Most experience reports for the Jab 3300 Gen 4 indicate no problems with CHT's/cooling.  I had a Gen 2 3300 built in the transition to Gen 3 with some, but not all of the Gen 3 improvements.  On my 750 STOL, I went from a 7-row to a 10-row oil cooler and still ran marginally high oil temperatures.  (I'm in East Tennessee, so it's a temperate climate with no extreme summer heat, etc.) I then ducted bleed air off the left ram-air duct and enclosed the fins over the front of the oil sump and ran the ducted air through this and it remarkably helped the oil temps:

I have a Gen 4 arriving next month and have heard they are much easier to cool with the all-aluminum cylinders and increased fin area, so I'm thinking it's likely the oil temps will be cooler, too.  So, for those with a Gen 4 in a 750 airframe, STOL or Cruzer, a few questions:

  • How many rows oil cooler are you running?
  • Have you made any modifications to cool the finned sump such as create an opening in the top baffling of the oil cooler air inlet to allow air to flow up to the sump, or, cut an additional slot in the cowl feeding a duct to the oil sump (I believe Zenith has a kit for this?), or, have found that no additional oil sump cooling is necessary?
  • If additional oil sump cooling is necessary, are you in an extreme heat climate - Arizona, etc?

I likely can re-fit my oil sump duct to the new Gen 4 if necessary, but it sure would be nice if no additional cooling was required!



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Update: Asked Nick at Arion Aircraft (the Jabiru engine dealer) the questions above and said he personally had installed at least two Gen 4's in 750 STOLs, both having the "old style" lower cowl design (same as I have) before it was changed to a more efficient design several years ago, and that they had no problem with cylinder or oil cooling.  He recommended going with the 7-row cooler and not adding any supplemental cooling as it was likely it wouldn't be necessary and could always be added later.


I have the sump cooling duct on my Gen 4 installed with the rectangular cowling hole to feed the air through the duct and past the sump just like the factory demonstrator, but the only problem I have ever had is with low oil temperature.  I have the oil cooler taped over for winter and probably won't pull the tape until 50° or 60° temperatures.  If I had it to do over, I would skip the sump cooling until I had a reason to add it.   

Update, I now have a few hours on the new Gen 4 and initially, the oil temps were running about 210 or so in our cool spring weather.  Between the cowl opening and the oil cooler, I cut an approximately 1" x 4" slot in the connecting rubber shroud.  The slot is immediately adjacent the top of the oil cooler, allowing air to flow up onto the engine's finned sump.  This immediately lowered my oil temps into the 190'sF and no large increases in temps in steep climbs.  I'm thinking I might frame this slot with some aluminum to reinforce it and perhaps prevent tearing in the corners, etc, and also to provide something to attach a block-off plate if it's needed in the winter.


Have a Gen 4 that just went over 500 hrs in the Pacific NW, with the 7 row oil cooler.  I have had success with partially taping the inlet most of the year to keep it 190-200F, except on very cold days...then it has to be totally blocked.  On the few hot (90F +) days it becomes a tradeoff between a high enough ias to cool the CHT's and a reduced rpm to cool the oil.  Have made multiple mods to address the cht and oil temp to limited success on the high temp side.  Just cant get enough airspeed on the stol airframe on hot days.  I believe the extra 5-10 kts on the cruzer makes all the difference.  Considering just removing the slats for the few "hot" days here...

John, my Gen4 in the Cruzer runs hotter than I would like (210-220) in the summer. I am getting around 190 in the winter in PA. I have the original 7 row oil cooler supplied by Arion. I have been trying to figure how to cool it down a little bit. With the newer cowling there is not enough clearance for a 9 row cooler.  I was flying yesterday at 3000 ft, the OAT was 88 and my oil was 221,, I like your fix with the slot, maybe I will try that before I start getting crazy with NACA scoops and scat tubing. I was raised in East Tn, Fountain City, probably a Knoxville suburb by now.. John W.

I'd definitely try the slot - you can always block it back off if there is some unforeseen adverse effect or you need to block it for the winter months.


Thanks John, I will definitely try the slot, my fix has been to climb to alt for 70 deg or less OAT, not always a good option.. John W


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