Rochester2bbl and hei combo question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

mateo1279 · 12-31-2007, 02:02 PM

Hello-

Just hooked up an hei and it is running better then the points.....
However I am concerned that I don't have the vac advance unit hooked up to the right place.

Does anyone know a location for a 'ported' vaccuum on a rochester 2bbl? All the vac locations seem to have full vac at idle.....

Thank you,

Matt

wild one · 12-31-2007, 08:56 PM

Here's something I stumbled on while surfing the web a while ago. Kinda lengthy, but it does explain this topic very well. Also gives a couple of links at the bottom of the article.

Hope it helps,
Pete

Distributor Tuning and Theory

There appears to be a lot of conversation on timing, vac advance, distributor tuning and, in general, hacking up a perfectly good ignition system to compensate for a poorly tuned carburetor.

The following two articles review the basics of distributor tuning quite well and have worked for years and years and are based on sound engineering principals. I thought it would be helpful for some to review these prior to hacking up their distributors. Hacking up your distributor to compensate for a poorly tuned, misapplied or defective carburetor is not very sound engineering, for a street application or otherwise.

Here's an interesting article on vacuum advance written by a GM engineer:

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.
Rick WI

http://www.fordmuscle.com/archives/2000/03/timing/
http://www.bob2000.com/carb.htm
http://www.jason.fletcher.net/tech/c...carbtuning.htm
http://www.futek.com/boltcalc.aspx
http://racingsecrets.com/

Ham-er · 12-31-2007, 09:19 PM

Wow, thanks for that Wild One. Mateo, most rochester 2bbl that I have seen Do have that "ported" vacuum port. It's just a matter of finding which one it is. But Uh....Do you still want to? After reading the above I wouldnt!

mateo1279 · 01-01-2008, 09:36 PM

Wow! That was very educational.... I will keep it hooked up to manifold vaccuum!
Thank you very much for the article, I have cut & pasted it to word for safe keeps.

-Thank you again!

-Matt

GMC Jim · 01-01-2008, 10:20 PM

Another important point---- HEI distributors that are quite commom are those on the early '80s engines. As emmisions requirements were in effect for all vehicles of that era; the ignition advance was retarded , ( severly retarded, in some cases) to meet government mandated specs for that year and model of engine. So, using an HEI distributor on an older engine can actually improve the performance very little.
Yes, the much improved spark voltage and the spark plug gap set to .045 does offer a substantial degree of performance and gas mileage improvement. But, the mechanical advance of the distributor is also very important. The system of weights and springs needs to be adjusted for the proper mechanical advance for the engine on which the HEI unit is installed. The best way to acheive this is to have a shop with a distritubor machine install the proper combination of weights and springs to acheive the proper advance for the entire operating RPM range of the engine... The specs for your engine will be contained in the section devoted to ignition and distributor specs. For the years of our trucks; these specs are listed in the back section of the GM factory shop manual
The combination of vacuum and mechanical advance is all important for the performance and fuel economy desired

mateo1279 · 01-02-2008, 10:06 AM

That makes sence. I bought a unit from a friend that was supposed to have already been tuned to improve performace.... I did feel a pretty good increase in throttle responce after the change. My enigine is kinda restricted, so I don't think I got much of a power boost, if any. I have a new intake manifold and carb I plan on bolting on in the next few months.... That should gimme a nice little boost ( going from stock 2bbl to a holley 600 cfm 4bbl)

Thanks for the tips

-Matt

12-31-2007, 02:02 PM	#1
mateo1279 Registered User Join Date: Jan 2007 Location: Oregon Posts: 169	Rochester2bbl and hei combo question Hello- Just hooked up an hei and it is running better then the points..... However I am concerned that I don't have the vac advance unit hooked up to the right place. Does anyone know a location for a 'ported' vaccuum on a rochester 2bbl? All the vac locations seem to have full vac at idle..... Thank you, Matt __________________ 1962 Chevy Fleetside 283/TH350

12-31-2007, 08:56 PM	#2
wild one Registered......... Join Date: May 2006 Location: Livermore,CA Posts: 288	Re: Rochester2bbl and hei combo question Here's something I stumbled on while surfing the web a while ago. Kinda lengthy, but it does explain this topic very well. Also gives a couple of links at the bottom of the article. Hope it helps, Pete Distributor Tuning and Theory There appears to be a lot of conversation on timing, vac advance, distributor tuning and, in general, hacking up a perfectly good ignition system to compensate for a poorly tuned carburetor. The following two articles review the basics of distributor tuning quite well and have worked for years and years and are based on sound engineering principals. I thought it would be helpful for some to review these prior to hacking up their distributors. Hacking up your distributor to compensate for a poorly tuned, misapplied or defective carburetor is not very sound engineering, for a street application or otherwise. Here's an interesting article on vacuum advance written by a GM engineer: 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. Rick WI http://www.fordmuscle.com/archives/2000/03/timing/ http://www.bob2000.com/carb.htm http://www.jason.fletcher.net/tech/c...carbtuning.htm http://www.futek.com/boltcalc.aspx http://racingsecrets.com/ __________________ 1966 C10 panel-400 small block/TH 350 combo Nothing is fool proof to a talented fool! Popcorn: Pops at exactly 444 degreese Expands to 44 times it's original size Has a 4 year shelf life

12-31-2007, 09:19 PM	#3
Ham-er Registered User Join Date: Apr 2007 Location: north dakota Posts: 50	Re: Rochester2bbl and hei combo question Wow, thanks for that Wild One. Mateo, most rochester 2bbl that I have seen Do have that "ported" vacuum port. It's just a matter of finding which one it is. But Uh....Do you still want to? After reading the above I wouldnt!

01-01-2008, 09:36 PM	#4
mateo1279 Registered User Join Date: Jan 2007 Location: Oregon Posts: 169	Re: Rochester2bbl and hei combo question Wow! That was very educational.... I will keep it hooked up to manifold vaccuum! Thank you very much for the article, I have cut & pasted it to word for safe keeps. -Thank you again! -Matt __________________ 1962 Chevy Fleetside 283/TH350

01-01-2008, 10:20 PM	#5
GMC Jim Senior Member Join Date: Dec 2002 Location: Portland, Oregon Posts: 901	Re: Rochester2bbl and hei combo question Another important point---- HEI distributors that are quite commom are those on the early '80s engines. As emmisions requirements were in effect for all vehicles of that era; the ignition advance was retarded , ( severly retarded, in some cases) to meet government mandated specs for that year and model of engine. So, using an HEI distributor on an older engine can actually improve the performance very little. Yes, the much improved spark voltage and the spark plug gap set to .045 does offer a substantial degree of performance and gas mileage improvement. But, the mechanical advance of the distributor is also very important. The system of weights and springs needs to be adjusted for the proper mechanical advance for the engine on which the HEI unit is installed. The best way to acheive this is to have a shop with a distritubor machine install the proper combination of weights and springs to acheive the proper advance for the entire operating RPM range of the engine... The specs for your engine will be contained in the section devoted to ignition and distributor specs. For the years of our trucks; these specs are listed in the back section of the GM factory shop manual The combination of vacuum and mechanical advance is all important for the performance and fuel economy desired __________________ '71 GMC;fleetside;PB/PS/AC/CC/402 Eng./Custom Paint/110,000act miles/3 fuel tanks(52gal). Last edited by GMC Jim; 01-01-2008 at 10:23 PM.

01-02-2008, 10:06 AM	#6
mateo1279 Registered User Join Date: Jan 2007 Location: Oregon Posts: 169	Re: Rochester2bbl and hei combo question That makes sence. I bought a unit from a friend that was supposed to have already been tuned to improve performace.... I did feel a pretty good increase in throttle responce after the change. My enigine is kinda restricted, so I don't think I got much of a power boost, if any. I have a new intake manifold and carb I plan on bolting on in the next few months.... That should gimme a nice little boost ( going from stock 2bbl to a holley 600 cfm 4bbl) Thanks for the tips -Matt __________________ 1962 Chevy Fleetside 283/TH350



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