Timing+pinging+Locked advance

D

DPC Racing

Member
Hey guys maybe somebody can make me understand somthing.A guy I did some work on his 70 GS Buick 455 [brakes front end elec]he had put in a built 455 engine, street cam, holle :?: y carb good exhaust the engine ran good but when the engine temp got to 180 or so it would ping really bad, total timing was at 34 degrees all in at 3000 rpms, MSD box no vacuum advance.When he set the timing back to 28 total it was better but pinging was still there.A guy in his buick club that has worked on alot of hot street set up and race ones told him to lock the timing in at 30 and set the MSD box so it would start,well I thought the pinging would be worst with the locked in 30 advance but it stopped it.Can anybody tell me why it worked.
Thanks DPC
 
obviously I was never there to verify but both the rate of the ignition advance and where the ignition advance started at can be just as important as the total ignition advance.
theres several factors at work, but piston surface heat, quench and fuel octane are major factors in when detonation occurs
Most automotive engines use aluminium pistons that move in an iron cylinder. The average temperature of a piston crown in a gasoline engine during normal operation is typically about 300 °C (570 °F)
The self ignition temperature of gasoline mixed with air , in about a 14:1 ratio is about 495° F,, but it takes time for the incoming charge to absorb heat, theres much more time at lower rpms, at 3000rpm theres about 25 intake strokes per second, but at idle or 1000rpm theres three times longer for the fuel air mix in each compressing cylinder to absorb heat , the ignition advance is normally not needed past about 3200rpm because both increasing cylinder turbulence and rapid compression, plus increased heat levels speed up the combustion process.
it takes about 30-40 thousands of a second to have a cylinder burn during low rpm operation, leaving time for compressed gases to self ignite from absorbed heat and pressure building in the area near the spark plugs ignition point, at higher rpms theres far less time for the fuel to absorb heat and self ignite before the far more turbulent and rapid burn reaches the trapped gases, effectively making self ignition or detonation a non-issue.
BTW this is one area where a IR temp gun can be useful, if both the ignition timing and fuel air ratio are reasonably near ideal exhaust manifolds tend to run in the 500f-670f range depending of course on displacement,compression and cam timing, a retarded ignition timing allows part of the burn sequence to occur while the exhaust gases exit the cylinder, resulting in glowing headers


watch video
http://www.youtube.com/watch?v=UYGU7mTw ... r_embedded
KEEP IN MIND IT TAKES TIME FOR PRESSURE TO BUILD AS THE FUEL/AIR BURNS AFTER THE IGNITION PROCESS STARTS, BUT THAT TIME TENDS TO BE REDUCED AS RPMS INCREASE
ignitiontiming.png

redheaders.jpg

file.php

engbal8a.gif

timingsdia.jpg

quenchl.jpg

the dotted line shows pressure and heat during detonation
the solid line shows normal combustion pressure and heat
the inconsistent lines indicate missing or partial ignition
0311phrm_compress_07_z.jpg

hypothetically the ping or detonation your hearing could be the result of not the total timing but an ignition advance coming in to rapidly that over heats the cylinder to the point that even after the ignition advance is correct for the 3000rpm plus cylinder speed the cylinders already heated to the point it continues to detonate due to piston surface temps raised earlier in the pistons acceleration to 3000 plus rpm, so the cylinder temp just continues to increase further increasing the tendency to detonate, it doesn,t take a huge increase in cylinder heat to cause a cascade effect,pushing the conditions into detonation, if the compression and fuel octane levels are very close to reaching detonation to start with

related info

viewtopic.php?f=70&t=4683

viewtopic.php?f=50&t=208&p=487&hilit=detonation+piston#p487

viewtopic.php?f=70&t=4683

viewtopic.php?f=70&t=840

viewtopic.php?f=57&t=4974&p=15425&hilit=infrared#p15425

viewtopic.php?f=52&t=4081&p=13147&hilit=quench#p13147
 
Grumpy thanks it's clear to me now what was going on.Would it have been better to work on the curve of the dist or just go with the locked advance as they did ?I really don't have any info on the cam or compression to go by.Thanks again for all your help and info.
DPC
 
You must log in or register to reply here.
Back
Top