GASGZLR

I live at almost 6000 feet and I noticed that when I go down to 3000 feet my car pulls better at low speeds and has more vacuum. Up here when I push down the gas my vacuum seems to right away drop from about 15 hg to 10 before I get ANY power and not just bog. At the lower altitude with the exact same tuning I gently press the gas around town the same and it pulls hard and the vac only drops from like 16 to 13 and I get good power and NO bog. My carb is tuned all the way lean for the cruise mode and the A/F is just right on my gauge, so I thought that the vacuum advance on the distributor might help if I adjust it. Any suggestions on how much to increase it? I can put all 23 degrees of vac advance in and still get no ping at all ever. Should I bump the initial timing? I can also put it to almost anything with no ping. Someone said to bump the initial untill the motor fights the starter then back it off a hair.

Timing now is:
16 initial
36 total by 3000
vac advance adds 20 with 15 hg

BTW the motor was noticably louder@ 3000 feet.

five7kid

iTrader: (14)

Bad news: You can't change the laws of physics.

Give it as much timing as it will take without starter kickback or ping. Do not use higher octane fuel to reduce ping - it doesn't help (octane is primarily compression related - if it pings because of the high-altitude timing you've put in it when using the same octane that you use at 3000 feet with "normal" timing, you've got too much timing in it).

What we call "vacuum" is really the difference between the atmospheric pressure and the pressure in the intake manifold. When the atmospheric pressure is lower, the difference will be less. Plus, when the atmospheric pressure is lower, the air density is lower, so the throttle has to be opened more to get the same mass of air into the engine. And, when the throttle is opened more, the pressure in the intake manifold goes down...

It's a vicous cycle, and about all you can do is accept it.


(FWIW, if you have solid lifters, you can change the lash setting on the exhaust to "fool" the intake a little. But, it's only a minor effect.)

ronterry

Also remember your not going to have the same HP/Torque up here.
Breaking out my good ol auto math book, and it states;

bhp loss = elevation/1000 * .03 * bhp @ sea level.

So basicly a loss of 3% per thousand feet.

So at my altitude (6300), my Vette only produces about 195HP @ the rear wheels ...and guess what that's exactly what my G-Tech Pro Comp shows...
300 (rated hp @ sealevel) - 56.7 bhp loss = 243.3 - 48.7( 20% drive train loss) = 194.6

Ron

five7kid

iTrader: (14)

I found the NHRA elevation factors to be dead-nuts on. For 5800', ET times .9276 = sea level ET, and 1/4 MPH times 1.0773 = sea level MPH.

With jetting adjustments, of course (I set the timing for total mechanical, which for WOT purposes, doesn't change with altitude).

GASGZLR

Nicely put "professor" five7. That's what I wanted to know, what a bummer huh? I am just going to leave it where it is. I just tried moving the timing AND the accelerator pump shot and NO results same thing. I never noticed the bog until I got a taste of low elevation. :hail:

Morley

One other thing, carbe were never good at high altitudes. Remember the "old" Pikes Peek callenges? Back before FI a full 1/3 of the cars entered in it had crapped out before reaching the top...carbs can't cope with extreme altitudes the way FI can.

owned

I completely disagree that your timing curve is not altitude dependent. As you are starting off with lower air pressure, in the end, you get lower cylinder pressures as well. My car has 355 with north of 10:1 compression at 6300 feet, and it runs with zero knock counts at 87 octane, because the cylinder pressures are so much lower.

Lower pressure in the cylinder makes for a slower flame front, meaning that you need to start burning earlier. I'm not sure how useful this is to you with a carbed car, but I've found that my TPI car likes to run best at around 11-12* initial timing. This is at 6200 feet in Colorado Springs. And yes, it runs that with no knock.

I know guys running iron head cars up here with 11:1 that do just fine on 91 octane.

owned

Wrong. Nothing about a carb really limits it as far as alitiude. As long as there is a pressure differential to pull gas out, a carb can be tuned to work.

The issue with the pikes peak runs is that the race starts at 7000 feet, and climbs a little less than a mile and a half straight up. At the top, you are dealing with around 20% less air pressure (I don't remember the exact figure. Thus, the tune thats optimal for the bottom changes drastically, and on a full out race car, that can have disastrous effects. You are either lean at the bottom, or rich at the top. A carb just can't tell this because it's only metering of off that pressure differential.

Hoowever, it's not like a carbed car tuned for sea level won't run at 10000+ feet, it just won' run as well. I have an old subaru wagon with a fair amount of 4x4 mods (don't laugh, they are suprisingly capable with the old dual range 4x4 setups) and taking it from 6000 to 14000 feet doesn't really hamper it's driveability (no stutters, bogs, etc). It just doesn't have **** for power (most due to altitude anyways) and will black smoke a bit at the REALLY high elevations. Buts its been way above 13000 feet and still kept on chugging just fine. (See pic)

five7kid

iTrader: (14)

I run more base timing on my street cars. The general rule is one degree per 1000' above sea level. My Camaro is currently at 10 base (LG4 sticker says zero, L69's say 6 degrees).

For the "race car", I adjust for total mechanical timing and pretty much ignore base or initial timing level. The group I hang around with at the track do the same, settling on what produces the best results. We typically run the same total here that we do at sea level.