pcard

For a 1970 455.
I just spent some time recurving my distributor. I found a place that has a Sun machine. The exercise was fun, but I was only partially successful in that I was not able to reduce the mechanical advance as much as I wanted.
The spring set I bought had no stronger springs than those on the distributor already, and although I ground down the weights quite a bit, in the time I had I was only able to take off about 2 degrees (crank) of advance.
Here are my final results (no vacuum) (all numbers at crank reference)(error 1*)
1000rpm - 2*
1500 - 6
2000 - 14
2500 - 18
3000 - 22
3500 - 26
4000 - 28 all in
Has anyone any suggestions where I can find some heavy springs or light weights? I would like to take about 4* off the curve.

The main reason I am posting this is because I think I used a cool method to clock the distributor before removal.
I cut a bit of cardboard (see photo) that fits over the rotor and is snug against the firewall. I then marked the positions of the rotor before lifting and after lifting off the spiral gear.
I used this as a reference when installing the distributor and was able to slide that puppy down on the right gear on the first try (I did put a bit of grease on the gears first).
The engine fired up straight away and timing was within a few degrees.

I did have a moment of panic - I removed the coil to make things easier, and forgot that I have a spacer washer to lift the coil a bit to clear the manifold. Well, when I went to put the distributor back in I moved some wires and the wires knocked the washer off the ledge and I watched in horror as it slid down right towards the distributor hole. It stopped right on the edge of the hole. Lesson - stop up that hole until the last moment!

Cheers.

MDchanic

Neat tool. I like it!

- Eric

pcard

The circle cutout is the inside of a roll of masking tape - perfect fit over the top of the rotor, but is held up by the flange around the edge.

RandyS

Just curious why you want less centrifugal advance? I did just the opposite in my 66 L69. I don't run any vacuum advance at all, initial around 16 deg, heavy weights and light springs, and all in at about 3000. Runs pretty darn good on high octane gas. Learned that trick back in 1968 when I owned my first L69. (Learned it from a guy who also owned a fast L69 named Billy Gordon in Vallejo, Calif. Anybody care to guess who is son is?

nsnarsk65cutlass

Ya Jeff.

pcard

detonation at 2000 - 2500 rpm under load. I want to reduce mechanical vacuum advance, increase vacuum advance.

MDchanic

Only at those engine speeds? Not at higher RPMs?

What is the vacuum when detonating?

- Eric

garys 68

Are you sure it's not a carb problem?

pcard

Hi Eric: it is only detonating at 2000 - 2500rpm, about 50mph-65mph, when going up a hill, or accelerating.
I do not have a in dash vacuum gauge, but this is happening when I have opened up the throttle so would imagine the vacuum to be fairly low.
At cruise with partial throttle there is no pinging at all.
I have a vac advance that is all (16*) in at 13 in, and falls off quickly below that.
It is connected to the manifold. I have 8* initial. Which I am going to move down to 6* to see how she runs (will have about 22* at 850 rpm).
The idle is good and steady, although the carb is not on the idle circuit (vacuum at the port is the same as manifold). Cliff Ruggles believes the carb cannot keep up with the new cam on idle, and has recommended some modifications which I will do this winter.

oldcutlass

So your saying at part throttle @ 2000-2500 rpm it pings. Have you tried the same exercise with vacuum advance disconnected?

What did the guy with the Sun machine say about your distributor settings?

MDchanic

+1.

I would recommend driving with a vacuum gauge and seeing whether this takes place at a certain vacuum - this may be solved by reducing the level of vacuum advance.

- Eric

pcard

The pinging at 2000 - 2500rpm only happens when I open up the throttle, not at cruise.

The mechanical advance at 2000 - 2500 rpm is 14 to 18 degrees. With my initial set at 850 rpm to be 8 (with about 1 degree of mechanical at this rpm) degrees, I calculate the total advance (mechanical + distributor) to be about 21-25 degrees.


I did not test without the vacuum advance.
I will jury rig a vacuum gauge on the outside to see if I am unloading the vacuum advance or not. This will be in two weeks.

Thanks for the input!

The guy working the Sun machine would be me .

garys 68

Do you know your compression ratio?
What kind of carb, sounds like power circuit might be too lean.

MDchanic

I hate to say this to a guy who's just put so much time and effort into his distributor, but I've been tuning an engine for the past few weeks, on and off when I've had the chance, and I've solved a similar problem by turning out the adjustment screw on the Crane vacuum advance cam about 2 turns.

It's all about the interplay between the vacuum and the mechanical advance, and you need to have a sense of exactly how much advance each contributes to the overall picture under a given set of engine speed and load conditions.

It's a question of the exact specifications of your engine -
We know it's a 455. High or low compression?
What sort of cam are you running?
Stock distributor?
What is the exact mechanical advance curve you've created?
What is the code on the flat part of the vacuum advance unit (something like "B7")?

I would recommend using that SUN machine again and mapping out the vacuum advance curve, then seeing what your vacuum is while driving, and using that information to figure out whether a change in the vacuum advance curve would be beneficial.
Also, in a fairly stock engine, changes as large as the ones you've made to the mechanical advance are seldom necessary, so you may want to consider picking up another pair of weights, in case you want to switch back.

- Eric

pcard

Hi Eric: thanks for thinking about this. Here are some answers:
The engine is an L31 - high compression, rebuilt in 2012, .030 over.
Pistons: KB277
Cam: MC2101: duration @ 0.05 214 224, 280 290 Lift:.472 .496 Lobe:107 117
Engine displacement is now 461ci, stroke is 4.25 (from dyno test). I do not know the current compression ratio.
Here are my final mechanical advance results (no vacuum) (all numbers at crank reference)(error 1*)
1000rpm - 2*
1500 - 6
2000 - 14
2500 - 18
3000 - 22
3500 - 26
4000 - 28 all in

Distributor is not now stock since I took some metal off the weights. It also has a Breakerless.com points replacement (which adds NO advance), stock coil. I do have a spare set of weights.

I had a Crane Vac advance, but could not get it to go lower than 12 degrees, all in at 11 in Hg (tested on Sun machine)(I think it was defective since it would not go to zero). The detonation occurred with this unit also (with the same mechanical and initial).
I replaced it with a B26 unit: 16 degrees all in at 14in Hg (confirmed on tester). I did that for two reasons: one so I could reduce the initial distributor advance (I have vac advance off of manifold), and two so that when I open the throttle the vac advance will be released earlier (14in vs 11in).

I am running a stock carb, with 70 primary jets. On the secondary side I have a G hanger and CK rods. Secondaries open smoothly with good response.

When cruising at 50 mph I now have about 40 degrees of advance (initial 8, mechanical 16, and vac 16). The engine likes that. But when I open the throttle to pass she pings, not badly but it is noticeable. I can accelerate moderately on the flat and there is no pinging, even up to 90mph. I am using 93 gas, with one can of octane booster per tank.

I really like your suggestion of monitoring the vacuum. That will tell me if in fact the vacuum advance is being released when I open up the throttle. I will do that once I get back from overseas.

I really appreciate the time you are putting into this for me.

BlackGold

Something is wrong besides your timing. With only 28 degrees of advance all in, you shouldn't have any pinging. You do understand that 36 degrees of mechanical is normal, right? And that with vacuum advance most cars run over 50 degrees of advance at cruise?

Does your engine run excessively hot, pointing perhaps to too tight of clearances on pistons or pins? Is your exhaust plugged up? Or perhaps the outer ring on your balance has slipped and you're actually running much more advance than you think.

That cam is fairly small; if coupled with a real high static compression ratio it will be prone to pinging. But I doubt that happened unless you really tried to make it happen.

pcard

Hi Brian: the engine does not overheat, and the dyno test had good temperature results. The exhaust is all new and there is a good flow out of the ends.
The total advance (mechanical,28 + initial,8) is 36 degrees all in, at 4000.

It is possible the balance ring has slipped, but that is not something I would expect from a new engine fresh off of a dyno. I have not checked the No. 1 piston for TDC, but I can do that. If the ring has slipped it is not by much since the rotor and distributor are lining up pretty much right where they should be. For a temporary short cut I will bring back the distributor back until there is no detonation under load, and see how she behaves at start, and low and high cruise.

As an aside, the dyno test gave the best results at 32degrees all in, so I would like to take 4 degrees off anyway.

Thanks for helping. If you can think of anything else please let me know.

oldcutlass

Ok, that makes more sense Peter, I was thinking along the same lines as Brian. The rule of thumb has been for the total timing to come in much sooner, around 2500-3000 rpm. Have you tried running without the vacuum advance and see if it pings?

pcard

I haven't tried without the vac advance, but I think that would be a good idea too, at least a good data point.

So here is what I am going to do:
1: take off the vac advance, confirm initial at 8* and see how she runs: idle -cruise 1000-3000rpm, and under load at 2000 - 3000rpm. This test will give me at most/about 32* total at 3000rpm. I would think there is no under-load detonation with this setup. If there is I will back off the initial until it stops. If I have to go far then that will be an indication of a slipped balance ring.

2: put back vac advance, jury rig a vac gauge, set initial to 6* and see how she runs: idle, cruise, under load, noting vacuum at all points.

3. Confirm balance ring position by going TDC

With all that information, along with the mechanical advance curve (known) and the vacuum advance curve (known), we can figure this out. I will do this all on July 17.

I cannot thank you guys enough for taking the time to think this through with me.

pcard

Just an update on this thread. I have not had the chance to take the steps I outlined above. I was pretty busy just driving out to Springfield and then coming home, 2,500 miles.

However, I did do some on the road adjustments while on the drive:
with the vacuum advance connected to manifold, I set the advance to 20-22* at idle at 850rpm in Park. This would correspond to about an 4* "initial" setting.

The engine is very happy with this setup; it runs cool, strong and is responsive. I get about 14mpg cruising at 70mph.

I still have mild pinging on acceleration at 2000-2500 rpm. At this rpm range my advance is 4* initial plus 16*-18* mechanical, plus whatever the vacuum is giving me (I do not know what the vacuum advance is because I do not know what my vacuum is under acceleration - I need to install that gauge). If I go to WOT when this happens, the pinging stops and the engine responds nicely, I think because my vacuum advance drops out when I open the throttle up.

I am visiting the car for a couple of days later this month, and will go through the procedures above and report.

Cheers, Peter

MDchanic

Hi Peter.

Sounds like your vacuum advance is pulling a few too many degrees at mid-throttle / high vacuum.

There are a number of ways of changing this, but the simplest is probably to install an aftermarket (Crane) adjustable vacuum can and follow the instructions to dial it in.

It sounds like your initial and your centrifugal are both good or close.

Let us know how it goes.

- Eric