LOW COMPRESSION on nitrous motors

[flyinphill]

...

[alpinesupra]

i wouldnt hold your breath on any factual updates in here.
i will just stick with its a lack of dyno time for whatever reason, because i did learn that its a old small block trick for the every weekend bracket racers. They would all run lower compression to safely stuff as much dope into the cylinders as they could and hand tune it before or during the races at the track because lets face it if your a weekend bracket racer you dont have time to hit the dyno during the week before the weekend race.
so for us bike racers who dont have shops and access to dynos we do it by hand with the same concept as above just translated into motorcycles terms.

[chuck]

Why do you need a dyno so much?? Pro Mod bikes never see a dyno, mine included. Wonder why that is?

[chuck]

And I got a dyno sitting right beside the bike.

[MR AC]

Why do you need a dyno so much?? Pro Mod bikes never see a dyno, mine included. Wonder why that is?

why is that?

[Pruitt'sBusa]

Mr. OLEPUT Wilburn,

If you don't mind sharing some info on Pro Mod motorcycle engines and setups. Please enlighten us.

Thank you,
Jason Pruitt

[Pruitt'sBusa]

Joey what diet are you on...

NOS energy 200hp

[chuck]

That's funny- there use to be an OLEPUT festival here every year, that's how it was actually spelled! My junk is slow- maybe I should put it on the dyno!!

[flyinphill]

OK, here we go:

Before I start, let me make it clear that I am going to WAY oversimplify this discussion. Before someone gets on here and tells me I forgot this property or didn't include that principle, I know already. I am going to use rounded, easy numbers, and make lots of assumptions and simplifications to make the concepts easy to follow. The first thing I am going to do is ignore all properties of heat. I am assuming an adiabatic and isothermal process, which in simple words means no transfer or generation of heat. Now this is clearly not accurate. Even the act of compressing the air-fuel mixture induces heat, but we are going to ignore it. There will be plenty of other things I ignore and I will point out some of the obvious ones as we go along just so people know where we are.

So first, let’s look at an engine with cylinders that have a displacement of 90cc (cubic centimeters). This engine also has a compression volume of 10cc. Compression volume means the volume of the cylinder with the piston at top dead center (TDC). Compression volume (or clearance volume) is the amount of space in the cylinder created by the combustion chamber in the head, any volume changes from pistons, head gaskets, and deck height. Now since the cylinder has 90cc of displacement, the volume at bottom dead center (BDC) will be the compression volume plus the displacement, or 10cc+90cc=100cc. To calculate mechanical compression ratio, you simply take the ratio of the cylinder volume at BDC to TDC:

100cc/10cc=10

Also written as 10:1

So this engine has a 10:1 mechanical compression ratio.

Hopefully all of this is clearly understood by everyone on here. If not, look it up in the internet machine for more details and lots of pretty pictures.

Now to a little more advanced stuff:

So let’s look at this engine with the piston at BDC at the end of the intake stroke. We are going to make some assumptions here, like the intake valve opens and starts filling the cylinder with air-fuel at exactly TDC. Also we are going to assume that the intake valve closes and stops filling the cylinder at exactly BDC. (Neither of these really happens) We are also going to assume that, at BDC when the intake valve closes, the compression process begins (also not accurate). Also, since atmospheric pressure is 14.7psi, the pressure in the cylinder is going to be somewhat less than atmospheric pressure, so let’s assume 10psi for an easy number. So with these assumptions, the cylinder at BDC just at the end of the intake stroke and the beginning of the compression stroke has a volume of 100cc and that volume is filled with 100cc of air/fuel mix at 10psi.

Now here comes the important stuff. Everyone should already be aware that the amount of power that an engine can make is limited by the amount of oxygen in the cylinders for each power stroke. So for the sake of simple math, let’s assume that the 100cc of 10psi air/fuel in the cylinder at the beginning of the compression stroke contains 100 grams of oxygen. If no spark were to occur, when the cylinder goes through the compression stroke, it forces that 100g of oxygen occupying the 100cc space into a 10cc space, which causes a 10:1 pressure increase. In other words, the 10psi cylinder pressure at the beginning of the compression stroke becomes 100psi at the end of the compression stroke.

But having an ignition source alters this pressure number. Typically the ignition occurs about 30-40 degrees before TDC on the compression stroke. The flame front created by the spark requires a few milliseconds to actually burn through the cylinder volume, so the ignition lead allows near-peak pressure to occur at TDC, and peak pressure to occur a few degrees after TDC on the power stroke. For this discussion, let’s just assume that peak cylinder pressure occurs at TDC. And let’s also assume that the process of burning the fuel causes the pressure in the cylinder to increase by ten-fold. So that means the 100psi of pressure at TDC without spark becomes 1000psi with ignition. If the pistons and head gasket can withstand a peak cylinder pressure of 2000psi without failure, then this engine setup is well within it's limits.

This post is already getting long, so I will let anyone interested read this much and let me know if they are following all of this. Next post I will discuss how increasing compression ratio increases peak cylinder pressure.

[rcrecovery]

I follow you so far and taking notes

[STREETDRAG]

Why do you need a dyno so much?? Pro Mod bikes never see a dyno, mine included. Wonder why that is?

you guys mostly have loggers and standalone with cutouts to prevent damage, and always set a little on rich side. the budget racers usually don't have none of that as option nor the knowledge...

[flyinphill]

I know it has been a while, but I have had a couple people ask me to keep going with this. So go back and re-read my previous post, then continue on:

At the end of my last post, we had an engine with 10:1 compression. This engine had a volume of 100cc at BDC and 10cc at TDC. At BDC, the engine had 100cc of 10psi air/fuel mixture, which was compressed to 100psi without ignition. Ignition caused a further 10:1 pressure increase, so the combination of compression and ignition meant that the 100cc of 10psi air/fuel became 10cc of 1000psi air/fuel at TDC. Now to the next things to consider:

Now suppose that we put some high-compression domed pistons into the engine giving a compression volume of 5cc instead of the previous 10cc. So now at BDC we have a total volume of compression volume plus displacement volume, or 5cc+90cc=95cc. So the new compression ratio is:

95cc/5cc=19
Or 19:1 compression

So lets look at what this does to peak cylinder pressure:

The engine has the same 10psi air/fuel at BDC. It is a lower volume of air/fuel (95cc now instead of 100cc), but there is one finer point I did not discuss earlier. When the piston goes to BDC at the end of the exhaust stroke, it does not push out all of the exhaust gasses. For the 10:1 engine, it still had 10cc of exhaust in the cylinder when the intake stroke begins. So that 100cc of air/fuel at BDC also includes 10cc of leftover exhaust gases, so it actually only contains 90cc of fresh air/fuel. The 19:1 engine leaves 5cc of exhaust gases, so the 95cc in the cylinder at BDC includes the same 90cc of fresh air/fuel but half (5cc) of the exhaust gases. So for the sake of this discussion, lets just assume that both the 10:1 and 19:1 engines have 100 grams of oxygen in the cylinder at BDC.

So back to the 19:1 engine. The 95cc of 10psi air/fuel compresses to become 190psi without combustion. The 10:1 combustion rise takes this to 1900psi of peak cylinder pressure. Now this is just under the 2000psi mechanical limits of the engine as discussed in the first post.

We will also add in here an additional assumption that the fuel we are running can also handle a maximum of 2000psi cylinder pressure without detonation. If we had a lower octane fuel that could handle only 1500psi without detonation, then we would have to drop the compression down to limit the peak cylinder pressure to 1500psi. These nice, rounded numbers probably make it obvious that, for this theoretical engine, 15:1 compression would be the limit for this fuel. So this shows you in a very basic sense the relationship between compression ratio, cylinder pressure, and octane rating.

So I will stop here for now. If anyone still cares, let me know either on this thread or by PM. If there is any interest, I will take this next to how this applies to a turbo/supercharged setup, and then finally to how it applies to a nitrous setup.

[Ike Turner]

Keep the info coming Phil

[Pruitt'sBusa]

Phill,

Let's keep right on Flying! I'm on page and paying attention.Thanks for the lessons.


Merry Christmas Psychobike Family!

[chuck]

you guys mostly have loggers and standalone with cutouts to prevent damage, and always set a little on rich side. the budget racers usually don't have none of that as option nor the knowledge...

My stand alone is carburetors and if you can't afford a WEGO you shouldn't be racing. :)