Boost is calculated into effective compression ratio. Let me find a calculator.

Here:

http://www.rbracing-rsr.com/compression.htm

Oh, and I think that graph represents certain static spark and valve timing conditions as well. Since the ECU learns and adapts, I don't think that's completely indicative of the octane fuel you should be using. From the looks of things it's more of a guideline.

 

Is that what the other measure of boost, BAR, would be?

Nevermind, I was thinking maybe bar would be the effective compression and you would just add it to the stock compression. (7psi ~ .5 Bar, so it would be an effective 10:1 ratio).

That calculator says with a 9.5:1 stock ratio and 7 pounds of boost, the ratio is 14:1, can that be right?

 

I'm not sure what you're asking.

 

Yeah I know, that question was worded worse than how some of the ESL people I work with ask questions. Check the edit, I corrected myself after checking the calculator. A 14:1 compression ratio seems a little high, but I am a noob so to speak, so maybe it is right.

 

Yeah, so you don't just add it on to the CR. Neat calculator though, eh?

 

just one little note, I know eboy and most ppl already know this but for the newbs, the quote from eboy said a high air fuel ratio causes an explosion in the combustion chamber, just to clarify, there is always an explosion in the chamber, how ever with higher a/f the mixture entering the chamber becomes more volatile and will ignite easier. Then when the piston compresses the mixture it causes the temperature to increase past the detonation temperature, and the explosion occurs before the spark plug even fires. Like eboy stated this is called engine knock (same as what happens if u put too low of octane gas in ur car).

 

He is right in a sense though. The knock is caused when the fuel explodes before spark, but it turns into a more controlled fuel "burn" the more rich the mixture becomes.

So, what is the relationship between compression ratio and A/F ratio, or are they one in the same?

 

Yea hes right I just wanted to clarify to some who didnt know that there is always an explosion but he was describing pre detonation

compression ratio is defined as Vmax / Vmin. This is just telling you how much the volume is being compressed. The AF ratio is just how much air is mixed with the fuel. The mixture gets more volatile as the AF increases. there really is no direct relationship for the two, meaning that having a higher compression ratio doesnt affect you AF ratio. However if u were to change to high compression pistons you would be heating up the mixture more due to the higher pressure in the combustion chamber therefore u may want to lower ur AF ratio, or do what most due and use a higher octane gas.

 

there IS a relationship between compression ratio and AFR : P*v=t (removing the constant R and the number of moles n from Pv=nRt. I know we're not dealing with ideal gasses, but work with me. There aren't formulae that are simple enough to explain what's goin on inside the engine in the amount of time I'm willing to give the subject, yet again ) The higher the pressure, P the greater the volume of gas is required, v to keep the temp inside the combusion chamber, t, constant.
If P gets too high, then t raises and the gas explodes. As v decreases (leans out the mix) T also increases (and increases more if P increases) which can also cause an explosion.

Octane is the rating for how well gas resists ignition, and therefore resists explosion. Change something to raise the temperature (i.e. lower v or increase P) then you need to increase the octane.

And no, you don't have an explosion everytime your plug fires, it's a controlled burn. That's why it's critical to get your AFRs at least on the rich side, if not close to whatever "magic" number your engine prefers.

 

There are also fuel requirement calculators out there for those interested. I think RC Engineering's site has one, but I've long since taken a break from trying to understand that fully.

 

Didn't I tell you guys not to go over our heads? I'm f*cking lost now!

 

DJ: step away from the vehicle

 

*sobs*

What part of "for n00bs" didn't you people understand?

*starts walking back to civilization...*

 

What engineerboy was talking about was simple physics, it wasn't very advanced at all. Even though I forgot all about that equation. Damn it's been a long time since I took those classes (5 years now).

You can decrease the temperature by adding fuel right? So what we're really trying to find out is the best trade off between octane, A/F ratio, and compression, right?

 

Timing and A/F mixture, two things to look for.