How to Build a V8 Engine for Fuel Mileage

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There are many different ways to build an engine, but performance in any arena generally boils down to one thing: efficiency. Most automobile engines extract only 30 percent of the energy contained in the gasoline they consume; the rest of the energy is wasted through heat, light and high-velocity exhaust gases. While the common perception is that the smallest possible engine displacement yields the best fuel economy, this is only true if the engine still produces enough power to move the vehicle. The ideal would be to build an engine that changes displacement to fit the circumstances.

  • Acquire a used 305-cubic-inch displacement small-block Chevrolet, a 302cid (5.0-liter) Ford, or a 318 Chrysler LA-series engine. Send the engine block to a machine shop to be stripped down, cleaned and tested for cracks. Have the machine shop assemble the block using steel connecting rods, and forged 9-to-1 compression ratio pistons, powder coated with ceramic for thermal insulation. If you're aiming to build over 350 horsepower with a 305 or 318, you'll probably want to install a steel crankshaft and steel rods. You could assemble the block, but paying the extra money for an engine shop to build it will buy serious peace-of-mind.

  • Bolt a set of modern, fast burn, high-swirl cylinder heads to the engine block. For the Chevrolet, use a set of 1990s LT1 Vortec heads; the Ford should get the GT40P heads from a late-1990s Explorer, and the Chrysler 318 engine will be most efficient with a set of late-model Magnum heads. Seal the heads to the block using a set of chrome-moly head studs. Note: If you build a 318 with Magnum heads, order a set of custom-length pushrods from Mopar Performance.

  • Slide an hydraulic roller cam into the block using the widest possible lobe separation angle: at least 114 to 116 degrees. Otherwise, your cam should have about 190 degrees duration at 0.050-inch lift on the intake and exhaust, and no more than 0.300-inch lift on the intake and exhaust. Any number of stock LT1, 5.0-liter or Magnum camshafts will work, so shop around to find the smallest one available. Drop the matching aftermarket or stock roller lifters into the lifter bores, and bolt a set of fully-rollerized rocker arms to the heads. Use 1.5-to-1 rocker arms on the intake, and 1.6-to-1 rocker arms on the exhaust.

  • Bolt a long-runner, multi-point fuel injection intake manifold to the heads. For the Chevy, use a Tuned-Port Injection (TPI) setup; use a standard 5.0-liter EFI manifold on the Ford, and a late-model Magnum manifold on the Chrysler. Plug a set of stock fuel injectors into the manifold. Install the remaining fuel-injection equipment and ignition system; the procedure will vary greatly by engine.

  • Install a twin-turbo system using two small turbochargers, flowing no more than about 350 cfm (cubic-feet per minute) of air apiece. They should use internal wastegates. The aftermarket is rife with turbo kits for the Ford and Chevy engines, but kits for the Chrysler are more rare. In any case, the basic install procedure is to bolt the turbo manifolds to the heads, bolt the turbos to the manifolds, and then duct the turbo outlets to the intake throttle body, using the supplied plumbing.

  • Install a water/methanol injection kit to cool the intake charge, and provide supplemental fuel to keep the engine from exploding under boost. Installation procedures will vary by engine and kit, but generally involve inserting one or more water/methanol injectors into the boost tube, connecting the injectors to the electronic water solenoid, and running a water feed line from a reservoir tank to the solenoid.

  • Top the engine off with an electric cooling fan and electric water pump. If you can do without power steering, eliminating the power steering pump will net you noticeable improvements in fuel economy.

Tips & Warnings

  • An engine built to this specification can produce 250 to 500 horsepower (depending on the size of the turbos, and the amount of boost you opt to use), but high power output is not the point here. Using a pair of small turbochargers, electronic boost control, fuel injection and water injection will allow you to custom tune the engine air intake, and to best match power output to vehicle requirements. The small engine displacement allows you to idle around town and cruise efficiently on the freeway, while the turbos ensure that you always have enough power on tap, so that you never over-rev or over-stress the engine. Massive torque at low rpm is ultimately more efficient than revving your engine high just to get up to speed. This is a big part of the reason why a 5.7-liter 2001 Camaro can attain over 30 miles per gallon, while the high-revving, 1.3-liter Mazda RX-8 rotary engine struggles to achieve more than 20 mpg.
  • Higher compression typically offers better fuel economy, but forced induction engines require lower compression to prevent detonation. Those 9-to-1 pistons are low-compression enough to work with the turbos, but are really only suitable if you run less than seven- to eight-psi of boost, utilizing the suggested cylinder heads. If you run old-school cylinder heads, drop the compression to about 8.5-to-1. Drop it by the same amount if you run nine-to 14-psi with the modern heads. Anything over 14 psi will produce massive power, but will require 8-to-1 pistons, which will hurt your fuel economy, either off boost or at low boost levels.

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