Vacuum Advance Canisters

[BMERDOC]

I've been studying every aspect of my truck to better understand it and increase its performance. The engine is a boat anchor integral headed 250 out of a '78 Nova/Camaro. I've got it running really good. I've been studying ignition timing hoping to squeaze any extra I can get out of it. It has an HEI in it and I'd like to get a re-curve kit to change out the springs. My research indicates that if I add the springs for faster advance I need to change out the vacuum advance because the stocker adds too much advance sooo...my question is this: Will the V8 HEI vacuum canisters physically bolt into the big cap/external coiled inline 6 distributor?
I do not have all my info yet. I know that I have set it for 8* initial but I dont yet know what "all in" timing is at and at what RPM or what timing is with the vacuum canister plugged in. I'll get that info later if I find that a tuneable vacuum canister is available for me.

[68gmsee]

The vacuum advance has little or no function when the throttle is wide open. So if you're trying to coax more power out of it and you floor it from a stop or while trying to pass, you are equalizing the atmospheric pressure between the outside and the inside of the intake.

Re. fit: If no one else knows, I think I have my 69 HEI somewhere in the garage. I'll compare it with the one in my 250 tomorrow.

[68_chop_top]

Don't know about the fit but other than a stock vacuum advance canister is not required unless you have a big cam in it with lower than stock vacuum.

[BMERDOC]

This is a quote from one of the articles I read:

TUNING FOR PERFORMANCE: Changing the advance curve.

Centrifugal advance: Stock the advance mechanism is pretty good but the stock springs are usually way too strong, causing the advance curve to come in too slowly, if it ever gets fully advanced at all. All you need are the right springs and the right initial advance setting. Most Small Block Chevys like about 32-38 degrees total advance at WOT. Since we know already that the HEI has 20 built into the stock mechanism the first thing we need to do is set the initial advance correctly- that means you need an initial advance between 12 and 18 degrees (you might want to retard it 2-4 degrees for daily street use just to build in a little safety margin).

Now all we need to do is make sure the centrifugal advance comes in at the right RATE relative to engine RPM. You want it "all in" by about 2800-3200 RPM for a street motor. You do this by changing the centrifugal advance springs to lighter ones. If you use the Crane advance kit like I do you are looking to install one Blue (heavy) spring and one Silver one (medium). These springs are located directly under the rotor and are easy to remove/replace by hand or with needle nose pliers. These springs will give you an advance curve that starts at about 800 RPM and ends at 2800. Easy so far, right? If you don't have the Crane kit then install whatever springs you have and check the advance curve with a dial back timing light and a tachometer, swapping springs until you get it close to these specs. It doesn't matter if the springs are not "matched" side to side- you can install one heavy one and one light one.

Vacuum advance: Stock cans typically provide 22-24 degrees of advance. This is WAY WAY WAY too much once you have recurved the centrifugal and initial advance the way I describe above. You will get "3 rocks in a coffee can" kinda detonation. Can you just leave it unplugged? Yes. Your highway mileage will be off by about 5 MPG and your plugs will load up with crud within just a few thousand miles. For a race car or a weekend street/strip car this is probably no big deal. For a street car, forget it.

Vacuum advance for the street: You want about 12 (crankshaft) degrees total vacuum advance if you run WITHOUT a functional EGR system, 16 degrees if you run WITH a functional EGR system. Regardless, you want it to come in between about 5 and 15 inches of manifold vacuum. I have found the most expedient solution to be the Crane advance kit once more. Install their can with about 9 turns on the adjustable advance can spring. IMPORTANT!- Also, use the little "lockout" cam that comes with the Crane vacuum advance to lock out AS MUCH ADVANCE AS POSSIBLE. This will still leave you with about 12 degrees of available vacuum advance. If you set it with 2 notches LESS lockout than maximum you will end up with about 16 degrees available vacuum advance
for those of you running an EGR valve.

[Wrenchbender Ret]

The 6&8 HEI vacuums interchange. You can get an adjustable vacuum advance & set it up like Docs post. That looks like some good info.

[BMERDOC]

Thanks Guys.
I have 8* initial and testing showed that the centrifugal advance is only advancing 10-12*. I think the springs are so stiff they aren't letting the weights fully advance. Weight advance seems to start after 700 rpm but doesn't advance past 20-22* and stops advancing before 2000rpm. I noticed a shiny spot on one of the weights, it looked like it was rubbing on one of the ribs on the underside of the rotor so I mildly clearanced that rib. I retested and it showed no improvement. I did test the weights by hand and they are smooth. Looks like I do have room for improvement.

[BMERDOC]

BTW, the canister was marked with 550 24 on the top of the arm.

[oldblue1968chevy]

I think the 24 is the degrees in the can?..

[BMERDOC]

I'm not sure it is and I'm not saying your wrong, oldblue1968chevy, but if thats the case I'm running 20" hg at 650 rpm. If that IS the case the diaphram would not bottom out and the timing would flutter.

[oldblue1968chevy]

My vac can says yada yada 20

I read somewhere that 20 stands for the * it puts in

[edhtrd71]

your vacuum advance should be hooked to manifold vacuum causing advance when at idle or light to no throttle. You shouldnt have both advancing at the same time. Set your total timing with the vacuum plugged of. Sounds like your springs are to tight and you will gain performance from more mechanical advance. Once you have total timing let the engine idle and plug in your vacuum advance look for mid 20s and notice the rise in idle rpm. Vacuum advance was designed to make the engine run cleaner not make more power.

[fleetsidelarry]

Originally Posted by edhtrd71

Vacuum advance was designed to make the engine run cleaner not make more power.

"vacuum advance" was around long before running cleaner was a gleam in anybody's eye. hooking it up to the "ported source" IS a way of cleaning up HC emissions at idle, part of a whole miss-mash of fix-its that manufacturers tried.

the fact that a vacuum advance retards the timing at critical times is equally, if not more, important.

here's a great paper on the subject of "vacuum" and timing:

http://home.comcast.net/~chadwick.ro..._explained.pdf

BMERDOC, if the canister you have is an original, then (it's my understanding) the 24 is indeed the amount of crankshaft degrees in it. these cans (there are something like 40+ different GM ones available) achieve their respective max degrees at different vacuum levels. if you're making 20" Hg, then your can ought to be full-in.

you can limit the amount of vacuum advance in several ways: buy an adjustable unit, buy a "stop", buy a OEM replacement can with the amount you want built into it, or fabricate a simple stop.

[BMERDOC]

Thanks for everyones thoughts and suggestions!
Can anyone give me a sense of the gains I could expect from doing this? I don't expect to carry the front wheels for 60'. I know that mileage can increase and driveability should be enhanced. I imagine that it may have the same effect as a drop in K&N air filter, not a lot but a butt dyno can pick up the improvements.

[fleetsidelarry]

Originally Posted by BMERDOC

Thanks for everyones thoughts and suggestions!
Can anyone give me a sense of the gains I could expect from doing this? I don't expect to carry the front wheels for 60'. I know that mileage can increase and driveability should be enhanced. I imagine that it may have the same effect as a drop in K&N air filter, not a lot but a butt dyno can pick up the improvements.

BMERDOC, most all the stuff that's been discussed concerns setting up distributors and timing on non-stock v-8s. I don't know whether any or all of these "tweaks" even apply directly to your engine. the more potential and engine has, the more it can benefit. I just don't know whether your 250 is going to respond to tweaking the timing with a "seat-of-the-pants" power increase or mileage improvement.

I know there are some 6 cylinder tuners out there that can help. They may have missed your post. If you haven't previously done so, maybe if you asked specifically about changes to a 6-banger in your title you would get more suggestions.

[BMERDOC]

Accidentally erased my subcription to my own thread. Had to repost to re-add. I reread the thread and Oldblue1968chevy and FleetsideLarry were right about the numbers on the vacuum canister. It was also in the article I quoted "Vacuum advance: Stock cans typically provide 22-24 degrees of advance."

[HEI451]

There are a number of stock GM HEI vacuum canisters available, over 15 or so. Selecting the RIGHT one is tough at best, and takes someone that KNOWS curves and settings to make it all work.

That said, you want the vacuum advance to work on FULL MANIFOLD VACUUM SOURCING, vacuum strong and present at IDLE, and NOT ported vacuum. This source should be at the BASE of the carb, NOT above the throttle plate.

It is also correct that all vacuum advances produce too much timing, IF the limiter bushing isn't installed on them. The limiter bushing is a rubber tube, mostly just rubber vacuum tubing, placed over the pull pin, to limit the pin travel, which reduces the over all degrees of vacuum advance crankshaft timing. This said, the "24" referred to on the canister IS the crankshaft amount of vacuum advance degrees that canister will produce, far too much for our needs.

A nice idle timing we would like to see os between 20 and 22 crankshaft degrees, but, if the initial is set there, the engine is usually too hard to start, and this is where CORRECT vacuum advance souring comes in. You state you now have 8 crankshaft degrees of initial timing, and the canister has 24 available, IF the stop busing is in place. IF the canister is on full manifold vacuum, as it should be, the vacuum advance will take the idle timing to 32 degrees, tar too many degrees. If the rubber limiter isn't there, as they degrade and fall off, never to be replaced when a new advance is mounted, then, the sky's the lkimit for idle timing.

Now, IF we set the initial to your 8 degrees, and make a simple sheet steel limiter and attach it to the vacuum advance mounting bar with two 6/32 machine screws, we can set the pin travel to .120 inch travel, for 12 crankshaft degrees, and make sure the canister is full manifold vacuum sourced, we now have an idle timing of 12 plus 8, 20 degrees, exactly what that engine wants.

We also have a situation where every stock canister has a different vacuum pull, along with different curve numbers, and this is why there are so many of them available, who knows what is what. The extremely simple solution to all this is the Crane 99600-1 adjustable vacuum advance kit, has 3 sets of mechanical advance springs, a pin limiter stop and a fully adjustable canister, giving EVERY stock GM setting, And everything in between them. The only modification is, the Crane stop plate needs to be mounted NOT where Crane says to put it, but on the OTHER side of the pin, towords the canister, NOT the end mounting screw. This is very easily done with one threaded hole and an 8/32 machine screw. The Crane method changed the vacuum pull spring strength with every degree adjustment, my modification separates the vacuum rate from the number of degrees selected, making vacuum advance tuning work, instead of compromising with every adjustment change.

If you are interested, I have a series of pictures and an outline on just how to correctly modify both the GM stock, and Crane advances to the correct way the stop plate should be, for both HEI and stock point distributors, along with the simple home made stops as well. There are even pictures on how to mount the Crane limiter plate to a stock GM vacuum advance, points or HEI.

If you want a picture set, there is no obligation, no ad, just simply e/mail me and ask for the one that fits your distributor, in this case an HEI, and I will send them to you.

Use this e/mail address, please. info@davessmallbodyheis.com and I will send same.

Also, I really don't advocate using ported vacuum, if there are those that still do use it, they have NO clue as to how a "load compensator" (vacuum advance) actually works in a non-EGR equipped internal combustion engine. The ONLY engines that require ported vacuum are those with a functioning EGR valve for EMISSIONS use, and there aren't many of them still running around these days.

[BMERDOC]

Thank you HEI454 for your input. I have read your posts providing information to others here and feel confident with your knowledge. I am familar with the clear tube you speak of but cannot tell you for sure if it is still intact on this distributor as I haven't touched it since doing the research and starting this thread. I would like to see the pics of your modifications so you will be hearing from me soon. Thanks again for the input.

I feel kinda silly about asking about the gains expected but I know that I will notice a stronger pull from the engine and better driveability. The engine in question is not desireable but I know that ANY engine will improve you just gotta know where to tickle it!

[HEI451]

For the V8 setups, I advise finding a different set of weights, marked 41, and a different center, marked 375, will give you 22 crankshaft degrees of mechanical advance, and when used with 12 degrees initial, total mechanical will be 34 crankshaft degrees. You can find these parts in wrecking yards, and they came in a lot of late 1970's/early 1980's HEIs in trucks, Suburbans, and ALL carburetted ZZ crate engines, along with a lot of complete replacement engines that came with the distributor installed. I do not know what the stock curve was for that engine, but, for performance, and stock/mild street, these parts help immensely. Mix them with a combo of springs from the Crane kit, and you should have the mechanical advance down to working. I would assume the stock mechanical curve was set up for a very conservative initial, like 8 degrees or less.

Match that kit up with the Crane advance, and it should work great for that engine.

This should be a large, coil in cap HEI. The weights, center, vacuum advance all interchange with all vacuum advance early HEI's.

As far as that engine goes, it did have an EGR valve in place, but it should be removed, and the boss it bolted to, have a gasket and block off plate in its place, and the plate can be an aluminum plate, cut to fit and bolted in place of the EGR valve. Of course, I can't recommend this if your state requires a functioning EGR valve for emissions testing/certification. removing it is up to you.

I got your e/mail, will sort out the pics and send them later tonight.

[crazy longhorn]

Originally Posted by BMERDOC

Thank you HEI454 for your input. I have read your posts providing information to others here and feel confident with your knowledge. I am familar with the clear tube you speak of but cannot tell you for sure if it is still intact on this distributor as I haven't touched it since doing the research and starting this thread. I would like to see the pics of your modifications so you will be hearing from me soon. Thanks again for the input.

I feel kinda silly about asking about the gains expected but I know that I will notice a stronger pull from the engine and better driveability. The engine in question is not desireable but I know that ANY engine will improve you just gotta know where to tickle it!

I have a copy of Chevrolet Inline 6, (Leo Santucci)....I will sit /read a little tommorro, if I can help on the curve, I will. Its not one i have played with, but I will try to send it on crazyL

[HEI451]

A basic info on selecting spring rates for the mechanical advance.

Your engine will have an out of gear idle speed. lets say it is 550 rpms. This speed will drop insignificantly if you have an automatic trans, when the trans is put into gear, no problem.

We want the mechanical advance to begin between 100 to 150 rpms above the out of gear idle speed, so, curve should be sprung to start between 650 to 700 rpm's in our example engine.

Limit of the mechanical curve in an HEI is set not by the spring combo as much as the weights/center selection. A points distributor will have positive stops in the mechanical parts, a post and bushing in a slot. There is an idle stop, limit stop and the weights and center set the curve path. Changing the stops changes the start/stop points of the mechanical curve.

The large cap HEI doesn't work that way. With these distributors, the selection of weights and center sets ALL mechanical advance degrees, start and stop parameters. This is why the "slots" many erroneously insist need to be "brazed up", are NOT the right way to set the start/stop and rate parameters.

If you think that is somewhat complicated, then realize that there are just at 437 different weight/center combos for the large cap HEI's, selecting one set that works is tough without knowing all that. The only GM performance sets are the 41 weights/375 center pair, which I outlined in the posts above, as they are the ONLY non-emissions curve parts for these distributors, and that performance spec was also used in those trucks, crate engines and every carburetted ZZ engine made.

Also, the after market curve kits for the large HEI are usually dismal at the every best, they don't work. Almost EVERY after market HEI manufacturer uses the 41/375 combo in their performance HEI's, so, the 31/375 combo IS the one to use for anything from dead stock to performance. These parts are not easy to find, but are in a lot of wrecking yard distributors, just have to take a day and go pull caps and rotors, and look for 'em. If you are really bucks up, then the shaft assembly for the ZZ engines is available directly from GM, for a paltry $151.90/EACH. Lotsa bucks for two weights and a center, and a shaft and reluctor you can't use.

Just more FYI for you on the large cap HEI's.

[crazy longhorn]

Thanks Dave, lots of good read there After reading the last post, it is my understanding that the HEI ,v8 weights/center do work with the I6 HEI? crazyL

[webfoot]

(Copied from another document)

The following listing (HEI) is as follows: The first four part number listings are the 4 numbers that are most commonly used in a Chevrolet performance application.

The “AR12” can is the most versatile and user-friendly unit for a good performance street engine.
The AR 15 and AR23 are almost identical, with only slight variations in their “start-stop” specs.
The “AR31” can is the HEI equivalent to the “B28” Hi-Perf can used on the early engines:
The advance comes in very quick on this unit – too quick for many performance engines.

Do not use this very quick unit unless you have a cam/engine combination that really needs an advance like this. It can be used as a tuning aid for problem engines that do not respond well to other timing combinations, and can be successfully used in applications where direct manifold vacuum is applied to the can (see paragraph and discussion on this above)
After this, the listing is by Echlin part number.

All GM HEI vacuum advance units are interchangeable, so you can use a Cadillac or GMC Truck unit on your Vette, if that’s what you want to do ....

HEI Distributors:
P/N ID# Application Starts @ “Hg Max Adv (Distr. Degrees @ “Hg.)

VC1838 AR12 1975 350 Buick 7-9 7 @ 10-12
VC1853 AR23 1976 350 All Calif. 5-7 7.5 @ 11-12.5
VC1843 AR15 1977 305 All Exc. Hi Alt, Exc, Calif. 3-5 7.5 @ 9-11
VC1862 AR31 2-4 8 @ 6-8
VC1703 N/A 1978-79 Vette Special Hi Perf 3-6 5 @ 7-9
VC1825 AR1 1976 454 Caprice, Impala 3-5 9 @ 6-8
VC1826 AR2 5-7 12 @ 10-13
VC1827 AR3 5-7 9 @ 9-11
VC1828 AR4 1975-76 350 Buick & Olds 6-9 10 @ 12-14
VC1831 AR7 6-8 12 @ 14-16
VC1832 AR8 1975-76 455 Buick Electra 4-6 12 @ 12-14
VC1833 AS1 1975-76 500 Cadillac Exc. Calif. 4-6 14 @ 15-16
VC1834 AR9 4-6 13 @ 13-16
VC1835 AS2 1975-76 350 Olds 5.5-7.5 12 @ 15-17
VC1836 AR10 1977 305 All Hi Alt, Exc. Calif. 3-5 9 @ 11-13
VC1837 AR11 1976 305 Blazer, Exc. Calif 6-8 12.5 @ 10.5-13.5
VC1839 AR13 4-6 12 @ 11-13
VC1840 AR14 1975-76 350/400/455 Pontiac Firebird 6-8 10 @ 9-12
VC1841 AS3 1975-76 500 Cadillac Calif. 5-7 10 @ 13-14
VC1842 AS4 1976 350 Olds Cutlass 5-7 12 @ 13-15
VC1844 AR16 3-5 12 @ 13.5-15.5
VC1845 AS5 1978-79 425 Cadillac w/F.I. 4-6 14 @ 14-16
VC1846 AR17 1977 301 Buick Skylark 3-6 13 @ 10-13
VC1847 AS6 1978 403 Motor Home 4-6 12 @ 12-14
VC1848 AR18 4-6 12 @ 9-12
VC1849 AR19 4-6 12 @ 7-10
VC1850 AR20 1977 350/400 Pontiac 4-6 10 @ 8-11
VC1851 AR21 1977-79 350 Buick LeSabre, Century 5-7 12 @ 11-13
VC1852 AR22 77-78 305/350/400 Chev Truck, Heavy Duty 7-9 5 @ 12-14
VC1854 AR24 3-5 13 @ 10-13
VC1855 AS7 1977-79 260 Olds Cutlass 3-5 15 @ 10-12
VC1856 AR25 3-6 15 @ 10-14
VC1857 AR26 3-6 12 @ 13-16
VC1858 AR27 1978-79 305 All 3-6 9 @ 11-13
VC1859 AR28 1979 350 Vette Exc Hi Perf 3-6 10 @ 9-12
VC1860 AR29 3-6 12 @ 10-13
VC1861 AR30 1978-79 301Buick 3-5 13 @ 11-13
VC1863 AR32 2-4 10 @ 11-13
VC1864 AR33 1978 305 Chev Truck, A/T, Light Duty 4.5-6.5 13 @ 11-13
VC1865 AR34 1973-74 350 Vette Special Hi Perf 3-5 15 @ 8.5-11.5
VC1866 AS8 1978-79 425 Cadillac w/carb 3-5 14 @ 13-15
VC1867 AS9 2-4 10 @ 8-10
VC1868 AR35 1979 305 Chev Truck & El Camino 2-4 10 @ 6-9
VC1869 AS10 2-4 12 @ 8-11

Good reading (and the source of this info)
http://www.lbfun.com/warehouse/tech_...ance_Specs.pdf

[HEI451]

Crane 99600-1 adjustable vacuum advance kit gives ALL the above specifications, and everything in between, only needing ONE kit to fix the distributor.

[BMERDOC]

Dang! Look at all the info!!! This is a good example why you guys rock!

[HEI451]

That info should be held on the site, in some sort of easily accessible topic forum for tech info, and NOT be lost/deleted.