High(er) Compression Because of Altitude - Page 7 - TriFive.com, 1955 Chevy 1956 chevy 1957 Chevy Forum , Talk about your 55 chevy 56 chevy 57 chevy - Belair , 210, 150 sedans , Nomads and Trucks, Research, Free Tech Advice

Click here to see our full list of sponsors for all your Parts and Restoration Needs

Click here to find Out all of the Latest information on the Danchuk Tri-five Nationals


Go Back   TriFive.com, 1955 Chevy 1956 chevy 1957 Chevy Forum , Talk about your 55 chevy 56 chevy 57 chevy - Belair , 210, 150 sedans , Nomads and Trucks, Research, Free Tech Advice > Hi Performance & Racing > Engine Building, Hi Performance
User Name
Password
Register FAQ Top Posters Mobile Members Search Today's Posts Mark Forums Read
HomePage Todays Post Mark Forums Read upcoming Events
Newsletter 55-56-57 Chevy Parts & Services Official Sponsors
Photo Gallery Garage Photos Trifive Classifieds Trifive on Facebook Calendar Tech Articles Arcade Oldies
Trifive Cards Car Certificate Trifive Brochure Help with Photos Website Help Videos Contact Us

Engine Building, Hi Performance Please tell us about your engine builds. What component combos and horsepower output. Let's help each other build better faster engines. Ask or answer questions.

Parts on Ebay            
Reply
 
Thread Tools Display Modes
Old 06-05-2019, 02:23 PM   #61
alhewitt
Senior Member 1 Blue Star



 
alhewitt's Avatar
 

Join Date: Jun 2015
Location: Federal Way, WA
Posts: 265
Member #37639
Default

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
alhewitt is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 06-05-2019, 04:17 PM   #62
Vet65te
Senior Member 1 Blue Star



 
Vet65te's Avatar
 

Join Date: Feb 2010
Location: Prescott, Arizona
Posts: 358
Member #12099
Default

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
Vet65te is offline   Reply With Quote
Old 06-05-2019, 06:02 PM   #63
Rick_L

Trifive Leaders Club




 

Join Date: Mar 2007
Location: Katy, TX
Posts: 33,011
Member #2256
Default

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.
Rick_L is online now   Reply With Quote
Old 06-05-2019, 07:33 PM   #64
hutchenc
Senior Member 2 Gold Stars



 

Join Date: Sep 2007
Location: Laramie, WY
Posts: 2,326
Member #3786
Default

Quote:
Originally Posted by Vet65te View Post
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.

Last edited by hutchenc; 06-05-2019 at 07:35 PM..
hutchenc is offline   Reply With Quote
Old 06-05-2019, 07:52 PM   #65
Vet65te
Senior Member 1 Blue Star



 
Vet65te's Avatar
 

Join Date: Feb 2010
Location: Prescott, Arizona
Posts: 358
Member #12099
Default

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
Vet65te is offline   Reply With Quote
Old 06-05-2019, 08:35 PM   #66
Rick_L

Trifive Leaders Club




 

Join Date: Mar 2007
Location: Katy, TX
Posts: 33,011
Member #2256
Default

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 60F. Automakers correct to SAE conditions which is 29.50" Hg and 25C (at least I think that's the temperature they use). You can also correct for vapor pressure (humidity) but most don't.
Rick_L is online now   Reply With Quote
Old 06-05-2019, 09:07 PM   #67
hotrodg726

Trifive Leaders Club


hotrodg726 has made a donation to the forum!

 
hotrodg726's Avatar
 

Join Date: Feb 2010
Location: Tehachapi, California
Posts: 11,566
Member #12165
Default

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.
__________________
Thanks

hotrodg726 is online now   Reply With Quote
Old 06-06-2019, 02:06 PM   #68
Vet65te
Senior Member 1 Blue Star



 
Vet65te's Avatar
 

Join Date: Feb 2010
Location: Prescott, Arizona
Posts: 358
Member #12099
Default

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
Vet65te is offline   Reply With Quote
Old 06-06-2019, 03:37 PM   #69
Rick_L

Trifive Leaders Club




 

Join Date: Mar 2007
Location: Katy, TX
Posts: 33,011
Member #2256
Default

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.
Rick_L is online now   Reply With Quote
Old 06-08-2019, 01:39 AM   #70
hutchenc
Senior Member 2 Gold Stars



 

Join Date: Sep 2007
Location: Laramie, WY
Posts: 2,326
Member #3786
Default

Quote:
Originally Posted by Rick_L View Post
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 60F. Automakers correct to SAE conditions which is 29.50" Hg and 25C (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.
hutchenc is offline   Reply With Quote
Sponsored Links
Advertisement
 
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
TBI recommendations for high altitude application jppohlen Fuel Systems, Fuel Injection, Carburation, Supercharging 16 03-04-2014 08:42 PM
High Altitude Carburation michaelfgoss Fuel Systems, Fuel Injection, Carburation, Supercharging 51 11-20-2012 10:23 PM
Pikes peak run with altitude and speed displayed joninomader R.I.P. 3-19-2017 Videos from You Tube, Bing, etc. 4 06-09-2011 11:04 PM
Altitude and spark plug problems fishGuy03 Stock Chevy Discussion 55-56-57 10 06-24-2008 10:51 AM
Betsy has a BAD attitude and a new altitude johnny99 Trifive Steering , Power and manual- Rack & Pinion - 605 16 04-25-2008 01:15 AM


All times are GMT -4. The time now is 01:10 PM.

Click here to see our full list of sponsors for all your Parts and Restoration Needs

Websites
www.tri-5.com www.tri-fives.com www.trifive.com

Powered by vBulletin
Copyright 2000-2009 Jelsoft Enterprises Limited.
1957 CHEVROLET BEL AIR Information, 1957 CHEVROLET BEL AIR Specifications, 1957 CHEVROLET BEL AIR Resources, 1957 CHEVROLET BEL AIR Pictures, 1957 CHEVROLET 55 chevy 56 chevy 57 chevy 1955 55 chevy pictures 55 chevy parts 55 chevy truck 55 chevy pickup 55 chevy nomad 55 chevy gasser 55 chevy pro street 56 chevy pictures 56 chevy parts 56 chevy truck 56 chevy pickup 56 chevy nomad 56 chevy gasser 56 chevy pro street 57 chevy pictures 57 chevy parts 57 chevy truck 57 chevy pickup 57 chevy nomad 57 chevy gasser 57 chevy pro street 55 chevy photos 55 chevy parts 55 chevy truck 55 chevy pickup 55 chevy nomad 55 chevy gasser 55 chevy pro street 56 chevy photos 56 chevy parts 56 chevy truck 56 chevy pickup 56 chevy nomad 56 chevy gasser 56 chevy pro street 57 chevy photos 57 chevy parts 57 chevy truck 57 chevy pickup 57 chevy nomad 57 chevy gasser 57 chevy pro street 55 chevy 56 chevy 57 chevy 1955 Chevy 1956 Chevy 1957 Chevy Bel Air 210 150 and Nomad belair Tri-5 Tri-five Tri5 55 chevy 56 chevy 57 chevy 1955 Chevy 1956 Chevy 1957 Chevy Bel Air 210 150 and Nomad Tri-5 Tri-five belair Tri5 55 chevy 56 chevy 57 chevy 1955 Chevy 1956 Chevy 1957 Chevy Bel Air 210 150 and Nomad Tri-5 Tri-five Tri5 55 chevy 56 chevy 57 chevy 1955 Chevy 1956 Chevy 1957 Chevy Bel Air 210 150 and Nomad Tri-5 Tri-five Tri5 55 chevy 56 belair chevy 57 chevy 1955 Chevy 1956 Chevy 1957 Chevy Bel Air 210 150 and Nomad Tri-5 Tri-five Tri5 55 chevy 56 chevy belair 57 chevy 1955 Chevy belair 1956 Chevybelair 1957 Chevy Bel Air 210 150 and Nomad Tri-5 Tri-five Tri5 55 chevy 56 chevy belair 57 chevy 1955 Chevy 1956 Chevy 1957 Chevy Bel Air 210 150 and Nomad Tri-5 Tri-five Tri5 55 chevy 56 chevy 57 chevy 1955 Chevy 1956 Chevy 1957 Chevy Bel Air 210 150 and Nomad Tri-5 Tri-five Tri5 55 chevy 56 chevy 57 chevy 1955 Chevy 1956 Chevy 1957 Chevy Bel Air 210 150 and Nomad Tri-5 Tri-five Tri5 55 chevy 56 chevy 57 chevy 1955 Chevy 1956 Chevy 1957 Chevy Bel Air 210 150 and Nomad Tri-5 Tri-five belair Tri5 55 chevy 56 chevy 57 chevy 1955 Chevy 1956 Chevy 1957 Chevy Bel Air 210 150 and Nomad Tri-5 Tri-five Tri5 55 chevy 56 chevy 57 chevy 1955 Chevy belair 1956 Chevy 1957 Chevy Bel Air 210 150 and Nomad Tri-5 Tri-five Tri5 55 chevy 56 chevy 57 chevy 1955 Chevy belair 1956 Chevy 1957 Chevy Bel Air 210 150 and Nomad Tri-5 Tri-five Tri5 55 chevy 56 chevy 57 chevy 1955 Chevy 1956 Chevy 1957 Chevy Bel Air 210 150 and Nomad Tri-5 Tri-five 55chevy 55 chevy pictures 55 chevy parts 55 chevy truck 55 chevy pickup 57 chevy 55 chevy nomad 55 chevy gasser 55 chevy pro street Searches related to 57chevy 57 chevy pictures 57 chevy 57 chevy truck 57 chevy parts 57 chevy pickup 57 chevy nomad 1957 chevy Tri5