Making Mopar Slant-Six Sense, Part 3

We Port and Shave a Slant Six Cylinder Head for Much-Needed Power Gains

If you missed the last two installments of “Making Slant Six Sense,” here’s a rundown of what we’ve been doing to build ourselves an enjoyable Slant Six street-driver combo.

In Part 1 (Nov. 2018;, we added Performance Distributors’ Tri-Power electronic ignition and Livewires ignition wires, which jolted its performance significantly. Then we installed and dyno-tested TTI’s new 2 1/2-inch exhaust for the early A-body (‘63-’66). It upped the performance to the tune of 9 rwhp and 10 lb-ft (a 15 percent gain). Those two bolt-ons helped our Dart improve drivability, but it still didn’t have enough performance for today’s roads, which are often jammed with displeased drivers and higher cruising speeds. At this point, total output on the Mustang chassis dyno was a paltry 71 hp and 119 lb-ft of torque.

For Part 2 (Jan. 2019;, we complemented the TTI exhaust with an Offenhauser four-barrel intake fitted with the Holley two-barrel (350 cfm). Back on the Tune Time Performance chassis dyno, we saw peak gains of 19 rwhp at 3,500 rpm (71 to 90 rwhp) and 17 lb-ft at 3,000 rpm (119 to 136 lb-ft). This mod added 26 percent more go to our tilted six-in-a-row. Those were big-time gains, but we still needed more. On to porting and shaving the stock head.

As you can tell, we prefer to test one modification at a time to find out how much power fellow Slant Sixers can typically gain from each specific change. That’s why we’re saving a cam upgrade for a future issue. A cam swap alone without porting the head would not enable the cam to move as much airflow through the engine. For this installment, we’ll port and machine a Slant Six head for added airflow and a point of compression increase. A chassis dyno test will confirm how much our efforts were worth. Some folks may feel it’s not worth the effort, while others, like us, feel significant power improvements (10 percent and more) are always worth the hard work, time, and expense.

We consulted with Dave Hughes from Hughes Engines and Slant 6 guru Doug Dutra on our intended buildup to help make our combination a fun street, daily-driver. For our upcoming cam swap, they helped us select the right camshaft based on our calculated compression ratio, rearend gearing, converter stall, and most importantly flow bench test results on our ported head.

We must mention that Dutra just wrote a new book, Chrysler Slant Six Engines: How to Rebuild and Modify. He put his decades of experience building many different street, street/strip and race combinations of the Slanted engine into this book. With these two knowledgeable engine guys’ help and advice, our Leaning Tower of Power will live up to its nickname.

Before porting and shaving began, we performed a cranking compression test and were disappointed to learn there was only 100 psi at each cylinder. We knew this was a sound engine that was rebuilt in the ’90s. It never used any oil, and the plug readings were always perfect. So we suspected the replacement pistons had a much lower piston-to-deck height than stock (typically 0.150 inch). When we removed the stock cylinder head, we found a ridiculously low piston-to-deck height of 0.220. At that low a piston-to-deck height, along with a 0.040-inch-thick head gasket and stock-size 60cc combustion chamber, our calculated compression was only 7.2 to 1—what a joke! Factory ratings were 8.2 to 8.4 to 1, ugh. The head we’ll port and mill for this test was the same casting number mounted on the rebuilt engine in our ’66 Dart GT.

Exploring New Territory

The porting work performed was labor intensive, averaging us two to three hours per port. Porting requires patience and the right tools, and is not for the faint of heart. Dutra told us the intake port was limited, so we used our 25 years of experience to add 25 and 48 cfm to the intake and exhaust ports, respectively. We also learned the Slant’s intake port is very responsive to back-cutting its intake valve. Here we’re exploring new territory, but what applies to a V-8 also improves flow on Mopar’s legendary six-cylinder head.

Check out the before and after porting pictures to see our handiwork. Notice on the flow-bench test results how the stock ports didn’t flow any more air after a 0.400-inch lift. After porting, take note of the big airflow improvements from 0.400-inch lift to 0.500, and also at the low and mid lift. The Hughes Engines’ high-lift cam (0.483-inch) we will be using in our follow-up test will allow us to really take advantage of all this extra flow.

After completing our porting to all of the ports, we brought our six-banger head back to IDM Speed and Machine in Manahawkin, New Jersey, for finishing machine work, assembly, and setting the new valve spring heights. These are our go-to guys for quality engine machine work and careful assembly. Ed Hickey Jr. handled the precision three-angle (60, 45, and 30 degrees) valve job on all the seats using IDM’s Serdii machine. He also back-cut the intake valves, which not only improved flow at low and mid lift but also at high lift on IDM’s flow bench. This is something we’ve never seen before, and it only happens on a Slant Six head. Go figure.

Before and after shaving 0.070 inch from the deck, Hickey cc’d the combustion chambers. Stock was 60cc; after head milling, the chamber measured 50cc. By using a stock 0.020-inch steel shim head gasket with the shaved head, compression can go up a full point, from 7.2 to 8.2. If we used the typical 0.040-inch composite head gasket, our calculated compression would only be 7.9 to 1.

After installing our reworked head and test running our inline-six, the cranking compression was now up to 120 psi. Ideally, if our cranking compression was around 150 psi, our Slanty would have a bunch more power, but we’re working with an abnormally low compression engine, so for now we’ll stick with what we have. We are thinking this winter about building a short-block with roughly 10 to 1 compression. It would make for   a really cool 93-octane, pump premium, daily-driver Super Slant Six. I suppose the good news is we can run 87-octane with our current combo and save a bunch of gas money.

Up and Running

With the engine now up and running, a test ride confirmed we definitely gained power with no tuning changes. Before going back to Tune Time Performance’s chassis dyno, we tried some seat-of-the-pants tuning with plug readings. It felt the Slant wanted about 5 degrees more timing (15 degrees initial, 40 total). Different jets other than the 64s didn’t indicate any improvement, and the plugs darkened with richer 68 jets. On our 40-minute ride to Tune Time’s dyno, improved drivability and power was noticeable. Our Dart was effortlessly cruising at 75 to 80 mph. Better acceleration onto the Garden State Parkway was also appreciated. This made us anxious to see how much power our efforts were worth.

On Tune Time’s Mustang chassis dyno we noticed the air/fuel ratio (A/F) was right-on at 14.7 to 14.9 cruising at different speeds. During the pull, it looked to be a bit rich at 12.2 to 12.3 A/F. The dyno showed us we were now up to 108 rwhp at 4,400 rpm from our previous 90 at 4,000. Torque climbed from 136 lb-ft at 3,000 rpm to 148 at 3,200. At points during the pull, it was making 20 lb-ft more torque than before.

For most naturally aspirated engines, a 12.6 to 12.9 A/F ratio is ideal for an optimum tune and the most power. That led us to decide to reinstall the Holley 350’s box-stock 61 jets from the 64 jets. The smaller jets help our A/F to be spot-on at 12.6 to 12.7, but we lost 2 hp. It looked like our low-low compression Slant Six combo liked to run a little bit rich (12.3) for most power. We also tried different timing settings, but our Six preferred the 15 initial and 40 degrees of total timing.

All in all, a 20 percent gain in power feels much better as we continue our quest to get our ’66 Dart GT towards 21st century performance standards. We don’t like the fact that the average minivan and econo-box has a performance advantage over our classic A-body Mopar. We’re getting there and hoping the cam swap will give us another 18-20 hp. If it does, we’ll have more than double the power of our baseline!

Flow Bench Test Results

Slant Six cylinder head, casting No. 169. Tested on IDM’s self-built flow bench at 28 inches depression.

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