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Old 08-14-2002, 05:38 AM
willys36@aol.com's Avatar
willys36@aol.com willys36@aol.com is offline
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If it is just for looks, go for it. "It is more important to look good than to run good!" However, if you want to enjoy driving the car just a little bit, I would advise against the tunnel ram. I designed (was in college taking physics at the time!) and built one of exhaust tubing and sheet metal in the early 60s B4 manifold makers started marketing them. I was real proud of it. Only problem is, it only ran worth a whistle at over 3000RPM! Tunnel rams are intended to turn a carburetor induction system into a pseudo-injection system. In all intake manifolds, there are pressure signals traveling up and down the intake runners, into the intake plenum under the carb and even up into the carburetor bores. These pressure waves are very predictable, can be positive pressure or negative pressure, and travel the speed of sound. All manifolds have a tuned optimal performance level due to these pressure waves, whether the designer intended one or not. By designing the runner length to take advantage of the pressure waves already in the manifold, one can achieve arrival of a positive pressure wave at the intake valve just as it closes. This gives a measureable boost in performance but only occurs at the designed RPM. Conversely, there is a speed at which the negative pressures arrives and performance suffers. Stock manifolds effectivelty squash these signals with unequal lengt runners and other tricks so there are no big positive or negative pressure arrivals. The higher speed range for desired boost the shorter the runners need to be to allow the pressure wave to reach the intake valve.

Tunnel rams are designed with a shoebox plenum between the intake runners and the carburetor so the pressure waves are isolated to the runners. This allows the designer to simply tune optimal RPM buy changing runner length. Optimum for a street engine is 18" to 20" - peaks @ ~1800RPM. Th runners in commercial units ar much shorter than that thus serve racing engines well at above 4000 - 5000rpm. In addition, that shoebox plenum really messes up fuel distribution at low RPM. You can have BIG differences between lean and rich cylinders at low RPM. The real killer in the setup you describe is the two 750 carbs. This is 4 times the carburation you need on the street for that engine.

All that said, I am a slave to looks and a tunnel sticking thru a hood is the best. If it were me, I would get a couple of quarts of Devcon plastic steel paste and mold the inside of the plenum to extend the intake runners to merge at a new fist sized plenum under a single 600CFM Holley. DON'T run with mechanical secondaries! Those huge carb(s) would just sit there and say "go get your own gas, I'm not giving you any!" I would go even further and offset this plenum under a single carb and put a second dummy carb on the manifold. That's just silly old me! Think that is stupid? Go see in my photo album the equal length, 6 individual runner, 180deg intake manifold I designed, made molds for, and cast in aluminum for a 230 Chevy 6. Talk about a LOT of work for dubious benefit!!

[ August 14, 2002: Message edited by: willys36@aol.com ]</p>
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