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Detonation?!?

Bolisk

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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.

So I agree that I need stronger springs. I guess my bigger issue is that I'm not getting enough advance in the fist place I should see at least 32, and not 28 right? What would cause that?
 
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So I agree that I need stronger springs. I guess my bigger issue is that I'm not getting enough advance in the fist place I should see at least 32, and not 28 right? What would cause that?

Bolisk,

If you are using the stock distributor, then it was not meant to reach 36* at 3,000 RPM. Your key is to play with the "ported" signal going to the canister. Those stock settings you posted earlier guide you towards this. I sometimes "forget" that 3,000 RPM is not even close to WOT. And this is even more true under no load conditions (in which we tune the engine -park/neutral on the transmission).

So it says that at 7" of vacuum, it should advance zero degrees but it will give you 12 at 15" HgIn. So when you reach your 3,000 RPM (and no need to venture into 4K RPM range), you should have part of that. The question is how much. So, attach a vacuum pump, pump it to 15", and see what you get at 2,800-3,000 RPM. That stock setting is matched to work with a cannister that will advance 12* at 15" HgIn. So I am guessing that around 1,500 you should see 15" of vacuum and as it climbs RPM's, it will go towards zero advance by 7" of vacuum.

This will let you know if the cannister is shot (stock cannisters are preset to work for the correct engine model, and aftermarket ones are adjustable). :D
 

Vettehead Mikey

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Gerry-

You're talking about vacuum advance. The OP is asking about mechanical advance.

He needs to modify the limiting slots or bushings to achieve more advance range than stock.
 
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Gerry-

You're talking about vacuum advance. The OP is asking about mechanical advance.

He needs to modify the limiting slots or bushings to achieve more advance range than stock.

Mickey, you are 100% correct. I meant to come back and add a "p.s." to my comments because I noticed I did not answer his question, but had to roll to work :(

But, I guess I was asking if he wanted to do that without seeing where 7" of vacuum is on RPM's for his motor because let's say it is at a higher RPM, then he could achieve the total timing of 36* with the assistance of the vacuum cannister. One other way would be to replace the distributor with an aftermarket one that allows bushings to restrict the timing. I thought he may want to keep his distributor a virgin one.
 

Bolisk

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Messages
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Crystal Lake IL
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1972 LS5 Convertible PS, PB, A/C
Gerry-

You're talking about vacuum advance. The OP is asking about mechanical advance.

He needs to modify the limiting slots or bushings to achieve more advance range than stock.

Yes, I'm asking about the mechanical advance. Please allow me to be more specific

As posted earlier, here are the specs according to my chassis service manual I need to see the following settings below. (BTW, my manual does not have my distributor listed. The tag on the distributor says 1112021. The 1970 Chassis service manual which says 1969 corvette on the cover does not list this distributor. The 1971 Chassis service manual. . .I do not have. . .and I don't know if it has the 1970 1112021 distributor in it.)

NOTE: My car was originally the L46 350/350 car, but now has an LT-1 short block in it (cam, pistons). Intake, heads, carb, an distributor are all original L46 parts.

I know that I may have to modify (curve) the distributor for my Frankenstein engine. . .the first question is which spec to go with? L46 or LT1. I do not know how lt1 cam and pistons factor into the distributor specs. Recommendations? I understand that I will also need to get a vacuum advance that matches the expectations of the spec I people recommed.

L46: LT1:
0 @ 1000 0 @ 1200
10 @ 1700 12 @ 2000
26 @ 5000 20 @ 4600

8 degrees initial BTDC 14 degrees initial at BTDC

Next question, I want to make sure I'm reading this right. The above specs are what the centrifugal advance should be at that RPM. . .and not the "observed" advance is at that RPM. . .correct? So for example, if I have 8 degrees of initial advance (according to the above info for the L46) at 1000 rpm on in the L46 spec. . .i should see 8 observed degrees because the centrifugal advance has not yet kicked in (e.g. it's 0). . .correct? At 1700 in the L46 I should see 18 observed because I have to add the 8 initial to the 10 centrifugal. . .right? At 5000 rpm I should see 34 observed (e.g. 26 + 8). I just want to check my understanding. I hope I have that right.

If so, then in my real world test, my observed maximum centrifugal advance was 28 degrees. When I subtract my initial 12 degrees I get 16. I believe this means that my existing centrifugal advance maxes out at 16 degrees, when it should be 26 for the L46, or 20 for the LT-1.

So at this point, I'm not sure my distributor has "original" components in it. Or something is may be binding. . .preventing full centrifugal advance. I find it unlikely because I'm seeing the full 28 at 1500rpm. . .but I may be missing something.

If it helps, the cam that is brazed to the top of the distributor shaft has the following markings on it. "183" and a "W" Not sure if that means anything.

If that is the case. . .then it seems like I either A) need to reduce the O.D. of the limit bushing. . .or B) increase the length of the advance limit slot (as I think was suggested above) in order to achieve the specs above. . .is that correct? Or instead of B are their different types (slot sizes) of lower cams (e.g the points cam)?

I think I cam play with the initial and centrifugal numbers as long I don't exceed the max combined numbers in the above chart. . .but I would like confirmation. For example: regardless of initial and centrifugal, the max observed should not exceed 34 degrees.

Lastly, if what I've read from other posters is true, it seems as if I want to get the all in number between 2800 - 3000rpm. . .so the specs should really look like this for optimum performance (correct):

L46: LT1:
0 @ 1000 0 @ 1200
10 @ 1700 12 @ 2000
26 @ 3000 20 @ 3000

8 degrees initial BTDC 14 degrees initial at BTDC


Thanks,
Bolisk

P.S.: I haven't given up on CCing the heads. . .and measuring true compression. I have just received the tools. Before I do that. . .I want to get the distributor acting correctly first.
 
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But, I guess I was asking if he wanted to do that without seeing where 7" of vacuum is on RPM's for his motor because let's say it is at a higher RPM, then he could achieve the total timing of 36* with the assistance of the vacuum cannister. One other way would be to replace the distributor with an aftermarket one that allows bushings to restrict the timing. I thought he may want to keep his distributor a virgin one.

You don't want to do that - the 36* "total timing" (sum of initial plus centrifugal) is measured with the vacuum advance disconnected and plugged; the vacuum advance is completely independent of centrifugal advance, and vacuum advance plays no part in "total timing".

Read this to understand the differences between centrifugal and vacuum advance and how they complement each other:

http://www.lbfun.com/warehouse/tech_info/timing & vacuum advance/Timing101Article.pdf

:beer
 
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1981 HD Suspension; ZN1 Option
You don't want to do that - the 36* "total timing" (sum of initial plus centrifugal) is measured with the vacuum advance disconnected and plugged; the vacuum advance is completely independent of centrifugal advance, and vacuum advance plays no part in "total timing".

Read this to understand the differences between centrifugal and vacuum advance and how they complement each other:

http://www.lbfun.com/warehouse/tech_info/timing & vacuum advance/Timing101Article.pdf

:beer

John,

Thanks for the remedial material. It was well written and you did a great job. :) I was aware of the principles in the article you presented me, but from a different source (one of the books that Dr.Jacobs wrote), but as always, it is good to read other sources to get a well rounded understanding.

I must be missing something concerning Bolisk's problem....because it seems that he just wants to get it running like it is supposed to and achieving 36* mechanical does not seem to be a goal intended by GM when they put that motor combination together... I have an Acel HEi distributor that when new would not produce more than 18* mechanical, and the factory distributors also produced about the same, so how were 36* mechanical advanced timing achieved on LT-1's, L78's, and L88's distributors in accordance to factory settings?

One thing was surprising to learn from your article was,"The incomplete burn reducedcombustion chamber temperatures, which reduced the formation of oxides ofnitrogen (NOX), and the significant increase in exhaust gas temperature ensuredrapid “light-off” and combustion of the hydrocarbons in the exhaust gas streamwhen the fresh, oxygen-carrying air was introduced from the air pump. " So with timed or ported vacuum they were seeking to reduce temperatures in the chamber and raise them in the exhaust? If this is so, then I really misunderstood that emission system. :(

Thanks for the clarification. :)
 
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Vettehead Mikey

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Gerry-

The poor AIR pump and system has been a whipping boy for the hot rodding crowd almost since the day it first appears in the '60s. It does little evil given that it takes only 1-2HP to drive the pump and allowed GM to install engine options that would not have otherwise been smog legal.

Bolisk's distributor has probably been played with over the years, but the limited total mechanical advance might be typical for early 70's lazy and boring advance curves.
 
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Next question, I want to make sure I'm reading this right. The above specs are what the centrifugal advance should be at that RPM. . .and not the "observed" advance is at that RPM. . .correct? So for example, if I have 8 degrees of initial advance (according to the above info for the L46) at 1000 rpm on in the L46 spec. . .i should see 8 observed degrees because the centrifugal advance has not yet kicked in (e.g. it's 0). . .correct? At 1700 in the L46 I should see 18 observed because I have to add the 8 initial to the 10 centrifugal. . .right? At 5000 rpm I should see 34 observed (e.g. 26 + 8). I just want to check my understanding. I hope I have that right.

If that is the case. . .then it seems like I either A) need to reduce the O.D. of the limit bushing. . .or B) increase the length of the advance limit slot (as I think was suggested above) in order to achieve the specs above. . .is that correct?

Correct on all four counts. :thumb


The factory spec for a '71 LT-1 (1112038 distributor) is:

0* @ 1060
2* @ 1340
17* @ 2400
24* @ 4800

In combination with the factory initial timing spec of 8* BTDC, that's a pretty conservative/lazy curve (for emissions) for a solid-lifter engine. A better setup would be total timing of 34*-36*, all in by 2800-3000. After setting up the distributor to that curve, select the vacuum advance can based on what your manifold vacuum is at normal idle so the vacuum advance (connected to full manifold vacuum, not "ported" vacuum) is fully-deployed at idle.

:beer
 
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One thing was surprising to learn from your article was,"The incomplete burn reducedcombustion chamber temperatures, which reduced the formation of oxides ofnitrogen (NOX), and the significant increase in exhaust gas temperature ensuredrapid “light-off” and combustion of the hydrocarbons in the exhaust gas streamwhen the fresh, oxygen-carrying air was introduced from the air pump. " So with timed or ported vacuum they were seeking to reduce temperatures in the chamber and raise them in the exhaust? If this is so, then I really misunderstood that emission system. :(

Thanks for the clarification. :)

Yes, that's correct - that's exactly how the A.I.R. system worked; remember that the catalytic converter hadn't been invented yet, so they got no downstream help in reducing emissions like they do today.

:beer
 

Bolisk

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1972 LS5 Convertible PS, PB, A/C
Update!

I pulled the distributor and disassembled. Very dirty. During disassembly I noticed the the post which holds the advance limit bushing was rubbing on the top of the vane assembly (part of the breakerlessSE electronic ignition system). Guessing that this could have been holding up the centrifgual advance I filed down the post after placing the new brass bushing. I made sure the the two parts had enough of a gap for heat expansion. Played with the new spring and determind that the gold springs gave me the strongest hold.

I put the distributor back in the car and found that I now have 38 degrees of advace all in just before 3000 rpm. Initial timing at 14 degrees.

Took her out for a drive, and all I can say is holly crap!!!! The car is a rocket. With 96 octane, no detonation, no noise just power out he gills. Much faster, much more torque, much more get up and go. I cannot believe how different this car is!

Next step is to take the fuel down to 93 and see if she detonates with this new cuve. If she does, off with the heads, and on to cc'ing the chambers.

I'll report back when I know more.

In the mean time, and chance my low advance numbers are the cause of the noise I was hearing?
 
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Congratulations! It is a bit of an art and blind providence for someone like Mickey and JohnZ talk you through a "landing" on a runway from hundreds of miles away and using only a radar!!!

Great job! You did all the work!
 

Bolisk

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Update.

Got it to make the noise again, under heavy load and hard acceleration.

Off come the heads after next weekend. :(

-JonR
 
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1981 HD Suspension; ZN1 Option
Update.

Got it to make the noise again, under heavy load and hard acceleration.

Off come the heads after next weekend. :(

-JonR

JonR,

38 degrees does seem a whole lot. On a dyno, you have an advantage to experiment with one or two degrees change effect on engine performance. Have you thought of backing off a couple of base degrees? I mean, since you have a bit of time before you pull the cylinder heads.

You are probably thinking that the engine made the noise with ~ 28 degrees and with 38 degrees, but perhaps the sweet spot could be somewhere in the middle?

p.s. You don't suppose you could post video of detonation?!?! ;)
 
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Bolisk

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1972 LS5 Convertible PS, PB, A/C
Excellent point. I for got the total number was centrifugal advance plus initial.

The reason it's high is because I was trying to get a number that was all in at 3000 rpm. No matter what springs I used, the advance was all in way before 3000, like 2500. I cannot find a strong spring set. I used the gold springs in the gr gasket 928g kit. So I adjusted the timing until the all in, what ever that is, occurred at or about 3000. That gave me initial of 16, and it never dawned on me that his was wrong. Now that you mention it, you are correct. 38 -16 = 22 of centrifugal advance. Time to pull the initial back to 12 which would give me 34. Is that ok?

If so, any one know of stronger springs so that I can get the advance all in closer to 3000?

Thanks!
 

navy2kcoupe

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Lighter weights would also help. Maybe get a "sacrificial" set of weights and
grind them equally until you get the desired advance curve. ;shrug;shrug
Andy :w
 

Bolisk

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1972 LS5 Convertible PS, PB, A/C
Lighter weights would also help. Maybe get a "sacrificial" set of weights and
grind them equally until you get the desired advance curve. ;shrug;shrug
Andy :w

Good idea!

With regards to earlier questions about video taping the noise. I can try but I'm doubtful that I will be able to record it. You can barely hear it over the engine noise, and only with the windows down .. .which would generate enough wind noise to make the recording imperceptible. It sounds like the engine is spitting in the engine compartment when it happens. :)
 
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Don't use the "advance kit" weights - they aren't hardened, they're the wrong contour and weight, and they're junk. Stick with the GM weights and back the initial timing down 2* at a time, starting at 12*, then 10*, then 8*, and see when the pinging stops.

:beer
 

Bolisk

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Yeah I heard the kit weights stink. I didn't order any.

Set it to 10 this am, and drove the crap out if it today. Could not get it to make the noise. I will beat on it some more tommrow.

At 10 degrees I'm all in at about 2500 rpm. Is that ok or too early?
 
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Vettehead Mikey

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I'll throw my 2 cents worth in.

Two wrongs don't make a right. Your advance curve is still 'too much too soon'. By having all the advance come in so early, the initial advance has to be retarded from optimum setting to compensate and you end up with less than ideal 'all in' mechanical. Being that the stock curve was far lazier than what you have now and you've got the strongest springs available, the weights must be some weird aftermarket set up.
 

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