Tech Q&A

The editors of Mopar Magazine answers reader's technical and general service questions.

I have a 1977 Gran Fury (ex patrol car). At the back of the exhaust manifold there is a small round hole covered with a screw and a cap cover. What in the world would this be? The bolt rusts and falls out and is difficult to replace. Was this a casting hole?

MM: Many similar holes of a curious nature have come and gone throughout the years. Some are casting/machining necessities; others are often for use with optional emissions equipment, which may have been needed on your vehicle. In any case, where the factory has plugged it, the hole obviously has no function and should remain plugged. If the original plug has not been treated well by time and the elements, we typically recommend replacing it with a thread sealant and covering it with a heavy application of engine paint.

I own a 1972 Dodge Swinger with a 340 engine and 904 transmission. Rear end is 8-3/4 and it has a 3.55:1 gear ratio. But at 65 miles per hour it’s taching at four grand. Did all the small blocks with this combination tach this high? And is this too much for cruising around town? How long would you keep it at this rpm on the highway?

MM: The calculator tells us you’re running 19.5" diameter rear tires, which would normally be something found on a tiny car. Run out and measure your rear tire diameter. If it’s 20" or larger, there’s something wrong. Possibilities are too numerous for the space provided here, but first make sure you’ve got a 3.55:1 gear ratio. Investigate speedometer and tachometer accuracy, then look into the transmission. Having said that, your 340 at 4,000 rpm produces piston speed of 2,200 feet/second, which actually poses no appreciable durability issues.

I usually see valve springs recommended along with camshafts. I understand more aggressive cam designs may need more valve spring pressure to control them, but often I see several springs recommended for a given camshaft, and pressure specs can vary quite a bit. What are the guidelines to help select the right springs? When is it OK to choose lighter springs and what conditions require heavier springs?

MM: If you see several springs recommended for a particular cam, they will all work. A good general rule in choosing a spring from within a recommended range is to consider valvetrain weight. Production valves are usually heavier than high-performance stainless valves. Production and chrome-moly retainers are heavier than are aluminum and titanium retainers. Stamped production rocker arms are heavier than aluminum roller rockers (with iron or steel mechanical or an adjustable rockers somewhere in between). If your valvetrain tends toward the heavier side, pick a heavier spring.

I’m putting together a 1968 Barracuda that will be a pretty serious drag car, but it will also be driven on the street occasionally. My approach is to build a mild motor (around 400 horsepower, small block) so I don’t have to run a lot of gear, converter or rpm. What are the little tricks you might be able to offer to make sure I’m not wasting power needlessly?

MM: We can break things down into three main areas to improve: drivetrain or chassis movement (flex), engine drag (or parasitic loss), and reciprocating weight. Getting the most engine output to the pavement means examining the path it takes to make sure little power is wasted along the way. We don’t want the engine, transmission, or rear end moving around, so use solid engine and transmission mounts to eliminate their movement. Polyurethane suspension parts will absorb less energy and let the car go where you want it. Chassis flex can also waste a lot of power. Think how much torque it must take to twist an entire car. This is where subframe connectors and roll bars or cages will really help stiffen the car up. Parasitic drag on an engine also robs lots of power. Accessories create drag in order to operate—things like A/C compressors and power steering pumps are driven by the engine, so we toss those right away. Opt for electric water pumps and cooling fans. In the weight area, hopefully we’ve already tossed anything unnecessary or unrelated to acceleration and traction. This is where lightweight wheels and tires, aluminum driveshafts and racing brakes come in. We once heard a story of a racer who picked up 0.3 seconds just by switching to a race brake kit from factory brakes.

I have a 1968 Road Runner and I would like to convert it over to front disc brakes. I would like a list of what parts I need to do the job and where I could get them. I prefer not to use aftermarket parts if possible. The car has 15" factory wheels.

MM: Try the following Web site. A lot of enthusiasts at Mopar® have used this set up on their personal cars and it’s worked out very well. Here’s the address: http://www.moparaction.com/Tech/archive/disc-main.html.

I have a 1973 Dodge Charger with a 400 cid engine and everything factory. Mileage is 63,000. I removed the 727 transmission and installed an A-518 with overdrive and lockup. Everything works great, but I have an engine vibration problem since I changed the torque converter. Is there a technique and/or device that will assist in balancing the engine and getting rid of the vibration?

MM: MM: First of all, the torque converter should be removed. Then a Mopar Weight Kit and Positioning Template Package (P4120241) should be added. Using the kit, two small weights are welded to the torque converter and the converter and trans are then reinstalled. This should solve your imbalance problem.

I own a 1984 3/4-ton Ram 2500 pickup with a 360 engine. I was planning on putting a 1975 440 engine (out of a motorhome) in my Ram, but there is a half-inch high ridge on the exhaust face of the head that keeps the header from facing up properly. Can I machine this lip off without damaging the head?

MM: It would be difficult to machine the lip off properly. Your best bet would be to modify the header flange.