Tunnel ram carb theory

I bought my first tunnel ram today and I'm excited as hell to get it on an saw a hole in my hood, but first I need carbs. I've been doing some searching and as far as I can tell there are two schools of thought as far as tunnel rams are concerned.

School 1- Two small carbs- This school says that two 390 or 450 holleys will work best becuase of the combined flow. This school also tends to say that vacuum secondaries are best because the motor will only use as much fuel and air as it needs.

School 2- Two big carbs- This school says that if it runs good with a single 600-750 carb, then it will run good with two. They say that it will make more power without any sacrifice in throttle responce. Some of them also say that mechanical secondaries work fine because it will give the motor as much fuel and air as it can take.

The engine in my Comet is a stock bottom end 289, but the ultimate goal is a forged 331 stroker. Currently I've got a 600 edelbrock mechanical secondary carb on it. It has Edelbrock heads with 1.9/1.6 valves and the cam is 224/234 @.050, .496/.520 lift. It's backed by a T-5 and 4.11 gears turning 235/60-15 tires. It's set up as a street gasser and is stripped down quite a bit. I figure 2850 lbs with me in it. It is a street car and I drive it a lot.

What are your thoughts? Would love to hear from the guys running a ram on the street.

 

I run one carb on my 302 tunnel ram in the Ranchero .If you plan to drive it much i wouldnt over carb it for sure,and i mean drive it.I dont mean backing on and off trailers.Im talking about sitting in traffic ,driving in 20 degree weather.All of which i do with mine.The carb thing isnt what kills low end ,its the size of the plenum or volume under the carb .I used the same carb on mine with the torker intake ,and all i did was switch to a tunnel ram .It lost its balls of an idle for sure ,but more than made up for it above 4000 by going to the tunnel ram.I am running a 4 spd and 4.57 gears now so it seems to be srong of an idle. I think with the 4.11's and stick shift you wont notice it as much as you would with highway gears and an auto .It all depends on what your gunna be doing with the car .Race only (2 carbs for sure and big ones),race 95 percent of the time but a tiny bit of street driving (2 small carbs or one biggun) ,driving it daily in any and all conditions (use the same carb you have or maybe a 650 with a bigger shooter and 50cc pump ,and maybe jet it up in the primaries a bit to help smooth out the stumble ,then drive the crap out of it).You have to decide what rpm range its gunna be used in most of the time to decide what carb / carbs is best .Running two 750's on the street with that little 289 is gunna be worse that if you had a 600 on it.Thats why tunnel rams got a bad rap on the street is because people simply get carried away with too many cfms .Just my opinion here .Some people tell me that a tunnel ram doesnt look right with only one carb.These are usually the guys that store their cars during the winter months and only drive their car on and off a trailer.Meanwhile im cranking my tunnel ramed car at 5:30 am in 20 degrees weather in Janurary and driving to work .I dont like to change fouled plugs all the time ,so i dont run too much carb .If it was a race only car i would run 2 carbs for sure...

 

Probably the best thing you can do, is to size the cfm rating for the 289-302-331, and carbs accordingly. Too much will wash oil off the cylinder walls, give poor throttle response, and generally loook grand. All depends if you are building for show, or for go...a real driver. I now size them for the intended driving and engine's disp.

 

Its a driver. I'm out there cranking it and waking up the neighbors at 6:30 on my way to work. I deleted the heat and they salt the roads here so it's a summer driver but still a driver. I'm sticking with duals, cause I love the way they look.

Shocker, your car is awesome, I gladly listen to someone who drives something like that.

All the advice is appreciated. Exactly what I'm looking for.

 

No problem ,and i agree 2 fours look coolest ,but i've been there and done that on a street car that i drove daily.Maybe you will have better luck than i did .BTW i like your Comet too...

 

What kind of T ram did you get ? You can always buy another top and switch back and fourth from dual 4's at shows and races to a single the rest of the time.You can change the top in about 15 minutes ...

 

I just got back from a little road trip with my hot rod. The motor is a ZZ4 crate motor with a ZZ430 hot cam kit, Dart pro 1 heads with 200cc runners, 3.73 posi, 700R4 over drive tranny, tunnel ram with twin 450 Holley carbs. I changed the power valves to 4.5 and still need to put 50cc squirters to help with the slight off idle stumble.

I do have to brag about the fuel mileage on the highway. I got 18.5 MPG. That's better than my daily driver. But I also have to say that the fuel mileage in town realy sucks.

 

Didn't Edelbrock make a device that went in the plenum and called it a Turtle?

Since tunnel rams give a weak vacuum signal to the carb, I wonder if it would help to install a device in the plenum that didn't interfere with flow, but would cut the plenum volume down for low-speed street use.

The right size triangular shape would be easy to do for most.

The medium riser manifold with dual quads on my late Buick engine worked well winter and summer as well as idled at 600 rpm with a fairly big cam.

Aided by the large 462" engine and MSD ignition no doubt.

And maybe running a tunnel ram on it would be easier than it is on smaller engines, but I do think an avenue to pursue is the smaller plenum volume for street use.

 

although a differant animal, I run 2 500 cfm edelbrocks on a tunnelram on my sbc with decent street manners. It is cold blooded without chokes hooked up but still manageable even in 20 degree weather. At a steady rpm of around 25-2600 in 4th gear it makes 17 mpg with 4:11 rears and 29 inch tires.I use a pertronix igntion with a accel 42000 volt coil that seems to do a good job.Biggest thing to get used to is the lack of bottom end power til it hits 2500 rpms or so. They are alittle hard to get used to but you will get used to it and probably love it.Mine is on to stay as long as it has the sbc in there.

 

It's an old weiand intake. I don't really have the money to have two tops and three different carbs. I'm going to have to hit the swap meets for some carbs as it is. I think I'm going to run holleys because they make them smaller than 500 cfm and because tuning help will probably be easier to get.

BinderRod, thanks for the tuning tips. I have heard about the 50cc pumps before and it seems to be the setup.

Keep it coming.

 

mine is an old Team G Super Ram for competition only but works well on the street. I do know my sbc runs and works fine with the Edelbrocks but it took alot of tweaking to get it there. If I remember right you'll have to mount the Holley carbs sideways right? I do know a guy here that has 2 450 Holleys new in the boxes that he'll probably never use. PM me if interested and I'll find out for you

 

Any carb setup you choose with take some tuning but it will be worth it in the end ,because you will freaking love the way a tunnel ram pulls at 3000 to 6000 ,especailly with those 4.11's and light weight .My Falcon accelerates from 3000 to 6000 in first gear so fast that its a blur on the tach when you floor it.Of course the 4.57 gears help alsnce you get all that velocity moving its on baby.I wouldnt use a box to take up velocity in the plenum but i have heard of it.It defeats the purpose of the T ram .Despite looking cool they have there advantages performance wise.I dont know what ign. system you are running ,but it better be a hot with 2 four's in traffic ,or you better keep plenty of spare spark plugs in the trunk.I am running a Mallory procoil ,and MSD 6 AL and i havent fouled a plug yet on my setup .You and i have and i have about the same car mechanically weight,heads,cam ,close to the same cubes.Keep me informed on how the two carb deal works .I may buy another top for my tunnel ram and try it with 2 as well ,but i will proly use the single top like im using on the street and save the two top for the track ...

 

I run a single 600 cfm with electric choke on my 55 chevy,(355,480 lift 292duration 7 degree lobe center cam, 9.5/1 comp. 882 heads with 202/194 valves and a home port job.hooker supercomps 1 7/8" primary into a 3" collecter, 3" pipes all the way back to a set of flowmasters. 350 turbo trans. 3000 stall/soon to be 4 speed, 9" ford with 4:11 gears) seems to work good. I had two 450 cfm on a basicly stock 350 back in high school. It was ok, but the throttle respose was a littly soggy of idle.

The 55

excuse the pic. it was from when she was first going together.

 

Personally I think Holleys are probably best. I've seen people run 2 600 double pumpers on smallish engines. I think the accelerator pump is the key, maybe not for economy, but for throttle response. For a daily driven car theory number one is probably best...

 

Glassthamesdoug here on the HAMB is running a tunnel ram on his small cubic inch (283,302,327)Chevy small blocks. He is running (2) Holley 6425 carbs (600cfm 2bbl) the engine runs great and looks good as well.

 

Make sure you have the correct power valves also. They are a must when coming off of the idle circuit. There will be people that say to plug them and bump up the jet sizes 5 to 10 sizes. I tried and it didn't work for me. I called Holley and they said to hook a vac gauge to the manifold, have the engine warmed up and in gear. Take the reading and divide by 2 and that will be the correct power valve to use. It sure helped my set up.

 

Go get yourself the December issue of Car Craft, they do a complete tunnel ram tuning guide. Oddly enough they run two 1850 series 600 cfm Holleys because they are vaccume secondaries and have no problems once tuned. They put the system on the dyno and compare it to 660 center squirters and tell all.

 

Mercury Kid, are the tops machined from flat aluminum stock?

If so, making your own from a piece of say, 1/2" - 1" aluminum plate is a fairly easy project.

You can probably use your present top as a template.

 

Tunnel rams actually have a very stong carb signal . In fact so strong that is is easy to over jet the carbs. Usually what comes in a carb stock is more than enough where as with a single plane usually 2 to 4 sizes increase is necessary generaly speaking two of what would be normal for a single carb set up works best. The power valve advice from the chevy guy is excellant. Sometimes they can be a bit cold heated for starting . Need a lot of fuel or pumping to light them off. Normal flow calculations do not apply . Similar to IR system in that respect . because of how they work they can handle a lot more than a standard singe 4 manifold. 289 302 ? I would use a pair of 600s

 

Based on your engine and use, I'd listen to school #1 - small carbs, vac secondaries, and non-progressive linkage. On the street, off idle response will be your biggest drivability issue. The smaller carbs will have a faster air stream and a better metering signal. You'll most likely have to play with the vac secondary spring, power valves, and acc pumps.

 

I've always read and heard that tunnel rams have a weak signal at startup and idle so I phrased that poorly.
Perhaps that's why the additional fuel supplied by the accellerator pump is required on startup.

There's a lot of info floating around about how poor tunnels work on the street, but conversely, I see a lot of them that run well.

Perhaps the requirement to work with them and find out what they want is more than a lot of guys want to get involved with.

Much of hot rodding seems to be bolt on and go with not too many really interested in the learning/experimenting end of things....

 

I run two 570 cfm 4150 I like how the sideways mount looks instead of in line and the dual feed holley 570 is the smallest dual feed vacuum sec carb. I have a 355 SBC mild cam and about 9.5 comp, had two 650's mech sec. on it and had a dead spot went to the 570 and no dead spot. I get 15 mpg on the highway and drive it everywere.

 

Also I only had to increase the primary jets two sizes and put the black spring in both sec.
and come standard with 8.5 power valves. And works flawless (start clean and shut off clean) on the street and at the track.

 

I put a Weiand 2x4 tunnel ram with two 600 Hollies on my '37 Chevy coupe with a 10 to1 350 SBC, 280H Comp. cam, 4:11s, 350 turbo with 2800 stall converter. The problem came in when the secondaries started to open. There was not enough vaccum to immediately pull the fuel from the secondary bowls. This created a lean condition and reduced power. The car, at the track, in this state (after experimenting with power valves etc.) was running 14.2 seconds. I wired the secondaries closed and went 13.7 secs. I then changed to two 390 cfm Hollies and the car eventually went 13.0 after some tuning with power valves and secondary spring combos. It ran great on the street and strip with the 390s and the fuel pressure down around 5psi. The problem is 390s are getting a little pricey these days, so 450s might be a good choice. Just my two cents, as all other opinions here have validity.

 

Ran my T coupe at GoodGuys in bowling green last week and ran a 8.60 at 81 mph in the 1/8 mile has a 3.89 gear and a 33" tire weighs 2220 lbs.

 

I ran this on an open car and the only thing I can add is that after a highway drive in cold temps the intake gets cool and the fuel does not atomize......when you pull off the freeway and start driving around town it will run like a cold motor in the morning.

other than that, it ran great.

 

I agree with the cold intake. It also takes a while to warm up on a cold morning, but this is the price we pay to be "cool". By the way, super trick "A" model you have there. Looks like serious business. Beautiful workmanship. Bob

 

I have a 289 with a Dual Quad tunnel ram and I have 2 holley 450's on it. The problem with the 450 carb is that it has mechanical sec. I would say to use the holley 390's or put on 2 edelbrock 500's with vacuam sec. .....

 

Bob37, I really appreciate the strip times. That's credibility right there. So it looks like I'm in the market for some 390 or 450 holleys on the cheap.

What I'm hearing a lot of is to run vacuum secondaries, lean them out accordingly, and put the big 50cc accelerator pumps on them. Mechanical secondaries are no good?

 

2 450s on that weiand will be right for the small block ford. It has a good shaped plenum. I ran 2 right out of the box on Falconstein and then only stepped up the squirt for better bottom end.

I will be running 2 vintage Carrer AFB 500 cfm carbs on the same intake on the 62 Comet, It is a flatop forged piston, long duration cam and heavily worked GT40 heads, also 4.30 gears and a 4 speed.


What cam and gears are you running?

Good luck. If I can help PM me.

Dave