Hot Rod Forum : Hotrodders Bulletin Board

Hot Rod Forum : Hotrodders Bulletin Board (
-   Engine (
-   -   Spark Knock Fix (

chevy_power427 09-09-2008 04:49 PM

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.

ssmonty 09-09-2008 05:15 PM

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

F&J 09-09-2008 05:15 PM

Sounds like you have not verified total advance yet. You should do that first before chasing things.

chevy_power427 09-09-2008 05:30 PM

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.

automotive breath 09-09-2008 05:48 PM

Whats the water temperature?

Double_v23 09-09-2008 05:49 PM

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.

chevy_power427 09-09-2008 06:34 PM

water temp is around 160. Colder plugs anyone???

4 Jaw Chuck 09-09-2008 07:24 PM

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.

automotive breath 09-09-2008 07:46 PM

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. .

chevy_power427 09-09-2008 07:47 PM

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.

automotive breath 09-09-2008 07:51 PM

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.

chevy_power427 09-09-2008 07:52 PM

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.

automotive breath 09-09-2008 07:56 PM

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.

chevy_power427 09-09-2008 07:59 PM

Good, now a way to hack my dist....
Afterwards I`ll see what happens

techinspector1 09-09-2008 11:38 PM

This is why I'm constantly harping on building a tight squish into the motor. Changing to a thicker head gasket to lower compression will just make it worse, widening the squish.

All times are GMT -6. The time now is 09:35 PM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999 - 2012. All Rights Reserved.