The Competitive Advantage
Name Withheld Upon Request: I am a student at UTI and we have many classes, but there are three classes that most people want: Hot Rod 1a, Hot Rod 1b, and Super Street. Super Street is mostly computer tuning, but the Hot Rod classes include getting a Summit 350 Chevy short-block, taking measurements, and then modifying it. While I subscribe to CC, I have no idea about Chevy 350s because I am mainly a Ford guy. I was wondering if you could give me some pointers so I could be ahead of my class—or in other words, so I can cheat. I am not too sure how the class will be carried out, but from what I have heard we put the stock engines in a T-bucket and do a chassis dyno pull and compare it with the modified one. I know we can either have a large shot of nitrous or a Roots-type supercharger and a small shot. What would you suggest? I know that it will change depending on engine setup, cam, piston, head, intake manifold, and carb setting, but I don’t have those figures yet. If you could tell me how to get the extra horsepower and torque out of a 350 by doing small things—clearances, timing and so on—that would be awesome.
Jeff Smith: What a great letter. I wouldn’t consider what you are doing cheating—I prefer to look at it as creating a competitive advantage. But it’s a double-edged sword because we’re publishing the letter. You wanted a private response, but as Car Craft has more than a million readers, I would rather share this information with all of them, including anyone who might also be taking the class! I really don’t have enough information to give you a solid, this-will-put-you-at-the-top-of-the-class answer, but I have some generic ideas that can point you in the right direction. Let’s start with a comparison of nitrous with the supercharger.
Setting nitrous fuel pressure must be done dynamically if you want to be accurate. This Ze
If I were the instructor, I’d want the blower and the nitrous choices to be as close in power as possible. It appears this supercharger versus nitrous comparison has occurred in this class before, so I’m assuming the instructor knows what’s going to happen. The reason I’d want both combinations to be close in terms of power is to ensure the winner would be the person who is going to assemble the engine properly and then—here’s the real key—tune it so he or she gets the most out of the combination. The big thing I see among those building high-performance engines is that once they have all the parts bolted on, they think the job is done. The truth is that they’ve taken the process only half way. Tuning to make the most power with what you have is the difference between the winners and the also-rans.
I’m not sure why the blower gets a shot of nitrous, though I’m guessing you’re using a mini-blower that can’t make the power of a typical 150hp shot, so the blower gets tickled with an additional 50hp shot or so. More than likely, both the nitrous and fuel jets will be fixed so you can’t alter them. I would do that to prevent a tuner using too lean a fuel jet in search of a hero horsepower number. Assuming this, I’d choose the straight nitrous setup because it is simpler, as you are managing only one power-adder rather than two. I also am going with the nitrous because while superchargers make positive manifold pressure, they also make heat. The classic mini-blowers such as the Weiand or old B&M blowers are generally not very thermally efficient and tend to put more heat into the inlet air (exceptions to this are the Magnuson and the Kenne-Bell). This reduces air density in the manifold. You will get boost, but the air is hot. Hot air is more prone to detonate, so you have to be careful with ignition timing, assuming you’ll use pump gas.
If you choose the straight nitrous route, there are plenty of tuning issues that can be addressed. For example, make double sure that the bottle you are using is full and up to pressure. Weigh the bottle on an accurate scale and compare the total weight as listed on the bottle label. For example, a full 10-pound nitrous bottle might weigh something like 25 pounds, with 10 pounds of liquid nitrous combined with a 15-pound weight of the bottle. For pressure, most nitrous systems use 900 to 950 psi as a starting bottle pressure. According to a chart listed by Nitrous Supply, a bottle temperature of 85 degrees F will produce 950 psi, while a lower temperature of 80 degrees F will drop the pressure to 865—almost 100 psi lower. Lower nitrous pressure will produce a richer overall air/fuel ratio, as the optimal air/fuel is based on the 950-psi number. If you don’t have an accurate pressure gauge (not one of those 1-inch-diameter gauges) to ensure proper pressure, use an inexpensive infrared temperature gun to measure bottle temperature. I would ensure the bottle pressure is adequate by immersing the bottle in a tank with 100-degree hot water to raise the bottle pressure. Once the pressure is up, keep the bottle warm until it’s time to use it. Never use an open flame like a torch or propane flame to heat the bottle. A full nitrous bottle is under immense pressure, and an open flame can create a crack and cause the bottle to split. It’s happened more than once. Nitrous is not a fuel, but any gas under this much pressure is equal to a small bomb that can cause major damage if not treated with the respect that it deserves.