compression question

[98-ls]

can some one tell me what compression ratios has to do with turbos, like whats good, bad and why

[SenseiAccord]

good is like 9.1:1 and below... meaning 9.1 running like 10 lbs and as u lower the compression the more boost u can run. Bad turbo compression is like 12:1. Ur engine will just knock like crazy and probably most likely blow ur engine like crazy. Smarter ppl can tell u more detailed information on why... just too hard for me to explain in a simple way.

[98-ls]

yeah i know that much, but i want to understand why. thanks for the info though.

[SRV BOY]

I dont know much about it either, the only explanation someone else gave me, is because honda's are already tuned to the max, meaning the engines are already pushing their limits, for example my prelude com ratio is 10:0:1, american muscle cars have low ratio, because their are low reving torque machines, thats the only explaination i can think of, i dont know much about compression myself, i just know how it works

[98-ls]

so is it lower compression higher torque?

and what exactly are the compression numbers related to?

[rubix777]

The compression ratio is the ratio of the volume of mass of air within the cylinder at BDC to TDC, Bottom Dead Center, and Top Dead Center, respectively. BDC is when the volume of the air is at a maximum. TDC is when the volume of the air is at a minimum.

Take for example the compression ratio 10.6:1
Let's say you have a certain amount of air occupying 10.6 m^3 of space. Now when the piston moves from BDC to TDC, The volume of the space changes from 10.6 to 1, so it is now 10.6 times smaller. That same amount of air is compressed, unless there is a leak somewhere, and the pressure increases respectively with decrease in volume.

So when they talk about compression ratio, they're talking about the ratio of the air within the cylinder and how much the cylinder compresses the air.


You can make torque in 2 ways:
1. larger bore, which is bigger diameter for each cylinder
2. high rpms.

Honda makes torque from high rpms and having small engines, 1.5L, 1.6L, 1.8L, 2.0L.. etc... meaning small displacement engines.

Domestics like to increase the bore and stroke, thus limiting how high the engine can rev. That's why you see Chevy bust out their 5.7L V8 engines.

[SenseiAccord]

Originally Posted by 98-ls

so is it lower compression higher torque?

and what exactly are the compression numbers related to?

When the piston is at TDC (Top Dead Center) there is X amount of space. Thats what the ratio is too. So if u have an engine that is 10:1 then at BDC (Bottom Dead Center) there is that 10 of X amount of space between BTC and TDC that is being compressed into X. Hard to explain. So its like 10 spaces being compressed into 1 space. So for example if u had 10 apples in ur engine... 10 of those apples will be compressed into one apple. Get it?

Anyway to get more torque usually requires a longer stroke of the piston. Raising ur compression will give u more torque in a different kind of way. For example like a truck engine could have a stroke of 90mm while a car will have a stroke of like 80 mm. The bigger the stroke.. the more torque while the shorter the stroke the more horsepower cuz the shorter stroke makes the engine move faster. Its like clapping ur hands... thats how a stroke is in an engine. You clap faster when ur hands are closer together... but when u extend ur arms as far as u can and try clapping like that then the slower u clap. Get it?

I could be wrong to someone correct me if im wrong on all this information. Im still learning too.

[SenseiAccord]

Originally Posted by 98-ls

yeah i know that much, but i want to understand why. thanks for the info though.

The reason why is because if u cram too much air/fuel into a space that gets compressed too much it will explode on its own. Ur car will be acting like a diesal engine. The way diesal is disigned... its has a compress ration of like 18:1 usually. There is no spark plug. On the intake stroke it only intakes air (which doesnt ignite on its own)... not an air/fuel mixture like cars. They way it ignites is when the piston is at TDC (Top Dead Center) it shoots gas and when it does it ignites on its own from all that pressure. The reason why it wont work on ur car because since it is being compressed with the air/fuel already in the cylinder... so when it is being compressed it will hit the point where it cant take anymore and just explode on its own even before it hits TDC and ur car will end up knocking... which is really bad and can lead to blown pistons, walls, valves, etc etc.

Its hard to explain but i know someone else can explain it better.

[SenseiAccord]

Originally Posted by SRV BOY

I dont know much about it either, the only explanation someone else gave me, is because honda's are already tuned to the max, meaning the engines are already pushing their limits, for example my prelude com ratio is 10:0:1, american muscle cars have low ratio, because their are low reving torque machines, thats the only explaination i can think of, i dont know much about compression myself, i just know how it works

Well u are correct because what Japan supposedly does is take american cars... make it better and sell it back to america. The way all cars are first made is to make the best emissions, luxury, and all that stuff. Of course u see people making their cars a lot better than stock but if u look at it.. it is loud, non-luxuriest, sometimes more worse gas mileage etc etc.


What i recommend to understand more is go to http://auto.howstuffworks.com/channe...under-the-hood
it is a good site to learn about compression ratio, turbo and all that other stuff. Honestly it makes u understand a lot better cuz they got illustrations and is well written.