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Official timing survey

Discussion in 'The Garage' started by 4DiggerDan, Mar 10, 2003.

  1. 4DiggerDan

    4DiggerDan 1/2 ton status

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    Just curious how everybodies timing is? I was just wondering a few things about ya'all's advance curve

    What do you set your initial at?

    Do you run your vacuum advance off of throttle vacuum or manifold vacuum?

    What distributor are you running? (stock HEI, Summit HEI, Davis Unified, etc...)

    How much total timing are your running?

    Where does your advance come in?

    Where does your advance top out?

    How much is from vacuum advance, and how much from mechanical?

    For example. In my 400 SB in my truck now (which is on its last leg, but I don't care because I'm building a roller cam 383 to stick in my new Blazer very soon) I run 10 initial, am hooked to the throttle vacuum, at 1350 RPM my advance curve climbs rapidly to 38 total at 2800 RPM, then slowly climbs to 43 total at 4300 RPMS. After that, it goes nowhere... This is with a stock HEI with components from several different manufacturers. Just in case you're curious, I'd like to see all that advance come in at 2800 and stay there, but with all the dinking around I've done, I can't make it happen.

    I just want to get an idea of what you guys are doing for timing in your Blazers and trucks strictly for informational purposes. I've been doing some studying on timing (AGAIN!!)

    Anybody that doesn't know... Answer too. I want to know just how many people are clueless as well as the ones that do know.
     
  2. MJB774X

    MJB774X 1/2 ton status

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    I set my timing at 10 with the vac hooked up at idle, mine starts advancing right away as soon as give it throttle, it tops at about 35 total around 3300 rpm and stays at that above 3300, I always have a hard time tuning dist when motor has a cam other then stock, things change dramatically because vacume is completely different, tri playing with your springs and get an adjustable diaphram, or just go with an msd with a box you can tune it how ever you want
     
  3. option#9

    option#9 Registered Member

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    Would you mind going into a little more detail on timing an aftermarket Cam MJB (or anyone)? I just bought a built 454, not really sure what's in it and it's my first built so I'm a little lost with the whole thing. I've been playing around with the timing for the last few days and just havn't been happy with it. It seems like it's just way to far advanced at an idle. It has an MSD 6AL/Blaster 2/MSD Pro-Billet with vac advance.

    It's going to sit in my driveway for quite a while it looks like until I can get the whole thing figured out. Sure does sound good though.
     
  4. MJB774X

    MJB774X 1/2 ton status

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    I'm no expert, like I say you just have to play with it, if you find out what cam is in it you can probably go and ask your local machine shop for there suggestion, guys who build motors understand all different characteristics and could point you in the right direction , last motor that I messed with that had a cam, had like a 292 cam with .505 lift,it was in a nova with a 4 speed, it also had an msd pro billet with the 6al digital box, that sh*T was to technical for my blood I'm used to working on simple stuff so I put the whole engine together but I couldn't figure out how to set the timing, maybe if I read the directions /forums/images/graemlins/rolleyes.gif but I hate to read /forums/images/graemlins/1zhelp.gif
     
  5. HarryH3

    HarryH3 1 ton status Author

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    43 degrees of total timing sounds a tad on the high side. When the hot rod mags do dyno tests on engines, it seems that they start losing power once the timing goes past 36-38 degrees total, so they bring it back to that range for best performance. They typically are using much hotter cams than what are run in most 4WD's, so that could make a difference in the outcome. But since my "dyno" is my butt and a stopwatch, it's difficult to test power gains at the high end of the RPM range. /forums/images/graemlins/eek.gif
     
  6. Lonnie

    Lonnie 1/2 ton status

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    There are many variables to have a correct answer.... but here are general guidelines. These are simplified as most do not have distributor machines, but still effective if done this way.

    First never check timing with the vac line connected... this has little to do with full throttle timing.

    Factory is generally 5-8 deg, but conservative. The bigger the cam, carb & ports, the less low end cylinder pressure is generated & will require more timing. Most near stock small blocks like 10-12 initial & 34-36 total.

    Proper initial timing will allow good throttle response while still allowing it to start when hot & no pinging off idle. If you have to pass emissions, remember extra timing will often increase your readings. I recommend a good aftermarket curve kit with a spring assortment. Try weaker springs first (weights should retract totally by themselves when rotating by hand for consistent timing) if not go stiffer. After finding where it idles happily (with vac line off) check total timing at approx 4000 rpm.... it should be under 38. If not back it off some to correct total timing & recheck your initial. Drive & check for full throttle pinging & adjust as necessary to eliminate. If pinging off idle use less initial, up high less total, midrange (under 3-3500) a stiffer spring is needed to slow advance curve. If no pinging, advance 2 deg. & retest, also try 2 deg less to see what it likes for best power. Only experimentation will give you the best results. If it likes more initial but the total is too high, you can limit the advance by adjusting the thickness of the sleeve on the stop pin in the distributor under the rotor. A thicker sleeve on this pin will give less total timing. Once you have your full throttle power at its best, move on to vac advance.

    Hook vac advance to manifold vacuum (not ported)& see how much timing is available at idle (you may be surprised to see 40) Vac advance helps mileage & part throttle response, but has no effect on full throttle. If idle falters when connecting vacuum, it has too much vac advance. You can get adjustable vac units & also a limiter to correct the total vac advance. If it pings or surges at part throttle reduce the vac advance. Remember turning the distributor back will hurt your full throttle performance from previous testing. Some engines (very few) will not require vacuum advance after these modifications.

    Hope this helps. Start conservative & reduce timing at any time you hear pinging.... do not drive it this way or engine damage can result.
     

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