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My 3A is going on a rolling road soon and I am looking for better MPG. Has anyone used another needle instead of the SM recommended for the H6 SU.

 

If anyone has had success please let me know. What else should I look out for?

 

Thanks Richard & H

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Hi Richard,

Using standard new needles and jets. You could save a fortune by having your carbs serviced and buying two Colourtune plugs and learning to use them. It is like a window on your engine and the provided flame colour chart does give great guidance. You save money and gain experience of your car, both of which are invaluable, the savings can be put to enjoying what is at best is a joyful experience and at worst a drain on resources.

 

To get the best from a rolling road tune you really should do it with known quantities as it is a very expensive way of finding out that you need to re-condition your carbs and or your distributer or engine.

 

IMHO the guy can only tune what you have and if you consider that when they were newly road tested by The motoring press back in the day you could get 35mpg in overdrive top gear from a TR2 and the TR3a was about 28mpg so you will never get 40 plus no matter what you do. FWIW I'm afraid that's about as good as it gets!!!! as these figures were probably in ideal conditions and with known parameters.

 

Wish you luck.

 

rgds

Rod

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Thanks for the reply.

 

I have a Racetorations standard engine, Distributor Doctor distributor, refurbished carbs, unleaded head, standard two box exhaust,Kenlowe fan, new radiator and last week a new battery 88 a/h

 

I tried the Colortune option and it gave me a very wide range of possible settings.

 

I hope that the rolling road will set the ignition and mixture so that I get better MPG.

 

I am open to any advice.

 

thanks

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Hi Richard,

Seems you have done it all but not sure what you mean by a "very wide range of possible settings" with colourtune. I use two colourtune units and have on occasion used four (just because I can) and Gunsen flow meter to balance the carbs. That way I can see what is going on at each pair of cylinders. Get the mixture and balance right and then play with the ignition side, works for me but throw the book ignition settings away!!!!

What sort of consumption are you getting?

When you say standard engine what size?

Are using cast or tubular manifold?

 

Rgds

Rod

Edited by Rodbr
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Rod.

Interested in what you say about the "book" ignition settings. I have suspected for some time that I could improve things by advancing the ignition setting. Do you know what your timing is at tickover ? (or static)

 

Bob.

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Hi Bob,

I have tried on successive occasions to use the factory settings and ended up with my own version of them. It seems to be dependent of the componets used and almost every engine seems to be different. Still working with mine and the fuel supply seems iffy for consistent results.

Will post when I get it the way I want it. Only done 1000 miles back on the road.

Rgds

Rod

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Modified engines usually do have unique tuning requirements. Colourtune is good, but a mate recently put his improved TR on a rolling road and after a few ignition and carby tweaks, they found 35 missing HP. It then gave the best fuel consumption that engine was capable of.

 

Viv

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Anything is a guess compared to a Rolling Road operated by someone who knows what he's doing. Both my Healey and my Bentley were set to the book and both got a noticeable increase in power, mostly from richer needles which modern fuels need in old engines in my experience.

 

I doubt there's a single owner on here that wouldn't benefit from a rolling road, so don't hold back. Just make sure the operator is familiar with Oldies. Spam in Stroud charges £185 and he works wonders with everything.

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For best cruise consumption and mpg you want about 40-50 BTDC spark and around 16 AFR. The spark advance should be given by the vac advance capsule and a butterfly edge take-off feeding it. The AFR is set about half inch from the top of the needle.

Both need to be measured at cruise loads and rpm. Measuring AFR on the road can be done with a UEGO sensor. Spark timing needs more experimenting - with a strobe light and camera looking at the crank pulley mark. If the spark and AFR are set around those above the engine will be working at its best bsfc. The settings are shared with most 'slow burning' (2-valve, central plug, bath-tub) combustion chambers and are determined by the speed of flame spread across the combustion chamber.

More on vac advance:

http://www.camaros.org/pdf/timing101.pdf

And best AFR for lowest brake specific fuel consumption:

Slide 3 : http://www.powershow.com/view/1e2c2-NmY0Z/Chapter_IV_Spark_Ignition_Engines_22703_powerpoint_ppt_presentation

note; equivalence ratio is the mixture. Best is 0.9 which is converted to AFR as 14.7/0.9 = 16.3

Also slide 4, which shows bsfc getting worse at cruise - ie consumption per horsepower gets worse !! But remember the hp needed to cruise are much less than at full throttle accleration. The' fish-hooks' show that the mixture still needs to be a bit lean to get best fuel economy.

 

Tuning for power - at wide open throttle - does not embrace tuning for cruise economy. Tuning for power gives a richer SU needle in cruise region - the top half inch- than desirable at cruise. The top of the needle determines mixture at around 1000rpm at wot and also at say 3000rpm at cruise. It has to do both: power tuning sets it rich, cruise tuning should set it around AFR16.So the choice has to be made it cant give both. And at wot the vac advance capsule isnt active, so its effect is not measured.

 

To tune for cruise economy on a rolling road you need to knpw typical rpm when actually driving at cruise ( easy) and the throttle position - not so easy. A gauge reading manifold depression can be read while driving. Then the rr has to be pegged to that rpm and the throttle opened until the dpression gauge reads the same as driving. Then measure AFR and measure the piston lift in the SU- you need to lean it there to give AFR 16. This can be calculated after measuring the needle diameter with micrometer and finding the new fuel annulus area. Data on torque from the rr may or may not be useful. I doubt that a 700hp-capacity rr with be precise enough to measure changes of 1 to 2 hp on a base of 20hp - thats asking for 0.3 % precision.

 

Peter

Edited by Peter Cobbold
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