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Unread 11-08-2013, 06:40 AM   #1
2Xtreme
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How to determine best timing...?

According to the Haynes manual, timing for my Jeep ('79, 258, originally had Carter BBD carb. Currently has a Holley 390cfm carb.) should be 8*-10* @ 500-700rpm.

My question is this:
How do you determine what is the 'best' initial timing for a particular engine? I mean, short of someone saying "Set it at 8* and call it a day" and someone else following that with "I run mine at 9.5*."

With a 2 degrees timing range and a 200 rpm range, there can be umpteen combinations. And I've read that some people are running at 7*.

Obviously, published specs are for stock engines running at optimum efficiency. I would think that is a relatively small percentage of members here.

So, is it a matter of timing that will give you the highest vacuum reading?

Thanks in advance!

2X

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Unread 11-08-2013, 07:12 AM   #2
Matt1981CJ7
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2X,

DO NOT set initial timing to the highest vacuum reading. It's likely to result in far too much advance. Use the vacuum gauge for setting the mixture. Use a timing light for timing.

Here's what I recommend. The total timing (initial and centrifugal) at the upper RPM ranges is what I've found to be the most critical for performance. IMO, you should strive for 30-35 degrees total at around 3000 RPM. Once you achieve that, then let the initial land where it lands. My 258 ended up around 11 BTDC initial.

Then hook your vac advance up to manifold vac, set your idle around 700 RPM and dial in the mixture, and enjoy.

Matt
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Unread 11-08-2013, 07:37 AM   #3
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Got it!

This is probably why setting initial timing at 8* didn't work so well.

Thanks, Matt!
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Unread 11-08-2013, 08:27 AM   #4
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Nice work, Matt

As usual, Matt leaves very little to add to. Most carbureted engines with stock cams fall in the 8* to 12* range. Here are a couple of things to shoot for:

Quick cold starts
Easy hot restarts that don't kick back on the starter
Maintain good idle speed when dropped into gear
Crisp throttle response

If you get the top end of the advance curve where Matt suggested and the base timing falls somewhere around 10* without screwing up the four points I listed, you can pretty much close the hood. If you've got the initial timing right, the manifold vacuum will be there, assuming the carb isn't jacked up.

The only proverbial thorn in your rump is if you have to pass the sniffer test. If that's the case, you'll have to drop the timing down from what's optimal to what the book says for your model year before your smog test. Low, lazy base timing is only for emissions.


Shawn
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Unread 11-08-2013, 08:44 AM   #5
Matt1981CJ7
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Shawn,

You're the one that should be complimented, since you taught me most of what I know about tuning.

2x, don't be alarmed when your idle jumps up about 150 RPM when you hook the vac advance to manifold. The initial will also spike up to around 20* BTDC. That's all normal and desirable, and is also why you will need to re-set the idle speed when you connect the vac advance.

Matt
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Unread 11-08-2013, 08:53 AM   #6
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Quote:
Originally Posted by Matt1981CJ7 View Post
IMO, you should strive for 30-35 degrees total at around 3000 RPM. Once you achieve that, then let the initial land where it lands. My 258 ended up around 11 BTDC initial.
Matt, how do you check 30-35* with a timing light? Do you put a mark on the HB at 30* and check it against the 0* mark?
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Unread 11-08-2013, 09:15 AM   #7
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Holy Crap!
I've created another one of THESE threads....!!
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Unread 11-08-2013, 09:15 AM   #8
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Quote:
Originally Posted by teedubbaya View Post
Matt, how do you check 30-35* with a timing light? Do you put a mark on the HB at 30* and check it against the 0* mark?
There are a couple of different ways.

I use a digital timing light with an advance feature. I know some old-schoolers don't advocated them, but the quality ones work great. My dyno guys use one, so that's good enough for me.

Another method is to get a timing tape for your balancer. Companies like MSD sell them.

Lastly, you can measure the circumference of the balancer and divide by 10. Then place a mark on the balancer at that distance from the zero mark. That mark will be at 36* (360*/10 = 36*)

Good luck

Matt
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Unread 11-08-2013, 09:17 AM   #9
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Quote:
Originally Posted by swatson454 View Post
...The only proverbial thorn in your rump is if you have to pass the sniffer test. If that's the case, you'll have to drop the timing down from what's optimal to what the book says for your model year before your smog test. Low, lazy base timing is only for emissions.
I wouldn't say that's the only one in my rump...

But thankfully here we only have to worry about the sniffer from 96 1/2 forward....
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Unread 11-08-2013, 09:25 AM   #10
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Edit: In response to deleted posts...

I've got plenty of stories about initial timing. Here are two:

A good friend of mine got a custom cam from Vizard (262 on a 108, IIRC). He calls David up and says,

Hey, this cam isn't at all what I wanted. It sounds like a damn race car.

David asks him where his timing is set.

8*, just like the book says.

David asks him where the distributor is hooked up.

Umm, to the port on the carb for timing.

David tells him to forget all of that stuff. "Set the timing at 20* and make sure your distributor is hooked up to the manifold."

He said it was like a whole new cam. His exhaust guy who was there for the build wouldn't believe that he hadn't installed another cam. Night and day difference.


Another friend of mine (the guy in the Corvette video I post on occasion) installed a new cam and called me up asking why the thing sounded like a sprint car and died every time he dropped it into gear no matter what the idle speed was.

I asked him where the timing was set.

At 8*, just like the book calls for.

I asked him where the distributor was hooked up.

On the port for the distributor, just like the Holley pamphlet says.

I told him to consider 15* an absolute minimum and don't be remotely scared to approach upwards of 25* on that engine and hook it up to manifold vacuum.

He calls back a few minutes later revving the engine and sent me a video of a burnout within the hour.

So yeah, initial timing is extremely important. It sets the baseline that the centrifugal and vacuum advance begin from. It's cam and altitude sensitive.

Enough typing. Me go now
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Unread 11-08-2013, 09:26 AM   #11
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Originally Posted by JeepHammer View Post
If you don't want the information, say so and I won't continue...
No, no, no....!

Was just saying that I seem to have a habit of asking a seemingly simple question that turns out being way more complicated than I originally had thought.

Knowledge is power, my friend. Fire away!

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Unread 11-08-2013, 09:35 AM   #12
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I set mine 8* with the vaccume advance unplugged and line capped while the engine was warm. Then I hooked up the vaccum advance up to the ported side of the bbd after the nutter. This is how I found mine to work flawlessly. no more computer.
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Unread 11-08-2013, 09:40 AM   #13
Matt1981CJ7
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Quote:
Originally Posted by 2Xtreme View Post
Knowledge is power, my friend. Fire away!

Be careful what you ask for.

Matt
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Unread 11-08-2013, 09:43 AM   #14
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Quote:
Originally Posted by Matt1981CJ7 View Post
Be careful what you ask for.

Matt
Point taken.....
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Unread 11-08-2013, 09:44 AM   #15
Matt1981CJ7
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Quote:
Originally Posted by seabass1858 View Post
I set mine 8* with the vaccume advance unplugged and line capped while the engine was warm. Then I hooked up the vaccum advance up to the ported side of the bbd after the nutter. This is how I found mine to work flawlessly. no more computer.
IMO, it would run even more "flawlessly" if you hook the vac advance up to manifold.

Take it for what it's worth....

Matt
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