How to Improve LXi Engine Cooling Performance

[davidbrady]

I had a chance to do some testing with my LXi yesterday. The static pressures inside the engine compartment with and without the under-radiator shrouding and with the bus stationary are very similar:

Shroud removed; Fan override set to ON:
Engine RPM: 1200, static pressure: 0.40 inch of H2O
Engine RPM: 1600, static pressure: 0.70 inch of H2O

Shroud restored; Fan override set to ON:
Engine RPM: 1600, static pressure: 0.80 in of H2O

I limited the testing to 1600 engine rpm's because that's the highest my engine fast-idle switch would allow. With a helper, I've previously measured my fan speed at an engine rpm of 1862 and it was 1850 rpm's, so at 1600 rpm's I may not be at peak fan speed. Nonetheless, I think it's safe to assume that we're seeing less than an inch of water static pressure in our engine compartments at peak fan speed. A quick glance at the flow rate chart attached to post (9) of this thread shows that a static pressure of 1" H2O will decrease fan CFM by maybe 10%. This eliminates the idea that the shrouding is creating high static pressures.

The increased cooling performance I see is probably a bus motion thing. When in motion there may be a low pressure area develop under the fan. With the bus stationary we don't see a benefit. This makes sense because the increased cooling I've seen causes the fan to cycle less (actually, it doesn't cycle at all on my routine workout trip). IOW's even with the fan "coasting" the air flow is improved. This hints at a low pressure area.

The next test is to put the shrouding back to see if I can recreate the higher run-time temperatures. If I can show that the performance is markedly worse with the shrouding in place then I'll experiment with louvers stamped into the under-radiator shrouding.