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Old 02-10-2012, 11:51 AM
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Can locked timing cause detonation?

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?

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Old 02-10-2012, 12:25 PM
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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.
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Old 02-10-2012, 12:25 PM
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Quote:
Originally Posted by Blown496
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.
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Old 02-10-2012, 02:01 PM
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What kind of curve do you recommend? I have all MSD ignition. I did replaced head gaskets with felpro 10171's that I have had good luck with before on other blower engines.
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Old 02-10-2012, 02:58 PM
8-71 blown 461 Rat on E85
 
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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.

Last edited by fatblock; 02-10-2012 at 03:04 PM.
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Old 02-10-2012, 03:20 PM
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Thanks. Sounds like a 6btm may be my next item. My engine builder said lock 32 and don't look back. I've never used locked timing and a lot of low compression blower guys like the locked over a curve. Decisions, decisions.....
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Old 02-10-2012, 03:31 PM
8-71 blown 461 Rat on E85
 
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Its a street deal...why not enjoy some added advance for cruising and let the map retard feature act like a vac can on a n/a deal.Out of boost..the added timing will aid your mpg.This is not an option if your locked out.
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Old 02-10-2012, 03:46 PM
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Who makes a map retard for my MSD application? I have wide band on it.
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Old 02-10-2012, 03:54 PM
8-71 blown 461 Rat on E85
 
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Hopefully the msd crowd chimes in..I run the crane hi6r with their 2 bar map sensor and in car timing controller.

As for the wideband..mines a heated o2 from aem.Mimics changes to the tune up reliably..just not at idle with a 246/254* @.050" cam at 7.5/1 static cr.

Last edited by fatblock; 02-10-2012 at 03:59 PM.
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Old 02-10-2012, 04:07 PM
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As far as I know msd makes two devices to retard timing. I have talked to them and 6btm is my choice. It is a little under $400.
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Old 02-10-2012, 04:13 PM
8-71 blown 461 Rat on E85
 
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Cheap insurance to keep a $10k blown bbc away from the death rattle,imo.
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Old 02-10-2012, 04:39 PM
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And 10k is a low estimate!
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Old 02-10-2012, 05:29 PM
8-71 blown 461 Rat on E85
 
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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.
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Old 02-10-2012, 05:42 PM
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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.
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Old 02-10-2012, 05:44 PM
8-71 blown 461 Rat on E85
 
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The short block after assy.
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