joshua dewitt
Well-Known Member
ok is it better to run factory vacuum advance off full manifold vacuum or ported manifold vacuum? mp 340 cam in 318 9.5:1 compression... old Chevy's like ported, but what about the mopars?
distributor vacuum
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Distributor vacuum advance is to be run off ported vacuum.
If not, you will have all of vacuum advance kicked in at idle...
If not, you will have all of vacuum advance kicked in at idle...
67Dart273
Well-Known Member
Ported is the way I prefer it, that is the way I've always done it, and that is the way all Mopars I've ever owned did it.
HOWEVER you can use manifold vacuum, but you need to use a different strategy. The guy at FBO insists there is no other way than manifold, or at least that's how I took it.
And, there most certainly has been cars which DID use manifold vacuum. Please don't ask me to enumerate them.
HOWEVER you can use manifold vacuum, but you need to use a different strategy. The guy at FBO insists there is no other way than manifold, or at least that's how I took it.
And, there most certainly has been cars which DID use manifold vacuum. Please don't ask me to enumerate them.
AJ/FormS
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While it's possible to use full-time vacuum advance to good effect on a big-cam engine; it makes no sense to do so on a streeter. A well-tuned advance system, including the can on the timed port, can tame a sbm with a 250*@050 cam.(biggest I've tuned). I wouldn't run a streeter without it, and I have best results plumbing it to the sparkport(timed).
NukeBass
Well-Known Member
He wrote an entire book about that. It's sitting in my files somewhere. This seems to be one of those big internet topics, too, where everyone has an answer and people doing it the other way are always wrong.
I think whichever way you go, you just have to tune for that way. The factory used ported, plus, it leaves the manifold vacuum port on your carburetor available for something else (that's how I do it. Just figure out which port is open and use it
)
jbc426
Well-Known Member
I've tried both ways on a handful of motors, all with mild to moderate cams. I now prefer manifold vacuum for timing advance. I later read the explanation on FBO's site. It's can be made to work either way.
Using manifold vacuum does increase your timing at idle, and it does falls away as you open the throttle (and vacuum drops) reducing the incident of detonation under load. It also improves off idle performance as a result of having more ignition timing out of the gate. However, it also increases NO2 output, which is why they used to add that little retrofit NOX delay box on late 60's and early 70's ignition and retard your initial timing. This clearly degraded engine performance and fuel mileage. I prefer performance increases.
This manifold vacuum timing strategy can be used when used with a properly set-up distributor to significantly increase off idle torque, drivability and increased fuel mileage.
Using manifold vacuum does increase your timing at idle, and it does falls away as you open the throttle (and vacuum drops) reducing the incident of detonation under load. It also improves off idle performance as a result of having more ignition timing out of the gate. However, it also increases NO2 output, which is why they used to add that little retrofit NOX delay box on late 60's and early 70's ignition and retard your initial timing. This clearly degraded engine performance and fuel mileage. I prefer performance increases.
This manifold vacuum timing strategy can be used when used with a properly set-up distributor to significantly increase off idle torque, drivability and increased fuel mileage.
Rapid Robert
Well-Known Member
I would try ported first & see how it goes. I would set the initial with the vac gauge method then shorten the slots to get 35 total (initial amt you determined +slots), either weld em up or the 25 dollar FBO plate or make one) then toss the heavy spring with the elongated loop on one end & sub in a mr gasket or MP light spring in its place for a start. mix and match the spring combo from there so the advance starts no less than 200 RPM above what your hot idle speed is (& that's why you need substantial initial so you can get the lowest idle speed+other reasons) and you want to stay just under the pinging point at WOT up thru the gears on your hottest/driest (most likely to ping) day & you want a cushion there cuz gas is crap & ain't getting any better (not in this lifetime) and quality can vary from the refinery and there is silent ping so need a cushion back from where it does ping. with the elongated loop spring out of there the weights adv in a linear fashion no matter what combo you use. Ace hardware has springs and you can grind the tips of the weights to slow the curve rate also. then plug in the can & same thing, want to stay under that pinging point in everday driving under varying load/rpm condtions on your hottest/driest day. CCW with the 3/32" allen wrench slows the curve but does not alter the total amt the can offers. You can epoxy strips of metal on the back side of the can on either side of the arm to limit how far the arm goes into the can before the notch "catches". No exp with manifold but I wanna try it. When I get around to it I'll just plug in the can to manifold & see how the eng likes the addit'l ~22 deg of timing at idle (can with 11 stamped on the arm). if all good then it might be good as is cuz off idle everything is the same so if I dont have to alter the initial then that is an easy deal. If it wont handle it then there's some changes to be done. I did read a borrowed copy of the FBO tuning book. (just started/no opinion yet) but is does have SOME good info but not worth its cost imnho. short version: in order: initial/total/springs/vac adv
mderoy340
Well-Known Member
I have my dizzy setup for manifold. I use the spring tension adjuster in the can to set when the vacuum advance starts and have added an adjustable max advance stop (see pic).
My motor with a [email protected] cam likes a lot of initial advance, cleaner idle and crisp throttle response. Full vacuum port allows this setup to work and I'm still able to heat soak start with no starter kick back.
My motor with a [email protected] cam likes a lot of initial advance, cleaner idle and crisp throttle response. Full vacuum port allows this setup to work and I'm still able to heat soak start with no starter kick back.
tanis4457
Well-Known Member
I'm still trying to dial in my timing as well, I have a 71 360 with I assume a stock bottom end and a semi lumpy cam (don't know the exact specs-but it's not stock) I dialed in the motor for 20* initial and 34* mechanical at 2500rpm and hooked it up to ported vacuum.
When I tried setting the initial higher than 20*, I would get starter kick back but the motor seemed to like it as it was idling even smoother than it was (not that it wasn't smooth already) and the vacuum kept increasing as I was increasing the initial timing.
Is the best way to set initial timing to set for highest vacuum at idle then back off 1-2*?
If that's the case maybe I need to run manifold vacuum so I can get more initial timing and adjust my mechanical advance to compensate for load changes at different throttle positions.
When I tried setting the initial higher than 20*, I would get starter kick back but the motor seemed to like it as it was idling even smoother than it was (not that it wasn't smooth already) and the vacuum kept increasing as I was increasing the initial timing.
Is the best way to set initial timing to set for highest vacuum at idle then back off 1-2*?
If that's the case maybe I need to run manifold vacuum so I can get more initial timing and adjust my mechanical advance to compensate for load changes at different throttle positions.
AJ/FormS
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tanis
Thats what they say.
But here's how that might work. Say you have the initial at 20* and the power-timing of 34*@2500, just as you say.That might work out so the mechanical is coming in at a rate of 1* per100rpm and starts at 1100. Suppose you have a 20* Vcan, and it's all in by 13inches.
So it idles at 20*. You step on it a little, and the semi-lumpy cam starts to build vacuum. Say it reaches 13 inches by 1300rpm. Now you have the initial 20, plus 2*centrifugal, plus 20vacuum =42*.
So you rev it up to 1800; now you have 20 plus 7 plus 20v +47.
So next you rev it up to 2500; now you have 20 plus 14 plus 20v +54*
Now some engines like this kind of curve and some don't. You can adjust the Vcan to delay the Vcans all-in point, a little bit, but by 2000 to 2200 most street cams have vacuum-peaked well past the all-in point.
Ok so lets start making the engine work. Say you are cruising in 2nd gear @2500, so starting from 54* total. With 3.55s and a 904 you might be doing 38mph. So let's roll in to it.So you press gently, and the timing stays at 54, and maybe you hear the dreaded spark-knock. Let's say you don't. Slow back down to 2500, and step a little harder, say hard enough to open the powervalve @11inches vacuum. Your vcan might drop out at a rate of 2* per inch, so at 11 inches it drops out 4* . So now your timing will be 54 - 4 = 50*. This will for sure knock. So you will have to press harder, to drop more out, but now the secondaries are opening and knock is still evident. So you floor it and all the Vcan timing drops out and you are sitting at 34* phew!
So what to do. Something has to be done.
Most sbms I have worked on are fine with 20initial, and are fine with 34* powertiming, and fine with 20 in the can. And are even fine with 54* at cruising speed. I have seen some like 60plus degrees at cruising speed.The problem is blending these 3 to make it driveable.
Well I really like to tune with the Vcan, cuz it let's me make some decent mpgs;so I bring that 20 to 22 degrees in as quick as possible. If you have a high cr the engine likes that. So for me I have to delay the power timing somewhat and usually start with a little less initial. If you have a slightly loose TC that stalls at 1800, we can easily sacrifice a bit of timing below 1800 and never even miss it. So lets back up the initial to 14*. We need the powertiming to be 34*, but let's delay it to 3000rpm, and let's say it starts coming in at 1000rpm. So the centrifugal curve will be 20* and 2000 rpm;that makes, 1* per 100rpm. And the can will stay as it was;20*,and dropping 2*per inch, starting at 13inches.
So idle timing will be 14*. Again at 1300/and over 13 inches vacuum, the timing will be 14 + 3 +20v =37. Thats just 5* less than before, and no detonation. At 1800 light throttle, the timing will be 14 + 8 + 20v = 42, and all quiet. At 2500 it will be 14+15+20v=49*. By 3000rpm the total will be again 54*. So far so good.
Now again lets roll into it and by 11 inches of vacuum @2500(same as before),timing will be 14+15+16v=45*. This is again 5 degrees better. And when you floor it, all the vacuum timing drops out and,you still have the 34 by 3000rpm .
This 45*Part Throttle timing at 11inches of vacuum may still be too high, but,with a little tuning of the Vcan, you might find that the rattle disappears.
If you value MPGs, you have to give the engine some serious cruise-timing.
Long story short; every engine is different.It's up to you to figure out what she wants, and when she wants it, and under what conditions. Then, since the dizzy is not programmable, you have to try and hit as many marks as you can. Since the powertiming is more or less fixed, one or both of the other two(idle and cruise timings,depending on which you value more),may have to be compromised. It's rarely possible to hit them all perfectly. I compromised the idle timing, and set mine to 12/14. And I run powertiming of 32 /34(alum. heads), but it's a two stage with a kink in it's rate, at 2800. It brings in about 16* by 2800 and then slows down bringing in another 3*or4* by 3200/3400.Then I run a 22* Vcan, bringing it in as fast as it is able. And since it still is not enough cruise timing(there's the compromise again), I installed a Jacobs Optotimer which allows up to 15 additional degrees of timing, depending on how it get's set up.While it's theoretically possible to run a total cruise timing of 14+20+22v+15timer= 71, I have never set it up that high. I typically run 14+12+22v+8timer= 56*of cruise timing at 2240rpm/65mph.
I know, clear as mud.
If your pistons are down in the hole., and you have no squish/quench, this may all be so much mumbo-jumbo, as low-cr engines have totally different timing requirements.
Thats what they say.
But here's how that might work. Say you have the initial at 20* and the power-timing of 34*@2500, just as you say.That might work out so the mechanical is coming in at a rate of 1* per100rpm and starts at 1100. Suppose you have a 20* Vcan, and it's all in by 13inches.
So it idles at 20*. You step on it a little, and the semi-lumpy cam starts to build vacuum. Say it reaches 13 inches by 1300rpm. Now you have the initial 20, plus 2*centrifugal, plus 20vacuum =42*.
So you rev it up to 1800; now you have 20 plus 7 plus 20v +47.
So next you rev it up to 2500; now you have 20 plus 14 plus 20v +54*
Now some engines like this kind of curve and some don't. You can adjust the Vcan to delay the Vcans all-in point, a little bit, but by 2000 to 2200 most street cams have vacuum-peaked well past the all-in point.
Ok so lets start making the engine work. Say you are cruising in 2nd gear @2500, so starting from 54* total. With 3.55s and a 904 you might be doing 38mph. So let's roll in to it.So you press gently, and the timing stays at 54, and maybe you hear the dreaded spark-knock. Let's say you don't. Slow back down to 2500, and step a little harder, say hard enough to open the powervalve @11inches vacuum. Your vcan might drop out at a rate of 2* per inch, so at 11 inches it drops out 4* . So now your timing will be 54 - 4 = 50*. This will for sure knock. So you will have to press harder, to drop more out, but now the secondaries are opening and knock is still evident. So you floor it and all the Vcan timing drops out and you are sitting at 34* phew!
So what to do. Something has to be done.
Most sbms I have worked on are fine with 20initial, and are fine with 34* powertiming, and fine with 20 in the can. And are even fine with 54* at cruising speed. I have seen some like 60plus degrees at cruising speed.The problem is blending these 3 to make it driveable.
Well I really like to tune with the Vcan, cuz it let's me make some decent mpgs;so I bring that 20 to 22 degrees in as quick as possible. If you have a high cr the engine likes that. So for me I have to delay the power timing somewhat and usually start with a little less initial. If you have a slightly loose TC that stalls at 1800, we can easily sacrifice a bit of timing below 1800 and never even miss it. So lets back up the initial to 14*. We need the powertiming to be 34*, but let's delay it to 3000rpm, and let's say it starts coming in at 1000rpm. So the centrifugal curve will be 20* and 2000 rpm;that makes, 1* per 100rpm. And the can will stay as it was;20*,and dropping 2*per inch, starting at 13inches.
So idle timing will be 14*. Again at 1300/and over 13 inches vacuum, the timing will be 14 + 3 +20v =37. Thats just 5* less than before, and no detonation. At 1800 light throttle, the timing will be 14 + 8 + 20v = 42, and all quiet. At 2500 it will be 14+15+20v=49*. By 3000rpm the total will be again 54*. So far so good.
Now again lets roll into it and by 11 inches of vacuum @2500(same as before),timing will be 14+15+16v=45*. This is again 5 degrees better. And when you floor it, all the vacuum timing drops out and,you still have the 34 by 3000rpm .
This 45*Part Throttle timing at 11inches of vacuum may still be too high, but,with a little tuning of the Vcan, you might find that the rattle disappears.
If you value MPGs, you have to give the engine some serious cruise-timing.
Long story short; every engine is different.It's up to you to figure out what she wants, and when she wants it, and under what conditions. Then, since the dizzy is not programmable, you have to try and hit as many marks as you can. Since the powertiming is more or less fixed, one or both of the other two(idle and cruise timings,depending on which you value more),may have to be compromised. It's rarely possible to hit them all perfectly. I compromised the idle timing, and set mine to 12/14. And I run powertiming of 32 /34(alum. heads), but it's a two stage with a kink in it's rate, at 2800. It brings in about 16* by 2800 and then slows down bringing in another 3*or4* by 3200/3400.Then I run a 22* Vcan, bringing it in as fast as it is able. And since it still is not enough cruise timing(there's the compromise again), I installed a Jacobs Optotimer which allows up to 15 additional degrees of timing, depending on how it get's set up.While it's theoretically possible to run a total cruise timing of 14+20+22v+15timer= 71, I have never set it up that high. I typically run 14+12+22v+8timer= 56*of cruise timing at 2240rpm/65mph.
I know, clear as mud.
If your pistons are down in the hole., and you have no squish/quench, this may all be so much mumbo-jumbo, as low-cr engines have totally different timing requirements.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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If you run the Vcan on manifold(full-time)vacuum, you will have to run a completely different dizzy cam with a different rate of advance.
This is cuz the manifold vacuum will instantly add its load to whatever you have set the initial to. Say your engine wants 24*, and the Vcan is able to give 20. Well you would set the initial to 24 - 20 = 4*. But you need 34 at 2500, so the centrifugal will need to be 34 - 4 = 30*. You will have to build that cam,cuz I imagine they are rare as hens teeth.
Now, the rate of advance will have to be 2500 less 1000=1500 rpm and 30* equals; 2*per 100rpm. So at 1300 you will have 4+20v+6= 30 to cruise. Thats way too much, and if you floor it you are left with a dismal 10*. At 1800 you will have 4+20v+16=40 to cruise(probably too much@1800) and 20 for power(pretty close). At 2500 you will have 4+20v+30=54 (looks good), and 34 powertiming(also good). From this it can be seen that at almost all rpms below 1800, you will not have enough power timing, and at below somewhere between 1800 to 2500 you will have too much cruise timing.
So again if you have an 1800 stall or better TC the total loss of powertiming below 1800 might not be a big deal, but at 38mph, or less, in high gear(about 1800rpm with 3.55s), the too much cruise timing may be an issue.
This type of plumbing works best with really big cammed engines, who need the extra idle timing, and don't care about mpgs, and have much higher stall TCs.I have only tuned engines with cams up to 250*@050, and have never gone this route.Never needed to.
This is cuz the manifold vacuum will instantly add its load to whatever you have set the initial to. Say your engine wants 24*, and the Vcan is able to give 20. Well you would set the initial to 24 - 20 = 4*. But you need 34 at 2500, so the centrifugal will need to be 34 - 4 = 30*. You will have to build that cam,cuz I imagine they are rare as hens teeth.
Now, the rate of advance will have to be 2500 less 1000=1500 rpm and 30* equals; 2*per 100rpm. So at 1300 you will have 4+20v+6= 30 to cruise. Thats way too much, and if you floor it you are left with a dismal 10*. At 1800 you will have 4+20v+16=40 to cruise(probably too much@1800) and 20 for power(pretty close). At 2500 you will have 4+20v+30=54 (looks good), and 34 powertiming(also good). From this it can be seen that at almost all rpms below 1800, you will not have enough power timing, and at below somewhere between 1800 to 2500 you will have too much cruise timing.
So again if you have an 1800 stall or better TC the total loss of powertiming below 1800 might not be a big deal, but at 38mph, or less, in high gear(about 1800rpm with 3.55s), the too much cruise timing may be an issue.
This type of plumbing works best with really big cammed engines, who need the extra idle timing, and don't care about mpgs, and have much higher stall TCs.I have only tuned engines with cams up to 250*@050, and have never gone this route.Never needed to.
tanis4457
Well-Known Member
That's kind of what I was thinking you'd have to tailor the vacuum advance curve to your initial and mechanical timing marks to find a compromise that works across the different types of rpm bands and driving styles. Thanks for the info, I'll see what I can achieve.
I can definitely see the benefit of timing maps with EFI now, it's much quicker to make some changes and see how it goes as well. At the same time achieving a more linear transition, I would assume. I've also heard that spark knock occurs well before actually hearing it thus the need for a knock sensor in the block and the computer reading this and adjusting spark accordingly.
I can definitely see the benefit of timing maps with EFI now, it's much quicker to make some changes and see how it goes as well. At the same time achieving a more linear transition, I would assume. I've also heard that spark knock occurs well before actually hearing it thus the need for a knock sensor in the block and the computer reading this and adjusting spark accordingly.
jbc426
Well-Known Member
As you point out, those high numbers will never work. I only use about 6 to 8 degrees of vacuum at the max, and totals in the mid to upper 40's all in. I guess its like anything, you have to want it to work and take time to dial in the details.
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