Should I retard the timing with a shot of nitrous?
Q Should you retard the timing with a shot of nitrous even if your air/fuel ratio is good?
A: Yes. Your A/F ratio might look great, but even with a good A/F ratio you can still have detonation and engine damage if the burn becomes uncontrolled because of too much timing. With pretty much all performance engines, and engines that have nitrous or forced induction, you need a good fat A/F ratio to keep the burn cool enough and to keep the engine in its safe zone (without it being TOO fat to cause cylinder wash down) as far as combustion temp is concerned. A typical stock engine will usually have an A/F ratio in the 14:1 to even up in the 15:1 area for fuel economy. That's simply too lean of a mixture for performance engines. Most naturally aspirated performance engines like to run in the A/F ratio areas of 13.5:1 and even down into the 13.0:1 area. Forced induction and/or nitrous engines will likely have a much richer A/F mixture in the 12:1 area. I have seen certain vehicles as rich as the 11:1 area and others as lean as 13:1, but every engne is different so ever "tune" will be different for the type of use it is going to see.
Now, timing is a whole other story. With pretty much all performance engines, you'll want a total timing to be in the 34 - 36 degree area. When you are using a basic shot of nitrous (or a street supercharger) I would say to retard the timing by 2 or 3 degrees when the spray is on or when boost is up in the 5-7psi or so area.
In reality, backing the timing off actually increases exhaust temp because the burn happens at a later time, so it is still combusting as the exhaust valve is opening. This is why headers glow red when the timing is too late, BUT with 2 or 3 degrees, you aren’t backing the timing off THAT far to cause any of that. All you are doing is trying to control the speed of the combustion process. Backing the timing off “softens” the blow to the piston top (for lack of a better description). You want the fuel to “push” on the piston, not “explode uncontrollably” on it. It’s exactly like the difference between trying to push your car or standing behind it and smacking it as hard as you can with a sledge hammer. Yes, the hammer will hit it with more force than you just pushing on it, but the car won’t actually move. All you did was cave-in the trunk or rear bumber. A smooth, steady, firm push will roll your car along MUCH better than smacking it with a sledge hammer, right? That’s exactly the kind of thing that happens inside a cylinder when the controlled burn becomes an uncontrolled explosion (detonation) that is too fast and harsh. It beats the hell out of the top of the piston, it often breaks pistons or blows holes in them, it kills head gaskets, etc, and it makes no power. You want the fuel to “push” that piston down, long, slow, steady and CONTROLLED, not smack it with a sledge hammer. Keeping the A/F ratio on the fat side (in the 12:1 or so area) and backing the timing off a couple of degrees will keep that combustion process under control, even when a bit of nitrous is intriduced or a little blower boost.
One thing people don’t seem to understand is that ALL engines and vehicles talk. They will tell you what they like and don’t like by how fast they go. If you do something it likes, it’ll tell you by giving you more MPH. If it doesn’t like something you did, it’ll tell you by going slower. Listen to what your car tells you. If you back the timing off a couple of degrees and you go faster than you did before, then it liked whatever you did, so try what you did a little bit more until it tells you it isn’t liking it by going slower.
ALWAYS go with less timing FIRST rather than more timing. Less timing you will go slower with no damage. Too much timing, or nitrous, or blower boost will also tell you it didn’t like it, but damage can occur in the process. It’s better to try to pick up something light and then add weight to it until you don’t like it anymore than it is to start-off trying to pick-up too much weight and throw your back out in the process, right?. Damage happened before you could really do any testing for just how much weight you are comfortable with lifting. Timing, fuel, combustion pressures, heat etc., are all kind of the same process. Go the easy / safe way first rather than the more dangerous way which can cause damage as it’s telling you it isn’t liking it. If you go too far, it'll let you know, so then back it off a little bit. They keys are; A) keep everything in MODERATION, B) do everything in small incriments at a time, C) PAY ATTENTION to what your car or engine is telling you, and D) don't make too many changes at one time. When you are messing with timing, air/fuel ratios, nitrous shots, blower boost, etc. try to do things one at a time so you will know what to focus on if the car liked it or not. If you try too many things at once, such as fuel & jetting changes, timing changes, nitrous jetting, blower boost, etc., etc., and the car tries telling you, it'll be more difficult finding out which one of those changes you made that it liked or disliked.