Altitude adjustment

[john.r.davies]

All,

At the recent Ten Countries Run, Jonathan Binnington and I in his SuperSix (2.5 Pi) found that at altitude above 1500 metres the engine began to lose power, and at the summit of Iseran, nearly 3000m, we could only progress in first gear.   I know that Lucas did make some 'altitude' metering units, but these are as rare as..... and we don't want to modify the car for something it will never face in the UK (Ben Nevis - c.1000m).

 

Is there any way to simply and temporarily adjust the MU and weaken the mixture for extreme altitude?   Like the 'full throttle adjustment screw' that moves the max. fuelling up and down?

 

John

[stephen cooper]

Did you not try taking the plenum chamber off?

[Tony.C]

When driving in the Alps last year I found that by opening the bypass valve this increased the tickover speed (races a bit) at lower altitude but kept us going well at higher altitude. We did the Grimsel, Furka, Susten, Gothard and several others up to over 2800 without issue (did run a bit rich) albeit we found there was one higher pass than what we did when we came back. Must say that I was pleased to finally find a good reason to have overdrive on 2nd & 3rd - makes a difference in the mountains.

Regards

Tony. C

'72 CP TR6

[Andy Moltu]

You can take the cap off the metering unit - mark the adjustment rings with a line of paint or tipex & then loosen the A1 locking ring & unscrew the A1 ring which effectively weakens the mixture, then lock up the ring. Put the cap back on & off you go.

About half a turn should do, perhaps a little more.

The marking should let you simply return to your sea level settings.

 

Having been over Stelvio a few times (2758m) the PI cars run mighty rich & will stall if you lift off the pedal.

[Peter Douglas Winn]

The Swiss have their metering units set up for 2.3 to 2.5 CO². They bomb up and down all their mountains without any problems. Where's the problem?

If you get stuck on the top of a mountain swap the plugs round (everyone has a set of plugs on board, if not clean them) and carry on driving when you get down to lower altitudes they will clean themselves.

[Andy Moltu]

The Swiss have their metering units set up for 2.3 to 2.5 CO². They bomb up and down all their mountains without any problems. Where's the problem?

If you get stuck on the top of a mountain swap the plugs round (everyone has a set of plugs on board, if not clean them) and carry on driving when you get down to lower altitudes they will clean themselves.

That's fine - but set up for 2.5% CO if done in Switzerland (presumably at altituder) would become a very lean mix at sea level.

 

Standard metering unit spec at sea level is about 4% CO.

 

Ultimately the settings for the Lucas PI are a compromise as the standard set up doesn't have the capacity to adapt to atmospheric pressure changes.

 

Modern EFi units measure airflow. The Lucas PI system doesn't - it estimates airflow by "measuring" the manifold pressure against atmospheric - the difference adjusting the mixture using 3 springs of different rates and a fuel cam. It assumes atmospheric pressure is constant.

 

Effectively the wider the throttle opening the less the difference between atmospheric and manifold pressure so it assumes more fuel is need. A closed throttle means greater difference between the pressures and a lower fuel setting is delivered.

 

However at altitude the atmospheric pressure is lower so the difference between manifold & atmospheric pressure at all throttle settings is relatively less than at sea level so the metering unit delivers a richer mixture across the rev range.

[Tom Fremont]

The so-called " constant depression " type carburetters may have an advantage here - my TR250 with the original Stromberg emission carbs ran beautifully over the Rocky Mountains ( ~9000 ft Loveland Pass ) giving a beige coating to the inside of the tail pipes - this was in the days of leaded fuel - with no adjustment. I therefore suppose the rest of the carburetted TRs don't suffer from altitude sickness either.

 

I wonder about Webers, never having taken them to such heights myself  ???

[Andy Moltu]

The so-called " constant depression " type carburetters may have an advantage here - my TR250 with the original Stromberg emission carbs ran beautifully over the Rocky Mountains ( ~9000 ft Loveland Pass ) giving a beige coating to the inside of the tail pipes - this was in the days of leaded fuel - with no adjustment. I therefore suppose the rest of the carburetted TRs don't suffer from altitude sickness either.

 

I wonder about Webers, never having taken them to such heights myself  ???

Generally the SU type carbs cope OK with altitude.

I have heard that Webbers tend to go rich at altitude but I'm not sure on that one