Ported or manifold? They ran vacuum advance back as far as I can remember. Unless you have a modern replacement carb meant to adapt to any system, smog engine or before you will have a hard time finding any ported vacuum on an old carb. Running 3 Rochesters from the 60s? Good luck finding any ported vacuum source on one of them...there ain't none. I kept reading on the internet about running ported for the vacuum advance. I couldn't figure it out. when we ran these carbs back then there was no such thing as ported vacuum sources. People get confused by where the vacuum advance port enters the carb. I have no idea why GM put the vac adv. port up high on the carb body but it is indeed manifold vacuum. You can not tell by where it enters the carb body. I only know for sure of one pre-emissions carb that has ported vacuum from the factory...the 49-56 Ford carb and it has to be used with the carb from one of those engines....It's the only carb that will work with this system. That is why most of the Y block crowd recommend replacing it with a 57up dist with a standard manifold vac. adv. so that a Holley or Edelb. can be bolted on with no problems and hook the vacuum adv to the manifold vac port. I don't know a thing about using the fit-all carbs that have both hook ups. Holley or Edelbrock but I do know that they have confused a whole lot of hotrodders about vacuum advance. I'm not a tree hugger so I want pre-smog performance. If you really care about the atmosphere that our small percentage of cars in the public may cause then by all means try to hook it up as the factory did when they were under pressure from the govt. to reduce emissions. I doubt very seriously that my few cars will damage the atmosphere enough to worry about. I lived through these days of the smog emissions being changed every year. Do you add an air pump to your hotrod? I don't.
Put .620 lift on your cam & 12.8 - 1 comp in your small block & live some, I do cause there's more to life than MPG ! My 5c worth
When you live paycheck to paycheck, MPG is a consideration!! But I am only .560 and 11:1. What does 100+ octane cost down in Oz?
Well I can't ad a vac cannister to mine I am using a mechanical distributer. I think that there has been a distinction or a mistake made here. It seems like most of the folks are talking about adding a vac cannister to their distributer, that suggests that it was a vac advance distributer to start with, as there is no way in creation that one is going to ad a vac cannister to a mechanical advance distributer. In that instance I am going to say yes adding a vac cannister to a vac advance distributer with the vac advance that has had it removed is going to make it perform better, they do not have the same amount of advance built into them. The OP was asking about if he would buy a vac advance distributer or a mechanical advance distributer. One or the other not if he should buy a vac advance distributer and remove the vac cannister. Now for the question name one vac advance small block that you know of that comes close to mine in fuel mileage. Now name one that will pull a 4000 pund brick of a pickup truck through the lights in the low 13s, and then drive the 3 hour trip home without any changes. My truck handles low and mid range well, it is completely drivable. But don't take my word for it, again drop the raven a note.
Well you should start. The pic resembles a old friend of mine from New York. Vic was not nice to 99% of the people he met but I always figured that those people didn't deserve nice, hell he wasn't always nice to me and we were friends. yea yea I know off topic. Just for general information I didn't build my engine or any other engine that I have ever built for economy. Once in a while I get lucky and get economy anyway. Guess I am blessed because I have led (sp?) a good clean life.
About the same as here. I drive my 55 quite a bit (5000 or so miles per year) year round, so I couldn't afford more than pump gas!!
He's a guy named Lemmy from a band called Motorhead. Check them out!! I try to treat people like I want to be treated. Some people just don't want to be treated nice, though. I didn't build my engine for economy, I built it to kick as much ass as I could afford to kick. BUT, If I can squeeze a little more street manners and fuel mileage out of her at the same time, I'm all about it. That and I can never leave anything alone, I've always got to tinker. Sometimes I even improve things!!
This particular engine was built on a dare and because I had a lower end that my now deceased little brother gave me for my birthday (long story not applicable here). Someone I worked with bet me 200 bucks that I couldn't build an 11 second car that I could drive to and from work. I started gathering parts, I already had a little OT pickup. I ended up with a 2800 lb pickup that I did drive to and from work rain snow or shine. I had it running for about 3 days and drove it to the track and knocked down an 11.90 first pass. Took it home with my time slips for the night and pulled the distributer, it was not comming on soon enough, took it apart put lighter springs in it and left it. I think the engine is actually in its 5th car now. I lost track but the raven knows. He is using it in his roadster for the time being. Every once in a while you just get lucky with an engine and you keep it. I think I have heard motorhead or maybe I have them in my collection. I am not a band guy so I don't recognize band folks as a rule. I am bad enough that I flew from here to San Diego seated next to the one armed drummer from def leppard and didn't know it. When we got there he said thanks for not treating him like he was someone special. I didn't understand until a little kid seated behind us showed me who he was on his lap top. D'oh. I shoulda got his T shirt or something. I live by one simple rule, treat me well and I'll treat you better . . . the end of that is not so nice no reason to tap it out. I try to treat everyone like they have treated me well until they give me reason not to. Now that I am older most people don't give me reason to treat them like they haven't treated me well, I like that.
My Sun504 is new to me and i am enjoying learning from it. One thing that i can share is that the vac pot on the distributor is casual about making a change. I haven't checked a mess of them but the ones i have checked takes a while, several seconds anyway, to get to full advance.
The reason that a vac advanced distributer had a secondary mechanical advance set up it that when you stab the gas your vacuum drops out until the system catches up with itself. You need the spark to advance when you stab the gas at low RPMs. If you think it is slow to act on your machine try it on an engine. With a full mechanical advance distributer you are not dependent in manifold vacuum to advance your distributer. I still stick by my original statement though, I run them because I like them.
PnBer, What degrees of advance are you running for initial timing and what is it at 1200RPM, 2200RPM, (typical cruising RPM± for most street cars) and all in?
I am all in by about 2K and running right @ 33 (useing the degree marks I made on the balancer, they could be a little off). I run 14-16 degrees initial, again my marks could be a little off. I marked every degree though 20 after that I only marked every 5 degrees though 60. Granted I am not running your average "belly button," the engine was assembled from pieces and I got lucky. I cruise @ 3K most of the time, I don't run OD and that is where the engine starts to climb up on the cam. If I am in a neighborhood I normally leave it in second, like the Los Angeles basin about every where you go around here is Freeway driving.
With a full mechanical advance distributer you are not dependent in manifold vacuum to advance your distributer. Porknbeaner, i thought that the mechanical advance start working per the springs/weights for both a mech only or one with a vac can, it is just that the can adds more on top of the mech curve, so all things being equal 2 dizzys with the same curve and advance rate would advance the same the one with the vac can would add more on top...i may be wrong tho
My understanding is that a vacumm advance distributer has limited mechanical advance and depends on the vac cannister to get it all in. A dedicated mechanical distributer does not depend on vacumm to get to full advance. Depending on the manufacturer they both advance about the same. For instance if you buy equal distributers from a manufacturer like mallory. One is vac advance and one is mechanical advance, they will both advance pretty close to the same. One will not advance any more than the other. The difference is that one uses a mechanical mechanism to turn the point plate and the other uses a vac pod to turn the plate. Either is adjustable, one uses a scew in the vac pod and the other you adjust by changing the springs and in some instances the weights as well. Now we think that one is better than the other, but in all actuality they are just different. When I am cruising with a mechanical I am all in at cruise, when I am cruising with a vac I am all in at cruising. Anyway that is my understanding. I am probably completely wrong.
my mallory had an adjustment to go from 20 to 26 mech all in... My GM single point has an adjustment bushing so that I can change the amount of advance depending on size so they would advance at different rates as well...mech all in as well. Both were timed at 36 total and the vacuum one had the vac plugged, so you subtract the total from the initial, I have a dial back light, and the middle is what is in the dizzy.... great thing about all of this is that there is no wrong way of running with or without vac advance....they both work...
I put a mark on my dampener at 36º advance and set the "all in" to that. then I hook up the vacuum advance to manifold vacuum for an additional 12º at "cruise" or idle. The mechanical advance advances 24º± so my initial is 12º± At "cruise" I've got around 48º± of advance but it's not a problem because there's virtually no load. If I start pulling a hill or accelerate the vacuum instantly drop off so it doesn't detonate. I'm just running about the same mechanical advance as PnBer is, maybe a little more. I get good mileage and at idle, like in traffic, I have that extra 12º of advance to keep it from overheating. Retarded timing will cause overheating. Chevy's really like the 24º total I run at idle. I left it running in the driveway, in gear, one day for about a half an hour and it ran as cool as a field pump engine.
You had a geritol moment and just walked away from it? Sorry I just couldn't resist. I go to great lengths to keep my engines form overheating. Maybe I over compensate. I don't know how it is out west, but with the recession here there is hardly anyone on the road. I seldom get stuck in traffic. There just isn't any any more. I have never noticed any difference between a vac advance engine and a mech advance engine as far as engine temp. I am going to take everyone's word that it makes a difference. I am not saying that it doesn't I have just never paid any attention. You guys have and that is good enough for me.
PNB - I don't think you understand how vacuum advance distributors work. The mechanical section is just that - mechanical. It works with weights and springs and in most popular V8 applications is fully tuneable by changing a combination of springs, weights, and limiters - no different than a fully mechanical distributor. You can dial in as much advance as you need and set the curve the way you want. The vacuum canister has nothing to do with the mechanical portion. It moves the breaker plate based on the vacuum signal, independent of what is happening with the mechanical, which relies purely on RPM. So for instance in our PerTronix distributors, if you run them without limiters they have about 24° of Mechanical advance available - add that to your initial and that is your total mechanical advance. So if you have 15° of initial, you will have 39° total. If that is too much, we include limiters to, well, limit it. Now the vacuum is totally independent. In our case the vac can will pull about 20°. So if you are cruising down the Highway at 3000 RPM, and you have set up your springs to be all in by that point with the scenario above, you will now have somewhere around 50-59° total advance based on how much vacuum your motor is pulling at that point. This will improve mileage and cause cooler running in most all cases. There is no down side to doing this! The only reason racers run distributors without vacuum advance is they are either idling or running WFO, in which case there is no vac signal so it's it one less thing to have on the motor and potentially cause a problem in a race car. And oj - if the vacuum advance mechanism is working properly, there should be No Appreciable lag in the vacuum advance. Now granted, if you have a distributor that wasn't designed to allow a vacuum advance, then all you can do is replace it with one that is designed for it, you can't add it to a distributor that doesn't have a provision for it. For a street driven car, there is absolutely no reason Not to run a vac advance distributor. You can have exactly the same mechanical curve as a mechanical only distributor, plus the added benefit of the additional vacuum advance.
Ha ha ha! I have been thinking that was "The Undertaker" for weeks! ... Anyway, vacuum and mechanical advance both have their place. They both work great, but for different reasons. I like both together. But you have to tune to take advantage of them, of course.
Thank you for all the great info. It appears that the general consensus is that the vacuum advance distributor is prefered for the street driven hotrod.
Very info but keep in mind, most aftermarket carbs are on the rich side out of the box, better to be to rich then too lean and burn valves so I think the first order is to get that correct and then play with the advance.
I have tested a good number of them and they are all kinda casual about movement. I recently tested the new msd street distr (drop-in with vacuum) and it reacted quicker because it was one of the type that starts around 7-8". Now, it might be that my machines' vacuum isn't as powerful as an engine, but what makes me think that the change is casual is that when the vacuum goes away the amount of time to react is about the same as when vacuum is applied. We live in a digital age and just assume that things happen immediatly. I see people make this mistake often..watch a guy 'adjust' his idle mixture screw. He'll back it out half a turn, stare for a moment then call it good and stick the aircleaner back on. I find it better to make a half turn adjustment, chitchat a bit and walk to the rear of the car and see if it is overly rich, it takes time.
I have a good understanding of a vac advance distributer. I don't need 50 degreess of advance to run down the highway. Perhaps a someone does. I started out by saying that mechanical is my personal preference. Someone else said that with vac advance the car runs cooler and gets better mileage. I had to go to a 190 degree T stat from a 180 just to get mine to make the defroster work. On the highway on the summer I never see temps above 190, with the 180 degreee in the summer it very seldom reaches 185. I think that is cool enough. Although I don't build a "hot rod" engine with mileage in mind I am quite happy with mileage in the 20 mpg range, other the Dr J no one has stepped up with their mileage, so we have no way to prove that the vac advance distributers make better mileage. I was not aware that the vac advance was to make it have more advance at highway speeds. So I have learned something.
Beaner, what heads are you using? Modern heads have a much more efficient combustion chamber design so they tend to need less total timing. There are so many variables involved such as engine size, rear gear, vehicle weight, torque convertor used if automatic, transmission gear ratios, cam profile, compression ratio etc. I've built a lot of engines of all sizes and brands and I have found that a properly set up distributor with a vac advance has better street manners and mileage. Just for grins beaner, why not try a vac advance dizzy in your engine and give us a report. Who knows, you may get 25 mpg.
I have, no real difference other than it falls on its face off the line. 355", one off heads, balance and blue print, good copy of a production performance cam shaft (slightly retarded) are the basics. I have been building healthy street engines and race engines for a very long time. Like I said earlier I have run them both ways and use mechanical by preference. I do not run mechanical because someone told me that my favorite racer does or that mechanical is better. My engine runs smooth except at idle, it is a little lumpy, not jump around in the chassi lumpy but someone would say that it isn't smooth. My power band is relatively broad, it doesn't fall below 400 ft pounds once it starts to climb up on the cam. I don't have the problems that I have been told in this thread that I am going to have, perhaps it is because I am willing to spend time with a wrench in my hand. Maybe I am just too stupid to know I am having those problems. Like I said earlier if someone questions my drivability or mileage claims or even if you want to ad operating temp to the list there is no need to take my word for it, raven is only a few clicks away.
With gas at $4 plus a gallon you want to run a vac advance on the street. Unless you enjoy watching money fly out your exhaust pipe.
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