compression

[kinkyllama]

Anyone wanna explain it to me? I know very very little about it. You dont need to go into great detail---although i dont mind---i just need to get the gist of it.

Danny

[MMamdouh]

didn't really understand what you are asking here man? are you asking about the consept of compression itself or what?

MMamdouh

[kinkyllama]

What the hell is it exactly? I have a small general idea but thats it

[PrecisionBoost]

Are you asking about the compression ratio???

If so.... it's simply the volume of the cylinder ( 0.5L ) at bottom dead center (piston down by the crankshaft) divided by the volume of the cylinder at top dead center.

So.... if our 2.0L has 0.5L at BDC and the compression ratio is 9.6:1 then that tells us that the volume at TDC is 0.052L

The higher the compression the more power the car will make when the air/fuel is ignited.

However.... a super high compression ratio like 13:1 is really bad.... detonation occurs very easily as the heat created during this extra compression can ignite very easily.

A turbo inflates the natural compression ratio..... so an engine with an 8:1 compression ratio might act as though it has a 10.5:1 compression ratio.

So.... that's why turbo cars allmost allways run lower compression pistons than naturally aspirated vehicles.

You can certainly run 9.6:1 pistons with a turbo but you have to be very carefull in your tuning and engine design and you must use high grade fuel.

Some of the big turbo drag cars use 12:1 compression with monster turbos but they are usually running extreemly high octane fuel or alchohol....... and they spend hundreds if not thousands of hours going over every single part of the engine to maximize power without getting the death stroke known as detonation.

I think someone posted a good link to a calculator which will show you the "effective compression ratio" on a turbo motor.

Hopefully your questions are answered.... if not....let me know what your after and I will try to come up with an answer.

[PrecisionBoost]

Here are some links....

http://en.wikipedia.org/wiki/Compression_rat

Oh.... my favorite site....

http://www.turbofast.com.au/javacalc.html

( pick turbocomp )

[PrecisionBoost]

Ok.... so the bore and stroke are 3.39" and standard compression is 9.6:1

So...

VE of 60% and 12psi --> effective CR = 10.4:1 and fuel should be 97 Octane

VE of 70% and 12psi --> effective CR = 11.9:1 and fuel should be 101 octane

VE of 80% and 12psi --> effective CR = 13.5:1 and fuel should be 106 octane

VE of 90% and 12psi --> effective CR = 15.1:1 and fuel should be 111 octane !!!!!!

So.... as you can see.... 9.6:1 with 12psi is fairly close to the maximum limit you'd want to go

The problem is that the VE will change based upon RPM levels..... so you might have an 85% VE at 4000RPM and 12psi and only 65% VE at 6500RPM and 12psi.

So.... next.... lets look at why we say 6-7psi is ok for the stock engine.

VE of 60% and 6psi --> effective CR = 8.3:1 and fuel should be 90 Octane

VE of 70% and 6psi --> effective CR = 9.5:1 and fuel should be 94 octane

VE of 80% and 6psi --> effective CR = 10.7:1 and fuel should be 98 octane

VE of 90% and 6psi --> effective CR = 11.9:1 and fuel should be 101 octane

[PrecisionBoost]

Now...... I'm sure your looking at that post and saying....... how can the effective compression ratio be lower than the stock 9.6:1 ???

Well..... the VE (volumetric efficency) effects the "real" compression ratio of both turbo and naturally aspirated vehicles.

If you have a 1L cylinder and 10:1 compression ratio it means that your going to stuff the air into a tiny 0.1L spot during ignition.

Now..... that's with a VE of 100%

At 90% VE your only getting 0.9L of air into the cylinder so your stuffing less air into that 0.1L spot. (thus lowering the compression)

If your VE is 50% your only stuffing 0.5L into the same spot.

In theory.... a VE of 50% gives a "real" compression ratio of 50% of the rated piston compression ratio

So if you have 10:1 compression pistons and you have a really bad cylinder head that has a VE of 50% the "real" world compression is only 5:1 !!!

To test this theory simply enter 0psi into the calculator I was using....

Hmmm..... that's strange..... 0psi.... 10:1 CR and 50% VE gives an effective CR of 5.5:1 instead of 5.0:1

Not sure why there is a difference.... perhaps there is something more that I don't know about.

Oh well....... this is just a general theory..... I'm sure there must be some other issue of the equations I've missed.

Hmmm if you use 50% and 0psi with a 9:1 you get an effective CR of 5:1

It seems it's more like (1+CR) X VE = effective CR

(10+1) X 0.50 = 5.5 and (9+1) X 0.5 = 5.0

( 9.6+1) X 0.60 = 6.36 and the program calculates it as 6.16:1

Hmmm now I'm confused..... I just came up with my forumulas off the top of my head.... I guess I will have to dig into my thermodynamics books and see what I missed.

Anyways...... even though I'm a little off (or the program is ) you can see that the real life Compression ratio does depend on how efficent your cylinder head is at flowing air.

So.... higher VE increases the effective Compression ratio which makes more power and requires higher octane fuel.

If anyone comes accross the true formula for calculating the "real life" compression ratio with the Volumetric efficency and boost level let me know as I'm interested in seeing what I missed.

[PrecisionBoost]

As for the VE of our cylinder head.... I don't know.... but it's probably up around 95% around 3000RPM and drops off from there.

Perhaps it's only 65% to 70% efficent at 6000RPM.... I don't really know.

It's very obvious from the dyno curve that our heads don't flow that great over 5700RPM..... the power drops off fairly quickly as more air is trying to get into the cylinder.

Longer duration.... more lift.... larger ports..... better "rod ratio" would help this on a naturally aspirated vehicle.

It would also help on a Turbo engine if your trying to make "maximum power"

But like I said before...... the boost level can equal out any poor VE issues.

The ideal boost controller would allow you to keep 200Whp all the way from 4000RPM to 6500RPM.

So.... it might give you 10psi of boost at 4000RPM (say that gives you 200WHP ) and the boost is increased as the VE goes down.

So at 6000RPM you might be putting out 15psi.... but your still only making 200WHP because your VE is not as good as it was at 4000RPM... so a smaller percentage of air is getting into the cylinder.

This is off topic.... but I'd like to make a "computer controlled" boost controller with integrated waist valve.

Then you could tune your car for a set horsepower level and it would keep it while maintaining a safe level of flow to keep the compressor from going into surge.

Basicly.... to avoid surge the waist gate would open up.... which allows the turbo compressor to flow air into the cylinder and out a valve..... which moves it away from the "surge line"

It does this without decreasing the psi to the engine.... so it shuts the waistgate on the turbine side to up the power going into the compressor which compensates for flow.

So..... at 12psi you might only flow 20lbs/min but with the waist valve dumping air out of the intake that same turbo gives you 12psi but it's able to flow 30lbs/min..... 20lbs/min into the engine and 10lbs/min out of the waist valve

It's kinda hard to explain..... but it's a unique design from what I've seen out there..... the problem is that it needs to be controlled very carefully to avoid boost spikes and changes in flow to the engine.

Basicly.... it would use a blow off valve that can open in varying degrees instead of totally open or totally closed.

The theory seems sound.... but I'm going to have to try it on my own car.

The benifit is that you can use a larger turbo with lower PSI without risk of hitting the surge line.

This is ideal for street/track setups where you might run 6psi on the street and bump it up to 15psi on the track.

Anyways.... I'm off topic.... hopefully you understand the roll of compression and Volumetric efficency on horsepower and "real life" compression ratio.

By the way.... pressure is related to volume..... so if you have a 10:1 compression ratio and it decreases the volume from 1L (BDC) to 0.1L (TDC) your pressure goes up by a factor of 10.

So the atmospheric pressure is 14.7psi..... so with a 10:1 ratio the pistons makes that into 147psi at TDC.

So in theory if you crank the engine.... you should see roughly 150psi on your compression tester.

This is just a general rule of thumb..... it's not an "accurate" number

There are all kinds of other things that happen..... as you compress the gas it heats up.... which increases the pressure even further.

There is also piston velocity to consider...... the faster you compress the gas the more energy you transfer.... so the more heat that is created.

I don't know how much of an effect piston velocity has.... I don't know how to calculate it..... but if it takes you 1 minute to compress that cylinder the heat generated will be way less than it would be if you compressed that air in 0.01 seconds.

Anyways...... this is getting long.... and I should stop now.... or I might get off on some other tangent.

[PrecisionBoost]

You dont need to go into great detail

Well.... you know me.... I can't shut up....especially if it's on a topic that hasn't been discussed before.

[PrecisionBoost]

I was thinking it would be cool to have a "technical" section where this stuff is posted.... but at the same time the info would have to be perfectly accurate so as to not mislead everyone.

Most of my posts are somewhat general.... they cover the topics but aren't necissarily 100% correct.

Most of the time I know the exact calculation.... but it's too complex and most of you would go crosseyed if I posted a four page explaination with calculus and other heavy mathmatics.

So.... take my technical posts with a grain of salt.... I know they aren't perfect.... I'm just trying to explain the general details without going overboard.

A good example is this post..... the real compression pressure depends on a couple trillion factors and the mathmatical formula for calculating the exact pressure you would see would be about 5 pages long.

Factors like temperature of the inlet air, heat exchanged from the cylinder walls, heat exchanged from the valves and cylinder head, pressure losses through the piston rings....and a whole pile of other factors come into play and make a difference when you take your pressure reading on your cylinder.

Here's a good one.... your pressure tester takes up a differnt volume than the spark plug...so your changing the volume of the cylinder head by a significant amount..... compression testers are to give a general idea.... you can't actually use them to do calculations on things like compression ratio..... for that you have to do physical volume measurements of the combustion area of the head, factor in the gasket width and then figure out where exactly the pistons ends up at TDC in relation to the top of the block (deck to piston height)

Ahhhhhhh.... I'm doing it again..... must shut up.... can't stop typing..... must crush hands with hammer

sorry.... I started to explain how everything isn't "exact" and ended up talking about compression pressures again.

Oh well.... my brain is in overload today for some reason.... must have been the 15 Chocolate Macadamia nuts I stuffed in my mouth before I came down here.

[PrecisionBoost]

I moved this out of general and put it into engines....

[kinkyllama]

No..this is great. I couldnt find any good info online so this helps me understand alot.

What are different ways to lower the compression that we have available on our car?

[Audacity Racing]

No..this is great. I couldnt find any good info online so this helps me understand alot.

What are different ways to lower the compression that we have available on our car?

headgasket (though it changes timing)
lower deck pistons

[tango]

One way to lower the compression ratio is to dish the piston. That means the top of the piston is cut away to create more space. Another way is to shorten the connecting rod. This will also has the potential of allowing the engine to rev freer (less internal mass from the shorter, lighter connecting rod).

But without boost lowering compression ratio isn't going to give you any gains, unless you just want to use lower octane fuel.

That said, neither of these modifications are for the faint of heart or pocket. Certainly dishing is cheaper but still not for the faint of heart.

[PrecisionBoost]

Hmmm.... I never thought about the idea of shorter connecting rods.... good point.

The gasket thing is ok.... but what Audacityracing was talking about is something that should be explored.

Basicly by going with a thicker head gasket your moving the cylinder head upwards.

As you move it upwards the timing belt rotates slightly.... which rotates both the exhaust and intake cams slightly.

I'm not sure of the exact relationship between increased thickness and number of degrees of rotation..... but it's something that has to be watched out for to ensure that your valves don't slap the piston.

I can't remember if the cams move forwards or backwards by a few degrees.

Usually this is easily fixable..... you simply install adjustable cam gears.

Unfortunatly... we don't have any "off the shelf" cam gears that will work on our engine.

There are cam gears available..... but our gears have metal on the backside which is used to trigger the cam sensor.

On the other european versions.... they don't have this metal on the back...so unfortunatly their adjustable cam gears don't work without modifications. ( I want to get a set and fool around to make a separate cam signal plate)