I chose the James Cowl & Plenum for looks and expected improvement in speed & cooling performance. I really like the concept of controlling the pressurized space above and around the cylinders vs. the relatively un-specific space created by sealing the engine baffles to the top of the cowl. I also appreciate not stressing the cowl structure with ram air but rather controlling in the plenum.
Please read the Update & LOP Update at the bottom of this page!
Unfortunately the Plenum and Cowl do not align well at all and significant modifications to the interface are required.

Update:
As stated above “I chose the James Cowl & Plenum for looks and expected improvement in speed & cooling performance” unfortunately what I expected and what I got were two completely different things. The James Cowl and Plenum seem to have no actual engineering behind them. We spent our entire 40 hour Phase 1 fly off battling EXTREME temperature issues even after upgrading our oil cooler to the Airflow Performance 2008 suitable for engines up to 450 Hp and creating a dedicated ram inlet to provide measurably 10 times the cooling inlet air to the oil cooling fins. Even this was not enough to being temperatures down the acceptable levels. The real issue is poor differential pressure between the upper and lower engine deck. There is not enough cooling air passing over the cylinders fins and out the bottom to cool the engine forcing the oil cooler to work overtime. To help solve this we added 4 sets of louvers to the underside of the cowl to increase the outlet area of the lower cowl. This along with the upgraded oil cooler system made for high but flyable CHT/EGT’s. Good enough to fly regularly while keeping a close eye on temps. Even in cool OAT’s I regularly see CHT’s either side of 400 degrees at altitude (now with 110 hours on the engine).

So I did not get the expected cooling benefits but do I see any additional speed from this aerodynamic looking combination? NO. In fact based on discussions I have with Tim Olsen I think his standard PLANS BUILT -10 with factory cowl is a few knots faster than my set up. This may partly be due to the fact my plane is heavier (lots of additional goodies) and that I generally cruise at 21.5 / 2360 so I pull the power back significantly. But I also have 33 more Hp than Tim. My TAS is generally 158 Kts at 21.5 / 2360 7,500-8,500’ MSL.
This exercise has been extremely expensive in a number of different ways:

Cost of the Cowl/Plenum & additional effort to install & R&D modifications to both Cowl & Plenum.
Cost of 14” spinner ($1,100.00 to $2,100)
Cost of flying 40 Hours of Phase 1 at basically full rich during spiking fuel prices of summer 2008 (Average $5.50/gallon)
Cost of replacement Oil Cooler, adding Lower Louvers
Cost of climbing slower and flying with power back during warm OAT’s
Future cost of running an engine at/above 400 CHT’s
Cost to Design & Install custom Turbo Nozzles for LOP Operations (see below)

LOP Update:
One of my goals for N110EE was to fly Lean of Peak (LOP). LOP operations have been well discussed on the RV-List but they are essentially running your engine on the back side of the power curve reducing the fuel consumption by about a third while only giving up about 6 Kts of speed.

Real Life Example:

ROP 21.5 / 2360 8,500’ MSL 15 GPH 159 TAS
LOP 23.5 / 2360 8,500’ MSL 10 GPH 153 TAS

This represents HUGE savings in fuel burn with many additional benefits like significantly extended range and cooler engine temperatures because there is less fuel being burned as well as lower internal combustion pressures. All these should make for a longer lasting engine. Unfortunately the design of the James Cowl & Plenum have made it impossible to fly LOP without significant modifications to the fuel injectors. When attempting to fly LOP with a standard James set up the engine runs extremely rough as the mixture is pulled back. After much research and discussion with other -10 James owners we discovered the air pressure at the throttle body entrance on the lower deck is greater than the upper deck where the injectors are causing some fuel to actually spray out of the injectors onto the cylinders. A number of us found slight blue markings on the cylinder fins from this phenomenon. To solve this the only solution was to equalize the pressure between the two locations. This is a common issue on turbo charged airplanes with the solution being what are known as Turbo Nozzles that go around each cylinder injector and are pressurized with the same ram air that flows into the throttle body to essentially equalize the pressures top & bottom. The bad news is another $1,000 in materials plus added weight and complexity to the engine bay. The good news is this is a fix that actually solves the problem! The first time I tried LOP operations the engine ran EXTREMELY rough at 13.8 GPH. So rough that it felt like the engine was going to quit. After installing the turbo nozzles the very first time I was able to pull the mixture back to 10.8 GPH with just a slight stumble. This was a HUGE improvement considering we had not yet balanced the injectors and had a spread of 1.6 Gallons between the first & last to peak. Balancing the injectors should get me down to a differential of 0.2-0.3 Gallons between all 6 cylinders. While it was nice to actually install something that solved this problem the standard Vans Cowl builders have no issues running LOP right out of the box. No heavy and expensive additional items like the turbo nozzles to install. (Injector Balancing always recommended regardless of cowl)

One final comment on the Plenum. Between all the R&D for temp issues and LOP operations we have had to remove & replace the cowl & plenum MANY more times that the average builder. Removing the cowl is a pretty fast operation taking 4-5 minutes on, 4-5 minutes off. The Plenum is another story. It takes us about 15 minutes to remove the plenum and a little longer to reinstall the plenum. So what is normally a 10 minute round trip to remove & replace a standard cowl has turned into a 40+ minute round trip when one includes the plenum. I would consider this acceptable if I were to see other positive benefits like better cooling or additional speed but unfortunately the opposite is true and so the James Cowl & Plenum are a significant net negative to my way of thinking.

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