Paul yes I am going to try 35" tyres legal side disregarded here for the moment but I will do the basis for my own experiment. Gearing is about response and feel so its very much engine torque and power related and at what point the torque peaks. For my engine which is now different with its torque response and amount of torque. I have decided to venture into loading up the rig using gearing hence the turbo will respond to this different load. So I have chosen to use 3.9 gears on 35" tyres giving me a 2050 ish rpm for 100 klms. Obviously this will make boost a bit higher at cruse speed, but if torque is high enough and volumetric efficiency is high enough there is the potential to gain economy and still have good 80 to 120 klm passing times. But also because I am using a GTX3071 I will have my boost level in its lower sweet spot hence torque rise is higher and I have the bonus of pre water injection to make the engine/turbo respond to change rather over fuel added to gain this response. This sort of thing can be improved in the lower revs/ gears if the cam and turbo is matched, there is always going to be a under boost point unless we use a switchblade type designed turbo with I am working on for future improvements.Originally Posted by RLI
But basically for this first bit we try and match our engine torque to our gearing, diesels have the added bonus of producing a wider more consistent torque curve so there is the ability to match our torque to the gearing to make the turbo more responsive and progressive so the rig feel a lot stronger without having to add higher revs and power KW's to do so.
Which brings us to your 6.5 which as I have told you is a lazy slow short torque engine, This is not a terrible thing but hopeless if you want a manual gearbox or a manual gearbox with only 4 gears and over drive. There is not enough gear ratio to spread across the rev torque range. This becomes even worse if you are trying to match a std type turbo too this engine, as you just don't get enough rev or engine load time to get the turbo to build torque to its potential because you are constantly running out of rev duration time.
The Allison auto box is very special it doesn't function like a normal old school auto where by it has to change gears in sequence of have oil flow to adjust its load transfer. So what we have here is a gearbox that is always in its right gear or can be adjusted to be in it right gear always. It sort of doesn't change down it just is in the lowest gear required for that rev/torque point of the engine when adjusted, then changes up as the torque drops off not so much engine rev related. The military combination of the 6.5 is very near as good as the duramax/Allison but not as good because that engine has many electronic to adjust and control how the box changes gear. If you ever get to drive a duramax with Allison or the newer Merc BMW you will understand about always in the right gear and near undetectable up changes its all so effortless. The military 6.5 has this type of gearbox and works with the same effortless moving the rig forward ability, sort off lol. But even for this the 6.5 is still to short in its torque response or range to be ideal so doesn't feel so effortless as the duramax combo. But I can tell you the military spec Allison combo will make the 6.5 slug feel so much stronger and will allow the turbo to actually find its legs to be a benefit.
But it will still need gearing to be selected to get the 6.5 and turbo enough time to build torque to revs to get the best potential relative to the weight of your rig. Which brings me to my other experimental very important criteria as to why I have chosen 3.9 gears to 35" tyres, gross weight, my rig will not weigh much more than 2200kgs fuel and water. For example my rig with the KKK which was a very quick and responsive rig to drive but it weighed 2750kgs with this weight loss with the KKK and my 180rwkw I have given back to my engine a full 65rwkws. So I can use some of this unused torque to drive my turbo into its sweet spot for longer so to speak. I know its a difficult subject but I am sure this will explain the concept.
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