Compression ratio

JakRex · 12-27-2001, 04:03 PM

This might be a stupid question, but I am driving a WRX but not familiar with a turbo engine. Could anybody tell me how think is the stock gasket?
Get this from some magazines that when N/A car need higher compression ratio, Turbo cars need lower ratio instead. What is going on there?
Any help would be great, Thank you !!

122ci · 12-27-2001, 05:19 PM

any factory turbo car will come with a lower compression motor, to allow more boost to be run, without detonation. on a factory NA motor that is having a turbo kit installed on it, a lot of times they will use a thicker head gastket to lower the static compression. like 3mm thick. the thicker head gasket can drop the compression as much as a point. (i.e. 10.5 to 9.5) stock wrx compression is 8:1, which is really low compared to other factory turbo cars, only one i can thick off are buick GNs. most are 8.5 to 9.0 . that help you out any?

Jung918 · 12-27-2001, 10:22 PM

suppose i had a stock WRX but only changed the gasket to make the comression 9 : 1... how much of a performance gain are we looking at? what are the saftey issues?

jeddy · 12-28-2001, 12:08 AM

I'm not a turbo specialist, but here's why it seems to me that you want lower compression ratios with a turbo.

Assume that the compression ratio is how much the stroke of the piston compresses the air/fuel mixture. The function of the turbo is in fact to cram more air into the combustion chamber. So, you in fact need a compression ratio that is lower than for a NA engine, so that you have room for the turbo to increase the pressure.

How the turbo can make more power this way, while simply increasing the compression ratio doesn't, i don't know. I'm guessing there are other disadvantages to higher compression ratios. Perhaps there is a minimum volume for efficient combustion. Too much movement of the pistons is hard to stabilize (especially at higher rpms). Things like that...

That's a long way of saying: i think the compression ratios on engines do not take the effects of the turbo into account. It's just the volume difference at the top and bottom of the piston stroke. You could in theory increase the compression, but (just as in a NA engine), you will start to approach the design limits of the combustion chamber.

This is why running too much boost is bad as well, it's the same idea.

Someone please correct me if any of that is wrong. =]

SubyTechMaster · 12-28-2001, 12:49 PM

To answer the other part of the question:

The WRX head gasket is .7mm thick. It's a 3-piece metal gasket.

Eddie Mcstiff · 12-28-2001, 01:29 PM

What would the negitive effects of lowering the CR to 8:1 in a 2.5 T? Positive effects?

SubyTechMaster · 12-28-2001, 01:48 PM

Consider WHY Subaru offers things the way they do.

Small displacement (2.0L) w/turbo = great power w/decent mileage. Downside - low-end lack of power.

Large displacement (2.5L) N/A (high compression) = good power, low-end torque w/decent mileage. Downside - not as much power.

When people bolt-on a turbo to a high compression 2.5L they run less boost (5-6psi) but retain the low-end torque.

If you also lower the compression so as to run higher boost you loose the low-end torque. You are basically creating a larger displacement Turbo Motor with the same disadvantages plus less mileage. If you want to build an engine, your best bet would be to build it upon a Legacy 2.2L Turbo block (basically the same as a 22B).

Eddie Mcstiff · 12-28-2001, 01:57 PM

Thanks, thats what I thought. It just seems like a waste to get a 2.5 RS then scrap the engine. It also seems like alot of work to start with a L. I wont have a Impreza till next fall, I'm looking at my options so I dont pay for things twice.

jeddy · 12-28-2001, 03:15 PM

You should talk to some of the RS-T guys around here about turbos on the 2.5 RS. It seems to me that you should be able (with proper engine management, etc) to get more power (low and high rpm) out of a turbo 2.5 than a 2.0. The combustion chamber is larger, therefore more mixture can be burned. Most of the problems i've heard about are due to some stock part failing or running too much boost (accidentally i assume).

JakRex · 12-28-2001, 04:15 PM

you guys' reply are really helpful !!

So, is that mean if I replace the stock head gasket to a thicker one, I lower the compression ratio -->make more room for the turbo to have higer boost --->but will lost a lot at the bottom-end torque.
So, if what I'm thinking is right, then lower the compression ratio is only good for track racing and bad for daily driving (maybe some weekend racing, hehe)

jeddy · 12-28-2001, 05:01 PM

Well, i think what you want to do is research the physics behind a thicker head gasket. Subaru designed it the way it is for a reason. ;] Just decreasing the compression doesn't mean you can automatically run more boost. There's got to be some compromise between compression ratio and boost. There's also got to be a compromise point that gives ok power, reliability, and fuel economy. Who knows which one Subaru picked?

There may also be some better, less potentially damaging, way to get more power. Things like exhaust, intake, chips, etc. are all proven, and are reasonably safe ways to get more power out of an otherwise stock engine. Also keep in mind that i've only installed an intake on my 2.5. I don't have the $$ right now, or a desire to mess with the warranty if something does go wrong.

If you've got the money to spend, find a local shop that has worked on the 2.5 engine, and see what they have to say. Remember, if you are thinking of doing it, someone else may have already. =]

On a side note, just because something seems like it should increase power, doesn't mean it will. Take the DC Sports intake on my old Integra for example. It sure was loud (that was a Bad Thing), and might have added a touch of high rpm power (up around 5500rpm), but the low end with that thing installed was _horrible_. I can't count the number of times i stalled right after i put it on. =\

JakRex · 12-28-2001, 05:39 PM

I totally agreed with you Jed. For the intake thing, I used to drive a Si and running with a Tenzo intake, but ended up I just put the stock air box back. I really can't afford to lose all the low-end torque when I do mostly daily driving.
You know for the Stock WRX, it's running a smaller turbo from mitsubishi. I want to replace that with a bigger ball bearing turbo in order to have more power and lesser lag. So I just wondering do I have to lower the engine compression ratio to make it safer.

122ci · 12-28-2001, 06:20 PM

take what i say in a srictly "most power possible, without blowing the engine" sense:

lowering the compression with a thicker head gasket on a 2.5 to run more boost will give you more power than higher compression and less boost. lowering it even one point can leave room for 15psi more with adequite air/fuel. especially with non-forged pistons. this is all in theory nothing says you can run 35psi on a stock motor 2.5 with 8:1 compression, there are too many variables to say that. i know there arent like aftermarker heads to swap right on like ford or chevy but what they would do is swap out the heads with larger or smaller combustion chambers (64cc lower compression, maybe 59cc higher)

you can just keep lowering the compression to be safer, air/fuel mixture is more important

12-27-2001, 04:03 PM	#1
JakRex Scooby Newbie Member#: 11351 Join Date: Oct 2001 Location: Toronto, Canada Vehicle: 2002 Impreza WRX Rally Blue	Compression ratio This might be a stupid question, but I am driving a WRX but not familiar with a turbo engine. Could anybody tell me how think is the stock gasket? Get this from some magazines that when N/A car need higher compression ratio, Turbo cars need lower ratio instead. What is going on there? Any help would be great, Thank you !!

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12-27-2001, 05:19 PM	#2
122ci Scooby Newbie Member#: 13584 Join Date: Dec 2001 Location: Mesquite Texas Vehicle: 2002 WRX Platinum	any factory turbo car will come with a lower compression motor, to allow more boost to be run, without detonation. on a factory NA motor that is having a turbo kit installed on it, a lot of times they will use a thicker head gastket to lower the static compression. like 3mm thick. the thicker head gasket can drop the compression as much as a point. (i.e. 10.5 to 9.5) stock wrx compression is 8:1, which is really low compared to other factory turbo cars, only one i can thick off are buick GNs. most are 8.5 to 9.0 . that help you out any?

12-27-2001, 10:22 PM	#3
Jung918 Scooby Specialist Member#: 10969 Join Date: Oct 2001 Location: Orange County	suppose i had a stock WRX but only changed the gasket to make the comression 9 : 1... how much of a performance gain are we looking at? what are the saftey issues?

12-28-2001, 12:08 AM	#4
jeddy Scooby Newbie Member#: 7711 Join Date: Jun 2001 Chapter/Region: SWIC Location: Tucson, AZ Vehicle: 2005 WRX STi Silver	I'm not a turbo specialist, but here's why it seems to me that you want lower compression ratios with a turbo. Assume that the compression ratio is how much the stroke of the piston compresses the air/fuel mixture. The function of the turbo is in fact to cram more air into the combustion chamber. So, you in fact need a compression ratio that is lower than for a NA engine, so that you have room for the turbo to increase the pressure. How the turbo can make more power this way, while simply increasing the compression ratio doesn't, i don't know. I'm guessing there are other disadvantages to higher compression ratios. Perhaps there is a minimum volume for efficient combustion. Too much movement of the pistons is hard to stabilize (especially at higher rpms). Things like that... That's a long way of saying: i think the compression ratios on engines do not take the effects of the turbo into account. It's just the volume difference at the top and bottom of the piston stroke. You could in theory increase the compression, but (just as in a NA engine), you will start to approach the design limits of the combustion chamber. This is why running too much boost is bad as well, it's the same idea. Someone please correct me if any of that is wrong. =]

12-28-2001, 12:49 PM	#5
SubyTechMaster Scooby Specialist Member#: 4423 Join Date: Feb 2001 Location: Placerville, CA USA Vehicle: '04 08 Tribeca, SVX Gold, Maroon	To answer the other part of the question: The WRX head gasket is .7mm thick. It's a 3-piece metal gasket.

12-28-2001, 01:29 PM	#6
Eddie Mcstiff Scooby Newbie Member#: 13449 Join Date: Dec 2001 Location: Boulder Co	What would the negitive effects of lowering the CR to 8:1 in a 2.5 T? Positive effects?

12-28-2001, 01:48 PM	#7
SubyTechMaster Scooby Specialist Member#: 4423 Join Date: Feb 2001 Location: Placerville, CA USA Vehicle: '04 08 Tribeca, SVX Gold, Maroon	Consider WHY Subaru offers things the way they do. Small displacement (2.0L) w/turbo = great power w/decent mileage. Downside - low-end lack of power. Large displacement (2.5L) N/A (high compression) = good power, low-end torque w/decent mileage. Downside - not as much power. When people bolt-on a turbo to a high compression 2.5L they run less boost (5-6psi) but retain the low-end torque. If you also lower the compression so as to run higher boost you loose the low-end torque. You are basically creating a larger displacement Turbo Motor with the same disadvantages plus less mileage. If you want to build an engine, your best bet would be to build it upon a Legacy 2.2L Turbo block (basically the same as a 22B).

12-28-2001, 01:57 PM	#8
Eddie Mcstiff Scooby Newbie Member#: 13449 Join Date: Dec 2001 Location: Boulder Co	Thanks, thats what I thought. It just seems like a waste to get a 2.5 RS then scrap the engine. It also seems like alot of work to start with a L. I wont have a Impreza till next fall, I'm looking at my options so I dont pay for things twice.

12-28-2001, 03:15 PM	#9
jeddy Scooby Newbie Member#: 7711 Join Date: Jun 2001 Chapter/Region: SWIC Location: Tucson, AZ Vehicle: 2005 WRX STi Silver	You should talk to some of the RS-T guys around here about turbos on the 2.5 RS. It seems to me that you should be able (with proper engine management, etc) to get more power (low and high rpm) out of a turbo 2.5 than a 2.0. The combustion chamber is larger, therefore more mixture can be burned. Most of the problems i've heard about are due to some stock part failing or running too much boost (accidentally i assume).

12-28-2001, 04:15 PM	#10
JakRex Scooby Newbie Member#: 11351 Join Date: Oct 2001 Location: Toronto, Canada Vehicle: 2002 Impreza WRX Rally Blue	you guys' reply are really helpful !! So, is that mean if I replace the stock head gasket to a thicker one, I lower the compression ratio -->make more room for the turbo to have higer boost --->but will lost a lot at the bottom-end torque. So, if what I'm thinking is right, then lower the compression ratio is only good for track racing and bad for daily driving (maybe some weekend racing, hehe)

12-28-2001, 05:01 PM	#11
jeddy Scooby Newbie Member#: 7711 Join Date: Jun 2001 Chapter/Region: SWIC Location: Tucson, AZ Vehicle: 2005 WRX STi Silver	Well, i think what you want to do is research the physics behind a thicker head gasket. Subaru designed it the way it is for a reason. ;] Just decreasing the compression doesn't mean you can automatically run more boost. There's got to be some compromise between compression ratio and boost. There's also got to be a compromise point that gives ok power, reliability, and fuel economy. Who knows which one Subaru picked? There may also be some better, less potentially damaging, way to get more power. Things like exhaust, intake, chips, etc. are all proven, and are reasonably safe ways to get more power out of an otherwise stock engine. Also keep in mind that i've only installed an intake on my 2.5. I don't have the $$ right now, or a desire to mess with the warranty if something does go wrong. If you've got the money to spend, find a local shop that has worked on the 2.5 engine, and see what they have to say. Remember, if you are thinking of doing it, someone else may have already. =] On a side note, just because something seems like it should increase power, doesn't mean it will. Take the DC Sports intake on my old Integra for example. It sure was loud (that was a Bad Thing), and might have added a touch of high rpm power (up around 5500rpm), but the low end with that thing installed was _horrible_. I can't count the number of times i stalled right after i put it on. =\

12-28-2001, 05:39 PM	#12
JakRex Scooby Newbie Member#: 11351 Join Date: Oct 2001 Location: Toronto, Canada Vehicle: 2002 Impreza WRX Rally Blue	I totally agreed with you Jed. For the intake thing, I used to drive a Si and running with a Tenzo intake, but ended up I just put the stock air box back. I really can't afford to lose all the low-end torque when I do mostly daily driving. You know for the Stock WRX, it's running a smaller turbo from mitsubishi. I want to replace that with a bigger ball bearing turbo in order to have more power and lesser lag. So I just wondering do I have to lower the engine compression ratio to make it safer.

12-28-2001, 06:20 PM	#13
122ci Scooby Newbie Member#: 13584 Join Date: Dec 2001 Location: Mesquite Texas Vehicle: 2002 WRX Platinum	take what i say in a srictly "most power possible, without blowing the engine" sense: lowering the compression with a thicker head gasket on a 2.5 to run more boost will give you more power than higher compression and less boost. lowering it even one point can leave room for 15psi more with adequite air/fuel. especially with non-forged pistons. this is all in theory nothing says you can run 35psi on a stock motor 2.5 with 8:1 compression, there are too many variables to say that. i know there arent like aftermarker heads to swap right on like ford or chevy but what they would do is swap out the heads with larger or smaller combustion chambers (64cc lower compression, maybe 59cc higher) you can just keep lowering the compression to be safer, air/fuel mixture is more important