High altitude power improvement?

[stevek]

I'm in the mountains for another month and I have a Tahoe and Yukon, above 6500ft almost permanently. They're both the 4 speed 5.3 engines. One of them we tow some jetskis with and it's unbearable to drive... acceleration is awful... the trailer is only 3500lbs or so (I guess I'm a little spoiled with my Denali at home ). Any performance suggestions to squeeze a little bit of acceleration out of these things?

Tune, intake, etc - exhaust isn't an option.

 

[Zed 71]

Supercharger!

Ok an intake with specific tune to account for the altitude would help (including shift/torque lockup management) would help (but not a lot). Not sure how to 'detune' if you come down lower in elevation though.

 

[SmallXL]

forced induction is usually the best option for altitude. It doesn't matter if you are at 0 ft or 6000 ft - pressure is pressure. Use low grade octane, 87 will give you more power than 91 (assuming your not pinging).

 

[Jay]

As said above, boost is your best friend. Intake and tune won't overcome what nature is doing to you at those heights.

At sea level you have 14.7psi of pressure (absolute) pushing into your intake/manifold/head/cylinder.

Using the chart seen here: http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html

You see at 6500 feet you only have around 11.5psi (assuming same temperature). A 22% drop! With fuel injection the computer can compensate to some degree with fuel and ignition timing adjustments so you don't loose 22% of your power. More like 18-19% by most online calculators. Either way it's a large loss and will continue to be so until you restore that lost pressure to the engine.

The pistons don't "suck" in air, it's movement just creates a void to let atmospheric pressure work it's way in there. This is why the GenIII and GenIV heads have much less "restriction" than the old small block V8 heads. They make the flow path from outside into the cylinder much easier... meaning less pressure drop and therefore larger mass of air entering the cylinder. But it's still dependent on what outside pressure is acting on the engine. You will see a close correlation to the lb/min of fuel you are burning and the power you are making.

BTW: superchargers are more affected by altitude than turbochargers (the reason most WW2 supercharged aircraft had multi-speed transmissions in the superchargers). At any impeller speed and pressure ratio (hint: outside pressure versus boost pressure in the manifold), a supercharger will only move a certain lb/min of air. Since a turbo's impeller speed isn't mechanically linked to the crankshaft it will naturally spin higher in response to compensate. Not fully, but a lot better than a supercharger. Most superchargers spin in the 50-80,000rpm range. 130,000rpm isn't uncommon in a turbocharger. And just like for an engine, shaft rpm means potential. Anyway, off topic. Sorry.

Intake and tune will help, but won't get anywhere near compensating for the altitude difference. Even the mighty 6.2 vortec will feel a lot more lethargic at those heights.

 

[stevek]

No forced induction... has to stay in factory warranty unfortunately. Doing it to two of them is not cheap - none the less one of them

 

[Jay]

Don't know what to tell you man. Air density is killing you up there.

For ****s and giggles you could mount a large nitrous bottle in the back and jet it for a small 15-20hp shot and have it come on over 10-15% throttle (mostly on accel and uphill pulls). That much nitrous at part throttle will give you a large benefit and a large bottle will last a long time. A 5.3 at WOT can make 300 lb-ft of torque at 2,000rpm. Which is only ~115hp at that rpm. Since you're usually towing at 30-40% or less throttle you aren't using near that much so a small shot would be a big help.

Not much else you can do short of boost or a larger engine swap. Or buy a diesel truck for the pulling chores.

 

[Danny_Z56]

Highflow airfilter and a good tune should fix you right up

Good luck and let us know what you decided to do and if you noticed any improvement.