Detonation?!?

[Vettehead Mikey]

At the risk of setting off a hijack bomb and diversion of topic at hand, please bear with me. No one else thinks that having 26.5 advance at idle is weird? So between idle and 3,000 rpm only a maximum of 9.5 degrees would be applied?

Bolisk, what was the timing at 3,000 rpm?

Sounds acceptable to me. 12* initial, no mechanical advance + full (14*) vacuum= 26*. The mechanical should add another 20 or so as revs increase, but vacuum would have less and less effect as the carb is opened up.

I've heard of a case recently where a guy was running just under 50* at cruise on the highway. High revs + small throttle opening.

 

[Bolisk]

boliskmail, what was the timing at 3,000 rpm

I'll try to get those numbers tommrow.

 

[GerryLP]

Sounds acceptable to me. 12* initial, no mechanical advance + full (14*) vacuum= 26*. The mechanical should add another 20 or so as revs increase, but vacuum would have less and less effect as the carb is opened up.

I've heard of a case recently where a guy was running just under 50* at cruise on the highway. High revs + small throttle opening.

I guess that is what I mean, Mikey. 3,000 RPM is hardly WOT, so plenty of that vacuum wil still be there applying advance, and normally, we (those of us always seeking more power ) would want the timing to be all in by 2,800 - 3,200 rpm. But it could also be the weight springs too light and the vacuum cannister too tight, que no?

 

[Vettehead Mikey]

I guess that is what I mean, Mikey. 3,000 RPM is hardly WOT, so plenty of that vacuum wil still be there applying advance, and normally, we (those of us always seeking more power ) would want the timing to be all in by 2,800 - 3,200 rpm. But it could also be the weight springs too light and the vacuum cannister too tight, que no?

Que si. I've been asking about the advance curve since post#3

 

[Bolisk]

Que si. I've been asking about the advance curve since post#3

Yes you have. Can someone point point me to a procedure on how to do that, or explain it to me.

Not really sure how to.

 

[Bolisk]

Ok. What I found it happened in my Vette was that it "jump" cogs between the cam and the distributor gear.

So, find TDC for your Vette's #1 cylinder, and check the location of the vibration/torsional dampener TDC mark to the timing tab and the relative location of distributor's rotor to the #1 electrode on the distributor cap.

You'll have to check for a sheared distributor pin or maybe the distributor gear cogs are worn. The distributor gear is replaceable. But at least, a jump on the cogs took place somehow. Also, check for broken flyweights springs in distributor. With a broken spring, it will automatically advance total timing at idle; however, the sign of this should be a higher curb speed on the engine...not backfiring like it is doing.

I went out just now and found what I believe to be TDC on the compression stroke. The balancer appears to be correct. I pulled the cap and rotor was just past 1 on its way to 8. I think this is correct based on the firing happening at 12 degrees BTDC.

So I don't think a cog has jumped. Sound right?

 

[Bolisk]

I know people have said don't put too much stock into cranking pressure. . .but I wanted to get opinions on my compression readings. Are the ok, too much. Too far out of wack?

 

[GerryLP]

I went out just now and found what I believe to be TDC on the compression stroke. The balancer appears to be correct. I pulled the cap and rotor was just past 1 on its way to 8. I think this is correct based on the firing happening at 12 degrees BTDC.

So I don't think a cog has jumped. Sound right?

To get my engines in the ballpark, what I do is mark the #1 electrode on the base of the distributor, so that when I remove the cap, I have a solid reference. I also use a straight edge to make sure that my eyes are not playing tricks on me and align the mark on the base of distributor and the rotor's tip.

I then set the dampener at 12 degrees BTDC on the timing tab, and I then adjust the distributor to align the mark I made for #1 and the rotor tip. Then I tweak it if necessary when I shine the timing light on the engine.

But yes, it sounds like you are not too far off, but 4 or 6 degrees don't look like much when looking at the rotor, so that is why I do it the way I am showing it.

 

[Vettehead Mikey]

Yes you have. Can someone point point me to a procedure on how to do that, or explain it to me.

Not really sure how to.

It's most easily done on a proper distributor machine, but in essence you want to measure how much mechanical advance the distributor adds as speed increases and in particular at what speed it starts advancing and at what speed the advance is 'all in'. Once the numbers are known and a curve is plotted, through a combination of different advance weights, limiting bushings and different strength advance springs the curve, can be tailored for optimum performance.

John Hinckley (JohnZ) wrote an excellent article on this.

So I don't think a cog has jumped. Sound right?

The 'jumped a cog' or 'one tooth off' idea is an old myth. As long as you can rotate the distributor housing sufficiently to achieve the desired timing, you're all set.

 

[GerryLP]

....The 'jumped a cog' or 'one tooth off' idea is an old myth. As long as you can rotate the distributor housing sufficiently to achieve the desired timing, you're all set.

I don't understand how or why it happened, and instead of,"jumped cog", we could call it,"gremlins", or,"the-same-entity-removing-a-sock-from-the-drier", but on the last week of August I suddenly found the timing way off on my Vette (hence the video), and until then, I also believed it was a myth...:L

 

[Hib Halverson]

(snip)No one else thinks that having 26.5 advance at idle is weird? So between idle and 3,000 rpm only a maximum of 9.5 degrees would be applied?(snip)

I think he said that at idle with the vacuum advance working he had a total of 26.5 or 12 initial plus 14.5 vacuum advance. Which, seems about right.

Bolisk, what was the timing at 3,000 rpm?

What was the timing at 3000 with the vac. adv. plugged?

I know that the sweet spot in timing is perhaps slightly higher, such as 32 - 36 degrees, but that could only be confirmed on a dyno or at the track, so my comments do not deny that perhaps 26.5 would be the sweet spot for Bolisk's setup.

A nice advance curve will have 10-12 initial and 22-26 deg. centrifugal advance all in by, say.... 3000-3500 rpm. Those readings are with the vacuum advance plugged. To get a curve like that you have to find an advance cam which has a relatively small amount of travel. As for vac. adv., the best bet is an adjustable vacuum advance can which you can limit to about 10 deg. Of course, that nice curve works only if you have enough octane to keep the engine out of detonation.

(snip)After several rebuilds, I suspect the OP's block (and heads) have been milled at least once (maybe twice), and he's probably running higher compression than he might think; the only way to know what the picture really is is to cc the heads and measure the piston-to-deck clearance so a calculator can make sense of the data to determine the static compression ratio. If he knows for sure what the installed cam is, factoring that data into the calculator will also yield the dynamic compression ratio, which is critical to the detonation issue.

What he said!;worship

I know people have said don't put too much stock into cranking pressure. . .but I wanted to get opinions on my compression readings. Are the ok, too much. Too far out of wack?

The second set of readings seem more in line with what a 70 LT1 should do but....what say you, "JohnZ"?

(snip)
The 'jumped a cog' or 'one tooth off' idea is an old myth. As long as you can rotate the distributor housing sufficiently to achieve the desired timing, you're all set.

Yep. That's it...it's an "old myth"

Forget about having the distributor body such that the tach cable lines up and don't worry about the vacuum advance can banging on the intake manifold, pointing towards the firewall or, much less, fitting under the distributor shielding. All that stuff just doesn't matter because the positioning of the distributor is...an "old myth".

Oh and if you have an F.I. distributor, don't worry about the fuel pump drive....you say the engine won't run without the fuel pump drive connected? Ha! A minor issue. Just remember, the importance of distributor position is...an "old myth".


Couple post scripts, here

"Bolisk", may I suggest you purchase a timing tape for your harmonic damper and install it per its instructions. Then, you can determine your advance curve without a distributor machine or a dial-back timing light. All you need a good "regular" timing light (MSD makes the best ones) and a tachometer you can read from under the hood. Disconnect the vacuum advance, then run the engine at idle, then, 1000, 1500, 2000, 2500, 3000, 3500 and 4000 rpm and record the advance figures. That will give you a rough idea of what your centrifugal advance curve is.

From your last few posts, it seems as if the centrifugal advance mechanism might be fouled up. I'd tear the distributor down enough that you can make sure the advance cam moves freely on the shaft. To do this you should only have to remove the weights and springs, then grasp the cam and make sure it moves freely. Then, make sure you've got the right distributor cam, advance weights, advance springs and advance limit bushing.

I recall that that the advance mechanism in a 62-74 Delco distributor was designed to work without oil or grease on it because GM didn't want to introduce oil or grease inside the distributor cap. This is why advance weights rode on a nylon "buttons". The last engine I had which used a 62-74 Delco dist. never had any lubrication applied to any of the advance weights or springs. With the exception of the distributor shaft where the distributor cam rode, I always kept everything related to the advance mechanism inside the distributor cap clean and dry. To lubricate the cam, I'd disassemble the distributor, slide the cam off, pack a little Red Line grease in the grooves in the distdributor shaft where the cam rode, then I'd reassemble the distributor and, finally, reinstall the advance mechanism.

When you disassemble the distributor, after you drive out the pin holding the gear, file the burrs off the pin hole in the shaft before you pull out the shaft then slide off the cam. Lastly, if you take the gear off and there is a bunch of shims between it and the end of the distributor body, remember to reinstall all those shims.

I hope you eventually get that LT1 running right.

Good luck!

 

[Vettehead Mikey]

Yep. That's it...it's an "old myth"

Forget about having the distributor body such that the tach cable lines up and don't worry about the vacuum advance can banging on the intake manifold, pointing towards the firewall or, much less, fitting under the distributor shielding. All that stuff just doesn't matter because the positioning of the distributor is...an "old myth".

Oh and if you have an F.I. distributor, don't worry about the fuel pump drive....you say the engine won't run without the fuel pump drive connected? Ha! A minor issue. Just remember, the importance of distributor position is...an "old myth".

Just for clarity Hib- is this your grumpy old man persona that keeps coming out or just your redneck side?


For the OP- and to make Hib happy, the distributor housing should be within the factory described limits which allow it to fit under the shielding and for correct connection of the tach cable. I didn't want to insult your intelligence by mentioning these things.

 

[GerryLP]

....Yep. That's it...it's an "old myth"

Forget about having the distributor body such that the tach cable lines up and don't worry about the vacuum advance can banging on the intake manifold, pointing towards the firewall or, much less, fitting under the distributor shielding. All that stuff just doesn't matter because the positioning of the distributor is...an "old myth"....

That is true, Hib. One can have the #1 electrode designated on any spot on the rotor cap and pointing at the #6 cylinder, but as long as all the corresponding wires are attached to the associated cylinder sparkplugs, and the firing sequence is maintained, the proper timing can be be set. It only matters as far as configuration of the engine compartment is concerned (with which purists also concern themselves)

I know that some of you guys find it amusing, but how can it be explained when the timing is way off, and the dampener is not loose or the distributor body is not loose and obviously the double-roller chain is not broken and the distributor gear pin is not sheared???? Like I said, I cannot explain it. The images I posted help show some folks how you can get a ballpark initial timing, specially after building an engine and first firing it. Working alone most of the time makes you resourceful and remember, that you only have two hands.

All things being equal, I still think that the problem Bolisk's is having is an elusive, but not a complex one. Unless, the previous owner sold him a Vette with a tankful of low lead aviation fuel, this problem should not be happening seemingly overnight, so it should not (hopefully) come down to a cylinder head removal.

 

[Bolisk]

I've owned this car for almost 6 years. Alays put 93 in it. Just seem to be worse after my heads had the valve job. Then even worse after the rockers.

Now having said that. I took the car out and beat on it. I cannot believe how much more pep it has with timing set to 12 degrees.

Now after a day of beating on it. I can say a few things. Cleaning the distributor, and getting the weights to move freely made a significant improvement. The problem is not completely gone, but it is MUCH harder to reproduce. I have a ton more power and I now only happens going up hill under load. The sound is very difficult to hear over the engine noise now. Sometimes the best way to hear it is to make it happen while driving between two cars, so the sound bounces back to my years.

Before I pull the heads and attempt to measure everything, I want to try to things.

1) map the distributor curve
2) put a full tank on 100 octane in it.

If the problrpem still exists on 100 should I start thinking about some other cause of he noise? I'm assuming that everyone believes that I should not have this problem with 100 octane.

 

[JohnZ]

Before I pull the heads and attempt to measure everything, I want to try to things.

1) map the distributor curve

Here's how to do that.


http://www.lbfun.com/warehouse/tech_info/timing & vacuum advance/MAPPING ADVANCE.pdf

 

[Bolisk]

Ok something is wrong with my distributor, or I'm doing something wrong.


I started the car and let it warm up. Got it to idel at 750. Pulled the vaccume advance, and plugged it. Used the dial back timing light to verify 12 degrees initial timing.

I then put my wife behind the wheel and told her to hold the throttle at the following rpm's while I measured the mechanical advance:

Initial = 12
1200 = 23
1500 = 28
2000 = 28
2500 = 28
2800 = 28
3000 = 28
4000 = 28

I was not expecting to be all in at 1500. To confirm that, I asked my wife to slowly accelerate the engine from idle. We verified that it's all in at 28 at near 1500 rpm.

So two questions. I thought all in mechanical is like 36 or something. . .not 28. Also I thought I should be all in some time past 2800 rpm. . .not 1500.

Opinions please.


Thanks.

 

[Bolisk]

Looking in the 1970 chassis service manual. The specs for the l46 (which the car was from the factory) and the lt1 which this car is built to using the l46 distributor.

L46:
0 @ 1000
10 @ 1700
26 @ 5000

0 @ 7"
15 or 12@ 12" (depending on part number)

8 degrees initial bBTDC



LT-1:
0 @ 1200
12 @ 2000
20 @ 4600

0 @ 7"
12 @ 12"

14 degrees initial btBTDC




So maybe, 28 doesn't sound so crazy, but it's past the 26 or 20 speed by GM.

And I didn't expect 8 initial for the l46. Or the 14 for the lt1. I thought 12 was the most GM ever speced.

I'm still concerned at the rpm that I'm all in at.

 

[Vettehead Mikey]

So two questions. I thought all in mechanical is like 36 or something. . .not 28. Also I thought I should be all in some time past 2800 rpm. . .not 1500.

Opinions please.


Thanks.

Ah ha! You are correct that it should be 'all in' at 2800 RPM or so and not 1500. This infers that the springs are too weak or the weights too heavy. This is not difficult to fix. To achieve the desired 36* total, this is mechanical advance (presently 28*-12*= 16*) plus initial (12*). The distributor can be modified to increase the mechanical advance beyond the current 16* to around 24* so that the 'all in' = 36*

 

[GerryLP]

Bolisk,

Barring a mechanical issue with the distributor (vacuum canister not connected to mechanism), you might just need stronger springs or lighter weights (like Mickey suggested) to help hold back the advance till it is needed.

Relatively low engine speeds/load will require less advanced timing for the same fuel charge. Now, obviously there are diminishing returns for adding advance timing, but your car will perform better if you swap the springs for stronger ones.

But thoroughly check the distributor for defects to make sure nothing else is interfering.

 

[Bolisk]

Ah ha! You are correct that it should be 'all in' at 2800 RPM or so and not 1500. This infers that the springs are too weak or the weights too heavy. This is not difficult to fix. To achieve the desired 36* total, this is mechanical advance (presently 28*-12*= 16*) plus initial (12*). The distributor can be modified to increase the mechanical advance beyond the current 16* to around 24* so that the 'all in' = 36*

Ok I think I understand. I'm wondering if the current setup might be advancing at idle. I'm going to put a rubber band over the weights and see if there is a timing difference.