joepenoso

Some many dumb questions...so few days....Anyhow my HEI distributor in my 1971 455/ 442 can total advance to 30 degrees then vacuum pot hits the firewall' I tried setting the distributor up as suggested by yesterdays dumb questions but this is the result. I know I have to slide the distributor one hyphoid tooth counter clock wise on the camshaft gears......right? Any insight will be much appreciated! I don't want to break out the cutting torch onto firewall to get the necessary couple of inches of adjustment!!
Thank in Advance
joepenoso

oldcutlass

Yes you can restab the distributor and then rewire your firing order accordingly to accommodate the new #1 position.

TripDeuces

Turning the distributor is going to give you more initial timing. Is this what you want?
I only ask because it looks like you want 6 more degrees and putting that much more into the initial side of the equation might make the engine hard to start. I guess you'll find out soon enough.
You want as much initial as you can but it has it's limits.

joepenoso

Advancing total timing means turning ignition clockwise.....it hits the firewall at 30 degrees. I am trying to lift the distributor out of the block and place it a couple of inches counter clockwise so now I can advance total the extra few inches without hitting the firewall. The initial should be the same according to my thinking....am I right?
Thx
joepenoso

RandyS

No, rotating the distributor affects all of your timing, initial and total. You will need to move the wires in the cap as well as re-stabbing the distributor. As mentioned previously, bring your #1 piston to TDC (timing mark at '0'. Stab the distributor so that the rotor points to where you want #1 plug wire to be. Adjust plug wires (in firing order) to coincide with the new position.

rjohnson442

If your wires are long enough you could move them all one terminal over and time it without pulling it completely out again.

Octania

When you turn the distributor, you are by definition setting the initial advance.

You can also alter the vacuum canister [they vary greatly in amount of travel = amount of VA offered, vacuum level required to attain motion, etc.], and/or the Mechanical Advance weights/ weight cam/ springs.

Don't forget that the engine may greatly prefer to have the distributor vacuum sourced from the intake manifold rather than a ported vacuum source at the carb.

MDchanic

This is very simple, so don't get bogged down with complex instructions:

• Remove cap.
• Note which wire rotor is pointing to.
• Unscrew and remove clamp.
• Lift distributor partially out of block. Rotor will rotate slightly as it climbs gear.
• Rotate rotor in the desired direction while holding it gently against the cam gear, so that you can feel when it drops into the next tooth.
• Push distributor back into engine.
• Replace clamp, tighten loosely, so that you can rotate distributor with a firm grip.
• move wires (if necessary) so that rotor now points to wire that it pointed to when you removed it.
• Replace cap.
• Set your timing with a timing light. You can get a rough idea while cranking without starting the engine.

- Eric

TripDeuces

If I'm correct I think HEI's have 20-22 degrees of mechanical advance built in. So you would need 14-16 degrees of initial advance.
Bring the #1 spark plug up on the compression stroke and stop when the timing pointer is on 14-16 degrees. Put the distributor body in where you want it and where you can move it easily with room. The distributor shaft and rotor position you can put anywhere (within reason since both the gear and oil pump shaft need to line up). You won't necessarily be able to put it one tooth at a time because the oil pump shaft will only allow you to drop down every 60* due to the hex shaped shaft. But you can get around that by moving the oil pump shaft with an extension and a socket if need be. Duct tape the socket to the extension.
Where the rotor is pointing will now be your #1 spark plug terminal. One again that can be anywhere you want to put it if you're worried about wire length.

With all that said I know HEI's are timed at something like 1100 RPM so I'm not too sure how that effects things. My procedure above was for a basic points system but it can work with anything.

joepenoso

Operation was very successful I got the advance following your instructions........3 step: 1 back 2 forward.....it woke the bottom end up...all across the board actually ....when it came time to set timing with my brand new Summit adjustable advance timing light bulb burned out.....set it to where runs nice no pre ignition....bad part when hot it cranks over slowly
Thanks
joepenoso

oldcutlass

Get another timing light and make sure it's where you want it. If its hard to crank and kicks back, you might have too much advance.

joepenoso

No kick backs only hard hot cranking

TripDeuces

Hard cranking is too much initial timing (as long as this is something new?) Taking out a couple of degrees should eliminate the hard cranking.

J-(Chicago)

I like where your head is at Chris, but it's a slippery slope.
I just did that same thing on an hei for my old lady's driver. Street cars run vacuum advance, so it was a big part of the equation.

my example: plugged the vacuum advance. set it to 14 initial.
with the added mechanical it was spinning up to 38 right off the bat at 2k rpm. Not what I wanted at all.

I put a pair of stiffer springs in it and found I tamed it down to 14 mechanical total all in around 2700rpm. fine by me.

I backed initial down to 10, and had 24.
Well, I plugged in the vacuum advance and, it shot right up to 54.
30 degree vacuum diaphragm in this thing.

I eyeballed just before the halfway point on the slider and welded it up. Couple swipes with the hand file, and I was at 12 degrees vacuum assist. (on manifold vacuum)

10 initial 14 mechanical, and 12 vacuum = 36 (at idle) 24 total

If you just go plugging in random hei's, and leaving them as is, you may be disappointed.
Even though the advance doesn't count as total, it's still something to think about while you're idling about at the car shows this summer.

Just my latest experience with an old 8:1 350. nothing fun or exciting. It probably would have taken it like a champ if I left it though haha

oldcutlass

Actually J, the total of mechanical and initial should be around 36, with vacuum should be around 50 @ around 3000 RPM more or less. Most aftermarket HEI's have any where from 17-21 degrees of mechanical advance. Non-adjustable vacuum cans can really drive your advance up past what the engine can tolerate and cause pinging and surging during cruise.

J-(Chicago)

Agreed.....for performance anyway.

I just don't see the reason why it should ever have that much for what it is. A 4300lb part throttle-mobile with a 2.41 gear.

That's why I just welded it up and avoided that pitfall all together and just run constant vacuum.
I'm not saying it's the "best" way to do things. and it might even be a little backwards, but it'll get the old jalopy to the cruise nights, and keep her out of the ditch.
I'm

TripDeuces

Vacuum advance is for emissions and gas mileage. It's fine on a performance vehicle that serves double duty on the street and an occasional track blast. 45+ degrees of advance is common with a vacuum advance hooked up. My 66 runs in the high 50*.
You have to remember that under load such as WOT the vacuum in the engine drops to near zero and any timing added by the vacuum advance is removed bringing the engine back to total timing which is initial and mechanical added together.
You want your total timing to be in the neighborhood of 34-36* for most vehicles.

RandyS

I was taught (eons ago) that as RPM increases, Vacuum drops in the motor, and Vac Advance begins to drop off, while Mechanical Advance begins to increase, so your total advance pretty much doesn't go over the max of 36 deg? Is this theory wrong?

J-(Chicago)

I had a problem with the weights not returning to center at idle when initial was 14 and total was 38. add the 30 degree vacuum at idle and i was up to 68 at idle.
Kind of a compromise situation i guess. I couldn't find any other weight springs in my junk drawer.

J-(Chicago)

Doesn't sound wrong to me. I can tell you from my experience that at 3000 rpm the vacuum gauge was still pulling 15 hg though.

However this was in park and not under load.

TripDeuces

Yeah Randy that's wrong. Initial + mechanical = Total. Initial is variable to what you set it at but mechanical is built into the distributor and can only be changed by modifying things within the distributor. Some aftermarket distributors allow you to change mechanical timing with bushings, screws, etc.
Vacuum advance is in addition to Total timing and helps mileage and emissions.
I'm not aware of vacuum decreasing with engine speed except under load. In fact I'm pretty sure it's a function of throttle opening overall. More opening = less vacuum. Anyone with a in car vacuum gauge can verify that.

TripDeuces

J as far as the springs they only control the rate at which the advance is applied. Your distributor still has 'X' amount of mechanical advance built in. Lighter springs allow that timing to come in sooner in the RPM band while heavier springs restrict that to a higher RPM. The springs do not control the amount of the timing, just it's 'rate'.

J-(Chicago)

I understand that.
I was merely pointing out that It would jump up to 38 total just off idle. That's why i put the stiffer springs in. they were so loose it would give it all at once under 2k rpm and then not return to home position

I don't doubt that it will eventually climb to the 20-22 mechanical built into it, but i'd really have to spin it up to get it to travel that far now. At 2700 it was only allowing 14 mechanical

J-(Chicago)

Assuming you're spot on with the 20-22 mechanical range, (which you probably are) at full potential total would be around 32. In it's normal putzing around state of existence, I'd call it 24.

65mph, 27.5" tall tire, 2.41 ratio.

I'd guess it probably runs 2000rpm on the expressway.

oldcutlass

Yes, your theory is wrong. At normal cruise your total advance with vacuum can get up to the mid 40's to over 50 depending on what vacuum canister is installed, amount of vacuum, or how it's limited. I don't run vacuum advance currently for fuel economy.


The amount of timing is controlled by the type of weight and shape combined with how the advance plate is setup. As stated above the springs control the rate. GM came out with dozens of weight sets for these distributors for different engines and models.

oldcutlass

I see, when I drink, I think, so I think... I'll have another drink. Now I understand Steve.

J-(Chicago)

Now that's funny.

J-(Chicago)

If you get buzzed enough, your spins could possibly generate more centrifugal force than the distributor i'm speaking of does.

joepenoso

Well put
LOL
joepenoso

HWYSTR455

That's correct, except IF the carb is too small for the application. In that case, you see vacuum at higher RPM.

Pretty common when using like 'Mr. Gasket' or other aftermarket springs and weights that you see advance at idle, making it difficult to adjust idle speed and mixture. Most are junk.

The weights actually should be matched to the center plate, which is part of the shaft. On a GM HEI, it controls rate to a certain degree, and total mechanical. I believe both Schurkey and SunTuned both have charts that show the plate relationship to advance, google it. (I've even posted URLs for reference on here before)

GM vacuum cans have a stamping on them that tell you how much advance they provide, and the inches of vacuum for the range.

Just to say too, vacuum advance isn't just for fuel economy, it is also for part throttle response and cruise conditions. In most cases, if you don't run it, while not at WOT, you will have a rich condition. Tune that out with the carb and most of the time you will have a lean condition in other areas.

Vacuum advance at idle helps cool the engine too, so in traffic, it's best to have it, especially with AC cars.

For initial advance, you want as much as possible, but usually starter load and idle speed/mixture will limit what you can run. New cars run 20, 24, and as much as 28 initial advance, as an example. On non-EFI/computer controlled car, usually 16-20 is the range people shoot for.

Running more initial, or base, means you most likely will have to limit total. As pointed out, that's an available option with bushings or whatever on aftermarket distributors. On HEIs, it's a function of the center plate and weights combo.

You always start with initial, then adjust the rate, or 'curve'. (with vacuum blocked off). Test by making WOT passes. Once you have that down, then you move on to vacuum, which is tested by 'normal' part-throttle and cruise driving.

What total advance an engine likes is usually a factor of the combustion chamber design, as well as dynamic compression. (Dynamic CR is higher with higher static CR, but many just reference static). Objective is to burn all fuel as fast as possible at a given relationship to TDC of the piston in the chamber. Slow burn = more advance is needed, and visa-versa. More compressed mixture = burns faster, hence why higher cyl pressures need less advance. Same with 'fast burn chambers/heads'.

Load is a factor too, so if you have 4.11 gears and a perfect tune, and switch to 2.56 gears, you're going to have to back off the curve.

.