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Hagerty Employee

Piston Slap: Tunnel vision on tunnel rams?

Wesley writes: Hi, I'm really hoping you can help me with this. I have a '68 Chevelle with an Ultra Street 540 built by Shiroff Racing Engines out of New York. I am wanting to put a dual quad Tunnel Ram on it. I want to do this for two reasons: I love the look!
Advanced Driver

Great advice Sajeev. Been there done that!

Hi much of what Sanjeev has stated is great advice. But I actually drove a Tunnel Rammed Duel Quad on the streets for a number of years. Yes and even to work in the summer time.

Here is what I learned and what you need to do.

As Sanjeev stated you can make the most usable power with a single 4 and an intake like a torquer or similar low rise single plane intake. So understand up front there is some compromise here for looks on the street.

#1 if you use a tunnel ram look for one with small runners. This will help on the street for low end power. You are not going to want to have to rev 4,000 RPM just to leave a light and smaller runners will help.

#2 If you are doing this for looks use two carbs. But if you go two carbs make sure they are the right size for the engine. Most people over carb an engine and that creates trouble. Smaller carbs will rejet more easily and you may not have to adjust the idle circuit.

#3 You will need a mid to high range cam for it to run right. 3000 RPM and up in most cases. Match it to the intake. I would recommend a Hydraulic Roller.

#4 Cultch or Converter. I used a 4 speed and it was no issue but if you are going Automatic you will need the right converter for the cam, gearing and weight of the car. If not this thing will be a royal pain to dive and will never run right. Call a performance shop and they can best match up what you need.

#5 Make sure how you want to run your linkage. Some intakes are inline and some are 90 degree turned on the carbs and need bell cranks. You must plan this out and make sure the accelerator cable is long enough.
#6 Make sure the intake will stick out of the hood. Mine was an SS but the engine was replaced with a 350 that sat lower and farther back. I had to run a Enderle like intake scoop to stick out of the hood.
#7 Run a Hood. Many times it is required by law but no matter how tight the engine is stuff will land on the windshield.
#8 Plan on weather. No more driving in the rain. Water will get in and make a mess.
#9 Plan out the choke. I used one on one carb and it worked fine but I made it manual.
#10 If the Idle circuit is too rich I was shown by an old time racer to use a wire bent in a u shape to cut the flow of the circuit to make it run right. At the time I did this small 4 barrels for tunnel rams were not common.
The way to sum this up is if you are wanting to do this and leave a little bit of performance on the table do it. I loved mine and had a lot of fun with it. But if you want every ounce of power then go single 4 or injection on a single plane intake.
I also highly recommend a 4 speed or 5 speed. I was able to drive it anywhere and was able to leave it geared so I could hop on the highway and drive anyplace.
Mine ran 13's on pump gas and with slicks and ladder bars could pull the left front tire off the ground. It was good for the street and reliable. I estimated the power was right around 375 to 400 HP with the compression I had. The engine other than the intake was nothing really radical. Flat tops with old 327 fueler heads. It was faster than the stock 396. I was not a street racer and with this set up few people ever even ask to race. It is not what you have but what they think you have.

Oh if you want traction on a Chevelle work on the suspension. Hotchkis suspension are great and will help with traction, handling and ride. Now if you want old school look then it is Ladder Bars. They will help you hook up but beware the ride is horrible. It limits suspension travel

My tires would bite good enough that If I hit the gas about three times both tires in front would come off the ground. But hit a bump my head would hit the headliner if you did not have your belt on.
I hope this helps and may prevent you from making any mistakes. I loved my time with the car and it was fun driving around with the intake shaking in the hood. But if I were going to race and wanted every bit of performance I would go Fuel Injection and a proper intake for it now.


Note even on a hot summer night I could get ice to form on the runners due to the air flow and moisture in the air. This is one of the reasons to run a hood. 


My neighbor has a similar Shafiroff 540. His is the 750/750 version and runs a Strip Dominator intake and Holley Dominator carb. He's meticulous about maintenance, but for some reason still managed to break a lifter roller. If you change the cam, put new lifters in it. When they break, they end up all over the motor. His required an 0.030" overbore, new pistons, cam/lifters, crank polish, bearings, oil pump, etc..

Well there ya go.  Don't bother with dual fours, 'cause your engine is just gonna  grenade itself pretty soon anyway!  🤣


[PS - if that happens to you, don't blame me, blame drm101's neighbor...]


The best advice is to speak with the engine builder! Now if you're absolutely set on running a tunnel ram, on the street, I'd go with EFI! Otherwise the cold starts can be a real pain if the ambient temp gets down much below 65f or so.
Pit Crew

The author of this post doesn't mention the most important factor in single plane intake manifold selection. Manifold plenum volume. The author doesn't have a basic understanding of intake manifold design and shouldn't be giving advice.

   I'm with Sajeev and Hyperv6 on most of their advice.  You are essentially contemplating building up a race engine for what is admittedly "mostly a street car".  Hardly ever a good plan (ya gotta trust those of us who've got that particular t-shirt on this).  Hyperv6 mentioned the hood aspect.  You really wanna cut a hole in the hood of a '68 Chevelle?  Or, since you love the look of the dual-quads, maybe you want to look at them all the time by going hoodless?  'Cause a stock hood is not going to fit.

  If you really are serious about "trading some low-end for more top-end", there are better ways - but you are talking about a street car, where you seldom are going to get to the top-end.  If you have too much torque for leaving a stop light and lose traction, go ahead and follow the advice on suspension and tire fixes - trading it for top speed on a street racer is not the best tradeoff.

  So I suspect that this is more about the cool looks than anything else (and hey, I've got nothing against that, I agree it looks Bad-A$$), but please take note of the dollar signs in that word, because it ain't gonna be cheap - heck, even decent progressive linkage can run you hundreds of dollars.  And with carb choice being a huge factor, it ain't gonna be easy, either.  Like others have noted, there are tons of people out there who have gone from duals to single 4bbl. or injection to get more consistent performance and less hassle.  Also noted by others: this whole thing doesn't just stop with carbs and intake.  For a combo to work as you desire, you've got to be thinking of cam, valvetrain, heads, exhaust, the whole shebang.  Dual quads dump a whole lotta atmosphere into an engine (even with smallish cfm carbs).  That engine better be able to swallow all that and do it efficiently, or the whole thing is just a waste of time and money and headaches.

   And finally, even though New User Chevrolet dissed Sajeev's advice-giving, he makes a valid point there - the whole plenum volume, runner length and diameters and shapes thing is science beyond the normal on-line blog.  It would take an entire column all its own just to get into the basics.  Of course, that level of science is really of best advantage for serious racers, and not so much for street cars, but if, as you suggested, you really are looking for better performance, you'd better take your eye off how it's going to "look" and think hard about the numbers of air/fuel mix, air-flow and lots of other stuff.

   You didn't ask me (but you did ask Sajeev and he mentioned that responses are welcome, so I'm gonna give mine 😁) - for this idea to have much chance of sucess (other than looking good), you should be doing a LOT more research and working a LOT of formulas and yes, as Sajeev said, probably time testing different setups on a dyno (also costing $$$).  And even after all that, my humble opinion is that any level of sucess you might achieve is going to be a disappointment given the troubles involved.

  Go ahead and ask the engine builder, as Swamibob says, and also look at YouTube videos (Uncle Tony's Garage just, coincidentally, put out one on multi-carb setups within the last few days).


I should add running multi carbs is not as difficult as some think it is. You just need to understand how to adjust and set up carbs which many do not know how to do. 


#1 keep the size of the carb down.


#2 on the street run vacuum secondary.  No need for double pumpers or dominators on the street. 


I have worked on multi side drafts, Duel Quades and Tri Powers and none are difficult to deal with if you just have a good understanding of the carbs and how they work and need to be adjusted. 


The greatest issue on most cars is the fact most do not understand how to adjust a new carb to a new engine even in a single 4. 


Most run too much carb  on the street. Few re jet the carbs to the specific engine. Too often they pull the carb out of the box and bolt it on. They adjust the idle mixture and idle speed and call it a day. They never re jet or even adjust the floats. 


Seldom are these carbs for a specific application so they need tuned and adjusted. Too often the old carb was so bad the new one feels good but often much is left on the table because it was never tuned. 


If you race a Holley or Holley based carb is great, On the street Edelbrocks are much easier to adjust and tune for the novice. FI is the way to go but on a Tunnel ram is not practical. 


The Intake in the photo also was designed for the street due to the small runners. This keeps the low end power up more for better street driving. Large runners of a Single plane need air velocity to keep the low end for the street. At the Strip they are fine if you are dumping at a High RPM and never letting it come down. You need to keep the air flow up. 

Like life Running a Tunnel Ram is full of compromise. If you want all the power single 4 if you want looks and ok leaving a bit of performance on the table a street ram will work. 


Again if you are doing an Automatic. Please make sure to visit the gear and stall on the converter to match the cam and intake or you will be hating life. 


I ran mine for a number of years with no issues after sorting out early tuning. I even got decent MPG around town vs my  stock big block in my daily driver. 

Pit Crew

Using a tunnel ram with 2 × 390 CFM Holley carburetors is the same as a single 780 CFM carburetor. Using a small plenum and a plenum divider on a tunnel ram can be more powerful at low speeds than a single 4 barrel single plane manifold with an open plenum. Intake manifold heat below the plenum is non existent on every single plane
manifold. I have dyno tested marine engines with tunnel rams with "reduced volume" plenums and plenum dividers and they have been more powerful at low speeds than the best single plane/single four barrels manifolds because of the direct shot of the air/fuel mixtures into the cylinder heads.

True, but the ask isn't about making low speed power - he's thinking he has too much of that now.  He's looking for WOT top-end poop.  Too much carb on top of too much intake manifold, without enough breathing below it - clear through to the tailpipe - results in one thing: bog.

Pit Crew

Bog is created by low manifold vacuum which can be created with a single plane/single 4 barrel manifold with a plenum that is too large.
Usually an amateur will simply increase the accelerator pump shot.
Pit Crew

Low speed torque and throttle response is directly related to intake manifold vacuum. Intake manifold vacuum is directly related to plenum volume.
If the intake manifold is divided into two halves with a divider the volume is reduced by half and throttle response is dramatically increased. This is the primary reason why dual plane manifolds increase throttle response and torque. A tunnel ram with a plenum divider can be a very powerful low speed manifold for a street engine.