High gear timing retard

Discussion in 'PSI Superchargers Tech Questions' started by Senior moments, Jan 2, 2021.

  1. Senior moments

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    I run an alky sbc, all iron, dry, 10-71 blown dragster as follows: 10-71, cam = .629/,669 280/290 @ .050, 112 lobe sep, 10.5 static CR, 180 glide, MSD 7AL3 w/trigger, 1750 lb wet, 33 deg lead, shift @ 7500, traps at 8000, ET +/- 6:80 @ +/- 196 I want to pull some lead out at the top end to relieve detonation possibilities so my questions are: 1) given that I clear the traps at about 8000, at what RPM should I pull the timing back and 2): how many degrees should I pull out as a starting baseline? Any help would be appreciated. Thanks ITA
     
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    Last edited: Jan 2, 2021
  2. Bjs344

    Bjs344 Member

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    Is it hurting bearings?

    I used to pull 1 to 1.5 starting around 4 seconds to try to calm down a combo that was unhappy. I never figured out why it wanted to detonate, just eventually changed enough stuff that it quit. I don’t recall 1.5 out on the backhalf making a substantial improvement but I think it helped a little.

    When I finally changed enough parts to make it happy it went a lot faster (almost .2) without pushing oil and hurting bearings and didn’t need the timing out at the big end.
     
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  3. Senior moments

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    Last engine I ran I had 35 deg lead and when I tore it down the splayed mains were hammered. This engine I'm going to back down the lead to 32. I don't really care about picking up mph or ET...I just want the engine to be totally happy. 32 lead isn't by any means radical on an sbc, so maybe I just should leave everything alone
     
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  4. gordon tarbell

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    What are you running for an A/F ratio @ 7000 to 8000 rpm and what size blower hat ?
     
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  5. Senior moments

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    Do not have an AFR gauge. The hat is BAU tall 3 x 5" blades
     
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  6. Ron C

    Ron C Jr. Dragster

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    What does the spark plugs tell you. They have the complete story....
     
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  7. Senior moments

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    Made only 1 pass in 2020...hung a valve because of a tight guide, but the timing plug witness stripe was right on the money on 7 @32 lead... but I ran it real fat to get the fuel tune-up right, so we didn't even touch the cad. I think before I start messing with retard, I'm going to run it straight up at 32 until I get the plugs the right colour, then I can start to play with timing. I was only concerned about top end timing because on the old motor we hurt the caps and flattened the bearings...but I was running 35 degrees. 32 is more conservative, so I might just leave it alone and not try to re-invent the wheel. Won't know until we take some hits in Spring....if the tracks open up
     
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  8. turbo69camaro

    turbo69camaro Member

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    I ran blown SBC for a long time .My weak link with a iron block was the main caps would walk and transfer material.The bearings looked perfect you could scotch brite them and re run . This was with Chevy bowtie race and the olds race block.never fixed the transfer issue Just flat filed the hi spots off and ran it. I ran 32 max timing with a 14-71 @ 45 psi think with the 10 71 had 35 psi .Any more timing it would try to lift the heads and push the gaskets out .
     
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  9. Mike Canter

    Mike Canter Top Dragster
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    Both the fuel and the timing will change the position of the blue line on the plug ground strap because it is the temperature in the chamber that moves it. So if you advance the ignition you have to add fuel to compensate for the increase temperature from advancing the timing. For safety I pull out 3-4 degrees at .3 seconds before the finish line.
     
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  10. Senior moments

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    Completely agreed...
     
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  11. Senior moments

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    Right now the proposed plan....after I get the fuel balanced, to set lead @ 33 with shift @ 7500 and pull 3 deg out @ 7600 with a 7950 trap rpm. which is consistent +/- 50 rpm. Make sense?
     
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  12. lucky devil

    lucky devil Member

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    Hi Mike , what performance difference does this make ?
     
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  13. Senior moments

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    I don't believe there will be any performance advantage at all....either ET or mph. It's simply easier on the engine at WOT to take away timing that it doesn't need. It's just an engine longevity issue for me
     
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  14. Mike Canter

    Mike Canter Top Dragster
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    No advantage in performance just in safety. Most motors go right before the finish like so have always pulled out some timing to make sure no detonation. Right at the finish line is where the engine is hottest and really pulling hard
     
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