Mech Advance Only Distributor Timing

[Argess]

I am having a heck of a time getting my 428 to run right. Since I first built the engine in 1996, it has been more or less fine with the non-vacuum Mallory YL distributor set up as follows:

Initial 10 Deg BTDC
Max 38 Deg BTDC

However, those numbers turn out to be all wrong for two reasons:

1/ My harmonic balancer outer ring had slipped, and

2/ some of the dist advance was already used up at an 850 rpm idle (5 degrees).

Regardless, I didn't realize those things, so late last summer I thought I'd try playing with the timing. First I set the intial back to 6 Deg and the engine sounded very smooth and free rev'ing all the way up through the rpms. Next I made a big mistake and tried 15 Deg. Engine was fighting hard at higher rpms and "oh-no", I broke a piston. ...........so now.... I've just finished my re-build which includes a new damper which I have triple checked to make sure it reads TDC correctly.

OK....now I fire it up with intial set to 10 Deg. Engine gets hot very fast with a lot of heat from the headers and a bad bog when I give it gas. Bump the timing up to 16 (14-16 intial is recommended for my cam) and it's better, but still a bad bog when I give it gas (although a little better). So I look at what my old timing figures would be if I corrected for the slipped damper and already slightly advanced dist mechanism.

So I set it up for 28 intial and 38 max (baseed on those old figures of 6 deg intial and a 22 deg slipped damper). This gives me 23 @ zero rpm. Much better, bog is basically gone, maybe a tiny hiccup. Engine runs cool.

Still not the same as before I broke the piston. A person should always write things down. I thought my damper was out by 22 Deg, but I just rechecked and it's out by 29 Deg.

So to correct for that, I should be at 30 intial and 38 max.

This seems way to high and I'm nervous about doing it.

Here's a tidbit. I adjusted my engine for max vacumm and rpm at idle. Timing was at 38 Deg BTDC. I am certain once the throttle opens at 38, I will get detonation until the rpms build, so that's out.

I am of the theorey that if an engine needs a ton of advance with no-load, and very little advance under full load, then a vacuum advance would be a terrific help. And since I don't have one yet, my car benifits from timing somewhere in-between. I just havn't figured out where yet.

Keep in mind, I only use 1/3 to 1/2 throttle zipping up through the gears, so there will be a partial vacumm to keep the ignition advanced. It's not like a heavy car or truck with an automatic transmission where you can successfully floor it..

So for now, I am trying to get this mechanical dist dialed in until I can sort out which vacuum dist I should buy (that will fit for one thing).

ooh...this is a long post....not in keeping with the KISS principle. Sorry about that.....

OK, so what I would like to know is where everybody who has a mech advance only distributor, has their initial and max timing set. Please let me know.

ps: Next I'll be asking how you can tell if it pings or not when the sidepipes are deafening you...I think may stepping on it in 3rd gear at 1000 rpms will keep the exhaust noice down and let me hear any pinging from the engine.......

[ERA Chas]

14 initial, 38 total all in by 2600. 427 Sideoiler on 93 octane. You don't so much hear ping as feel it. It feels like when the rev limiter kicks in.

[Argess]

Thank-you. I was afraid someone would say that. I guess before I do anything else, I better pull right side valve cover and check my cam timing. I've never made a timing gear error before, but maybe I did this time.

Mind you, here's an older thread that seems to uncannily represent what I am experiencing:

http://www.clubcobra.com/forums/arch...p?t-17348.html

[CJ428CJ]

20 initial. 38 total.

Chris

[Argess]

Thank-you. That sounds more like mine. Before I limited my advance mechanism, that's pretty much what I got with the stock YL....about 20ish intial when I set the max for 38. As I have been going up with initial, I have been adjusting the plate to maintain 38 max.

Anyone else?

[Bobcat]

38 total and all in by 2500 . 20 at idle ( 950 rpm ) . 427 aluminum engine stroked/bored to 482 .... 93 octane gas .

[patrickt]

10 initial, 35 total.

[Chaplin]

Geez, Patrick, not only are you using an antiquated cam, you're also running an antiquated advance curve! Actually, all kidding aside, you'd be amazed how much you can clean up your idle if you recurve your distributor to run 16-20 initial.

I'm around 17 initial and 35 total, all in by 2500.

[patrickt]

Originally Posted by Chaplin

Geez, Patrick, not only are you using an antiquated cam, you're also running an antiquated advance curve! Actually, all kidding aside, you'd be amazed how much you can clean up your idle if you recurve your distributor to run 16-20 initial.

I'm around 17 initial and 35 total, all in by 2500.

But I really like the way my idle sounds. And I was told that if I ran the old "K" cam that the SAAC people would let me sit in the back of the room. Turned out it was just another lie....

[PurpleVenom]

14 initial, 38 total - Stock 460 crate engine. You should be able to get all the timing you need out of your mechanical. I was just reading the box on my MSD last night, and you can get 16 to 28 total out of changing the springs and bushings. Check out this link, maybe it will help - http://msdignition.com/instructions/...utors/8577.pdf

[olddog]

Tom Kirkham started a thread in the EFI and tuning section. He left several links a few posts later. There is a lot of reading, but the best info I have read to date. I still have a few questions though.

Bottom line is that your engine needs a bunch of advance at low load. That is partly to do with the lean fuel mixture. So no **** your engine runs better with a lot of advance. It may like as much as 52 deg at 2500 rpm and low load conditions. The problem is that at WOT the rich dense fuel charge burns much faster and the engine can only tollerate 35ish deg. Too much timing breaks engines, as you have already learned the hard way. Too little timing will make it run hot and slugish.

For mechanical only advance you must tune it only for WOT operation and let it run poorly at light load conditions. Big cams due to exhaust dilution from the overlap may need 16 to 18 initial, but do not go higher.

If you want it to run good at light load, get a vacuum advance on it.

I think the articals will answer all your questions and support what I have said. Good luck!

[scottj]

My motors always ran best without any advance so I've always lock out my distributors, but my cams have always had 260+ degrees at .050". On my 11:1 motors I didn't need the MSD start retard box as they always started without issue, but it was required (-20deg) on my high compression motors or they would blow teeth off the starter ring gear. A dyno is needed to determine optimal timing.

Scott

[Michael C Henry]

You have given me boubts about my dampener timing marks accuracy. I just ordered a piston stop from Summit Racing, So I can find if TDC is a match on my harmonic dampener.

[olddog]

Originally Posted by patrickt

10 initial, 35 total.

Patrick,

Is this the graph of an actual engine or some representation of what it might be?

I definately do not understand what is going on around 800 to 1300 rpm.

I find it interesting that the timing comes in more quickly from 1500 to 2000 than from 2000 to 2500, in other words a curve.

[Excaliber]

Once you've established TDC, measure the circumference of your balancer. Divide that exact number by 10. THAT number is how far to measure from TDC to arrive at 36 degree's BTDC.

360/10=36 you see... Half of the measurment used to arrive at 36 would be 18 BTDC, etc. etc. That way you can verify your timing marks or add more marks because it's likely your timing "lines" on the damper stop before reaching 36. If your going to use vacuum advance as well, then carry forward some new lines out to about 55 degrees. Using tape or similiar on the damper face.

About 50-52 BTDC with base/mechanical and vacuum advance all in (10-15" of vacuum) is about right for a cruise setting and max mpg.

20 base with 18 mechanical advance on my 427. My vacuum advances to much and at to high a vacuum setpoint so I intend to replace it with an adjustable GM unit (for my "Ready to Run" MSD distributor).

Bear in mind: MY motor has a lot of "quench" designed into the combustion chamber/piston dome, I may be able to run more advance than the average 427. 38 is working very well, I suspect 40 would be "OK". In fact, I have about 7000 miles on my "new" motor now and it's always run at least 38, on occasion above 40 because I screwed up the timing a time or two. Run's great, keeps getting better with more miles on it!

[patrickt]

Originally Posted by olddog

Patrick,

Is this the graph of an actual engine or some representation of what it might be?

I definately do not understand what is going on around 800 to 1300 rpm.

That's a representation of what it is close to -- it is not exact. That line between 800 to 1300 was just kind of hand drawn... it's a crappy job, I know.

[olddog]

I have been trying to find out if the mechanical is straight line lenear or a curve. If a curve what does it look like.

Same question for a vacuum advance.

Basically I want to understand this to have better idea of what a EFI timing map should look like. When I enterview some tuners, I want to know true brillance from total bull.

[patrickt]

Originally Posted by olddog

I have been trying to find out if the mechanical is straight line lenear or a curve. If a curve what does it look like.

That hand drawn graph of mine is an estimation of the advance with those particular springs that I'm using as detailed in the MSD literature. If I were to do it really accurately I would just use my timing light and check the advance values at 100 RPM increments. That would be pretty easy. Vacuum advance would be more difficult. The graph might surprise you; I don't know. The "spin" could come on very quickly, almost in spurts, and give you a subtle stair-step graph, or it could be smoother and straighter. You won't know until you graph it (or put your distributor on a dizzy machine).

[Argess]

According to my Mallory Instructions, the selection of springs can determine whether you want a straight line advance or a curved advance. I don't have a spring kit, but it appears that some of the heavy springs install "loose", so they dont' start to affect timing until later rpms, hence the bump. Here's the link that explains it:

http://www.malloryperformance.com/pdf/29015.pdf

Piston-Stop Tool. I made my own. Knocked out the center of a spark plug and threaded the inside for a bolt. Using the tool, you advance from either side until it stops and read your balancer. 1/2 way between is top dead center. I don't like using the tool as the engine has some resistance to turning by hand (even with all the plugs out, there's some), so I don't want to have the piston hit the tool too hard. So, here's another way. Rotate engine to 10 BTDC and insert a long rod down the plug hole until it lodges up against the top of the piston and the far cylindeer way. Make a mark on the rod and a corresponding mark where it rests (you have to hold it) against the valve cover. Remove rod. Rotate engine to 10 ATDC (preferably form the other direction, but it's not that critical). The marks shold line up perfectly if your Balancer is correct. I use +/- 10 as a balancer usually shows both.

So....most people have timing from about 14 to 20 intial and never more than 38 max. I could live with the 20 initial as the engine heat problem is more or less gone by that point. I still have the bog though, and I guess I could tune that out. It's either getting too much or too little pump shot, starts to recover and then the secondary's open and then it's off. With further advanced timing, all those problems dissapear. However, with these timing settings, the enigne seems to be firing with late timing...engine sort of pounds. When it's too far advanced, it's different...like a reluctance to keep incresing rev's...sort of as described earlier ...like a rev limiter is at work.

Waht I expect at correct timing is a freee revving, light sounding enigne. It id that once, but as the balancer had slipped, I don't have much of an idea where that timing was....but it looks like 36 intial....yikes!

Curretnly, at 28 intial and 38 max, my new plugs are slowly turing gray (and I suspect, eventually will turn tan). When I had detonation problems before, I had another warning sign I should have paid more attention too....sparkling white porcelin plugs....gleaming...more white than new "out-of-the-box" plugs.

To get rid of the bog.....hmmmm. I am already using the light springs in my vacuum secondaries. I have the spring kit, and there is a set of lighter ones, but I tried installing one once and it wasn't long enough to even stay in position under the diapraghm. I also have a couple of pump cam kits, but it's set pretty light now. Maybe a couple more choices to limit the pump shot.

Just seems odd as it worked well before. Darned engine. I am sure I can get it to run great, but the timing numbers defy all data and experience. I kind of wonder if I seem to need such high timing as I am using a single plane 2x4 manifold with poor port matching to the heads. Poor mixture, so more advance needed.......

Anyway, thanks for all the replies. If anyone can think of anything else to shed light on the matter, I'd appreciate it.

[Bobcat]

Argess .... you might try turning your mixture adlust screws out about 1/4 turn ( richer ) and see if that helps the bog . Sometimes you have to have the carb adjusted slightly richer on the low end than what gives optimum idle .

Bob