At the O'Reilly Auto Parts Fall Street Machine Nationals in Springfield, Illinois, last year we spotted a dyno sheet on the windshield of Steve Engberg's '69 Mustang Mach 1. What demanded closer inspection was the sheet's 595 rear-wheel-horsepower (rwhp) number from a car that appeared to be more of a resto than a runner.
That led to a short piece in our Feb. '05 issue ("Make Mine Mach"), but what was appealing about Steve's 463ci screamer was the Nitrous Oxide Systems (NOS) Top Shot kit he had installed inside the repro Shaker hood air cleaner. Steve explained that he would have used a typical under-carb plate system, but the added height would not allow the hood to close over the Shaker scoop. Instead, he chose the Top Shot system, which worked extremely well. His normally aspirated power numbers measured 472 rwhp, while the nitrous pass revealed an astonishing 573 rwhp-almost a 100hp surge in power. Later, he tested the car again with a better tune-up on a much cooler day and squeezed out 592 rwhp on the bottle. We decided to learn more about this Top Shot system.
The idea behind the Top Shot is to make it quick and easy to install a nitrous system on a carbureted engine without removing the carburetor. Key components in the system are four nitrous and fuel discharge nozzles fitted into a round fixture designed to slip over the air-cleaner stud. The feed lines for fuel and nitrous are designed to easily route through the base of the air cleaner and could then be snaked through an innocuous rubber hose if total stealth is your goal.
What makes this kit even more fun is that the added power is adjustable between 75 and 150 hp using the same jets found in the more upscale NOS systems. The kit comes with jets for 100 to 150 hp. The kit is limited not just by the jet sizes, but also by the orifice sizes on the discharge nozzle. Replacement discharge nozzles are also available, so if you already have a plate system, you could buy the nozzle alone, add a few fittings and have a Top Shot system for a little extra fun.
Steve says the key to making this system work is to ensure the spray from the nitrous is not aimed directly at the carburetor booster. If the nitrous sprays into the booster, this will affect the booster signal and fool the carb into running full rich, which will kill power. Steve checked this by triggering the fuel side with an electric pump and aiming the fuel shot away from the carb boosters.
Combine an honest 125 to 150 hp with an incredibly easy installation and you have all the ingredients for maximum fun at the touch of a button.
Power ChartThese runs were performed on Steve Engberg's 463ci Mustang at Illinois Dyno Center, in Port Byron, Illinois, using an SAE correction factor applied to observed power during a 60-degree-F day in late October 2004. Test 1 is normally aspirated while Test 2 ran the Top Shot system with a 125hp jetting with an overly rich average air/fuel ratio of 9:1 to be safe. Keep in mind that the 125hp rating is for flywheel horsepower, so if you assume a 20 percent loss through the drivetrain, that makes the package worth 100 hp at the rear wheels. Steve's tune-up stepped up an honest 88 hp at the rear wheels despite the rich air/fuel ratio.
TEST 1 TEST 2 DIFF RPM TQ HP TQ HP TQ HP 3,000 450 257 - - - - 3,{{{200}}} 485 295 - - - - 3,400 488 316 542 351 54 35 3,{{{600}}} 486 333 585 401 99 68 3,800 494 358 610 442 116 84 4,000 530 404 618 471 88 67 4,200 563 450 615 492 52 42 4,400 563 472 610 511 47 39 4,600 543 476 619 542 76 66 4,800 527 481 623 569 96 88 5,000 525 500 617 588 92 88 5,200 525 520 598 592 73 72 5,400 515 530 564 580 49 50 5,600 489 521 - - - - 5,800 461 510 - - - - 6,000 397 454 - - - - Peak 563 530 623 592 Avg. 523 440 600 503Note: Only power values between 3,400 and 5,400 were used to create the averages for both tests.
As an interesting side note, Steve had performed a similar test on a humid 90-degree-F day when the car was not as finely tuned. Peak horsepower normally aspirated was a much lower 472 with 471 lb-ft of torque. When he hit the nitrous, the car responded with a much greater power differential, adding 121 more horsepower to 573 hp and 594 lb-ft of torque, a 122 lb-ft increase. This was due to the cooler and denser inlet air temperature improvement from the nitrous.
Parts List Component Part Number Price Top Shot kit w/10lb bottle 05090NOS $490 Top Shot kit w/5lb bottle 0509-05NOS 467 Fuel pump, electric, red 12-801-1 96 Nitrous bottle, 10lb 14700-SNOS 120 Top Shot module 13400-SNOS 151 All prices pulled from www.summitracing.com.All prices pulled from www.summitracing.com.
