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In the August 2011 issue, we completed the install of a full Nitrous Express late model Hemi system. In addition to the basic components we installed in the May 2011 issue, we added a bottle heater with automatic pressure switch, purge valve, MSD window switch, and a fuel pressure safety switch. We also swapped spark plugs for nitrous/boost friendly non-protruding two-step colder Brisk plugs. So it’s time to head over to our friends at DC Performance and get Chris to lay one of his magic tunes upon the Maulin’ Magnum.
We wanted a 150 horsepower shot, so we installed a 62 nitrous jet per Nitrous Express instructions. The manual actually calls for a 38 fuel jet, but we’ll get to that in a moment. When we arrived at DC Performance, they bolted the Magnum up to a Dynapack chassis dynamometer. Previously, only a cold air intake and a cat back exhaust away from factory stock, Chris had tuned a very reliable 417 hp/376 lb. ft. torque (a pickup of 12 hp/11 ft. lb.) into the car. If you didn’t see the September 2011 issue, running on the DC Performance tune the Maulin’ Magnum and our driver Alex Rogeo won their first race in the West Coast Hot Rod Association. So yeah, we like well tuned vehicles.
After talking to the folks at Nitrous Express and a few other nitrous suppliers, we can tell you this right now: if you run the jets they suggest, and you don’t go crazy on the size of shot you run, nitrous oxide will not hurt a healthy late model Hemi engine. The consensus seems to be that for 5.7 L engines, the upper limit you can push is 100 hp, and for 6.1 L you can push up to 150 hp, as we did. Stay with the suggested jets and you will have a “safe” if somewhat rich air/fuel ratio.
Knowing that, we made our first run on the dyno stepping down from the recommended 38 fuel jet to a 36. The air/fuel ratio was very rich. We decided to keep stepping down the fuel jets until we got close to our desired A/F with just the jetting, and then tune precisely from there. Now let’s be clear about this, we’re doing this on a dyno with carefully monitored A/F, knock, and spark advance readings. Playing with jets other than the kit maker’s suggested ones is not something you want to do by just guessing about it. (Unless you really want one of those cool broken piston ashtrays you see in every wrecking yard’s office.)
We ended up close to our desired 11.7 A/F ratio running 62/28 jets. Finally in the ballpark, Chris was able to tune it from there. In addition to trimming the fuel, he optimized the spark advance curves and all that other black art stuff that good tuners do.
Now, about that A/F ratio. There’s a lot of noise on the ‘Net about “ideal” A/F ratios for different cars, engines, and applications. It’s important to understand that there’s no such thing in the real world. There are so many variables involved that you can’t even say that, for a 2007 SRT8 Hemi engine with these bolt-on mods the ideal A/F ratio is (whatever). Variables include the engine design (OHC, Hemi, etc.), mileage (carbon deposits and/or compression loss with age), spark plugs (stock or changed at some point), etc. You can assume that the nitrous kit manufacturer, in our case Nitrous Express, has a high degree of confidence in their recommended jets for your car. And that’s all you can assume.