Alan,
Thanks for putting some interest in my posts. You are right, english being unfortunately my 3rd language and not my mother tongue, my lexical is then sometimes (often..) limited.
I can see from your question that my answer was not enough detailed and structurated but I can also see , that you are mixing some considerations wich have to be considered separately.
Gear Strength :
I maintain that an helical gear offers better load capacity than a spur gear. This considering obviously a set of 2 gears with exactly the same specifications in terms of material, width, and module (n° of teeth).
Keeping it simple, the reason is that the helical gear tooth is effectively larger since it is diagonally positioned. Also the contact ratio is higher (number of teeth involved at the mesh point) than a spur gear assembly.
You asked for "references to some reputable technical articles that support your assertion "...My library is full of mechanical books but most of them are unfortunately in french or italian. Anyway , I have some books in english wich I consider as "reliable" and I am happy to recommend you this one :
Manual Gearbox Design by Alec Stokes - SAE International ( Society of Automotive Enginers) - ISBN 0 7506 0417 4
You can buy it here :
Amazon.com: Manual Gearbox Design (9780750604178): Stokes: Books
My edition is 1992, I guess some went after but I doubt he denied what he wrote in its first publication.
Also, Alek Stokes has been involved in designing Formula 1 gearboxes for over 40 years, so we may think he can be classified as a "reputable source".
On page 37 of the 1992 edition, we can read :
"Helical Gears :
Single helical gears are an alternative to the spur gear, for transmitting power between parallel shafts, but the action of the helical gear with its teeth cut at an angle to its axis is different from that of the spur gear whose teeth are parallel to the axis. Standards of accuracy being equal, helical gears are superior to spur gears in the quietness of operation and load carrying capacity. As a result of the angular displacement of the helical gear teeth, the contact with the mating gear will run diagonally across the tooth face and not parallel as with a spur gear. Thus, the tooth engagement and load distribution is gradual and therefore quietness of running is an inherent feature of helical gearing and shock load is practically eliminated. This becomes a major advantage in cases where speeds are too high for the successful application of spur gears....."
Also "Google is your friend", and a quick search reported many links like this one :
What Is the Difference Between Spur & Helical Gears? | eHow.com
So with an helical design, we need less material for a predeterminned load capacity than with a spur gear. The diagonal design allows a thinner gear. Thus, the rotating weights are decreased and , beside the quietness benefit, this is why you find helical cutting in synchronized gearboxes as the less inertia the better to allow quick synchronization.
A negative point is the axial forces created because of the tooth angle wich then ask for a reinforced housing and bearings, so here weight and more friction (bearings).
Also, because of the angles there are friction losses and so the efficiency is affected.
Efficiency :
Spur gears offer a higher eficiency than helical. A common acceptance when doing raw calculations, for example to determine the reachable speed of a car, then to determine the gear ratio sequence is to take a 85% efficiency for an helical gearbox and 90% for a spur gearbox.
Obviously these numbers are not definitive and only the ones got from the dyno test are the truth but they help for the very first calculations.
For example, taking a gt40 of 1057 kg with the following specs :
Frontal area : 1.30989 square meter
Drag coefficient : 0.39
Air density : 1.22 kg/m3
Tire resistance coefficient : 0.015
Other resistances (brake pads, steering move ..etc) : 0.003
and to not make it too complex 0% percent Grade.
We can then calculate that for a speed of 280 km/h, the opposite drag would be of 2072 N.
With a 0.85 gearbox you will then need 254.2 hp to maintain this speed, and with a 0.90 : 240.1 HP. Considering , for example, that 254.2 hp is our maximum engine output, using a 0.90 gearbox you would be able to reach 285.7 km/h and only 280 with a 0.85 gearbox.
Also because spur gears do not offer axial thrust, housing can be lighter, and you don't care about gear inertia as the dog engagement is a direct clamp.
In a world where racing teams are chasing the best efficiency it is quite a normal move to go to spur gears, but you will notice that in racing gearboxes ,crownwheels are most of the time helicals ...
I hope to have clarified my previous answer.
Greetings
Stephane