Tuning For Altitude

Planning a cross-country trip? Hoping to get your car up to 5,280 feet or considerably higher? We know all too well the pitfalls that come along with driving vintage British cars from sea level to our great heights in Colorado. This article, written by Paul Dierschow preceding MG2009 for the North American MGB Register newsletter, the MGB Driver, describes how visitors might best cope with driving at our high altitude.

Driving at Denver Altitude

One thing that most people will notice about the time they arrive at the folksy Colorado border sign is a distinctly lower level of performance than they are accustomed to from their car. This is a very common complaint; that’s because it affects everyone unilaterally. There’s an easy explanation, and it’s found under your hood. If you open it and look, you’ll find that . . . someone stole your compression!!! Yep, while you were minding your own business during the mind-numbing drive across Nebraska, Kansas, or Texas, you wuz robbed. Even though your eyes were open, your air density was decreased by a full 10%, and you may have even missed the posted warning signs entirely. As if that wasn’t bad enough, you’ll find that the consequence of that invisible, meager 10% decrease in air pressure translates into a whopping 18% loss of horsepower. So your perky MG with 135 psi compression at home can now only muster about 120 psi in Denver at 5300 feet. And that’s just at the base of the Big Hill. Sixty miles further west, by the time you cross under the Continental Divide inside the Eisenhower tunnels on I-70, you’ll exceed 11,000 feet. Ay, caramba! Now we’re talking only double digit compression, and horsepower that won’t qualify as a moron! If you don’t believe it’s important, just get out of your car at the tunnel parking lot and test it for yourself by running a short sprint. You’ll have new sympathy for your car. (Just kidding, DON’T DO IT! You’ll be flattened. Now.)

So what’s the solution? The easy answer is to replace your lost compression with that produced by a supercharger. End of that problem. However, the answer for the rest of us is basically, to live with it, with some minor adjustments. The most important adjustment is to one’s expectations. Climbing the Big Hill requires patience and perhaps some strategizing, but you’ll never get your performance back until you head back toward home. As far as tuning for altitude, these are closely guarded secrets which have been passed down through incestuous generations of grizzled British car mechanics, akin to those of voodoo magic. But now, for the first time ever, under threat of excommunication from Brotherhood of Keepers of Anachronistic Transport Contraptions, I will reveal the secret. Here’s the magic formula, never before seen in print . . . . LEAN OUT THE MIXTURE! That’s it. That’s all there is.

Driving In the Rocky Mountains

Sort of a let down, huh? The principle involved is this: as you drive higher, your fuel supply through the carb remains the same, even though the air component of the mixture keeps getting less and less. This results in a mixture that continues to get richer as the air gets thinner, and before you know it you’ve got some serious drivability problems. Like smoking that rivals anything Detroit Diesel can produce. The fix is to simply reduce the amount of fuel to return the air/fuel mixture to normal. With a typical SU installation, the adjustment is simple: we plan on about a two flat adjustment of the jet adjusting nuts to make the mixture right for Denver; prorate for going higher. That translates to 10 flats down from having the jet flush with the bridge, instead of 12 at sea level. While the final tuning must always be done on the exhaust gas analyzer, this rule of thumb is good enough to get a car running from scratch. We are frequently asked about the use of lean metering needles in our area, and we find that they are not necessary IF (hugely important if) the jets are in good condition. With SU carbs, we do not consider using a lean needle unless a customer drives their car mostly above 8000 feet. Stromberg cars are another story; we usually prefer lean needles, but mainly for the purposes of passing emissions testing. Weber conversions require jet changes – that’s a whole different article.

Additional Adjustments

There is a further adjustment that can be made, perhaps for the skeptics who might believe there’s got to be more to it than just a simple mixture adjustment (trust me, you’re not going to get your performance back…). A small consolation prize can be won by advancing the timing just a little bit; 2 or 3 degrees is about it. The principle involved here is that engines produce more power the more advanced the timing is, but too much advance will be harmful, as the engine will detonate (ping), and that burns holes in piston crowns (rarely considered desirable). However, with your compression having been involuntarily lowered, you can advance the timing a little bit without incurring detonation, and therefore produce marginally more power. I almost hate to mention this, as timing advance should be regarded as a loaded weapon, and handled in an educated and measured fashion. If a little advance is good, more is not necessarily better!

The whole tuning for altitude discussion is really greatly overshadowed by a much larger issue; one that we have to deal with nearly every day in the shop. That is that carb adjustments are often the least of a car’s woes. More often than not, drivability complaints are explained by problems other than carb issues, instead they are almost always ignition related. When a customer brings in a car complaining of carb problems, the first place we look is at the ignition system. That’s because the symptoms involved are often misleading; carb and ignition problems can manifest themselves similarly. In fact, I would go so far as to say that 90% of “carb” problems are found under the distributor cap. If someone complains that the carbs won’t hold their tune for more than a week, the problem is most likely not carburettor related. The bottom line is don’t touch the carbs until the entire ignition system is proven to be good. And the best tuning won’t do much good if the basic condition of the engine is not sound. You must start with good basics first.

Tips for a Great Trip to the Rockys

So here’s some tips to increase your odds of making a successful trip:

  • Check out your car thoroughly before you leave – do not wing it and hope for the best (this actually does happen). Book your appointment now with your local LBC shop to be sure your car is up for the journey.
  • Avoid using fuel containing ethanol, especially in high temperatures. Our lower atmospheric pressure causes the fuel to vaporize easier, which induces vapor lock. The higher you go, the worse it gets.
  • Pressure test your cooling system, as the ability to maintain high pressures is the key to not overheating. Water boils at much lower temperatures out here: 202 at Denver, 192 at 11,000 feet. Also, remember that running hot (good) does not equate to overheating (bad).
  • If you encounter drivability problems along the way, don’t start by fooling with the carbs – most likely it’s explained by something else.
  • If something untoward does happen, don’t fret – you’ve got the world’s best classic car support system at your disposal. You’ve got highly qualified technicians available at the other end of the phone and you’ve got whatever part you might possibly need only a day away, thanks to the wonders of air freight, and suppliers who really do care about the well being of their customers. You’re also in the company of a great crowd of caring enthusiasts with similar interests who never fail to amaze me with their stories of generosity and support for their comrades.

I wish you the very best of luck on your journey. Please come visit us at the shop – you’ve probably never seen any place like it.

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