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  • Throw a Mean Curve-Reposted

    Thanks to Scott Allard for saving this for me and helping me repost it.

    A subject that has been of interest for some time is a distributor curve that is optimum for a given combination. I would like to open up a discussion here on this topic with an old school method that has been successful for me for many years. This is not complicated, and it is not one size fits all. There are many combinations that have a different need. Here are some generalities:

    1. A heavy vehicle needs a slower advance curve. Heavy means, "perceived" weight, which varies by displacement and many other factors. But the load dictates the curve. This can be a 3500 lb car with a 2.75 rear gear, or a 4000 lb car with 3.25s. A lighter vehicle allows a quicker, more aggressive curve, again the load perceived is the key. A 3000 lb car with 4.56 gears can lock out the curve to the max, where a 5000 lb truck with the same gears will need the timing broken up.

    2. Give the engine what it wants. If your compression ratio is too high, you need better gas. The timing can be retarded, but you can cause more problems doing this because retarded timing raises exhaust gas temperatures. You can crack heads, break ring lands and rings doing this.

    3. You can determine the initial timing with a vacuum gauge . BTW, confirm TDC and use the proper timing tape for your balancer diameter, dial backs are OK in a pinch but I have found them to vary. Disconnect the vacuum advance and block the port even if you are using ported vacuum (and you should be). Advance the distributor until you get the highest steadiest vacuum reading and write it down.

    3. Most wedge engines like 36-40 total depending on certain factors like chamber shape, piston speed, effective squish and plug location. In this example I am going to say 36 total is the optimum. You found that you need 18 to get the most vacuum you can get. So, you need to limit the centrifugal timing to 18. This can be done on a GM or MSD by swapping the centrifugal bushing in the advance system. On a stock Ford you have to weld the slot up that the advance pin is in and grind it to fit, or find a reluctor advance as close as possible to your desired timing. I measure the slot with a caliper, calculate length to timing ratio and size the slot accordingly. Ford arms will have a number with an L suffix on each side, i.e, 10L on one side and a 15L on the other. These numbers mean distributor degrees, see #4. Also, if you flip the reluctor from one slot to the other on a stock Ford, you have to re stab the distributor or your timing will be 180 out because the rotor is engaged to reference the side that represents #1 . The MSD Ford uses the same system as the GM, so the bushing is changed.

    4. Keep in mind that the distributor runs at half speed to the crank. The change that you make in the distributor will be half of what you want to see on the balancer The 10L-15L nomenclature above is an example. These will yield 20 and 30 respectively at the balancer. The 10L in our example is too much for 36, but would yield 38 total. If the engine will accept it, it will work, but if it really needs 36, you need to fill the slot. Generally, since it would very rare to use the 15L side (30 centrifugal) I weld it in (or I used to, I can't see anymore well enough, so I have someone do it for me) and grind it to fit. The advantage of this is that if you find yourself wanting 38 total, you can pull the dizzy, swap the reluctor arm to the 10L side and you have it. On the GM and MSD style, various bushings are available to fit the need and are quite a bit easier to swap.

    5. The rate at which you bring the timing in is important. I like to get it all in as quickly as it will tolerate it, but 2500-3000 rpms will fit most needs. You change this by changing the springs that control the advance rate. Lighter is quicker, and heavier is slower. Weight of the car, gear, converter will dictate this. You do not want detonation. After a change, pull the plugs and look for any shiny specks or uneven burn patterns on the plug. Even if you can't hear a ping, you can still detonate.

    6. Vacuum advance. Not necessary on a race engine or one with the timing locked, but mileage and drive ability are important on the street. Hook it up, chock the wheels, and steadily rev the engine up slowly watching the marks on the balancer until it stops advancing. You should not see over 50 or you need to limit the vacuum advance travel. On old Fords you can washer them on the inside of housing the spring outboard of your spacer. Other cans are adjustable with an allen wrench. See your directions with the cannister to see what you need to do because they vary. The rate that it drops out under a light pull is adjustable too. Whether it is the allen wrench or the Ford early style that unscrews you can tailor the advance to drop out under light to moderate load. You don't want ping. Check the plugs again after a drive to look for evidence of detonation.

    There you have it. Distributor tuning remedial grade. I hope this helps.









  • #2

    Re: Throw a Mean Curve-Reposted

    Re: Throwing a mean curve

    That is some awesome information :-) I have a question for you about vacuum advance. The distributor, I have in my street/racecar does not have a vacuum advance. What is the mileage and drivability difference? It seems like no matter how I drive I get between 10.5 and 11 miles a gallon. That seems really low in a small block Ford powered 2300 pound car with a T5 with .62:1 overdrive ratio and a 3.55 rear and 17 inch wheels. I was curious if this low consistent reading was due to the vacuum advance not being on there.
    Great post by the way!

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    • #3

      Re: Throw a Mean Curve-Reposted

      Re: Throwing a mean curve

      Excellent tech bit Bill.

      Vacuum advance really shines at part throttle or light load conditions like cruising around town in stop and go traffic. I agree that in these cases you could see a vast fuel mileage improvement as you can make more power down low with less right foot to get you moving.

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      • #4

        Re: Throw a Mean Curve-Reposted

        Re: Throwing a mean curve

        Excellent write up!

        To find my total vacuum advance I've always plugged a manifold vacuum source in (just for the test) at idle to see how far advance it goes above initial advance. Knowing what mechanical advance max's out at and adding the what was gotten from the vacuum advance test will tell me the total. I just don't like revving high torque engines against wheel chocks ;) Does that sound like a reasonable method?

        For the vacuum advance I've used the test and tune method. Go for a steady cruise at a constant speed (my case 70mph) and see where the vacuum needle sits. Pull over, adjust the vacuum advance a half turn one way or another and run down that same road at the same speed and note any positive or negative changes (going for higher vacuum at the same rpm/speed/road). I recommend the same road in the same direction unless you know it's flat both ways, slight hard to notice inclines will effect the vacuum.
        Escaped on a technicality.

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        • #5

          Re: Throw a Mean Curve-Reposted

          Re: Throwing a mean curve

          3. Disconnect the vacuum advance and block the port even if you are using ported vacuum (and you should be).


          6. Vacuum advance. Not necessary on a race engine or one with the timing locked, but mileage and drive ability are important on the street. Hook it up, chock the wheels, and steadily rev the engine up slowly watching the marks on the balancer until it stops advancing. You should not see over 50 or you need to limit the vacuum advance travel. On old Fords you can washer them on the inside of housing the spring outboard of your spacer. Other cans are adjustable with an allen wrench. See your directions with the cannister to see what you need to do because they vary. The rate that it drops out under a light pull is adjustable too. Whether it is the allen wrench or the Ford early style that unscrews you can tailor the advance to drop out under light to moderate load. You don't want ping. Check the plugs again after a drive to look for evidence of detonation.
          Nice write up, thanks!

          On the subject of vacuum advance at idle....I've heard several others say that they have better results with non-ported advance, and that ported advance is mostly an emissions thing that the carmakers used to meet federal requirements.

          I know a lot of hot rods have cooling issues when idling in traffic, seems having more advance could help this situation. What are your thoughts on this?

          My fabulous web page

          "If it don't go, chrome it!" --Stroker McGurk

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          • #6

            Re: Throw a Mean Curve-Reposted

            Re: Throwing a mean curve

            Using straight manifold vacuum to run the vacuum advance, especially with any kind of performance cam can lead to erratic timing down low and some hunting and surging.

            Comment


            • #7

              Re: Throw a Mean Curve-Reposted

              Re: Anecdotal BS that has no place here

              falcon - something is wrong - you're at least 3-4 mpg from where you should be.

              start with the basics - tire pressure, wheel alignment, brakes - anything dragging? can you push the car easily in neutral on flat ground?


              after the basics - does anyone you know have an a/f meter you could borrow?

              I suspect that if it is not a simple rolling resistance issue - it's running rich ... ALL the time.
              There's always something new to learn.

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              • #8

                Re: Throw a Mean Curve-Reposted

                Re: Anecdotal BS that has no place here

                What happened here????
                Escaped on a technicality.

                Comment


                • #9

                  Re: Throw a Mean Curve-Reposted

                  Re: Anecdotal BS that has no place here

                  apparently someone pissed off Bill and he edited/deleted all his posts? huh....
                  My fabulous web page

                  "If it don't go, chrome it!" --Stroker McGurk

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                  • #10

                    Re: Throw a Mean Curve-Reposted

                    Re: Anecdotal BS that has no place here

                    ???
                    i appreciate all knowledge, tips and shortcuts whether learned on a farm or in engineering school. as long as it's correct.
                    shoot, i'll take welding tutorials...there are a lot of good welders out there.

                    Comment


                    • #11

                      Re: Throw a Mean Curve-Reposted

                      Re: Anecdotal BS that has no place here

                      apparently someone pissed off Bill and he edited/deleted all his posts? huh....
                      I don't get it. His account is gone too. I was planning on rereading his post on timing tonight.
                      BS'er formally known as Rebeldryver

                      Resident Instigator

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                      • #12

                        Re: Throw a Mean Curve-Reposted

                        Re: Anecdotal BS that has no place here

                        That was wierd. ???

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                        • #13

                          Re: Throw a Mean Curve-Reposted

                          Re: Anecdotal BS that has no place here

                          i think Bills account has been hijacked....
                          Never kick a fresh Turd on a hot day.....Harry S Truman

                          Comment


                          • #14

                            Re: Throw a Mean Curve-Reposted

                            Re: Anecdotal BS that has no place here

                            Bill -
                            Sounds like you feel disrespected in some way. I'm sorry that happened to you. I can assure you that virtually ALL of us highly respect your skill and knowledge. If some Bozo blew you off, that's his/her loss.

                            As far as dynos - I spent most of my working life with them, and they are just tools, like a wrench or a screwdriver. They are of no use at all without someone who knows what to do with them. You are exactly the kind of person who knows what to do with one of these things. I'm pretty much useless - I know all about dynos and how to run them, but no idea of what to adjust to make the engine/vehicle on the dyno work better - that wasn't my job.

                            Again, I'm sorry if you feel unvalued or unappreciated on this forum. Most of us would love to have you here and value your input.
                            Dan

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                            • #15

                              Re: Throw a Mean Curve-Reposted

                              Re: Anecdotal BS that has no place here

                              Thanks Dan, you expressed my sentiments too.

                              My fabulous web page

                              "If it don't go, chrome it!" --Stroker McGurk

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