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Old 02-23-2010, 07:29 AM
Webb's Avatar
Webb Webb is offline
Join Date: Oct 2007
Location: Jackson, MS
Posts: 1,260
Default SJ Cowl/Plenum vs Vans cowl/baffling speed differences

I'm thinking about switching to the SJ cowl and plenum this summer. I'm wondering if anyone else has done this and has the performance numbers.
Webb Willmott
Jackson, MS
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Old 02-23-2010, 09:59 AM
yakdriver yakdriver is offline
Join Date: Aug 2006
Location: Twin Falls, ID
Posts: 683

I modified my stock cowl to SJ round inlets, ram air inlet and plenum similar to the SJ shorty cowl. I could never get my oil temps down and finally replaced it with a stock cowl and plenum with a Rod Bowers ram air inlet. My oil and CHTs run over 20 degrees cooler and my airplane is 10mph faster with the stock cowl. Don
RV 7 N212MD Flying as of 12/22/2007
Backcountry/TCOW Super Cub flying 03/12/2011
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Old 02-23-2010, 10:08 AM
rv72004 rv72004 is offline
Join Date: Nov 2005
Posts: 452

I put a sam James cowl on and found a 25 knot increase in speed. Also stall was down 10 knots. ............
Only kidding, did the exercise myself and found there could be a few knots in it depending on how well the installation is done. At the end of the day I opted for the standard cowl ,based on looks and ease of fitment.
One thing I have found is that the RV flies very nicely at low power settings giving about 7GPH and 135knots. I often putter around at these speeds.
I doubt the SJ cowl would make much difference at these lower speeds.
Sorry about not giving a proper answer, just thought my experience could be of some help with your decision.
RV7 flying
xp360, CS, All Glass cockpit
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Old 02-23-2010, 10:21 AM
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f1rocket f1rocket is offline
Join Date: Jan 2005
Location: Martinsville, IN
Posts: 2,284

I think reliable comparison numbers may be hard to come by due to the rigors involved with conducting repeatable and measurable tests. Most people agree that there is some cooling efficiency to be gained by various techniques over the standard cowl, including the SJ approach. I know of two builders who went with the SJ cowl and neither would do it over again due mostly to the work involved. Some people like the look of the round inlets and the minor speed gain is of minor consequence.

I think most of the gain that can be had from a SJ conversion can be gained in a standard cowl by using a plenum. If I remember correctly, some of the speed gain is due to cooling efficiency and some from the aerodynamic gain due to smaller openings and frontal area.

Bottom line, it will likely add a couple of knots of speed but the change is pretty intensive. I like to use plenums because it's the best of both worlds...improved cooling efficency without all the fiberglass work.

Good luck with your change. Take some good measurements before and after and let us know what you find!
Randy Pflanzer
Martinsville, IN (II87)
Paid through 2043!
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