GM might not have made the 305 as performance-oriented as its big brother 350, but they sure made a lot of them. The Chevrolet 305 was the engine of choice for millions of full-sized GM cars and trucks that were too large for a V6 but didn't require a 350 just to cruise around town. However, hot-rodders from the dawn of time have made a religion out of modifying just such pedestrian offerings and turning them into hard-core racers. While it may be GM's second son, the 305 is just as capable of producing 350-plus horsepower as any of its larger siblings.
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Things you need
- All required engine parts and engine overhaul kit
- Porting kit
- Socket set, metric and standard
- Torque wrench
- Gasket maker or silicone
- Phillips screwdrivers
- Flathead screwdrivers
- Allen wrenches, full set
- Assembly lube
- Air compressor and sandblasting gun
- Painter's masking tape
- Welder and welding supplies
- Basic cutting and fabrication tools
Begin with any late 1980s 305 engine and send it to a machine shop to have the block disassembled, cleaned, checked for cracks, bored 0.030-inch over and aligned bored to bring it into factory spec. Purchase a set of forged aluminium race pistons designed to deliver 10-to-1 compression with a 54cc combustion chamber and forged or powdered-metal connecting rods. Have the machine shop balance the assembly before you install it.
Purchase a set of 1985 to 1991 Corvette (L98) cylinder heads and perform some port clean-up to enhance flow. Gasket-match the intake ports and exhaust ports by laying an old gasket atop the mating surface, spraying it with a light coat of white spray paint and then removing the white ring around the port with a carbide cutter bit. Smooth the removed area about an inch into the port.
Coat the piston tops and the insides of the intake-exhaust ports with thermally-insulating ceramic powder-coating. You could do this yourself, but it requires a specialised oven and delivery gun, so powder-coating with ceramic is probably best left to a professional shop. Note: don't coat the combustion chamber roofs with ceramic for this application.
Pay your machine shop to assemble the engine's bottom end and install the cylinder heads. Another job you could do yourself, but the block will already be there and for this application you'll probably want the security of knowing the engine has a warranty.
Slide a 1989 to 1991 Corvette camshaft into the block, and bolt a high-performance dual-plane intake manifold to the heads. Install a set of ceramic-coated headers with equal-length, 1-1/4 inch primaries.
Bolt a wet-flow, plate-type nitrous system to the intake manifold and follow the manufacturer's recommendations for nitrous solenoid and plumbing installation. Install 75-horsepower jets for your first-stage tuning. Bolt a set of 1.7-to-1 roller-tipped rockers to the heads to actuate the exhaust valves, and use a set of 1.6-to-1 rockers to actuate the intake valves. Using two different rocker arm ratios will mimic the effects of a nitrous-specific camshaft, allowing you to use a stock camshaft for idle quality while optimising the engine for N20.
Bolt a 500 cfm carburettor to the manifold atop the nitrous plate, and install a GM High Energy Ignition (HEI) distributor with 8mm plug wires. Use a set of high heat-range plugs that can withstand the nitrous, and use a nitrous-compatible ignition control box to retard timing while running under nitrous.
Tips and warnings
- An engine built to this spec should easily produce at least 250 horsepower and 145kg-ft of torque off of the nitrous, and at least 350 on a 50 horsepower nitrous hit. If that math doesn't seem to add up, it's because nitrous jets are typically rated for engines not optimised for the juice. Using longer exhaust rocker arms and a nitrous-retard ignition box will add some power and an extra margin of safety.
- You'll need to replace the stock cast crankshaft with a stronger steel unit if you're planning to increase the nitrous to anything over about 380 to 400 horsepower.
- Consider injecting propane instead of gasoline with your nitrous system. Propane has a higher octane rating, will make more power than nitrous and will help to cool the cylinder for increased durability.
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