Timing, where are you?

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shellbuyer

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1973 Convertible Mustang
Hi there, I am running my electronic HEI 14 degrees before TDC on my 73 351 C. Where does everyone else run there timing at? 14 seems to be where it likes to run. Will it do any damage to engine running at 14? I THINK it is supposed to be at 10?

 
Hi there, I am running my electronic HEI 14 degrees before TDC on my 73 351 C. Where does everyone else run there timing at? 14 seems to be where it likes to run. Will it do any damage to engine running at 14? I THINK it is supposed to be at 10?
The OVERKILL and confusing guide to timing adjustment.

This guide will help determine not only what your engine timing should be, but how to determine if your vacuum advance and centrifugal advance are working properly. Springs in the vacuum and centrifugal advance can wear out(crummy over the counter parts) or get stuck, preventing the engine timing from being correct.

It also helps if you have a VACUUM GUN which will allow you to pull varying inches of vacuum on your distributor so you can curve your distributors VACUUM and CENTRIFUGAL advance. You can also measure the vacuum your engine is pulling. Logging your engines vacuum from time to time can help provide clues as to performance issues down the road. For example, if you have 20 inches of vacuum at the manifold today, but 12 inches 2 years from now, you have developed a serious vacuum leak, or your engine simply isn't sucking like a typical Chevy.

In any case,

0. Get under the car with Whiteout and mark a line at 0 and at the 10 BTDC mark on your damper. You may have to turn the motor over to see the marks from the ground.

BTDC is BELOW the TC(0 degree mark) on the damper, or as I like to say, the side of the TC mark that goes to 30 degrees.

1. Remove the vacuum hose(s) at the distributor.

2. Take this opportunity to measure the vacuum with your vacuum gun by hooking up the gun and starting your car.

A stock cam will generally pull 20 inches of vacuum near idle. A performance cam will only pull 12-17. If you are using a performance cam and not had your distributor curved to your cam, your vacuum pod may need to be adjusted/set for the lack of vacuum at idle. This is shown later.

Also, the vacuum should generally be steady. A jumping needle indicates a problem cylinder.

3. According to the specification bookelt, 1973 351C, timing should according to engine decal. On my 1973 351C 2v, the decal said:

6 BTDC at 600 RPM.

At 600 RPM the centrifugal advance should BARELY be active(maybe 2 degrees), giving you a true timing reading. You will probably have to decrease your engine idle at the carb to do this.

4. After setting the timing, you should pull on the throttle to increase your RPM to see at what RPM your timing changes. The table below details the increase in timing you should see based on the increase in RPM.

If you do not get a good match to the table, you need to perhaps look into your distributor and free up/grease friction points. Or you may need to change the advance springs.

5. Now, you can hook your vacuum gun to the distributor vacuum pod, set your engine to 1000RPM and start pulling vacuum.

Confirm your timing changes match the table below. (Because you are operating with the distributor on a car, you must factor in the the centrifugal advance. For example, if your centrifugal advance increased timing by 5 degrees at 1000 rpm, TOTAL timing you should see at 5 inches of vacuum should be 5 degrees more than what the book shows, because the book is assuming centrifugal advance has been disabled on the test stand)

If you do not get a good match to the table, you may need to adjust the pod. Or if you do not get any advance at all, you may need to replace your vacuum pod.

Good luck.

2cfa255.jpg


 
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Great info, James. Could you post the chart as an attachment? Even when I open it and maximize, I have to point my binoculars at the computer screen to read it. :cool:

Thanks!

Doc
The print is very tiny to begin with....see if this helps.

1973_FordShopManualSpecifications_Ignition.jpg

 
Asked my machine show guy...THE CLEVELAND MAN told me base timing is based on total advance, not to exceed 30 deg. So if your distributor has 20deg advance at about 2500 RPM base timing should be 10deg. 10 base plus 20advance equals 30 total. He advised me it's a must to get a timing light with an advance dial to be able to confirm total advance.

Alan L

 
Wolverine,

Absolutely a great post and thanks for the information and charts. I have a question though for my setup. I have a 73 351C that was originally a 2 barrel and the PO put on the correct Edelbrock Performer and Edelbrock carb to convert to a 4 barrel carb. It still has the original 2v heads and distributor. Would the chart for the 351C -2v still be correct for me to use to determine the proper settings or would I use something different?

Thanks again!

Scott

 
What is the vacuum?

My 351C 4V quench runs 16 degrees BTDC and 16 inches vacuum.

We found the car's sweet spot.

mike
I'm a bit embarrassed to say that I don't (yet) own a vacuum gauge. I'm shopping now!

Scott

 
What is the vacuum?

My 351C 4V quench runs 16 degrees BTDC and 16 inches vacuum.

We found the car's sweet spot.

mike
I'm a bit embarrassed to say that I don't (yet) own a vacuum gauge. I'm shopping now!

Scott
Get one that can also apply a vacuum. Comes in handy to be

able to pull a vacuum as well as read one.

mike

 
Okay, just using the info from this thread I've already figured I was about 10 degrees (yes 10!) off on my timing. The PO had mismarked the balancer and after sanding it down and cleaning it up I correctly marked it and for now, have set it with the timing light.

New car! Extra 50 HP at least! Woo Hoo, smoked a BMW that thought they were just going to blow past this 40 year old relic. Jokes on him!

:banana:

Scott

 
(See signature line for Engine details)

I discovered a while ago that the vacuum pod in my Mallory unite wasn't working. I didn't know it at the time so I had a LOT of initial timing. Pinged like crazy.

Then I replaced the pod and in doing so, using a hand vacuum pump, I discovered the pod adjustment is for how much timing it adds, not at what vacuum level. It seemed pretty binary, once enough vacuum was applied it operated. There wasn't an 'increase in advance with increase in vacuum'. (Again this is a Mallory distributor, I don't know if this is the same with other types)

With this understanding I lowered my 'initial' and non-vacuum connected timing. I am using manifold vacuum.

In gear with about 800 RPM I had 22 degrees advance with with the vacuum not attached. With it attached I was at 37. It pinged.

This weekend I dumped down the initial 'no vacuum' to 16 and adjusted the pod to pull it up to 37. It seems to idle well and the pinging is gone. I need to check my total mechanical advance.

 
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I lowered my 'initial' and non-vacuum connected timing. I am using manifold vacuum.
This is typically not the best way to do it.

 
Mike,

I am running 14 degrees base at 825 rpm and I start advancing at 925 rpm and all in at 2500 rpm for a total advance of 34 degrees. In addition I use 10 degrees ported vacuum advance so at freeway cruise I have about 46 degrees total.

Using manifold vacuum at idle you are adding additional advance to your base timing. Using ported vacuum there is no vacuum added at idle.

 
The use of manifold vacuum is a hotly debated issue. At idle it does advance your timing, but that has the advantage of keeping the heat from combustion from overheating the engine as advance lets it out of the exhaust side a bit better. If using manifold vacuum, it is important to adjust your timing for smooth operation with the vacuum advance unhooked as vacuum drop when you crack the throttle.

Jbojo I think you may be wrong. If all in at 2500, the ported vacuum will not pull the advance another 10 degrees as the stops in the distributor are the same for vacuum and mechanical advance. You'll get all in on your advance quicker with your set up on partial throttle, but I am almost certain you won't exceed the total advance number you see at 3000 rpm

 
Yes, there are two camps to the ported vs manifold vacuum. On a stock car I would stick with the stock configuration. On a modified engine my advice would be to play around with it and see which works best.

When I get a chance I will confirm total advance both with and without a vacuum draw. I have a timing light that lets me set the advance value. So for example, if it displays 20 advance and I am blinking at TDC mark then I am at 20 degrees advance. It is really handy. Just to confirm, the distributor doesn't care that I am in park and revving the engine to find the maximum advance correct? Here's what I do:

Rev the engine to about 3000 plus RPM and determine maximum advance. I leave the timing light on that advance setting and then I go back to idle and slowly rev the engine until the light is blinking on TDC again. That's the RPM where I am 'all in'. Is this a valid way of doing it?

I drove the car again today with a lower octane mixture and it still didn't ping but I can feel it is off a bit on performance so I will mess with it a bit more.

 
The use of manifold vacuum is a hotly debated issue. At idle it does advance your timing, but that has the advantage of keeping the heat from combustion from overheating the engine as advance lets it out of the exhaust side a bit better. If using manifold vacuum, it is important to adjust your timing for smooth operation with the vacuum advance unhooked as vacuum drop when you crack the throttle.

Jbojo I think you may be wrong. If all in at 2500, the ported vacuum will not pull the advance another 10 degrees as the stops in the distributor are the same for vacuum and mechanical advance. You'll get all in on your advance quicker with your set up on partial throttle, but I am almost certain you won't exceed the total advance number you see at 3000 rpm
Agree that this is a topic that is confusing. So let me see if I understand this correctly. Total timing is initial plus centrifugal which will be 34 degrees at 2500 in my case. So when I am above 2500 I am curious why the timing is at around 46 degrees. So are you saying that I should see 34 degrees max?

Edit: Jeff, I forgot to mention that all my timing is done by electronics, there are no weights or stops in my distributor. The ported vacuum is also simulated. On a side note though shouldn't the principles still be the same whether it's done mechanically or electronically?

thx

 
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