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Engine Performance

.. or put more correctly, the sensation of vehicle performance.

 As most of us know, the Auto Trans 928 models do have a number of different final drive ratios. (Differential ratios). Most Australian delivered were mainly supplied with the tall ratio. The best way to tell which ratio you have is to have someone poke inside the differential and count the teeth on crown wheel and pinion.

 A friend of mine calculated that his 928S '84 MY, if it could reach redline in top gear, would be doing over 320kph. This is obviously not possible. In reality, that car changes out of third gear at redline at about 235kph, but will not accelerate beyond that speed, as the revs are now too low in top gear (approx 4200) to generate the power required. A shorter diff ratio would be better suited to this car, but would possibly have an impact on fuel consumption.

 Manual cars do not have this symptom/problem for two reasons. Firstly, as a five speed, the ratios are closer spaced overall, and secondly the diff ratio is shorter on all manuals.

Which leads me to torque and where it is when you accelerate from a standing start. If you take for example the 310hp '84-'85 MY 4.7 litre 928S you will find its max torque is high up in the rev range. (4300rpm for a '84 MY).

 Just look in your manual at how much torque there is at 2,300rpm not a lot. If you look at the much-maligned lower horsepower (288hp) '86 MY 5.0 litre, its max torque is about 2,500 rpm. This is important.

If you do a Rear Muffler Deletion operation, you also get the sound and the acceleration!

I often hear the term RMB, meaning Rear Muffler Bypass. Remember RMB by definition means you leave the muffler there. I prefer to use the more correct term RMD for vehicles where the rear muffler has been removed.

 Note also that the '86 MY 5.0 was designed to run (from new) on 91 Octane unleaded as that was the highest rated fuel you could buy in 1986. Another reason for the lower quoted horsepower output. Premium unleaded 96 octane was introduced in 1987-1988 and then only sparingly.

 So if you run a '86 MY 5.0 on 96 or, even better, 98 octane, they certainly perform better and a bit crisper.

Bruce Buchanan
Buchanan Automotive
Unit 2/2 Paton Pl, Balgowlah NSW, 2093
(02) 9948 2651