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Old 09-09-2008, 04:49 PM
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Spark Knock Fix

I have a 10.75:1 Compression 355 Chevy with Vortec heads. I currently use 94 octane gas with no octane booster. The car seems to knock at around 5300 rpm. What could be a solution for this. I retarded the initial timing, richened the carb slightly, knocking is a little less bad. I was thinking about colder plugs and or maybe limiting mechanical advance. Could octane booster help?? I yes, which brands are recommended.

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Old 09-09-2008, 05:15 PM
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Never heard of an engine knocking at that high of an RPM. I would think that the cylinder charge turbulence, lack of time for heating of unburned fuel, and lack of time for excessive pressure build up would eliminate knock before RPMs got that high.
If its detonation, it would occur at a lot lower rpm than that I believe. If your mechanical advance is all in at say 2400rpm, thats closer to where the rpm knocking would start, probably less, then drop off as rpm increased.
I'm no expert.JMO
ssmonty
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Old 09-09-2008, 05:15 PM
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Sounds like you have not verified total advance yet. You should do that first before chasing things.
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Old 09-09-2008, 05:30 PM
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I have a total of ~34 of advance (initial+mechanical) at around 3500RPM. I reduced initial timing since and has gotten more quiet, but I feel my motor needs more initial advance.
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Old 09-09-2008, 05:48 PM
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Whats the water temperature?
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Old 09-09-2008, 05:49 PM
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I had the same problem with a 383 it would run fine until you got up in the rpm then it would start to rattle.

Put some 110 octane gas in there and see if it clears things up. Mine seemed to be worse on the hot days so you might want to run a cooler thermostat.

10.75:1 is pretty high for a street car generally 10:1 is the limit for pump gas.
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Old 09-09-2008, 06:34 PM
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water temp is around 160. Colder plugs anyone???
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Old 09-09-2008, 07:06 PM
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What plugs are you using? What carb and carb jetting? What cam and intake manifold? is the thing starving for fuel at 5300rpm? (lack of fuel flow volume at high rpm)
Did you verify true TDC using a piston stop to ensure you are setting the timing accuratly? Vortec motors often don't need much total timing. Some as low as 28deg.
29 to 34 is typical. The AFR where a motor will tend to knock easiest at WOT is often not very lean 13:1ish. You may have to go further richer (even at the cost of a few ponies). Especially if it is one cylinder of the 8 is knocking. May have to stagger jet. You are on the edge as far as cr is concerned. If you want lots of initial timing for idle quality and low rpm throttle response, (long cam duration) and conservative total timing, you'll need to recurve the distributor to limit the mechanical advance travel stop. Look for a manifold vacuum leak. One leaky intake manifold/ intake port gasket joint will cause a lean(er) running, knock senstive cylinder (especialy if the gasket leak is drawing in engine oil into the intake port) Look for oil in the PCV hose. (bad or no valve cover breather/PCV baffling) Look for trans fluid in the transmission vac modulator line. Vacuum advance amount and rate, will need to be carefully adjusted to avoid throttle roll in knock.

Champion RS9YC are colder than stock. Mixing in 110 octane unleaded race gas is cheaper and more effective than use less lil cans of octane booster.

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Old 09-09-2008, 07:24 PM
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At 5300 rpm it sounds like you are right into it with the secondaries wide open, have you tried going up one jet size on the secondaries? I would do a plug chop at rpm to see what you are getting for mixture at high rpm.

Colder plugs likely won't solve the problem unless you are really lean at which point a jet change on the secondaries should solve that.

10.75 is pretty high for a street motor, not much you can do about the poor fuel quality either so you might want to consider putting in a thicker head gasket or running fuel treatment or a gallon of race gas at each fill up.
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Old 09-09-2008, 07:46 PM
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Adding octane is an expensive option, is this a daily driver?

Reducing spark advance further is another option, it might
cost you some power, might not its worth a try.

The most logical option is to reduce the compression ratio,
normally when thy ping at high RPM, they ping at takeoff
you just don't hear it then. .
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Old 09-09-2008, 07:47 PM
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It's not at all a daily driver, I`m lucky if I drive it at least once a week. As for reducing advance at the cost of power, I don`t really mind it, rather lose horsepower than a piston.
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Old 09-09-2008, 07:51 PM
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It the cam has a good bit of duration, it would be best to do like F-BIRD'88
suggests, limit the centrifugal to keep the initial up.
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Old 09-09-2008, 07:52 PM
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That was my initial plan, limit mechanical advance while keeping initial advance. The car has a 300 deg 0.515 lift camshaft with 1.6 roller rockers.
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Old 09-09-2008, 07:56 PM
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It the cam was shorter duration, the problem would be much worse. Your
plan is good, the engine needs the higher initial to run right at idle/low RPM,
but less total to eliminate the ping completely.
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Old 09-09-2008, 07:59 PM
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Good, now a way to hack my dist....
Afterwards I`ll see what happens
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