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I've read quite a few older threads where someone describes their engine build and asks advice towards picking the right head gasket and while I do grasp the idea of keeping the quench to a relatively low number, in the case of the 400 SBC I'm building, it appears that the compression is going to be a bit higher than I expected. I believe that I 'should' be alright due to living at the 5500 foot elevation I should be okay but I'd like to hear what you guys have to say about this juggling act.

This is first paragraph from the original post asking about compression ratio and the effect of altitude or more correctly density altitude. I realize that the engine is an - in progress - build and many of the postings are about various components. I feel that the discussion about altitude and CR has been lost. So I would like to get back to CR and its effect on altitude.

With standard atmosphere you lose about 1" of mercury (HG) at WOT per every 1,000' of altitude up to about 10,000'. Starting with almost 30" HG at sea level, when at 5,500' you'll be down to less than 25" HG. Turbo or supercharging will compensate for this and many systems will allow sea level power to be available at close to 20,000'. At 5,500 feet your un-supercharged or un-turbocharged engine will suffer from reduced performance as compared to sea-level operations - with all things being equal. If your car is always going to be operated at this altitude you can run a higher CR than you could at sea level and leaner air/fuel mixture and improve its performance.

I'm not an aeronautical engineer but as an airplane driver for 50+ years this is well known... Al
 

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Discussion Starter #62
Has anyone actually figured out approximately how much horsepower is lost due to altitude? I'd heard numbers kicked about in the range of say 10-15% when talking about sea level versus our altitude of 5500 feet. I know it 'feels' like that much is lost but wondered if anyone has more definitive data on it.
Mike T - Prescott AZ
 

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It's directly proportional to the pressure ratio. So if the barometer at 5500' elevation is 25" mercury instead of the sea level's approximate 30", then power at 5500' elevation is the power at sea level times 25/30, about 17% less power. A 400 hp engine would be 333 hp.
 

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Has anyone actually figured out approximately how much horsepower is lost due to altitude? I'd heard numbers kicked about in the range of say 10-15% when talking about sea level versus our altitude of 5500 feet. I know it 'feels' like that much is lost but wondered if anyone has more definitive data on it.
Mike T - Prescott AZ
The general rule of thumb is 3% loss per 1000 feet in altitude. So at 5500 feet you're looking at 15%-17% down (ish). There are probably too many variables (barometer, relative humidity, temperature, for example) to come to a definitive answer.

To complicate matters more barometric pressure doesn't necessarily increase or decrease just because of altitude alone (although it generally does, weather affects it). Right now, where I live at 7200ft, the barometer is at 29.76. That doesn't mean my engine has the same power as it would at sea level though. Go figure.
 

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Discussion Starter #65
I checked our local Barometric Pressure for Prescott AZ and found it to be 29.92, almost the same as Los Angeles. Time for celebration, right? Uh, not so much. Too bad it isn't that simple, huh?
Mike T - Prescott AZ
 

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hutchenc and Vet65te, the barometric pressure you see on TV weather, etc. is corrected to what it would be at sea level (from your elevation). It is also corrected for humidity. As alhewitt posted, you lose about 1" Hg per 1000' of elevation.

When the weather is bright and sunny, the sea level barometer will be 29.80"-30.50". When it's stormy, the sea level barometer will dip to 29.60"-29.80", even lower in a big thunderstorm or especially in a hurricane.

At 5500', the raw barometer should be about 24.42"+/-, and at 7200' about 22.72".

Standard horsepower corrections on a dyno are made to correct for barometric pressure and temperature. These are widely accepted. Most race shops with dynos correct to "standard temperature and pressure", which is 29.92" Hg and 60°F. Automakers correct to SAE conditions which is 29.50" Hg and 25°C (at least I think that's the temperature they use). You can also correct for vapor pressure (humidity) but most don't.
 

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I would be surprised to ever see the density altitude below 6000' on any given day in Prescott. Physical altitude is one thing but density altitude is what is important here. and it is higher than the physical altitude 99% of the time. for example at Famoso raceway the physical altitude is 635msl but on my weather station in May in really nice 70 degree weather the density altitude was 1800' according to my weather station and several others I compare with.
 

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Discussion Starter #68
It's been a while since I heard that term 'Density Altitude'. When I lived in Northern California, about 40 miles from Sears Point Raceway (locals still commonly referred to it as Sears Point versus the later names of Infineon or Sonoma Raceway), I had friends who were serious racers with personal shops out at the track and they all seemed to keep accurate records of their runs along with background info like temperature, wind speed and 'Density Altitude'. They all relied on their portable weather devices for the data but I never did quite understand how Density Altitude was determined. Is there a way to figure this out without having a device that actually specifies DA?
Mike T - Prescott AZ
 

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It's a calculation based on barometer, temperature and humidity. Same one described on the dyno correction factor, just with the results expressed in terms of relative altitude, which is what the pilots use.

If you ever watch the NHRA race coverage on FS1, they usually mention density altitude sometime during a broadcast, usually near the beginning of the show so that you can see if the track will be relatively fast or not. Since those races are usually in the warm months, density altitude is almost always higher than the local elevation as hotrodg726 said. This is due to temperature for the most part. In coastal areas or the summer Midwest it is also humidity.
 

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hutchenc and Vet65te, the barometric pressure you see on TV weather, etc. is corrected to what it would be at sea level (from your elevation). It is also corrected for humidity. As alhewitt posted, you lose about 1" Hg per 1000' of elevation.

When the weather is bright and sunny, the sea level barometer will be 29.80"-30.50". When it's stormy, the sea level barometer will dip to 29.60"-29.80", even lower in a big thunderstorm or especially in a hurricane.

At 5500', the raw barometer should be about 24.42"+/-, and at 7200' about 22.72".

Standard horsepower corrections on a dyno are made to correct for barometric pressure and temperature. These are widely accepted. Most race shops with dynos correct to "standard temperature and pressure", which is 29.92" Hg and 60°F. Automakers correct to SAE conditions which is 29.50" Hg and 25°C (at least I think that's the temperature they use). You can also correct for vapor pressure (humidity) but most don't.
Thanks for that Rick...I didn't actually know that. Explains a lot.

Looked up the baro pressure at my altitude. 22.91 inches. :rolleye0009:
 

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Aviation over the years went through many growing pains and altimetry is just one of them. A lot of people paid the ultimate price in the established usage of barometric pressure and how it is used. Someone commented that at their home of 6500 feet of elevation the reported pressure was 29" HG or thereabouts. What is done in aviation today is the reported barometric pressure has been corrected for the altitude that it is taken at. The actual barometric pressure at 6500 feet is probably close to 23" of HG. In the early days of aviation pilots used this 23" HG reported pressure and set the altimeter accordingly. With this procedure the altimeter would then read zero when he was at 6500 feet. Many accidents occurred with this procedure and eventually aviation adopted the corrected from sea level reading where the hands of the altimeter read the altitude about sea level. Unless it is a day when the barometric pressure is very high you can expect your performance to deteriorate as you go up... Al
 

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The standard sea level pressure/temperature is 29.92 in. (1,013.25 mb) and 59°F (15°C) and the average lapse rate lies between the dry adiabatic and the moist adiabatic at about 3.3°F per 1,000 feet.

So at 6,500 feet the standard temp would be roughly 38 degrees. If the temp is above that, your motor will think it is higher than the actual altitude. Yes, there are minor things that also affect density altitude, but temp is the main factor.

That's what a supercharger, blower, or turbo charger takes care of. If gives you sea level power at higher altitudes. A normalized turbo (airplanes) gives you roughly the 30 inches of sea level and has a waste gate to release pressure over that. An engine not built for over that pressure can't take the higher pressure. An engine built for higher pressure can take a much higher pressure than 30 inches.

Turbo charged engines loose power as altitude goes up as well but they are starting with more pressure than a normally aspirated engine.
 

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Mike, I'm kinda surprised about your part choises for your build, if you are worried about vacuum you would have choose a cam with alot wider lobe and then you could run alot more timing to it. I understsnd that the 'sound' is one factor but there is more to it.

Your drivetrain is as generally prepped for rpm happy engine and ultimate low end torque isn't actually needed for your combo, you are building alot bigger engine than 327 after all. Just trying to say that all the effort building 400 sbc with roller cam and aluheads is half way wasted. Just like choking the cubes with performer intake, small runner heads and small carb

Don't get me wrong, I'm not bashing your build anyway but just my humble opinion. Good luck for your build! :tu
 
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