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MG MGB Technical - Compression at higher elevations

Does elevation affect compression in an engine? Is 140 lb./sq in. at sea level the same as 140 lb./sq in. is at an elevation of 3,000 feet? I know that when you test for vacuum, every 1,000 feet in elevation is equal to a 1 hgs loss on a vacuum guage. Does this apply to compression, and does that mean that my engine with 140 lb./sq in. at 3,000 feet is actually got higher compression than indicated?
I had this conversation today and was told that, but I'm not sure if it is sound logic. Any thoughts?
SteveO

Hi Steve

Two things -

At a higher altitude the air is less dense, and there is a lessor weight of air in the cylinder at the start of the compression stroke. It follows that the absolute pressure of the compressed volume would be less.

But to measure this brings in its own error.

At a higher altitude a bourdon tube pressure gauge reads fractionally higher because there is a slight variation due to the reduced external pressure on the guage. (Imagine if you were to connect the guage to an 'empty' dive tank, it would read zero. If you were then to put the whole tank and guage into a chamber and suck out all of the air in the chamber, the guage would then read thecpressure onf the air in the dive tank, - approx 15 lb/sq in. )

Now wether these two things exactly cancel out - I dont know, I would have to get out my old textbooks and check it out.

But there is no doubt that at higher altitudes a normally aspirated engine looses power.

What you need is a supercharger!


Cheers
Ian F

Ian Fraser

Steve

The effective compression ratio is dependent on the density of air

A pressure gauge usually measures the relative pressure between the cylinder and ambient , and therefore I think Ian is correct that there is no great difference in readings caused by altitude

However, 2 things affect hp

1. at 3000 feet amsl you are about 100 millibar or 10% down : this represents nearly 10% loss of power bcos the only compensating factor is that there is less exhaust resistance

2.nearly always, the mixture strength will be too rich for max hp at your altitude : a change to weaker needles should help this ; this is why piston engined aeroplanes have mixture controls which weaken the mixture as the craft climbs - it will not overcome the loss of hp but it will optimise your setup at a given altitude

Chris
chris

Hi Chris

Mmm --
To be absolutly correct, the compression ratio is a result of the mechanics of the engine ( function of piston displacement and combution chamber volume), and is constant whatever the altitude.

The effective compression pressure varies with altitude and temperature ( or the denisity of the air )

(unless of course you fit a supercharger where the effective overall compression ratio is is additionally affected by boost pressue :} )

Cheers

Ian F
Ian Fraser

A mechanic told me that even though I'm running 8.0:1 pistons, and the compression measures 140lb./sq in., which is in line with a low compression motor, that I actually have closer to 10.0:1 compression due to the elevation. I'm struggling with that theory, since someone who is actually running 10.0:1 at sea-level would have their engine blow up if they ever travelled to the mountains at say, 7,000 feet. I definitely agree that power falls off at elevation, but if it increased the compression ratio, wouldn't that give you a bit of a gain in power? The info seems to contradict itself, and I was just wondering if anyone had a better feel for this.
I'd love to have a supercharger under the bonnet, but that doesn't fit into the equation any time soon.
SteveO

In past travels when I needed 92 octane to keep from running on (non MG) I always found I could run on a lower octane in the higher the elevations, and at the same time the higher octane was more difficult to find. However the lowest commonly found octane where I usually drive is 87 which became 85 at higher elevations - which leads me to believe that increased elevation effectively lowers the compression ratio of a normally aspirated engine. This is especially obvious when driving a non-turbo deisel up a mountain.
.

Doh!
Now I'm really messing up. I should have written that a car with 10.0:1 at 7,500 feet might have problems at sea-level if the logic holds true. Thanks for straightening me out.
SteveO

the accent in my last post was on the word effective

as air density falls, so does the filling of the cylinder

hence a cylinder at 3000 ft is less full at the end of the intake stroke than it is at sea level

as a result the effective (tho not the mechanical) comp ratio falls

the same happens when the throttle is not fully open - the effective cr is much lower than the nominal cr

this is why diesels, which have no throttle, are much more efficient volumetrically under light load - the effective cr is much higher than the equivalent petrol (gasoline) engine

you could in fact safely run a much higher nominal cr ) by about 10% or a factor of 1 ratio , at your altitude without detonation, but you could not risk taking the car to a lower level unless you drove it very gently

Christopher Storey
chris

This is a really interesting question. I don't have any answers but a few comments. I agree that the answer lies in how the density of the air affects the compression. As you increase elevation, the air gets thinner, so in order to maintain the correct air/fuel ratio, the mixture has to be leaner. On the other hand, last weekend I put carb kits in my son's 70 midget, and it still uses AAC needles, which are the standard mixture. I don't think SU's are affected by altitude like a Weber or other carb. I've lived in Colorado since 1985, at about 4500 ft above sea level, and the question of mixture comes up a lot, but the question of compression never comes up. But you're right, the regular unleaded gas is 85 octane, and as you climb higher in elevation less octane is needed. Last October I drove my 74 MGB over Trail Ridge Road, which reaches 11,800 ft and never noticed any difference other than running cooler. That was with HIF4 SU's. Now I have a Weber but haven't had the chance to drive into the mountains.
What I wonder is if the engine will take in more volume of air to compensate for less density. With the SU's, the pistons might float higher (allowing more air) at elevation than the same conditions at sea level. I'm not sure if that's possible with a Weber or other carb because there are no floating pistons.
Comments, anyone?
Ken Thompson

This thread was discussed between 11/08/2002 and 13/08/2002

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