Good Info
Bob,
You are absolutely right about the adaptability of different intake systems on the RV-4 (and Rocket). The original vinylester extended hub cowlings had a small scoop which the FAB would not fit. When Van introduced the FAB back in the 90's, many "old timers" were skeptical as to the performance gains/losses. The standard up until that time was a "home-made" RAM-AIR box fitted to the small cowl scoop. Even the larger scoop on later cowling only allows a very tight fit for the FAB. Based on the photos, the SJ product would have a difficult time fitting in a stock RV4 cowl in my humble opinion.
When the FAB was introduced, Van did extensive tests with the prototype RV4 FAB-off/on to prove that it worked every bit as well as RAM-AIR. The results were posted along with extensive test data in the RV8R. On paper, the FAB looks like a win-win.
However comma, when I flew a friend's FAB equipped RV4 on a long XC with the exact same engine as mine I noticed a definite loss of MP above 10K. Minimal, but even 1 inch is very noticeable up high. I decided to stay with my RAM-AIR and installed a by-pass with a K&N filter per the 89' RV-4 builders manual! I would simply close the by-pass when entering the traffic pattern and open it when climbing above 1000' AGL. Opening the by-pass always yielded 1-2" of MP increase. Many times above 10K I would experiment with airflow by closing the by-pass. 2" of MP loss was routine. I know it isn't a true FAB comparison since my by-pass was underneath the cowling with much lower airflow. My Rocket had a very similar system which worked even better due to the HR2 cowling design.
Now that thousands of FAB equipped RV's are safely flying, most builders view the FAB as an "acceptable compromise". Since the Sam James system also has a filter between the airflow and the fuel atomizer, I am doubtful it could match RAM-AIR any more than the FAB.
V/R
Smokey
Smokey, Thanks for the info, after reading your comments about history, I pulled out my 27yrs of the RVator and read the "Induction:Carb Heat and Airboxes" section from 1991. Quite detailed too. Not sure how much has changed or been discovered in 20 years though.
Vans Quotes: " Ultimately it seemed wise to start over and develop an all new, full time, filtered airbox." and further on...speaking of the current standard airbox.."Tests thus far have shown that this filtered induction system provides a manifold pressure equal to the Ram Air system."
More from RVator. Titled OUR FILTERED AIR INTAKE BOX:
"The new air intake systems we have just released for the RV-4 and -6 are based on the pressure recovery principal of achieving a high manifold pressure. In principle it works like this: There is a small air inlet in the cowl, about the same area as the carb throat, positoned away from the cowl so that it is exposed to free stream of air velocity. The air passes through the inlet at high velocity and enters an expansion chamber where the air velocity decreases smoothly so the energy of the incoming air is converted into pressure. A cylindrical filter is mounted approximately centered below the carb inlet. The air passing through the filter enters the carb from all directions promoting an even airflow."
So....Pressure Recovery airbox = same as Ram Air ??
When thinking about this and investigating it, seems to me the induction issues might be like cooling issues....all depends on your execution of the details. Like, how airtight is my induction system? not very, when I think about the connection to the cowling and how about the carb heat valve area, does it leak air too? If you are using the James induction system and looking for more performance, you may pay more attention to detail and the system itself seems to be more leakproof with the neoprene seal at the nose, etc. The James induction seems to do the same thing Vans does in regard to the Pressure Recovery, it just flows the air in a tube from the filter box to the carb, either system has the air turning or flowing a 90* turn into the carb.
While exploring options and seeing James induction system, I also found Rod Bowers induction system. I emailed Rod and asked about using his system on my -6 with a James extented cowling, his reply was interesting. Kind of follows Vans Pressure Recovery thoughts.
"Bob I have not tried to adapt my system to carburation. Field experience indicates that a Plenum and filter is needed to make the engine run properly. Carbs are mounted vertically and an adaptor would be needed to organize the air flow and attach the filter cannister. Rod"
Actually Rod's system is a ram air system with filtered bypass air. First thoughts made me think it might have been a plenum chamber like James's system. Honestly, I didn't think about the plenum chamber until I read the 20 yr old article in the RVator.
So what is better on a carb'ed set up in an extended James cowl?
A. Well executed Vans induction connected to James cowl intake ring.
B. James induction system connected to James cowl intake ring. (this is like the -7 you flew from Ill to Fla and liked it too.)
C. Bowers Ram Air system with elbow to connect to James intake ring. (seems this is closest to your RV-4 set up.)
I know better is subjective, really just looking for your thougts !
Thanks in advance for reading and helping me and others with this.
Personally, if your current induction/cowling setup is working well, I wouldn't change anything. However comma, since you asked, my RV performance improvements top three are:
