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Old 11-22-2011, 04:29 PM
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Pinging And Detonation Loads Sbc

Hi all .
wondering if when timing is too far advanced and pinging is taking place under load if you lift off the throttle lightly and cant hear the pinging any more does that mean that it has stopped or is it still doing it but just not that bad to be audible . Also when pinging under load then when you change down a gear and revs pick up has it stopped pinging or is it still doing it just you cant hear it .
Reason im asking is if you have a high stall converter and low gear rear end and revs are consistently higher can timing curve be more agressive in coming in .
I realise maximum advance shouldnt exceed 36-38 degrees
Thanks for input .

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Old 11-22-2011, 05:20 PM
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Just what is the timing set at, and how did you go about setting it by ear or light ? And what exactly are the engine spec ?
Back it off some and see if it stops . JMO


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Old 11-22-2011, 05:35 PM
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Back off on the timing to 34- 36*. You may have to replace the advance springs in dist. for some heavier springs to reduce the advance. Try replacing the plugs. Go one step colder. Increasing the gears ratio to a higher level could help. hope this helps.
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Old 11-22-2011, 06:05 PM
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Quote:
Originally Posted by theg
Hi all .
wondering if when timing is too far advanced and pinging is taking place under load if you lift off the throttle lightly and cant hear the pinging any more does that mean that it has stopped or is it still doing it but just not that bad to be audible.
Once you back out of it, the detonation (pinging) stops.

This is because (if using a distributor w/mechanical advance) the mechanical advance will decrease the timing. It does this because the engine RPM has dropped and the springs that control the mechanical advance are then able to pull the weights in- which lessens the mechanical advance.

Also the engine is under much less load. Remember how at a stand still, you can dial in all sorts of advance w/o detonation- because there's no load. Same thing when you back out of it, you are lessening the engine load, that makes the pinging stop.

Quote:
Also when pinging under load then when you change down a gear and revs pick up has it stopped pinging or is it still doing it just you cant hear it .
By downshifting, you are again lessening the load on the engine.

Quote:
Reason im asking is if you have a high stall converter and low gear rear end and revs are consistently higher can timing curve be more agressive in coming in .
I realise maximum advance shouldnt exceed 36-38 degrees
Thanks for input .
Yes, all else being equal a looser converter and lower rear gears very often will allow the total to come in sooner than with higher gears and a tighter TC, so you'd be correct.

Now, there can be what is sometimes called "silent detonation". It's not actually silent but if you cannot hear it by ear, it might as well be. This is the kind of thing knock sensors will detect on the computer controlled engines. But in OUR case- i.e. modified (for the most part, anyway) engines and a tune that we know the values on (like gear ratios, compression ratio, how much initial and total timing, carb jetting/calibration, etc.), we can keep ahead of the problem by using a safe, sane amount of timing, listening for ping, checking the spark plugs for signs of detonation, using appropriate fuel, and using good engine build techniques like a tight quench distance.

Obviously if the exhaust is loud enough that it covers up the sound of pinging, you could get into trouble and checking the plugs and feeling for any hint of power loss are the things that might keep you out of trouble. Fortunately, the limits are well known for the various types of engines/heads, so having too much total timing isn't likely to kill the engine. A bad tank of gas can cause problems if the timing is on the edge, as will anything that causes the engine temp to rise or the vehicle load to increase. That's why dropping a couple degrees on a street driven vehicle has its place.
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Old 11-24-2011, 04:45 PM
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If you drive the car with more revs ( keeping the revs up ) instead of highway labouring then a more agressive timing curve would be good ? If you drive like an old lady ( labouring in gears )then a conservative curve is needed .
Basically if I know my engines going to labour and ping but I change down and drive the car with more revs I can afford a more agressive advance curve but without exceeding total advance limits ?
I can drive it to ping or drive it so it doesnt . I should have more performance with this advance curve !
Is this a correct line of thought ???
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Old 11-24-2011, 11:07 PM
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Quote:
Originally Posted by theg
If you drive the car with more revs ( keeping the revs up ) instead of highway labouring then a more agressive timing curve would be good ? If you drive like an old lady ( labouring in gears )then a conservative curve is needed .
Basically if I know my engines going to labour and ping but I change down and drive the car with more revs I can afford a more agressive advance curve but without exceeding total advance limits ?
I can drive it to ping or drive it so it doesnt . I should have more performance with this advance curve !
Is this a correct line of thought ???
There are MANY things that combine to dictate the ping resistance an engine has, and many things, too, that can cause ping.

Basically, a timing curve is used so that the correct timing will be in place for all engine conditions. If the RPM were to always be "X", you'd only need one timing setting, but since the engine has to work well- and safely- across a large variation of RPM and load, the advance curve comes to the rescue.

When you ask if you can use a more aggressive curve if you drive around in a lower gear, it wouldn't necessarily be true that a more aggressive timing curve could be used- after all, you have to get the engine up to the revs in the first place. And that's what the curve allows you to do: get the engine up to higher RPM w/o detonation. Once there (to the higher RPM range), the curve is basically out of the picture. The engine will be running at whatever timing the advance curve allows for at that RPM and engine load (provided you're using a vacuum advance).

As for the performance you're now getting w/the curve you have in place, like was asked earlier- give a run-down of the engine and vehicle specs:

type/size of engine
camshaft specs
intake type
the timing (initial, total, "all in by" RPM, vacuum advance or no, how much vac. adv. supplies, hooked to manifold or ported vacuum)
the CR
engine vacuum at idle
current carb specs (jetting, power valve, accelerator pump cam and squirter size, turns on the Idle-Eze (if a BG), turns on the idle mixture screws, any changes that have been made from how the carb was set up originally)

Along w/the vehicle specifics like:

weight
gear ratio
trans type/ratio
the way you use the vehicle (race, w/e toy, parade, DD, etc.)

Then describe:

the symptoms of what the vehicle is (or is not) doing that it should be doing
what RPM the problem occurs
what you're looking for overall
etc.
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Old 11-25-2011, 07:58 AM
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Richen up the fuel mixture. You might be running on the ragged edge of being too lean. That will raise combustion temps. Your timing curve might be perfect as it is, but the fueling could be way off. If you run a holley go up 3-4 numbers on the main jet and see what happens.
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Old 11-25-2011, 01:15 PM
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Joelster is correct about the fuel mixture. A "fat" fuel mixture will allow for more advance. If you really want to get it right and protect the engine from expensive damage, you need an air/fuel ratio monitor set up. If you have headers on the engine, buy a collector with an O2 sensor port. If you want to do this on a budget, just by a single wire O2 sensor from your local parts store(most engines used these early on in the mid 80s to early 90s). Then just connect to a DVOM set to DC voltage and watch the numbers. .450v is an ideal mixture. Anything above is rich(over 450mv), and below is lean(below 450mv) It is difficult to have an ideal mixture at steady cruse speeds with a tall rear end gear and a high stall converter. To obtain an ideal "cruse" mixture you will have to increase primary jetting and reduce the overall advance. However, over jetting the primaries will cause it to bog slightly during launch. If you want it to launch hard and not run lean, you will always have to "drive it like you stole it" and only run it at steady cruse for short periods of time. Leaner primary jetting with an aggressive timing curve will give the best launch. How high a stall are you using? Many use too high a stall for street driving because they like the wheel spin coming off the line, however the distributor timing will be maxed out when the converter "couples". For example, lets look at a 350HP SBC with a 2100 stall and a 373 Auburn posi. The distributor pulls 36deg of advance at 2600RPM. It will boil the tires off with little effort from a dead stop. If I recurve the distributor to come in at 2200 and reduce the primary jetting it leaves even harder, but pings like crazy at steady speeds and the O2 voltage is less than 200mv. The engine is relying on the secondaries to fatten up the mixture as the throttle is wide open. It performs best at these settings, but It can only run at idle and wide open. This combination performs well on the street with more primary jetting and a later timing advance. It can run a "lean cruse" mixture that reads 380-400mv without causing any damage. It has very good performance without excessive fuel consumption. The carb is a properly tuned Quadrajet which is much better for street driving than a square bore Holley, it just needs to be set up properly.

You will just have to play around with first, the jetting, and second, the advance curve. The fuel mixture is more important, as a lean cyl environment will cause the most damage. Always run high test as well. Lower octane makes for a leaner burn. There will have to be a trade off between the launch and safe cruse driving(no pinging or engine damage)
I would start by simply increasing the primary jetting. If you can not get the results you desire, it may finally require a lower overall advance, like 32deg and or a lower stall speed. Have fun. Let me know if I can help in any way. I have played around with my and friends engines to get the most out of every combo without having to spend gobs of $ on fuel. Of course nothing beats higher HP if you have extra $ to pour into your tank!
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Old 11-25-2011, 05:55 PM
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Good post Dadwrench.
Could we interest you in doing some wiki articles?
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Old 11-28-2011, 11:51 PM
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learning the tricks

Lots of learning here in this thread . Thanks Heaps to everyone .
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