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compression ratio #94787 Feb 1, 6:23am Feb 1, 6:23am
Joined: Oct 2003
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stefanoc OP
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compression ratio
i know that its 11.5:1
but wut are these numbers stand for >
wut does compression ratio do or is?
lower compression better?
thanx

Re: compression ratio [Re: stefanoc] #94788 Feb 1, 2:06pm Feb 1, 2:06pm
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Lower compression is better for cars with Turbos and S/C's. High compression is good for NA cars.

I believe the ratio numbers mean how many air molecules to fuel molecules.

11.5 air molecules to every 1 fuel molecule.

Correct me if I'm wrong.

Re: compression ratio [Re: stefanoc] #94789 Feb 1, 5:36pm Feb 1, 5:36pm
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Rave669
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Sorry man, but you are waaay off. It has nothing to do with air/fuel, What you are referring to is air/fuel ratio. In the instance of compression ratio, it's all about how much compression of the air and fuel in the cylinder is occuring in each combustion cycle. picture it like this:

you have a certain volumetric capacity for a cylinder bore when the piston is at it's lowest point, this is represented by the number 1. see the pic below:
Linked Image

Now, when the piston is at the top of the bore, that volume of air/fuel mixture is compressed by the piston at the top of the cylinder, in the case of the 2ZZ engine, it is compressed by 11.5 times the full volume of the cylinder. Again, see the pic below for an idea of what this means:
Linked Image

So, when you are figuring out how much you are compressing the a/f mixture in the cylinder for a given application, you represent it by putting how much the cylinder volume is compressed, followed by a colon, and the number 1. so it is notated for engineers as 11.5:1 (or when saying what you want your mechanic or engine builder to do to your motor, you tell them, "eleven-point-five to one")

it is possible to have an overbore (for example: 11.5:1 could be changed to 11.5:1.5, but this would mean the overall cylinder bore would be lengthened over stock, however, since this is impossible in most engines without some serious retooling, you usually don't see this, in all automotive performance applications, the last number in the compression ratio notation will always be 1.

the compression ratio is important because the more you compress the air/fuel mixture prior to igniting it, the more force the resulting explosion (yes, the mixture is exploding every time your plugs fire) the more force will be generated when the mixture combusts.

However, there is a fine line between too much, and too little compression. If you put the A/F mixture under too much pressure, it can detonate without a spark (pre-detonation) and this is bad, and where knocks and pings come from. too much compression under forced induction, such as a turbo or supercharger, can have the same result, which is why the boosted guys run lower compression (8.5:1 is common on turbo 2ZZ celicas)

Running too high of a compression can damage connecting rods, piston tops, valves, piston rings, and may even crack your cylinder walls, so knowing the basics of engine building, or knowing someone who does, is essential when upping the pressure in the cylinders.

However, on a N/A car, higher compression can be benificial. TRD Japan makes a 13:1 compression piston set for the 2ZZ, but please note, you must run 98 octane fuel or higher to prevent pre-detonation from occuring (higher octane fuel is more stable, and can be compressed more before detonating on it's own.
Hope this clears it up for you thumbsup


[Linked Image]
Re: compression ratio [Re: Rave669] #94790 Feb 1, 5:57pm Feb 1, 5:57pm
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Great info Rave


To deny the simple pleasures in life is to desire death in the most horrifing way.
Re: compression ratio [Re: stefanoc] #94791 Feb 1, 8:02pm Feb 1, 8:02pm
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Rave669 Pretty much summed it up. anything over 10 is good!


Knowing the path and choosing the path are two different things.
Re: compression ratio [Re: stefanoc] #94792 Feb 2, 3:32am Feb 2, 3:32am
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so celicas compression isnt really high? i heard ppl saying its high. does it affect the speed or anything or is it only bad when turboing the car? are the harms only physical/mechanical or if the compression was lower on our gts stock, our cars would have been faster.

Re: compression ratio [Re: Rave669] #94793 Feb 2, 7:53am Feb 2, 7:53am
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Originally Posted by Rave669
Hope this clears it up for you thumbsup


it def. did, great info thumbsup thumbsup thumbsup

Re: compression ratio [Re: Celicaracer18] #94794 Feb 2, 7:58am Feb 2, 7:58am
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Originally Posted by Celicaracer18
Originally Posted by Rave669
Hope this clears it up for you thumbsup


it def. did, great info thumbsup thumbsup thumbsup

Re: compression ratio [Re: stefanoc] #94795 Feb 2, 10:22am Feb 2, 10:22am
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Oops. I knew I was thinking about something different, but just couldn't get my tongue on it.

Re: compression ratio [Re: Grand-Touring-Dan] #94796 Feb 2, 10:31am Feb 2, 10:31am
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Originally Posted by Grand-Touring-Dan
Oops. I knew I was thinking about something different, but just couldn't get my tongue on it.


that happens to me alot, especially when I'm around hot girls frown

Re: compression ratio [Re: stefanoc] #94797 Feb 2, 3:51pm Feb 2, 3:51pm
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Originally Posted by stefanoc
so celicas compression isnt really high? i heard ppl saying its high. does it affect the speed or anything or is it only bad when turboing the car? are the harms only physical/mechanical or if the compression was lower on our gts stock, our cars would have been faster.


On the GTS, the compression is quite high in stock trim, 11.5:1 is usually reserved for race engines to begin with; in fact, it's one of the higest stock compression ratios you will find on a factory setup anywhere.

There should be no harm in lowering the compression on the GTS, but unless you go turbo or add a supercharger, you will lose power in the long run. the toyota/yamaha engineers knew what they were doing when they designed the 2ZZGE. the stroke is shorter than the 1ZZ, and the compression is much higher as well. You shouldn't need to lower the compression at all, unless you go Forced induction. even then, you can boost the motor so long as you leep the PSI dialed down and run a fuel with an adequate octane rating.

Lowering the compression on the 2ZZ will allow you to run higer levels of boost without pre-detonation, but on a naturally aspirated engine, higer compression will make more power. Unless you are adding a turbo or supercharger, you should keep the stock compression ratio, or even raise it a little.

Now, if you plan on going forced induction, you don't really need to swap in lower compression pistions to lower the compression ratio, you can simply add a thicker metal head gasket, this will increase the amount of space between the top of the cylinder and the head, thus reducing the compression a little bit over stock (so, rather than an 11.5:1 compression, you will end up with anywhere between 11:1 or 10.5:1) Companies like Paeco can custom make metal head gaskets for any application.


[Linked Image]
Re: compression ratio [Re: stefanoc] #94798 Feb 2, 4:25pm Feb 2, 4:25pm
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the higher the compression, the harder it's to work with...such as turbo's.


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