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Old 05-31-2010, 05:05 PM
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Opinions, dumb idea: Using vacuum canister + vac advance to avoid hardstart 38'

Quickly: I have a 383 stroker weekend driver with 38' total distributor locked full advance and using an MSD timing computer for 20' retard on start and adjustable idle timing (I can idle anywhere under 38'). Been like this for at least 10 years, maybe 15?? The engine calls for 38' total timing.

No weights, no vac advance used for years.

Even with the MSD computer retarding my start timing 20' (goes down to 18' when the starter is cranking) I still get some hot days where the engine wants to spin in reverse and the starter is crying a little. I have a switch to kill the ignition which works. Usually its after driving, getting gas, and then going to restart the car (all warmed up and hot and gas flowing too well)

Today I tried this setup which appears to work well so I ask opinions:

I have a vacuum canister (for brakes) that holds somewhere around 18hg after I rev the engine. Need that for my brakes with the cam I have. My hg never goes below 10 when I press on the brakes and stays there when I hold the pedal down. At idle the engine wont get below 10hg but cant get higher because of the cam, thats just the average hg at idle give or take. But the brakes hold well and stops fine

I just hooked my vac advance on the dizzy, adjusted to kick in instantly, to the vacuum canister for the brakes. THEN, retarded my timing something like 20' (just a guess). The point is that when I first start the car I am NOT locked at 38', my distributor is probably locked at 18' now because I turned it back. After about 2 seconds as the vacuum canister fills (empties, you get the point), the vac advance kicks in at 5hg, raises my total timing back to 38' (which I checked to confirm). The timing holds because my vac advance never releases.

I ran the car around and never stumbled because the vacuum canister always had well over 10hg...enough to hold the vac advance canister in total advance. And, at WOT where I have no vacuum, it doesnt matter because the vac is coming from the canister which is still holding enough to pull the vacuum advance in.

Long winded but I wanted to cover the story. My idea is when I start the car I have maybe 18' on the distributor because its locked, the MSD timing computer throws in additional retarding for a better start, and then the vacuum advance pulls in once the car is started to get me back to 38' total locked timing. The MSD computer is still dropping my idle timing down and doing what it should.

YES, I know ideally I should have weights and vac advance canister hooked up. But aside from fuel economy, does anyone see this as a really dumb idea? Its nothing different than any other radical cam riding at total timing locked out except that my total locked out timing isnt in effect until the car runs.

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Old 05-31-2010, 06:12 PM
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Why is it different using your vacuum method instead of the MSD start retard function? Unless your vac solenoid is putting more than 20 degrees into the dist.
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Old 05-31-2010, 08:04 PM
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Without using this new method the MSD computer would try to start the car at 18' timing (thats 38 locked minus the 20' retard it creates). The 18' is too much sometimes and sometimes causes a hard start. The engine is 9.7:1 compression, 93 octane gas, and the somewhat radical cam. So 18' is not really an ideal timing to crank the engine when its all hot and warmed up.

With using the vac advance now, my timing on start is close to ZERO before TDC. Since the vac advance is "off", my dizzy is locked somewhere at like 18' TOTAL now (no longer up at 38') and the MSD computer will retard from that new setting of 18'. The computer says "upto 20' retard" so Im sure its close to 0' on cranking.

See my point? My dizzy is now locked at maybe 18' instead of 38' total advance.

Its like turning my dizzy before I crank the car, then advancing the dissy once its started. The vac advance is doing that for me
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Old 06-01-2010, 08:47 AM
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It sounds like a rather elegant solution if you ask me. In any event, I like how you think! lol
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Old 06-01-2010, 12:17 PM
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"does anyone see this as a really dumb idea? "

not I...more like hot rodding creativity. It would be better IMHO if you could get the MSD to take out more on start up just because there would be fewer components involved, is this do-able? probably not since you went this route but it may be worth checking into since just about any micro-chip can be manipulated. And a question to appease my own curiosity...How does the MSD box know how long to hold the timing retarded? what tells it that the engine is running, tack input or something?
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Old 06-01-2010, 12:30 PM
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Put a switch on the MSD....with power off to the MSD start cranking.....flip switch to give power to MSD.....engine fires. Not a new problem
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Old 06-01-2010, 12:39 PM
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The MSD box has a wire that goes to the ignition 'start' wire. It retards only when the starter is cranking then puts timing back up again. It can not be adjusted for more retard, its fixed The newer msd boxes can do 25' total retard though and they too are maxed out.

I do have a switch to kill the msd spark but even that is not ideal. The engine will turn freely and as soon as that switch is turned back on you can hear the starter struggle. However, thats been my past solution on hot starts without fail.
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Old 06-01-2010, 01:26 PM
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Quote:
Originally Posted by bubbahotep
Quickly: I have a 383 stroker weekend driver with 38' total distributor locked full advance and using an MSD timing computer for 20' retard on start and adjustable idle timing (I can idle anywhere under 38'). Been like this for at least 10 years, maybe 15?? The engine calls for 38' total timing.

No weights, no vac advance used for years.

Even with the MSD computer retarding my start timing 20' (goes down to 18' when the starter is cranking) I still get some hot days where the engine wants to spin in reverse and the starter is crying a little. I have a switch to kill the ignition which works. Usually its after driving, getting gas, and then going to restart the car (all warmed up and hot and gas flowing too well)

Today I tried this setup which appears to work well so I ask opinions:

I have a vacuum canister (for brakes) that holds somewhere around 18hg after I rev the engine. Need that for my brakes with the cam I have. My hg never goes below 10 when I press on the brakes and stays there when I hold the pedal down. At idle the engine wont get below 10hg but cant get higher because of the cam, thats just the average hg at idle give or take. But the brakes hold well and stops fine

I just hooked my vac advance on the dizzy, adjusted to kick in instantly, to the vacuum canister for the brakes. THEN, retarded my timing something like 20' (just a guess). The point is that when I first start the car I am NOT locked at 38', my distributor is probably locked at 18' now because I turned it back. After about 2 seconds as the vacuum canister fills (empties, you get the point), the vac advance kicks in at 5hg, raises my total timing back to 38' (which I checked to confirm). The timing holds because my vac advance never releases.

I ran the car around and never stumbled because the vacuum canister always had well over 10hg...enough to hold the vac advance canister in total advance. And, at WOT where I have no vacuum, it doesnt matter because the vac is coming from the canister which is still holding enough to pull the vacuum advance in.

Long winded but I wanted to cover the story. My idea is when I start the car I have maybe 18' on the distributor because its locked, the MSD timing computer throws in additional retarding for a better start, and then the vacuum advance pulls in once the car is started to get me back to 38' total locked timing. The MSD computer is still dropping my idle timing down and doing what it should.

YES, I know ideally I should have weights and vac advance canister hooked up. But aside from fuel economy, does anyone see this as a really dumb idea? Its nothing different than any other radical cam riding at total timing locked out except that my total locked out timing isnt in effect until the car runs.
If you're running locked in timing, having a vacuum advance is inconsequential. Vacuums intent is to pick up the low to mid RPM timing to overcome the loss of mixture density under partial throttle conditions. This does more than improve gas mileage in this condition, it greatly improves power in this stage. The problem with this is that long duration cams knock down induction vacuum regardless of throttle position. Lock in mechanical advance is used to overcome this deficiency in vacuum, but has a dark side which makes the engine difficult to start.

In your case the set up would probably be better with putting in 6-8 degrees mechanical to come up say starting from 2000 or 2500RPM and all in by 3000 RPM. Set the base at 10-12 degrees. With 30-32 locked in, the electronic retard dropping 20 will leave 10-12 degrees for starting. The final 6-8 degrees would come in on the top end of the mid RPM rev range where it can be used. Then run no vacuum advance.

Keep in mind that locked in timing is a feature of race engines, it's old school technology to overcome timing problems with big cams before the age of computer driven timing. It was also done to remove another failure prone part on race engines, especially those that see a lot of G loading in frequent and sharp corners, beyond that it serves no useful purpose.

In your case, running a total of 30-32 degrees up to 2000-2500 RPM is plenty. Frankly with 9.5 compression and 38 degrees of timing from the git-go, I'm surprised this will stay below the detonation threshold at moderate RPMs on 93. I'd guess that it does because the vehicle is light and perhaps "low geared" against it's weight and rolling/aero drag factors. Low geared in this case is a relative term variable with the work the engine has to perform against the load of the vehicle to be moved.

Bogie
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Old 06-01-2010, 02:56 PM
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This motors ideal timing 38' total, anything less is a loss of power on this motor. I idle at 30'

I know about vac advance and mechanical but had to remove the centrifugal weights to use the MSD timing box. Its their suggestion to lock the dizzy only because the msd computer does the timing.

All that aside I dont yet see why using the vac advance to give me my full timing is a bad idea. I know its not being used properly. Im using it more as a 'full advance' after startup with no regards for fluctuating engine vacuum
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Old 06-01-2010, 03:32 PM
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Quote:
Originally Posted by bubbahotep
This motors ideal timing 38' total, anything less is a loss of power on this motor. I idle at 30'

I know about vac advance and mechanical but had to remove the centrifugal weights to use the MSD timing box. Its their suggestion to lock the dizzy only because the msd computer does the timing.

All that aside I dont yet see why using the vac advance to give me my full timing is a bad idea. I know its not being used properly. Im using it more as a 'full advance' after startup with no regards for fluctuating engine vacuum
Mostly the problem is a lack of vacuum. In a"normal" motor manifold vacuum is quite high at idle and part throttle. Under that circumstance the vacuum can pulls in it's advance. As the throttle is opened the the vacuum falls and the advance it creates goes away while the centrifugal takes over with advancing RPM. At 10 inches of manifold vacuum at idle you're in a situation where a normal vacuum advance has retreated. The reduction in vacuum just falls further as the throttle is opened, so the resource you need for the vacuum advance keeps becoming less. The vacuum brake reservoir saves up a bunch of low pressure air, but it can't make the vacuum deeper than what's being produced in the manifold. It will, however, be able to supply some advance based on how deep the vacuum can be retained, this is a good idea and will work, but you'll have to trim an adjustable vacuum advance to work with the available low vacuums.

Way back you said this engine was cammed but not with all that much-do you know how much? Your description of this engine speaks to a lot of cam, or a miss timed cam if it isn't a big whomper stomper stick, or the compression is lower than you think it is. An engine that needs 38 degrees to run good also speaks to a slow burning chamber. So some or all of these could be players if this isn't a huge cam, what ever that really means. Please comment to these details if you know them. such as

-Cam duration and lift, timing events if you have a card or the cam maker and part number so they can be looked up if the cam is fairly recent and not an obscure maker.

- Cylinder head, whose and what chamber size.

- Pistons need crown configuration (flat, dome, dish & whether that dish is round or D, compression height.

- Rod length

- Cam coordination to the crank, straight up, advanced or retarded and by how much.

- Status of the distributor gear and if a roller cam status of the distributor drive gear on the cam,,i.e. wear.

- Carb mixture ratio, lean will give also you some of the headaches you have.

- Vehicle weight, gearing and rear tire size.

Bogie
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Old 06-01-2010, 07:16 PM
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F-bird, you think just dropping back 2' on my total will make much difference? The msd timing box can give me 26' idle and have an adjustable curve to eliminate weights. Thatd match your suggestion.

Id still be firing the engine at 16' with the 20' retard

But youre suggesting also using the vac advance as it was intended? Meaning, let the vac advance raise the timing to 48' (using 10' vac advance) at idle? Or are you suggesting i use the vac advance to get me to 26' for idle running only?

Also, you said about waiting to flip the ignition on to get better gas/air: isnt that what causes the starter to strain? I thought the starter strain was from the combustion pushing against the piston...cause im so far advanced.
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Old 06-03-2010, 05:35 PM
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FBIRD - Im totally agreed with everything youre saying and Ive read tons of your posts over the past year and learned a ton.

Aluminum heads, I was told 38' was acceptable, you still say 36? I'll take your word.

With the timing, in my case, Im not confused just in a bind with the MSD. I think this was mentioned in an older post of mine and is related this this new post this week: I use an MSD 8981 timing box for my advance curve because its so dead-on-balls precise. Its biggest downfall, as you're already pointing out, is that it requires the distributor to be LOCKED at full advance. It does the retard for hard starts but 20' from 38' on my engine is still sometimes a hard start.

So, thats what Im trying to use my vac advance for more initial start retard BUT keep the vac advance "advanced" even for WOT.

100% I see that I SHOULD go back to the weights and the vac can etc etc. But for now, isnt my only loss the use of the vac advance for cruising light load since my vac can is just stuck at advanced fully? (not going over 36 or 38 total advance ever)
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Old 06-03-2010, 05:51 PM
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What kind of starter do you have? With gear-reducted starters I dont even see this prob anymore, even on oval track cars with conventional headers after running at peak power for 30 minutes. I would stick with the locked mech adv if I had the timing setup you have and try to get some vac advance if you can, but if not its really not a big deal.. and get a better starter with big cables going to it.
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Old 06-03-2010, 06:25 PM
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Quote:
Originally Posted by bubbahotep
my only loss the use of the vac advance for cruising light load since my vac can is just stuck at advanced fully?
That, and if the reservoir ever goes below the vacuum that is needed by the vac. adv. can to stay fully advanced, you will obviously lose timing advance until the vac. in the reservoir comes back up.

I had thought at first you were running a gnarly cam but a 236/242 @ 0.050" doesn't qualify for needing a locked-out advance dist., IMO.

If you set it up w/36-38 total timing all in by 2500-3000 RPM, w/as much initial timing as the engine "wants" (initial could be anywhere from 16 to 24, ballpark- the rest from mechanical), and use a vac. adv. as a vac. advance and have 10-12 in the can, the engine will run great for you. Use the timing retard to lower the initial for starting, like it was intended.

The ACCEL #31035 is an adjustable vacuum advance can for GM HEI that allows infinite adjustment to BOTH the amount and rate of advance. Comes w/instructions and tool.

To limit the amount of vacuum advance w/an OEM or some adjustable cans like the ADJUSTABLE VACUUM ADVANCE CAN KIT- Crane #99600-1: 99600-1 INSTRUCTIONS, you will need to physically limit the vac can's travel w/a VACUUM ADVANCE LIMITER PLATE- Crane #99619-1: #99619-1 INSTRUCTIONS.
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Old 06-03-2010, 09:46 PM
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Vac Advance

Hey Bubba, While your solution to your problem at hand is pretty clever. I think that FBIRD&COBALT can help you set up for an engine that likes alot of timing. I have dealt with two big blocks that liked the timing but the starters didn't. If you can't crank your sbc without an interrupt and a timing box, you really should consider a gear reduction starter. I cured both the big blocks mentioned with gear reduction starters. Back in my Mopar days, cranking big cam, high compression, high timing engines was never an issue due to the factory g/r starters with no interrupt or box. Big cable, big battery, g/r starter= easy start. Worked on my 454 motorhome too. Used a later model 350 g/r on it. On the other two I used Powermaster. Food for thought. olnolan
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