[73GrandStang]

I know absolutely NOTHING about compression. I've seen 9:1, 10:1, 10.5:1, but what do all these numbers mean? What compression is it measuring? Whats good? Whats bad?

PLEASE ENLIGHTEN ME ALL-KNOWLEDGABLE VMFers!

 

[390fastback]

Its a ratio of cylinder volume compared to combustion chamber volume. Example, 9:1 means the size of the cylinder with piston fully down is 9 times bigger than the combustion chamber of the cylinder head. High comp. ratios generally require higher octane gas. If I'm wrong I will be corrected.

 

[Hemikiller]

Essentially the compression ratio is how much the air/fuel mixture is compressed. If your 302 had 10:1 compression, each cylinder has 37.75 cubic inches of volume at bottom dead center. When at top dead center, your cylinder volume is only 3.775 cubic inches. Thus the ratio of 10:1. Generally the more compression, the more power, but that also depends on many other factors.

 

[roadracer]

It's really just the degree of "squeeze" the engine has. ::

the ratio is just the maximum volume for one cylinder of your engine (this occurs at BDC or "bottom dead center") divided by the minimum volume for one cylinder of your engine (taken at TDC "top dead center").

So if you max cylinder volume was 100 cubic inches and the minimum was 10 cubic inches then the compression ratio is 100/10 = 10 .....expressed as 10:1.

Now, this is static compression which is purely based on geometric volume and does not take into account mass flow effects and thermal losses (this stuff can get really complicated so just ignore it)

These volumes are total volumes meaning not just the "swept volume" that the piston displaces when it moves but also the small volume in the cylinder head (and the piston too), the thickness of the head gasket and the small annular area from the top of the piston down to the top piston ring. ( you can go crazy with this stuff.

On ratios...

9:1 (good for pump gas)
10:1 (more power but also more prone to preignition)
10:5 (even more power but also even more prone to preignition)

 

[sharkmouth]

The piston makes the volume of a cylinder larger and smaller as it moves up and down...as it moves up the mixture in the cylinder gets compressed to 1/9th (in the case of 9:1 compression) the size it used to be.


Take it too far and the gasoline auto ignites in the cylinder like a diesel...but gas engines arent built for that!!


How much compression an engine can have and not run into probs depends on fuel octane, ignition timing, cam profile, combustion chamber shape and "squish", engine load (vehicle weight, transmission, and gearing), fuel system limitations, etc.

 

[roadracer]

Funny!

We all jumped one that one at the same time!

We need an alert that pops up when someone is busy replying to the same message at the same time!

Bob???

 

[sharkmouth]

LOL....I launched headlong into a very technical dissertation concerning compression....and my big fingers suddenly hit some crazy combination of keys and wiped it out before I could post.

So I threw out the condensed version

 

[dvturnupseed]

Isn't it true that you can "tame" ( not shure if that is the right word that I'm looking for) preignition with a water injector, and can even use alcohol in the injector and do the same thing?

But it is a big PITA

 

[73GrandStang]

Dont you all laugh at me at once...but...whats "preignition"? I get it now, but I got completely thrown off by "preignition"

 

[nylhtak]

9:1 (good for pump gas)
10:1 (more power but also more prone to preignition)
10:5 (even more power but also even more prone to preignition)

I guess this is why I've heard that the higher the octane rating, the lower the combustive nature of the fuel. Ie. 87 octane ignites quicker than 91. And that pinging is largely due to the lower octane fuels igniting in multiple areas of the compresing cylinder...

Thus, the higher compression rating of the engine-the higher octane needed to run it.

How far off am I, besides the fact that the higher octane fuels have more detergents in them?

 

[sharkmouth]

preignition=dieseling of the gasoline which obviously occurs before the intended time for ignition

 

[DAnson66]

I'm not really smart about this like the other guys are, but a simple definition, IIRC, is that the fuel/air mixture ingnites before a spark is produced by the plug. Like, a deisel engine has no spark plugs, because the compression is so high, that the fuel/air ignites automatically.
That's extremely crude, and i probly got a few things wrong, but hey, i tried. ::

 

[dvturnupseed]

Dont you all laugh at me at once...but...whats "preignition"? I get it now, but I got completely thrown off by "preignition"

A very good question. Preignition, ignition of the charge in an internal-combustion engine earlier in the cycle than is compatible with proper operation. Right out of Websters Dictionary

 

[Johnbear]

preignition is when the gas ignites itself and starts to burn then the sparkplug fires and starts to burn then they meet and create a crash witch can distroy the bottom end of an engine.