We've had a few cars on the dyno & in for service this past month with problems caused by or caused by bad and/or practically defective coils that we thought, let's try again at a (hopefully) informative post!
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A coil is generally not a part that is often replaced preventively, while that is a good idea for more than one reason. When the car drives without noticeable complaints it is easy (and also understandable) to think that the coils are still good, while there are also noticeable steps in functioning between a new and completely defective coil.
If you google the recommended replacement period you will find enough sources and opinions but in general you can conclude that somewhere between 80,000-100,000 and up to 150,000 kilometers it is a good idea to install a fresh set. We strongly advise to be on the low side of this advice (so 80-100dkm) and to prevent that you are on the side of the road with a car that runs on 1 leg less, will misfire with possibly enormous consequences as a result. For tuned engines (and if you have had 2step launch control/flatshift or hardcut limiter programmed by us) it is even more important to be on top of this because more is then asked of the coils.
I have a lot of fun with it
We have seen so many brands in customer cars and tested them ourselves over the years, but the answer is very simple, the original Bosch ignition coils (1.4 t-jet, 1.4 multiair & 1.75 TBi engines) are the only ones we fit and recommend.
We have done 400HP (1.4 t-jet) on the original Bosch coils, we also tested Japanese (brand 'highspark') coils years ago on a Giulietta t-jet with forged engine without any noticeable difference on the dyno (in a fair comparison, both new) and all 300+ HP setups that we have built also have Bosch coils in them. Of course, so many people, so many opinions and we have also seen the kits with the NGK redtops ('Audi R8/2.0' coils) pass by (and we would like to compare them) but so far the original coils work best for us, we have not yet tested any other brand/type that gave a noticeable difference (both in 'feeling' and demonstrable performance).
And speaking of upgraded/performance coils, many people experience a difference that they wrongly attribute to that specific brand/type of coil after replacement. When your car has done 150k and you buy a new set you will most likely feel a positive effect of course, but the reality is that this effect would have been there too if you had installed a new set of original Bosch coils. So don't let them drive you crazy is our advice!
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As written above, there is still a phase between a perfectly working and defective, no longer functioning coil. For example, we recently had a Giulietta QV TCT on the dyno for software where the car, when accelerating (on the dyno) at low revs (so with practically no load, low turbo pressure), took 3-5gr ignition from all cylinders (even -7 seen on all, log not saved 
) which is 100% attributable to purely & solely old, worn coils (correct ones, from Bosch). An almost new set of coils in and the problem is immediately gone.
) which is 100% attributable to purely & solely old, worn coils (correct ones, from Bosch). An almost new set of coils in and the problem is immediately gone. Another example was from last week, an Abarth 500 135HP (IHI) that performed very poorly as standard (now that was for more than one reason, that in another post) but that car used to have NGK coils (which the current owner had very wisely changed to the original Bosch coils) that had probably been installed by the previous owner/garage.
After installation probably had problems with misfires (have already solved many complaints of bad idling, bucking, etc. by simply swapping the NGK's for the original Bosch) and instead of installing the correct coils, the diagnosis, detection of misfires in the engine computer was made impossible by turning off the associated error codes. Now that doesn't surprise us much anymore these days, but we hadn't seen this one before either. 
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If you are still driving around with the coils that were originally in the car and your car is 8-10 years old, has run more than 100k and you have the idea that it responds a bit more sluggishly to the gas, runs less nicely, etc. then consider installing a new set! This also applies if you open your hood and find NGK's, Denso's and other 'strange' brands..
Want to buy the right coils? We're here to help:
Alfa Giulietta:
Abarth 500/595/695:
