![]() ![]() The amount is determined by the movement of the pin in the slot. That system of weights and springs determines not only the rate at which the advance occurs with rpm but also the total amount. Mechanical (or centrifugal) advance is a system that is purely determined by engine rpm. It appears there is some misunderstanding about the relationship between mechanical advance and vacuum advance. We’ll also assume this is a purpose-built engine with aftermarket heads, cam, headers, and a carburetor. This discussion will be mostly aimed at optimizing the part-throttle timing curve on a street engine. ![]() We’re not talking about a drag race engine – we must emphasize this. So let’s take a look at a 10.5:1 compression small block Chevy street engine. Total max timing on it is 28 degrees and it has 10.5:1 compression. Look up the timing on a 2002 Z06 Corvette. If you have a high performance engine you don’t want to ever see more then 38 degrees. ![]() I have a cracked cylinder in my garage to show you what happens when you allow the advance to reach 48 degrees or more on a high compression engine. On pump gas and 10.5:1 compression you can’t have a max total timing more than 28 degrees. If you’re running 10.5:1 compression you will detonate with vacuum advance hooked up. Here is the link to the original question and answer dated March 27, 2015. But since this gentleman brought it up, we’ll take that ball and run with it! Nowhere in my answer did we talk about a 10.5:1 compression engine. The original question and my answer related to a mild small block Chevy with low static compression. So I thought this required us to go a little deeper to shed more light on vacuum advance. This may have resulted from my answer not being complete enough. The following comment reveals some confusion about how vacuum advance works as opposed to mechanical advance. Some comments are right on target – others not so much. All this activity indicates there is still quite a bit of confusion about ignition timing and how it works. The original post was roughly a year ago and has resulted in over 20 comments on the answer. One reader posted a comment (below) in response to my answer about ignition timing curves. Our tech column seems to be gathering quite a following of guys who want to know more about their cars and engines. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |