[Firebob]

So I'm going from sea level to around 5000 ft for a week long show in August and last year I noticed the car ran like a slug when I got there. I have the carb tuned in pretty well for sea level now.
I'm wondering what kind of changes I need to make to make it run better at the increased altitude? Is it possible to make it run as well there as it does here?

[Tom Vaught]

If you lean it out a bunch it will run "ok".
Problem is the density of air 5000 feet is 17% less oxygen (vs at sea level).
So you would need to lean out the air/fuel mixture by 17% in order to make the engine run even close to the right air/fuel mixture just cruising.
It would be even worse under hard accelerations.

I went to Denver a few times with a sea level calibration and as I climbed in altitude I removed a vacuum cap on a vacuum fitting "tree" I had hooked up to the intake manifold. Think I had a "vacuum tree" with 6 nipples and caps.

So I would drive with the sea level calibration until it was a bit doggy rich then remove a vacuum cap. Drive again for a while and then remove another vacuum cap. Not great by any means but the engine ran OK (even at 7400 feet of altitude) with all of the vacuum caps removed. Ate Dinner at "The Fort" which was higher up vs Denver.

Other than that deal if you messed with the mixture screws/ the jets, the power valves, etc you could do the same thing as you went up in altitude, Leaning out the final mixture was just easier for me and a trial and error deal. Quick to add less air on the way down too.

Tom V.

If you had more time you could set up a second carb that was dialed in by a member who supplied his Denver calibration.
Still would be a one step swap vs the multi-step function with the air bleed system.

["QUICK-SILVER"]

For cruising: I'ld be tempted to drill and tap the APT plug. Then add a screw to limit 'how high' the front metering rods can raise up.

Idle: Mixture screws should handle that well enough.

WOT: Swap out to fatter secondary metering rods.

A few minutes under the hood with a screw driver would take care of all of it.

Just a thought
Clay

[Firebob]

I do have an extra Holley 750 sitting around and I have a couple of months to figure out the throttle linkage when I swapped over. Hmm. I have never had a Holley before so I'm not sure where I would start with it. Something to think about.

[TheSilverBuick]

Like Tom Vaught says, you won't gain 100% back, but odds are it's running rich, so leaning it out a jet size or two will probably get you some of that back. Unless you are comfortable changing jets regularly I'd probably leave it alone. Maybe advance the timing a few degrees because it's easier to do.

[johnta1]

The density of the air is what will kill your HP.
No way can you retrieve that N.A.
(blower, yes to an extent)

Try this calc:

Jetting calc

Probably need this one 1st though:

ADI Calc

This uses absolute pressure, not gauge.
If your barometric pressure at 5000' is around 29.97 in/Hg it's wrong.

[Tom Vaught]

using John Wallace's calculator

26.5 Hg - 78 degrees F - 35% Humidity gets you a calculated altitude of 5283 feet
close enough the the "Mile High City's" average altitude.

It says you need to lean out all of the systems of the carb by 17 percent.
http://www.wallaceracing.com/adi.php

Funny, my post number 2 said exactly the same thing so you can take the 17% deal to the bank. So that means all of the Holley circuits would have to be 17% leaner.

You not being a Holley Engineer, unless you had background in that carb and a GOOD Air/Fuel meter you probably do not have time to calibration much.

Like I posted, sometimes "simple" works a lot better vs overthinking the deal and just gets you confused. But an air/fuel meter and a vacuum leak might get you real close very easily.

Carbs work off of Physics and Density Calculations, They do not care whose name is on them. I did High Altitude Calibrations for my Company (at Denver) several times so I have some actual knowledge on this deal. Maybe you can find someone else with more than a calculator program who has done the same work.

Tom V.

[Formulajones]

I deal with this on a daily basis. I live at 5,000 feet, and drive to sea level and even higher up above 7,000 feet. I can go any direction 15 miles from the house and the elevation changes 1,000 feet or more. Things I've found....

Roughly every 1,000 feet up will suck about 3% of power out of the engine (general slide rule here) So going up to 5,000 feet from sea level will hurt HP about 15%, very similar to the percentages that Tom posted. You can make these percentage changes in the carb circuits to get the AFR in line, but you don't get your HP back, not all of it anyway. But you will get a much crisper running engine at altitude.

I've also found that if I tune for sea level using the wideband, the car will go rich about 1.2 to 1.4 on the AFR using the wideband when I climb to 5,000+ feet. So yeah it can run fairly fat if you're already tuning it safe and on the rich side at sea level.

I've also found the Holleys with replaceable air bleeds are a very nice feature for instances like this. I can swap the high speed air bleeds in a couple minutes. Take out the 32's at sea level and swap in 38's at 5,000 feet (adds more air to the main jet circuit) and the AFR is pretty darn good up here. I keep them in the glove box. If I plan to run down to sea level, it will run good enough until I make a pit stop, I'll swap them out in a couple minutes.

Same deal can be done with the Q-jet if you have an APT adjustment like Clay mentioned.

I've also found that I've gotten a bit lazier, lol. So what I've done, knowing what the change is in AFR from 5,000 feet down to sea level with my wide band, is I run them a little rich up here at 5,000 feet and just live with it. Not overly rich, just 3/4 of a point on the AFR. I'll jump up a heat range on the plugs so they stay clean. Then when I drive down to sea level, it's only about .5 or .6 lean on the AFR gauge, which isn't enough to hurt a thing. So I compromise and just leave the hood shut

It is pretty eye opening though, when you drive around at 5,000 feet on a daily basis and on the weekend buzz down to Phoenix near sea level and suddenly the car is like you've just flipped a light switch and it's gotten a real kick in the pants. You realize just how well the car runs. Roughly a 15% increase in HP can be huge even if you're talking about a mild 400 hp engine, you're gaining roughly 60 HP. You can definitely feel it.

[Tom Vaught]

Great Post, Thanks Formulajones.

.028" is pretty typical on a Holley High Speed Bleeds so you are already a few numbers leaner vs the stock calibration with your .032" sea level bleeds.

Going to .038" air bleeds on the high speed circuits seem to work for you.

Not really that difficult to remove the press-in High Speed Air Bleeds and drill/tap the holes for the screw in air bleeds. As you said the newer Holleys come with that feature already: Adjustable Air Bleeds.

So if Firebob is attending very many higher altitude shows might be worth the time to mod the Holley carb for the easy to change air bleeds.

So did you leave the Primary Idle Air Bleeds alone or just mess with the air/fuel mixture screws?

Tom V.

[Formulajones]

Yes I leave the idle air bleeds alone and find that as long as the mixture screws are responsive enough, I can simply adjust the idle mixture screws to make it happy.


Yep great suggestion to remove the fixed air bleeds and tap for removable air bleed jets.

[Formulajones]

On the subject, I've been playing with our 69 Z/28 with this very scenario. It still runs it's original 4053 Holley. Since you worked for Holley Tom you may be very familiar with this. I believe the factory calibrates the original carbs to a sea level calibration. Kind of funny but after a lot of tinkering and off the car multiple times I've settled on pretty much the factory calibration

68 front jet
6.5 power valve
76 rear jet
6.5 power valve
I only swayed from the factory calibration with the pump circuit, running a 28 squirter (stock is 25) and a different pump cam.

Being an original carb dated to the car (used one will fetch $1500+) I'm reluctant to do any major mods that are difficult to reverse, so no drilling has been done. The idle circuit is plenty fat so it has a lot of adjustability.

I've found it runs pretty darn good as is up here at 5,000 feet, it's a little fat but not bad. I just simply stepped up a heat range on plugs and it stays clean, and I drive it down near sea level without worry.

Just shows that back in the day, the Holley techs and GM knew what they were doing. Almost 50 years later and I still really don't need to change much.

[Tom Vaught]

I have a book with a lot of the factory calibrations listed one at a time in it.
It took me a long time to put it together.
Holley did not deviate much with their Base Orifices:
Emulsion Holes were typically .028" on both holes in each Emulsion Circuit.
Kill Bleeds for the same circuit stayed the same size on many carbs.
High Speed Air Bleeds stayed the same size on many carbs.
The blade diameter was basically the same for all of the 650 to 800 cfm carbs.
It was 1-11/16" diameter.

Only the old 4781 (850 carbs) got the 1-3.4" Throttle Blades.

The Carb Venturis (changed with the different carbs) which changed the Delta P across the Main Circuit Jets and Emulsion Circuits.
Lots of different Carb Venturi Sizes.
Metering Blocks changed as far as Single Squirter, Dual Squirter, Center Squirter, Jetting, and Two Corner or 4 Corner Idle capability.

So when you have all of those variables, without a quality Air/Fuel meter you would be in the dark. We had Horiba Portable Exhaust Gas Analyzers back in the early 70s that were used on all of the factory calibrations which is one reason why your factory calibration on your Z-28 carb is so good. Years and years later lots of enthusiasts have similar devices. Our Stuff was over $10,000 per unit and that was in 1970s dollars.

Tom V.

[Lee]

I live at sea level, and had a friend that lived in Denver that I talked with quite a bit about 15 or so years ago. We both drag raced quite often. When talking about new cars we would each see at the track, normally aspirated cars that ran about 14.0 here were running about 15.0 up there.

A good turbo system will pretty much negate the power loss, but N/A cars are going to lose a significant amount of power. EFI can help, otherwise you'll be stopping and changing jets and air bleeds all the time if you want to minimize losses!

[Blued and Painted]

The engine will tolerate a little more timing at altitude. Two or three degrees.
I would not be monkeying with the carb before or during a trip.
Your only talking one or maybe two jet size leaner.
You may learn more on the trip leaving things the way they are.

[Firebob]

Yeah I was really surprised last year when I pulled the on ramp to get on the frewway and the car just didn't respond like it normally did. It pretty much ran ok for most of the time there but I could really feel the difference. I got back home at the end of the week and laid into it a couple of streets away from the house and I was like, oh hello, youre back baby! Then I was thinking, I wonder how those guys that drag race there do it? Mine certainly isn't going to impress anyone running the way it does there.

[carbking]

If you are going to the higher altitude to race, then reread the really good comments in this thread.

If you are going to the higher altitude just to visit and drive around a little, AND you are currently running real gasoline; then fill up with E-10 at around 4000 feet. There is enough less energy in the E-10 to make a sea level calibration on real gas fairly close at 5000 feet.

Once you get back below 4000 feet, run the tank almost dry before filling up with real gasoline.

And you ARE going to feel the difference. Physics cannot be ignored, although there are many that try!

Tom V.'s "controlled air leak method" is another relatively easy way to save your spark plugs.

Jon

[Tom Vaught]

Originally Posted by Blued and Painted

The engine will tolerate a little more timing at altitude. Two or three degrees.
I would not be monkeying with the carb before or during a trip.
Your only talking one or maybe two jet size leaner.
You may learn more on the trip leaving things the way they are.

Help me out on that deal, B&P, typical difference in flow between Holley Jets per size is about 3 percent. Formulajones and I, (as well as the Wallace calculator), have stated that the air/fuel ratio would have to be 15%-18% leaner. So how is changing ONE jet size going to give you a 18% change in Air/Fuel Ratio? Always interested in learning something new about carbs.

Tom V.

Jon Hargrove's method would also work as you are changing the chemical compound of the fuel mixture.

[Formulajones]

Originally Posted by Lee

I live at sea level, and had a friend that lived in Denver that I talked with quite a bit about 15 or so years ago. We both drag raced quite often. When talking about new cars we would each see at the track, normally aspirated cars that ran about 14.0 here were running about 15.0 up there.

A good turbo system will pretty much negate the power loss, but N/A cars are going to lose a significant amount of power. EFI can help, otherwise you'll be stopping and changing jets and air bleeds all the time if you want to minimize losses!

Yep, when you're a mile up, a second lost in ET is pretty typical on average. It's definitely a change you can feel when you drive the car down to sea level.
Sometimes I forget about it, then when I'm down in Phoenix I'll jump on the gas at an on ramp getting on the highway and it's like a different car.

[Formulajones]

Another little piece of information that changes is vacuum. Engines will make less vacuum up here. I drop about 3-4 inches of vacuum when I'm at 5,000 feet.
Cars that run vacuum advance will see slight changes, making the vacuum can more sensitive or less sensitive, which can also affect how the engine feels at part throttle.

If you have it hooked to a manifold vacuum source, your idle changes with elevation. As you're increasing vacuum driving down in elevation, the idle speed will go up. Typically I've seen 200-300 rpm changes. Not a big deal but definitely noticeable. Hooked to a ported source and the idle is stable at any elevation, but the vacuum can will be less sensitive while up in elevation so it typically adds less timing at light throttle cruise conditions.

Those that run a 1970 Q-jet that doesn't have a ported source (like I do) have no choice but to deal with idle changes with elevation changes. Just something I've come to accept on that particular car, and have gotten used to. To compensate for that on this particular car, and to keep from popping the hood all the time, I find a happy spot with idle. I have it set fairly low at elevation up here, idling about 600 in gear, and pops up to about 800 out of gear. When I drive down to sea level it idles more like 800 in gear and 1,000 out of gear, which is acceptable enough to not worry about changing it all the time.

[Tom Vaught]

Originally Posted by Formulajones

Another little piece of information that changes is vacuum. Engines will make less vacuum up here. I drop about 3-4 inches of vacuum when I'm at 5,000 feet.

29.92 Hg is Sea Level Pressure in Inches of Mercury.
26.5 Hg is Denver Pressure in Inches of Mercury.
So your 3-4 inch lowered vacuum at altitude vs sea level is spot on as far as the physics goes, as it should be. Great Info Formulajones.

Tom V.