Nitrous and Timing.... Whats the purpose???

[obengston]

I know some my be like if you don't know then you should not be using it..(and possibly so). My question here is what is the reason/purpose of pulling timing when your on Nitrous. I have a really nice set-up that I built a couple of years ago and have never had a issue, but was just curious as to the timing part. Does it have anything to do with not running lean?? I run a 200-250 shot and always pulled x amount of timing out, but the other day I hit the nitrous and forgot to pull the timing (got lucky I guess nothing got hurt that i know of), but my air fuel ration did not change or lean out it stayed about the same as it does when I pull timing, and thats why I'm asking this question. So I'm guessing it has to due with something else besides the AF. My normal AF is 12.8, and when on Nitrous its low elevens.high tens. Thanks

[obengston]

Or does it all have to do with where the piston is in order to compensate for the extra pressure when ignited??

[Ron Sutton]

So many things are mislabeled, like "shocks" which are actually suspension dampers.

I love the fact it is called "timing" ... because that is exactly what you're doing. You are "timing" when the spark plug fires & starts the flame propagation of the air/fuel mixture inside the cylinder. More correctly, it is called "ignition timing" or "spark timing."

To be very basic, with every engine, you are searching for the optimum time to light the flame. Each engine has it's own sweet spot based on many factors (rod/stroke ratio, fuel type & quality, atomization of the air/fuel mixture or lack thereof, cylinder pressure, cam profile, etc.) but it basically comes down to combustion efficiency.

Most already know this, but for beginners reading this too, you are always starting the flame (with spark from the plug) BEFORE the piston reaches top dead center (BTDC). The only question is how far before. As the piston is coming up the cylinder on the compression stroke & the intake valve has recently closed (creating dynamic compression) ... you want to start the fire early enough to create maximum heat expansion .... EXACTLY at the point to drive the piston back down the cylinder with the most force (power). If you miss it and are too late ... and the piston has already started it's way back down ... you will not create optimum power.

So, you can start the flame too late ... by simply running less spark timing ... and not fully burn the air/fuel mixture. This results in less power output & unburnt fuel. You are wasting fuel & power.

So let's advance the timing ... but don't get greedy. :hammer:

You can start the flame too early ... by running too much timing ... which causes the point of maximum heat expansion to happen BEFORE the piston has finished traveling up the cylinder. So instead of all the power driving the piston down the cylinder after top dead center (ATDC) ... some of the power created by the combustion process is actually trying to stop the piston from coming up the cylinder. Not good! This always wastes power ... but the real concern is it causes a hell of a shock on the bearings, crank & rods ... continuously. The engine is fighting itself inside the block. It is a ticking time bomb, and parts will break ... blowing up the engine. It's just a matter of when. How much damage & how quickly it happens is a matter of how far past optimum are you with "too much timing."

As an engine builds more power, there is more heat being generated inside the combustion chamber. This is simple and an absolute. When an engine is built to make more power, it is making more heat inside the cylinder.

This makes the engine more susceptible to pre-ignition, which is when a hot spot inside the engine, lights the flame before the spark plug. These hot spots can be from sharp edges on the piston, sharp edges in the combustion chamber and/or too hot of a spark plug. this is why you see the piston domes & chambers radiused smooth in race engines & of course we run colder plugs. If anything lights the fire before the spark plug, that is considered pre-ignition and often times the driver can not hear it over the engine running in loud race engines.

And that is when you're running GOOD Fuel.

When you're running fuel with too low of an octane ... like on the street ... it lights too easy causing detonation. Detonation is, when after the spark plug has started the burn ... the end gas in the chamber spontaneously combusts. This is caused by too much heat & too much cylinder pressure ... with too low of an octane fuel. Reducing the timing will decrease the heat & cylinder pressure.

And lastly there is detonation induced pre-ignition, caused by an engine detonating for a significant period of time until the spark plug gets so hot it becomes a glow plug and induces pre-ignition. Then the engine fails.

All of these 4 conditions are ugly. Some uglier & more destructive than the others. But having the ignition timing right is CRITICAL. When in doubt ... test. When you can't test ... err on the conservative side.

Each engine has it's own personality & therefore it's own unique sweet spot for the spark plug to fire. This is based on many factors (rod/stroke ratio, fuel type & quality, chamber shapre, piston shape, atomization of the air/fuel mixture or lack thereof, cylinder pressure, cam profile, etc.) but it basically comes down to combustion efficiency.

The better designed an engine is, the less timing it needs for optimum power. Here are three examples.

Bad: When I had an engine development shop in the 80's designing mountain motors for outlaw Pro/Stock racing, even with custom cast big blocks with 2" taller decks than normal (11.800" versus 9.8000") ... the massive stroker cranks led to ugly rod/stroke ratios. These engines built incredible power, but the power per cubic inch (Volumetric efficiency) was way less than we could achieve with 500" big blocks or any shorter stroke/longer rod engine. These 632" mountain motors ... making 1180hp almost 30 years ago (1.87 hp per cubic inch) ... ran best with 44 degrees of timing.

Good: When we built 360" small blocks with big bore & short stroke ... with long rods ... state of the art induction ... making 815hp ... these higher volumetric efficient engines (2.27 hp per cubic inch) ... made optimum power with 32 degrees of timing.

Nitrous example: In 1987 my partner & I had the fastest single carbed drag doorslammer in the country. Simple, well built 468" big block Chevy, made 780hp without nitrous & liked 42 degrees of timing. With a 300 hp multi-stage nitorus system, it built 1080hp, running 7.60's in our 2250# drag car. Optimum timing with nitrous ... 38 degrees.

Why? Because the Volumetric Efficiency (V/E) went up ... the cylinder pressure went up ... and the heat went up with the significant (300 hp) power increase. It LIKED less timing ... and NEEDED less timing. it has nothing to do with A/F ratio .. but of course A/F ratio & timing affect each other ... and should always be worked out together in harmony.

*The Volumetric Efficiency (V/E) is not everything. So don't make timing assumptions based only on V/E #s.

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So, when you run your engine without nitrous, and find its timing "sweet spot" ... it will need 3-5 degrees less timing when you add 200-250hp, increase the V/E, cylinder pressure & heat. I use 4 degrees less as a rule of thumb" ... but we always test.

Also, if you have too much timing, building hot spots in the engine ... and pre-ignition occurs on a nitrous run ... you will get a lesson in what engine parts cost.

P.S. Your nitrous system is running on the rich (safe) side at 11.0 +/-. If it was tuned for optimum power, your A/F ratio gauge would read 12.5. If you find you need that extra power, sneak up on the ratio. I'd rather be 12.0 & safe ... than 13.0 & blown up. :cool:

[Todd in Vancouver]

Now that was well answered Ron, thanks for the lesson.

[obengston]

Ron,

Great read.. Appreciate the time you took to spell that out for me. Answered allot for me. An yes I do realize my A/F is pretty rich (in fact It's probably a little lower than what I said). I plan on taking it out and tuning it up some so I can get into the higher 11's/12's A/F while running nitrous. I just recently added a progressive controller, so I can dial it in more for my application, as well I just installed a 200R4 Stage 3, built by Lonnie from Extreme Automatics (with a trans brake and a convertor). So far tranny feels good, waiting to see how it really performs (hey he gave a 2yr warranty with it), and is only about a hour from where I live.

[Ron Sutton]

Some nitrous systems are tunable with jets (like my current unit). But you can always tune the fuel volume with a pressure regulator & tune the nitrous volume by cooling or warming the bottle to change the nitrous pressure (using the pressure gauge on the bottle).

Can't say this strongly enough ... sneak up on it.

[andrewb70]

.....

Can't say this strongly enough ... sneak up on it.

Nah...Spray it until you break a piston and back off a jet size...LOL

Andrew

[Ron Sutton]

You're funny Andrew. :lol:

[obengston]

Yeah I have plenty of jets for both the Gas, and Nos side. As for pressure I have the pressure gauge inside, with a remote opener and a warmer to get the pressure where I need it. I guess I need to start going down on my gas jet, because I always run at the same pressure, and always in the 10's. So I guess I will try going down to the next size on the gas and see how that changes it.

[Ron Sutton]

I want to reiterate ... you don't "need to" ... unless you need more power when you are running nitrous. The current 11-1 AFR is "safe & builds power".

You will realize some more power, if you get to 12-1 or 12.5-1. But if you lean it out too much ... there are no mulligans ... just burnt engines.

Changing jets is more precise than adjusting fuel pressure, so I agree with that method. I would talk to the nitrous system manufacturer ... tell them the jets you have now ... and get clear on how big or small of steps to make to sneak up on the optimum set up.

Best wishes.

[Mr_Roboto]

Just a few things I would like to add, first off the reason why high octane fuel makes a difference is it burns slower than low octane fuel. This may seem somewhat counter intuitive but it lowers peak cylinder pressure that can cause issues.

As mentioned there are a ton of characteristics that go into how tolerant an engine is and what timing it needs, quench swirl and head design are critical for this. It's one of the reasons why everyone thinks TBI heads suck. They have a huge amount of swirl due to the ramps but a lot of the people that run them try to run them with "wedge" SBC head timing of 30+ degrees when they tend to "want" around 24 degrees.

Another thing to keep in mind is MORE TIMING DOES NOT EQUAL MORE POWER. As odd as it sounds, there are plenty of engines that will go several degrees past where they make more power with more timing before they hit detonation. A dyno or drag strip are probably the easiest ways to tell the difference.

Running lean is bad unto its self, fuel cools an intake charge. More heat in the intake charge makes your engine more susceptible to pre-ignition. That's why a lot of "chips" have a person change the stat to a lower temperature one. It lets them reduce the risk of this happening so they can bump timing and make more power (theoretically.) The reason OEMs don't do this is they have to worry about stuff like emissions, mileage and so on. Power is pretty far down the list for an auto maker over all.

[72RBMonte]

I am glad I found this site, a lot of wisdom here and one thing I find with so many new hotrod goers compared to car guys of decades where carbs were more dominate and laptops and EFi were new is the AFR guage makes too many lazy, often times if you changed spark plugs more often especially when tuning a new setup the plug will always show signs of a problem definitely before you loose a head gasket or piston, yet all this fancy technology is cause for less looking at what your engine is telling you, spark plugs always show some signs, just takes patience and even humbleness to learn the ropes from those older guys who actually keep logs of Atmospheric conditions as well as what their timing was for each situation. I feel humbled and this is just advice not concrete yet trust what your motor tells you and you'll always be happy, and as timing allows more efficient power to be made there is a degree where HP/Tq level and additional power isn't yielded yet too many punch the timing too close to detonation and trash that safety zone of 3-6 degrees where power didn't improve much, yet now your into heat soak detonation and on long harder runs you may experience knock. There are different tunes for short burst runs and endurance type runs where extra fuel is required for thermal dynamic heat dissipation to remove heat from each cylinder to recent detonation.

Just an opinion and learned from some experiences, and I stayed at a Holiday Inn last nite!!

Don't hurry power or you'll build it again!

[Ron Sutton]

if you changed spark plugs more often especially when tuning a new setup the plug will always show signs of a problem definitely before you loose a head gasket or piston, yet all this fancy technology is cause for less looking at what your engine is telling you, spark plugs always show some signs,

I think of the spark plugs as my "spies on the inside" ... because the plugs will tell you what is happening in the combustion chamber if you look & know what you're looking at.

[Chadman27]

Once you dial in your fuel whether it be via jet change or a different regulated pressure, I would highly recommend wiring in a fuel pressure safety switch to disarm the system if you ever lose the pressure. Nitrous gets an ugly reputation for ruining motors, but using it smartly and safely you can't beat it imo!