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John Morrison

Four Pot at Altitude

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Just curious,

going to be taking the TR4 up 2500M, thats 8,000FT plus.

Can I expect running difficulties, on Strombergs and a pretty standard set-up?

John.

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The depression above the jet will stay constant as it is set by the mass of the piston and spring force divided by the are: those wont change with altitude. So we need to know if the lower atmos pressure will affect lift and hence needle position. At 8000 feet (= 2400 m  ) atm press is   75 % of sea level, and air density likewise.

 

So if the piston/needle stays fixed the mixture will richen by about 25%.

However, the atm pressure below the piston has fallen by 25% too, so the force too lift the piston against that constant depression is less. So at 8000 feet the piston will assume a lower position, the needle delivers less fuel and that 25% richening wont happen. It might evn go leamer dependong upon the needle profile. However there is a possibilty of richening due to a greater throttle opening. No free lunches, at altitude you will need more throttle!!  If at cruise at sea level the throttle is open less than 30 deg and open more than 30 at altitiude the mixture will richen, but still to nearly the same mixture as that >30 deg opening at sea level.

Overall a sllight leaning or richening might happen but not affecting driving.

In short the SU Stromberg is a brilliant design because there is no venturi.  Webers at altitude cant sense the air density change as their venturi operates on air 'particle velocity': they will go rich, badly.

 

Peter                                                                                                                       

 

 

image.png

 

 

Edited by Peter Cobbold

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John,

A couple of years ago I took our 4 on SUs over the Col d'Iseran at 2764m with out any problems. Given the similarities with Strombergs I would say just enjoy the trip!

Mike

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Brilliant Peter,

Always known my Stromberg cars were more robust than my highly strung Weber cars........... Now Ive got an inking as to why

 

Thank you.. and for all your wider contributions on here

Tony 

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Webers will cope with altitude its just that they go a little bit richer but no where near as much as a PI will.

Stuart.

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22 minutes ago, stuart said:

Webers will cope with altitude its just that they go a little bit richer but no where near as much as a PI will.

Stuart.

...and that is why there was an high altitude use type metering unit with an additional atmospheric capsule fixed below the main control body of the MU.  I guess it worked the fuel enrichment lever in the opposite direction (pushed the choke in further)

We had a box of new ones under the stairs at Cox and Buckles in the 1980's.  KMI had some too when they were in Fulham.

Peter W

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Summit loop of Col de la Bonette (2,802 metres or 9,193') no problem for the 4A with SU HS6s and standard needles on the Club Triumph 2013 Ten Countries Run. Idled happily and restart after photo stop (at left) was instant.

Tim

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5 hours ago, Tony_C said:

Brilliant Peter,

Always known my Stromberg cars were more robust than my highly strung Weber cars........... Now Ive got an inking as to why

 

Thank you.. and for all your wider contributions on here

Tony 

Thank you Tony. I had to ponder the fundamentals of SU/Strombergs when feeding the supercharger with one HS8. I gave a talk at an IWE , here:

https://supertrarged.wordpress.com/2017/07/10/how-does-an-su-carburettor-work/

The explanation of "mixture spread" is not as far as I know available elsehwere. Mixture spread is how that < or >30deg butterfly opening gives a different mixture at the same station on the needle.

Peter

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Yes thanks to Prof., and the other comments.

I do plan to enjoy this trip.

John.

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20 minutes ago, John Morrison said:

Yes thanks to Prof., and the other comments.

I do plan to enjoy this trip.

John.

John, Have  a good trip. Fuel consumption at altitude will rise, and not only because of the gradients ( slide 34) . Best carry spare fuel ? Peter

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I should have said - the Col de la Bonette is a superb road. We were very lucky in early September not to see another vehicle - or mad cyclist- between Jausiers and the top and, at least in 2013, the surface was excellent all the way up, no 'nids de poules' to worry about! A perfectly dry road and mostly open, sweeping bends with great visibility allowing a nice rhythm to be established. Thoroughly recommend, at the summit you really feel you are at the top of the world.

Tim

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Thanks Guys,

Tim won't be doing the Col de la Bonette this time.

We have six cars from Cumbria group doing 1050 mls. over three days to an overnight in Obergurgle, and then

we are straight onto the Timmelsjoch, billed as 60 spectacular bends to 2500Mtrs, thats only no 1 and another 12 days after this!

Think I will enjoy the trip.

John.

N.B;- Moving this to out and About, seems more appropriate now. John.

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You will enjoy the Timmelsjoch John, although subject to a toll. Attached shot of three Michelottii TRs was taken on the 2011 Club Triumph 10CR, looks like the Spitfire was running a little rich! I always use the  hard top on serious events - there's nothing wrong with the chassis but with the top firmly bolted in place it does stiffen the car and the handling is sharper. The light colour reflects the sun's heat and I find the face level vents provide very effective ventilation. 

I am off on this years 10CR on 3rd September. Many Alpine and Dolomite passes are included, I am particularly looking forward to the Grossglockner, which I haven't done before. At present the entry list includes a 3, a 3A, four 4s, two 4As, a 5, thirteen 6s and three 7s out of a total of eighty three remaining entries, a very pleasing 30%. 

Tim

IMGP2462.JPG

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Having just got back from doing the Liege Brescia Liège Rally, ( also goes to Ljubljana) we have done some serious passes. My car on standard H6 carbs and setup on a Rolling road to check mixture all the way though the Rev range under-load performed brilliantly. A little rich at the top but no issues at all. 

Here we are at the top of the Stelvio

 

EA097137-3176-459B-B276-C2B6A58CAE45.jpeg

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Congratulations Ian, great achievement.

Regards,

Waldi

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Emergency stop to buy Lycra Iain ???

 

sounds like a great trip. 

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Hamish, me Lycra....oh what a thought!:lol:

No this was a Passage Control on the rally.

 

 

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20 minutes ago, iain said:

No this was a Passage Control on the rally.

 

 

:o that’s a tough rally if you have to stop to prove you are not incontinent  !!!!!!:ph34r:

 

think the Lycra would be better !!

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On 20 July 2019 at 7:41 PM, John Morrison said:

Just curious,

going to be taking the TR4 up 2500M, thats 8,000FT plus.

Can I expect running difficulties, on Strombergs and a pretty standard set-up?

John.

We experienced no engine difficulties at all on the recent Liege Brescia Liège Rally, a 2200 mile plus course including 8 Alpine passes.

We have twin 175 CDSEV Strombergs on a standard US spec 76 TR6.

However perhaps this Porsche driver couldn’t say the same as we watched him trailered away down the Passo di Pennes (7254 ft) …

Martin

Porsche descending Passo di Pennes on a flat bed truck.jpg

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Oh dear me,

how sad,

never mind. (Said in a Welsh accent)

John.

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On 20 July 2019 at 7:41 PM, John Morrison said:

Just curious,

going to be taking the TR4 up 2500M, thats 8,000FT plus.

Can I expect running difficulties, on Strombergs and a pretty standard set-up?

John.

John

You asked if you can expect running difficulties at high altitudes.

I don’t expect your Strombergs to give you any problems but that’s potentially the least of your worries.

I found that I was back-firing all the way to the top of some of the high Alpine passes on our recent European rally.

The cause? – Drinking too much carbonated water before the ascent because the bubbles expand at the higher altitudes….

And so in the interests of marital harmony, in the close confines of a TR cockpit – stick to plain water if you want to avoid those embarrassing bottom burps…

Or take a dog so you can shift the blame…

Martin

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Martin,

many thanks for this,

'Its always the little things!'

John.

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