Detonation problem
Thread Starter
Instructor
Joined: Jun 2014
Posts: 178
Likes: 3
From: Pensacola FL
Detonation problem
Mild 383 with Accel HEI and an early Q-Jet carb.
With total timing advance around 36 to 38° all in @ 4000 RPMs I have a bad detonation problem at wide open throttle. I have to decrease total advance to around 32° to get rid of it. No detonation but the car runs like an absolute pig at 32° total advance. Part throttle and cruise is not a problem. I am running 93 octane fuel. The engine temp runs between 170 and 180°s
I think the engine is about 10 to 1 compression.
2 valve relief flat top pistons .010 below deck height, 68CC heads and .015 "thin" head gaskets.
I am not crazy about pulling the heads and using thicker gaskets to drop the compression some as a fix. I was thinking about going richer on the secondary side of the carb first, then trying a MSD programmable ignition.
Any suggestions would greatly appreciated!
With total timing advance around 36 to 38° all in @ 4000 RPMs I have a bad detonation problem at wide open throttle. I have to decrease total advance to around 32° to get rid of it. No detonation but the car runs like an absolute pig at 32° total advance. Part throttle and cruise is not a problem. I am running 93 octane fuel. The engine temp runs between 170 and 180°s
I think the engine is about 10 to 1 compression.
2 valve relief flat top pistons .010 below deck height, 68CC heads and .015 "thin" head gaskets.
I am not crazy about pulling the heads and using thicker gaskets to drop the compression some as a fix. I was thinking about going richer on the secondary side of the carb first, then trying a MSD programmable ignition.
Any suggestions would greatly appreciated!
Pro
Joined: May 2011
Posts: 722
Likes: 58
From: Karlsruhe (Germany)
Well, the car runs better with detonation?
36° is always a good value btw. timing usually belongs where engine runs best.
If you have to retard until you can feel a lack of power, then I would say you need better fuel. Maybe you have more compression then you think. The engine shop could have decked more then they should. Just as example.
36° is always a good value btw. timing usually belongs where engine runs best.
If you have to retard until you can feel a lack of power, then I would say you need better fuel. Maybe you have more compression then you think. The engine shop could have decked more then they should. Just as example.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,170
From: Hermosa
2 valve relief flat top pistons .010 below deck height, 68CC heads and .015 "thin" head gaskets.
Give us some more detail. Head chamber size, aluminum or cast iron, and cam specs. The devil is usually in the details.
Just quick computation on CR, if you're using 65cc heads you would be around 11.1:1 CR. With 76 CC heads you're at 9.87, but your DCR is unknown without the cam specs.
Where does it detonate, what RPM range?
edit: oops I see you gave the CC of the heads LOL. Those heads put you at a 10.76 CR with a 7 cc piston, and a .015 gasket.
Last edited by REELAV8R; Mar 30, 2015 at 09:51 AM.
Thread Starter
Instructor
Joined: Jun 2014
Posts: 178
Likes: 3
From: Pensacola FL
Wow! You sure about that? That would be a .025" squish. Sounds dangerously like piston hitting head territory.
Give us some more detail. Head chamber size, aluminum or cast iron, and cam specs. The devil is usually in the details.
Just quick computation on CR, if you're using 65cc heads you would be around 11.1:1 CR. With 76 CC heads you're at 9.87, but your DCR is unknown without the cam specs.
Where does it detonate, what RPM range?
edit: oops I see you gave the CC of the heads LOL. Those heads put you at a 10.76 CR with a 7 cc piston, and a .015 gasket.
Give us some more detail. Head chamber size, aluminum or cast iron, and cam specs. The devil is usually in the details.
Just quick computation on CR, if you're using 65cc heads you would be around 11.1:1 CR. With 76 CC heads you're at 9.87, but your DCR is unknown without the cam specs.
Where does it detonate, what RPM range?
edit: oops I see you gave the CC of the heads LOL. Those heads put you at a 10.76 CR with a 7 cc piston, and a .015 gasket.
I did make some progress. I fattened up the secondaries and went to a colder spark plug. My total advance is 34° and I have minimal ping. I added more initial timing via the adjustable vacuum advance hooked to manifold vacuum. The engine/cam combo likes a lot of initial timing.
I will keep tinkering and might pull the distributor and work on the centrifugal advance in an old Sun distributor machine.
Performance is better,but not where it should be. Last option is an airport a mile from home that sells 100LL for 4 bucks a gallon. I can add a few gallons each fill up to get the octane where I need it. The car is not a daily driver anyway..
Thanks and feel free to add comments.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,170
From: Hermosa
It's tight but with Lunati internals and a 6500 red-line I feel safe with clearance issues.
I did make some progress. I fattened up the secondaries and went to a colder spark plug. My total advance is 34° and I have minimal ping. I added more initial timing via the adjustable vacuum advance hooked to manifold vacuum. The engine/cam combo likes a lot of initial timing.
I will keep tinkering and might pull the distributor and work on the centrifugal advance in an old Sun distributor machine.
Performance is better,but not where it should be. Last option is an airport a mile from home that sells 100LL for 4 bucks a gallon. I can add a few gallons each fill up to get the octane where I need it. The car is not a daily driver anyway..
Thanks and feel free to add comments.
I did make some progress. I fattened up the secondaries and went to a colder spark plug. My total advance is 34° and I have minimal ping. I added more initial timing via the adjustable vacuum advance hooked to manifold vacuum. The engine/cam combo likes a lot of initial timing.
I will keep tinkering and might pull the distributor and work on the centrifugal advance in an old Sun distributor machine.
Performance is better,but not where it should be. Last option is an airport a mile from home that sells 100LL for 4 bucks a gallon. I can add a few gallons each fill up to get the octane where I need it. The car is not a daily driver anyway..
Thanks and feel free to add comments.
what cam you got?
Iron or aluminum heads?
Piston CC's. I just wagged 7 cc's.
Melting Slicks
Joined: Jul 2006
Posts: 2,282
Likes: 483
From: Salmon Arm, BC
Make sure your vacuum advance mechanism is functioning as it should. It's could be holding onto some advance (possible binding or sticking) as the vacuum goes away.....just something to look at.
