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Old 07-09-2009, 06:23 AM
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Ignition Timing a Modified V8

I have a Ford 428 installed in a light car, manual transmission, and am having some issues with timing.

Cam is a Crane Fireball, about 294 advertsied and 227 at 0.05" if my memory is correct. Distributor is mech advance only.

It has always worked well with 10 Deg intial and 38 Deg max advance, however it was detonating and finally broke a piston. I can't hear it ping on account of the sidepipes being so loud.

Now, it turns out the reason for this was a slipped outer ring on the harmonic balancer. Out by a whopping 22 Degrees which made my timing very, very too much advanced.

So, I've rebuilt the engine and put in a new balancer, but now my timing seems to be affecting off-idle performance. When I give it gas, say 1/3 throttle, it bogs, vacuum secondaries seem to open, and eventually, once the rpms are up a bit , it seems fine.

Doing a bit of research and some testing, I can pull 16 in Hg at an 800 rpm idle when the spark advance is at 16 Deg BTDC. Below 14 Deg, it doesn't idle well. So at 16, it bogs badly. Bumping up the advance to 20 degrees, the bog clears up significantly, but is still not acceptable.

Before, when I thought I was at 10 Deg, I was actually at 32 Deg due to the slipped balancer and there was no bog. But that seems a lot of advance for an idle, so I dont' want to set it there.

So I suspect the previously incorrectly set high initial advance was masking a carb setting. Vacumm secondarys have the lightest spring in them now, and I think I have the accelerator pump cam set for a small squirt. Either short, or standard pump cam is in now.

I'm a little unsure where to start. As it worked fine before, and all that's changed is the timing, it seems to me I should be focused on the timing. But the numbers arn't adding up. Any idea what's going on?

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Old 07-09-2009, 07:56 AM
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It would seem pretty obvious that the engine is asking for a manifold vacuum-fed vacuum advance can to be installed.

A vacuum advance will give you back 10-15 degrees of advance at idle, of the 20 degrees or so that you were getting "extra" from the slipped damper ring.

As long as you don't exceed the total advance (mechanical plus initial) of 38 degrees or so (or whatever the amount is, that the engine likes), the initial can be as high as you need it to be to get a good idle and off-idle response with the vacuum advance included.

One other thing to check is that the timing markers- both the tab if so equipped and the balancer TDC mark- are actually correct. I'd want to verify them by manually finding TDC and checking the accuracy of them.

Last edited by cobalt327; 07-09-2009 at 08:18 AM.
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Old 07-09-2009, 06:08 PM
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Yes, I do believe you are right. Seems to me when I had the timing too far advanced and it worked well would be the same as the proper tuning with a vacuum advance added.

At idle, I set it for say, 16Deg. It runs hot, manifold vacuum is adequate, but not peaked. Idle has just become barely acceptable. But this is probably the best spot for full power.

With no load and a vacuum unit, idle timing could be as much as say, 45 Deg, but may fall to the 16 if the vacuum drops when the throttle opens.

On a light car, I probably need something in-between if I don't have a vacuum unit. This may match up closely with where it was set before (when it worked fine).

I'll do some more tinkering, but in the long run, I think I'll try to find a distributor with a vacuum can that offers adjustability for centrifugal advance amount, centrifugal advance rate, vacuum advance adjust, and maybe vacuum advance rate and/or amount.

ps: trust me, I really double-checked the new balancer to make sure it read ZERO when the piston was at TDC...LOL
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Old 07-10-2009, 10:07 AM
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Success!!!!!!!!!!!

Finally, it runs like it should. But against most timing numbers, and against basic rules on dist adjustment.

Here's my final numbers:

0 rpm (23 Deg BTDC) and it cranks fine.

800 rpm idle (28 Deg BTDC)

Max advance (38 Deg BTDC)

Operation is smooth and powerful. None of that pounding you get when the ignition is too retarded. And as far as I can tell, no knocking under acceleration. At least engine isn't fighting me as the revs rise, which is good. Bog is gone.

At 800 rpm, max vacuum was 20 in Hg at 38 Deg advance. Too much to accelerate and not expect knock.

At 800 rpm, lowest good idle was at 16 deg, which is textbook for the cam I am using. Engine ran hot there, and pounded when acclerating.

Not having a vac dist, I went in-between 16 and 38 and based on old data, I went with 28 as listed above.

Not sure this would work with a heavy truck, but it's interesting none-the-less.

And for those who say vacuum advance is only for economy (like I always thought), you are wrong. It will adjust for the best advance dependant on the load, not just the rpms like a mech adance.

I think I'll get me one of these in the near future:

http://www.shopatron.com/product/par...26688.2096.0.0
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Old 07-10-2009, 10:54 AM
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Quote:
Originally Posted by Argess
Success!!!!!!!!!!!

Finally, it runs like it should. But against most timing numbers, and against basic rules on dist adjustment. *SNIP*
And for those who say vacuum advance is only for economy (like I always thought), you are wrong. It will adjust for the best advance dependant on the load, not just the rpms like a mech adance.

I think I'll get me one of these in the near future:

http://www.shopatron.com/product/par...26688.2096.0.0
Good looking distributor. I've yet to hear anything negative about Pertronix.

A sea change towards vacuum advance, huh? You're right IMO, to have nailed down its usefulness- in the right application.
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Old 07-10-2009, 11:49 AM
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Quote:
Originally Posted by Argess
I have a Ford 428 installed in a light car, manual transmission, and am having some issues with timing.

Cam is a Crane Fireball, about 294 advertsied and 227 at 0.05" if my memory is correct. Distributor is mech advance only.

It has always worked well with 10 Deg intial and 38 Deg max advance, however it was detonating and finally broke a piston. I can't hear it ping on account of the sidepipes being so loud.

Now, it turns out the reason for this was a slipped outer ring on the harmonic balancer. Out by a whopping 22 Degrees which made my timing very, very too much advanced.

So, I've rebuilt the engine and put in a new balancer, but now my timing seems to be affecting off-idle performance. When I give it gas, say 1/3 throttle, it bogs, vacuum secondaries seem to open, and eventually, once the rpms are up a bit , it seems fine.

Doing a bit of research and some testing, I can pull 16 in Hg at an 800 rpm idle when the spark advance is at 16 Deg BTDC. Below 14 Deg, it doesn't idle well. So at 16, it bogs badly. Bumping up the advance to 20 degrees, the bog clears up significantly, but is still not acceptable.

Before, when I thought I was at 10 Deg, I was actually at 32 Deg due to the slipped balancer and there was no bog. But that seems a lot of advance for an idle, so I dont' want to set it there.

So I suspect the previously incorrectly set high initial advance was masking a carb setting. Vacumm secondarys have the lightest spring in them now, and I think I have the accelerator pump cam set for a small squirt. Either short, or standard pump cam is in now.

I'm a little unsure where to start. As it worked fine before, and all that's changed is the timing, it seems to me I should be focused on the timing. But the numbers arn't adding up. Any idea what's going on?
To start with, the FE is sensitive to detonation. You need to be careful about ignition timing and advance rate, compression ratio, mixture ratio, and operating temperature. If your pipes are so loud you can't hear the engine when it pings, I'd surely put a detonation sensor in the distributor circuit to just shut the thing off when it pings. Greater rocket science would to put an MSD computer in there for a more graceful retard of the spark, but either way is better than what you've got.

Big cams need a lot of base advance in lower to mid RPM range, because the mixture is of low density at this time and needs more burn time. A properly adjusted vacuum advance can be helpful in this range. If you run the setup you have (base and centrifugal) you need to add 5 to 15 degrees in the base, an amount equal to the excess base advance has to be removed from the centrifugal to keep from over advancing the engine at high rpms; this is done by restricting the slot length of the advance cam which also operates the breaker points. This is a cut and try process of modify and test, if it pings, modify and test again.

People are mostly confused about the purpose of vacuum advance, they see it as a gas economy thing that isn't desired by real racers. Nothing is further from the truth. The engineering and rocket science that puts that device there is that when operating at partial throttle openings, the mixture does not have a lot of molecules in it (low density), this causes the burn to proceed slowly compared to the burn speed at WOT which increases mixture density and with it burn speed. Without additional advance in these low thru cruise RPMs, the engine looses power, which the driver makes up for with greater throttle opening for the RPMs and engine load. This kills fuel mileage, increases operating temps as the engine reacts like the spark is retarded from ideal, which it is, and the extra rich fuel mixture wipes out what little top end lube there is greatly reducing engine life. Race engines dispense with vacuum advance and often centrifugal as well since they operate mostly about WOT and don't need to CYA at low thru mid RPMs. Really rad engines put all the advance in the base and have no variable of any sort. This requires the ignition be separately switched as the starter needs to get the engine spinning before spark can be applied, otherwise it just can't be cranked against the early firing cylinders.

Bogie
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Old 07-10-2009, 12:32 PM
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"And for those who say vacuum advance is only for economy (like I always thought), you are wrong. It will adjust for the best advance dependant on the load, not just the rpms like a mech adance."

There are too many factors in figuring out the base timing, and total advance timing, when fine tuning an engine. Using a vacuum advance, hooked to manifold vacuum, will allow better fuel efficiency, thus an increase in mileage at a steady cruising speed. When you get back on the throttle, the vacuum advance will drop out, greatly reducing the chance of pinging.

That being said, you have a 'big' block in a 'light' car, so your total timing can be set to a lower specification, because of the larger cubic inches, and the amount of torque it puts out. Once you add the Pertronix distributor you linked to, you will be able to increase your maximum total advance, without the chance of taking out your pistons again, and still having the same performance you are enjoying.

The total timing specifications being recommended, by many members here, are just basic guide lines that have been proven to work, from "experience".

The reason the basic guidelines do NOT work on every application, is mainly the camshaft selections RPM operating range, and the rear gear ratio selected.

The most common 'procedure' used, is to increase horsepower in the engine until it performs like you 'thought' it should. This can cost "big" bucks in the long run, and many headaches trying to fine tune it. Then experiment gearing it to the engines RPM operating range.

I have always used, and recommended, changing the rear gear first, and watching the engines RPM operating range, and performance gains, of the engine that is in it. Then selecting the camshaft, and other performance build changes, to have the best power, and performance within that RPM range.

Any, and all comments to this, are welcome. Bring it on.

Stephen
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Old 07-10-2009, 03:52 PM
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Thanks guys....a lot of good info there. I am happy with the performance right now, but it's limited a bit. I think with the addition of a vacuum dist, my choices on how hard I want to accelerate will broaden dramaticaly.

I suspect that I can reduce the basic mech curve to say, 16 intial and 38 max, but with the vacumm addition, I can set it for the 38 Deg at idle where I find the best vacuum and highest rpm (only co-incidence the advance for best idle is the same number as the max full-load advance). With some tinkering, the advance should then move appropriate to the amount of throttle I want to use.
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Old 07-10-2009, 04:11 PM
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Argess, this is how I would set your timing, with the Pertronix distributor installed.

With the vacuum advance disconnected and plugged, raise the rpm over 3000 rpm, or above where the mechanical advance is at it's maximum. Adjust your timing to 38*, and lock down the distributor. Bring it back down to idle, and record your new base idle timing. Then hook up the vacuum advance, as you concur to manifold vacuum, and readjust your idle speed, you will need to drop it between 100 & 200 RPM.

Then enjoy your power gain.

Stephen
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