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Discussion Starter #1
Hello, I recently reset my timing on my 305 sbc which has a comp cam and edelbrock performer intake and carb. I was told by comp cams to run 8 to 10 degrees advanced. Which i did, and now my car runs at 210 degrees or hotter. For some reason when it was set at 20 degrees advanced it ran at 190 constantly. I also wanted to know what total timing means. I know its initial+ advanced timing or something like that, but quite dont understand it. Thanks much.

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Did you disconnect your vacuum advance when you checked your timing?
 

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Total timing includes the initial plus the possible centrifugal advance. It does not include vacuum advance. Vacuum advance is above and beyond total timing.

-Mark.
 

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Discussion Starter #4
I did set my timing with the vacuum advance line plugged.Also, I am still learning what advance really means. I think it has something to do with a full spark, or with the pick up coil delivering full spark to my distributor at a given rpm, but im not sure about the idea behind this, I am still learning. Thanks much. Peters76
 

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Peters -

In basic terms, spark advance refers to when the spark plug fires relative to the position of the piston as it comes up on the compression stroke. The air-fuel mixture in the cylinder takes a little time to burn once it's ignited, so the spark needs to fire before the piston reaches top dead center in order for the burning mixture to develop maximum cylinder pressure by the time the piston starts down on the power stroke.

The higher the engine rpm, the less time there is during each compression stroke, so the spark has to fire sooner at higher rpm to "light the fire" sooner than it does at low rpm in order to develop maximum cylinder pressure at the same point of piston travel as the piston starts down on the power stroke; that's why the spark has to be advanced as rpm increases to maintain efficiency.

Also, lean mixtures (like at steady cruising speed or while at low speed around town) burn slower than rich mixtures, so the engine needs more spark advance under these conditions for best efficiency and fuel economy - that's what the vacuum advance does - adds advance (over and above whatever the distributor centrifugal advance mechanism provides at that rpm) under high vacuum (low engine load) conditions to burn lean mixtures efficiently. Then when you accelerate, the mixture richens (and can burn faster), engine load increases, manifold vacuum drops, and vacuum advance decreases, as the extra advance it provides isn't required any more. When you ease off the gas, the mixture leans out to cruise levels again, vacuum increases, and the vacuum advance comes back into play to fire the leaner mixture earlier again for fuel efficiency.

Spark intensity remains essentially constant at all times and isn't related to advance, which only determines WHEN it fires. Hope this helps you understand the concept.

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JohnZ
CRG
'69 Z28 Fathom Green
 

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Discussion Starter #6
Thanks for the explination, I am a visual and hands on person, so to hear in basic terms really helps me understand whats going on inside the engine.I think this is the key to me understand what the engine is doing in a troubleshooting situation. I really appreciate your explination, it helped me a lot. Now I know why timing is so important to fuel economy and performance. Much appreciated. Peters76//Thanks man..
 
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