I have a 67 L71 Vert that is pretty much factory under the hood. I am always reading about detonation knock/ping on the forum and need some clarification. The only "knock/ping" I ever get comes at the beginning of pull-away under load from a stop - or - very slow speed accelleration in 4th gear. Other than that there is nothing except the solid lifter clatter. Is this the detonation KNOCK that is talked about? I definately don't drive it hard, but I am worried about this low speed "knock". What is the adjusted timing reading (retarded)that everyone eludes to for the L71 and pump premium fuel? Is this noise potentially damaging?It doesn't happen all that often but it's there. Thank you in advance for your input and direction on this.
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Re: C2- Detonation questions
Detonation - the metallic rapping sound is more likely to occur at low revs.
The first thing you should do is convert what I believe is a ported vacuum advance source on the L-71 to a full manifold vacuum source. This will cool down the combustion chamber surfaces by increasing total idle advance and reduce the tendency to detonate. It will also make the engine run cooler if it has a habit of running on the warm side at idle and low speed driving.
The ignition map specs I have for the L-71 are from Corvette News are:
Initial: 5*
Centrifugal: 0*@900,(no intermediate spec is listed), 30*@3800
Vacuum: (no start spec) 15* @ 15.5"
(How do these compare with what's listed in your service manual and MVMA specs?)
I don't like the vacuum can specs. Your engine should pull about 14" at 900 and this is not enough to keep the diaphragm pulled to the stop with full manifold vacuum to the can at idle. A better can would be the NAPA/Echlin VC1765, 0*@6" 16*@12".
The initial timing is not at all aggressive, but the centrifugal curve is not too bad. A proper vacuum advance with full vacuum advance at idle should eliminate or mitigate the low speed detonation, and you might even be able to run more initial timing - 8 to 10 degrees for total WOT timing of 38-40 should be near ideal. If you get this far and it doesn't detonate, you can try quickening the centrifugal curve. The quicker it comes in the more low end torque, but the limiting factor is detonation.
Duke -
Re: C2- Detonation questions
Too much science!! I think I can work with the timing adjustments. The advance curve methodology escapes me, but I do know wizards that can help. Can you suggest the manifold vacuum source point. I really don't want to modify the overall look of the engine to any degree. I will try to aquire the alternate vacuum can from NAPA and set it up accordingly. Just to be sure....the detonation is actually a KNOCK? This is what I hear,sort of scary really. Thanks for the direction, truly appreciated.GlennComment
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Re: C2- Detonation questions
Okay, knock. However one describes it, it is not a good sound. I'm not intimately familiar with your engine configuration, but Several L-72 owners have implemented full vacuum advance by teeing into the choke vacuum break line. L-72s also have a threaded port on the inlet manifold for the vacuum brake option. You can buy a threaded fitting with a 1/8" nipple to run a rubber hose for the vacuum can, or tee into the brake vacuum line. You need to poke around the engine compartment with a vacuum gage to see what other full vacuum sources may be available. The carb can also be modified to convert the ported line to full vacuum.
DukeComment
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Re: C2- Detonation questions
Thank you immensely. I'll go about finding full vacuum ASAP and see how things go. I am in Canada and the weather will soon stop the driving road test. Might be next spring getting to the winning combination. I'll keep in touch accordingly. GlennComment
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Re: C2- Detonation questions
I'm confused here. It sounds like these changes will INCREASE the spark advance at lower speeds/loads. I would think that more advance would INCREASE the amount and severity of the detonation or "knock". Am I missing something? Are you actually just using more vacuum advance at idle and less initial to eliminate the detonation? I also thought that increasing advance increased cylinder temps but decreased exhaust temps, as the fuel was burned more fully inside the cylinder before being ejected.... am I missing something here too?
Thanks,
KevinComment
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Re: C2- Detonation questions
The ideal total idle timing for a SHP engine - the timing that minimizes EGT and fuel flow is in the range of 25 degrees. This requirement is met on most SHP engines by the conbination of initial timing and full vacuum advance. If the vacuum advance signal line is "ported" - above the throttle plate at idle - then total idle timing will only be initial timing,which is less than ideal.
For any engine operating condition, if total timing is reduced from the ideal that minimizes EGT and fuel flow, EGT will increse as will fuel flow. The engine is operating at less than its potential maximum thermal efficiency. In addition to increasing EGT, the combustion chamber boundary temperatures will increase, which will tend to increase detonation. It won't detonate at idle, but when you open the throttle the hot chamber boundaries will increase the propensity to detonate, but the cooler the boundaries, the less propensity to detonate. This is why converting from ported to full time vacuum advance will often eliminate light detonation (and low speed overheating) with no other changes.
You have to think of timing as a "map" that is a function of speed and load. Speed is represented by RPM and load by manifold vacuum. Initial timing is just a starting point. If you plot this on a XYZ axis with X and Y representing speed and load, and Z the ideal timing you will create a surface. There are essentially and infinite number of speed and load combinations, and we can't optimize them all, but the two most important are "total idle timing" and "total WOT timing". Total timing at any point is some combination of initial, centrifugal, and vacuum advance.
If you juggle the initial, centrifugal, and vacuum advacne to achieve 20-25 degrees total idle timing on medium performance engines and 25-30 on SHP engines, and 36-38 degrees total WOT on SBs and 38-40 on BBs, then the virutally infinite number of part load engine states between idle and WOT will usually be somewhere in the acceptable range.
DukeComment
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