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Permalink Reply by John Austin on November 21, 2013 at 2:04pm

Engines after #2331 have completely revised plenum, diffusers and intake runners. This gives much more even mixture distribution and even EGT's, which in turn results in more even CHT's. Combined with an X-vane insert (available from Jabiru USA) to straighten the airflow into the carb, the EGT and CHT spreads get even tighter.

Cooling problems are exacerbated in high drag, low speed airframes like the 750 - it doesn't seem to be much of a problem to get air into the ram air ducts, the problem is having enough negative pressure in the lower cowl to get the air out. As the engine installation manual suggests, extending the cowl lip to increase the negative pressure is helpful. After that, it's an exercise of trimming the ram air duct baffles, carefully sealing and containing the airflow in the ram air ducts so that it actually flows through the cylinder fins and cools them. Also, in "air-cooled" engines, actually a substantial portion of the cooling is done by the oil. Of course, an oil cooler is a must (supplied in the FWF kit), but most have found that ducting cooling air over the heavily finned sump is very effective, also. Zenith has a kit for a Jabiru 3300 to create an "oil sump duct,", but some have found it aggravates cylinder cooling because it dumps additional high pressure air into the lower cowl. I developed my own design that takes bleed air off the left ram air duct and ducts it to the oil sump - works great and no additional high pressure air is being introduced. In the winter, when I don't need the oil sump cooling, I route the bleed air to the heater for better airflow vs the rather passive NACA duct in the cowl.

I think the heater is adequate for Michigan - Pete (Jabiru USA) developed the heater to work in the Wisconsin winters before seeing the light and moving to the more temperate climes of Tennessee!

John

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