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Old 12-11-2011, 10:50 AM
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No vacuum advance needed?

I just finished breaking in a new 290hp 350ci from GM performance. It states in the paperwork that this engine doesn't require a vacuum advance? It says to set timing at 10 degrees at idle and centrifugal should be all in around 3000 at 32 degrees. Why would a vacuum advance not be required?

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Old 12-11-2011, 10:56 AM
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it doesn't pull any vacuum at 3,000 rpm anyway, you really dont need it.
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Old 12-11-2011, 02:28 PM
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Quote:
Originally Posted by cullencombs
I just finished breaking in a new 290hp 350ci from GM performance. It states in the paperwork that this engine doesn't require a vacuum advance? It says to set timing at 10 degrees at idle and centrifugal should be all in around 3000 at 32 degrees. Why would a vacuum advance not be required?
The timing recommendations you quote are conservative and tells me they are more worried about a warranty return than for you to get the most from the engine.

10-14 degrees BTDC is a place to start from for the initial. Max timing (w/o adding the vacuum advance) should be about 36 degrees, all in as soon as the engine will allow it w/o detonating and that depends on a number of things like fuel quality, gear ratio, vehicle weight, vehicle use, etc. The "usual" all in by RPM is between 2500-3000 RPM.

There is not one valid reason for not using a vacuum advance. Unless getting the best MPG that you can, a cooler idle temp and a smooth cruise is of no concern to you, that is. If you're using a GM HEI, I'd suggest something like Crane's adjustable vacuum advance can kit- p/n 99600-1 that has the limiter plate, springs and an adjustable vacuum advance can.
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Old 12-11-2011, 06:02 PM
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X2 ten characters
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Old 12-12-2011, 08:26 AM
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X3 on Cobalt. Lack of vacuum advance will make it a lazy turd in the low end. Sluggish throttle response, slow to get up to speed, use twice as much fuel, causes throttle plates to be off the idle circuit causing idle and off idle tuning problems. By all means connect it to a full time manifold source that pulls vacuum at idle. Your looking for 18 to 24 degrees advanced at idle.
Since you`ve told me that`s what the GM source says now we`ve learned the next source of bad information other than just MSD ignition.
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Old 12-12-2011, 08:28 AM
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http://www.jegs.com/i/Proform/778/67022C/10002/-1
what about using this?
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Old 12-12-2011, 10:49 AM
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Quote:
Originally Posted by cullencombs
I just finished breaking in a new 290hp 350ci from GM performance. It states in the paperwork that this engine doesn't require a vacuum advance? It says to set timing at 10 degrees at idle and centrifugal should be all in around 3000 at 32 degrees. Why would a vacuum advance not be required?
Can you spell DOG? This engine is long on cam and short on compression, which probably means it's low on vacuum. If ever a dog needed vacuum advance this one's it.

For a street engine especially when combined with high gearing with an automatic transmission, you need vacuum advance. With stiff gearing and a stick, the engine is either spun up to where the centrifugal comes in or it can be manipulated with the gear box to bring in the advance.

The reason for advance is that the burn speed inside the cylinder is variable with the density (number of molecules) in the cylinder and the RPMs which limit the available time in which to burn the mixture such that the best pressure happens at the best piston location to effectively transmit the energy of the burning fuel onto the crankshaft.

The concept of mixture density is trickier idea to get your arms around but not all that complicated. The speed of the burn is dependent upon the number of molecules and the absolute pressure/temperature in the combustion chamber. Taking this apart, first the burn speed is related to mixture density, more density nets fast burn and less density results in a slower burn. So to position the best cylinder pressure time with the best piston location against the burn time, a certain amount of early initiation is required which varies to mixture density, engine speed, and absolute cylinder pressure.

On the street at cruise speeds the typical vehicle is moving with very little throttle opening at pretty low RPMs. This results in very low mixture density that burns slowly. While the time available for the burn at low RPMs is relativly long as these things go, it is not proportionally long enough to time best pressure and piston location properly so the burn is started early (advanced). Since the RPMs are low and the manifold vacuum high it is preferable to use the manifold vacuum to produce the needed advance. Preserving centrifugal for high RPMs where the throttle is open wide and the manifold vacuum disappears into atmospheric pressure, unless there's blower on the engine which is a different set of more complex issues.

Your 290 horse 350 happens to suffer from a double whammy of poor parts selection against the power produced. It is a low compression engine with a long duration cam. The long duration cam bleeds off a lot of lower RPM cylinder pressure which greatly reduces the burn speed. This is the effect of Dynamic Compression Ratio (DCR) which is a reduction in the Static Compression Ratio (SCR) by the amount of lost stroke length that positions the piston on the compression stroke in crankshaft degrees when the intake valve closes. This results in reverse pumping the mixture back into the intake manifold until such time as the engine's RPMs generates enough mixture velocity in the ports to overcome this phenomenon, that happens about the
torque peak RPM. Under this point the engine needs to be crutched with additional advance than one would think necessary.

The subject of burn time with high cylinder pressure is also a high engine speed event where above about 3000 RPMs and WOT, the burn speed naturally speeds up in direct proportion to RPMs such that further advance is not needed. In the case of a supercharging or nitrous injection may even need to be retarded. Supercharging and nitrous injection are two things that increase mixture density by force. These will speed the burn much earlier in the RPM range and to a much greater extent. They always require some form of mitigation to the ignition advance to prevent the engine going into severe detonation which will destroy the engine.

When you get to the 290 horse GMPP engine, while the power is decent, the fuel it will burn to make that power is ugly big because the engine lacks efficieny. Without changing the cam, the best you can do is mill the heads to get some compression up. Replacing them is even better; the substitution of Vortecs lifts this motor to 330 horses with no other changes for less fuel consumption at that. This is because the Vortec is bringing more compression, not mention better ports and a way better combustion chamber. A piston change to flat tops or D dished to dial in the compression ratio really dials this motor in. But as it comes it leaves a lot to be desired.

Bogie
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Old 12-12-2011, 11:25 AM
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Bogie`s post about this matter should be placed in the knowledge base. He breaks down the cause. This will give many insight to why vacuum advance is needed at idle and off idle. How I always explained it to others was simple as I am not technical. I would tell them idle and off idle mixtures are lean. Leaner mixtures burn slower than do rich ones so the fire has to be lite off sooner due to the leaner mixtures slow burn time. Perhaps when we tell guys this and they think were crazy they can read this and understand it better.
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Old 12-12-2011, 12:14 PM
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Quote:
Originally Posted by DoubleVision
Bogie`s post about this matter should be placed in the knowledge base. He breaks down the cause. This will give many insight to why vacuum advance is needed at idle and off idle. How I always explained it to others was simple as I am not technical. I would tell them idle and off idle mixtures are lean. Leaner mixtures burn slower than do rich ones so the fire has to be lite off sooner due to the leaner mixtures slow burn time. Perhaps when we tell guys this and they think were crazy they can read this and understand it better.
Absolutely!! Great Post!

Bogie, Do you mind copying this to the wiki?

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Old 12-12-2011, 01:20 PM
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Look at the torque rating of this engine. It tells the whole story.
Low compression ratio engines do not like big duration cams.

Needs 10:1 cr or a smaller cam. The cam in this motor was designed for 11:1 compression. (1969 L-46 350-350hp corvette motor)

Its a dog at 8.5:1 cr.

Advancing the crate motor's cam by a full 7-8 deg will help.

Get a 3000 stall and some gears. A head swap will wake it up.

Yes you want to employ vacuum advance. approx 10-12deg at hiway cruise speeds and a max of around 15deg.

You are much better off buying the base 350-260hp crate motor and getting your own aftermarket performance cam for it.
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Old 12-12-2011, 02:35 PM
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I plan on running it for a while, maybe until the warranty is up. Then i will do a head and cam change. I knew it felt like a turd without vacuum advance and just wanted some input as too why GM said dont run it. Thanks!!
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Old 01-07-2012, 10:51 AM
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Well I got one of these turds for my pickup too. Is there anything I can do short of tearing into this motor ? Maybe something with the distributor and timing, vacuum and mechanical ? I could change the cam but don't want to go there yet.
Thanks ,Gene
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Old 01-07-2012, 02:33 PM
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Quote:
Originally Posted by 57halfton
Well I got one of these turds for my pickup too. Is there anything I can do short of tearing into this motor ? Maybe something with the distributor and timing, vacuum and mechanical ? I could change the cam but don't want to go there yet.
Thanks ,Gene
If you're not willing to go into the motor, pretty much the same thing, add vacuum advance and work with the ignition timing, it's not much, but it would help.
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Old 01-07-2012, 02:59 PM
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Ive got the timing lined out and it did help the response off idle! Its ok for know but will get some top end work maybe next winter!
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Old 01-07-2012, 03:02 PM
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A couple thoughts...

Quote:
Originally Posted by 57halfton
Well I got one of these turds for my pickup too. Is there anything I can do short of tearing into this motor ? Maybe something with the distributor and timing, vacuum and mechanical ? I could change the cam but don't want to go there yet.
Thanks ,Gene
The best bet for making do w/what you have (besides timing and vacuum advance) is to not make things worse. Off the top of my head:

Use a dual plane intake. Do not use an air gap-type intake.

Supply the carb w/cold air from outside the engine bay if possible.

If you use any spacer, use a divided or 4-hole spacer.

Don't over carburate it. A 600 CFM carb like Holley or Edelbrock or a Q-jet is plenty. The Q-jet will offer the best compromise, but don't use a Q-jet w/an adaptor on an intake w/a squarebore flange.

Be sure the carb is in tune. What you don't want to have happen is for the power enrichment system to come in too early because the engine is down on vacuum. Checking and correcting for this depends on the type of carb you are using. Holley uses a power valve, Eddy uses step up springs and the Q-jet uses a power piston spring. Regardless of the name they all do the same thing.

This engine is going to like a relatively long duration accelerator pump shot to cover up the lean 'hole' in the a/f mixture that's created when the throttle is opened quickly. This is caused by the same basic lack of vacuum that can also cause the power circuit to kick in too early as mentioned above. Again, this is adjusted in different ways depending on the carb you have.

Advancing the cam 4 degrees might make an improvement.

Lower rear gears will make the engine snappier, especially if you are running tall rear tires. Or use a 4-speed OD auto trans like the TH700R4 that has a low first gear.

A looser torque converter will help things as well. You don't have to go crazy here, I'd consider a 3000 RPM stall speed more than adequate.

Good luck.
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