How do you lower the compression ratio

[obi_waynne]

I thought we needed a thread on here about how to lower the compression ratio of an engine as it comes up quite a bit.

So if you have any comments to make on this subject please post up your musings here. What works and what doesn't? (Thick gaskets, rebores, stroker kits, low comp pistons,cylinder head work etc...)

Also any pointers on which are the best methods on lowering the compression ratio of an engine.

 

[HDi fun]

shorter rods and shallower pistons are the first obvious choices in my opinion.

 

[obi_waynne]

Now there is something I struggle to get my head around here. It's a bit like the phrase "more viscous" - fine when you understand it but confusing other wise!

Shallower pistons, increase the air in the cylinder so surely these increase the compression ratio? or does it not work like that? I suppose they short rods and shallow pistons work together to lower the CR overall.

 

[SLEEPER]

compression ratiio is the cc of a cylinder compared to the cc of the combustion chamber

So with shorter rods the cc is the same but you increase the size of the combustion chamber so lower you get a lower compression ratio. Same with shallower pistons cc is the same but the combustion chamber is larger.

You can do it teh other way with a larger head gasket or stroker kit if installed above the block - if its fitted below the block it would increase it.
I imagine this may not be a good idea on all engines but it will work on some.

 

[HDi fun]

Divide the swept volume (which will not change unless the engine is bored out and/or the crankshaft is replaced with one of longer throw) into the combustion chamber volume.

 

[kain35m]

Low comp. pistons are best IMO

 

[obi_waynne]

Have a read of this article then chaps and let me have your thoughts? Any glaring omissions or errors?

http://www.torquecars.com/tuning/lower-compression-ratio.php

 

[kain35m]

Have a read of this article then chaps and let me have your thoughts? Any glaring omissions or errors?

http://www.torquecars.com/tuning/lower-compression-ratio.php

On the "head work" section, you may want to add that head work is very complicated, and somewhat costly. If one knows not with which one is dealing, on can seriously screw things up. On the other hand, it requires the least amount of pulling the engine apart. With an experienced machinist, this option requires the least time and parts. It can potentially be not-too-costly as well, however, your gains can prove minimal.

Same with all of them. Add some pros and cons:

Pistons: Not for the faint of heart, this requires almost a complete teardown and rebuild of the engine. Only do this if you're already pulling the engine anyway.

Boring: Again, only do this if you are rebuilding the engine. You can combine this with new stronger rods/low compression pistons, and perhaps a forged crank with a different throw.

Gaskets. Some companies sell gasket kits which allow you to choose the thickness of your head gasket. This is the cheapest/easiest, but it is also the easiest way to create a leak, especially with high boost applications.

 

[jarrus]

You can get decompression plates for some engines which is stronger than using a thicker gasket. but yeah, dished pistons, some engines can have there pistons skimmed slightly.

Lowering the compression of an engine allows to safely add more air in the combustion chamber without rasing the cylinder pressures to unsafe levels.

 

[xacro]

You can use a thicker head gasket and lower compression height pistons

 

[WhiteNoise]

Couldnt be bothered reading it all but a compression plate perhaps???

 

[WhiteNoise]

haha, looked 2 posts above and BOOM!!! compression plate, lol.

 

[jc denton]

This thread is beyond my feable mind.

I would like to know more about increasing compression though!

 

[pgarner]

basically the opposite as lowering
skim the head slightly or fit higher compresion pistons

 

[jc denton]

Higher compression pistons? would they protrude up into the combustion chamber?

 

[jarrus]

you can also take some material out of the heads combustion chamber to decrease compression...

 

[HDi fun]

Drill some holes in the cylinder head :-D

 

[jarrus]

Ha ha,

Yes then your compression will be really low.....

 

[old-git]

With regard to the article, reboring the engine will INCREASE the compression ratio, not lower it!!!

Compression ratio is worked out by dividing the volume above the piston when it is at TDC into the volume above the piston when it is at BDC.

 

[old-git]

Divide the swept volume (which will not change unless the engine is bored out and/or the crankshaft is replaced with one of longer throw) into the combustion chamber volume.

Wrong way round However, even the other way round it is incorrect. See post above.

 

[obi_waynne]

Thanks for the heads up OG! Much appreciated! Is the article looking a bit better now?

 

[old-git]

This reads a bit funny:

Reboring or increasing the size of the cylinder with low compression pistons.

and it has already been covered earlier I think this particular paragraph should be removed as this article is about reducing compression ratios.

 

[obi_waynne]

You're right, I hate deleting text though, it makes the article shorter! Ah Well!

 

[old-git]

Yes, but more accurate. Less is more in this case

 

[obi_waynne]

Once again thanks for the heads up. The feeback boxes on the article pages are the quickest way to reach me about innacuracies. If you spot any more or have anything to add please let me know.

 

[hicusdicus]

The best way to reduce compression ratios is to use gasoline with ethanol in it. Who is going to save us from the greenies?

 

[claymore]

The best way to reduce compression ratios is to use gasoline with ethanol in it. Who is going to save us from the greenies?

that would increase the octane, how would adding ethanol reduce your compression?

 

[hicusdicus]

Claymore, Ethanol increases the octane and loses a couple of miles to the gallon. I am glad to see you have a sense of humor. Do you live in the UK?

 

[claymore]

Claymore, Ethanol increases the octane and loses a couple of miles to the gallon. I am glad to see you have a sense of humor. Do you live in the UK?

I don't understand, what has that got to do with lowering the compression ratio?

 

[ninjasta]

Apart from changing the basic displacement and combustion chamber volume no one has mentioned changing the cam.
The effective or dynamic compression ratio determined by calculation of the piston position at IVC (Intake Valve fully closed) and deriving a ration from the actual volume displaced.
DRC calculator can be found here.
http://www.projectpontiac.com/ppsite15/compression-ratio-calculator
or
http://www.wallaceracing.com/dynamic-cr.php

BTW I hae no connection with these companies - I just used them to work out the effective CR before Supercharging my VXR8

 

[claymore]

Apart from changing the basic displacement and combustion chamber volume no one has mentioned changing the cam.
The effective or dynamic compression ratio determined by calculation of the piston position at IVC (Intake Valve fully closed) and deriving a ration from the actual volume displaced.
DRC calculator can be found here.
http://www.projectpontiac.com/ppsite15/compression-ratio-calculator
or
http://www.wallaceracing.com/dynamic-cr.php

BTW I hae no connection with these companies - I just used them to work out the effective CR before Supercharging my VXR8

I can't see how changing the cam will lower your compression ratio, can you please explain in more detail, the links don't help me, they basically just talk about valve timing, more specifically valve overlap.

 

[ninjasta]

I can't see how changing the cam will lower your compression ratio, can you please explain in more detail, the links don't help me, they basically just talk about valve timing, more specifically valve overlap.

I'll try.
The first thing is to understanding displacement volume is to plot the Valve timingand lift graphically.
The standard for this is use 0 degs the represent tdc at the beginning of power stroke. The point of maximum compression. BDC at the end of the power stroke occurs at 180 degs then exhaust stoke goes from 180 to 360, intake from 360 to 540 and compression from 540 to 720.
The static CR is the ratio of displaced volume / chamber volume ie the volume between 540 and 720 degs.
Having established the 4 stroke timing cycles examine the attached jpeg (ValveTiming.jpg) showing the valve timing as function of lift and angular position in the 4 stroke cycle. If we look at the position of IVC it is 60 degs ABDC at 600 degs.
This means the actual volume compressed is LESS than the Total displacement of the cylinder. This is called the Dynamic CR and is a measure of the usable compression. If we move IVC to a later position the Dynamic CR drops even though static CR is unchanged.
I have also attached an plot of Dynamic CR v Theta = IVC close position after BDC (540 deg) in the compression stoke for a static CR of 9.5/1

 

[old-git]

Well, that has saved me some typing

 

[wigguy1]

Now there is something I struggle to get my head around here. It's a bit like the phrase "more viscous" - fine when you understand it but confusing other wise!

Shallower pistons, increase the air in the cylinder so surely these increase the compression ratio? or does it not work like that? I suppose they short rods and shallow pistons work together to lower the CR overall.

Anytime you drop the height of tdc you will increase volume of the combustion chamber, and because you can only suck so much air on the intake stroke, the compression ratio drops.

 

[old-git]

With compression ratio discussions, you need to say whether you are talking about the static or dynamic ratio. Most people are talking about static, which is the ratio between combustion chamber volume and swept volume & combustion chamber volume. Dynamic compression ratio is a movable feast as it is dependant such things as valve timing and intake and exhaust effeciencies (as explained by ninjasta earlier)