Sorry for the delay in responding, I have been away from the computer for several days. I will weigh in on the lower air duct from an engine builders perspective.
You should be running one; PERIOD.
Now, lets get down to the technical side of why you should be running one. The lower cooler duct was designed was back at the inception of the Legends series. Many people do not like them because they believe they "hold the hot air in".
The cooler duct was designed during the days of the non-Siamese cylinders of the 1200. There was air gaps between the cylinders and the closed off end of the cooler duct forced the air through the cylinders cooling them. I found on the 1200 that the rear tab should be bent away from the engine about 30* to get the best cooling.
Now, for use on the sealed 1250 engine with Siamese cylinders, you do need to open up the back of the duct for air flow. This airflow, because the duct is in place, contacted the cooling fins and the hot air is transferred out the back of the cooler duct. Depending on what type of racing you are doing determines the size of the hole.
I also recommend that you run an inline cooler fan in the hose going to the duct. You can run that fan when the car is in the pits, slow speeds under yellow or after the race to cool the engine. At race speed you turn off the fan so it does not act like an air brake.
That cooler duct is a requirement if you want to keep you cylinders round and the rings alive longer. Now, you should also be plumbing air from the bumper duct to the fins that are under the factory carb heat shield. The heat shield acts as a duct forcing the air past the rearward cylinders.
Guys, this is an air-cooled motorcycle engine that we have put into a car and turned it sideways. The rear cylinders need air to cool properly. If they do not cool effectively, you are costing yourself horsepower.
Now, I see this question has also lead to oil coolers as well. Please remember you have two things on this engine you need to cool, #1 the engine via the cooling fins and #2 the oil to maintain an acceptable operating temperature. I personally do like care for the upper head fan unit, because at speed the fans are working against the natural air flow. If you are really concerned about head temps you should be running the head cooler duct to force air the back two cylinders. 600 used to make them, but they do not anymore.
Now, oil coolers are used only to control oil temperature and I personally am not a dealer for any oil cooler company such as Setrab, Fuildyne, or XYZ coolers. I sell only what I have found to work the best and nothing else. If I sell it I use it. Why does 600 put on Setrab, because they are a Setrab dealer. Why does Dennis sell Fuildyne, because he is a dealer and what he likes. Coolers just like, what oil to uses, what carb kit to use, or what tire cutter to use becomes a personal search for what works best for you and does the job you need it to do at the price you can afford. Do you need to run a fan on a oil cooler, it really depends on the cooler that you choose to use.
Ask questions of the guys offering you the parts they are selling. Ask to speak to some of their customers, or post on the forum(s) for input from racers who use a specific product. I will gladly put you in contact with any of my customers and you can ask them how my cooler system works. They may not be able to answer the technical questions of why it works, but I can do that.
So, back to the root of the question...YES, you should be running a lower cooler duct with an inline fan and modified slightly for the sealed 1250 engine.
If you have any questions please let me now here or you can send me an email.
Randy - RPM
randy@rpmracingca.com
