Best book(s) on SU tinkering

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RobThomas
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Best book(s) on SU tinkering

Post by RobThomas »

Has anyone got any reviews on the various SU books that could tell me which ones best describe the 'science' behind the design and the actions required to adjust profiles etc? I understand the practical aspects of them but not the in-depth stuff required to be able to modify profiles, if you see what I mean. Des Hammill stuff?

Thanks

Rob
Cardiff, UK
philthehill
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Re: Best book(s) on SU tinkering

Post by philthehill »

Rob
I would suggest :-

Cars & Car Conversions...…………….Tuning SU Carburettors (including full needle charts).

Car and Car Conversions...…………..Tuning the Mini Pt 1 & More Mini Tuning Pt 2...…...Author of both Clive Trickey.

The later inclusive Tuning the Mini does not go into as much detail as Pt 1 & Pt 2 with respect to modifying the SU carb.

I obtained my copies off 'e' bay.

Phil

RobThomas
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Re: Best book(s) on SU tinkering

Post by RobThomas »

Thanks, Phil. :D
Cardiff, UK
MikeNash
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Re: Best book(s) on SU tinkering

Post by MikeNash »

Rob,
I've found "the science" behind SUs difficult to find but I have some ideas - however I certainly don't want to "teach granny how to suck eggs"! If I remember right you're an aviator and have a "spare" lambda fuel/air measuring device so you're ahead of most of us. So I'll plunge on.

One of the big difficulties of altering needles is determining which part of the needle is at fault and the only method I've seen described to find it is here https://sucarb.co.uk/technical-su-carburetters Scroll right down and you'll find a section entitled "piston lift measurement" using a manometer. I've not done it yet but intend to.
As a aside, many tuning manuals, etc state that the upper part of the needle is used at full throttle but, of course, if you demand full power in top gear at, say, 35/40 mph you may have your foot to the floor but the SU piston may only be half way up with the needle in it's nominal "cruising" and weakened zone. At this point the enrichment required is provided by the effect of the intake pulsations on the venturi that are now undiminished by the butterfly. These pulsations are easily changed when an engine is altered which is why different needle profiles are then so necessary.

Another need is airflow measurement for which I suppose modern mass flow systems would be the way to go. (I'm trying to grapple with one!) But a stab at it can be got by measuring the pressure drop down the air-cleaner intake pipe; so see the graph below from my previous (and completely standard) 1098. My intake pipe is one of those that is cut straight across and not angled and its been cleaned inside; I'd expect the orifice coefficient to be very close to one and the mass flow shouldn't be too difficult to derive. (Note, the two lines projected up in speed will intersect at near 75 mph, about right for my Trav! The speeds are GPS read.)

Another tool is fuel flow measurement which I've done by using a simple electro-mechanical counter wired to the SU petrol pump thereby counting the pulses with calibration to a given volume. (Alas, I've put my graphs of speed versus consumption "somewhere safe".) This technique works well on smooth roads but is upset by bumps etc due, I believe, to the sensitivity of the pump's valves. I'm modifying one now to overcome this.

Finally, I've found that the SU needs a degree of vibration to get the piston and needle to a consistent position for an given airflow. Running an SU on a "vacuum cleaner" suck rig it will stabilize over 1/8 of an inch for a given throttle opening and of course as the piston gets closer to the balanced position where the air pressure above and below it together with the effects of the internal spring and the piston weight are now all in balance, the actual forces up and down decrease almost to nothing. Thus even in a very free assembly the tiny frictions still present can have a disproportional effect, but on a running engine this seems to be overcome by the very small "shaking" from the vibration. (As an aviator you may remember the small electric motor vibrators fitted to bariometric altimeters in days of yore.)

My overall interest is in economy aiming at a 50mpg Minor. I believe its possible but each Minor needs its individual tuning due to the varying accuracy of their build. (Mine for instance has valve timing varying from cylinder to cylinder which'll have a marked effect on the intake pulsations.) And the first thing to do is to get a grip on what's gomig on inside.

Please let us know of your progress! Regards from MikeN.
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