GearheadJoe

For what it's worth, after reading extensively on this topic, I set up my distributor and engine as follows (this is a 383 with Dart heads and a 224/224 degree cam):

Mechanical advance: 20 degrees, all-in at 2600 rpm
Vacuum advance: 10 degrees, all-in at 10" vacuum
Initial advance: 14 degrees

So, the total mechanical advance is (14 + 20) = 34 degrees, and the vacuum advance adds 10 degrees at idle and light-load cruise. Other than idle and light-load cruise, the vacuum advance is not active.

This is a pretty conservative tune.

I should mention that this engine is not yet in the car, but for dyno testing, maximum power was at 34 degrees total mechanical advance with the Dart heads. OEM heads would probably be 36-38 degrees.

I think the ZZ4 heads would be similar to the Dart heads (slightly faster burn than OEM heads).

wmf62

here is the chassis dyno results as my ZZ4 is setup:

Bill

FWIW, initial timing at idle (approx. 800) is 20*

Stan's Customs

JohnZ..I know you have a lot of experience in this field.
What would be your best guess/estimate for a good base timing and "all in mechanical" starting point on the ZZ4.... and at what RPM ?


GHJoe..
Thanks for the input...those numbers are pretty conservative as you say...but so is this engine. Hopefully it will be a pleasant all around driving experience...with a/c, aod, and strong acceleration. Do you have any part numbers for the advance pot..and distributor weights/springs by chance? (if you have a GM hei..if not disregard)

Bill...
Thanks for the input and dyno results. 20 degrees initial timing...how do you like that?

That would be about 10 degrees initial and 10 degrees with a low advance vacuum pot all in on a HEI ..... (also what GH Joe noted). My initial break in /start up tune with no vacuum advance required 800 rpm to idle right with 10 degrees initial...and no advance with the stock ZZ4 HEI. You are idling at 800 rpm @20 degrees on your set up...correct?

Thanks gain...Stan

wmf62

Stan
yes, 20* with no apparent problems; BUT you have to consider that I have a computer controlling all the operational parameters.
Bill

Plasticman

Bill,

When the Vette was Dyno'd, was the trans in 3rd (1:1 ratio) or 4th (.73 OD) (knowing that you had the MY6 4 speed OD trans at the time)?

John

Stan's Customs

10-4.. ...Thanks

wmf62

OD (I think) with a 3.36 RER

Bill

Ironcross

Performance engines do not require a VC Can....

All manufactures, GM, Ford, and Chrysler have engines that don't require them....I know mine don't.....and I drive most of them on the streets.......

63 340HP

The 10 degrees initial and added 8 to 10 degrees idle vacuum really helped the responsiveness of my 350, and improved the ability to hold a low idle speed. Without the vacuum advance the stable idle is 850 or more at about 12" vacuum, and very temperature sensitive. With limited vacuum advance the idle is stable as low as 600 with the Duntov lope and 14+" vacuum, and not as sensitive to engine temp. My 350 design is more high rpm like a SHP than the ZZ4, so I would think it would help the ZZ4 more if a stable idle is desired.

Achieving the same stable idle is likely possible with no vacuum can using 18 degrees initial, with 16 degrees centrifugal, but the work to close the advance slot and then set the springs correctly to minimize detonation at the passing rpm range is time consuming (and you lose the response advantage of the advance kick off when tipping into the throttle on the highway, along with the safety factor of the lower vacuum advance under load).

If you have the spare distributor there is not much to lose other than an advance kit, and possibly a new vacuum can. Is there a vacuum advance can similar to then B28 for the HEI?

Stan's Customs

If you have the spare distributor there is not much to lose other than an advance kit, and possibly a new vacuum can. Is there a vacuum advance can similar to then B28 for the HEI?[/QUOTE]

Good question..and one I was hoping to get answered here...along with availability of springs and part numbers for both.

I did noticed the my vacuum was lower than motors back in the day..11/12 hg.

I'm hoping I can get a 10 degree pot... and only need springs to set the curve at "all in at 2800" or so...or after that, like you said, it gets to be work!

I do have two new distributors which helps a little...

Stan..

Stan's Customs

Hello Wally..

...are any of your cars automatics with air and HEIs?...or are they mostly manual transmissions, with point distributors.

Thanks ..Stan

63 340HP

Good question..and one I was hoping to get answered here...along with availability of springs and part numbers for both.

I did noticed the my vacuum was lower than motors back in the day..11/12 hg.

I'm hoping I can get a 10 degree pot... and only need springs to set the curve at "all in at 2800" or so...or after that, like you said, it gets to be work!

I do have two new distributors which helps a little...

Stan..[/QUOTE]

Late timing will lower vacuum.

HEI Advance Springs

If the link does not work search HEI Advance springs. The kit at Summit is $5.

None of my distributors are HEI so I have never looked at vacuum advance cans for them. The regular distributor B28 can is available through Rock Auto, you just have to cross reference the Echlin VC1810 number (NAPA no longer lists it in their computer, only in the old counter books, and only stock on hand).

Ironcross

A couple or two are here

Two 68 Camaros, one a GM crate LS-7 454 with a T'400 and a 75 Hei ignition, another {My favorite, a 68 with both AC and a T'400} with a L-72 427 crate engine but to keep appearances looking stock has the OE distributor

My 62 Vette is the point type, No VC can there and the 68 is a OE GM pulse amp type ignition firing a L-88 that the distributors VC port was sealed from the factory........the pieces are either 4.11 or 4.56.

One of my other toys is the 70 Cuda automatic convertible with A-990 Race Hemi and naturally no Vacuum advance here ither.....I did a stupid stunt on it.... placed a 513 gear in it......

More information can be found by google

GearheadJoe

Hi Stan:

Maybe I'm not understanding the wording here, but to clarify the terminology I was using in my post, I do not consider vacuum advance to be part of the initial advance. As I see it, there are three completely independent timing systems. All three of these operate independently of each other, so the total advance at any given moment is the sum of all three:

1) Initial advance, which is set by rotating the distributor at idle with vacuum advance disconnected.

2) Mechanical advance (the weights and springs), which is purely a function of engine speed and should never be active at idle.

3) Vacuum advance, which is only active at idle and high speed cruise.


At wide open throttle (WOT), the vacuum advance is inactive, so all you have at WOT is initial advance plus the maximum mechanical advance. At WOT redline, most engines want to see 34-38 degrees total. Many GM designs used, say, 6 degrees initial and 30 degrees mechanical, for a total of 36 maximum.

These days, a lot of guys think it is better to have more initial timing and less mechanical advance. So, you could have 20 degrees initial if you limit the mechanical to 16 degrees, and you still end up with 36 degrees at WOT redline. The idea is that this provides better throttle response with today's gas.

Another thing that people often do is to make the mechanical advance be all-in at 2500-3000 RPM, rather than the 3000-5000 RPM range that GM typically used. Again, this is believed to improve throttle response.

Note that I have not yet mentioned vacuum advance. Vacuum advance is only active at idle and low-load cruise conditions. Under these very specific conditions, most engines are "happier" with some extra advance thrown in. At idle, the engine will idle more smoothly and will run cooler. At low-load cruise, fuel economy will improve.

So how much vacuum advance is too much? If you set your mechanical advance to be all-in at 2500 RPM and your gears are such that you cruise at 2500 RPM or higher, your total timing at cruise will be the sum of all three systems (initial + mechanical + vacuum). Some engines will tolerate 52 degrees total under these conditions, but Lars has advocated on this board to keep it under 48 degrees to avoid surging. So, if your initial advance is 20 degrees and your mechanical advance maxes out at 16 degrees, your vacuum advance should max out around 12 degrees (20 + 16 + 12 ) = 48 degrees.

My Corvette is a '67, so I don't have HEI and I am not experienced with modifying HEI distributors. In general, though, the simplest modification is to change the advance springs to change the RPM where the mechanical advance is all-in. However, this does not change the amount of total mechanical advance.

So, if your distributor has, say, 30 degrees of mechanical advance, you probably can't run more than about 6 degrees initial advance (30 + 6) = 36 degrees. There are ways to reduce the mechanical advance so that you could use more initial advance, as long as the sum of the two remains in the range of 36 degrees.

In my view, vacuum advance is always a good thing, as long as the total advance at cruise does not exceed about 48 degress (per Lars' recommended limit).

Vacuum advance sure has a nice effect at idle. My guess is that anyone who is running without vacuum advance will find that their engine idles better with more initial advance, but unfortunately they can't run that much initial advance without exceeding a safe level of total advance at WOT redline, or without getting pinging when they accelerate at low RPM

It's worth noting that vacuum advance has nothing to do with peak power because it's not active at WOT. Furthermore, even the mechanical advance curve has nothing to do with peak power as long as the initial advance and mechanical advance add up to the same value. Both the vacuum advance and the mechanical advance have more to do with driveability and throttle reponse than with all-out racing applications.

Stan's Customs

Thank you John...that's a "lot" of good information.

Best regards...Stan

Stan's Customs

Ok boys and girls..

More information on a stock ZZ4 distributor ...on a stock ZZ4 crate engine.

After checking all my frame and engine grounds...transmission fluid, rack and pinion fluid fluid, differential grease...and digging out a vacuum gauge ...complete retune of the new carb and timing the engine with much more advance.

Here's the results of my distributor in stock form, using a Snap-On adjustable timing light.

36 degrees all in (mechanical (+) initial) at 2800 rpm.
18 degrees initial at 750 rpm (after the above setting).
38 degrees at 750rpm with vac advance hooked up (the vac can added 20 degrees)
17 " vacuum at 750rpm idle

I haven't driven the car yet...because I have some temporary exhaust suspended under the car for break in tuning. My new Sanderson ceramic coated headers aren't covered under warranty on a new engine break in...supposedly because the assembly lube burning out of the engine will saturate the ceramic coating before it has a few hot and cold cycles to cure the coating.

The engine sounds great (without vacuum advance)...but may ping with this initial adjustment. I believe they recommend 32 degrees at 4,000 rpm.

In order to run vacuum advance I'll need a VAC limiter or another can that doesn't pull 20 degrees at the crank.

But... I can see where this distributor could be driven with some minor timing adjustment if it pings, without a vac advance. I am surprised that the all in mechanical is not more...but I shouldn't be. John (Plasticman)said as much when he tested Bill's distributor on his Sun machine (21 mechanical).

If I'm understanding everything right...I should be pretty much on the money with a vac can that pulls 10 or 12 degrees...yes/no? Comments?

...Stan

GearheadJoe

Hi Stan:

Yes, I think you are on the right track. It looks like the mechanical advance in the distributor is only 18 degrees, so your initial advance can be set to 18 degrees if your goal is 36 degrees total at WOT redline (or actually, any RPM above 2800). The ZZ4 heads might be happier with a bit less total advance, but all that requires is backing off the initial advance setting just a bit.

So, the main problem here is that the vacuum can supplied with the distributor adds WAY too much vacuum advance (20 degrees) at cruise, bringing the total advance under cruise conditions to 56 degrees.

I'm not sure how much andvance the engine likes at idle, but it might tolerate the extra 20 degrees at idle.

In any event, I think all you need to do is to limit the vacuum advance to about 10 degrees, and you will be able to have the "best of both worlds." Namely, good throttle response due to the aggressive initial + mechanical advance, but also a smoother idle, lower idle temp, and better fuel economy at cruise, all of which are benefits that the extra 10 degrees of vacuum advance will provide.

One cautionary note is that if you simply limit the travel of the existing vacuum can to get 10 degrees max, that 10 degree value will occur at a much lower vacuum than the original 20 degree value. If you have something like a MityVac hand pump, you can use the MityVac to characterize the vacuum can while you watch the timing at idle.

Plasticman

Distributor Advance, Vacuum, Adjustable, GM, HEI, Kit:

http://www.summitracing.com/parts/crn-99600-1

Been using the Crane adjustable vacuum advances for many years. They adjust in vacuum pull-in setting, and in number of degrees.

To check and set your total idle timing (both initial and the added vac. can # of degrees - cent. should not be adding any advance at idle), adjust the dist. hold-down so that you can rotate it with some drag.

Then run the engine, and rotate the dist. until you see highest vacuum and smoothest idle. Then check the number of degrees of ignition timing at that point (both with and without vac. advance. Then do the adjustments (both initial timing and total vac. can degrees) to get where you want to be.

Plasticman

Stan's Customs

John...does it just adjust the stop point...or will it adjust the stop point and the vacuum pressure start point to engage ?

Question is clear as mud probably...but I can't think of another way to ask the question...?

Thanks...Stan

Plasticman

Stan,

The Crane units have 2 separate adjustments.

#1 is the vac. pull-in point that is set by using an Allen wrench inserted into the vacuum port (after the vac. hose is removed.....). They provide you with the Allen wrench.

#2. is total vac. advance degrees, that is set via a plate added inside the dist. under the cap, and this plate has several adjustment degree "valleys" to limit the vac. advance rod movement. The black plate is visible in the above Summit ad (webpage) just to the left and down from the vac. can.

Here is the writeup that Summit has:

These Crane adjustable vacuum advance kits let you tailor your ignition to the advance you want. The adjustability provided by these kits permits you to run the maximum amount of ignition advance throughout the rpm range, without incurring detonation. Once they're installed, you can quickly compensate for changes in fuel quality and altitude. Each kit comes with a vacuum diaphragm, an Allen wrench, advance weight springs, and instructions.

John