HEI conversion tuning

[Guest]

Hello all, I recently did the hei conversion on a 73 Cutlass 350, using a 77 distributor. Problem is I have to run alot of initial advance to get the car to run strong. I know the 77's were timed at 20 btdc, but its way over that.(yes, vacuum adv is disconnected) And still it doesnt pull as strong as the original set up. I'm sure the balancer hasnt shifted because it used to run perfect at 12 btdc w/ the points distributor. Is there something I'm overlooking?

Also, I'm using the Crane advance kit w/ the adjustable vacuum unit and both heavy springs rated at 3200 rpm. Its a stock 73 350 4bbl, auto trans and 3.70 gears, running on a 1/4 mile oval track (pump gas too). Should I switch to lighter springs? Thanks -Eric

[Sgt Art]

I found this on Olds FAQS, it's good site with a ton of info. Here's the conversion info http://www.442.com/oldsfaq/ofign.htm#IGN%20PointsHEIIgnitionConversion

BTW, are you getting a full 12 volts to the distributor? Don't use the resistance wire for points type distributor.

Tuning

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Advance Curve / Recurving

Introduction:

You have an "initial" timing setting for idle and as the RPM's go up, the weights inside the distributor overcome the force of the springs preventing the weights from moving, due to centrifugal force. As these weights extent outward the timing advances, until they hit their stops. This is called mechanical advance. Between your initial and total mechanical settings, you have the advance curve or specific amount of timing at a certain RPM. If you plotted this on a graph it would look like a curve that starts adding just a little advance at first and more towards the total end. Vacuum advance can add a bit more timing advance over this curve. You need an adjustable timing light to really play with it, or a degreed balancer so you can watch the curve happen as you raise the rpm. By swapping in lighter or heavier wieghts and stronger and weaker springs, you can manipulate this curve in several different ways, but it is sort of a black art, and takes a bit of talent to get the most out of it.

The recurve is easy enough to do yourself, but a lot of trial and error is involved. You need a graduated timing light as mentioned (or timing Tape). I used the Moroso kit which is cheap and has lighter weights and two weights of spring. With your stock springs that makes three different weights of spring. You can calculate all the combinations and permatations. After a lot of messing around with varying combos, I ended up using both stock weights and a light spring on one side and the stock one on the other side. I also used a bushing that came with the kit to limit the amount of vacuum advance. Ended up with 34° total timing, all in by 2500 RPM. this is on a 69 350/350 with Edelbrock Total Power pkg. (heads excepted). I was originally after 36 deg at 2500 RPM and the car ran like a raped ape at mid to high RPM, but I couldn't get rid of a real rough idle and it would baff out starting off at light throttle. set it back to 34 deg. and it runs a lot smoother but I think I lost some performance. It takes a lot of patience (or luck), to get the curve where your car wants it.

If you plot the amount of centrifugal advance vs. rpm, you get a "curve" that is usually linear up to a maximum amount of advance. Different springs will adjust how soon that curve begins from the initial advance, as well as how long it takes to get to maximum advance. You can also include the vacuum advance in that curve, but because that advance is proportionate to engine vacuum, it's extremely variable, and usually not counted in what's referred to as the distributor "curve".

Recurving or setting the advance curve involves springs, and in extreme situations, new weights (either heavier or lighter) and limits or extensions to the slots or pins that regulate maximum advance. Springs and weights are included in a recurve kit.

You can do it by machine, where you match the amount of advance at certain rpms to some ideal curve. Or, you can do it yourself in about an hour's worth of testing, and create a custom curve that's ideal for your particular engine. There are many advance kits out there; I was happy with the Crane adjustable vacuum advance kit, which cost me $22.

The existing distributor curve may be ideal for your engine, or it may not. Usually, you want to have as much advance as you can, before detonation (spark knock) sets in. You want to find out how much total advance you can run, and try to get that much advance in there as soon as possible. The exact numbers are not all that important, which is why curving a distributor on a machine may or may not improve your car's performance, since the ideal curve that the distributor is set to on that may not be the ideal curve for your car. It's not just the cam that affects the ideal curve, but also gearing, transmission, engine condition and other modifications, jetting, fuel quality, driving style, atmospheric conditions, etc.

I'd recommend something like the Crane kit; follow its instructions, and you'll likely be happier with how the engine runs afterwards.

The various combinations of springs and wieghts available will determine the speed and rate of change of the mechanical advance. This is not a linear change (ie, a straight line if you graph it rpm by advance) but has a more rapid rate of change at either the lower or upper part of the RPM band. This is your curve. What is important is not to set the curve to exact specs but to what the engine wants. Because of this, you will eventually have to play with it on the car to get it exact. Variations in your combination (carb, cam, CR, ignition, etc) will make the optimal curve different for just about every engine. One curve may work from one engine to the next but still not be optimal. The adjustable vacuum advance comes after the distributor curve for fuel economy and drivability reasons. All you really need to curve a distributor is a fundamental understanding of how the advance operates, an adjustible timing light, an accurate tach, a curve kit with various springs and wieghts, and lots of patience. Most of the good curve kits come with instructions which will set you in the right direction. As long as you don't enter the detonation range just getting the curve close should be enough, since driving around town you really won't notice a difference of a few degrees here or there. If you were seriously drag racing however, you might find a few tenths. Hope this helps and wasn't too confusing.

A friend has a Sun distributor machine he picked up at an auction. Dates from the 1950s but everything works and it has all the documentation. It is quite a thing to see. It has a vacuum pump for the vac advance it drives the distributor with an electric motor. It has all sorts of dials to measure different parameters as it spins the distributor. I can tell you from watching it that you wouldn't want one of those advance weights to ever come loose at speed... it would be quite a violent collision with whatever it hit. A distributor really spins a lot!

He did mine last year. With a service manual for the specs you want it actually is pretty straightforward to check. With the machine you can see how the different weights will affect how much advance kicks in at a given RPM. Similarly with the vac advance. Also you can see how any variance or wobble in the bushings affects advance or dwell. Neat. I wonder how many of these machines are still in shops?

Race engines don't need vacuum advance, because they're never at part-throttle anyway. Any street engine spends more time at part-throttle than WOT, and can always benefit from having vacuum advance. Magazines like Hot Rod test engines at WOT (Wide Open Throttle) on a dyno, where vacuum advance plays no role, so they leave it off the engines. Then they tell you that you need 36 deg, or 32 deg, or whatever, of mechanical advance. They totally fail to inform you that your engine will run better on the street with additional vacuum advance over and beyond that 36° or whatever.

[ Thanks to Mick Gillespie, Robert Barry, Mike Bloomer, Greg Beaulieu, others below for this information. ]

Factory Setup and Information:

Delco produces a specification book called the "1.2" manual. The good news is that all distributor advance curves are listed. The bad news is that they are listed by distributor part number. You cannot just look up the curve for a "70 Toro". Fortunately the number is stamped into the distributor housing.

The centrifugal mechanism is subject to wear, which alters the curve. This is common on all distributors. The weights will cut into the pivot pins, sometimes shearing them off! The proper fix is to buy a new distributor mainshaft from GM. The last one I got was $60, had a "performance" curve built in, and fit a Che*y. Incidentally, that mainshaft "fixed" a number of "carburation" problems! There are repair kits to salvage the worn pins in the original mainshaft, they are a pain in the tuckus if you don't have a torch to heat the weights cherry red before trying to drill the pivot holes oversize. HEI's are especially bad for having worn centrifugal mechanisms because there was no need to remove the cap and rotor to replace points. No mechanic on commission is going to rip off the cap and rotor to lube the advance mechanism out of the goodness of his heart. Point style distributors needed the cap pulled off to replace the ponts, and so sometimes the advance was lubed at the same time.

The distributor bushings especially on points style distributors also wear, altering the timing in erratic ways.

Virtually all distributors from GM have WAYYYYYY too much end play, and should be shimmed so that end play is around .008-.015. This prevents dynamic timing changes due to the distributor gear jumping up and down on the cam gear. Since they are helical gears, vertical movement of the distributor gear causes the shaft to rotate, changing the timing.

[Guest]

Thanks Art, thats a good site. I've got 12V from the ign terminal on the fuse block. Gonna get a timing tape for the balancer to see where the actual timing is. On the Crane instructions it says that the advance starts at 800 rpm, I'm setting it in park which is probably 1000-1200 rpm, could this be the cause of the high initial reading? -Eric