When is a tune a good tune ?

There are a lot of different factors that make up a good tune, most people concentrate solely on the easiest part of the tune and the bit that can be talked about down the pub POWER but there is a lot more involved than just the outright power.

WOT tuning

Wide Open Throttle tuning is the part that is most often talked about and what the average person sees as the outcome of any tune but in reality this is only a small part of what goes into doing a good tune.

Take the graph shown below, both tunes are making 267kw @ wheels peak power, but the important difference is that tune B is making more power on average than tune A due to the tuner spending more time optimizing the tune at every WOT load point to get the best fuelling and spark timing outcome, rather than just chasing the peak output number. In reality tune B will be a nicer car to drive, have more low down power and ultimately will be quicker down the quarter since it has more usable power in the area the vehicle spends most of it time.

Fuel tuning

In the case of the above vehicles (which are the same vehicle but with 2 different tunes in it) there are a total of 380 tune-able load points just in the fuel mapping. During the WOT tuning part of the session only 20 of these are accessed to achieve a good WOT tune, leaving 360 other points that still need to be calibrated for optimum performance, economy and that's just on the fuel matrix. Each of these load points has to be optimized to give the correct amount of fuelling for a balance of power, durability and most of all economy. The beauty of a good EFI system in that you can have the best of both worlds in the one package.

All too often we see a car come in that has been tuned and the operator has relied on the Fuel trims to fix the fuelling rather than spending the time to get it right.

Fuel trims

The fuel trims in the computer are there to compensate for minor differences in the factory calibrations and as the engine wears, they are not designed as a band-aid tuning tool that allows you to forget about getting your Volumetric Efficiency tables right, to achieve good fuel economy and drivability it is very important to get this so the fuel trims are less than 2% everywhere, if the engine has to compensate too much for inaccurate tuning it will overcompensate and use more fuel in a effort to keep the engine safe.

A little known fact is that these fuel trims are carried over into WOT as the designers of the system decided that if the engine thought it had incorrect fuelling via the trims it then carries these over into the WOT fuelling strategy to safeguard the motor , this has been one of the things that has fuelled the myth that the engine needs to learn the mods that have been done - basically the PCM sees an error has occurred and tries to correct the error, in a properly modified car tune this doesn't happen.

Spark Timing

One area we see a lot of damage occurring is in the amount of spark timing that has been put into tunes with the mistaken belief that more is better. this is especially dangerous in a supercharged application .

In the old days of carby Chev and Ford V8's they required a total timing of 38 deg to get efficient combustion, nowadays with efficient combustion chamber design and lean burn combustion less is best . The more efficient the combustion process is, the less ignition advance that is required to propagate combustion effectively, the modern V8 requires approx only 20 deg of timing at WOT to get maximum power, any more than this and detonation can occur. Add a cam into the equation and cylinder efficiency drops off at low rpm and more timing can be accepted by the engine.

Some tuners believe that it is alright to run more timing as the knock sensor will reduce timing to take this into account , this is  very dangerous practice as the knock sensor is a reactive device rather than a predictive device so for it to sense knock it actually has to see knock which in any motor is a bad thing.

The newer design engines have safety feature built in such as dual spark tables so if  any knock occurs due to poor quality fuel,  it will lower the spark timing  by a percentage to the low octane spark table so the engine will not knock next time it comes into this load are .Unfortunately a few tuners remove this feature when tuning in Speed density mode as it requires a custom operating system which cost them money to buy for each engine  , leaving the engine with no form of protection if poor quality fuel is experienced.

As with the fuel tables there is a full matrix of load points of the same size which only 20 of these are played with during a WOT tune the other 300+ load sites need to be addressed to gain optimum control.

Even at part throttle the timing curve is very important , too much timing and power is lost trying to drive the piston up against the combustion explosion ( even more timing and detonation results) , too little timing and engine efficiency is compromised causing poor fuel economy .The timing map requires very precise control for optimum performance and economy , being out by as little as 2 deg can have a huge effect on power, economy and engine durability

Cold start

One of the most important parts of the tune for long term engine life and economy is the cold start strategy , it is also one of the hardest areas to address properly as the tuner can only have about 1 min to log what is happening and make changes accordingly , with a cammed car this process takes days to get right - remember to give your tuner the car for a few days to get these issues sorted .When the factory did the original tune it took a team of tuners months to get a acceptable cold start routine in place , change the engine in any way and this whole area needs a complete retune to suit .Unfortunately harsh economic reality dictates that most tuners will ignore this area as the engine doesn't operate here for long  but it will have a big effect on the day to day economy of the vehicle.

Deceleration fuel cut off

This is another very important area where a good tuner can make some significant fuel savings by customizing when ,and by how much the fueling is cut during overrun .Too much too early and the engine will stumble when you back off , too little too late and fuel is wasted out the exhaust pipe.

Transient fueling

This is also a important area that can make or break a good tune , all too often we see modified cars that have had no changes at all made to these area's as it isn't part of the big horsepower number  but all these points must be addressed to get the car running to its potential. Acceleration enrichment needs to be finely tuned to suit the characteristics of the engine to achieve maximum performance and economy on the road at all times.

Idle strategy

Another very important part of the tuning process that gets overlooked , this is critical to get the transition from idle to part throttle right to help eliminate stumbles  , having the idle parameters right can take a considerable amount of the tuning time to get right but without this the car will be a pig to drive  , add a big cam into the mix and it gets harder to satisfy all these requirements as the engine becomes more inefficient at low rpm , to the point it will not like running at all below a certain rpm , careful tuning can help eliminate these issue to a degree.

Compensation factors

The PCM has a myriad of compensation factors to allow for changes in temperature and air density , these all need careful setting up to ensure efficient and safe operation under all conditions , just a small change like a tune done on a 20 deg day then not compensated for on a 40 deg day can have devastating effects.

These are just some of the engine parameters that need to be looked at during a tune , in total there are over 4000 programmable tables in most late model computers. It takes  a very good understanding of how each interacts with each other to obtain a good tune  .

We haven't even touched on the huge area of auto trans tuning control that must be addressed in a car equipped with a auto trans  or the finer points of a tune but only the basics in this article.

What most people fail to realize is that when the factory do their initial tune it is a process that takes a team of engineers months to create a workable tune .Change one little part of the engine such as the exhaust and a huge amount of work is required to get all parts of the tune operating correctly .Getting the WOT part of the tune right is easy and all that most novices ever address,  sure the car will still run ok but not to its full potential and it can be left in a dangerous state given the right conditions. The classic example is with the shortage of premium fuel recently , some tunes had been done without the protection of dual spark and knock sensing capability , instantly the owners of these cars where having huge issues and in a couple of cases engine failure resulted .

The only advice I can recommend to anyone contemplating a tune on any car is to seek out a qualified tuner that has been educated  on engine management systems and how has years of experience before they start practicing on your pride and joy. The computer is the heart of your engine would you go to your local GP for heart surgery or would you entrust the job to a specialist heart surgeon.

Source:

Chipmaster.

9452-0213.