Altitude effects on carb

[MaineShark]

Realistically, if I tune my car at sea level, how high do I have to go before I'll notice a problem due to the altitude change?

Joe

 

[AKRAY4PLAY]

Joe,
around 4000 ft you will start noticing a power loss, at 6000-6500 the engine will run very rich and at 8000 you will notice a condition similar to a stuck open choke. the higher you go the less oxygen you get to the motor. air density also plays a part because the higher you go the, lower the atmospheric pressure. at sea level standard pressure is 14.7 psi, but up high it drops. if a given motor makes -10psi intake pressure, also known as vaccum, and add in the atmosphere of 15 psi, the net is 25 psi differential across the carb. now dropping the atmospheric pressure to 10 or even 5 psi and the net drops 20 or 15 psi and drastically reduces the venturi effect resulting in an exponentially worsening state of tune. the numbers are not real, but they express the idea of what is happening a little more clearly. don't forget that less oxygen plays an even bigger role in combustion characteristics, Brian

 

[Bob Chadwick]

Joe,

You are probably too young to have experienced this. When I lived in California in the early 80's I had a 73 Capri. It was a peppy little engine but when I used to take it to Lake Tahoe I would get up about 4,000 feet and notice all power go out of it and it would continue to get worse over the pass at somewhere north of 6,500 feet.

I would take the same run in my new Plymouth K-Car. It was gutless at sea level but because of the computer controls and sensors it would adjust itself as you went higher with no noticeable loss of power.

Bob

 

[Stallion]

Realistically, if I tune my car at sea level, how high do I have to go before I'll notice a problem due to the altitude change?

Joe

How would you tune your carb? And what would you tune it for? I assume the fuel/air mixture?

 

[Bob Chadwick]

As I recall, the carbs were metered differently for high altitudes. I don't hink you could adjust it out.

 

[Stallion]

Then what would the "tuning" be?? :confused

 

[Bob Chadwick]

I'm assuming he is referring to carb adjustments. There might be different gaps on the plugs, etc. but I don't know "nothing bout dat".

 

[MaineShark]

Thanks, guys. I guess that is the advantage of modenr technology. I'm going to drive up Mt Washington (~6300 ft) at some point, and I was curious how much of an effect I should expect due to the increased altitude.

How would you tune your carb? And what would you tune it for? I assume the fuel/air mixture?

Correct.

Joe

 

[Vette/Berlina-coupe]

-regarding Brian's 400+cid Small-block...

Brian: anyone wondering why i so viciously attacked another member's 454/Big-block '70-C3 (see jSimpson's: "Headers/in the way of everything"), --they need only check-out your extensively modified SB-v8 specifications (by clicking on Brian's AKRAY4Play header-name and looking down at his forum discussions); --all that muscle without BB's adverse weight-penalty! --way to go Brian...
~Bob vH

 

[sscam69]

Joe

You would have to adjust jets to compensate for the thinner air. If your just passing through don't worry about it.

Bobchad

I notice a power drop on my EFI Firebird. Check my sig it has my E.T. at 5600 ft (don't know what the density alititude was that night) and the corrected if I ran at sea level.

I lived in El Paso where the elevation was at about 4000ft and now I live in Albuquerque where its about 5200-5600ft. The track is at 5600ft according to their website.

DAMN is there a difference!!! Mind you my stock tires need to be replaced but in El Paso I can take off normaly and then gun it and the tires will break loose and I haved bounced off the rev limiter a few times because it revs so quick. In Albq. I can't do that unless I pop the clutch. And thats only at ~1500ft difference!!

Typically these LS1 F-bods dyno at 260rwhp here and 300rwhp at sea level.

 

[MaineShark]

Joe

You would have to adjust jets to compensate for the thinner air. If your just passing through don't worry about it.

Oh, believe me, I wouldn't. I was just curious how much of an effect I should expect.

I may be moving to a higher elevation in the next few years. If so, I'll definitely go to some form of EFI (even TBI, if I have to - something that can compensate for atmospheric pressure).

Actually, that's something to check into - some EFI systems only have one pressure sensor (the MAP sensor), and take a reading when the car is first turned on (before it is started), and never take another reading. If you start driving up or down a mountain, that could negate many of the advantages of the EFI. It's something I would like to check into, with any manufacturer, before considering their product.

Joe

 

[AKRAY4PLAY]

Joe,
most EFI systems utilize oxygen sensors. by doing that they change the fuel/air mixture to suit any altitude or atmospheric pressure change. some systems have a limited range of adjustment though, so check into that before you buy. Brian

 

[GerryLP]

It was gutless at sea level but because of the computer controls and sensors it would adjust itself as you went higher with no noticeable loss of power.

This is so in most late model car computers, but in the L81's, for example, one has to disconnect the battery for about five minutes, and then once reconnected, the computer should re-sample the athmospheric pressure and apply a new timing curve and adjust the mixture accordingly. Bob, you might remember how in the old days the car companies had an option for cars that would normally be operated at altitude...I think even some "computer" cars were included....:L

Stallion and MainShark,

I agree with SScam. Typically in a Holley carb, one would adjust one size smaller jet for every 2,000' of altitude. As long as the humidity remains the average (15 - 18% in the summer) I won't need to change the jets while racing.

Holley makes a carb that comes with an open or closed-loop system. The open loop accepts input from a dialer (called a quarter mile dialer) and the closed-loop uses the signal from an oxygen sensor.

Now that I have my drag project car, I plan to get my L81 back to original powerplant and enjoy the benefits from computer controlled mixture and timing.

 

[sscam69]

Now that I have my drag project car, I plan to get my L81 back to original powerplant and enjoy the benefits from computer controlled mixture and timing.

Hey you didn't mention anything about a drag car!! So when are we going to the track!?:L

Once I find employment I am going back to the track to get a better baseline E.T's before I start making changes on my T/A.

 

[GerryLP]

Oh Yeah.....I forgot to tell you that my sister-in-law found-out that I was looking for a car body to race (I was too scared of wrecking my vette at the track) and she offered me her 71 Camaro. It has a 396 or 400 engine, 400 tranny, posi rear end, and it sat since 96 in storage (open air). But the interior is not assembled and the stering column is missing (don't know why). So if it turns-out that its not a rare Camaro, then I plan to make a track racer out of it. You'll see it for sure, and it has alot of work, but its work that I have always wanted to do (like patching new skin on rusted-out panels). Alot of what it has maybe option-related, but I'll have to check the numbers. I'll probably need a good Camaro information source on the web(Hopefully a Camaro version of CAC -I love the

I spoke with my neighbor today, and he hopes to take his 10.50's Baracuda back to the track this coming weekend. At the latest on the second weekend from today. If you like I'll let you know when we'll be there and you can accompany us or meet us there. He's hoping to get down to the 9's.

 

[sscam69]

Oh Yeah.....I forgot to tell you that my sister-in-law found-out that I was looking for a car body to race (I was too scared of wrecking my vette at the track) and she offered me her 71 Camaro. It has a 396 or 400 engine, 400 tranny, posi rear end, and it sat since 96 in storage (open air). But the interior is not assembled and the stering column is missing (don't know why). So if it turns-out that its not a rare Camaro, then I plan to make a track racer out of it. You'll see it for sure, and it has alot of work, but its work that I have always wanted to do (like patching new skin on rusted-out panels). Alot of what it has maybe option-related, but I'll have to check the numbers. I'll probably need a good Camaro information source on the web(Hopefully a Camaro version of CAC -I love the

I spoke with my neighbor today, and he hopes to take his 10.50's Baracuda back to the track this coming weekend. At the latest on the second weekend from today. If you like I'll let you know when we'll be there and you can accompany us or meet us there. He's hoping to get down to the 9's.

That would be saweet Gerry!! My family ended up coming for the 4th and we went to the track Sat. Night. Got to watch jet engine funny cars and REAL dragsters. the 6 & 7 second kind!! Almost got knocked off the stands as they roared by. Also went to a car show at Hooters. We had a good time.

I have a new email frperez792000@yahoo.com. Just let me know when.

 

[motorman]

carbs are calibrated for sea level and you must reduce the AREA of the jetting 2 % for every 1500 feet you are above sea level. some Q jets have a altitude compensator built into the carb.

 

[igasper10]

carbs are calibrated for sea level and you must reduce the AREA of the jetting 2 % for every 1500 feet you are above sea level. some Q jets have a altitude compensator built into the carb.

I may be moving to colorado soon so I'll be having to change my q-jet set-up. I've heard a good rule of thumb of down-sizing you primary jets 1 size per 2000 ft of elevation. How about the primary metering rods? Also, I have the stock q-jet in my 74 Vette. Are these altitude compensators something that can be installed easily?

ian

 

[motorman]

no they can not be added to non equipted carbs. you could try a carb with it. i do not know how good these work because they only used them for a couple of years

 

[igasper10]

no they can not be added to non equipted carbs. you could try a carb with it. i do not know how good these work because they only used them for a couple of years

ok... how about my metering rods question? How should they be adjusted?