Exhaust manifolds - big bolt and 'labirinth'

One downside to less vacuum is that the fuel might not get atomized as well and the speed across the jet is slower. Could that be? :thinking:

Indeed, David - with lower manifold vacuum less ‘fresh’ air is drawn into the engine; EGR is sort of a vacuum leak - only the leak is exhaust…

As fuelling is directly related to air ingress, whether carbs or FI; less fuel is delivered resulting in loss of power - which the driver can regain, if required, with more pedal. Increasing fuelling…

Of course the amount of exhaust is strictly controlled - it’s not like a heap of exhaust is introduced. But the EGR was not introduced to save petrol - and like all emission control devises; it benefits the environment but not the engine…:slight_smile:

There are no free lunches…?

xj6 85 Sov Europe (UK/NZ)

It’s rare when I disagree with Frank Anderson, so let me emphasize the IMHO at the start. I think you’re confused with what is meant by “lower manifold vacuum.” As David mentioned above, one fourth of the four-stroke cycle (1.5 cylinders at any particular time) have their intake valve open and are “sucking.” This sucking (which you confuse with vacuum) is what speeds fuel across the jet and determines admission of mixture into the cylinders. The more sucking, the more fuel. It’s just like a vacuum cleaner–the more suction, the more dirt is sucked in. In contrast, vacuum is a measure of air (or mixture) evacuated from a container (the manifold, the vacuum cleaner hose). If present, it means there is a restriction on entry of mixture (or dirt). Work is done evacuating the container but mixture (or dirt) are not sucked. So a vacuum gauge inserted into the manifold (or into a vacuum cleaner hose) will read close to zero with the throttle completely open (or hose unclogged) but will read high if the throttle is closed or the hose clogged.

So with lower vacuum MORE fresh air is drawn into the engine, not less.

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I might need more time to understand exactly what each of you say but Frank says…

Introducing exhaust is like an air leak, it reduces the amount of fuel that is sucked in because there is less draft across the jet. There is less oxygen as well, because the exhaust is inert. Therefore, all the carb sees is a certain oxygen demand, and it adds the correct amount of fuel to it: mixture correct. All the oxygen is metered and therefore the mixture should be spot on.
But since there is less oxygen going in less fuel is added.
Less fuel - air means less power means MORE fuel - air needs to be introduced to regain engine output. The throttle needs to be opened more, getting us back to the same amount of petrol burned. Almost:

This is true, but since the throttle is open further, the engine is throttled less, so it has to create less vacuum. This is the point exactly.

We want the throttle to be open as far as possible while burning as little mixture as possible. Same power output, throttle open further.

Details - less vacuum could mean less fuel is sucked in, which may not be linear. Perhaps requiring a different needle.

Sucking and vacuum are the same to me, the piston on the intake stroke has to pull down against the vacuum, that is work, less vacuum sucking it up means less work means more efficience.

I see petrol and oxygen as two fuels (like in a rocket, propellant and oxidizer) that need to be combined. The engine is controlled by liliting one of the fuels and the other is always mixed in at a certain ratio.

That limiting creates a loss (it‘s not a diesel) because the engine creates more vacuum (lower pressure) when the throttle is open less. So if the loss can be decreased by introducing an inert gas, it will need to work less at the same engine speed.

I’m not confused, Robert - using ‘manifold vacuum’, which is the difference between manifold and ambient air pressure. So 18" Hg manifold vacuum is high - 12" Hg manifold vacuum is lower…:slight_smile:

Actually, one can argue about ‘sucking’ - it is actually the air pressure that forces the air into the manifold. So absolute manifold pressure is arguably more ‘scientific’; it is what forces mixture into the cylinders. But since ‘vacuum’, the difference between ‘ambient pressure’ and ‘manifold pressure’ is so easily measured - it’s what is commonly used. Bearing in mind that pressure is inverse to vacuum.

So with low manifold vacuum, logically, less air is drawn into the manifold. Nomenclature may be confusing but I’m not confused re EGR…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

Carbs, and EFI, responds to air flow, David - which is some 20% Oxygen, so the mixture going to(!) the manifold is correct; but the cylinders(!) which does the work get a lean mixture (EGR induced) - which drops power.

A similar air leak does the same, reducing ‘pumping loss’ and leaning out the mixture. Neither leaning out the mixture nor introducing an air leak is recommended to improve fuel economy - and EGR is in the same category. Jaguar certainly did not introduce nor claim EGR as a fuel saving item…

‘Pumping loss’ is the price the engine must pay - but is but a fraction of the hard work 1,5 cylinders must do to keep the engine running…

EGR has its just purposes, but fuel saving is not one of them…

xj6 85 Sov Europe (UK/NZ)

The cylinders do NOT get a lean mixture because there is no excess of oxygen!
An air leak introduces excess oxygen!
Exhaust contains almost no oxygen, so it does not cause the engine to run lean as it is not at all like an air leak!

See fuel and oxygen as carburant part A and B respectively.

A:B stays the same if you introduce something inert, C. But obviously, if you introduce C you need to add more A and B to create the same power.

If you have more ABC going through the engine it has less power per ABC volume, so you need to feed it more ABC, the throttle is open further. Then less vacuum is made as C takes up the space, and it is inert so it does nothing else.

If you need more power, the C valve closes and all available A, B can burn, they make power.

So as you say, A:B is correct in the manifold, add C, then you have a new ABC mixture, still with A:B intact.
An air leak would introduce B, not C. A leaking cold start injector would introduce A. A hot air intake lowers the amount of AB, so a cold intake makes more power and slightly more pumping losses.

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I’m still confused as to what you’re saying, Frank (sorry about previously misspelling Andersen BTW). Agree that we are both using the convention of “high” vacuum meaning lower absolute pressure.

Again, completely agree. To elaborate:

A. If the manifold is completely evacuated of air or other gas, the vacuum is maximum, and the absolute pressure is zero. As you say above, no mixture is getting into the cylinders.

B, If the manifold is throttled so that its absolute pressure is less than atmospheric pressure (e.g. 5 PSI = 10 mm Hg) then some mixture gets in.

C. If the manifold has full access to the atmosphere (not throttled; WOT) then its absolute pressure is nearly that of the atmosphere (15 PSI = 30 mm Hg).

D. If the manifold is supercharged, then absolute pressure can rise higher than atmospheric (e.g. 30 PSI = 60 mm Hg).

So in A through D above, as manifold pressure increases (manifold vacuum gets lower) more air is drawn into the manifold. [And more power is produced.]

The above is what I disagreed with in your earlier post, and what you repeat here. As I argue above, IMHO the opposite is true. It follows that EGR (although introduced for NOx reduction) does indeed increase fuel efficiency under cruising conditions.

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But isn’t he right:

Ambient = 10 (David units)
WOT pressure = 8
Low vacuum = 5
High vacuum = 1
Completely evacuated = 0

Low vacuum means less air is sucked past the same throttle opening. Less suction aka pressure differential with lower vacuum = higher absolute pressure.

I think you approach it the other way - if there’s more air drawn in, the vacuum must be lower because the throttle must be open further in relation to engine demand. You‘d both be right.

With or without EGR the amount of A, B going in should be exactly the same as before, the only difference would be that the throttle has to be open further because the pressure differential is less when C is added into the manifold.

Less differential means less work means less waste. So adding C means needing less A, B.

Technically yes, David - but too narrow…

The heat generated by the petrol burning is the producer of power. Less petrol is induced by the higher manifold pressure. And the total volume to be heated, air and EGR is the same; the power generated is less. I would call it ‘lean’ - there is not enough petrol to heat the ‘blend’ enough for full power production. But less petrol lowers combustion temps - which was the intent of the EGR. It’s semantics involved, but argument still valid whether the addition is air leak or inert/EGR gasses…:slight_smile:

Again, if power is adequate no action is needed - otherwise more power, pedal/petrol, is required not beneficial for economy…
induces more petrol in both cases.
As an aside; V12 EFI use manifold pressure as a parameter - so higher pressure induces more petrol - though useful only with an air leak…

Purely academic…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

That is not lean because lean is a lack of combustible A and excess of B. An air leak is like spraying liquid oxygen into the engine.
Because C is inert, the only thing that changes is that there is more mass total in the cylinder. Maybe harder to ignite, not sure, but lower temperatures allow for more (vacuum) advance or a leaner mixture.

More throttle does not equal more fuel used (as you say less vacuum, lower velocity), if the same work is done, the consumption is identical, but if that includes lower pumping losses because the throttle is further open to do the same work, we‘re wasting a little less.

Just for the record Robert - ambient air pressure (sea level) is around 30" Hg = around 15 psi.

As said before; if power produced (with EGR) is enough to maintain cruising speed; no action is required - otherwise; more pedal/more fuel is required. And if less power still maintains cruising speed it raises questions - and cruising is anyway only part of fuel economy…:slight_smile:

Anyone is free to install EGR of course - it would be interesting to know the result in practice to end theories - my opinion should be well known…:slight_smile:

As an aside; cylinders are filled to manifold pressure fairly rapidly (700 cc). At 1200 rpms this must be done in 1/10th of a second - and four time shorter at full throttle. It’s rather amazing…

Manifold pressure is a function of both throttle position and engine rpms. This is dynamic and the pedal is the sole control the driver has over power production to increase, maintain or reduce car speed. The power required is car, wind and road dependent - and the driver’s whim…

xj6 85 Sov Europe (UK/NZ)

No! If manifold pressure can be increased, more pedal for the same power means same air, and fuel.

Same power = same consumption.

And slightly less loss from throttling.

[quote=“davidsxj6, post:34, topic:442466, full:true”]

No! If manifold pressure can be increased, more pedal for the same power means same air, and fuel.

More pedal gives higher manifold pressure, David - forcing more air (and fuel) into the cylinders. The ‘air’ does not stop in the manifold - it increases cylinder fil, and consumption. This is how power is increased with constant rpms. At higher throttle settings, the pressure rises further powering the engine to higher revs - producing further power increases for acceleration…

You can increase manifold pressure by external means like compressors, even using inert gasses, but it’s the amount of petrol burnt that dictates power - and consumption. If there are other means of increasing manifold pressure I’m willing to learn…

xj6 85 Sov Europe (UK/NZ)

Using the same amount of fuel and air, adding an inert gas increases manifold pressure, I keep saying it, it is logical.

If power output is the same, doesn’t matter what the throttle position is, consumption is also the same.

Interesting discussions.

I only add that on my E-type V12 1973 US specs, the EGR is simply a pipe connecting the exhaust manifold to a plug in the connection of the Strombergs to the inlet manifold. There is a restrictor in the pipe, regulating the amount of exhaust gases allowed to get it.

Useless to say, that pipe is known to be always clogged , and a previous owner already had blocked the input at the Strombergs. I removed all that piping.

I guess that modern devices, electronic controlled according to engine temperature and load, may be more effective and may also have some theoretical impact on fuel consumption, if associated with other engine management systems.

In my view, all these old devices ara of no use nowadays and are a source of problems, they were used mainly to reduce emissions along the US legislation , but they worked mostly when the car was new and were needed for the homologation. Same as with the recent VW Dieselgate.


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That makes sense, the vacuum at cruise pulls more exhaust than at full throttle: why the $£*# is there a valve for the 4.2?

These EGR things are always a hassle but a little more maintenance wouldn’t kill me for a free tank of gas a year. But is it worth it, definitely not.

EGR is still used today.

Whatever gas is added, David; it either needs more petrol, pedal, to heat the increase to the same working pressure, which is power. Or the temp will drop - giving less pressure/pow er…

And adding a gas, inert or whatever, just to increase manifold pressure is an air leak - which reduces the fuel-giving air flow through the throttle

xj6 85 Sov Europe (UK/NZ)

It’s still effective in some aspects of emission control, David - but I doubt if the claim is as a fuel saving devise…?

xj6 85 Sov Europe (UK/NZ)