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Old 08-14-2002, 04:27 AM
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Post Street Tunnel Ram

Hey Ya'll,
I have an Edelbrock Street Tunnel Ram, that I want to put on my 350cid. I don't want to build a racing engine, but I wanted to put the tunnel ram on there just for looks. I was told that i could put two 750cfm carbs on it, but unhook the vacuum secondaries, and I wouldn't have to go much more than stock with my engine. Can anyone confirm this?

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Old 08-14-2002, 05:38 AM
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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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Old 08-14-2002, 06:06 AM
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Thanks for the advice! I thought the 750's my be too big, but what about two 600's. I got the tunnel ram and scoop in a simple trade and i really don't know what to do with it. I have a 1950 chevy truck that i am doing a frame swap on, and I thought it would look good having that big tunnel ram under the hood. The truck will not be a daily driver, nor a drag racer. I just didn't want to build a racing engine for something I didn't plan on racing. I may just try it out and if it don't run like i think it should, I will just go back with a single 4bbl and aluminum intake. If I do go through with this, what carbs would you suggest with a stock bottom half 350? I plan on putting a new cam and heads on the engine.
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Old 08-14-2002, 06:37 AM
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It'll run, just not very well. It may be great for an occasional cruise to Sonic but would not be a nice daily driver. The smaller carbs are a good idea. For a stock engine consider two Holley 390 4bbl carbs. For a never raced street engine, smaller is definitely better.
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Old 08-14-2002, 08:24 AM
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HRM had an article back a few months, you could find it online, it was the feature article on the summit street tunnel ram and it actually ran great-according to them with 2 600 holleys or was it 450's i dont have it here but it's online. i want to do the exact same thing with my 66 tempest and would like to know if anybody knows if somebody makes one that would fit a 326 motor.
IT'S NOT ONLINE!!! But it is in the summit catolog. stop me from recommending to anyone to go to the HRM site for old issues. all you can get to are the covers!! only current issue on line <img src="graemlins/spank.gif" border="0" alt="[spank]" />

[ August 15, 2002: Message edited by: bullheimer ]</p>
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Old 08-14-2002, 05:33 PM
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You can make this work using the 450s and run a vac balance tube or hose between the secondary housings and use the heavy diaphragm springs. you need atleast a 3.70 - 4.11 gear in the rear or so. This will work a lot better with a stick shift or a high stall converter,I have had a set up like this and run it right though the winters never changed a thing for 3 years on a 327 loved it!Thats not the only one i have set up over the years.Have Fun!
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Old 08-15-2002, 04:43 AM
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Bullheimer,
Where would I go to get that magazine article?

Roy63,
Are talking about leaving the secondaries hooked up? I was told to unhook the vaccum secondaries.
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Old 08-15-2002, 11:15 AM
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Old 08-15-2002, 11:55 AM
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Yes! the heavy springs stop the opening till higher RPMs, you do want it to have some balls when you stomp on it! RIGHT!
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Old 08-15-2002, 12:18 PM
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Everyone's different but I don't like leaving secondaries disconnected either. I don't want to take any chances on the secondary throttle plate siezing up from not being openned every now and then. The heavy secondary spring is a good idea. With all that carb you want to hold back on those secondaries till you really need them.
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Old 08-16-2002, 04:27 AM
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That makes alot of sense. Thanks for your advice!
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