MAKING SLANT SIX SENSE, PART 2
WE SWAP THE STOCK 1-BARREL FOR A HOLLEY 2-BARREL CARBURETOR ON A BONE-STOCK SLANT SIX. NOW OUR SIX-IN-A-ROW HAS MORE GO FOR TODAY’S TRAFFIC FLOW!
Swapping the stock 1-barrel for a Holley 2-barrel carb on a bone-stock Slant Six
In our last installment, we proved the old-reliable Slant Six in bone-stock form isn’t anywhere near the performance standards of a modern sixor four-cylinder engine. When we put our 1966 Dart GT on the road two and a half years ago after a 20-year hibernation, we realized just how sluggish the old leaning tower of power was in this day and age.
If you missed Part 1, we added Performance Distributors Tri-power electronic ignition and Livewires ignition wires, which jolted-up its performance significantly. Then we dyno tested TTI’S new 2 1/2-inch exhaust, and it also upped the performance to the tune of 9 rwhp and 10 lb-ft. Those two bolt-ons helped our Dart be a more compatible driver, but it’s still not enough performance in today’s world. Total output on the chassis dyno was still a paltry 71 hp and 119 lb-ft of torque. We needed more.
Our plan for our early A-body is a reliable daily driver in our (rapidly approaching) retirement years. We want to be that “old guy driving a Dodge Dart.” But we don’t want to be that old-timer holding up traffic, because we don’t have the performance to go and flow on today’s roads, which are jammed with impatient drivers. Before we get to the 1- to 2-barrel conversion, future plans for the leaning tower of power will be a cam and cylinder head swap. The camshaft will be of the smooth idle type to broaden the power band from idle to 5,000 rpm. The stock cylinder head will be ported and .080 inch removed from its deck. We thought
swapping to this Holley 350 two-barrel might be too much for a stone-stocker, but the Slant Six will be ready to breathe in the added airflow when we perform the cam and head swap.
So here we’re exploring new territory by bolting on a big Holley 350-cfm two-barrel instead of the Super Slant Six, Carter BBD two-barrel with 260 cfm. We figured since the TTI 2 1/2-inch exhaust is getting the flow out, the Offenhauser four-barrel intake fitted with the Holley two-barrel would get the flow in more effectively for more power. Our theory is the six-in-arow will go better with a two-barrel than a small four-barrel Holley 390 or Edelbrock 500 cfm. There’s no waiting for the secondaries to kick in.
We think this is a very worthwhile upgrade for the Slant Six enthusiast. Just be sure to have a high-flow exhaust and strong ignition before planning to perform this intake/carb swap. Also, we need to mention, gas mileage has improved. The tilted engine must be getting the right amount of the airflow in and out to make it inherently more efficient. All the performance parts needed for this one- to twobarrel adventure are available through Summit Racing. The restoration supplies
were from Eastwood. Now our cool-looking Slant Six is the star on cruise night when we open the hood. Follow along and see how to add 26 percent more go to a six-in-a-row.
We think this is a very worthwhile upgrade for the Slant Six enthusiast. Just be sure to have a high-flow exhaust and strong ignition before planning to perform this intake/carb swap.
We decided a two-barrel carburetor like this Holley 350 cfm would perform better on our stone-stock six over a small four-barrel Holley 390 or an Edelbrock 500. We intend to daily drive the Dart in our retirement years and chose this electric choke Holley Street Avenger 2300-series two-barrel (PN HLY-0-80350, $373.95).
In the previous episode, our ’66 Dart’s original Slant Six improved its get-upand-go from Performance Distributors electronic ignition and TTI’S unrestrictive 2 1/2-inch exhaust. For this adventure, the stock Holley one-barrel, a 150cfm choker, will be replaced with a better-inhaling Holley two-barrel 350-cfm carburetor. The results were better than we expected. We found out that even a bone-stock Slant Six enjoys freer breathing.
We’ve been testing and using K&N X-stream air cleaners since their introduction in the late ’90s with great results. K&N’S 11-inch X-stream Airflow assembly (PN KNN-66-3110, $137.99) will provide unrestrictive breathing, fit with the factory air conditioning compressor, and under the Dart’s low hood.
This Summit Racing 1-inch-thick two-barrel carburetor adapter plate (PN SUM-G1406, $53.99) allows use of a Holley 2300-series type two-barrel carburetor on a square-bore four-barrel intake manifold. Three Holley 2300-series two-barrel carbs were standard equipment on our beloved 340 and 440 Six Pack engines. Our 225 Slant Six only needs a Two Pack.
Instead of swapping to a 40-somethingyear-old factory Super Slant Six two-barrel intake manifold and carburetor, we chose this proven, tried-and-true Offenhauser four-barrel manifold (PN OFY5270LK, Summit, $357.99). This cool Offy intake bolts up to the factory exhaust manifold, allowing us continued use of the 2 1/2-inch TTI Exhaust bolted to the stock manifold.
When we removed the intake/exhaust manifold assembly, we discovered the exhaust manifold was cracked right where it was once repaired/welded. The ’60s Slant Six exhaust manifolds were notorious for cracking. Luckily, we knew of a parts car nearby, a 1972 Dart, and were able to get the needed manifold.
Or if you’re handy and have the tools, like us, you can use a cutting wheel, carbide bit, and sanding rolls to open up the carburetor base area. To promote good airflow velocity, we rolled the roof of all the runner entrances and blended the outer sections of the four holes to help direct airflow toward the outer cylinders (1, 2, 5, 6).
Mounting the two-barrel adapter plate into position showed a major restriction underneath from the four holes for a four-barrel carburetor. An advantage of running a two-barrel on a four-barrel intake is the two venturi holes will be centered better to the intake’s runners. A machine shop can cut away the four holes and turn it into an open plenumtype intake.
After extracting the bolts, IDM also steel-shot blasted and tapped the threads for the three boltholes in the manifold. Your author smoothed-down excessive casting flash using his die-grinder equipped with sanding rolls. Using a sanding block, we resurfaced the gasket area where the manifolds bolt together.
Notice the top of our Dart’s 1966 thinner-looking exhaust manifold (shown upper). Ma’ Mopar beefed-up the Slant’s exhaust manifold in 1970. They made it a thicker casting with extra reinforcement ribs (shown lower) to strengthen it and prevent the cracking issue of the ’60s Slant Six exhaust manifold.
Now, after our handiwork there’s no longer airflow interference under the two-barrel adapter plate. The two nostrils will now have unrestrictive airflow!
The three bolts holding the manifolds together broke off inside the exhaust manifold. We tried soaking the bolts with penetrating oil for three days, but they still snapped. Our local machine shop (IDM Speed and Machine, Manahawkin, New Jersey) extracted the bolts for us.
Also, at IDM the flange area was resurfaced to remove pits for good gasket seal. We smoothed-out all the port entrances, going an inch into each port.
Notice the exhaust port exit also received the sanding roll treatment. We welded the heat riser shaft to keep it in the open position for best exhaust flow. The Slant Six manifold has longer port runners for superior flow compared to the other straight six’s of the time.
First, the 13-nuts were progressively tightened (2, 5, 10, 15, 20 ft-lb) from the middle to the outside of the intake/ exhaust manifold assembly. Then, the three bolts that were finger-tight holding the manifolds together were secured to 18 ft-lb. High-temp antiseize was utilized on the threads of all the fasteners. Next, seen here, is one of the four screws being tightened to hold-down the four-barrel adapter plate.
At first we tried using the combination of the stock throttle cable with a fourbarrel bracket we had laying around. The Lokar trans kickdown cable and bracket worked well and looks great. Using the stock throttle cable made for excessive gas pedal travel (an extra inch) to obtain full throttle. At that, we ordered the Lokar throttle cable giving us the right ratio for the throttle and kickdown cable to function properly.
The new 350-cfm Holley Street Avenger is mounted with the fuel bowl forward and the throttle lever on the left side. The throttle lever faced rearward on the stock one-barrel. This necessitates the use of aftermarket throttle and kickdown brackets/cables.
Here the Holley 2-bbl adapter is mounted, and the pencil is pointing at the included tapered head screw. This screw needs to be installed in the left/ front hole for accelerator pump clearance. The tan-colored two-barrel carb gasket is the same Fel-pro gasket (FEL-60124, $4.99) listed for the 340 or 440 Six Pack Holley carburetors.
With the three-bolts finger-tight, we were able to move the manifolds around enough to see if they were square to the edge of the ruler. The manifolds met up square except for the amount removed (.015) from resurfacing the exhaust manifold flange. See the orange washer? There are 10 of those triangular-shaped washers to help evenly secure the two manifolds to the cylinder head during assembly.
We cleaned up the side of the block with Eastwood’s Pre Painting Prep before painting. Eastwood’s High-temp Ceramic Engine Paint is the most durable engine paint we know of. For our application, we used Chrysler Red (PN 51621ZP, $31.99 quart) and their Urethane Activator (PN 21854Z, $14.99) for added durability and gloss.
While we were at it, the starter and these two brackets were cleaned and prepped with the Pre. Eastwood’s 2K Ceramic Underhood Black (PN 14147Z, $22.99) has two-part spray gun technology, and it’s as close as it gets to powdercoating. We used the Aluma Blast (PN 10109Z, $12.99) on the starter’s housing for that extra detailed look.
The thick Remflex gasket lined up well with all the ports. No need to gasket match the ports or gasket itself. For Slants with pitted and/or warped flanges, this thick (.125 inch) gasket can prevent leaks that the other thinner gaskets can’t.
Mock assembly showed us there wasn’t quite enough hood clearance using the 1-inch-thick Summit two-barrel carb adapter (left) with the K&N X-stream air cleaner in place. Using the Holley 3/4-inch-thick two-barrel adapter plate (PN HLY-17-90, $106.95), there was just enough clearance to close the hood.