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High Horsepower GM Crate Engine

Think of this as a Saturday Night Specail small-block with the amp turned up to 11.

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Hands On

Despite what the boo-birds on the Internet think, there are hundreds of small-block Chevy stories yet to be told. This is one of 'em. Car Craft staff meetings are great places for "what-if" musings. The question at one such meeting was whether we could make a survivable 500 hp from nothing more than a brand-new Goodwrench GM crate engine, a budget camshaft, and a single-plane intake--topped with a simple nitrous system. Right there on a yellow legal pad was born the Saturday Night Slayer.

For the benefit of the few car crafters who may not know about the Goodwrench GM Crate engine, it's only the most affordable crate-replacement 350ci small-block Chevy ever offered by GM. This GM crate engine comes with four-bolt mains, a two-piece rear main seal cast crank, cast pistons, a very low static compression, iron heads, and a very-short-duration, flat-tappet camshaft. It is the essence of the original small-block Chevy, and the best part is it's ridiculously affordable. We got GM crate engine from Scoggin-Dickey Parts Center in Lubbock, Texas, for $1,500 plus shipping. You could probably rebuild a small-block for that price, but who would give you a three-year, 100,000-mile warranty and all-new parts? GM does if you leave the engine stock. Our plan killed those warranty fantasies as soon as the engine cleared its crate.

Did we mention this GM Goodwrench engine has no compression? While GM Goodwrench claims 8.5:1 (Scoggin says 8:1), we measured 7.9:1. That's the reality, and it allows this motor to run on moose piss for fuel, but it's hardly conducive to excellent performance or mileage. We've had some experience with these engines, and each one we've beat on has survived to live a long, prosperous life. This latest effort plans to tax that longevity reputation to the max with a hefty whiff of nitrous.

The plan is to make 500 hp with minimal cash. That sounds simple enough, but it's a bit more challenging on the execution side. The easiest way to increase the GM Goodwrench compression is with cylinder heads, but that slaps the budget around a bit, so for this first effort we decided to retain the anemic 1.94/1.50-inch valve heads. A camshaft, however, was certainly within our modest means when we found a Summit flat-tappet hydraulic with appealing specs priced at barely more than $50. We didn't even need to buy lifters because we could use those that came with the engine. We're still smarting from the flat lobes used during a recent engine build, so we opted for 5 quarts of Comp's new Break-In 10W-30 that ironically cost almost as much as the camshaft. That's the price of ensuring cam survival these days, as 5 quarts of regular oil and a can of EOS is virtually the same price.

We also included a few other necessary pieces to complete the engine, such as a harmonic balancer, a carburetor, an ignition, and a set of headers. These invisible pieces can drive up the price of a complete engine and are often overlooked. In a second installment, we intend to bolt this motor into our Orange Peel '66 Chevelle to evaluate how quick we can make this entry-level power burner run while also revisiting some basic performance- tuning recommendations that anyone can do. But first, let's get into the game by loading up our Saturday Night Slayer and hitting the button.

Cam Specs

Camshaft Duration at 0.050 Valve Lift lobe-separation angle
Stock cam, int. 195 0.390 114
Stock cam, exh. 202 0.410
Summit, int. 224 0.465
Summit exh. 234 0.485

GM Goodwrench Tuning Notes

NOS recently changed its tuning recommendations based on input from a recognized nitrous-tuning expert. Note also that there are different ignition timing specs depending on combustion efficiency. We took this to mean the combination of compression and quench. We used the low-efficiency number because of our engine's weak compression. We've listed only the two tune-ups we tried and changed the NGK spark plug recommendation to Autolite heat ranges for the plugs we used. Your actual plug part numbers will depend on the GM Goodwrench engine application.

Nitrous HP Combustion Efficiency Ignition Timing Spark Plug Fuel/Octane
150 Low 29 AR133 100+
150 Medium 27 AR133 100+
150 High 25 AR133 100+
200 Low 24 AR132 110+
200 Medium 22 AR132 110+
200 High 20 AR132 110+

To make the most of our test day, we skipped the entry-level baseline but found an earlier test of this engine using the stock cam, Q-jet intake, a 750-cfm Q-jet carburetor, and a set of 1-5/8-inch headers that produced 348 lb-ft of torque and a wheezy 254 hp. After we added the new cam and intake, we were ready for our normally aspirated baseline. We expected the bottom end of the curve to be weak, as we were adding 29 degrees of intake duration and a single-plane intake manifold. Despite those changes, the Goodwrencher delivered nearly 50 lb-ft over the original package at 2,500 rpm and twisted 355 lb-ft of torque at 4,100 with 332 for horsepower at 5,500. We chose a single-plane because it promised to work well with nitrous, but a good dual-plane such as a Holley or an Edelbrock Performer RPM would pump the torque as much as 20 lb-ft with no significant loss of peak horsepower. This would be the best choice if nitrous were not part of the equation.

We were pleasantly surprised with the torque and horsepower numbers from the normally aspirated combination, but our goal was 500 hp, so it was time for the squeeze. Nitrous Oxide Systems has recently changed its tuning recommendations for all its nitrous kits by leaning the overall air/fuel ratio. For example, older NOS Cheater kits recommended an 0.063 nitrous jet with an 0.071 fuel jet for its 150hp combination. But the new kit uses 0.063 jets for both fuel and nitrous while maintaining the same 5- to 6-psi fuel pressure recommendation. NOS also offered new insight into how much timing should be retarded. The new tune offers timing retard recommendations based on three levels of combustion efficiency (low, medium, and high), with the highest timing allowed for low-efficiency combos. Since our Goodwrench engine barely pushed 8:1, the recommendation for a 150hp shot was to pull the timing back to 27 degrees.

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1 comments
mrduck05
mrduck05

Very informative but I do have one bit of criticism. Whenever you guys build an engine with stock heads you never list the casting numbers. That info would help alot.

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