compresion ratio?

[mazda3zoom]

ok so i get what compresion ratio is.....but what do the numbers mean? there are usually 3 sets of numbers and i was hopin someone on here could tell me what they mean, also does anyone know they #s for my 3s? thanks (uhm)

 

[Black Majik MSP]

There are 2 sets of #s. To use the MSP as an example, it's 9.1:1. If you are familiar with suck/squish/bang/blow, you know the 2nd cycle is compression. Basically all it means is that the compression inside the engine is 9.1 times more than the normal ambient atmosphere.

MazdaUSA lists the 2.0L at 10.0:1 & the 2.3L at 9.7:1.

 

[goldwing2000]

Basically all it means is that the compression inside the engine is 9.1 times more than the normal ambient atmosphere.

Uhhh... not exactly correct. If that were true, compression ratio would be dependent on atmospheric pressure, which changes with altitude.

Compression ratio is the relationship of cylinder volume with the piston at the bottom (bang stroke) to the cylinder volume with the piston at the top(squish stroke) with all valves closed.

ie. say for example, with the piston at Bottom Dead Center, cylinder volume is 10 cubic inches. Then, with the piston at Top Dead Center, the cylinder volume is 1 cubic inch. This would be a 10:1 compression ratio.

Clear as mud? (thumb)

 

[Black Majik MSP]

There's your expert explanation...I'm never any good at the technical stuff. BLAH

 

[goldwing2000]

LoL... you get marks for effort, though. (2thumbs)

 

[Da 6]

when you start boring intakes and heads and changing out pistons and rods and valves cams ext...compressin goes up or down?

 

[goldwing2000]

Could do either, depending on what exactly you do.

Porting heads has no effect on C.R., since it's on the other side of the valves.
Boring a cylinder has little effect since you are increasing the cylinder volume uniformly.
Milling a head or decking a block will increase C.R. because you are decreasing the size of the combustion chamber
Stroking an engine (increasing piston travel) will also increase C.R.
Cams and valves have no effect on C.R.

Generally, the only time you lower C.R. is when you're pressurizing the intake system, as with a turbo or supercharger. This is sometimes done by doubling up head gaskets but can also be done using different pistons, heads or crankshaft.

Increased C.R. requires higher octane fuel, so it's not a large benefit on daily drivers unless you're willing to spend the $$$ on gas.

 

[Da 6]

ok got it now...loweing compresson requires higher octane to prevent detonation from changing the system from normal compression on N/A

 

[goldwing2000]

Reverse that. Higher compression ratio requires higher octane.

Boosting has the same effect as raising compression ratio, though. Increases pressure within the combustion chamber. That's why boosting also requires higher octane.

Compression ratio is lowered on boosted cars so the pressure doesn't get too high and so you can get a larger volume of air into the combustion chamber.

 

[mazda3zoom]

so if u put a turbo on ur car do u have to use higher octane fuel? (uhm)

EDIT-oops didnt read all of goldwings last post- so when cars come from the factory with turbos ur supposted ta use higher octane fuel? and what happens when u use 87 octane with a turbo?

 

[Da 6]

so if u put a turbo on ur car do u have to use higher octane fuel? (uhm)

EDIT-oops didnt read all of goldwings last post- so when cars come from the factory with turbos ur supposted ta use higher octane fuel? and what happens when u use 87 octane with a turbo?

most cars that use higher octane as a normal fuel will cause the pcm to seriously retard timming to prevent too damage and severely degrading peromance... so on a N/A car you can deffinantrly feel it and on a turbo/superchatged car turbo lag becomes turbo draaaaaaaaaaaaaag

 

[goldwing2000]

Good question. According to theory, you are supposed to use higher octane with a turbo but I've never owned a turbo, so I can't say for sure. If you are supposed to use it and you don't, I imagine it would cause detonation (spark knock) just like any other engine. I guess it's really all dependent on what the vehicle was designed for when they programmed the ECU.

Any of you turbo guys care to comment?

 

[Da 6]

gota forgive me i'm not femilar with the other mazda engine lineups other than MZ3-6 so I'm gonna walk down the hall to where I have grounds to compare...

(peep) we'll take a dodge neon R/T for example....it's 2.0 H.O. requires 91 and up or it retards timming to prevent spark knock

and srt 4 it's 2.4 H.O. is the same process but seriously retards timming...

both engines are designed to run on 87...but is seriously not recommended


in otherwards if you cheap out on the gas then the car gives you the finger till it rids its self of the lower octane in figure of speach terms....

 

[mazda3zoom]

so when and if they come out with a turbo kit for the 3 does the ecu have to be reprogramed or how does the car know theres a difference?

 

[Da 6]

there is a kit for the 6i and they are still in the works for both 3's and IIRC if the boost is below 6-7 psi you don't need to run a piggyback...but since 6 throws cels on just a cai...thats still up in the air for grabs on the piggyback.

http://www.protege5.com/vbb230/showthread.php?t=43745
link to the 6i turbo thread and picts and person(s) making the 3 turbo

 

[boostaddict]

Could do either, depending on what exactly you do.

Porting heads has no effect on C.R., since it's on the other side of the valves.
Boring a cylinder has little effect since you are increasing the cylinder volume uniformly.
Milling a head or decking a block will increase C.R. because you are decreasing the size of the combustion chamber
Stroking an engine (increasing piston travel) will also increase C.R.
Cams and valves have no effect on C.R.

Generally, the only time you lower C.R. is when you're pressurizing the intake system, as with a turbo or supercharger. This is sometimes done by doubling up head gaskets but can also be done using different pistons, heads or crankshaft.

Increased C.R. requires higher octane fuel, so it's not a large benefit on daily drivers unless you're willing to spend the $$$ on gas.

Well actually boring the cylinder will also up the CR a little since, with the piston at the bottom of the stroke the volume is increased, but at the top of the stroke the volume of the cylinder head hasn't increased and this is where the vast majority of the volume is when the piston is at the top of the stroke.

 

[mazda3zoom]

there is a kit for the 6i and they are still in the works for both 3's and IIRC if the boost is below 6-7 psi you don't need to run a piggyback...but since 6 throws cels on just a cai...thats still up in the air for grabs on the piggyback.

http://www.protege5.com/vbb230/showthread.php?t=43745
link to the 6i turbo thread and picts and person(s) making the 3 turbo

yeah i have no idea what that means da6- what is a iirc and whats piggyback?

 

[Da 6]

yeah i have no idea what that means da6- what is a iirc and whats piggyback?

dammit...did it again


IIRC=If I Recall Correctly

and piggy back as in computer systems to go around the stock computers.

and that 6-7 was in general for a 50 trim

 

[goldwing2000]

Well actually boring the cylinder will also up the CR a little since, with the piston at the bottom of the stroke the volume is increased, but at the top of the stroke the volume of the cylinder head hasn't increased and this is where the vast majority of the volume is when the piston is at the top of the stroke.

Yup. That's why I said "Boring a cylinder has little effect..." and not "no effect." (thumb)