At higher altitude, the air is much less dense (thinner). Less dense air = less horsepower.

 

You take that 1/4 of whatever from sea lavel to 10000 feet above seal level, and you will have more than a quart by measured volume.

The potential compression area of a given chamber volume is affected by the actual amount of air you are can put in chamber. The air cannot get into the chamber in like speed or volume at 5280 ASL as it will at 0 ASL.

I guarantee you that you can dyno your motor in San Fran, then dyno it in Reno and you will lose hp. It is a direct function of lost air density due to gain of altitude. Your chambers, intake runners, everything mechanical about your motor is exactly the same.

FI motors actually need larger chambers to reduce potential chamber pressures to offset the increased air density provided by the boost.

Of course, I'm just an old non-degree holding gear head who has been playing with motors since 1974. So potentially i may have missed something along the way.

Very interesting stuff, to say the least!

 

The air density has nothing to do with the compression RATIO. It has to do with density. A ratio is a ratio. It's a percentage. The compression ratio is saying,
"Here's the volume when we started, and here's the volume when finished." That PERCENTAGE is the same percentage whether at sea level or on Mt. Everest. It's not like your compression stroke starts at sea level then you move the car in altitude by the top of the compression stroke. I don't know how to get it clear to you. You're confusing density with fractions. Totally different issues man. Until you understand these basic concepts, there's no way to explain it to you.

 

Which is irrelevant unless your compression stroke starts at sea level then ends in Denver. That's one fast car!


Exactly! Which means that compression will be the same! Air density may be different, but compression RATIO will remain the same.


And I have a degree and then some. But middle school or high school is where I learned that density and fractions are two different things.

 

Okay, I see what you're saying. Yes, the ratio (mechanics) stays the same, but the results do not based upon availability due to air density. So the CR is the same. I mis-stated.

What I SHOULD have said was that the actual chamber pressure is will not be the same at sea level versus 5280 feet above sea level. Thus you will have less power available.

Due to the lack of air density, increasing the CR is a constant way of making up for the lost density.

 

Well, at least you told me I am the *** I am in a nice way...better than a few of the other posters I have come across....

lol

 

LOL! I also have never heard of anyone increasing compression based upon elevation. Not that it's a bad idea, but I think you want the car drivable everywhere in most cases, so 8.5-8.8 DCR is the norm across the board. But that's a good thought. You should start a thread asking how one's compression goals would change based upon altitude. That's interesting.

 

It is something to consider (if one is reaaaaaly bored or want the most from the motor), if you know that 90% of your driving is at a certain base altitude. I plan on never living/driving my SS at near sea level again. So I am going to build heads that will bump my Cr up to something near 11:1. This will help make up for some of my lost chamber compression. The right cam will help make up the rest.

Of course, if I ever take it down to Sacramento (50 feet ASL), I will have to add fuel and pull some time to nip detonation in the bud. <sigh>

 

Some info on the discussion of compression and altitude from here: http://www.nitrophotos.com/240z/nitrous.html

"Compression Ratio" as a term sounds very descriptive. However, compression ratio by itself is like torque without RPM or tire diameter without a tread with. Compression ratio is only useful when other factors accompany it. Compression pressure is what the engine actually sees. High compression pressure increases the tendency toward detonation, while low compression pressure reduces performance and economy. Compression pressure varies in an engine every time the throttle is moved. Valve size, engine RPM, cylinder head, manifold and cam design, carburetor size, altitude, fuel, engine and air temperature and compression ratio all combine to determine compression pressure. Supercharging and turbo-charging can drastically alter compression pressures.

The goal of most performance engine designs is to utilize the highest possible compression pressure without causing detonation or a detonation related failure. A full understanding of the interrelationship between compression ratio, compression pressure, and detonation is essential if engine performance is to be optimized. Understanding compression pressure is especially important to the engine builder that builds to a rule book that specifies a fixed compression ratio. The rule book engine may be restricted to a 9:1 ratio but is usually not restricted to a specific compression pressure. Optimized air flow and cam timing can make a 9:1 ratio but is usually not restricted to a specific compression pressure. Optimized air flow and cam timing can make a 9:1 engine act like a 10:1 engine. Restrictor plate or limited size carburetor engines can often run compression ratios impractical for unlimited engines. A 15:1 engine breathing through a restrictor plate may see less compression pressure than an 11:1 unrestricted engine. The restrictor plate reduces the air to the cylinder and limits the compression pressure and lowers the octane requirements of the engine significantly.

At one time compression pressure above a true 8:1 was considered impractical. The heat of compression, plus residual cylinder head and piston heat, initiated detonation when 8:1 was exceeded. Some of the 60's 11:1 factory compression ratio engines were 11:1 in ratio but only 8:1 in compression pressure. The pressure was reduced by closing the intake valve late. The late closing, long duration intake caused the engine to back pump the air/fuel mix into the intake manifold at speeds below 4500 RPM. The long intake duration prevented excess compression up to 4500 RPM and improved high RPM operation. Above 4500 RPM detonation was not a serious problem because the air/fuel mix entering the cylinder was in a high state of activity and the high RPM limited cylinder pressure due to the short time available for cylinder filling.