Last month, we showed you how we turned The Big Block From Hell from a pump-gas thumper into a nitrous-swilling beast of an engine. This time around, we'll dive deeper into the tune-up of our engine, because it represents one of the most difficult examples of tuning an engine to run safely on N2O. The tips here apply to most any engine you'd like to tune, and although there is a multitude of ways to arrive at the same result, these are just the steps that worked for us. Entire books have been devoted to the subject of nitrous tuning and we won't pretend to be experts on the subject, this is simply what worked for us.
As you recall, we rebuilt the bottom end of the engine using a piston design and ring package that will withstand up to a 400 shot of nitrous. We then added a Nitrous Supply direct port fogger nitrous system to the intake manifold and an Aeromotive fuel pressure regulator for the system. We also wired up an MSD Digital 7 ignition system and HVCII coil, which gave us a ton of tuning options and the firepower needed to light the mixture of air and fuel.
The nitrous system was flow-tested and blueprinted by Steve Johnson of Induction Solutions. Steve provided the baseline tune-up for our system, which gave us a safe starting point for the nitrous and fuel jetting and ignition timing. We then took that info and the engine to Westech Performance Group, where we utilized the shop's Innovate ST-12 wideband O2 air/fuel ratio data-logging system to aid in tuning the engine.
When all was said and done, the engine cranked out 1,284 horsepower at 7,100 rpm without grenading on the dyno. Along the way, we learned the effects of jetting and timing changes and how we could fix a bad fuel distribution problem with some creative nitrous tuning. Here are the results of our latest engine experiment.
Part One: Firestarter
We last tested this motor using an MSD 6AL box and a blue HVC coil. The 6AL is capable of firing the engine even with the nitrous oxide, but we'd have to add a separate retard box to pull timing advance from the engine when the nitrous system is activated. We have to pull out some timing to avoid detonation when the nitrous is activated so we looked for a box that had that feature built-in. The Digital 7 box (part number 7531) not only contains several different programmable retard functions, but it also offers a start retard, programmable rev limiter, individual cylinder timing, and data-logging capabilities. The new box requires switching to the red HVCII coil though. The blue coil will work on a naturally aspirated engine, but it's not optimized for the Digital 7 box.
1. This screen shot from our laptop will give you some idea of the level of power of the D
2. For our test, we utilized the pink output wire of the Digital 7, which is a trigger wir
3. If you want to get really tricky, you can ramp in and ramp out the amount of ignition r
Part Two: Well-Timed Sparks
When adding the nitrous system to our engine, we re-phased the distributor rotor so that it was closest to the contact terminal when the ignition was retarded for the nitrous oxide. When we hit the go button, the base timing of 30 degrees advanced will be retarded 12 degrees by the Digital 7 box. To ensure the rotor was not in between terminals when the nitrous was activated, we moved the Jesel belt-driven distributor rotor so that the rotor pointed at the number one terminal when the balancer read 18 degrees before top dead center, which is 12 degrees retarded.
PART THREE: TINY JETS & BIG TIME POWER
1a. The air/fuel ratio of each cylinder is controlled via these small jets. Our assortment
2. Another determining factor in the air/fuel ratio of the engine is the flowing fuel pres
Determining Fuel Jet Sizing
Before flow-testing the fuel system, we needed to simulate the fuel flowing though the tiny orifices of the jets in the nitrous nozzles. This handy formula told us that with an .024 fuel jet in each nozzle, we needed a .067 flow jet for our flow tool.
Using a .0678 orifice flow jet, we're simulating the combined orifice of all eight jets in our system and can set the flowing fuel pressure pretty accurately with our tool.
PART FOUR: FAILSAFE MONITORING SYSTEM
1. With the ignition and nitrous systems set up, we strapped the engine to the dyno and th
2. The ST-12 gave us the ability to install a wideband oxygen sensor into each primary tub
3. (See below.)
3. We’ve used the ST-12 before so we knew that we had a fuel distribution problem that resulted in the corner cylinders (numbers 1, 2, 7, and 8) running very lean, while the middle cylinders (numbers 3, 4, 5 and 6) were running very fat. After talking with the designer of the manifold and the guys who ported it, we settled on the idea that the manifold, exhaust, and camshaft combination was to blame. Rather than replace a bunch of expensive parts, we chose to use the direct-port nitrous system to correct the air/fuel ratio when the motor is at wide-open throttle and racing, which is when the air/fuel is most critical to us. As you can see from Innovate’s software, the corner cylinders are as much as three air/fuel ratios leaner than the middle cylinders. Our solution to our distribution problem was to richen the mixture in the corner cylinders via carburetor jetting changes and to lean out the middle cylinders using the nitrous system. This was accomplished by going bigger with the nitrous jets in the number 3, 4, 5, and 6 nozzles. What we learned through our testing was going up .002 in nitrous jet orifice size leaned the air/fuel ratio of the middle cylinders just under a half a ratio. We began the day with the middle cylinders nearly fouling the spark plugs with a low 10:1 air/fuel ratio. By the end of the test, we had staggered the fuel/nitrous jetting from .024/.026 to .024/.030, which brought the middle cylinders to a more reasonable 12:1 air/fuel ratio. At the same time, the power output of the engine increased dramatically, showing about a 20hp gain with each jet size increase. Going bigger with the nitrous jets while keeping the fuel jets the same resulted in leaner cylinders that made more power. Each time we went up .002 in nitrous jet size in those overly rich middle cylinders, the air/fuel ratio went down. We stuck with the two jet spread in the corner cylinders and ended our tuning session with a happy engine that made awesome, safe power.
Part Five: Spark Plug Detective Work
As cool as the ST-12 is to use, it's a pricy tool that not every dyno facility has at its disposal. That is why confirming what we saw on the computer with spark plug readings was so important. Reading spark plugs is an art with different ways of interpreting the colors and nuances of each plug. Our plugs indeed confirmed what the computer told us. Here are a few examples of spark plugs from cylinders running in various states of tune.
1. While you can clearly see the effects of ignition timing on the ground strap of this pl
2. The fuel ring is clearly visible on this plug. It’s a dark, sooty, color and stretches
3. Now this spark plug looks like it’s barely ever been run. Note that there’s a very fain
4. Nitrous tuners used to think that running the engine with a rich air/fuel ratio was saf
The Final Word
Since 2006, we've run this engine on pump gas and on race fuel as a guinea pig for dyno testing, and now we've added nitrous oxide to the mix for some real fun. Thanks to the expert advice from the guys at Nitrous Supply, Induction Solutions, Westech Performance, and a few savvy members of various Internet message boards, we were able to add almost 300 horsepower to the engine without making eight new ashtrays out of the pistons.