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Can locked timing cause detonation?

10K views 19 replies 5 participants last post by  Blown496 
#1 ·
Am I destined to repeat this problem over with locked timing? I have a show car blown 496 set up for 5lb boost. It has 8.25 compression. Cam is 114lc on 109icl 240/245 @.050 and .647 lift. Locked 32 timing. I have Edel perf RPM heads. The Cometic head gaskets were leaking oil in the back on both heads. The lower half of the cylinder rings on both sides were separated on the gaskets. Cometic tells me detonation causes this from the heads lifting. How do torqued heads lift? I drive this thing at 2000rpm at 70 other than around town shifting through the 5 gears. I have never hammered it more than a little rolling burst to break the tires loose in first and second. I did run it up to 3lbs of boost one time. The right side leaked from day one and the left about 500 miles later. Engine has 1500 miles on it. I had leaking teflon guide seals that I replaced with viton at 300 miles. When I pulled the heads all the pistons and combustion chambers were heavily coated with baked oil and carbon from those leaking seals. I cleaned all that off and everything looks brand new. No signs of damage anywhere. Where do I go from here?
 
#2 ·
As to whether locked timing can cause detonation, it depends on where it's locked at. I don't think you mentioned whether or not you replaced the head gaskets when you pulled it down for the valve seals but you probably replace them anytime you have the heads off. You really should check the block and head deck surfaces for straightness while it's down, and you might want to use studs rather than bolts.
 
#3 ·
Blown496 said:
Am I destined to repeat this problem over with locked timing? I have a show car blown 496 set up for 5lb boost. It has 8.25 compression. Cam is 114lc on 109icl 240/245 @.050 and .647 lift. Locked 32 timing. I have Edel perf RPM heads. The Cometic head gaskets were leaking oil in the back on both heads. The lower half of the cylinder rings on both sides were separated on the gaskets. Cometic tells me detonation causes this from the heads lifting. How do torqued heads lift? I drive this thing at 2000rpm at 70 other than around town shifting through the 5 gears. I have never hammered it more than a little rolling burst to break the tires loose in first and second. I did run it up to 3lbs of boost one time. The right side leaked from day one and the left about 500 miles later. Engine has 1500 miles on it. I had leaking teflon guide seals that I replaced with viton at 300 miles. When I pulled the heads all the pistons and combustion chambers were heavily coated with baked oil and carbon from those leaking seals. I cleaned all that off and everything looks brand new. No signs of damage anywhere. Where do I go from here?
Your problem is your cruise RPM. If you can move your cruise to 3,000+ rpm you'll be fine, if not you can't run locked timing. A heavy low rpm load and locked timing do not mix. If it were mine id fix the timing and keep the low rpm cruise, but either approach will work.
 
#5 · (Edited)
With all that oil and carbon deposits...no wonder it was detonation prone.You may want to consider a 2 bar map sensor that pulls some timing out when boost builds.
Fwiw..I run 22* initial and another 14* centrifugal all in at 2500 rpm.I pull a degree out for every lb of boost...about 29* at wot/6k rpm.
 
#13 ·
In my case..I only needed the machine shop to plate hone the block and balance the rotating assy.The rest was done by myself and came right in at just over $10k..carbs to pan.Things like Lunati pro mod rods/ati balancer and sfi flexplate for $700 when someone else fell on hard times.But you are correct..building a stout blown bbc is no place for a penny pincher.
 
#14 ·
This is the first time I paid a pro to build the engine. Always had machine work done and assembled myself. Spare no expense, the wife said, so I picked up the long block ready to go. I never thought I would be working on it constantly. I ran a blown 350 for 16 years and never had an issue. Go figure.
 
#19 ·
fatblock said:
In my case..I only needed the machine shop to plate hone the block and balance the rotating assy.The rest was done by myself and came right in at just over $10k..carbs to pan.Things like Lunati pro mod rods/ati balancer and sfi flexplate for $700 when someone else fell on hard times.But you are correct..building a stout blown bbc is no place for a penny pincher.
In my experience building ANY engine that will outrun anything at all isn't for penny pinchers.
 
#20 ·
I raced big blocks on gas and alcohol from late 60's through late 80's and had none of the issues I have had with this set up. I have never flogged this thing anywhere near the way I did in those days. At any rate it is up and running again and I'm moving on. Next time I'm going to supervise every detail and get it done locally. Very upsetting to see an engine builder make 15 plus 900hp pulls on a dyno and send $50k plus big blocks to offshore boaters and not believe he knows his stuff. Perhaps he does and my deal was a fluke. At any rate it is up and running again and I'm moving on. Next time I'm going to supervise every detail and get it done locally.
 
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