TheDieselStop.Com Forums: Let's get the Duramax in on this "longest life" argument

The single largest weighted factor, and to a high degree, for peak torque production, no matter the rpm at which it occurs, is displacement. Forced induction acts a sort of false displacement, and complicates major factors such as where and how much TQ/HP is made because of wide variances in how induction tract and supercharging devices are designed and operated. For a 'given displacement AND naturally aspirated induction tract design', a main reason why the engine with the longer stroke and narrower bore makes more low end torque, is the effect of a stonger vacuum signal, if you will, created by the higher average and peak piston speeds, especially important at low rpm where the intake tract is being 'under taxed' relative to some average middle of the road engine speed the tract is compromised in design to operate at. For this reason the extra signal created by the longer stroke piston speeds, helps motivate sluggish intake tract air at low engine speeds. At some engine speed, this point differing depening on a mulitude of engine design factors, the larger bore shorter stroke will begin making more TQ/HP, due to lower frictional losses due to lower average/peak piston speeds for a given engine rpm no matter how the rest of thengine is designed. Remember, HP/TQ are tied mathematically. At a 'given rpm' if an engine makes more hp it also makes more torque and vice versa.

I do not have the statistics and would like to see them if someone has access to them, but one of the things often found in engines specifically designed to be of a longer relative stroke and often undersquare design is to have larger deck height (distance from the crankshaft cenerline to cylinder head/engine block mating surface) figures, so that longer rods can be used, to keep con rod length to stroke length ratios from getting smaller than reasonable. This reduced piston to bore side loading and therfore friction among other things. So, I know the stroke is longer and the bore larger in Cummins vs 6.0 mainly because a similar displacement needs to be attained (5.9L vs. 6.0L) with only 6 vs. 8 cylinders. The deck height of the Cummins should be significantly larger because the rod length is almost certainly longer even if the rod/stroke ratio is potentially smaller, which I am not sure of in the case but would like to find out. So, at face value, boost, fueling amount and timing etc., valve area/timing, relative piston crown surface area to stroke ratios etc. etc. not included, a 6.6L diesel should have more torque at most usable diesel rpm ranges than a 5.9L or 6.0L engine. Add all that other stuff in and it's a whole new ballgame, but given similar technology it still ought to be easier to make torque throughout the rpm range with the 6.6 or the 7.3 just given the displacement advantage. Now the fact that the induction layouts are different in all these engines advantages will show up. Clearly in most cases the new 4 valve setups are going to get more boost converted to actually getting air in the combustion than most of the old 2 valve designs. Boost in the manifold is still second to actual air into the CC on each combustion cycle whether low or high rpm or everything in between.