Why is it not so pointless to compare the Cummins ISF2.8 engine to the Volkswagen 3.0 TDI
It's impossible to deny the Cummins ISF2.8 is quite a crude engine, as it caters mostly to commercial utility vehicles for which cost-cutting measures are given a higher priority than utmost sophistication. Due to its marketing approach more focused on the so-called "emerging" markets, being fitted to light trucks such as some versions of the Volkswagen Delivery trucks range, at a first glance it might seem quite pointless to compare it to something way more complex within a somehow closer displacement range, but once we look at a simpler engine with a different perspective it may highlight a handful of features which would otherwise remain overlooked. Even though some lower states of tune usually applied for the sake of improving reliability may render it easier to underestimate an engine like this, there are some reasons to consider the viability of some higher-output approach to render it suitable to fancier vehicles.
Considering their displacements as not too far to the point of rendering a direct comparison unfair, the Cummins ISF2.8 may look inherently disadvantaged towards Volkswagen's own 3.0L variants of the V6 TDI which is fitted to the Audi Q8 among other models. While the 4-cylinder Cummins resorts to iron block and head, a wastegated turbocharger and a chain-driven single overhead camshaft (SOHC) valvetrain, the TDI gets a pair of aluminium cylinder heads with double overhead camshafts (DOHC) accounting to a total of 4 camshafts and an electronically-controlled variable-geometry turbocharger. Besides the inherent weight penalty of an iron cylinder head, the smaller amount of camshafts already provides a decrease on inertia and internal frictions which could be already beneficial, and since both engines feature 4 valves per cylinder it would be pointless to point at the ISF2.8 as disadvantaged to what the intake and exhaust flows might concern. When it comes to bore and stroke, which are often pointed out as extremely relevant to how rev-happy an engine may become, the Cummins with 94mm bore and 100mm stroke for a 0.94:1 R/L (radius/lenght) ratio could seem not so unsuitable to operate at higher RPM bands because it's less oversquare than the TDI which 83mm bore and 91.4mm stroke lead to a 0.91:1 R/L ratio. Undersquare engines have a bore smaller than the stroke, square ones have identical bore and stroke, while an oversquare features a bigger bore, usually leading to an ascending order of how rev-happy an engine might be even though it's not unarguable.
Sure the turbocharger also plays an important role when it comes to the aptitude of an engine to reach a broader RPM band with the fixed-geometry one fitted to the ISF2.8 requiring a smaller size in order to avoid a turbo-lag even though it leads to some flow restriction at higher engine speeds. Meanwhile, the variable-geometry turbocharger fitted to the 3.0 V6 TDI has adjustable blades on the exhaust side leading to a quicker low-end response and setting peak power and torque to higher RPMs which may allow the engine to proportionately increase its performance. Even though the Cummins rated at 153 hp at 3200 RPM and 430 Nm between 1500 and 2400 RPM may not seem so attractive compared to a TDI with 228 hp at 3250 RPM and 500 Nm from 1750 to 3250 RPM, it's still worth to look at some eventual advantages from the 4-cylinder layout which could be conciliated to a different turbocharger configuration in order to remain competitive against a high-end V6 within a similar displacement class.