Texas Classic Chevy Experience: Engine Building for Power and Gas Mileage

The Perfect Hot Rod/Mileage Maker Engine for Your Classic Car

-Alan Arnell

I have been thinking about engine camshafts and I wonder what exactly kills gas mileage? Is it total seat duration, duration at 0.050 inch, valve overlap, valve-timing relationships in general, or what? My mind went further to ponder what causes an engine to be poor on gas mileage? Is it too little low-end torque? What could it be? I mean, I think I know that the bigger cams kill gas mileage but exactly what and how? What would be the perfect cam for a hot daily driver?

Here is what I researched and found out about finding that happy medium between horsepower and gas mileage:

First, the most important thing to help get good fuel economy is lower engine speeds. Bottom line the fewer times the engine breathes in fresh air and fuel, the less it will burn. Getting good fuel mileage is all about making good torque at engine speed it uses running down the road. This is expressed in “road-load horsepower.” Most Classic V-8s run at highway speeds of three to four thousand RPM’s Thus, that is why those cars only get 12 to 18 miles per gallon. Since cars of the 2000 era were tested for emissions at 55 mph hour those engines were engineered to run at the most efficiency at 55 mph.

Side bar: I don’t know how it is in you neck of the woods but in Dallas if you drive 55 on the freeway you will get run over. I drive on average five miles over the speed limit and have to stay in the slow lane! I think if I were to build an engine I would have the “road-load horsepower” set to cruise at 70 miles per hour. Just saying.

If you convert the “road-load horsepower” number into torque at operating RPM, it will give you an idea of the torque required to get down the road. For example, an engine running down the road at three-thousand RPM only takes around 28 to 42 lb-ft to keep going 55 mph. If you lower the engine speed to 1500 RPMs, the torque needed jumps to 56 to 84 lb-ft. Now you can see why it’s so important to have excellent part-throttle torque.

Creating part-throttle torque requires a combination of the correct-size intake parts and primary header and correct compression ratio spark advance along with camshaft timing. Where the camshaft comes into play is in the overlap cycle and how it affects cylinder pressure. When an engine has more overlap, this increases overlap airflow. This is when the exhaust system aids in starting the intake flow. When the engine has a high overlap flow, the intake charge can flow right through the chamber and out the exhaust. High overlap also allows the vacuum in the intake system to draw exhaust back into the intake system at slow engine speed. This dilutes the inlet charge with the exhaust that can’t burn again. This is what is called “built-in EGR.” Thus-performance engines several year ago were able to pass emissions without the use of a EGR system.

The intake valve closing point is very critical in building low-speed torque. This point is critical in building cylinder pressure. If the engine can close the intake valve early (advanced), it gives the cylinder more crankshaft degrees to build cylinder pressure. If you close the intake valve later the engine will have lower cylinder pressure. When the engine closes the valve early it will rock you torque curve to produce higher slow-speed torque or the expense of higher RPM power This is where the balancing act comes into play.

Smaller engines produce better fuel economy numbers because of their smaller displacement. This is also at the expense of high power levels. Although you gain with smaller engine with lower frictional losses, and you can make better horsepower per cubic inch, this rarely makes up the difference for the larger displacement engines.

That being said, to build a performance/mileage monster, the engine must hav EFI. This delivers all the benefits that the OEMs have grow to love including accurate fuel/air metering at low engine speeds. It is very difficult to produce correct air/fuel ratios with a carburetor at lower revs with performance engines. It would be best to build a 350 over to a 383. Go with the highest flowing, smallest intake-port aluminum cylinder head you can afford. Use 1 ⅝-inch primary tube headers and a good performance dual exhaust. Run you compression ration in the 10.0-10.5:1 range.

For the camshaft, it is suggested to keep the 0.050-inch tappet lift duration numbers in the mid to high 210s on the exhaust side. It is a must that the engine run a hydraulic roller design. You will be leaving too much on the table if you do not . Use a lobe separation angle of 112 degrees and an intake centerline of 107 to 109 degrees. With the above engine package, running the engine between 1.600 to 1,800 rpm will yield a low -to-mid 20-mpg potential. This engine will also build 400 to 425 hp and 420 to 430 lb-ft of torque. This will give you a very good performance car and give you your dual-purpose hot rod.

Wow, no wonder I could not get that engineering degree! To do this I would have to lose my 3:42 rear gear and my Borg-Warner 4 speed. More to think about.

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