will be twice than at the crank because the dist. turns half as fast fast as the crank, so you will have 32° at the crank with full advance in. Now with 32° advance and 6° static advance, your total is 38°, just what you want.

 

Actually, the EGR does NOT function at idle, or at least it's not supposed to. If the valve opened at idle, it would cause the engine to run very rough. It is set up to open (by means of an electronic operated EGR solenoid on newer cars, and a mechanical means like vacuum on older engines) at an rpm above 1500 or so and introduce exhaust gases that are basically stripped of oxygen molecules, and this in turn has a cooling effect on combustion, which allows higher ignition timing. (nitrogen and oxygen are removed)
Now, since the EGR is removed/deactivated, timing must be retarded to accomodate a now, non-EGR engine. If not, you get detonation. NOX levels soar.

With the right timing setup, you can run without an EGR, and the recurve is something I also recommend.

 

I took some mechanical advance readings today (with the vacuum advance disconnected).

6°@500 RPM
6°@1000 RPM
18°@2000 RPM
24°@2500 RPM
30°@3000 RPM
34°@3500 RPM

Even with this advance curve, and the adjustable pot adjusted all of the way out (minimum advance, I guess) I still have detonation. I even connected the pot to the ported vacuum on the carb, no change. Disconnect the pot from vacuum, no detonation. My next step is to use a hand vacuum pump and document how much advance the vacuum pot is creating. I should be able to get those measurements sometime Sunday. Again, thanks for all of your input!

 

Quote; Also, the detonation doesn't happen until I let off the pedal as I reach crusing speed.

I have never heard of this, Detonation should only happen when hard on the throttle. What do you guys think?

 

Sounds like your vacuum advance is starting too soon.

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