Dual Quad Tunnel Ram

[50 F1]

On my 50 F1  I have front sub from a 72 Monte carlo.   The motor sits really low in the engine compartment. So it really looks out of place with a single 4 barrel carb and intake.  Years ago I got a good deal on a dual 4  barrel tunnel ram. It ia a Weiand intake with 2 edelbrock carb. I put it all on way back when and got it running ( have not drove it yet).

I have done a little reading on the HAMB. They have a good thread about 2-4's set ups. Everybody says use progressive linkage. My intake has the 4 runners coming up for the front four cylinders and carb mounting and 4 runners coming up for the rear four cylinders and carb mounting. The two groups of runners are not connected in anyway except for a cast in brace in between the two groups of runners.

How does progressive linkage work? Will it open both carbs at once or just the front or back leaving 4 cylinders not getting any fuel.

I know the truck will run better with a single 4 barrel but I really like the looks of this and am willing to put up with a few driveability problems.

I have it set up now with the linkage working both carbs at the same time. It works good with the pedal. If I can I  would like to leave it like it is because progressive linkage set-ups are not cheap.

I know I know let me have it    The good and bad of what I am doing.

Thanks Mike

[39deluxe]

With the front four cylinders isolated from the rear four you will have to run both carbs simultaneously like you have it. Otherwise only 4 cylinders will get fuel. Calibration can be a little touchy with this setup but it can be made to work well with some playing. Idle speed and mixture will have to be adjusted on both carbs. Butterflies will have to open together for smooth acceleration and part throttle driving. I had a '56 Chrysler 300 hemi in my '47 Ford that ran this way on a stock cast iron log type 2x4 intake.

Progressive linkage runs on the front 2 throats of the rear carb up to about 1/2 throttle then the other 6 barrells open at a rate that allows all 8 to be wide open at the same time. Early Corvettes, Super Duty 421 Pontiacs, 409s and most '60s muscle cars like 427 Fords and street hemis were progressive. This is a better setup for fuel economy and drivability but fuel distribution is not as balanced as simultaneous operation.

Tom

[kb426]

Is it possible to connect both plenums so you could have true progressive running only on the front carb until above crusing rpm? What manifold is this? This may not interest you but I have seen many setups running Holley 2 barrels front and rear.

[50 F1]

A couple pictures to show you what I got going

[50 F1]

another pic

[wayne petty]

whats important as described above..

equal throttle openings at idle.... and equal opening rates

how well do you understand AFB or AVS idle circuits and idle transition circuits..

if you flip the carbs over..  you will see a round hole below the throttle blades for idle mixture fuel to be pulled out of..

just barely exposed is the idle transition slot..  this is only to be exposed when you open the throttle slightly..   the idle transition circuits maintain the air fuel ratio between closed throttle and when you are well into the main circuits operation.. but it also effects rpm ranges from 750 to about 2200 on most motors. allowing the engine to maintain air fuel ration just above idle with the idle feed and the idle transition circuit feeding.. then the primary starts flowing but the idle transition slot is still flowing up till about 2200 rpms..    wow... i am writing a circle..















PCV flow will need to be divided between the 2 carbs somehow..  as it greatly effects idle circuits..

and all that i have posted.. only gets you to idle.. and to get it part way off idle..

you will need a vacuum gauge and 2 sets of primary metering rod lifting springs..

[Glen]

I ran the tunnel ram on my 27 with great success.  the car was lite so that helps.   I did not have progressive linkage.

I ran the two small holley carbs that are sold to work as a combination with the tunnel ram.

The secret is understanding what circuits do what.  If you can grasp what Wayne just posted and follow it you can have success.  When people blend these and start idling off the wrong circuit, things get ugly.

[wayne petty]

wow.. i did not realize how sleepy i was.

http://forums.chevyhiperformance.com/70/9510419/general-chevy-technical-discussion/edelbrock-2-4-tunnelram-rich-out-of-box-bbc/

http://forums.carcraft.com/70/9585390/general-car-craft-technical-discussion/bbc-tunnel-ram-tuning/

most of the info duplicated.. but almost everybody has a struggle to get these to idle without fouling the plugs out..  

and to get them to come off idle without backfiring because of going LEAN..

if possible.. please invest in an oxygen sensor bung in the exhaust..

a 3 wire oxygen sensor from a 94 chevy truck 5.7 application.. and a narrow ratio air fuel ratio meter..  the wide ratio air fuel meters are several hundred dollars..    you need to use a heated sensor when you install them down by the collector..   you can install the air fuel gauge and vacuum gauge in a rubber mount  power the air fuel gauge off a plug into the lighter socket..  a tap off the ignition power circuit to the oxygen sensor heater circuit..

the ideal thing.. is.. can you take it to 2500 RPM.. and slowly release the throttle.. can you hold any rpm above 1200 to about 1800 and accelerate or slow it down without it stumbling..

with mom gone.. perhaps i will get some time to drive over to edelbrock.. i have some questions to ask them in person about some of the tuning parts.. since i don't have a mule engine or a dyno or right this second a shop to work in.. i will have to drive the 20 miles over and knock on their door..

i had a look a while back at one of the rare 400 CFM carbs.. and the bolt in boosters filled more of the venturi.. the changes are less between the next sizes up.. but are replacement primary boosters available.. so one could order another set with larger or smaller idle feed restrictions..  to assist in tuning idle transition circuits to smooth out the flat spots off idle..

[50 F1]

Thanks for all the info. It looks like I have allot of learning to do to get this set/up working good. I am going to try though.

I am going to get it running this winter. This motor has not been started in about 17 years.

In 1997 we trailered our 30 coupe out to AZ to the Goodguys show. Spent a week out there running around with the car, had a great time. Put the car in the trailer headed home to Michigan, backed the trailer in the building when we got home and it has not moved since. Life and work takes up to much time

[rumrumm]

Wayne is right about the O2 sensor. Tunnel rams by nature run rich, and I found with mine numerous years ago, that 390 Holley carbs ran best on my 350. Having an 02 sensor would have helped immeasurably in getting them properly tuned. It looked awesome, but eventually I got tired of the low rpm throttle response, sold the eye-candy top end, rebuilt the engine, and went with a B&M 144 supercharger. WOW! What a difference that made!

[50 F1]

Finally got it running Friday night. It runs just OK Right now I have two problems. One is eratic idle. I checked for vacuam leaks and could not find any. The other problem is when I rev it up a little off idle it takes a long time to come back down to idle.

What I have done so far. Adjust the idle screws, just by ear. I have taken them down tell the motor starts to slow down then back them out a half turn. Also have tried taking them out tell the motor starts to slow down and then take them in a half turn.  I can get it to run real good and have it idling around 600 rpms and it will sit there for a couple minutes then it will start to idle higher then it will go back down.

I have run fuel pressure from 2psi to 6psi with no difference.

I put the accelerator pumps linkage on the lowest setting to give it less of a shot off idle

I have taken the carbs apart and adjusted the float levels and  float drops.

I made sure the slot in the base of the carbs was barely showing with the butterflys shut.

Both carbs have the stock (base is what Edelbrock calls them) metering rods and jets.

Mike

[kb426]

My first thought is you have a vacuum leak. I've had some carb problems mimick that before. Check your float levels also. My memory has left the building so that's all I have at the moment.

[50 F1]

Thanks KB   I have set the float levels.

One thing I have been curious about is for some reason when I put this together years ago I plumbed the PCV into the side of the tunnel ram plenum.  Would this have anything to do with the vacuum problem I seem to have?

[kb426]

As an experiment, unplug the valve and plug the manifold. Pull the pcv out of the cover so there's no back pressure and start testing. Is your connection in the common plenum in the tunnel ram?

[1800guy]

Sorry for a slight hi-jack, but re-reading this brought a question to mind:

Wayne - Do I read correctly that an O2 sensor is safe at a full 12 volts?  Can it run 12 v full time or is that just for set-up/testing?