Nitro Facts
 

The power potential of nitromethane as a fuel comes from the fact that it is introduced into the engine as a liquid instead of a vapor. Since the liquid is more dense, more molecules of fuel can go thru a given port size. As a rough comparison, a 500 inch Pro Stock engine might be making 1500 hp and the Top Fuel 500 incher makes 10,000 horsepower; so using liquid fuel (instead of vapor) gives you over 6 times the power potential.

But....liquid fuel won't burn. So nitro has to phase change to vapor before it can burn. If all is well, the spark plug finds enough vapor to start a flame and then the flame front heats the liquid, creates a bit more vapor, burns some more, creates more vapor, etc. This phase change makes the burn process last a long time. So the spark has to fire anywhere from 54 - 70 degrees BTDC to get started. On the other end, combustion is still going on when the exhaust valve is opening, which explains the noise and flames out the pipes. This also means that the exhaust valvetrain has to be much stouter than the intake since it is opening against combustion pressure. The other effect that happens is that combustion is pushing on the piston for a lot longer after TDC so the engine pulls much harder.

Common misconception....the blower (on a nitro engine) isn't there to make power. The fuel quantity injected sets the power level. The blower is just there to push in the corresponding correct amount of air.

Recall that normal combustion in any engine is a smooth burn of fuel across the combustion chamber. Detonation is the sudden combustion of the entire slug of fuel because the temperature/pressure in the chamber gets too high. From our previous calculation, detonation in a liquid-fueled nitro engine has over 6 times the energy potential of an NA gas engine. So the gas engine detonation beats up the pistons and bearings, and nitro detonation grenades the entire engine and tosses the cylinder heads at the spectators.

Nitromethane is shock-sensitive, so a sudden spike in cylinder pressure can cause the entire load to detonate.

Tuning nitro is walking a tightrope. If the mixture is too rich, the extra fuel cools the combustion chamber too much, then there isn't enough heat to vaporize fuel and the spark plug can't fire anything. Then you have a "dropped cylinder". Watch video closely of nitro cars and when you see mist suddenly appearing at the pipes, you know a cylinder has dropped. On the other hand, if you lean the mixture too much, it accelerates combustion too quickly, a violent pressure spike occurs and the next thing to happen is detonation; recognized from the stands as a wallet-draining engine fireball.

Decades of experience have taught nitro tuners that compression has to be low (6:1 in our case....mid-7's for the big show cars that can afford to blow stuff up), in order to spread out the combustion event and prevent the engine-killing pressure spike.

One more thing to help you wrap your head around what's happening. The typical shower head in your home runs about 3 gallons per minute. Our fuel pump is pumping about 25 gpm of 90% nitro at 8000 rpm....so your home shower head shows you about the flow of nitro in each individual cylinder.

Hope this was entertaining,
Eric

A few words about our tuning approach on the Boss Bird:

Since we don't have the budget to grenade engines, we started rich and accepted that fact that we would have dropped cylinders. Each run has the overall mixture being leaned in baby steps; as well as leaning individual cylinders to get them to run more evenly.

At Norwalk last year, we had all 8 cylinders lit for the first 330', then one cylinder had cooled too much from being too rich, and it dropped. Once we get the cylinder stagger adjusted to eliminate that dropped cylinder, then we can lean the entire engine a bit more to make the real power that this combo is capable of. Our best ET of 6.39 is on 7 cylinders with the overall engine still lazy.

Stay tuned...
Eric

Good stuff!

Spud Miller website:

Nitro Notes

:)

Good reading.

Thanks Eric.

I've been around drag racing for over 50 years and I didn't know that stuff.

K

Eric,
What's the current cost of a barrel?

Nitro is made in China and comes over here by the container load. The price can vary depending on who's selling it and whether they just got a big shipment or they're trying to move the last few drums. Freight charges are sometimes the most significant variable.

Given all that, the cheapest we've bought it has been about $800 (including shipping) for a 53 gallon drum.

The most expensive I've heard of was the official NHRA-sanctioned VP nitro that was about $1200 for a 40 gallon drum plus shipping. Edit: I just remembered that there was a nitro shortage a few years ago and the official VP stuff jumped up to something like $2000 for 40 gallons.

I haven't priced it yet for this year.

Eric

I have been curious since we spoke last summer at the Motel.
Who were the early pioneers of this technology?

And I bitch about $8.50 a gallon. 🙄

Great read Eric and thank you for sharing!

With that much fuel,if it does not light is the a chance to hydrolic a cyclinder?Tom

Weren't the Surfers one of the originals in the learning curve?

With the amount of fuel we use, the large combustion chamber size and the valve timing, it is unlikely our particular engine would hydraulic. I guess anything is possible, though. The professional cars, have a little higher compression, 2 fuel pumps, each larger than our single pump, so it does happen with them and lots of damage results. One of the big advantages for us is having a very experienced tuner who is willing to teach us all the safety precautions many learn the hard way. An example would be you will never see spark plugs in our engine unless the engine is ready to run, period. You don't want the nitro to ever be contained in a closed chamber where it can be shocked and detonate. Even when the car is going down the road in the trailer, the spark plugs are out. Fuel left in the system could drain down into a cylinder with the valves closed, you hit a big pothole, and when you get to the track part of the cylinder head is missing. Lots of little details like this help us stay out of trouble.

OK,that's what I had heard about the top fuel cars that they were on the verge of hydrolic.

IIRC,, the average A/F mixture of a regular car is around 14 to 1,,, Top fuel Nitros average 1.7 to 1.

Also, if the engine is running and dies, with warm nitro vapor in the chambers, it's like a bomb waiting to go off. It's mandatory to immediately ground the coil so no spark can occur. A single stray spark at that moment will blow the side out of the block.

After the coil is grounded, we either "back it down" by rotating the engine backwards a few revolutions with a 1/2" ratchet or crank it with the starter to clear the nitro out....before attempting a restart.

Eric

Thanks for the knowledge Eric.

So, if the tune is right combustion continues even if spark goes away? You have to cut fuel to shut it off correct?

Alot more complicated than my teenage experiments with mixing ether with methanol on my mini-bike lol.

The only 100% reliable way to shut it off is shutting off the fuel. The spark plug is probably still needed to initiate combustion, but there's always a chance it could diesel. And shutting off the ignition loads the hot engine with lots of unburned fuel....a bomb just waiting for somebody to make it mad.

Eric

I have watched that stuff at those cackle fests.Everyone always seems very nervous.You see them turn the fuel off and the engine runs down.Tom