Does valve size matter?

[Ratau]

If we look at the cylinder head valve size on the 1982 6.2 heads comparing to the smaller ones off the 6.5 heads, which will give the best mileage using the same fuel settings if I install a turbo? It sound like the 6.5 heads gives more power but doesn

[TimK]

Desiel engines are just a big air pump and the rule of thumb is that the easier you make for it to breath the more horsepower you can create. Larger valves would make it easier for the engine to breathe. I haven't had a chance to compare the 6.2L valves with the 6.5L valves. I don't know why they made them smaller other then to increase the life or performance of the heads.

When adding a turbo you should turn up the fuel delivery from the IP in order to get the maximum power available from the increased air from the turbo. Keep in mind the diesel engine always runs lean. That is, on every stroke the cylinder gets a full gulp of air but only enough fuel to fire the cylinder. The turbo doesn't kick in unless the exhaust gas flow is high enough to spin the turbine. This doesn't happen unless you are feeding it fuel or running at high rpm.

Let's assume you have a truck like mine where I have added a turbo and turned up the fuel delivery. If I were to drive exactly the way I did before adding the turbo with the same amount of acceleration and speed going up hills, then I would not expect to see lower mileage. There may even be a slight increase in mileage. However, now that I have all that extra power, I tend to use it. I now find myself zipping up hills at 65 mph and stomping on the accelorator leaving stop lights. All that power feels great. I am sure that will contribute to worse mileage.

TimK