AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
- Joined
- Jan 19, 2014
- Messages
- 25,807
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Ted; No "fix the distributor" is not plugging the vacuum line.Did you reread post#5? This is the 3rd time I have referred you to it.The engine has many different timing requirements, depending mostly on LOAD and RPM. The two most important are POWER timing,and IDLE timing.Then are; tip-in, part throttle, and cruise.When we "fix" the dist., we take it apart, modify and adjust it so that it will address as many of those requirements as possible. Its not always possible with factory type dizzys to hit all the marks, and compromises will have to be made. It is possible to buy a dizzy that is more user-friendly than the stock Mopar one. Its also possible to purchase an adjustable electronic box to install between a locked out dizzy and the coil which more closely meets the engines needs. And finally, you can get a fully adjustable computerized box, that will get really close to ideal.It kinda depends on how much you want to spend or how perfect you want/need it to be, or how much you are able/willing to do yourself and how much time you have to get the job done.
-A typical scenario goes like this: the engine on the dyno with non-octane limited gas makes best hp&tq with 37*. But on the street with a lessor fuel starts to rattle at 34*.So right there is the first compromise. Next, a little experimentation, finds a best idle timing of 22*.But wait, like your dist., theres 20* in the dist. So 34-20=14; 14*is what we had to set the idle timing to get the 34* of power timing.This compromise can be changed. We need to take the dist. apart and modify it to get the 22* the engine wants, and the 34* it is limited to. Next on the list is tip-in and part throttle.I lump them together because they are somewhat inter-related.So lets say the dist has been modded to get the 34*power and the 22*idle timings. Next we will have to figure out how early/late the engine will accept timing increases.The more quickly and accurately you give the engine what it wants, the nicer it will respond to load increases; until you get rattle.Rattle is not allowed.So for this theoretical engine, it might accept a timing rate increase of 12* in 2000 rpm.So if it idles at 1000rpm, then by 3000 it will accept the 22* initial plus the 12* equals 34*.This is typically called the "all-in timing". It might also be that this theoretical engine would accept 12* in 1000rpm, or 12* in 500 rpm. In every case the dist would need to be modded to make those numbers. Are you understanding this yet? Reread as many times as you need to, to kinda get it.
-So now, lets say the dist. has been modified, to deliver the correct power and idle timing, and rate of advance. By this time the engine will be operating pretty good already.And on a track-only engine, which pretty much operates on only two parameters; namely WOT and idle, you could pretty much run it as is. Now a street-engine, spends most of its life in the lower rpms, and load settings, and is saddled with street-friendly gears, and converters, and spends a very significant time at idle, or lightly accelerating,and not to forget; cruising. This is a whole different world.This is the world where the vacuum advance lives and works.It is also the place where the most compromises have to be made.
-For instance, going back to the theoretical engine as above, the power timing was limited to 34*. And light throttle vacuum peaks at 2500 rpm.So it will want to cruise around there. 2500rpm happens to be about 60mph with a tire that rolls out at 83inches(just like your 14 inchers) and 3.23 gears.So thats pretty good. So you take it for a cruise with vacuum advance disconnected, and find the fuel useage unacceptably low. The engine wants a ton of more timing to boost its cruise efficiency.It may want 50 to 60 degrees, at 2500rpm.So you put a timing lite on it, and rev it to 2500, and see something like 28*.We are only half way there(to 55*). What to do? Well all vintage factory "cans"(vacuum advance units) have a number stamped on the actuating arm, ranging from about 8 to 20.( thats all Ive seen). These numbers indicate the number of degrees the can is able to advance. So you determine that you have the 20* one. That means this can is able to add 20* to the 28* the theoretical engine currently has at 2500. Doing the math, thats a total of 48*. Right there is another compromise.We are about 7* short. Not a huge problem,for cruising, unless you are planning a cross country trip.It does, however, also compromise all low rpm/low load operations.So we are stuck with that can.Unless you modify it, which to a degree, can be done.But lets leave it for now.
-This can also serves another purpose. It has an adjustment screw down in the vacuum nipple, that can be adjusted to alter the rate of advance, proportional to the amount of applied vacuum.If you set it on the soft side, it will begin to bring the advance in earlier,at a lower engine vacuum.If you set it on the hard side it will take more vacuum to get it to begin advancing.Knowing this,we can now experiment a bit.Also remember that the can only works at light loads/ higher vacuum numbers. Its possible to map the can.(might get into that later).Its also possible for it to operate at higher rpms/light loads.For most applications though, it should NOT operate at idle.
-The optimum operation of this device can only be determined by trial and error testing, often involving a great deal of time.As it gets closer to optimum there will be much improvement; in tip-in, as will as low rpm cruise quality, and of course, fuel consumption.
-As camshaft durations get bigger,the low rpm engine vacuum generally suffers.And the compromises are more numerous.The cam in your engine is probably bigger than most guys would be willing to run, except maybe as a weekend bomber.I ran a Mopar 292/509 cam, for a bit, with 1.6 rockers.I measured it as 239*@.050 intake and about .540 lift. I ran it just long enough to get the tune done. Then a neighbor kid liked it so much he wanted it.I sold it without much convincing. Then I switched to a 223*@.050 cam.I loved that cam in my daily driver. When it died(due to zinc phase-out), I switched to a 230*@.050 cam.Its ok I guess.
-So thats the end of todays lesson.What an amazing device.
-A typical scenario goes like this: the engine on the dyno with non-octane limited gas makes best hp&tq with 37*. But on the street with a lessor fuel starts to rattle at 34*.So right there is the first compromise. Next, a little experimentation, finds a best idle timing of 22*.But wait, like your dist., theres 20* in the dist. So 34-20=14; 14*is what we had to set the idle timing to get the 34* of power timing.This compromise can be changed. We need to take the dist. apart and modify it to get the 22* the engine wants, and the 34* it is limited to. Next on the list is tip-in and part throttle.I lump them together because they are somewhat inter-related.So lets say the dist has been modded to get the 34*power and the 22*idle timings. Next we will have to figure out how early/late the engine will accept timing increases.The more quickly and accurately you give the engine what it wants, the nicer it will respond to load increases; until you get rattle.Rattle is not allowed.So for this theoretical engine, it might accept a timing rate increase of 12* in 2000 rpm.So if it idles at 1000rpm, then by 3000 it will accept the 22* initial plus the 12* equals 34*.This is typically called the "all-in timing". It might also be that this theoretical engine would accept 12* in 1000rpm, or 12* in 500 rpm. In every case the dist would need to be modded to make those numbers. Are you understanding this yet? Reread as many times as you need to, to kinda get it.
-So now, lets say the dist. has been modified, to deliver the correct power and idle timing, and rate of advance. By this time the engine will be operating pretty good already.And on a track-only engine, which pretty much operates on only two parameters; namely WOT and idle, you could pretty much run it as is. Now a street-engine, spends most of its life in the lower rpms, and load settings, and is saddled with street-friendly gears, and converters, and spends a very significant time at idle, or lightly accelerating,and not to forget; cruising. This is a whole different world.This is the world where the vacuum advance lives and works.It is also the place where the most compromises have to be made.
-For instance, going back to the theoretical engine as above, the power timing was limited to 34*. And light throttle vacuum peaks at 2500 rpm.So it will want to cruise around there. 2500rpm happens to be about 60mph with a tire that rolls out at 83inches(just like your 14 inchers) and 3.23 gears.So thats pretty good. So you take it for a cruise with vacuum advance disconnected, and find the fuel useage unacceptably low. The engine wants a ton of more timing to boost its cruise efficiency.It may want 50 to 60 degrees, at 2500rpm.So you put a timing lite on it, and rev it to 2500, and see something like 28*.We are only half way there(to 55*). What to do? Well all vintage factory "cans"(vacuum advance units) have a number stamped on the actuating arm, ranging from about 8 to 20.( thats all Ive seen). These numbers indicate the number of degrees the can is able to advance. So you determine that you have the 20* one. That means this can is able to add 20* to the 28* the theoretical engine currently has at 2500. Doing the math, thats a total of 48*. Right there is another compromise.We are about 7* short. Not a huge problem,for cruising, unless you are planning a cross country trip.It does, however, also compromise all low rpm/low load operations.So we are stuck with that can.Unless you modify it, which to a degree, can be done.But lets leave it for now.
-This can also serves another purpose. It has an adjustment screw down in the vacuum nipple, that can be adjusted to alter the rate of advance, proportional to the amount of applied vacuum.If you set it on the soft side, it will begin to bring the advance in earlier,at a lower engine vacuum.If you set it on the hard side it will take more vacuum to get it to begin advancing.Knowing this,we can now experiment a bit.Also remember that the can only works at light loads/ higher vacuum numbers. Its possible to map the can.(might get into that later).Its also possible for it to operate at higher rpms/light loads.For most applications though, it should NOT operate at idle.
-The optimum operation of this device can only be determined by trial and error testing, often involving a great deal of time.As it gets closer to optimum there will be much improvement; in tip-in, as will as low rpm cruise quality, and of course, fuel consumption.
-As camshaft durations get bigger,the low rpm engine vacuum generally suffers.And the compromises are more numerous.The cam in your engine is probably bigger than most guys would be willing to run, except maybe as a weekend bomber.I ran a Mopar 292/509 cam, for a bit, with 1.6 rockers.I measured it as 239*@.050 intake and about .540 lift. I ran it just long enough to get the tune done. Then a neighbor kid liked it so much he wanted it.I sold it without much convincing. Then I switched to a 223*@.050 cam.I loved that cam in my daily driver. When it died(due to zinc phase-out), I switched to a 230*@.050 cam.Its ok I guess.
-So thats the end of todays lesson.What an amazing device.
