Detonation on part throttle!
Thread Starter
Melting Slicks
Joined: Jan 2003
Posts: 2,877
Likes: 13
From: Doha
St. Jude Donor '08
Detonation on part throttle!
I've noticed that nasty detonation sound lately, it comes only on part throttle, am I having too much vacuum advance?, can detonation occur on only part throttle on the first place? or do I hear something else and its not a detonation? .. I know my questions always sounds silly, but hey, you can't have enough of silly questions on the internet forums anyway! 
Drifting
Joined: Mar 2011
Posts: 1,886
Likes: 9
If the initial timing is too advanced, then the extra 15 or so degrees of vacuum advance, on a light throttle, might cause your detonation. Check the timing first, and set it correctly. You need the vacuum advance for a street car. Hook it back up when you are done testing.
Always go back to the basics, first.
Always go back to the basics, first.
Race Director
Joined: Dec 1999
Posts: 11,096
Likes: 990
From: Sierra Foothills CA
Look for things that could have changed. Can you rotate your distributor either way by hand? If so it needs to be re-timed and cinched down tighter. Are the distributor weights and springs all in place and moving properly? Have any vacuum fittings or lines disconnected? Did this happen only since you last got gas? Is your fuel filter clogged? It's usually something simple.
Le Mans Master
Joined: May 2006
Posts: 9,993
Likes: 1,136
From: League City Tx
too much vac advance. The stock ignition curve with the vac advance is likely 50*+ at part throttle cruise.... way too much for a 10-1 comp engine on todays crap fuel.
Disconnect the vac advance and plug the port on the carb. Drive it and see if its better.... If it is, then you need to limit your vac advance with a plate. You may have to try a few different total advance settings to see what is going to work.
I have mine limited to only 4.5* vac advance. Stock vac advance is like 20*. With my vac advance connected, I run a max of 36* at light cruise..... I have no pinging what so ever.... 10-1 496", iron heads and a relatively mild cam.
Disconnect the vac advance and plug the port on the carb. Drive it and see if its better.... If it is, then you need to limit your vac advance with a plate. You may have to try a few different total advance settings to see what is going to work.
I have mine limited to only 4.5* vac advance. Stock vac advance is like 20*. With my vac advance connected, I run a max of 36* at light cruise..... I have no pinging what so ever.... 10-1 496", iron heads and a relatively mild cam.
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Crane Cams sells an adjustable vacuum advance module. You can decrease total mech advance, use octane boosters. But what it really comes down to is you need more cam duration. If it not something out of whack all those other steps are just band aids.
