Gas HogI have an '83 Dodge Ram with a .030-inch overbored 318. The engine came out of a Diplomat police car of the same year. It came from the factory with a windage tray, double roller timing chain, and the big heads. It has a cast-iron spread-bore intake and a Holley rebuilt Thermoquad carb, which actually works well. It has the EGR on the intake also. The cam is a Melling brand. I would have to go get the card to give the exact specs, but off the top of my head it was a .440 lift. I don't remember the advertised duration. I know it was not more that 270 degrees. The 727 trans has the stock low-stall torque converter that was in the truck originally.
I did rebuild the trans and used a Trans-go shift kit with stock gearing in the rear.
With the original 318, the truck got around 15 mpg and the police car engine got around 14 mpg when it was in the police car. I cannot get better than 10 mpg with this engine in the truck. I would not think that the cam change would cause me to lose 4 mpg. I used the stock distributor that was on the truck instead of the system that was on the police car. I wonder if I have a problem with the EGR valve. Yes I have checked for fuel leaks, and the Thermoquad seems to be the only one.
Do you guys have any suggestions on how to get better gas mileage with this old beast? It's eating me out of house and home at the pump. The engine seems to run fine, but at less than 10 mpg it should leave the paint job laying in the street every time I stomp on the foot feed. When I hook up my trailer gas mileage gets even worse. You guys have a great magazine, keep up the good work.CCAVia e-mail
It's hard to narrow down just where you went wrong, but there are three areas that are the most likely contributors to your problem, and only two of the three are things you can readily do something about. First, the one you can't do much about now, compression ratio. You didn't say which pistons you used in the rebuild, but I suspect they were the basic rebuilder's cast pistons. Unfortunately, these are typically as much as .090-inch in the hole once the engine is assembled. With the police 318, which uses the large chamber 360 heads, the stock compression ratio typically measures in the high 7s, and if you didn't pay attention in selecting pistons, you might be in the mid-to-lower 7s now. It's pretty hard to get good fuel efficiency with a ratio as low as a depression-era Fraiser. Add a performance cam, and cylinder pressure comes down even further, making the mileage situation worse. You can crutch the ratio upwards with a small chamber head like the Magnums and some thinner Mopar Performance head gaskets, but really that's no substitute for the appropriate piston choice.
If my hunch is wrong, and you have a decent compression ratio, the problem is in the tune. The basic components here are fuel and ignition. ThermoQuads will still run well at very rich air/fuel ratios, masking an overly rich condition. The TQ is a complex piece, and there are several conditions that can cause excessively rich running. Our recent article on tuning the TQ explains this carb in detail. On the ignition side, the advance curve of your truck is controlled by the computer. Make sure the system is functioning properly, providing the needed spark advance. Diagnosing these electronically controlled spark advance systems gave expert dealer techs fits, so be prepared with a factory service manual and plenty of hours if you want to troubleshoot the stock system. The quick and easier fix if the ignition advance system is the problem is to swap the ESA system for the conventional Mopar electronic ignition with a kit from Mopar Performance.
I'd put the truck on a chassis dyno with a wide-band lambda sensor to zero in on the mixture and tune. Otherwise, getting it dead-on is a very hit or miss proposition.
Do you have a tech question for Steve Dulcich? Send it to Mopar Muscle, Performance Clinic, 9036 Brittany Way, Tampa, FL 33619; mopar.muscle@primedia.com.