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Old 04-17-2010, 05:20 PM
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Timing... vacuum or no vacuum advance?

I disconnected the timed port vacuum hose from my Mallory Unilite distributor and plugged the port at the carb. I set my initial timing at ~ 17 degrees. Then I slowly raised the speed of the motor and watched where the timing leveled off. This was at 40 degrees. I verified my idle rpm was at ~ 1000rpm and dropped to about 800rpm when I put it in drive. So far so good. I then reconnected the vacuum to the dizzy and I rechecked the initial timing. I was amazed when I saw my initial timing sitting at 30degrees. I slowly rev’d the motor again and the timing topped out at 52degrees!!! That’s about the same 22-23degree change in timing. (with and without vacuum advance)

What is going on here? Do I need to fix something? Should I leave the vacuum advance disconnected?

The vacuum port I’m hooked up to on the Edlebrock carb is in the front and to the far left. I measured 10psi vacuum at idle. It goes up to over 20 when I slowly rev the motor up. Drops to 0 when I pop the throttle to WOT.

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Old 04-17-2010, 05:47 PM
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Hook the vacuum advance up to a non-timed port. Timed port is for emissions purposes only. You want the vacuum advance hooked to a full time source so that it drops off with any movement of the throttle but is high at idle and cruise rpm. Keeps the plugs cleaner and increases gas mileage.
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Old 04-18-2010, 03:39 AM
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Both the mechanical and vacuum advance curves have too much travel.

You need to physically limit the travel of both.

BBC motors want 38deg total mechanical advance at WOT. 40 is way too much.

14 to 26 at idle base timing. 38 total mechanical

10 to 12deg max limit of vacuum advance for cruising at part throttle.

The bigger the cam duration and overlap the more the motor wants increased idle base timing for a clean idle and crisp throttle response.
The thumpr cam you have wants 24-26deg at idle. That requires that you limit the mechanical advance travel to a max of 14deg so that the total advance does not exceed 38deg at WOT.

The single plane manifold wants a 3000-35000 stall conveter.

Last edited by F-BIRD'88; 04-18-2010 at 03:49 AM.
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Old 04-18-2010, 08:31 AM
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your timing curve looks good to me. how does your engine run?

You can try different initial settings to see what timing your engine likes. Most BBC dyno tunes use 38 to 42 total mechanical timing for max power. Try 38 total mechanical, try 40 total, and try 42 total.

48 to 52 degrees total with the vacuum advance will be good for highway mpg (low load, high vacuum, low rpm)

is your vacuum advance canister adjustable?
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Old 04-18-2010, 08:42 AM
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Hey guys, thanks for the feedback.
The car seems to run pretty well at 40 degress total. I will experiment with 38 & 42 just to see how it feels in the seat of the pants and throttle response.

My big confusion is should I run with the vacuum advances connected or not? It suprized me when I saw it was at 30 at idle. Is this a bad thing as far as performance goes? I also end up at 52 total with the vacumm connected.

It does launch better with 18 initial and no vacuum advance. What I do notice now is while cruising at say 40mph and I punch it there is a massive stumble and bog. I guess this could be a carb issue and not timing.

Thanks again.
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Old 04-18-2010, 08:46 AM
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The vacuum canister is a stock unit that came with the Mallory unitlite distributor. Dont know if its adjustable (i'll check). If it is, How would you go about making the adjusments. I mean I know there is an allen key screw up the vacuum snout (if it has it) but what procedure do I follow? Do I adjust it so that at 10psi theres no advance on the timing or just till it feels good?

I really think its the vacuum advance that is the key to the problem here, but what do I know.
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Old 04-18-2010, 09:08 AM
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yes, 30 at idle seems right. 17 initial plus 13 from the vacuum advance.

yes, could be a carb issue. what carb are you using? could need more accelerator pump or just a bigger squirter.

what heads, cam, compression, gear, ect........
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Old 04-18-2010, 10:00 AM
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So you're saying I should connect the vaccum advance and let it go even though my final timing is going to end up at 52? Just seems contradictory to everything I've been told. (I'm not trying to be rude, just trying to understand)

I'm just about fed up with that Edlebrock carb and am about ready to by a Holley. Any suggestions?

Equipment:
454 .030 over

Edlebrock 800 cfm carb, Wieand Team G single plane intake with oval ports

Comp Cam Thumper hyd roller cam (11-600-8) with 283 intake and 303 exhaust duration 107 lobe separation, .547 intake and .530 exhaust lift.

Brodix RaceRite oval port heads with 270cc intake runners. Flow: Intake 314cfm @ .500”, Exhaust 196cfm @ .500”

About 10.25:1 compression, all gaskets and ports are matched up.

3.31 12bolt posi rear end

700R4 Automatic Trans, 2400rpm stall

Headman hearders and 2.5" exhaust system
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Old 04-18-2010, 08:23 PM
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After listening to 454C-10 (and one day I will have to run him down to meet him), and a few others on another site, taking the time to absorb and comprehend what they are saying---and why-------My 454 runs much better.

I am very close to 40* mechanical and a bit over 52 at cruise RPM
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Old 04-18-2010, 09:16 PM
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Quote:
Originally Posted by 454ChevelleMalibu
So you're saying I should connect the vaccum advance and let it go even though my final timing is going to end up at 52? Just seems contradictory to everything I've been told. (I'm not trying to be rude, just trying to understand)

I'm just about fed up with that Edlebrock carb and am about ready to by a Holley. Any suggestions?

Equipment:
454 .030 over

Edlebrock 800 cfm carb, Wieand Team G single plane intake with oval ports

Comp Cam Thumper hyd roller cam (11-600-8) with 283 intake and 303 exhaust duration 107 lobe separation, .547 intake and .530 exhaust lift.

Brodix RaceRite oval port heads with 270cc intake runners. Flow: Intake 314cfm @ .500”, Exhaust 196cfm @ .500”

About 10.25:1 compression, all gaskets and ports are matched up.

3.31 12bolt posi rear end

700R4 Automatic Trans, 2400rpm stall

Headman hearders and 2.5" exhaust system
You have to remember that the vacuum advance only gives timing when the vacuum is high enough to provide it. Under high load conditions, the vacuum advance is supplying NO added advance.

The adjustment on the vacuum advance is where the advance starts or drops out- not the total amount that is supplied.

The instructions as supplied w/the Crane adjustable vac. can KIT is- http://cranecams.com/pdf/210g2.pdf
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Old 04-19-2010, 08:32 AM
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yes, the timing from the vacuum advance drops off with no vacuum (wide open throttle).

revving the engine in park is not a load so you get the full timing (mechanical plus vacuum). That is why you are getting 52 degrees when you rev it up the driveway.

your engine is a fairly stout build. I'm sure you will do better with a holley.

an 850 would be a good size for your engine. Use a vacuum secondary carb if you want better mpg or a double pumper for better performance.

I like the classic 3310 750 vacuum secondary carb. very durable, cheap, and they work great. Hard to beat and they only cost about $270. However, it might be a little small for your setup. An 850 vacuum secondary holley goes up to $460! Is 100 cfm worth another $200? That is your call. But on the other hand, you can get by will a little smaller carb since you are using a single plane intake. In any case, a 750 holley or bigger will run better than an edelbrock carb on your engine.
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Old 04-19-2010, 08:45 AM
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Quote:
Originally Posted by 454C10
However, it might be a little small for your setup. An 850 vacuum secondary holley goes up to $460! Is 100 cfm worth another $200? That is your call. But on the other hand, you can get by will a little smaller carb since you are using a single plane intake. In any case, a 750 holley or bigger will run better than an edelbrock carb on your engine.
This brings up a question I have: Does the stub stack get used anymore? I realize a lot of carbs have gone to different castings w/smoothed airhorns, but like on the "traditional" DP's and carbs like the 3310 that still have the choke air horn, etc.

I have one (stub stack), and when used on a 455 Pontiac w/a 4777 650 DP Holley, my ET and trap speed didn't change. I thought sure it would- IF it actually did what it was supposed to do.

Opinions?
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Old 04-19-2010, 09:02 AM
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I have also tried those snub stacks at the track without any noticeable improvement. furthermore, it seemed to interfere a little with the carb linkage.

I guess you might see improvement on the dyno or on a carb flow bench, but I'm thinking the overall gains are so small the track improvement would be insignificant.

If cobalt is running a 650dp on his 455 then that might also be a good carb option. again, the single plane intake gets more out of a smaller carb.
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Old 04-19-2010, 09:29 AM
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Quote:
Originally Posted by 454C10
I have also tried those snub stacks at the track without any noticeable improvement. furthermore, it seemed to interfere a little with the carb linkage.

I guess you might see improvement on the dyno or on a carb flow bench, but I'm thinking the overall gains are so small the track improvement would be insignificant.

If cobalt is running a 650dp on his 455 then that might also be a good carb option. again, the single plane intake gets more out of a smaller carb.
A bigger carb might've helped my top end- at least that's what everyone always said when I told them what I was running- but because my Pontiac 455 peaked so low (my best ET's were run shifting @ 4900 RPM w/a 3.31:1 diff!), I couldn't justify using anything any bigger.

But that was w/a Performer intake, had I had an RPM intake (they weren't out yet, back then) I would've tried a bigger carb.

But that said, the OP's 454 will have the breathing ability to surpass what the 650 DP can deliver, so I'm w/everybody else AFA using a 750-range carb for good all-around use.
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Old 04-19-2010, 09:38 AM
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I just looked at the engine specs again. Combo's real good, if it wasn't for the Thumper cam and the Team G open plenum intake. That cam has too much exhaust duration w/a too late closing point- just to get a choppy idle. Those heads do not want or need that exhaust timing, IMO.

A different cam would make more power w/as good or better vacuum @ idle. A high rise dual plane would pick up vacuum and have more power under the curve- and possibly on top as well. Something like the RPM.

Last edited by cobalt327; 04-19-2010 at 09:43 AM.
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