compression ratio?

[schotty46]

Alright so, im doing a build on my 4G63T. I was reading around and noticed that someone said the thickness of the headgasket makes a difference in the compression ratio. I was wondering what changes change the compression ratio? Also, how do the pistons change the compression ratio? This is something ive never really messed with. So please help. Not looking for jerks to bash me on this question. Thanks.

 

[jwahl16]

This is how i take it:

Compression ratio compares the amount of space in the cylinder when the piston is at its lowest point, to the amount of space at its highest point. So if pistons are raised lower or higher on the rods they can alter the cr. And if the head gasket is thicker, there is more space in the cylinder so the compression ratio decreases.

I hope that helps a little

 

[noV8undrDIShood]

the compression ratio is mainly based off the piston... the top of the piston is the ultimate factor, the same amount of air/fuel is going to be in the combustion chamber when piston is at top of it's path.. so to increase your compression the top of the piston must be raised or "domed" thus creating less space for the air/fuel and creating more pressure.. the head gasket, depending on thickness will raise the head off the block ever so slightly to give the air/fuel more room in the combustion chamber and therefor lowering compression a little bit..

overall.. for an example if you have a piston with a 9:1 CR with a mitsu MLS headgasket.. you will be at about 8.8:1-8.9:1 CR

correct me if i am wrong someone but i think some people actually kinda polish or smooth (whatever you want to call it) out there combustion chamber to lower compression and create more room for more air/fuel as well but not 100% sure on that one.

hope this helped

 

[GRAVEDIGGER]

If a cylinder head is resurfaced it takes some material away and in turn compression will INCREASE. A thicker head gasket can help restore the compression back to where it was before the material was taken away. The pistons help change compression too depending on their design. Just because you have pistons that are 9.0:1 doesn't mean that would be the exact compression ratio though. It would most likely be close, but every setup varies a bit.

 

[schotty46]

so when i have my block and head resurfaced ask them how much theyy took off and compensate that with the headgasket? also, what is a decent CR that i should go with? im plannin on runnin the HX 40 with 950s and dsmlink V3 to tune. somebody told me 10.5:1. does that sound right?

 

[habitatguy187]

10.5:1 is too high for about 99% of boosted street cars. 9.5:1 would be fun, even 9:1 would be great. Stock 2g CR is 8.5:1, stock 1g CR is 7.8:1.

 

[schotty46]

So that leads me to another question. I have a 1G block. A 90 with a 2G head. So do i go with the 9.5 still?

 

[nightmares GS-T]

Just know that the higher compression you run the less forgiving its going to be against knock. Even the slightest error in your tune could lead to disaster on really high compression engines. Just something to consider.

 

[schotty46]

So i should probably go with 9:1 huh?

 

[jrohner]

so when i have my block and head resurfaced ask them how much theyy took off and compensate that with the headgasket? also, what is a decent CR that i should go with? im plannin on runnin the HX 40 with 950s and dsmlink V3 to tune. somebody told me 10.5:1. does that sound right?

If you're running E85 all the time that compression ratio would be great.

 

[nightmares GS-T]

Well like the guy above me said if you were to use e85 all the time you would be golden since it resist knock so much better.

 

[schotty46]

No,im runnin 93 with a snow meth kit.

 

[GRAVEDIGGER]

I'd stay in the 8.5-9.0:1 range on pump gas.

 

[Absit]

I'd stay in the 8.5-9.0:1 range on pump gas.

I concur, especially with the HX40, you can easily make up the CR with more boost, and when you're out of meth (for a prolonged period of time), you can turn the boost down and still drive. It's hard to turn the pistons' compression ratio down.

 

[schotty46]

I think ill go with 9:1. Any oppinions? Pros vs cons of going lower or higher?

 

[Absit]

I think ill go with 9:1. Any oppinions? Pros vs cons of going lower or higher?

The pros and cons of going higher or lower have been discussed in this thread, as well as specific recommendations based on your fuel type and goals.