Tuning power/economy with a limited budget

	Mixing it on your Own Dyno
	by David Vizard

	The average home tuner's most frequent problem, if the letter
	we get are anything to go by, is getting the fuel/air ratios
	(mixture) right after having tinkered wit hthe motor.  After
	printing this article, we do not expect to get, ever again, a
	Technical Query asking what needles or jets to use on a
	modified motor, becuase this really should be the end of your
	mixture problems!

	When tuning the engine, it is more than likely that the
	fuel/air demand of the engine is changed.  Becuase of this it
	is neccessary to "calibrate" a carburetter so that it passes
	fuel into the engine in the correct quantities for the amount
	of air being consumed.  Over its operating range, a
	carburetter may have to pass a volume from say two to two
	hundred cubic feet of air a minute!  At tickover, the
	consumption of air is very low, but at full throttle the
	opposite applies.

	At every point between the two extremes the air flow
	requirement is different, yet at all these points the
	carburetter is expected to mix the correct amount of fuel with
	the incoming air.  There is little point in trying to set up
	the mixture at tickover becuase very few, if any, of us drive
	on tickover and there is absolutely no guarantee that the
	mixture is right throughout the range.

	How do we know or how can we tell what the mixture is like
	anyway?  Up until now, the most common method has been to
	"read the plugs".  If you have had years of practice and are
	an expert plug reader (not the print on it) you can just about
	pull off the job of mixture setting.  The trouble, however,
	with a plug count is that one is never too sure what the plug
	is saying.

	For instance, a dark, very slightly sooty plug can look like a
	mixture that is just a shade rich, but it could be that the
	mixture is just right and the plug type is too hard (runs too
	cold) for the motor.  Unless you are an expert then, reading
	the plug is, to say the least, just shade dodgy.

	An alternative to reading a plug is to get your engine set up
	on a dynomometer; either the rolling road type or the engine
	type.  When an engine is on a dyno, it can be run at various
	rpm and against various loads, thus simulating the conditions
	met on the road.  While all this is going on, an electronic
	gas sampler can be analysing the exhaust and indicating the
	fuel/air ratio.

	Going from reading plugs at a dollar a time to using three
	grand worth of dyno might seem like going from one extreme to
	the other or to use a comparison, going from an abacus to an
	electronic computer.  What is needed to use the analogy again
	is a slide fule ie: a method between the abacus and computer,
	or to come back to reality, a method of setting the mixture
	which is more akin to the dyno and mixture analyser than the
	plug reading method.

	A dynomometer measures horse power, commonly called brake
	horse power becuase a dyno is nothing more than a brake,
	calibrated to read out the work absorbed and the rate of
	absorbtion.  Here we have a clue; all cars have brakes (or
	should have) so what we have, in effect, is an uncalibrated
	dyno fitted to our car.  To simulate road conditions whilst
	standing still, all we need do is to jack up the driving
	sheels of the car, put it in gear and use the throttle as if
	we were driving along the road.

	To simulate road levels we need only apply varying pressure on
	the brake pedal.  Such action will, of course, get the brakes
	hot.  For our purposes we need to be able to hold full power
	for about fifteen seconds maximum.  If your brakes cannot cope
	with this from the heat point of view, then you are sadly
	lacking in that department so see to it.  It's probably better
	brakes you need and not more power!

	Okay, so we have our dyno.  The fact that it does not read out
	in horsepower is, for our purposes, irrelevant.  What we need
	now is a mixture analyser.  Up until a few years back this
	would have cost a tidy sum.  These days we have a device known
	as a "Colortune" and within the price range that can be
	afforded by the enthusiast, this is the _only_ device we know
	of that will do the job in hand.

	For those who man not know, a Colortune is a device which
	replaces the sparkplug in the cylinder.  The top of the
	Colortune is made of a Borosilicate glass and this allows you
	to see what is going on in the mixture combustion chamber.
	Different mixture strengths burn at different colours.

	By looking into the combustion chamber through the Colortune
	we can get a good indication of the mixture strength
	prevailing.  Inspection of the flame color shows that four
	fairly distinct stages occur.  when the flame colour is
	blue/white, the mixture ration is between 16 and 14:1; a blue
	colour indicates a mixture strength between 14 and 12,5:1; a
	blue/orange colour indicates 12,5 to 11:1 and orange indicates
	11:1 or less.

	The Colortune, then, is indicating at the colour transition
	points the mixture strength of the ingoing charge.  Maximum
	power occurs when the fuel/air ratio is between about 12,5 and
	13,5:1.  The exact point varies from engine to engine, but
	most cast iron tuned production engines seem to be best around
	12,8:1.  The best economy is achieved on weaker mixtures than
	that giving maximum power, and fuel/air ratios between 14 and
	16:1 seem to be the easiest on the pocket.

	To set up the mixture in the manner about to be described you
	will need an accomplice.  The first and essential step is to
	part the car in some place which is poorly lit, so that you
	can see the combustion colours.  Jack up the driving wheels of
	the car until they are just clear of the ground and in the
	interest of safety, securly blockthe car so that it cannot
	move under any circumstances.

	At this point, warm up the engine, the remove a spark plug and
	replace it with a Colortune.  Set up the mixture so that you
	have a fuel/air ratio of about 12 to 12,5:1 (orange/blue) at
	normal tickover revs.  If you have multiple carbs you will
	have to do this for each cylinder or set of cylinders having a
	carb.

	Next, get your accomplice to put the car into gear, usually
	third gear is best, and increase the throttle opening but at
	the same time put on the brake.  (Continue opening the
	throttle and increasing breaking pressure until your
	accomplice ends up with the throttle wide open and the revs
	pulled down by braking to 2000 rpm).  You can now look ath the
	Colortune and at this point it will reveal what the fuel/air
	ratio is under the prevailing conditions.

	It can then be noted, preferebly by colour rather than
	reference to its fuel/air ratio.  After this, let the brakes
	cool for a few minutes, then repeat the procedure at 3000 rpm
	then at 4000 rpm and finally at 5000 rpm, stopping to let the
	brakes cool between each run.

	As far as brake overheating is concerned, it should not take
	more than fifteen seconds to ascertain the mixture ratio at
	each rpm interval, so they will be well within their capacity.

	Once you have an indication of the state of affairs of the
	fuel/air ratio up the rev range to 5000 (5000 rpm is the limit
	on the Colortune) the necessary corrections can be made to get
	it right.  Not only can the full throttle conditions be
	catered for in this manner, but so can part throttle and
	transient conditions.  For instance, a hesitant pickup when
	going from parth throttle to full throttle could indicate that
	the mixture is too weak during the transition from one state
	to the other.  During the transition period and for a short
	while after the Colortune should show a rich mixture
	condition.  If it doesn't, then you can bet your life that on
	a fixed jet type carb, the accelerator jets or pump stroke are
	inadequate.

	On carbs like the SU and Stromberg CD variety, a lean mixture
	during the acceleration phase would indicate that the damping
	is insufficient.  A thicker oil is usually required to
	compensate this.

	A couple of cars were used as guinea pigs to test the method.
	Both cars were modified and therefore required different carb
	settings.  In each case the carburation has been originally
	set up by the owners who had only an average working
	knowledge of what was required.  After use of the Colortune by
	the mothod just described, both power and economy were better
	between 5-8bhp and 10-15 miles per gallon.

	The reason that consumption was so much better after use of
	the Colortune stemmed from the fact that the mixture was
	originally set rich for maximum power and unfortunately
	becuase of the guesswork method of setting, it was too rich.
	The performance increase was better than the power increase
	alone suggesting that the mixture was right (within limits)
	throughout the rev range.  As a side effect this led to a
	smoother running engine with a snappier throttle response.

	To sum up, the Colortune proves to be a very useful device.
	It can, at a price of 4-87,5 UKP easily justify its place in
	the tool kit of any self-respecting enthusiast, and its
	intelligent use can only bring about an increase in
	performance.

	Editor's note:

	Whilst David Vizard has used this tuning method successfully
	with both a Mini and Austin 1100 its use on many other types
	of car could possibly be dangerous.  Firstly therefore we
	recommend that the suspension characteristics and drive
	shaft-prop shaft geometry of the car in question be checked
	carefully ebfore even considering use of this method.

	On rear wheel drive IRS cars which may have large wheel
	angularity, attempt to jack the car at points on the
	suspension that will allow the wheels to assume a position in
	angle similar to that which exists in normal use.

	On a non-IRS rear wheel drive car jack under the springs on
	either side at the axle location point.

	Make sure jacks are very secure and will not move under
	testing vibration, also make sure any car to be tested has
	alternate props underneath in case of jack failure -- ie:
	spare wheels and tyres which should be first tested using the
	full weight of the car.  One advantage of the DV method is of
	course that during load tesing the wheels on the ground have
	the brakes applied.

	On a Mini the best method Vizard found was to put a piece of
	wood on top of trolly jack lift point and jack up from the
	Mini sump.  Once having jacked car up, jam wheels and tyres
	underneath car for safety.  Car does rock but cannot thus fall
	over.  Drive shafts will assume peculiar angles during testing
	and this can be minimised by supporting bottom suspension arms
	on axle stands --these can however move and you may have to
	take the risk of the odd shaft angles for the few moments of
	testing.

	This method does not apear to be practical with swing axle IRS
	cars ie: Herald, Spitfire, Vitesse Mk 1, GT6 Mk1.