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Building a 400 HP 357 SBC - Quick Draw .357

We Build an All-Star, Buck-Bangin', 400hp 357 SBC Using CC's Budget-Tested Parts

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This left us with the selection for a carburetor. Here's where the choice was really easy. If you recall, our original test was confined to vacuum secondary carburetors costing $299 or less. For fun, we also tossed in a vacuum secondary 0-3310 Holley 750-cfm carb we bought at the swap meet for $85. Of course, it didn't run when we first tried it, and it took an idle circuit roto-rooting to make it work. But once we tuned it up, it went from the doghouse to the shady spot on the front porch just beating out the new Holley 750 carb. So we brought the old dog out of retirement and planned to test our swap-meet hero against a 750-cfm HP mechanical secondary carb to see if there was any difference.

To round out our All-Star small-block, we added a set of Hedman 13⁄4-inch headers for a Chevelle and added a pair of Dyno-Max mufflers with 3-inch lead pipes to get closer to a typical street application. We retained Westech's electric water pump because it could cool the engine quicker in between runs, and we also used an MSD distributor and 6AL ignition because it plugged right into Westech's dyno ignition. We also decided to try the Voodoo 1.6:1 roller rockers. Once the cam was formally broken in by running it at various load levels for a solid 30 minutes using the Lucas SAE 30 Engine Break-In oil, we were ready to make some noise.


Unholstered Results

We started with the dual-plane RPM intake in place along with the swap meet 750-cfm vacuum secondary carburetor. Once we determined that this engine liked 40 degrees of total timing and an air/fuel ratio of around 12.8:1 at peak horsepower, the first pulls on the engine indicated we had a problem once the engine got around 4,600 rpm. It was clear that the small-diameter valvesprings were not able to handle the higher acceleration rate of the 1.6:1 Lunati rocker arms. Since time on the dyno was limited, we decided to go back to the standard 1.5:1 roller rocker ratio. After a little more tuning, the 357 picked up to 402 peak horsepower at 5,200 rpm with a peak torque of 434 lb-ft at 4,400. This was both good and not-so-good news. The good news was that this little engine was making more than 420 lb-ft of torque at 3,000, which means it will accelerate well even in a heavy car with an automatic and tall gears. But the peak power was certainly not as strong. We were shooting for more like 425 to 430 hp, which would be a true 1.2:1 hp per cubic inch. But looking at the graph of the power curve (page 18), it was obvious that our small valvesprings were holding us back. If you look at the torque curve, at 4,400 the curve takes a sharp downturn, which indicates that the springs were having trouble controlling the valves, causing valve bounce, which immediately begins to lose power. Our recommendation would be to go with the 195cc heads with the larger-diameter 1.440-inch valvespring or, if you have a similar problem on your engine, choose a spring similar to the spring recommended by Lunati. The Jegs head spring was rated at 100 pounds on the seat with around 300 pounds of load at the 0.525 max lift with a rate of 380 pounds per inch. A good spring like the 1.440 that Jegs offers would have a 333 lbs/in rate with 130 lbs of load on the seat and 300 lbs with the valve open. In discussing this with Steve Brulé at Westech, a dual spring with a slightly better rate could be worth an increase of peak horsepower rpm to 5,800 with perhaps as much as a 20 to 25 hp gain. This would put this engine at 430 hp at 5,800 rpm, which is what we expected. The larger spring would also allow us to return to the 1.6:1 roller rockers, which should also improve the overall power curve.

To eliminate variables, we also dropped on the Holley 750 HP mechanical carb for a quick carburetor evaluation. The HP offered a slightly richer fuel curve but really didn't give us any significant power increase, which just proved to us that our budget Holley 750 vacuum secondary carb was worth all the trouble to make it work.

Next, it was time to test the Holley Strip Dominator. Typically, a single-plane intake trades low-speed torque for more peak horsepower. The key is whether the trade-off will allow the car to run quicker in the quarter-mile by shifting the power curve into the higher engine speeds. With our 357ci engine, the valvesprings limited the engine's top-end potential, and unfortunately, the big single-plane really hurt the midrange power curve. As you can see by the dyno graph, at 4,200 rpm the single-plane lost 38 lb-ft of torque with a similar difference all the way through until peak horsepower, where the single-plane finally made more power. As far as this combination is concerned, the dual-plane is a far better choice. We would need bigger, higher flowing heads, and a bigger cam to justify a single-plane intake.

Perhaps the best part of this All-Star package isn't that the peak horsepower is all that impressive in these days of simple 500-plus-hp LS engines, but that we have a very simple small-block with mild heads, a budget flat-tappet cam, and pump gas compression that offers excellent midrange torque that should make a typical street car really fun to drive. In terms of quarter-mile times, we plugged the curve into our copy of the Quarter Pro simulation, and in a conservative 3,550-pound car with 3.55:1 gears and a three-speed auto with a 2,800-rpm converter, like our Orange Peel Chevelle, the program says that even with a slow 1.90 60-foot time, this engine should be capable of 12.50s at 104 mph. That's what we'd call a Quick Draw ride on a Shirley Temple budget.

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