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The Hummer Knowledge Base
Well I just got back from some wind tunnel testing, a few hours of which included testing the hummer at highway speeds. I have to first establish that although I do have a little experience, by no means am I an expert in this subject. The majority of my wind tunnel testing is an iterative trial and error process, loosely directed by the few fluids courses I took in college . Any conclusion I do provide are simply suggestions. The majority of the hummer testing was in regard to airflow over the hood, into the engine compartment, and finally under the truck. The truck in question is a 1994 four door hardtop, with aftermarket slantback installed. I have a milemarker mounted in the front bumper, using a bracket I made myself. It does closely resembles the mile marker bracket. The truck has the factory brushguard installed, factory spare tire carrier, short PVC snorkel (6 inches), and rocker panel protection. (no driveline protection) The truck was tested at highway speeds, on a stationary tunnel floor, and instrumented using various flow measurement and visualization techniques. I'll try to summarize some of my findings.
First off, as we all know, the Hummer has the aerodynamic qualities of a 6000 lb brick. The first few tests were to visualize the flow of air over the truck at various speeds. All I can say is WOW, things are TERRIBLE. In the aforementioned configuration, air initially strikes the front grill, moves over the hood and essentially "splits" to either side. The area in front of the windshield exhibs a very turbulent flow, in which air moves both to the sides and FORWARD. To get an idea of this try using your washer at 40 mph +. It should be noted that air does flow into the louvers, and very little is bounced upward as some seem to think. I'll talk about this a little later.
At this point we varied the length of the intake extension, to determine if air was actually being sucked out of the engine at higher speeds at certain heights. My conclusion is, if possible, do not extend the intake above the windshield. At higher speeds, with the intake above the roofline, a fair vacuum effect is created, essentially making it more difficult for the engine to ingest air. I have not had a problem with my NA, but those TD's out there probably need a much air as possible. CONCLUSION: Keep those intake extensions short.
We also removed the factory brushguard, to see if flow improved a bit. No such luck, the factory brushguard seems to have little effect. Next was an investigation into the cooling stack. First off I seperated the coolers from each other, usiing spacers, in an attemp to increase air circulation. A slight increase in flow around the individual coolers resulted, but no noticeable increase of flow THROUGH the entire stack (and into the fan shroud) CONCLUSION: seperating the coolers with spacers might result in a few degrees of temp drop, as it allows more potential paths for radiation. It is cheap enough to do, so if you have an afternoon free then give it a shot.
Next was an investigation of the aiflow at the back of the engine, in the hump area. In this area air tends to stall, resulting in "low energy" air movement. This results in the heat buildup in the hump we all have experienced. This area has the potential for some SERIOUS improvements. Converting this area into a low pressure zone would result in a vast reduction in interior heat soaking. I have a few ideas which I want to play with in regards to this. Unfortunately, tunnel time ran short, so it will have to wait. CONCLUSION: The design of the firewall/hump results in serious lack of flow into and out of the area. The potential for improvement is immense, and will probably solve most of the problems people experience. It is, however, a difficult area to experiment with, both because of space limitations and current design.
Next came some part swaps and experimental part installation. First was a prototype hood scoop we all have heard so much about. Well the things do work.. Essentially they just intercept the air flowing towards the windshield (on its way over the front edge of the hood) and channel it into the engine compartment. This method to increase flow into the engine compartment is UGLY, but it does get the job done. We are, after all, not dealing with a slippery race car here so results are what we are after. The only requirement for scoops is the overall height. As air passes over the front edge of the hood it is seperated from the surface by an inch or so.
The scoop has to be high enough to "grab" some of this air. I tested both a 3.5 inch and 5.5 inch scoop which I had welded up earlier. My reccomendation is simply try to stay near the 5.5 inch height. The difference between the amount of air channeled by the 3.5 and 5.5 is quite large. The scoop did have some other benefits. As it channels air through the radiator, and fan shroud, it forces more into the previously stagnate area along the firewall. This is not really the correct method to get air to the hump area, but the results are beneficial. If it ain't broke........
CONCLUSION: Hood scoops simply "ram" more air into the engine compartment. This results in some helpful imporvements. I can only expect that a hood scoop will help drop fluid temps, and aid in heat transfer. (by increasing volume flow rate past the heat exchanger you therefore increase convective heat transfer rate) Some other benefits include cooler air conditioning (see above reasoning) as well as heat reduction in the area under the hump. Note that these improvements only exist at highway speeds where the "ram effect" can exist.
****I am going to post my findings, using my 5.5 inch scoop, in real world settings in the near future.***
The final testing involved a prototype "scoop" for the transfer case/transmission. This idea can be attributed to Mike Selig. Well it too looks like it might be worthwile, but more difficult to impliment. I welded up a quick triangular scoop and tacked it to the transmission cross member. It hung about 6 inches below the level of the rocker panel protection. With was approx 14 inches. It definitly did "catch" some air, and deflect it upward toward the tcase. I have not tested it onroad to determine if it will actually drop temp in the case. If it does work, I am unsure to what degree it will be beneficial. Increasing airflow over the tcase results in an improvement in convective heat transfer. (Transfer due to a fluid (air) passing over a surface) To heat the surface, however, the fluid inside the case must transfer heat through the metal. (called conduction) With each type of heat transfer, however, there is an associated degree of resistance. ie Resistance from the metal to heat passing through it. Adding up these resistances heat transfer becomes less and less efficient. In my opinion, the best way to cool the tcase is with either a fluid to fluid cooler (from the factory) or an external air to fluid cooler (like the tcase).
I'll be sure to post my conclusions with the scoop tesing in the near future.
Well I've finished my degree, but you're right this would have been quite a thesis paper. Now onto business.
The underhood scoop was approx 14 inches long. It was simply a very ugly mockup, made out of some scrap material I had laying around the shop. For testing I simply tack welded it to the cross memeber. It doesn't get much uglier than that. The height was about 6 inches below the rocker panel protection. Your thinking is corect when you considered building up a low pressure are in the tunnel, which would draw air through. Unfortunately this is a very difficult thing to do. I am in the process of instrumenting my tunnel with an assortment of pitot tubes and pressure taps. I do have to build a larger manometer rig, however, so this might take a few weeks. I am planning on using a few strategically placed gurneys (they are simply a little L shaped pieces of metal) and vortex generators to move air around a bit and develope flow. I'm not too sure if this would work, since physically the tunnel becomes a high pressure reservior due to the design of the truck. If it does work, however, we all can say goodbye to the constant flow of air up the shift lever. I'm also going to play with the underhood scoop idea a little more. I'm considering moving it back, near the fuel tank (on diesels at least). Doing this I think it might be possible to virtualy suck air out of the front of the tunnel, with the benefit of also increasing convective cooling over the tcase/trans. I'll be sure to post my findings.