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vacuum advance theory question


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I wasn't sure where to post this, but it's engine-related so I'll start here. My question is in relation to where the vacuum that creates the advance in the distributor (among other things the car uses it for) is really made.

It stands to reason that it's the pistons creating the physical vacuum during their intake strokes and that vacuum is translated through the intake manifolds as air flows to fill that vacuum. Yes? If it turns out I'm wrong already, someone please correct me. Anyway, with the pistons generating a vacuum, does it also stand to reason that installing an oversize block (a 2.8 in place of a 2.4, for example) can offset the function of the devices that use the vacuum since the pistons would be mechanically larger and thus generate a slightly stronger vacuum as they operate? An example of this could be a vacuum advance dashpot on a distributor or a vacuum brake booster not working exactly as intended. Are these ideas correct? Are the effects of such a change small enough that one wouldn't notice them? Are they measurable?

This isn't a troubleshooting question, it's just a curiosity that hopefully some our veteran tinkerers can explain. Thanks, everyone.

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The things that influence engine vacuum are;

A running engine will always see some 'vacuum' in the intake due to inherent flow restrictions from port walls, runners, intake valve and the throttle plates. This is even true at wide open throttle. Although the guage shows 0 vacuum, get a sensitive enough manometer and you will see some vacuum. The other way to look at vacuum is to think "restriction" The greater the intake restriction, the greater the vacuum. Reduce restriction, reduce vacuum and theoretically you increase intake flow and build horsepower. Think individual runner triple carbs as minimum restriction max power setups. Have you also heard about those guys having enough vacuum to run vacuum advance mechanisms on their distributors? Tapping all six runners is needed to gain a 'dampened' signal to run one effectively. Brake boosters also require vacuum to operate well. Some resort to vaccum tanks or accumulators that save the vacuum when the throttle is closed and the engine is on the overrun, changing down gears for example.

The camshaft profile. Specifically it's the valve overlap period of the cam profile which will affect vacuum. Ask any guy that puts a big cam into an engine what happens to intake vacuum. (and ask the guy that runs a Holley what he needs to do to the power valve after the cam change!)

The mechanical condition of the bores, rings and valve seats effect the vacuum seen in the intake too.

Change the size of the engine doesn't seem to make any real difference. The throttle needs to be cracked open slightly to let sufficient mixtures into the engine so that it can at least idle. Put a lawn mower carb on an L28 and you may get it to run with a throttle that would be fairly open. At 800 rpm. (idle) the vacuum would read around 20 inches of vac with a stock cam and that's the same as with the original units with the engine at the same idle rpm's.

Don't forget that vacuum is important for mixture vaporization. It reduces the boiling point of the fuel so it can vaporize better in the induction system which results in better fuel mixture homogenization and cylinder distribution. Torque is generated this way. In a carbed car, that's why acceleration can 'feel' stronger at part throttle than at full throttle. Gas speed is involved here as well, and vacuum is maintained with a partially open throttle with its inherent advantages. That's why double pumper carbs are not suggested for street use, too little vacuum (WOT and low rpm's) and the fuel falls out of suspension and the engine loses torque. Too much carb mate. At WOT and higher rpm's (gas speed), the problem before becomes an advantage now! (low restriction)


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So the final comment you made about fuel falling out of suspension due to insufficient vacuum also explains (at least in part) why a set of triple Webers is less street-friendly than the pair of SU's our cars come with; becuz with significantly reduced vacuum due to significantly reduced restriction, fuel vaporization is affected...fascinating :D

This is exactly the type of explanation I was hoping for, thanks!! So many new bits of knowledge to research now. I love it.

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You're welcome. :)

The Sus run a 'variable jet' and Webers run a 'fixed jet', like a Holley.

Sudden opening of the throttles at low induction airspeed will cause a stumble because vacuum returns to atmospheric and the fuel falls out of suspension. That's certainly the case with a Holley, for example. The great thing about the SU's is they don't suffer with the same problem and don't have or need an enrichment device like the fixed jet carbs. SU's maintain some of their manifold vacuum by the piston rising slowly in response to airflow increases through the manifold. A simple design that can provide a very healthy level of performance when setup correctly. (says the man who runs a modified Holley under the hood of his own car :stupid:)


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A Manifold Absolute Pressure (MAP) EFI setup relies primarily on the vacuum seen in the intake manifold along with other imputs to determine how much fuel is needed over a range of operating conditions. Vacuum in the intake manifold is an excellent indicator of engine load.

When talking about older throttle body injection, the fuel is added to the air close to the throttle blades, usually a long way from the intake valves in the cylinder head. With these setups, the 'wet manifold' is effected by manifold vacuum in exactly the same way as a carburetor setup.

More modern multipoint or direct injection EFI systems don't have a wet manifold to deal with. Fuel is sprayed by the injector under high pressure as a 'mist' of very fine droplets straight onto the back of a hot intake valve in the head. Some fuel enrichment is still needed, but its much less than any of the other systems mentioned when the throttle is cracked open and the intake manifold vacuum drops towards zero.

And when the turbo spools there isn't any vacuum that I know of in the induction system LOL Is there?


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