PI versus altitude

[Yodel]

Hello fellow Triumph enthusiasts!

 

Of all the frustrationts to be experienced with a TR6 PI, this must be it: Out here in the mountanous part of Austria it is just so important that your car is capable of going up hill (up to 8000 feet and more, if necessary). This is not possible with my TR: Long before this height is reached, plug 5 or 6 have fouled due to over fueling, due to a reason which is not clear. There is sufficient vacuum pull for the PI and in general the condition of the engine is good. In fact on a good run at lower altitudes, the car returns a good mpg. And so my question: Could it be that the Lucas PI unit is overly sensitive to altitude change? Or is the standard induction manifold and air filter a hindrance at lower ambient air pressure? Does anyone have experience with the PI unit at altitude?

[John L]

I took my 5 for a hoilday in the Pyrenees this year and into Andorra, and we did some quite high passes but I think the highest was 2240meters, in Andorra Arcilis. The car still ran ok, but was a bit short of breath. On the whole trip it returned 27mpg, but much of it was never over 60-70mph.

In fact I think the road between Luchon in France and into Spain and thru the mountains to Andorra beats the Stelvio, not many cars or motor bikes and the scenary was superb.

I would like to add some pictures but I'm not sure how it's done.

I have been playing with my metering unit and have been using a wide band oxygen sensor, and set it run about 14 AFR, this has improved the overall consumption from what it was. There still seems plenty of pull, and is lively off the mark.

John

[john.r.davies]

Yodel,

Lucas Pi modulates the amount of fuel injected according to the pressure in the inlet manifold. As the throttle opens, the pressure falls and more fuel is injected. At altitude, the low atmospheric pressure causes the whole range of throttle opening to be shifted towards more fuel, so that the mixture becomes richer and richer as you ascend. On the Club Triumph 10CR, Bin and I in his SuperSix could only just get over the Col 'd'Iseran, at 2770meters the highest road pass in the Alps.

 

This effect may be more or less apparent depending on how rich/lean your Pi is normally set, because the standard metering unit can be adjusted within quite a wide range, but far from easy even in your home garage - to do so at the road side, and progressively as you ascend would be very time consuuming.

 

Lucas produced for Triumph some special 'Alpine' M/units, but they are rarer than a hair on Kojak's head today, but you can pick one out easily - see pic.

[Peter Cobbold]

At altitude the mass air flow into the cylinders is reduced but the MU still meters out the same amounts of fuel as at sea level. So not only is the engine starved of air it also runs over-rich. The richness can be cured,if you are inclined to experiment. First fit a wideband AFR gauge to tell you what the mixture strength is as you drive ( AFR 12.5 for max power, 14-15 at cruise). Then at altitude we need a way to push the diaphragm in the MU upwards to lean the mixture. So use a 12v tyre pump to keep a small tank of air at say 1 atmosphere pressure. Take that air pressure to a precision 'relieving pressure regulator' such as this:

http://www.norgren.com/document_resources/usa/11-018.pdf

Connect the outlet pressure from the regulator to the vent just above the balck plastic cover on the MU. As you climb and the AFR gets too rich wind on a bit of pressure on the valve (it can go under the dash) until the AFR is correct. When you descend dont forget to take off the pressure or it will go too lean.

 

Cant help with the low air density at altitude --- unless you fit a blower...

[harrytr5]

I believe the works Saloon Pi metering units were tuned with the choke (enrichment lever) either half or fully out when at normal sea level then slowly pushed back in the higher you got up in the Alps.Very clever idea from the mechanics at the time.

I had to change my plugs in a snow storm when driving from Italy through Switzerland to Germany when we were at 2668m above sea level and it was running like a pig.My best mate in his tripple webber TR6 ran with no problems at all.

Now I am on EFI I will not have any more issue's in that department (famous last words!)

Regards Harry TR5 Nutter

[rpurchon]

if you drive at altitude a lot. fit some carbs, SUs or webers. would be the best option.

plus theyre would be a lot less to go wrong long term once fitted maintance wise.

put all the pi stuff in a box and save it if you ever decicde to sell.

i have humble SUs on my car and it goes great,wouldnt even consider pi.

richard

[Tom Fremont]

I went over 11,000 ft. with my triple Weber DCOEs with no issues; plugs clean ( I checked ). Power loss due to less oxygen and pressure is to be expected; just move the throttle another 1/4 " to compensate when cruising .

 

The options for dealing with drastic altitude changes without loss of performance appear to be EFI or triple DCOEs. Mine have done 92,000 trouble-free miles since I bought them in 1995, quite used ( 1960s vintage with brass throttle shafts ). Hard to beat that kind of reliability

[Andy Moltu]

If you mark the 3 rings beneath the cap on the metering unit with paint - then loosen the lower lock ring & turn the whole thing half a turn anticlockwise & nip up the lock ring. This has the effect of globally weakening the mixture across the rev range and can be reversed on return to lower altitudes.

 

2 things to remember - don't forget to mark the rings and don't turn it too far.

If it's newly rebuilt you will break the seal and invalidate your warranty.

[john.r.davies]

I'm grateful for the ideas above, esp. if I go pass-storming again. Well worth considering, esp. Peter's suggestion which fascinates me, for it's fiendish cunning!!

But Peter,

At altitude the mass air flow into the cylinders is reduced but the MU still meters out the same amounts of fuel as at sea level.

Your own work-around would seem to contradict that ,in that it will clearly compensate for the low atmospheric pressure and make the M/u go leaner. Did you mis-speak yourself?

 

JOhn

Edited September 1, 2011 by john.r.davies

[Peter Cobbold]

I'm grateful for the ideas above, esp. if I go pass-storming again. Well worth considering, esp. Peter's suggestion which fascinates me, for it's fiendish cunning!!

But Peter,

Your own work-around would seem to contradict that ,in that it will clearly compensate for the low atmospheric pressure and make the M/u go leaner. Did you mis-speak yourself?

 

JOhn

 

John,

Thanks for the moral support! I dont think I made a mistake..

This is the way I look at it, thinking aloud. At wot, the diaphragm is pushed fully down by its springs and allows the fuel flow to go to maximum squirt. Altitude has no effect on fuel pressure or flow per se: each squirt is the same volume as at sea level. But up a mountain the amount of air taken into the cyinders will be down because of the lower atmospheric pressure - so the mixture goes rich ( because same fuel squirt but less air to mix it into). At part throttle it will stay rich as the atmospheric pressure pushing the diaphragm up against the springs is less at altitude, so the 'suction' from the manifold will not lift the diaphragm up as much as at sea level. ( by 'suction' I mean the pressure difference across the diaphragm).

So the auxilliary air pressure is used to help the diaphragm up a bit, reducing the fuel squirts, and so restoring correct mixture. Note that the air pressure is applied to the under-side of the diaphragm.

 

How much air pressure to apply? At 3000m atmospheric pressure will be around 10psi, about 4.5psi down from 14.7psi at sea level.

(apologies for the quaint units, I've spent too long reading american stuff about blowers. metric units here:

http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html

As air flow into the engine will be linear with pressure - ie 30% down- we need to set the pressure difference acros the diaphragm to its sea level setting, by adding about 4.5 psi under the diaphragm. This will push the diaphragm up against its springs, reducing the squirt volume.

Now at altitude the driver will tend to open the throttle more to recover that lost 30% of power, but at least now the MU is compensating: the MU now 'knows' that the max fuelling per squirt has to be 30% less than at sea level.

 

And the air system is stand alone, cheap ( except for the wideband AFR gauge) and easily removed , and will not break any warranty seals etc.

Peter

[alec]

Hi

 

I had this problem going over the Stelvio Pass last July plugs 5 & 6 had to be cleaned at the top but a few days latter replaced with spares

 

Even in the UK 5 & 6 are black if engine left to idle

 

UK Car PI good mpg 30 on a motorway 22 running about

 

I since discovered my 12 volt distributor coil feed was via the ballest resistor feed hence only getting 8 / 10volts at low revs

 

(This became about after the starter motor was replaced 5 yeras ago with a new High Torque type)

 

Have rewired with 12 volt feed from ignition circuit in Fuse Block. Not able to try altitude run in UK but plugs 5 & 6 stay a lot cleaner

 

Worth a check before messing around with the mu unit

 

My replacementt / reconditioned mu unit has done 35K miles over last 6 years without any problems

 

Alec

[Peter Cobbold]

Hi

 

I had this problem going over the Stelvio Pass last July plugs 5 & 6 had to be cleaned at the top but a few days latter replaced with spares

 

Even in the UK 5 & 6 are black if engine left to idle

 

UK Car PI good mpg 30 on a motorway 22 running about

 

I since discovered my 12 volt distributor coil feed was via the ballest resistor feed hence only getting 8 / 10volts at low revs

 

(This became about after the starter motor was replaced 5 yeras ago with a new High Torque type)

 

Have rewired with 12 volt feed from ignition circuit in Fuse Block. Not able to try altitude run in UK but plugs 5 & 6 stay a lot cleaner

 

Worth a check before messing around with the mu unit

 

My replacementt / reconditioned mu unit has done 35K miles over last 6 years without any problems

 

Alec

 

Alec,

Stelvio's altitude alone will richen the MU mixture - by around 25 to 30%- and so blacken plugs. It is inevtitable in its design, not a MU fault.

 

My MU did 150,000miles without attention - but car was daily driver. Your 6k pa will ensure you'll have another trouble free 115000 miles...

 

Peter

[Peter Cobbold]

A couple of general points about altitude.

 

The lower air pressure - 30% down at 3000m - means that max power will be down by at least that, more if the mixture is not corrected. Obviously the cylinders will have 30% less air in them. But that also reduces the effective compression. The overall effect is a slower burn, so more spark advance can help at altitude.

 

Carbs such as SUs ( my choice) or Webers do nothing to correct the power loss from lower air pressure - max power will fall by 25-30% at 3000m - thats physics. But they have a big advantage in that their metering mechanism involves the venturi effect. The amount of venturi 'suction' over the jet depends in part on the air density (the symbol 'rho' in Bernoulli's equation). The less dense the air the less the venturi suction. So at altitude we have lower atmospheric pressure, and hence less dense air, so the venturi suction lessens and the fuel flow out of the jet lessens, keeping the mixture within reasonable bounds.

 

Recovering that lost 30% in max power requires forced induction, the simplest being a supercharger. But even that would suffer from the 30% drop in its inlet pressure. If an engine is to run at altitude all its life then gearing the blower to restore full power is easy. But for engines that have to perform at sea level and at altitude the blower output has to be variable to keep boost pressure within bounds at sea level. Variable gearing would be ideal but tricky to engineer. More feasible would be to fit a restrictive air filter that limits max boost at sea level. Then at altitude that is by-passed to a less restrictive air filter.....

[Tom Fremont]

Keeping the power up at high altitude along with optimum mixture is increasing the burden on the poor old fueling system rather unreasonably, IMO, for a vintage car. Does any modern car offer that combination ?

 

Redirect to the Weber DCOE option for trouble free touring in the Alps or by the sea

[Peter Cobbold]

Keeping the power up at high altitude along with optimum mixture is increasing the burden on the poor old fueling system rather unreasonably, IMO, for a vintage car. Does any modern car offer that combination ?

 

Redirect to the Weber DCOE option for trouble free touring in the Alps or by the sea

 

 

Modern engine management with MAP sensor would I'd expect adjust both mixture and AFR at altitude, but would not correct the power loss from less dense air, unless it had a turbo with dump valve correction of boost. Not sure if the supercharged moderns can do that.

 

Vintage fuelling: If our old engine is supercharged just enough to recover the lost 4.5psi atmos press at 3000m, the fuel requirement will be nearly the same as the unboosted engine at sea level for a given power output, and max power will be restored too. ( I say 'nearly' because some power will be needed to drive the blower - around 5-10hp and that needs a bit extra fuel). So easily within capability of the Lucas PI.

Peter

[k_raven_smith]

hi all, a quik note on this lucas had three calibration test plans for EU (the true green top unit0 I have these test plan so can set a unit for altatude driving.i hope this is of help to you. neil

[Denis]

Peter makes me feel so dumb!

In 1973 I drove my Pi 6 down through France, before they had any Motorways...which makes my arms ache just thinking about it now!

Then over the Pyrenees through Andorra to go to the Grand Prix in Barcelona (Stupid youth!)

Even more stupid...I don't recall noticing any loss of performance, only an overheated fuel pump that wouldn't go again until it cooled down after I stopped at the summit to admire the view!

 

How do we add a new profile picture now?

Edited September 3, 2011 by Denis

[Alec Pringle]

Hi Yodel,

 

please see PM

 

Cheers,

 

Alec

[Peter Cobbold]

Peter makes me feel so dumb!

 

 

 

 

Denis,

Oh dear, thats not the intention. Just trying to pass on accumulated knowledge. Its the scientist in me that wants to understand the whys and wherefores, and drives the 'I wonder if that would work...' approach.

 

I guess we both have had years of reliability from our daily driver PI 6s in the '70s . I carried a spare fuel pump amd injector for many years and never needed them, nor a bag of frozen peas to cool an overheated pump.

 

Peter

[Tr4Tony]

Apparently the World Cup 2500 PI's used a metering system connected to an Altimeter (metering unit as above), but crucially the altimeter had a mechanism to vary the mixture attached to it ..... so all the thinking is done if you can get one of the PI's guys to show you how it looked.

[Brett]

Very intereting thread indeed!

 

My TR5 usually lives around 500m altitude, where it already runs a bit too rich. On my recent trip to the Stelvio and 15 other high passes in the Alps, when in addition to altitude the weather was very hot, of course it ran significantly richer, but I had no other consequences than fairly thick black smoke at the exhaust, and a slight loss of power, so it wasn't too bad!

[Willem]

<p>Yep, very interresting.</p>

<p>This spring I drove my PI TR6 over Timmelsjoch, Grossglockner Stelvio and several other high Alpine passes during a classic car challenge.</p>

<p>Of course, at high altitude the engine was less powerful and other participants smelled the unburnt fuel from my exhaust.</p>

<p>Luckily the engine kept running.</p>

<p>I too would be very interrested in any (PI) solution to manage the AFR more efficiently</p>

[Yodel]

Hallo all,

 

The idea of compensating atmospheric pressure loss by backloading the underside of the diaphragm with up to nominal 4.5 psi as Peter suggests above, seems quite an intriging solution. However, I don't quite understand the technical requirement of of this atmosheric vent port. Surely the underside of the diaphragm should be at a constant reference pressure of 1 atmosphere which could be maintained at all heights above sea level by simply bunging the port shut? Or have I overseen a vital reason why Lucas designed the MU as it is?

 

Would be pleased if you could give me an answer before I do anything silly.

 

Yodel

 

Many thanks to you all for your informative answers. This was my first question placed in the forum and I impressed by the high level of technical help offered.

[Peter Cobbold]

Hallo all,

 

The idea of compensating atmospheric pressure loss by backloading the underside of the diaphragm with up to nominal 4.5 psi as Peter suggests above, seems quite an intriging solution. However, I don't quite understand the technical requirement of of this atmosheric vent port. Surely the underside of the diaphragm should be at a constant reference pressure of 1 atmosphere which could be maintained at all heights above sea level by simply bunging the port shut? Or have I overseen a vital reason why Lucas designed the MU as it is?

 

Would be pleased if you could give me an answer before I do anything silly.

 

Yodel

 

Many thanks to you all for your informative answers. This was my first question placed in the forum and I impressed by the high level of technical help offered.

 

Yodel,

Sealing the underside at sea level to keep a constant reference pressure would I think be impossible as there are air leakage routes past the 'choke' control etc. (Modern MAP sensors seal the reference vacuum in a hermetically sealed chamber, but the Lucas MU would be too leaky.) But say you could keep the chamber under the Lucas diaphragm at sea level atmospheric then as you drive up a mountain- what would happen?: For a given throttle opening the 'suction' from the manifold ( fed by the lower atmospheric pressure) would get relatively greater at altitude which would pull the diaphragm up too much and so lean the mixture more than normal. The auxilliary air feed under the diaphragm that I proposed would allow feedback control via the AFR gauge and the dash regulator that allows the driver to compensate.

You could try a 'lash up' - a test rig. Connect bicycle pump with plastic screen washer hose to the underside of the diaphragm so that your passenger can apply air pressure - T in a pressure gauge ( say 0 to 1 atmosphere). When at altitude get her to gently pump up the pressure; see if richness goes , engine runs better..

 

A better way of looking at the MU's reference parameter: it is the springs under the cap on top of the diaphragm that are the fixed reference. Its the 'suction' from the inlet manifold that weakens at altitude and make mixture rich. 'Suction' is not an engienering term - there is no such thing as negative pressure ( a vacuum is zero). The ' suction' force acting on the diaphragm is the difference between the manifold pressure and the atmospheric pressure under the diaphragm- at altitude the pressure under the diaphragm lessens so the 'suction' force is less. But the spring pressure is unaltered. So the diaphragm assumes a position at altitude that is lower than at sea level, for the same pressure in the manifold. Result: a over-rich mixture. Applying air pressure from an auxilliary supply adds back the missing pressure under the diaphragm and restores the correct mixture.

 

 

Peter

Edited September 6, 2011 by Peter Cobbold

[Nick Jones]

There is to be a group test of this next Friday and Saturday as the Club Triumph 10 Countries run passes through the Alps.... and then turns around for another go. 20 Passes, 14 over 2000m, including Stelvio. Not cars all are PI but there are several in various forms. Hopefully my own electronic (and barometric compensating) system will work......

 

Nick