Words And Photos: Richard Holdener

Is there anything better than a big-block powered Camaro, Chevelle, or El Camino? Heck, stick a big block in just about any vehicle and watch the instant transformation from mundane to maniacal. What could be better than a big-block powered classic (or modern) Chevy? How about a big block equipped with a dual-quad tunnel ram? Let’s face it, on the induction cool scale of one to 10, a tunnel ram rates a solid high nine. About the only thing that rates higher is a big, nasty roots blower poking out of your hood, but a bitchin’ dual-four barrel, tunnel ram is right there in terms of street presence.

I can remember seeing my first (working) tunnel ram back in high school. Like most t-rammed street motors, the small (283) Chevy surely didn’t need all the airflow (and fuel) supplied by the dual 660, center squirter Holleys, but that kind of logic really wasn’t the point.  This particular early Nova had the right stance, courtesy of a set of air shocks, the right big rear/little front-tire combo and even the requisite set of traction bars. Was the 283-powered Nova the quickest car in the parking? Not by a long shot, as a host of other, considerably less extravagant machines could easily clean its bowtie clock, but not many drew as much attention on cruise night. Rolling down the boulevard, (okay, so it wasn’t exactly Woodward, but it was the main drag in our little town) with a pair of Holleys sticking out of the hood practically guaranteed a double take. What more could a Chevy enthusiast ask for?

Call me greedy, but even a high nine on the cool scale might not be quite enough when it comes time to choose performance parts, especially for my driver. One school of thought favors the notion that form follows function. The application of this logic with regards to the tunnel ram dictates that the only reason that a manifold should stick out of the hood is that it makes more power than one that doesn’t. Sounds reasonable, right? Of course this completely ignores the ability to actually watch the linkage open those eight butterflies as you roar off from the light—talk about way cool!

Still, wouldn’t it be nice to have a motor that actually takes advantage of the benefits offered by a tunnel ram or for that matter, to discover what those benefits might actually be? Taking things even further, how about discovering the power differences between different tunnel rams? One thing we discovered during testing of a trio of different tunnel rams on our 540 Dart SHP motor was that all manifolds are not created equal. Truth be told, like most induction systems, tunnel rams are actually designed for specific applications and rpm ranges.

A great deal of confusion exists about tunnel ram manifolds, the first of which is that all tunnel rams are basically race-only manifolds. Actually, there are a great many different tunnel ram manifolds available (at least for Chevy applications) that offer decidedly different power curves. Like any intake manifold, the power produced by a tunnel ram is decided primarily by the runner length. Sure, the plenum and runner volume affect power production, but the real key is the runner length. A number of tunnel ram intakes, like ours from Weiand, Dart and Wilson Manifolds actually offer longer runner length than a typical single-plane intake. The tunnel ram combines the common plenum of the single plane with the long runners typically offered in a dual-plane intake–kind of the best of both worlds. The other benefit to the tunnel ram design is that it offers near-identical runner length for each cylinder.

Take a look at a typical single (or even dual) plane intake and check out the difference in runner length for each port. This difference in runner length optimizes power production at different rpm ranges in each cylinder (based on cylinder filling). The tunnel ram offers improved power over a typical single-plane intake by having all the cylinders produce the same effective power curve. In our testing we even added porting to the mix in the form of an as-cast Dart tunnel ram massaged by the flow experts at Wilson Manifolds. Would our low-compression, 540 big block respond to the improvements made by porting the Dart tunnel ram? Only the dyno would tell.

With the cool factor and design theories established, the only thing left to do was get them on the dyno. Our trio of manifolds included a Weiand Hi-Ram tunnel ram, a Dart as-cast tunnel ram, and a second Dart tunnel ram massaged by Wilson Manifolds. According to literature supplied by Holley (Weiand), the Hi-Ram was designed for use on a performance big-block Chevy with an effective operating range of 2800 to 8,000 rpm. Of course we had no intention of running our big block anywhere near 8,000 rpm, but our calculations indicated that the runner length of the Hi-Ram would allow a power peak past 6,500 rpm on our test motor.

Holley shipped the Hi-Ram with a pair of 950 HP carburetors. Some enthusiasts might see a pair of 950 carbs as a tad overkill on the big block, but the pair of HP-series Holley’s worked perfectly on our 540. If anything, the aggressive cam timing had more of an effect on low-speed power and drivability than the use of dual quads. Properly tuned, tunnel rams offer an impressive combination of power and drivability thanks to their all-important average power production.

While there is no denying the visual appeal of a tunnel ram, you shouldn’t just go hog wild and pop one on your stock 396 and call it good. Well, you could (and it would look cool), but it wouldn’t work as well as if you built a motor capable of using the performance offered by the unique design. To that end, we resisted the temptation to install one on an otherwise stock 396 motor or, for that matter, a 454 crate motor. Instead, we assembled a suitable test mule capable of the kind of power and rpm suitable for our tunnel ram test. Rather than run a smaller 454 or even 496 stroker, we selected a big block sporting 540 cubic inches.

The Dart SHP short block was equipped with a bevy of forged internals including a 4.25-inch stroker crank, rods, and forged flat-top pistons. The SHP 540 was topped off with a set of CNC-ported, Dart Pro 1 heads, and a healthy COMP roller cam offering .742/.715 lift split, a 271/280 duration split (@.050) and a 112-degreee lobe separation angle. The combination also featured a Moroso oiling system including a billet oil pump, pick up, and oil pan with integrated windage tray.  We also employed an MSD crank-trigger ignition system, Hooker headers and Ultra Pro Magnum roller rockers.

Prior to running our trio of tunnel rams, we established a baseline by running a single four-barrel, single-plane intake. The Weiand Team G was designed to accept a 4500 Dominator carb. After jetting and timing, the 540 produced 770 hp and 663 lbs/ft of torque. Torque production with the single plane exceeded 600 lbs/ft from 4,200 rpm to 6,700 rpm. Next up was the first of the tunnel rams, the Weiand Hi-Ram and pair of 950 HP Holleys. Once again jetting and timing were optimized, though the 950 HP Holleys were very close right out of the box.

The Weiand tunnel ram improved the peak power numbers to 777 hp at 6,600 rpm and 681 lbs/ft at 5,600 rpm. Though a peak gain of just seven horsepower doesn’t sound like much, the differences were much more significant elsewhere in the rpm range. Measured peak to peak, the Weiand tunnel ram offered nearly 20 additional lbs/ft of torque, but the difference was more than 30 lbs. ft from 4,500 rpm to 5,000 rpm. In fact, the Weiand tunnel ram offered more power from 3,500 rpm to 6,800 rpm, something that can’t be attributed to running an extra carburetor. Credit the long runners in the tunnel ram for the additional low and mid-range power (up to 6,800 rpm) over the single plane, but also note that the shorter runners in the single plane offered slightly better power at 7,000 rpm.

Having run this test previously on our 540, we expected the Weiand Hi-Ram to shine compared to the single-plane, Team G intake, but how would it now compare to another tunnel ram? To illustrate the differences, we replaced the Weiand Hi-Ram with a Dart tunnel ram. According to Dart literature, their tunnel ram was designed to combine the performance of a custom-built, sheet-metal intake for the price of a cast manifold! The tunnel ram featured curved runners designed to meet the cylinder head ports at the proper angle to reduce turbulence during the transition.

The runners in the tunnel ram were also tapered to maximize intake charge velocity for more complete cylinder filling. Interchangeable top plates are available for both inline and sideways carburetors mounting as well as electronic fuel injection (with the fuel injector housing cast directly into the manifold). Both tall and short-deck manifolds are available and designed to use standard length distributor (small cap required). For our 540 BBC, we chose a top designed to accept a pair of 4500 Holley Dominator carbs.

To feed our new tunnel ram, we installed a pair of Holley 1050 Ultra Dominators. The Ultra-series Dominators featured many improvements over the already-impressive standard Dominators, including aluminum bodies, billet metering blocks and fully adjustable, external linkage. The trick Ultra Dominators also featured high-signal, billet booster inserts, a tumble-polished finish on the main body and fuel bowls and even hand-polished venture inlets to maximize airflow. Naturally the Dominators also featured adjustable idle feed, emulsion, and power valve restrictors in the metering blocks for near-infinite tunability.

Run with the Dart tunnel ram and Ultra Dominator carburetors, the 540 BBC produced peak numbers of 806 hp at 6,700 rpm and 694 lbs/ft at 5,500 rpm. Measured peak to peak, the Dart tunnel ram improved the power output by 29 hp and 13 lbs/ft of torque, but the gains were as high as 51 hp at 7,000 rpm. Given the big gains at the top of the rev range, it would seem the Dart tunnel ram was tuned for top-end power, but such was not the case. The 540 produced more power with the Dart tunnel ram even down at 4,000 rpm, though the Weiand offered more power in a narrow range from 4,600 rpm to 5,100 rpm.

The final test was to run the Dart tunnel ram supplied by Wilson Manifolds. Wanting to test the power potential offered by porting, we sent a second Dart tunnel ram to the flow gurus at Wilson Manifolds. They cut, welded and ported the daylights out of the tunnel ram, but the finished product was something you’d be proud to show your friends. In fact, we hated having to hide the precision porting with the carburetor lid. Critical changes to the as-cast manifold included alterations to the taper ratio, port entries and transition between intake and cylinder head. Basically they took the casting and improved it everywhere, without any major external alterations, save for an aluminum badge proudly displaying Wilson Manifolds and the attending serial number.

While a street racer might not want to advertise, what Chevy enthusiast wouldn’t be proud to display the fact that their big block was sporting something more than an as-cast tunnel ram? Impressive visually, the Wilson-ported manifold performed even better, as the massaged tunnel ram improved the power output of the 540 to 833 hp at 6,800 rpm and 712 lbs/ft at 5,600 rpm. More than just peak power gains, the Wilson porting improved the power output right from the get go (nine lbs/ft at 3700 rpm), though the major gains (as high 27 hp over the as-cast tunnel ram) came higher in the rev range. Compared to the Weiand tunnel ram, the Wilson-ported Dart intake improved power production by as much as 78 hp, though it should be said that this extra power will cost significantly more than the as-cast Weiand intake.

Sources

Dart Machinery
dartheads.com

Holley/Hooker/Weiand
holley.com

Wilson Manifolds INC.
wilsonmanifolds.com