The answer to that question really is a matter of degree and what you want from your hood. The first phase would be to not worry about sealing the air path. Your new Suncoast hood will provide a clean path for the cold air from above the hood to justabove the intake box in the engine compartment. GM engineers ran some tests with one of our Silverado hoods to see if they would getboth a boost in Hp and a cooling effect in the engine compartment if they did not seal the path. Their conclusion was to not seal the path. They got enough of a Hp boost by providing that cold air in the vicinity of the air intake and blowing away some or all of the hot air from that area. That alsogave them theexcess cold air being forced into the engine compartment to cool engine compartment electronics. Their installation recommendation to their dealersis using an open air path. They want the extra cool air in the engine compartment so they can cool some of their electronics that are concerned with on their big diesel in the hot months. Running your hood and cold air intake without a sealed path is much like blowing on the hot coals of a campfire. They glow brighter with the added fresh air flow.
The next phase of improvement is segregation. Keep the hot air out of your intake. Of course, the better the segregation the better the performance. The relationship of horsepower to air inlet temperature is 1% of horsepower gain (or loss) for each 10 degrees (F) decrease (or increase) in inlet air temperature. With a ram air system, the effect on performanceand the level of separation basically becomes a function of speed. At a standstill, in addition to the cold air brought in through the ram air path and the normal air path, small amounts of hot air from the engine compartment could be sucked through openings in and around the heat shield. Once the vehicle is moving forward and the ram effect is pushing fresh cold air into the intake compartment, It will also eventually overpower any sucking effect from the engine and the excess pressure will not only push air through the air filter but also push the hot air away from any openings in the heat shield. The guys at Premier Performance have done some testing and found that on heavy acceleration, the intake air temperature jumped up as hot air got sucked into the openings in their cold air intake. Once they were cruising and definitely when they were slowing down, the ram air overpowered the demand and the intake air temperature dropped to nearly ambient.
The ultimate in performance is the next phase, a truly sealed air path. This is an air path where in addition to sealing out the hot air from the engine compartment we can contain that pressure generated by the forward motion and force cold air from the hood scoop through the filter media. The better the seal the higher the pressure it will allowand the better the Hp boost.
But now lets move beyond this “text book” answer in this last phase. Is your cold air intake sealed now? It can’t be. It is open to the normal air intake path. If you leave that path open, which is wise to do, and add another opening for the ram air from the hood, the actual pressure increase generated around the filter is due to the opposing force of the cold air from the hood flowing into the top of the intake and the mixture of cold and hot air flowing in from the fender well opening on the side or bottom. At a stand still and on hard acceleration, you’ve effectively doubled the “throat” of your intake. In other words, it has twice the area to suck air from. At speed, the cold air from the hood and the warm air from the fender well will quickly overpower any hot air from the engine compartment that leaks through the heat shields. As your speed increases, the cold air from the hood air path will eventually overpower the warm air coming from the fender. I want to run a test to determine if it makes more sense to actually close off the fender intake or should it be left open. As soon as I get some data, I’ll be writing a magazine article on it.
So back to your question on how well does it need to seal. Unless you plan on closing the normal air intake path, you don’t truly have a pressure vessel to gain 100% of the theoretical maximum ram effect at high speed anyway. Leaving the normal air intake path open allows for a ton of air available to the high flow filter allowing it to breath even easier regardless of speed (zero, low or high). Unless you’re going for that last 100th of a second in your quarter mile, a reasonably sealed air path will minimize the effects of hot engine compartment air at most speeds and provide a reasonable amount of containment for the ram air pressure. Also,leaving both air paths available will give your engine its maximum “throat”at all speeds while still providing a good ram air effect at higher speeds.
One discussion on performance with both sides open is available at http://www.clubtitan.org/forums/showthread.php?t=35580&highlight=ram+air
(Note: Many models of Suncoast Hoods interface using a sealed air path to the stock air box or through the use of a ram air replacement air box. These configurations offer the truest in ram air functionality. )