I currently run two 600 vac sec. holleys on an offenhauser medium rise single plane intake so its not exactly a tunnel ram but pretty darn close as far as tuning goes. The motor is a 10.5 to 1 454 with large tube headers, 4.10 gears and a 5 speed. The following is what I have learned about tuning this combo. Hope it helps. 1. The velocity which the air moves through the carb is key to good throttle response. Two factors that affect this is the runner/plenum size of the intake and size (cfm) of the carbs. There are some TR intakes that are marketed as street rams. Typically this will have smaller runners and plenums that a typical race ram which help with velocity. Check out the various manufactures catalogs as they will have this info in their port dimensions. I would error on too small a carb than too big. The velocity of the air past the boosters in the carb is what will dictate how much "pull" the main circuit will see once off idle. This velocity is also what helps keep the fuel in suspension instead of puddling on the intake floor. 2. As stated I run two 600 holley's but the car came with edelbrock 600's. The edelbrock carbs are a great carb for a mild set it and drive car but had a terrible stumble under hard braking or moderate cornering due to their float set up. It was double bad with two of them. They were also alittle less adjustable due to their accel. pump/spring set up. With the holleys I ran the linkage 1 to 1 instead of progressive as the progressive set up had a hick up that I could not tune out. The progressive set up was always a compromise to feed the 4 cylinders farthest from the primary carb. I know some people run progressive linkage and many of the factory multi carb set ups came this way but it did not work that well with my set up. 3. The vac. secondaries will allow you to run larger carbs that you might not get away with otherwise however a smaller carb will maintain better velocity which typically will translate into better throttle response. I connected both vac. cannisters with a vacum line much like they did with the ThunderBolts so both secondaries open at that same time. Holley still sells the vac. secondary quick change kits that have the nipples on them for this purpose. I put an o ring on the rod that is attached to the rubber diaphram to verify how far the secondaries were opening and played with the springs from there. To figure out which spring will give you the best drivability and not opening too soon find yourself a nice long hill with your car in top gear (overdrive, 5 gear) and roll into the throttle. If the spring is too light the car will hesitate as the secondaries open and close as vacum drops. If you have to back out of the throttle to get the car to run right while going up the hill go to the next stiffer spring. 4. My distributer does NOT have vac. advance. I run a bunch of initial timing ( about 22 degrees) and have an additional 14 degrees via centrifical advance for a total of 36 degrees. I have large roller cam so this helps with idle also. 5. Even with relatively small (600 cfm) carbs I found that my idle circuit screws were very insentive when adjusted. I eventually restricted my idle circuits by inserting a .010 gauge wire into the brass idle circuit tubes found in the metering block which made a huge difference in the idle adjustability. You could play with the gauge of the wire to dial in more or less restriction. I used digiatal calipers to measure the wire. Before this mod the exhaust would burn your eyes it was so rich in the idle circuit and plugs would not last very long. 6. If the car is an automatic make sure you check your vacumn at idle with the car in gear as this will give you your vac. reading so you can properly size the power valves. If the power valve opens at idle you are dumping another 6-8 jet sizes of fuel into the mixture which you will never be able to tune out. 7. Most folks that run dual quads or t-rams run the small and short 4 inch air cleaners which do NOT flow enough air for a big block or high winding small block anything. Period. I went with K@N 5 in tall 4 inch in diameter air cleaners to get the flow I needed. Look on the web for CFM flow requirements of your engine combo (which is derived from cubic inches times max RPM) to see how much CFM your combo will need then go to the K@N website to see what each filter will flow. Remember you will be running two filter so each filter should will need to provide half of this number. 8. In my experience holley's like about 5.5 to 6 psi of fuel pressure. Make sure your fuel system is able to deliver the volume that is needed with two carbs to ensure this pressure. A float level that is too high will also confuse the idle circuit tunning process as it allow for fuel to dribble out of the booster. There is more but this should point you in the right direction.
I didn't lean out my secondaries at all on the 390s. As I recall, I think I richened them up a little by drilling them out, then I played with the secondary diaphram springs, primary jets and power valves. Try the stock jetting and springs first, but make sure you know the power valves are right. Correct power valves are must with these carbs.
Good thread going here, whats everybody thinking on mounting them, sideways or inline? I have 2 600's and a early Weiand going on a SBF over the winter in my '46. Will the vac. secondarys provide even air flow mounted sideways, I've been wondering about that part of it. Just seems they'd be more "user friendly" , read tuneable mounted sideways to keep from having to pull them to make any changes
The article in Car Craft where they put a tunnel ram on their '67 Ramblurr said that mounting them sideways can lean out one bank, but I don't know because they mounted theirs sideways. I listed my motor and drivetrain specs in my first post. Bob37, I meant that most people have to lean the carbs out as a whole, not neccesarily the secondaries. gofaster, thanks, that is really detailed and I will be coming back to that frequently when I get carbs and start tuning.
You are hearing a lot of vac secondaries because on a street set up you are not doing a WOT launch, but rather need to drive it up off idle and into the RPM band with no bog. The vac secondaries only open when the engine is actually pulling a CFM that requires them. You can tune them as to when they dump it by playing with the springs. A pair of 390s is your best bet with your small engine.
I always thought T-rams robbed low end performance too. Boy...was I wrong. Granted my set up is on a race car/508CI Pontiac.. I have to run Carter AFB's per class rules. They have a secondary air valve. I tried removing air valves and it did stumble off idle. It very responsive off idle and my motor only has 5.5"hg of vacuum at idle. The T-ram was worth 53HP on dyno over Offie 2X4 with no other changes/same carbs...surprised me !! I made a top plate out of 1/4" aluminum plate to get more hood clearane so I guess I'm guilty of reducing area under carbs.. My carbs are Comp 750's per class rules. A buddy of mine has a street/strip 63 GP and put a T-ram on his. Had to have it. He wound up using Edelbrook 600's...He says he can smoke the hides (MT Drag Radials)very easy and lights them up on 2nd gear change.He's chomping at the bit to go to track.His motor is street/strip 462 Pontiac with Hyd roller cam and Aluminum heads.. With a smaller motor I wouldn't go with overly big carbs.. Pics are of our cars, GP has air cleaners.
I don't know much about tunnel rams. But I have a theory about the problems they are known for on the street. Here's my theory and my solution... PROBLEM: Due to the long runner lengths the well atomized fuel that travels from the carb down to the intake valve reforms into larger droplets at low engine speed. This inhibits it's ability to burn correctly. SOLUTION: Make the fuel break up into fine particles again. My brother made a custom stainles steel screen and placed it in the air/fuel path of his 418W and we believe it is further atomizing the fuel and aiding burn.
good question. I know the 600's have the side mounted float instead of the cathedral floats that the more prevelant float set up with the bigger carbs. Under hard accelerataion the fuel is going to slosh to the side of the float bowl which is not how these floats were meant to meter fuel level so this will be a concern. You could convert to the cathedral float bowls and remedy that though. The only other thing that I think you would want to do is add jet extension so that you do not uncover the jet that will be at the "shallow end" of the fuel flow once gravity and force take affect under hard acceleration. Leaning out one bank would be possible as your primaries are now located closer to to one bank of cylinders vs. the other, particularily if you have any kind of a divider in the plenum. However a t-ram has a pretty good open plenum that should allow for the mix to get "sorted out" before it hits the runners. I would think that sideway mounting would lend itself more to racing and double pumpers in regards to set up. Drag racing is pretty much either idling or WOT. At WOT all the butterflys are open so the leaning out of one bank is pretty much a mute point. Besides, you really only have to pull the front carb when changing jetting because you can then get to the primarys on the rear carb while it is still on the car and the linkage is much simpler when mounted traditionaly.
Which Offy dual quad were you running? The 360 low or mid or the funky one that had the ports with dividers in them. I ask as I always figured I was giving up something with this intake (360 mid single plane) but 40-50 HP would get me to change.
Good point and I believe that you are correct in regards to the importance of keeping the fuel in suspension with the air. I know there used to be some intake gaskets that were sold that had the screens sandwiched in the gaskets for this purpose. Anyone ever try them?
I'm curious if you Tunnel Ram guys are running progressive linkage? I've always run straight linkage on any multi-carb install, but lately have been thinking of making a progressive linkage and giving it a try with my medium riser dual quad setup. Along with that thinking is a very lean jetted primary carb and a jetted for optimum HP secondary carb. Some of the sorting out vis a vis carb jetting could be done one at a time on my single quad intake. A Tunnel Ram may be a good candidate for a lean/rich carb setup, but in my particular case I think it's real workable cuz the 462" Buick always ran a little lean on the front four cylinders when both carbs were jetted/rodded identical.
A guy I know actually ran them in a 302 street strip drag pickup (Ranger). They worked like crazy mad....for about one summer. They pulled the intake to fix a sucked rear valley gasket (common problem with 302's) and found the mesh was gone. It was very thin mesh. Seems the fuel wore it away. The stuff my brother used was very heavy and we've been watching it. No noticeable wear yet.
"It was very thin mesh. Seems the fuel wore it away. The stuff my brother used was very heavy and we've been watching it. No noticeable wear yet" How did your brother install the screen? Did he stretch it across the carb mounting surface and if so did he sadwich the screen between two gaskets? I ask because I want to try this. Any pics?
There are a few differences in the sideways mounted tunnel ram carburetors vs the one mounted in line.With the sideways mounted tunnel ram carburetors you gain the metering block on the rear which allows you the ability to jet all 4 corners. You also now have the larger center hung float bowls and with our tunnel ram calibrated stuff we use a special cut float in them that allows the fuel to still slosh back but not cut the needle and seat off when this occurs.
I noticed several of you were discussing the hot vs cold and awhile back we had posted some pics of our new Pro Twins which uses our new AeroRam intake which is a warm design vs the cold design of some of the taller tunnel rams. With this set up we are using the 2 carburetors in line and one of them has a choke for easier start ups when cold. These are both vacuum secondary carburetors which are fine for street use. I'll look and see if I have any individual pics of it but it is in the center back row of this one.
For street use definitely stay with the smaller carburetors. They will be more responsive and have better drivability manners. They will also keep the signal up with the larger plenum tunnel ram and the lower rpm range the motor will see vs a race application where the motor stays at a higher rpm.
Do not run mechanical secondaries unless you are going to disconnect them! That's a big cam for a street 289 and a TR. I ran a 340 that was typical 70s: Over cammed, over carb'ed and high compression. I tried 3 different sets of carbs and was happiest with fresh 600s. Typical swap meet carbs have some slop, so unless you can find 2 that are nice and tight, you will have issues. Find out how much vacuum you currently have at idle, and build the carbs for around an inch less. How much timing are you currently running? You may need to bump that up some. Make sure the secondaries are open the same. You may have to adjust them so the primaries are open to the right spot. Then get the carbs even, then mixture. It's a balancing act. I had to run a small wire in the air bleeds in one set. Overall, I loved the look and they were fun when right, but they did require lots of effort and it was soggy under 22-2500 RPM. A smaller cam would have helped, but I was after a period look and sound.
intake gaskets that were sold that had the screens sandwiched in the gaskets Yes, J. C. Whitney sold quite a few of those. My grandfather bought them for his Model T. They still don't work.
I us 2 AFB 575 cfm carbs on my 351W. So are you going to be at Thompson Ohio in the spring for the Gasser Reunion? your car would be a hit out there.
If you are talking about my Offie it was the 360...."low" riser..I had to weld up/raise the ports as they were not tall enough for my KRE aluminum heads with 2.40X1.50 ports...It was also "ported".. I have been told it's a better intake than the Edlebrock 2X4
Unfortunatly there are no pictures. But here is what he did for his Victor Jr. single plane 418W. 1.) He took a 4x4 (3.5"x3.5" actual dimension) post and cut one end into a 4 sided pyramid. 2.) He rounded the pint of the pyramid 3.) He formed the stainless mesh over the 4x4 post pyramid and left a 1.5" or so flange. 4.) He sandwiched it between two .25" carb spacers and sealed it with epoxy. 5.) He installed it as if it were just a carb spacer. He got/gets crazy throttle response at all rpms.
For a street tunnel ram set-up, it's really hard to beat the lowly, common old 600cfm vacuum secondaries Holley carbs (#1850). They aren't exotic, trick or "cool", but they are common, cheap, easy to tune and they do well in dual carb applications. 1200cfm SOUNDS like a bit much for a street small block, and it is...in a way. But, with a vacuum secondaries operation, you're initially coming off the line with two 300cfm carbs...which is about the same effect as if you were just using a 600cfm 'double pumper' carb. As RPM increases, the resulting vacuum signal created by air rushing through the carb pulls open your secondaries and delivers additional air and fuel to the engine. These carbs are VERY tuneable, and with a little effort, you can dial a couple of them in to work really well on a street-driven vehicle. They cost a good deal less than the 390cfm offerings, too. The 450cfm carbs are a little bit cheaper than the 600s are, but they are mechanical secondaries carbs (without a second accelerator pump) and don't always respond as well as a couple of dialed-in 600s do. When tuning the carbs to work together, a stiffer set of secondary springs will give you better mileage and street manners at lower RPMs, while lighter springs will give you more power sooner (assuming you don't over-do it!). In addition, Holley offers new secondary diaphram covers with a vacuum nipple on them so that a small hose can be run from one carb to the other to help 'balance' the vacuum signal to the secondaries. It's just another tuning aid to bear in mind. Running the 1850s is a win-win situation...they are plentifull, cheap and easy to work with...how can you go wrong?!
I am planning on running progressive linkage, since this is a street car and I don't make the kind of money where mpg isn't a concern. I've done my research on that too, and with the vaccum secondaries it will be much easier. I really appreciate all the technical help, as you can tell I like to ask a lot of questions to make sure I get things right. This car has been a long string of first times for me. Falcon Gasser, I wanted to make it up there this year but I was still getting the straight axle and my tire/wheel combo sorted out. I will be there next year for sure. RodStRace, I'm running all the timing it will take. I haven't even put a light on it yet, I just turned it until it idled well and turned it back until it stopped breaking up on the top end. Had to retard it about 4 degrees when I swapped in the 4.11's. I don't think the dist. had ever spun up that fast in it's life. The CFM formula for my engine now is 289x6000*/3456= 500cfm The formula for the planned engine is 331x7500/3456= 720cfm 390s should give me what I need, but if there are 450's with vaccum secondaries, then that is the alternative. Fat Hack, I like the way you think. Simple and common is always easiest. The guy I bought the intake from had a pair of 600 cfm holleys that he said were matched and tuned for the tunnel ram. He still has them and wants 300, so I at least have something to turn to if the search for smaller carbs comes up empty. *Above 6k the motor starts loosing power like the valves are floating. Edelbrock says the RPM package makes power to 6500, and I have the whole package, so I don't understand this.
The screen is a huge reduction in effective area. The improvement is similar to what you'd get with a smaller venturi. It's like getting backpressure by stuffing a banana in the tailpipe.
The CFM formula for my engine now is 289x6000*/3456= 500cfm The formula for the planned engine is 331x7500/3456= 720cfm There is no formula for tunnel ram. In fact, no one would plan a race carburetor using that "formula" - it's too small. Go back a ways in this thread, and you'll find a post that explains how a tunnel ram works. Reason not to use really big carbs: if they're too large, you open the power valve all the time. 5 mpg. "Wait - my friend cured that by taking out those stupid valve!!" 3 mpg.
I used 2 Q-jets, Not the best look'n but works awesome, Think about it, small prim. large secds. My set up is also cold blood'd as hell. Pontiac Slim
and there is something wrong with a phalycally (sp?) equipped exhaust system? seriously im planning to run dual tunnel on my sbm 360 - summit sells a complete kit which I planned to do (SUM-CWND310) to take some trial and error out of this. They spec the holley 4160 450cfm carbs for it - I imagine that carb setup for your ford application would be right in the ballpark and also fairly easy to come by - new or used - theyre mechanical secondaries though - Im intersted in what others have said about staying away from mech 2ndaries and going vacuum, is this strictly for fuel management/savings reasons or compensating for a larger carb cfm or....
is this strictly for fuel management In my experience it's easier to jet. You can adjust how quickly the secondaries open (Holley: diaphragm; AFB: air valve weight; AVS, ThermoQuad, QuadraJet: air valve spring) rather than change the jets, rods, etc. Also tolerates WOT better if not on the money. Easy way to de-tune a TR: reduce the plenum volume with a stuffing block. Bill Jenkins made them out of wood on his pro stock car with varnish to prevent gas from eating them. Less volume = better response, richer mixture but will reduce max power unless the carbs are large. The reverse is also true: large engine with small carbs? Put a big spacer in between the top and body to add volume.
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