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MG MGB Technical - more compression=more vacuum?

this is more general than just MGB but here goes...

All things being equal (e.g. no leaks, etc.) does a higher compression engine provide a significantly higher vacuum ?

cheers
p
Philip

Hi Philip.

I would have thought that if the swept volume remained the same the manifold vacuum would also stay approximately the same.
I guess that there would be a slight modification due to the (presumably) reduced unswept volume.

I'm sure that someone more knowledgeable will contribute.

Don
Don

Hi Phillip

I think that Don has it right, the main decontributing factors are leaks in the manifold, leaks in the carb spindles, leaks from the inlet valves, leaks from the distributor advance pipe, and depending on the particular smog setup, leaks from that as well.

If you have a greater vacuum following work carried out to increase compression, it is as a result of improved seating of the valves and elimation of other leaks that will have done this.

Cheers
Ian F
Ian Fraser

-more-

While I was lying on my back in the mud under the house doing some plumbing work today, it crossed my mind that yes, compression ratio will affect manifold vacuum. But probably not to any great extent if you are making only a nominal change of say a few decimal points in the range of say 8.8:1 to 9.5:1

But - Consider this -

If the CR was infinite, ie there was no dead space in the cylinder when the valve opened and the piston started its travel downwards, it would suck in a certain volume of mixture, not quite the swept volume of the cylinder, but somewere near it.

Now if the CR was very very low, say 2:1, at the top of the stroke there would be a volume of gas about equal to half the swept volume of the cylinder.
As the piston travelled down, this gas would expand somewhat, so the volume of mixture taken into the cylinder from the manifold would be less than in the first case. The vacuum in the manifold would then be less.

I'm not sure if this is of any practical use on our MG's, but it may come into play on the variable compression engines that are now under development. (Saab, Volvo, Honda???)

Cheers
ian F

Ian Fraser

Vacuum is produced by pumping losses accross the butterfly. If increased CR means more power, we have to close more the butterfly to develop the same power at a given engine speed, so more vacuum.
JM Thély

Hi all.

I agree with all points raised.

An amendment :

I would have thought that if the swept volume remained the same the manifold vacuum would also stay approximately the same FOR A GIVEN THROTTLE SETTING.

Don


Don

all,

thanks. vacuum is a bit of a mystery to me since i guess i'm always finding/plugging a leak somewhere :)

At least now i can relax and stop trying to find that 'excess vacuum' that doesn't even exist....

cheers
p
Philip

Other factors being equal, yes, the higher the mechanical compression ratio the better the vacuum/signal to the carb.

Personally, on my performance toys I like to run Moroso,s "Evac system".
This system runs two high temp hoses down to each exhaust collectors from both valve cover breathers, be they built in or not (breathers).

This systems avoids diminishing the vacuum (crisper throttle)and keeps the incoming charge clean and unburned. It also promotes a cleaner chamber, valves and plugs.

The exhaust pulses do an exceptional job of creating negative pressure in the crankcase.



http://store.summitracing.com/default.asp?target=esearch.asp&N=110&Ntk=KeywordSearch&Ntt=evac
jegawatt

It did not route directly to the item detail..just scroll down and click on MOR-25900, you can then view a larger image of the system.
jegawatt

The vacuum in the inlet manifold is complex but in a perfect engine depend of two major concept :
A ) Empirical relation :
Coeff. of filling (%) = (P th (Bar) - 5) / (0.6 * CR)
For the * same * power (ie same average theoretical pressure in the cylinder : P th), if the CR rise, the coeff. of filling decrease (and the vacuum). This consideration is done at the same power, but normally with a better CR, the power increase, so the filling is better.
B ) Resonance
The Coeff. of filling (and the vacuum) depend of the resonance effect (1/4 wave) in the inlet system :
global geometry of head + manifold + carb + ram + air filter
and homogeneity + density + temperature of the air / gas mixture
and frequency of the inlet valve (trumpet effect).

The vacuum in the inlet manifold depend of these two phenomenon, the CR contribute, but the resonance effect is the main.
Arthur

Arthur

This thread was discussed between 10/10/2004 and 12/10/2004

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