Join Date: Apr 2005
Location: Pensacola, Fl. U.S.A.
Ignition timing isn`t just a function of engine size, compression and fuel.
The cam design creates through valve events compression changes that have to be addressed or the results could be expensive.
Typical Alfa Factory Cams have a very slow ramp rate with lots of overlap. These cams as well as old design performance cams bleed off a lot of combustion pressure and allow "linear" spark advance rates as max combustion pressures are at higher rpms.
Modern cam lobe designs build higher combustion pressures lower in the RPM range and ignition timing through the mid range requires testing to get at optimal points. This is a function of several things, piston dome shape, static (starting) compression ratio, induction length, and specific cam lobe design. Commonly these motors above 4500-5000 RPM need a reduced timing advance due to less loss of "bleed-off" of combustion pressure as reduced overlap is severly increasing the CR due to "charge effect" as happens with all induction systems.
Commonly pump gas motors need about a 3 degree retard at 7000 RPM beginning linerally about 5000 RPM with my cams.
Since programable engine controls have become the norm in racing this isn`t a issue, but with higher compression ratio street motors running on pump gas most settle with a reduced "total advance" curve with adequate initial advance and mid range to keep output happy without detonation at high RPM which is evidient only on tear down to see what is damaged.
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
"When you are dead you don`t know it.
Only those around you are distressed.
Same with stupid"
Last edited by Alfar7; 08-06-2014 at 12:53 PM.