Nitromethane will not ignite at all.

It's a oxygen carrier and helps to bring much more oxygen into the combustion process.

Exactly, and it will make tunning harder, and since it brings more oxigen to the cumbustion process you need to open up the needles a lot to use it or you are running lean, so fuel consumption goes way up too.

Also, you cant just put 215 deg to all engines, rod length is an equally big part of timming, you have to consider it just like you consider port opennings

your experience make you talk about 215 intuction?or something else?

The problem is that crank timming is proportionally dependent on rod length, the longer the rod, the "tamer" the timming, and vs, in other words, you can have dynamically the same timming with a 212, 215, 218 or whatever if only you have different rod lengths, obviously moving the sleeve up or down in accordance to the rod length difference, by that what I mean is that you can have a rod that is .25mm longer, so you have to raise the sleeve .25mm, so everything aligns perfectly as designed originally piston/sleeve wise but unintentionally you just changed the crank timming becouse the rod is longer. Sudenlly everyone is hoping they never skipped math class in 7th grade LOL

I for one basically don't fiddle to much with the top of the line engine timmings, I find it to be obtuse of us IMHO, just if I think that Novarossi has like 50 years of experience since they started doing engines(Rossi), they have to be abble enough to find the best timming for all general racing situations, not that if you have a drag applycation or a very short or very long track you can fix it here and there, but for all general racing situations - on the very best engines from all companies - I tend to leave the timmings as they come unless they are way off of my liking and I usually test them first to see if what I saw in the "numbers" its what its seen on the track. Just fix it with header/pipe/restrictor selection

On the very high end engines, I tend to work on flow, which when done right gives a lot everywhere on the power curve, also bluprinting, squishband, compression and rotating assembly weight and balance.

On the other hand milder engines can benefit from timming a lot becouse they are made tame on purpose. Its like bolting a hot cam on your daily driver.

But there you have it what "hot cam" can you put on an F-1 engine?

X

if we speak with maths in mind when you are putting longer rod you change the other timings too.the piston stay in the tdc-bdc more time and accelarate more in the middle of the bore.i don't know how much difference it can make a so short rod.
in the big engine world it can make a difference in some things.

Well, the experience is otherwise, more nitro makes an easier tuning of the enging because with a slight rich setting the engine is still performing strong and fast. With a low nitro content the window where in the engine performs well is smaller and if you do need power there is a risk you will go to lean.

The fuel consumption has also another but quite simple reason: there is less methanol inside the tank with more nitro, and you do need methanol for the combustion. You can calculate the avarage methanol needed per combustion, with more methanol (=less nitro) more combustions can be made, thats why some people are using less oil.

Changing timing by the length of the conrod is something no one (without tools and the right materials) can do, it is not the easiest way. Placing the cilinder lower or higher is much easier to do and is adviseable if you want to change timings without grinding, you can always go back.

More interesting is playing with the boring/stroke numbers. Long stroke will give more play in the hight of the ports, for sure with the exhaust. With a .21 square stroke it is 16.4mm, with a NR long stroke it is 16.8mm, you can make the exhaust poort 0.4 mm higher on a long stroke without affecting the combustion stroke. More port space and more rpm without loosing power (= combustion stroke)

Although its very good the piston will stay in TDC more time and mechanically a longer rod will remove stress from the stroke. Thats ALL ENGINE talk, big or small, but on a 2-stroke engine port timming also changes with rod length, in a big way.

Let me put it like this, you have an engine with 1inch stroke, you have a 2inch rod length, when the crank is at 90 deg the rod's angle - which is proportinal to the rod's length - will be 22.5deg, this angle is one of the componets of the pistons position since it changes the relative length of the rod in temrs of the stroke , or the "Y" axis, imagine now you have the same 1inch stroke with a 1 inch rod length, the angle of the rod will be twice at 45deg!!!, the piston for that reason and since it moves on the same "Y" axis has to place it self farther down on the stroke, (In fact, that's the only thing that can happen since shortening a rod will make a piston accelerate and deccelerate quicker, and since it moves faster specially through the mid section of the stroke) so mechanically, you openned all the ports WAY sooner, you have just installed a race cam!

That's the reason OS v-specs need such a high timming on their cranks, the engine has a short stroke and a long 30mm rod, so the real dynamic timming is still conservative.

This is the main reason that some modders get different results from the same timming numbers on different engines, the quick numbers may be the same but when you put the rod/stroke relation in the equation the two engines have different timming.

X

Its not hard to change rod legth, just get a different one. They come in many sizes, just look a littler harder

No man I'm sorry, more nitro makes it harder to tune, it closes the gap of actual fuel you have in methane, the more nitro the more methane you need to make the burn and since the amount of methane per volume is minimised, it makes it harder, there you have your math again

If you just leave it rich and run it as is becouse it has enough power thats another story, but to be able to get the perfect setting is easier with the leeser nitro %'s.

The amount of methanol needed per combustion can be obtained from the most efficient Air/fuel ratio for alcohol based fuels which is around 6:1. So in a 3.5cc engine it should be around 0.3cc of fuel per stroke's power section at the best volumetric efficiency. But thats not the point I made. What Stefan and I said is that nitro HAS oxigen in its chemical ecuation, and in a higher % than regular breathing air, so when you add more nitro you are fisically pushing up the % of oxigen that you have "available" to burn, so the fuel mixture leans and since the needles must be opened to add fuel and burn it with oxigen. Thus more fuel consumption occurs.

X

If you believe that, pour nitro all over yourself and light a match.

Nitro is flammable and will ignite. Drag racers have used 100% nitro for years, only the rules have forced them to drop down to 90%.

Yes, rod length will change port timings but that is not something most of us have the ability to change.

Here is the wiki page on nitromethane.

http://en.wikipedia.org/wiki/Nitromethane

As you can see from this info, nitromethane is explosive. Also, since it is an oxygen carrier, it requires less oxygen to burn. Since we don't usually reduce the carb venturi when going up in nitro % this will require a richer needle setting (same amount of air will require more fuel) therefore fuel mileage is reduced.

I have to agree with Roelof that more nitro is easier to tune up to a certain point. It will be much easier to tune a motor on 30% nitro than it will be on 5%. However, it would probably be quite difficult to tune them on really high % nitro simply because our motors are not designed for it. Now if we increased head volume and made numerous other changes it would probably work OK but no one I know has ever tried it.

LOLLL!! that's a good one

But hey guys, I'm not saying you have to change rod length, although you can, as easy as puting a rod from a different engine, its even easier than modding!!! , what I say is that you cannot precisely just "put a timming number" on an engine just because you had good results in another one without considering rod length and rod to stroke ratio.

X

Oh well..... experimenting is my middle name

- A Novarossi piston and rod will fit in a Mega (1st model)
- A Basic N21-3T set will work very good in a JP R1 crankcase with the JP crankshaft (just raised and widened the exhaust port)
- currently I am busy to fit a GRP P/S set into a NR engine, that is more difficult. The piston pin is 4.2mm instead of 4mm. So I ordered a piston pin and have to drill up the NR conrod. When it works it saves me 150 euro for rebuilding a good engine!

I need to do some measuring for myself but in the meantime, any idea if a Nova .21 rod will work in a Sirio?