Filed under: 1992 Ford Thunderbird (Silver Thunder)
I’ve started diligent work on my new intake manifold. I have discovered that not only will it require a much longer supercharger belt, but I will also have to either relocate my coolant temperature sending unit (for the eec) or tap into the manifold and build the boss where it is on the stock manifold. In addition, I discovered that the thermostat housing is somewhat different than stock, so I will also have to modify that as well. It is all possible, but just adds to the complexity of the overall project. It’s already hard enough to weld to the impure aluminum casting, and this just means more welding. And, because of the nature of the casting, additional welding can potentially cause warping or even cracking of the casting. It will be critical on this project to insure that the manifold does not reach excessive temperatures and that it is not cooled too rapidly to prevent warping and cracking. I guess it’s a good thing that I’m not the one doing the welding, but my good friend who has a number of years (I don’t want to divulge how old this person REALLY is…) of aluminum fabrication. I will always defer to “experts” in certain areas…this being one of them. I’m attempting to limit the amount of welding on the manifold as well by designing a three piece manifold. The design is relatively simple. I have a lower manifold that bolts to the heads. On this lower manifold is welded a mounting plate for the upper manifold. The upper manifold will consist of two parts. The upper manifold is basically an open plenum that feeds the lower which will have runners that lengthen the intake runners of the heads. The second part of the upper is simply the top that will bolt onto the plenum and provide for a place to mount the supercharger in its (relative) stock configuration. I will have to rework the intake plenum that bolts to the upper plenum of the intake manifold and the intercooler tube. Based on my design, I may be able to incorporate it into the upper portion of my manifold. At this point, it is too early to say if I can do that or not as I have other steps that need to be completed prior to that design step.
My current effort is to design the upper plenum of the manifold to have enough volume to adequately feed the high flow capability of my heads without “scavenging” from the manifold. Of course, under boost, there’s really not a scavenging effect, but it can have an effect on manifold pressure and supplying enough air to adequately fill the chambers. The volume of the stock manifold is approximately 1 liter. I have not taken accurate measurements, but estimated based on a few measurements I have taken. Given the approximate volume of 1 liter, each cylinder displaces approximately .633 liters of air. Therefore the manifold does not have enough volume to adequately supply enough air to a pair of cylinders filling at the same time. Considering how fast valves open and close is relatively easy to see how a manifold’s volume can end up starving cylinders of air and as a result leave potential horsepower on the table. I estimate that a manifold volume of 1.5 to 2.1 liters of air is sufficient to provide enough air to fill the cylinders without incurring a large pressure change within the manifold. And, of course, that increase in volume (prior to the combustion chamber) will result in lower boost levels at the same engine rpm. And, given the fact that my new heads flow significantly better than stock or even my ported stock valve heads that will further reduce the overall boost pressure. That is good, because then I will be able to further increase boost pressure further increasing overall performance (although that may take the use of a different supercharger).
Ultimately, I believe my manifold design is on track to provide enough airflow to properly supply my custom heads. Obviously flow testing will reveal that assumption for sure. I still have a significant amount of work to complete before my design is ready for flow testing, but that is a significant portion of the R&D process.
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