Attn car guys... i got some Q's

[HoppysMag]

ok all you car guys, is a higher compression ratio better or worse?

[Creative Mayhem]

Depends what you want to do with it. If racing, you want the higher ratio, if not you really don't need it. It does however produce more power in most cases. Imports usually have the higer ratio for power reasons. Smaller engines trying to produce the same power as larger engines need the higher ratio to even things out. The higher ratio can cause more impact on the internal parts(ie, pistons, con.rods, crank and associated bearings) there could be failure down the road but usually the extra power/force is accounted for during design. Is this a prob? not really, if it's stock, but if it's aftermarket... there could be problems. I say could because as we all know, not everyone know what they're doing.


C Mayhem

[GreasyPigeon]

depends

it depends on what you have. my compression ration on my chevy 350 is 10.2:1 what type do you have under your hood?

[rhetor22]

compression is there so that you can cram 50lbs of crap in a 4lb bag.

It really depends on application. To get high compression, you need a longer stroke. Longer stroke = lower max rpm.

Diesels need high compression cuz they have no spark plugs! They use the compression to create enough heat to blow up the mixture.

[bofh]

Originally posted by rhetor22
To get high compression, you need a longer stroke.

Everything else you say I agree with, except this part.

Conpression has very little to do with stroke length. Normally you'll change the compression by gasket size, or combustion chamber changes, perhaps with a domed or flat piston, almost nobody changes the stroke for compression changes.

As for what compression ratio you should run. Don't worry about it, if the engine has the power you need, it doesn't matter what's going on inside. If you're actually going to muck with the inside bits... you shouldn't be asking in a paintball forum, really.

[nic m85]

high compression pistons are good for running all motor applications

if running a turbo, you want lower compression to avoid damage to the engine because the turbo is forcing more air/fuel mixture into the cylinder than normal.

[krafty]

Just remember that as the compression goes up, so does the octane requirements of the fuel you use.

[HoppysMag]

one of my friends who actualy knows alot about cars said the lower the better, but mabey i mistook him...? iv got a stock 1985 BMW 535I. and i cant rember exactly what the compression is on mine but he said his was like 9 or something... i think i might have heard wrong... thanx for the info guys.

EDIT: and my friends got a Camero that he, "Supped up". i think he has like big block 400 or 350? im not sure im ignorant when it comes to cars and stuff

[bornl33t]

the higher the compression the faster a car runs...PERIOD.

The only time you don't want high compression is when you're playin with boost or nitro. Cause they artificially create high compression and if you use high compression+high compression you get blow headgasket

the reason imports generally have higher compression is because they have smaller engines so they have to use high revs and high compression to make the same power.
V-8's can do that too with stuff like o-ringed heads and copper headgaskets ( I THINK it's copper ), but what good is a 500hp 350 to anyone? ahaahah

Why?

[bofh]

Originally posted by Heat
the higher the compression the faster a car runs...PERIOD.

Care to back that up with any data, or perhaps some common sense? or should we all be running 19.5:1 ratios? :)

However, the germ of truth in the above statement is, the you'd want to run the highest comprssion ratio you can get away with. With forced-induction engines that ratio becomes lower. :)

The "get away with" part is the clause. There are a huge number of factors which limit the compression ratio. bad things happen when you exceed that limit. More is not always better.

{edit} and of course "Better" is even debatable. Better = more power? = reliablity? = drivablity? = efficency?

[Zumina]

Originally posted by bofh

With forced-induction engines that ratio becomes lower. :)

Then mechanical ratio becomes lower, but the final-drive ratio is higher due to the pre-compressed air entering the combustion chamber.

At any rate, higher compression motors require truer burning fuels (high octane). Also, end power isn't in compression alone. Proper valve/cam timing plays a LARGE part in the manipulation of highly compressed fuel.

[rhetor22]

Originally posted by bofh


Everything else you say I agree with, except this part.

Conpression has very little to do with stroke length. Normally you'll change the compression by gasket size, or combustion chamber changes, perhaps with a domed or flat piston, almost nobody changes the stroke for compression changes.

As for what compression ratio you should run. Don't worry about it, if the engine has the power you need, it doesn't matter what's going on inside. If you're actually going to muck with the inside bits... you shouldn't be asking in a paintball forum, really.

I was refering to stock engines. Stroke and bore in the end determine compression, not necessarily stroke alone, so you're right about that.

I can't quite remember what the ratio in my truck is...

Either way, if you're not a big gear head engine junky, you shouldn't be conserned with compression ratios.

I think its becoming one of those "yea well i have DOHC" thingies where no one knows what that stands for, or what a cam rod really does, but its printed on the top of your V-Tec so it must be good.

[bornl33t]

Originally posted by bofh


Care to back that up with any data, or perhaps some common sense? or should we all be running 19.5:1 ratios? :)

However, the germ of truth in the above statement is, the you'd want to run the highest comprssion ratio you can get away with. With forced-induction engines that ratio becomes lower. :)

The "get away with" part is the clause. There are a huge number of factors which limit the compression ratio. bad things happen when you exceed that limit. More is not always better.

{edit} and of course "Better" is even debatable. Better = more power? = reliablity? = drivablity? = efficency?

I don't think 19.5:1 is feasable... as far as I can remember, and I may be wrong, but there is not a production motor on earth that has more then a 13:1 compression ratio. Mostly because it's too hard to keep the enging from flying apart. You are right about everything.

And since you missed the common sence in the first post I'll do it again. Compression IS denser air. More air=more fuel= more power. But yeah, we'd all be running 19.5:1 if we could afford that kinda of technology. But like you pointed out, durability would suffer.

[Mango]

The 427 Tri-Power in our vette is 13:1. :) 14:1 is combustion ignition isent it? haha

[bofh]

Originally posted by Heat
And since you missed the common sence in the first post I'll do it again. Compression IS denser air. More air=more fuel= more power.

The only flaw here is that more compression is not more air/fuel in the cylinder.

Compression only make denser air/fuel, but the 13:1 has the same ammount of fuel and air as a 8:1 does. {Technically, less fuel and air, given the same engine with changed heads or pistons or gaskets to make the compression change, but close enough to not worry about.}

There's a subtle difference between more air/fuel (which properly used, *always* makes more power) and a denser combustion event. A denser combustion event allows more energy to be extracted, but the same amount of energy is released in both a 13:1 and 8:1 engines.



Speaking about compression, and head design and the ilk. I was going to make a seperate post about this, but I started to delve a little deeper into this than I thought and now's a good a time as any.

Two things. One,... beg, borrow, steal or go to a good libary, and look at John B. Heywood's book "Internal Combustion Engine Fundamentals" it'll tell you more than you want to know about engines. Everything is in there. *Everything*

Second, Check this out. http://theses.mit.edu:80/Dienst/UI/2...8/1?npages=286 this is a paper from a student of John Heywood, about differences in spark plugs and flame kernel propagation. It doesn't go into chamber design, but a lot of the groundwork is there. Very surprising stuff.