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Old 03-10-2013, 09:36 PM
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1976 built Pontiac 455 pinging!

I have a 1970 cadillac with a 455 pontiac motor in it. stock short block & rotating assembly, large cam, high rise intake manifold ported and polished matching the ported and polished 6x heads, roller rockers, hei distributor, electric feul pump, demon 850 carb with no choke on it. currently the vacuum secondaries are not working . i am having pining issues under aggressive acceleration, backed down the timing a bit, tried increasing the fuel, still pining. will higher octane (100+) solve this, or could it be the secondaries not opening when needed?

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Old 03-10-2013, 10:46 PM
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What's your c.r.? If you have a large cam like you say, you probably don't have enough compression for the cam. If the secondaries aren't opening, it's seriously lean/limited. I assume by pinging you mean detonating. Fix the secondaries, pull out some timing and go from there.

It's not that simple, but it's a start to find the problem.
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Old 03-10-2013, 11:04 PM
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i do not know exactly im just starting out with engine building the guy that helped me put the motor together has the specs on the cam lift and c.r. i will get that as soon as possible. him and i were adjusting the timing getting the most out of it last night. the diaphragm seems to be leaking (the rubber seal on it), ordering the part tomorrow. the advanced isnt working on the distributor as well. fuel pressure is about 7-8 psi, floats are both half way as visible through the little window glass. idle is about 700rpm. do you think it would have to do with the secondaries not opening and leaning out when stepping on it? Pontiac's are thirsty haha or its just over timed and detonating to quickly? or something to do with the advance? could it be the cam isnt degreed correctly? it is straight mark to mark. sorry for the confusing description.
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Old 03-10-2013, 11:29 PM
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I dont think your cam is "that large" if its idleing at 700RPM... Detonation is one of two things, either pre ignition or too much timing. How much total timing is it currently at? Also how do you know that your secondaries are not opening? You can not physically see them open because they only do so when the engine is under a load. Even if for some reason they are not opening, I dont think this would cause your pinging. Really need to know C.R. and how much timing you have. My guess is too much total timing.

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Old 03-10-2013, 11:44 PM
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it was idling at 500, but we raised it a bit. its definitely not the largest cam possible, for large i mean to large for the set up. how do i know? well manually the vacuum is stuck, took the diaphragm apart and the plunger seems to have a minor tear, and previously when it did work i could feel it when i stepped on it it would jump from 3500 to 5000 almost instantly it still jumps but not nearly as much as when they did open. i dont think it would cause pining either but then again, it might be leaning out as i progressively accelerate. i believe its 12 and 27. cr is about 8 or 9:1 i dont know exact cant find out untill tomorrow. i have a feeling its not getting the fuel when needed, or im wron on the timing numbers and its too high, i can find that out tomorrow as well.
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Old 03-11-2013, 12:04 AM
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it was idling at 500, but we raised it a bit. its definitely not the largest cam possible, for large i mean to large for the set up. how do i know? well manually the vacuum is stuck, took the diaphragm apart and the plunger seems to have a minor tear, and previously when it did work i could feel it when i stepped on it it would jump from 3500 to 5000 almost instantly it still jumps but not nearly as much as when they did open. i dont think it would cause pining either but then again, it might be leaning out as i progressively accelerate. i believe its 12 and 27. cr is about 8 or 9:1 i dont know exact cant find out untill tomorrow. i have a feeling its not getting the fuel when needed, or im wron on the timing numbers and its too high, i can find that out tomorrow as well.
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Old 03-11-2013, 12:24 AM
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27 degrese is low, IMO. If you are at 27, then its not too much timing. I would have to say that you are higher than 27. With a good vacuum secondary set up (proper tension spring, good diaphram, etc) you should not be able to feel when the secondaries open. It should be a smooth trasnition. IMO the carb will have little to do with your pinging issue. if you C. R. is close to what you believe it is, you should be able to run 32-36 degrese of advance with 93 pump gas with out any pinging. Also a "bigger" cam will bleed off compression, so its probably lower than you think. Cam specs, and C.R will help us narrow it down.
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Old 03-11-2013, 01:10 AM
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Quote:
Originally Posted by PontiLac View Post
I have a 1970 cadillac with a 455 pontiac motor in it. stock short block & rotating assembly, large cam, high rise intake manifold ported and polished matching the ported and polished 6x heads, roller rockers, hei distributor, electric feul pump, demon 850 carb with no choke on it. currently the vacuum secondaries are not working . i am having pining issues under aggressive acceleration, backed down the timing a bit, tried increasing the fuel, still pining. will higher octane (100+) solve this, or could it be the secondaries not opening when needed?
You cannot just twist the distributor to add a ton of timing. That might make it run better- but that's because it wants more INITIAL timing. But adding timing by turning the dist. adds to the TOTAL timing as well as the initial.

You also may have the timing coming in too early (springs too light). And you need to limit the vacuum advance to around 10 degrees added timing.

The engine could be running too lean under a load. The plugs will be a good indication of this. I suggest you get the secondaries working and check the power valve rating compared to the idle vacuum to be sure you are getting enrichment soon enough.

Timing info.
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Old 03-11-2013, 07:41 AM
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There are two different 6X castings. One has a secondary marking of -4, the other, -8. It's located on the raised rib, just below the valve cover to the left (when looking at the head). If you have -4s, you're around 94 CC chamber volume, which is too small for a 455. It WILL detonate on 93. Retarding timing is NOT the "fix", as it does nothing to relieve the problem, just "masks" the symptom.

NOTE: "Spark knock" and "detonation" are two completely different issues. The only similarity is the "sound" they make. Spark knock comes from timing being too far advanced. A simple adjustment (retard the timing) can "cure" this. Detonation is when octane is too low for the compression ratio, and the fuel self-ignites, or detonates. The "fix" is either higher octane or lower compression. A cam with a lot of "overlap" can help "bleed off" cylinder pressure at lower engine speeds, but does nothing "up high". Your only ally there is the "clock", where the parts are moving so fast, it may not have TIME to detonate... But I've disassembled many engines with ring and ring land damage, where the owner said "It was quiet as a mouse..."

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Old 03-11-2013, 08:19 AM
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IIRC 6X-4 heads are advertised as 6X-4 93cc, mine measured 95cc. Using FT pistons and a 0.040 quench the CR is right at 9.5:1, and my engine had NO detonation. Had many thousands of miles on this one, and literally hundreds of 1/4 mile passes. Best power total timing was 34 degrees.

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Old 03-11-2013, 08:58 AM
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Quote:
Originally Posted by PontiLac
its definitely not the largest cam possible, for large i mean to large for the set up.
If the cam had an unusually (for a performance build) early intake valve closing point, the dynamic compression ratio might be high but that doesn't sound like what you're running. But to be safe, let us know what the cam specs are, or do the http://www.crankshaftcoalition.com/w...pression_ratio ]DCR[/url] calculating yourself.
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Old 03-11-2013, 12:23 PM
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I am not home to fully respond, I will do so when I get home. I did get some more specs though.

Timing: 20 initial 27 total
Cam lift: .510
Power valve: 3.5
I believe my heads are 6x 4s I will take a look and get back with that.
Also Thanks for helping !
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Old 03-11-2013, 07:27 PM
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Sounds like the cam may be to short on duration/overlap or the static CR is too high to start with. also how warm does the engine run?
Heat..lean condition...high CR.......
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Old 03-12-2013, 12:11 AM
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Okay so i was doing my research about the 6x 4 and 8 they both supposedly fit even though the cc is different just with correct air to fuel and timing should be able to run either heads. i could be wrong thats just what ive found @ Mr. P-Body

@cobalt327 i got what i could for spec still havent determined compression ratio, got the timing, cam lift, power valve up. i have the fuel pressure and electric fuel pump rating if that would help.

@LATECH please explain overlap im just starting out with building engines theres alot i still do not fully understand, and for temp it runs 175 on a good day on a hot day it will run about 190, only once has it almost hit 200. average about 180.
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Old 03-12-2013, 02:57 AM
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If you have a small cam the dynamic compression ratio (DCR) could be too high. If the heads are 6X-4 and have been milled, the static compression ratio (SCR) could be too high. Or a combination of the two could be happening. This is why it is always recommended that the chambers of both heads be measured (called "CC'ing" the heads) in order to know that the two heads match each other, and that the chamber volume is compatible w/the CR target you have, and that the cam is compatible w/the SCR. It's ALL interrelated.

Pontiac engines using production D-port iron heads (like your 6x heads) are going to be fine in most applications as long as the SCR is right around 9.5:1. Higher CR than that, or if using a very mild cam having an intake valve closing point that's excessively early, can cause problems w/detonation.

Regardless of whether the chamber size is known or not, you'll want to set up the ignition advance curve, as well as the initial and total timing. Then if you still have a problem w/the engine pinging, you can at least have eliminated the timing as the problem.

I cannot imagine that the total timing is only 27 degrees, but if it IS only 27 degrees and the engine is pinging, there's something serious going on that need addressing. Might be pointing to excessive CR, possibly from the heads being milled too much for the 455. Could be a hot spot in the chamber causing preignition, could be stale or diluted gasoline or a too lean air/fuel mixture (as LATECH mentioned already), or even a too hot spark plug heat range. But whatever it is, it needs to be corrected.

I would start by seeing exactly what the initial and total timing is, and at what rpm the total is all in by. The distributor mechanical advance mechanism needs to be evaluated to be sure it's working as it should, and that there's no other related problems like the vacuum advance hanging up.

Be sure there's no vacuum leaks that could be leaning out the A/F ratio.

Get the carb working both primaries and secondaries. If there's any question, baseline the carb to stock or at least see that it's not been recalibrated too lean- that includes jets as well as air bleeds. If it has been sitting for a long time dry, the power valve should be replaced.

Last edited by cobalt327; 03-12-2013 at 03:14 AM.
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