Not Your Average Overheat Question

[Andy4639]

Vacuum is constant at idle. No you don't want ported vacuum you want manifold. Once your up to around 3000RPM's the vacuum advance is where it is and want change any. At wide open speed you have no vacuum.
Hook it to the manifold vacuum.

This is the article I have and read. It's long but has all the info you need to make the right decision.
This was written by a former GM engineer as a response to a similar question on a Corvette board:


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.

[Boog]

There are and always have been varying ideas on correct ignition timing and engine performance. I read many years ago the ported vacuum was more for emissions purposes and an easy performance mod was to switch to manifold vacuum. I tested this on a 71 Pontiac Ventura with the 307 engine. It did make a difference in that application as the 307 was not power house anyway. It started easier and idled smoother with the additional advance at idle and felt peppier on take off.
Another item to mention was the ignition spark control which limited ignition advance until the vehicle was in high gear. All aimed at improving emissions, certainly not fuel mileage or performance. Less idle timing lowers some part of the bad emissions, I forget which one. NO or HC ?
The article by that GM engineer above explains the difference and benefits of manifold over ported vacuum very well. I had to learn this the hard way. And as was said over and over, "get the ignition timing right first THEN begin adjusting the carb" is so true.
In this case the op cannot set his timing yet due to a damper and pointer change so first on his list of things to do is verify tdc and adjust the pointer tab then get that timing dialed in. Of course he will have to adjust idle speed down as he brings up the timing to get an accurate setting. An engine with retarded timing will run hotter even at idle and more so under load.
Big blocks were designed to keep cool with a fan clutch, 7 blade fan and big radiator and shroud and they do, did so under most all operating conditions.
At this point the op has it running but lacks the fine tune to smooth it out.

[HO455]

I will respond with a few things and then I will be done as this will not help the OP with his problem. Yes, you are correct vacuum is steady at a fixed idle speed. My quote was "engine load affects vacuum". My information was incomplete as I failed to point out that with an automatic transmission it is difficult to obtain the same load on the engine in gear as out of gear. Thus as the load affects the idle speed the manifold vacuum changes. If you have a manual transmission then the ported source is of less value to you, but if you have an automatic transmission then a ported source is a good way to a better idling vehicle. That was lazy posting on my part and I apologize for it. Now your cut and paste says :

[quote=Andy4639;8016082][COLOR="Red" 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 [/COLOR][/quote

He is correct in his statement that after years (since the mid to late 1930's when vacuum advance began to replace manual advance methods) of using manifold vacuum they switched to ported vacuum. (Up to that point there was no reason to do otherwise.) However he strays from the facts at that point. In the 50's automatic transmissions started becoming common and as they became more commonplace customers wanted them to idle like their old manual shift cars. They expected their brand new car to idle well and not jump when shifted into gear. Remember that most of these early automatic transmissions came in the higher price range cars which were supposed to be like driving a cloud and shouldn't scare the wife every time it was put in gear.
So prior to the arrival emission regulations ( California required PCV in 1961) the auto manufacturers were well past designing ways to solve the idle problem. Ported vacuum started appearing on automobiles in the late 50's as one of the solutions. By saving the manufacturers money on initial set up and warranty work ported vacuum was a good fix. Any solution that worked on all cars across the line up was a winner. It soon became the industry standard.
His description of how ignition advance systems operate is correct, but he starts grabbing random examples to prove his point, like the statement below, where he carries on about mechanical advance numbers for smog motors that were not envisioned in the 50's. (Heck we were promised jet packs and rocket cars by then! ) He is only correct with his numbers if you are looking at select high performance engines of the seventies like a 1974 Pontiac 455SD motor. Which made horsepower with low compression and required lots of timing as he describes. (Our SD motor runs 38* of timing with 12* intial and runs 12.20 in a 3100 lb car with 8.4:1 compression like it did when it left the factory.) The rest of the seventies GM grocery getter cars had high initial timing with limited the mechanical advance in the distributor (14-18 degrees) then loads of vacuum advance to try to get some MPG. Engine emissions are affected by combustion chamber design and the bore and stroke combination. These things were not concerns or even known facts at the end of the sixties and when the crunch hit Detroit was not ready and everything was a band aid to try and survive. Ever wonder why every size of small block Chevy in the seventies has such different timing specifications. It was all to get through the emissions nightmare.
He also completely skips over the whole deceleration preignition issue that is drastically reduced by using a ported source. Ever hear a vehicle backfiring as it coasts down hill? Manifold vacuum. I don't doubt this mans claim to be an engineer at GM, but I doubt he was there back in the day. As most of them have passed away by this time.

Rest in peace Mr John Sawruk.

Originally Posted by Andy4639

[COLOR="Red" 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.

. [/COLOR]

I am done with the soap box. Thank you for your time.

[Andy4639]

My 64 Impala had the vacuum advance on the ported vacuum it ran like crap. I have since swapped it over to manifold and it's now running fine. No more backfiring through the tail pipes as before. The idle is down below 1000RPMs where it should be.

I tried everything when it was hooked to the ported side of the vacuum and nothing worked.

The over heating could be from to much timing to begin with.

[cleszkie]

I have found that you really need to play around with both ported and manifold vacuum when trying to determine which will give you the best perceived performance. I've had engines that loved ported, and I've had engines that loved manifold. There are just so many other variables that come into play that interact with the type of vacuum advance that is chosen that you can't purely rule one or the other out. Trial and error is your friend.

With that said, I run a built 400 SBC, with an MSD Atomic throttle body fuel injection system that also controls timing in addition to the A/F ratio, in front of a manual tranny. The system gives you a choice of selecting ported or manifold vacuum for the vacuum advance, based on the MAP (manifold air pressure) readings. The MSD instructions pretty much point you to ported vacuum as the way to go. So I set it up with ported vacuum and ran it for quite a while. Never totally satisfied after much tweaking and fine tuning, so I switched over to manifold vacuum, reset my initial timing, closed down the throttle plates at idle, and now it runs noticeably smoother at idle, little better throttle response, and runs a tad cooler overall. I have not yet checked the mpg yet, but I suspect that has also improved. Trial and error.

I should also note that I got rid of a slight dieseling condition after turning off ignition when I switched to manifold vacuum. Probably due to the increased timing at idle maximizing time for complete fuel burn in the cylinder combined with cooler cylinder temps.

[mosel70]

So here is my conclusion to this thread story. Big CFM Derale fans and fan controller completely solved my overheat problem and also give me 10-20 degrees cooler temp on bottom rad hose. Many thanks to all of you for the great help, invaluable!!

Thread summary - I changed every component in the cooling system. Tried 3-core alum rad, hoses, two temp sensors, 2 gauges, 2 water pumps, 2 thermostats. Checked timing, carb mix screws, etc etc. A really annoying overheating problem all year here in Las Vegas.

Mishimoto's twin fans (2600 CFM total) couldn't keep my '70 C10 402 from overheating at slow speeds/idle and seemed to provide little temp difference between top & bottom rad hoses.
https://www.mishimoto.com/chevrolet-...oud-67-72.html
Their fan controller did work well though.
https://www.mishimoto.com/adjustable...oller-kit.html

So, those of you who said I needed more airflow were dead ... right!!
I just put in two Derale 14" fans (2100 CFM each, 4200 total) and wow, those little hurricanes have no problem keeping the engine cool. Finally after a year of this!!
I also installed Derale's PWM fan controller which automatically adjusts fan speed to match temp I set and also has a soft start ability. And yes, they run after ignition off but only for 1-2 mins at very low RPM, no problem to the battery.

So worth the $150 more for the Derale fans and controller than what I paid to Mishimoto. Perhaps other Chevy big blocks cool down well with the Mishimoto fans, but mine sure didn't. I guess I know now why there are no Mishimoto reviews on Summit or Amazon for the Mishimoto fans I got . They don't do the job. Their customer support is really good though.

This thread also had some great discussions on connecting vacuum advance to ported vs manifold vacuum on my Edelbrock 750 carb. I've had some difficulty getting a decent idle RPM in and out of gear so I tried manifold vacuum the last fews days. It was terrible. Engine knocked like crazy. Clankity clank clank. It's a '70 C10 EEC truck built in CA and I do have my EEC charcoal canister installed and PCV valve btw. Also am using a new MSD HEI distributor. I'll work more on getting timing set right even if I have to take it to a performance shop somewhere here in Vegas for help.

http://derale.com/products/electric-...fitting-detail

http://derale.com/products/electric-...n-probe-detail

[Ironangel]

Glad you nailed down to the air flow through the radiator. I was going to concur with kwmech and Andy on the cooling issue and Geezer on the timing. I have a factory copper 3 core with a modified to fit small block shroud. Thats modified for a tight fitting 7-blade fan on a long water pump. The draft through the radiator is strong enough to suck a pizza box (dont ask) against it at idle. Thats all to keep a built L-34 396 cool. Your idle rpm is way high but some good advise has been given on how to bring it down with the idle (butterfly) screw and the mixture screws together with a timing light. My 396 is .060 over with a Comp 268H cam, domes with closed chambered heads and I can idle her down to 500 rpm in park no problem but keep it at 600 and stays there when I put in gear. Learning your carb and how to tune it is key. My initial timing is where the motor likes it, 16 degrees @ 600 rpm and 37 degrees all in @ 3000. I dont trip on the ported vs. manifold vacuum because I like to keep those butterfly's open where vacuum gets replaced with fuel and air, lots of it! MSD Pro Billet 8360 keeps it all in time and limits the rpm to 6K. https://www.youtube.com/playlist?lis...FwXZgV5iVEXyeB