Compression Ratio/Boost/ Octane Question.

[SinisterSHO]

Compression Ratio/Boost/Octane Question.

I was poking around on SHO Shop's site, and I was looking at the s/c kits. They say that with 12 psi, on a 9.0:1 compression 3.2 you can run with 93 octane gas. However, I know there are people running higher than 12 psi on pump gas. Knowing that you can run more boost with lower compression, how much? Is there anyone running 18 psi on pump gas?

 

[AutoSHO]

Pat runs 15 psi on the street with pump gas & water injection... On the track he runs 100 Octane to help combat knock in high temp/rpm situations.

The real issue is that with no charge cooling, you've got air 200+ degrees going into the engine... Which makes it very hard to get rid of knock. Lower compression helps this because, by compressing the air less, it does not get as hot. It also reduces peak cylinder pressures, which also reduces knock to some extent.

The real solution to knock is to lower the intake charge temps. Inter/Aftercooling is the best way to go. You will be able to run increased boost, timing, and compression without getting spark knock due to excessively hot mixtures.

 

[MotorMouth]

Pat runs 15 psi on the street with pump gas & water injection... On the track he runs 100 Octane to help combat knock in high temp/rpm situations.

The real issue is that with no charge cooling, you've got air 200+ degrees going into the engine... Which makes it very hard to get rid of knock. Lower compression helps this because, by compressing the air less, it does not get as hot. It also reduces peak cylinder pressures, which also reduces knock to some extent.

The real solution to knock is to lower the intake charge temps. Inter/Aftercooling is the best way to go. You will be able to run increased boost, timing, and compression without getting spark knock due to excessively hot mixtures.

Knock is not just a fucntion of intake air temps. Well, actually knock is not a function of anything. It is the end result of a combination of processes with knock (detonation and or preignition) as the end result. Here is the definition of detonation (knock).

The secondary ignition of the air/fuel mixture in the combustion space causing extreme pressures. Detonation is caused by low gasoline octane ratings, high combustion temperatures, improper combustion chamber shape, too-lean mixtures, etc. Detonation produces dangerously high loads on the engine, and if allowed to continue, will lead to engine failure. Detonation, unlike preignition, requires two simultaneous combustion fronts (fuel burning in two or more places in the combustion chamber at once); whereas preignition occurs when the fuel-air mix ignites (with single burning front) before the spark plug fires. Both preignition and detonation produce an audible "knock" or "ping," but detonation does not produce the rapid "wild pinging" noise that is typically associated with preignition. The extreme pressures of detonation can lead to preignition, but even worse the high temperatures of preignition can cause detonation.

Detonation can occur even with a proper air/fuel mix at optimum tempretures. Carbon build up on combustion chaimbers or on piston crowns or hot spots on either can cause mulitiple uneaven flame fronts to occur. Preignition is more of a result of the combination of initial high intake temps and poor air/fuel mixture or low octaine. Same thing that happens in a diesel motor but there it is supposed to happen.

A proper and complete combustion is called Stoichiometric Combustion.

Stoichiometric or Theoretical Combustion is the ideal combustion process during which a fuel is burned completely. A complete combustion is a process which burns all the carbon (C) to (CO2), all hydrogen (H) to (H2O) and all sulfur (S) to (SO2). If there are unburned components in the exhaust gas such as C, H2, CO the combustion process is uncompleted.

So, basicly lowering intake charge temps is one of many things that help in preventing knock. While air/air and air/water is the prefered way, alky/water or methanol injection is a very practical way to go as well.

You will be able to run increased boost, timing, and compression without getting spark knock due to excessively hot mixtures.

These don't all go hand in hand, especially the increased compression ratio. My advice would to do alot of research before getting into or thinking of forced induction. I will freely admit that I have blown up many motors on forced induction because at the time I did not have a full understanding of what I was trying to do.

Remember, horsepower cost money. How fast do you want to go and how much can you afford to spend.

 

[SuperchargedSHOguy]

What about the current fad...Spraying the CO2 into the intake before the TB??? Anyone know if this is an efficient way to lower in intake air temp? I believe it consists of a "bulb" that when sprayed with CO2 freezes and cools the incoming air very well. Of course this only happens when you have the system on. Does anyone want to share a pic (or p.m) of their intercooler system used with the S/C system? I am thinking about upping the boost alittle more (about12 currently) and am exploring my options for keeping pump gas.

 

[2L8V8_4sho]

To cool the air the bulb would have to be in the center of the tubing...am I correct? And if the bulb was in the center of the tubing it would be an obstacle for the air to go around which would eat horsepower, albeit not much. Am I right?

 

[Hack]

Motor Mouth, I think that Chris(AutoSHO) was saying that inter/after cooling is generally considered the best method because it is low maintainance. No worries about refreshing your water/alcohol/methanol supply.

 

[MotorMouth]

Motor Mouth, I think that Chris(AutoSHO) was saying that inter/after cooling is generally considered the best method because it is low maintainance. No worries about refreshing your water/alcohol/methanol supply.

I wasn't disputing the effictiveness or usefullness of an intercooler or aftercooler (be it A2A or A2W) and or water injection. I was more or less trying to put a little clarification and insight to his statement that "The real solution to knock is to lower the intake charge temps."

Just trying to help inform......

Brian

 

[Hack]

Understood. Welcome to the forums by the way!

 

[SHOMurph]

Been running 12lbs for over a year now on 93 octane. Tony runs up to 18psi on 93 no problems.

I cool mine by running a 55 shot. Poor man's intercooler.

Boost love nitrous.

 

[Lupo]

Been running 12lbs for over a year now on 93 octane. Tony runs up to 18psi on 93 no problems.

I cool mine by running a 55 shot. Poor man's intercooler.

Boost love nitrous.

I've been running 13psi with California crap 91 octane and octane boost.
Any more boost, and I'd probably run 100 octane, or mix with race gas.