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Old 04-21-2008, 06:25 PM
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Tunnel Ram on a Chevy 350

I have recently purchased a brand-new tunnel ram with two 600 cfm holley's for my GM 350, with double hump heads. I know the two 600's are too much for my motor, two 450 cfm would possibly work. My question is, since I already own the 600's will the engine perform if I only connect the one, and the other one just set there for looks.

Thanks, Mark

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Old 04-21-2008, 06:29 PM
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No! It will not distribute the a/f mixture to the other cylinders.
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Old 04-21-2008, 06:37 PM
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Quote:
Originally Posted by markedwarde
I have recently purchased a brand-new tunnel ram with two 600 cfm holley's for my GM 350, with double hump heads. I know the two 600's are too much for my motor, two 450 cfm would possibly work. My question is, since I already own the 600's will the engine perform if I only connect the one, and the other one just set there for looks.

Thanks, Mark
Tunnel rams are just plane doggie on the street, they are a race engine item and as such like to work way up in the RPM range. The problem isn't so much of excessive carb CFM (although its present) but is more of a lack of signal in the venturis. This is the result of several factors, the port runners being too big softens the signal, the plenum being really too big softens it more, and all the reversions from too many overlapping cylinders causes gaps and unwanted augments to the metering signals.

So it's really tough to get these things to function on the street with all this chaos going on. It can be made acceptable but it is a pain to set up. Basically you've got to reduce port and plenum volume. And still---------?

It works best with a manual gear box and really stiff gears out back, which takes you back to where I started, high revs.

Bogie
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Old 04-21-2008, 06:42 PM
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I was running two 600's and went to two 450's and it ran a lot better.My motor was a 11 to 1. In a 2300 pound 32 roadster.
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Old 04-21-2008, 07:31 PM
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Most tunnel rams have adapters to run a single carb. Never used one but it might be worth a try. If its a mild 350 you might be able to get away with one of the 600 carbs you already have.

Jordon
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Old 04-21-2008, 07:34 PM
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Even thou your 350 engine will not consume 1200cfm of air at WOT, the two 600cfm carbs will work. Vacuum secondary carbs are very forgiving because you can adjust the secondary opening rate to dial in the total carb air flow @ wot to suite your engines needs.
Your 600's may need rejetting. (start with the stock jetting) Because a tunnel ram manifold has more internal runner and plenum volume than a single 4bbl manifold has you'll need more accelerator pump shot volume.Your 600's will need the accelerator pumps cams and nozzles set up properly. Your 600's will need the Dual quad secondary vacuum top lids with the vacuum balance tubes to allow you to connect the two secondary vacuum ports together so they open in a balanced fashion.
You'll need the secondary vacuum spring kit to allow you to dial in the secondary(s) opening rate by selecting the right spring pair for your motors airflow needs. . The secondaries will not completely open at WOT, but thats ok. Once properly dialed in the carbs will open exactly as the motor needs as you floor the gas pedal.
You want to set the carbs up so they are even and the idle is shared by all 8 barrels. This will take some playing with to get it right. First thing to do is to flip the carbs over and look at (ALL of) the throttle blades. You want to preset them all the same (primary and secondary curb idle throttle opening).
This is your starting point. You'll probabily have to take the carbs on and off a few times to get the throttle curb idle setting right at idle for your motor.
Look at the throttle blades in relation to the idle fuel transfer slots.
The idle fuel transfer slots edge should be just exposed under the throttle blade edge at idle. They should all be adjusted the same.

A tunnel ram manifold by nature is a mid to high rpm manifold.
What it lacks (for street use at idle and low rpm) is manifold plenum heat under the carbs.
Normal conventional 4bbl manifolds under carb plenum area gets a lot of heat
from the motor and extra heat from the exhaust heat riser passages in the heads and manifold. This plenum heat greatly aids fuel vapourization and drivability.
So, your tunnel ram manifold will take a lot longer to "warm up" than a single 4bbl manifold does.
Once it warms up fully so the runners are warm, fuel vapourization (and low speed drivability) will be much better.
But until some heat gets in the manifold runners and plenum the fuel will not vapourize well at idle and cruise, making the drivability and idle quality a chore.
The fix is either manual or electric chokes.
The chokes will allow the flexability of rich(er)er fuel mixture necessary for good cold engine idle and drivability while allowing relatively lean(er) jetting and set up for once the motor and manifold is fully warmed up and fuel vapourization is good.

You want to set up and jet the motor to run well once its fully warmed up and the manifold feels hot and use the carb(s) choke(s) together to aid idle and driving quality when the motor and manifold runners are cold, untill the motor gets some heat in it and warms the manifold runners and plenum.
Cold manifold runners and plenum = poor fuel vapourization, rough idle, bad throttle response. (use the chokes to aid warm up)
Warm manifold runners and plenum = much better fuel vapourization, idle quality and throttle responce.

A tunnel rammed motor will want a very agressive ignition advance curve with lots of spark timing at idle. A stock distributor advance curve will not work.
That could require up to and including locking out the mechanical advance curve and running full advance at idle ( 34-36deg BTDC) + vacuum advance 10-15deg)

Once set up properly and allowed to fully warm up to operatiing temperature a tunnel rammed motor has very good drivability and makes big mid to top end power.
The twin 600cfm carbs with vacuum secondaries will "size" them selves to your motor once the secondaries are dialed in properly.

A tunnel ram motor will run fine with a stock or mild cam but needs a cam in the 225 to 260@.050" range to get best power (similar to what you would use with a vic JR manifold. Something like a comp 280H magnum or Comp XE284H-10 cam wil work very well.
A tunnel rammed motor needs a higher than stock stall speed torque converter and a high rear gear ratio to keep the cruise rpms up.
A 10" 3500stall converter and 4.10's will work well.

Last edited by F-BIRD'88; 04-21-2008 at 07:55 PM.
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Old 04-21-2008, 07:48 PM
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If you can do without the tunnel ram look for a street motor I seriously recommend selling the tunnel ram and getting a performance dual plane manifold.. search for test comparisons etc and you can find some intakes that will make you driving on the street much more enjoyable.. unless you already have a high stall converter, gearing, the know how to tune that thing right.. and a love high rpms, engine wear, and money for more gas. Tunnel rams can be rather cranky with the warm up as posted above as well. You can get dual plane manifolds that perform very well from off idle to over 6k rpms. the throttle response and power band were 90+% of street engine driving occurs is substantially better with the dual plane manifold..
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