i've worked on both types ported and manifold. i've never really had a overheating problem at idle (not to say that is doesn't happen). on quads they have a ported vacuum port and run my cars @ 10-13 initial. i've also not had one hooked up to ported vacuum stumble unless the advance isn't working. the factory had ported vacuum in the late 60's and 70's. so i guess i'll agree to disagree . but i do understand what your saying and i'm not saying that only one way will work (so i guess my answer would be to try both and see what works better for you).
It may help your mpg on the street. I admit to being an evangelist about this. My 32 had the advance hooked up to ported and it would overheat in 5-10 minutes of idling/slow driving summer or winter. Big Walker radiator, boiled out block before the build etc. Where things have gone wrong is the instruction sheet for the Carter and Edelbrock carbs that indicate the ported front carb bib is the one to use for vacuum advance. Lot of guys do the plug and play bit with carbs and if it runs ok they're happy. I was at the time until Skip Readio set me straight. I don't know why guys who hit the track don't want to run vacuum advance. It backs the vacuum timing all the way off when the throttle is open. Maybe we could call it the drag racing only vacuum retard can. Since many - if not most - of the aftermarket distributor manufacturers are making vacuum advance distributors for most engines, seems to me that Hot Rod et al ought to run an article about this. Since most magazine articles like to reflect kindly upon their advertisers - and they should - now is the time to do it. Getting back to the original poster . . . 12.5 ET? 1.67 60 footer sounds like you're running slicks. Would be a fun race with my vacuum advance Buick powered roadster that runs street tires. Now where did I put that locker?
I had an auto tech instructor years ago that was a bit of a fanatic about using a vacuum gauge when doing tune-up work. The two of you would have gotten along famously! He did his best to pound his beliefs into our heads and the majority of his students ended up with vacuum gauges clamped onto the steering columns of their cars. We may not have had a tach, but we knew how much our cars "sucked" and when! Being able to pull the highest cruising vacuum became a bragging point. It was also a simple matter to connect the gauge hose to different vacuum sources under the hood to see the differences between ported and manifold vacuum readings. I don't think most people realize the changes in engine tune (from stock) that can happen by swapping in even a relatively mild cam or a larger volume intake manifold. As a result, a lot of the stock tune-up specs go out the window and you have to spend some time deciding which new tune-up works best for you. It may end up running rich and not want to idle in gear, and this is when a vacuum gauge can help to sort things out. But you also have to know how vacuum affects things like carb enrichment circuits, distributor vacuum advance, trans vacuum modulators, HVAC vacuum actuators, power brake boosters, vacuum wiper motors, concealed headlamp vacuum motors, your Hollywood Wolf Whistle and other items. For most street driven cars, the more vacuum you can make, the easier things will be. So you stick in a bigger cam for more performance but you've also taken away some of the vacuum that runs the items listed above. You can maybe get some of the lost vacuum back with your tune-up choices (a little more initial timing, a little less total advance) or by altering cam timing (advance cam timing a bit or adjusting solid lifter cams a couple thousandths looser on the intakes; replacing a worn, stretched timing chain might give you similar results). Beyond this you'll need to consider changes in power valves or power piston springs, distrubutor advance curves, the elimination of certain vacuum powered accessories or the installation of a vacuum reservoir and maybe an electric vacuum pump. Just some stuff to consider before installing the great sounding cam and the cool looking tunnel ram intake. Install a vacuum gauge in your car, connected to manifold vacuum, and in no time at all you'll understand why it runs too rich with 8.5" power valves when it wants to idle at 6" in gear with the idle mixture screws closed. You'll see why the power brakes feel like you've converted them to maual brakes. When the auto trans (in drive, equipped with a vacuum modulator) makes harsh, delayed upshifts, you'll realize it's because the trans thinks you've got your foot to the floor, even though you're just cruising in stop and go traffic. Maybe the total timing is dead-on, but you've got a miss at idle because the vacuum advance is trying to give you too much initial timing? So where should you hook up your vacuum advance? Hell, I don't know! The point I'm trying to make is that you should add a vacuum gauge to your timing light and other tune-up tools to help decide which vacuum source to use. Hook it up to different places and pay attention to what it's trying to tell you about how your engine is running. Watch how ignition timing affects vacuum, but keep in mind that advancing the initial mechanical timing will also increase your total timing, maybe more than you want it to, to the point of a high speed miss or detonation. And after you get it all sorted out and running right, run a vacuum hose back into the car, plug in the gauge and hose clamp that baby onto the steering column. Just like all the cool kids did back in auto shop class!
Lots of great posts here guys. I think different applications may require different sources of vacuum, but it sounds to me like the majority of us should be pulling our vacuum advance from manifold vacuum. I am running a three deuce intake on an otherwise stock 1972 Chevy 350. My carburetors are early Holley 94's with no vacuum ports, so I have pulled my vacuum advance straight from the intake manifold, and with the timing adjusted correctly and the carbs tuned it idles great and has good throttle response. I do have about a 100 rpm drop going from park to drive, but other wise it runs cool all day long and performs well. Thanks C9.
so what your saying is.......by changing to full manifold vacuum for the advance, I will only see that change and extra advance at idle......after I take off at wot it goes back and follows the spring setup in the dist and the initial advance? I am still tuning my 396, 10.75-1, 292H 110 LCA 550 lift.
A forum search turned up this previous post by C9 which explains things better than I could. If this isn't the one you're thinking of, post a link here for it if you find it again. I'm sure it would be well worth the time to read thru it too. The effect of cam timing on vacuum is interesting to experiment with. Years ago a friend had a '67 Barracuda with a stock solid lifter slant six engine. It had enough miles on it that it had a little valve train noise and I told him that if we adjusted the valves it would be so quiet he'd barely hear it running. We set the valve lash to the stock specs and with the hood closed it sounded like we'd put a new engine in it! The car had a factory installed vacuum gauge in the dash though, and we noticed that it lost like an inch of vacuum at idle. But it ran so smooth and quiet that we didn't give it much thought. Well he drove this car about 30 or 40 miles or so a day to and from work and always kept close track of his fuel mileage. A couple weeks later he tells me that his mileage has dropped slightly since we adjusted the lifters and wondered if there was a relationship between the two events. I knew that what we'd done had dropped his vacuum so I figured it could have affected his mileage as well. So the next weekend we pulled the valve cover again and added a few thousandths more clearance just to the intake valves if I remember correctly. Some of the valve clatter came back but so did the idle vacuum and more importantly, so did his gas mileage. And the little extra noise didn't bother him at all.
Getting back to the original poster . . . 12.5 ET? 1.67 60 footer sounds like you're running slicks. Would be a fun race with my vacuum advance Buick powered roadster that runs street tires. Now where did I put that locker? [/quote] The rear gears are 3:73s in an otherwise stock 1979 Trans AM disc rear end 8.5 rear end, stock th350 with a 3,000 hughes converter, and I bet Im around 1000 lbs heavier than your roadster. Oh and I did run BFG Drag radials. Oh and because of the non ported heads, the 12.5s are done with a 105.6 mph. I installed a nitrous set up and am now 11.8s at 114 mph but am hitting the rev limiter hard at the 1000 ft mark. So now I put on some taller tires which will lower RPMs by about 9% so no more nitrous rev limiting hits!!!!This car is very quick rev'ing even with a Holley street dominator open plenum manifold and an 800 DP. I use the nitrous just in case some light Buick powered roadster thinks they can beat me. He He he He. My car isn't set up for racing with a 1 1/4" front sway bar, polyuerethane bushings, 7/8" rear sway bar, full stock interior full 2.5" exhaust. Heres a quick video from about 5 years ago on a 12.8 second run on cheap ARCO gas. As you can see, no hesitation without vacuum advance. It wouldnt help anything since I have to power brake to 2800 and load the converer or its spin , spin and more spin off the line. I'm hoping with the taller tires, I can leave off the line at a lower stall and use the flash to speed my 60ft times even faster. Typically I run all my Pontiacs with ported vacuum, with initial at 14 and all in at just under 3,000 with a total of 35-36 degs advance . If you notice the mph, its very low for the ET. From traffic light to traffic light Pontiacs are great, with ported heads and a bigger cam, they can all out fly with street gears and low stalls. So I can agree that it all depends on how the engine/car is set up if you use ported, manifold or no vacuume advance in your car. Its all in the set up. If you gain some pep from going from ported to manifold, did you try advancing the initial on the ported and take out some mechanical? Like I said, its all in the set up. You want good initial and a max of about 35-36. Dont check your total timing with no load and vacuume hooked up as you will probably see over 50 degs as the vacuume advance will be pulling full when you rev with no load. I have people come to me and say they are way advanced when they checked their timing for total advance. http://www.youtube.com/watch?v=jpEgC6G12Sk
Correct - at full throttle. Part throttle gives you some or all of the vacuum advance available. Take a look at the distributor timing figures in a motors manual. Vacuum advance doesn't start until 7"-8" or so. That allows the lean idle mixture and part throttle mixture time to burn. You're running say, 15"-18.5" or so at highway speeds with a light car and that helps mileage cuz you're burning all the fuel you can. Note that the above paragraph is true even with a big cam that may idle at 9"-11" vacuum. Spin em up a bit to get out of reversion at low idle rpms and the vacuum gauge will indicate normal levels of vacuum. Keep in mind too that every 1000' of altitude gain will drop the vacuum level 1" at idle. My 32's 462" Buick that used to idle at 18.5" at 350' now idles about 16.0" at our present 3300' altitude. There are charts in the better motor manuals as well as on the Internet. They'll show what different indications mean. The on-board vacuum gauge is a marvelous tool. You can use it to drive for mileage, take note of engine tune at any speed as well as see if you have a leaking valve, blown head gasket etc. Fuel pressure gauges are another good on-board diagnostic tool. (Be sure to use a pressure isolating device between gauge and pressurized line. You don't want fuel under pressure in the cockpit and it's against the rules to do so in any racing organizations rulebook.) With a vacuum gauge, fuel pressure gauge and the other instruments on board, most times when the car quits or starts running badly you can diagnose quite a few things before you come to a stop.
Thank, I will hook up the vacuum guage and test it. I dont get 7" of vacuum at an idle in drive, but I probably do if I slip it up in Neutral. Since I am in Phoenix, Im always looking for ways to survive a traffic jam at 5pm on Friday in July.
Your cam is a 292H from what I see. Is that a .050 duration figure? Which makes it a pretty big cam with perhaps 330 degrees of advertised duration. Or . . . the 292 figure is the advertised duration figure. If the .050 figure, then I can see 7" or so vacuum. If it's a 292 degrees advertised figure then it should have around 10-12" vacuum at idle which assumes you have an idle rpm of 500-600 rpm. And . . . you're getting only 7" vacuum, any chance you're plugging into the ported vacuum port? If the engine is idling 500-600 rpm and you're reading ported vacuum the reading will be zero. Since my engine idles at 1000 rpm when on the electric choke and ported reads 12" it seems like your engine, if it idles 800-900 rpm would read about 7" vacuum warm etc. Keep in mind that I run a pretty mild cam. Aside from that, I'm wondering why you're running a trans-brake? No arguments, but it seems to be an added complexity for a car that's mainly a street runner.
its the comp cam 292H...... 244@50 and 550 lift, with a 110 LCA Im definitely in the right vacuum port. I am still tuning but when I was checking vacuum for my power valve It was bouncing aroun 6-7" while in drive. I was implying above that if I can get 10-12" in Neutral at a light I could actually see the benefit of the vacuum advance. Like I said, I am always looking for any benefit it all starts to add up. In regards to the trans brake, I want to see a 1.4 60ft. time...yes it is a street car but right now If you add up my passes and the drive back down the return road I probably have more miles at the track than on the street. The line lock and brake are powered thru a switch on the dash so I can disarm them when not in use.
I'm firmly in the "manifold vacuum" camp. Here's a long winded but very good explanation, apparently written by a GM engineer. Borrowed from HOTRODSRJ who borrowed it fronm the original poster on Camaros.net: Here is a discertation from a fellow techy (JonhZ) over at Camaros.net that says it all and moreover was involved with the very design......enjoy! "As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative. TIMING AND vacuum advance 101 The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency. The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation. At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph). When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean. The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic. Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it. If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more. What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone. vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam. For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts. And that's all I have to say about that! Ralph
Great information! I've often wondered if my vacuum advance on the dizzy wasn't working out with my cam. I put a vacuum can from a lincoln on the car for a bit, I don't think it had any effect. I wonder if a caddy vacuum pump would help any. Does it just plug into the system, or is there any cans or valves that need to go with it?
I don't think you'll want a distributor vacuum advance connected to a vacuum reservoir system. The stored vacuum in a reservoir is great for operating brake boosters, HVAC actuators, vacuum wiper motors and the like. But the vacuum advance needs to be able to sense engine load and throttle position changes quickly in order to operate properly. The vacuum storage cannister may continue to store 15" to 18" of vacuum even after you've added load to the engine by mashing the throttle or starting an uphill climb. Or a pass down the drag strip. You can leave the reservoir on the car for other purposes, but find another source for pure, direct manifold vacuum for the distributor. And strap a vacuum gauge inside your car for a while and watch how it works. You may want to make it a permanent installation.
I'm gonna throw another curve ball into this game! And for want of the proper name, I'll call it "venturi vacuum". It was OEM to '49-'56 FoMoCo cars. There were no advance weights to create a centrifugal advance mechanically, all accomplished by this venturi vacuum. The pickup point for the vacuum was well above the throttle plates, just under the smallest point of the carb's venturi. The faster the rpm, the more vacuum present under the venturi, if the load remained constant. Increase the load, and the vacuum will decrease from the same rpm at a lighter load, and the timing will drop back. IIRC, this was named "Loadamatic" spark advance by Ford, and it was followed with all the carbs used during that year span having a vacuum souce from this spot just below the venturi. This really fouled up the deal when you bought a manifold for multi carb use with no vacuum port at all, or a port that really picked up manifold vacuum, and then tried to hook up the vacuum lines to an 8BA, etc. distributor that was calibrated to operate from venturi vacuum! Friend of mine back in the early 60s or late 50s had a '51 Merc. and installed a dual carb setup with 97s and manifold vacuum hooked to a "Loadamatic" distributor. Had more flat spots in the power curve than you could believe and just generally was a PITA to try to tune.
Shock, go back up to post #19 and you'll see what cooks vis a vis vacuum sources and readings at different throttle settings.
Very nicely written and very true! I hope a lot of guys will read this and take it to heart, I can't tell you how many Hotrodders I've met that will run a non Vac distributor on the street or block of the vacuumcan on their distributor because they think it will help the performance of their car. I've given up on trying to explain to them why a vacuum advance is important just to have them tell me that all the drag cars do it so it must work. I think every body should print this out and keep it in their records. Thanks for posting this!
Well, there goes traditionallity! Sounds more like ricers with more gauges you can look at than driving. JMO but four gauges a right foot and a good ear is all you need to tune a car "traditionally". Or alot of money if you forgot where your wrenches are.
Almost every factory setup used ported vacuum. Ported vacuum the vacuum created by the venturi so you get more vacuum with more air flow. So more air flow more advance. The old Ford loadomatic distributor used both. If you have an engine that has detonation problems if you use maniford vacuum you will have more advance at light loads and less under heavy loads helping your problem.
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