OK so I think I need to re-ask in a different way. I only brought up the GTO because that is what spawned the discussion in the first place.
So compression ratio is the ratio between the amount of volume at the bottom of the stroke in relation to the amount of volume at the top of its stroke. So if you could hypothetically place the same psi in a high compression motor and a lower compresion motor why is it that you would get more HP out of the low comp. motor? Or I could be completely off base. Thanks for the replys so far.

 

Whats with all the hate....i know the basic answer but i surely cant explain it in detail...chevykiller or others with some very nice built engines will be able to explain this better than the a holes above could dream of. Google would be a great place to start, maybe wikipedia or how stuff works also. I would like to hear some actual answers to this question though.

 

Sorry about the dog breath comment from the other guy.
It just proved my point and it didn't take long did it?


The answer to your question will not fit in a simple phrase, I'm afraid.
Each motor has it's own limit and combo. It's that fine line between blowing it up and making a 1000 runs (if this is even possible).
It's hard to see it or find it.

 

I love the word SMARMY!

 

I would answer your question, but unfortunately i'm a technical dummy to.

 

 

That makes practicle sense.

Thanks 99flhr!

So the gasoline is the ultimate factor. Higher octane means higher knock ratio therefore higher C/R and more HP potential. Obviously there is a balance here but it is all becoming clear! Obviously there are other factors that you have pointed out but I understand your explaination.

 

First of all, there is no absolute right answer to this type of question. I’ll start with the basics. The function of any engine is to create heat energy and put the energy to work. First, about the fuel itself… (Mostly the fuels knock resistance). Most modern cars are designed to run on regular (87 octane or so, depending upon the state you live in) and have been designed to meet all emissions laws and if a performance variety, make good or even exceptional power while running on the cheap stuff. Knock resistance in a nutshell is at what temperature the fuel will self-ignite. The higher the octane rating, the higher the temperature the fuel will ignite by itself. As you probably know, the engine’s compression ratio is the number of times the whole volume of the cylinder is compressed when the piston is at the top of its travel. When you add forced induction, you are in effect increasing the engines compression ratio. Normally aspirated motors draw the air in at no more than one atmosphere (at sea level). FI can multiply that depending upon the amount of boost produced. You may or may not realize that when you compress the air (by the piston raising in the cylinder towards the now closed valves on the compression stroke) you are creating heat. The tighter the squeeze, the more heat is produced. Our engines are designed to not create enough heat during the compression stroke to avoid igniting the fuel/air mixture before we want it to (such as when the piston is actually where power will be produced and the spark plug fires). When you add FI you are adding a much larger amount of air/fuel to the cylinder and you can easily exceed the knock resistance of the fuel – which will then ignite at the wrong time and can then cause engine damage. Even if you buy high octane fuel that will allow the higher effective compression (by not “knocking�) engine damage can occur when the structural limits of the components are exceeded. And the reason you added the FI is to make more power, right? So when you actually get more power, you end up putting more strain on the engine components (not to mention transmission, drive shafts axles and tires…). This probably isn’t as much of a problem with current engines and FI kits as it used to be. Some of this is because the kit designers have already tuned the kits to work within the limits of the stock parts that remain, some is because the stock parts seem to be better designed to work in a variety of situations. The cam timing (and lift) control when and the quantity of fuel/air drawn into the engine on the intake stroke of the engine’s four cycles. And since on a normally aspirated engine the piston has to ‘suck’ the air/fuel mixture in the amount of time and the total lift of the valve are very important to the mixture. The longer valves are opened increases the amount of the air/fuel mixture in the cylinder. The more air/fuel in the cylinder the more power can be created. Personally given the choice between a high-compression engine or a forced induction engine I’d choose the FI. Then you can adjust the amount of power desired (by changing the amount of boost created) and purchase appropriate octane fuel for the situation. This is all a pretty simplistic look at the question you asked. But it (I hope) gives you something to think about.

 

First your right about C/R
C/R is the ratio of the differance in air volume at the bottom and top of the stroke
10.75:1 is a (now old knock sensors made higher commpression possable on 93) limit of 93 oct
if you were to put in 118 unleaded (if you can find it) you could up the commpression

Gas pre-ignites when there's to much pressure on it

now add F/I to the equation
now at the bottom of the stroke the air pressure is (lets say 10psi of boost) 10psi instead of 1atm (atmosphere ie normal air pressure at sea level)
true 0psi is only in space
1atm is 10psi so youd add 10psi(of boost) to 10psi(of air pushing down on the earth)
so now you have 20psi at the bottom of the stroke and multiply 20 by C/R (lets go with 9:1) now you hace 180psi at the top of the stroke (if you could measure it you might get 170psi because its compensating for the 1atm)
and your 93oct can only take so much before it pre-ignites and your gaskets can only take so much as well

now the gas explodes (spark) and the psi growes exponentially (at this point pre-ignition is not an issue any more) but gaskets are
as you know the piston goes down due to this force which lowers the pressure on the gaskets


the 1atm is something alot of people forget but you have 10 pounds of pressure on every inch of your body (less at higher altitude)

 

So you are saying 10.75:1 is the limit of 93 octane gas but motors like the LS2 are running around 11:1? Not sure how that works on 91 octane, which is what is recomended? I think I just don't understand your meaning of "limit". Anyway I am glad some of you guys were able to chime in and give us the low down on C/R. It is amazing how much there is to know about a motor and how everything works together. I am looking to stroke and blow my motor so I will be needing to find that delicate balance here in the future. So thanks for the information guys, I am glad I didn't only get made fun of for asking a technical question.