# can somebody explain this compression thing to me?



## kaptainkrollio (Jul 29, 2003)

I know that lower compression will aloow more boost, but then I heard that for NA, engines will be more responsive to bolt ons if compression is higher. I know what it is, but could somebody explain to me the pros/cons of high/low compression and what you can do to change it?


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## James (Apr 29, 2002)

for one thing, you're confusing a prophylatic safety measure and increasing power. lowering the compression makes you lose power no matter NA or turbo. all lowering the compression does for turbo is allow you to run higher boost before detonation and higher boost ='s more power, not the lower compression.


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## kaptainkrollio (Jul 29, 2003)

i actually figured that out a few minutes ago while reading something, and thats what i thought all along, but the rest?


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## vsp3c (May 15, 2003)

pros of higher compression ratio : more power
cons of higher compression ratio : more damage to the engine in the long run
pros of lower compression ratio : less damage
cons of lowr compression ratio : less power

im sure that there's more to it than this but i think these are the major points.

somebody correct me if i'm wrong but when you start playing with different comp ratios, aren't you also dealing with bore/stroke, fuel injection rate/amount, and timing??


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## FLIP (May 18, 2004)

The compression ratio of a motor is basically how much the pistons compress the air. For example: 12:1 12units of air compressed into 1unit, anyways, when air and fuel are compressed they ignite easier as with allmost all gasses. With enough compression, the air/fuel mixture in the cylinder can detonate before its meant to = knock/ping/detonation = engine death. In other words high compression causes more strain on the motor. A turbo creates more compression by stuffing more air into the cylider, so basically raising the compresion ratio.
Now, if you can get a high compression ratio with GOOD tuning and no knock, you produce a larger explosion in the cylinder therefore creating more power (more air = bigger explosion).
Thats basically it, hope it helps.


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## esyip (Dec 19, 2002)

www.howstuffworks.com


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## UNISH25 (Aug 12, 2002)

FLIP said:


> The compression ratio of a motor is basically how much the pistons compress the air. For example: 12:1 12units of air compressed into 1unit, anyways, when air and fuel are compressed they ignite easier as with allmost all gasses. With enough compression, the air/fuel mixture in the cylinder can detonate before its meant to = knock/ping/detonation = engine death. In other words high compression causes more strain on the motor. A turbo creates more compression by stuffing more air into the cylider, so basically raising the compresion ratio.
> Now, if you can get a high compression ratio with GOOD tuning and no knock, you produce a larger explosion in the cylinder therefore creating more power (more air = bigger explosion).
> Thats basically it, hope it helps.


P1V1/T1 = P2V2/T2
Got it?


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## vsp3c (May 15, 2003)

FLIP said:


> The compression ratio of a motor is basically how much the pistons compress the air. For example: 12:1 12units of air compressed into 1unit, anyways, when air and fuel are compressed they ignite easier as with allmost all gasses. With enough compression, the air/fuel mixture in the cylinder can detonate before its meant to = knock/ping/detonation = engine death. In other words high compression causes more strain on the motor. A turbo creates more compression by stuffing more air into the cylider, so basically raising the compresion ratio.
> Now, if you can get a high compression ratio with GOOD tuning and no knock, you produce a larger explosion in the cylinder therefore creating more power (more air = bigger explosion).
> Thats basically it, hope it helps.


wow!! i never knew that!!!  you guys still haven't answered my question


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## Harris (Nov 11, 2002)

vsp3c said:


> somebody correct me if i'm wrong but when you start playing with different comp ratios, aren't you also dealing with bore/stroke, fuel injection rate/amount, and timing??



Yes and no. It all depends whether the motor is being designed for naturally aspirated or forced induction purposes.

So say I have an SR20DE which has 9.5:1 C/R. Say I want to stay NA and want to increase the C/R of the motor by using UK SR20DE 10:1 pistons. This will mean that I have to do nothing to the bore/stroke, fuel management and timing as they are all the same.

However, say I were to turbocharge that SR20DE and will be using some JE 9.0:1 pistons to lower the C/R. Because of the extra amount of air that will be introduced into the combustion chambers by forced induction, I have no choice but to up the amount of fuel (and also using higher octane fuel) and retard timing in order to avoid detonation. 

C/R doesn't really dictate all this. What C/R dictates is how much boost you can make. So if you have an SR20DE with a T25 and and an SR20DET with a T25, the SR20DET will be able to take more boost than the SR20DE. However, one thing to note is that you can gain hp with higher compression on a turbo'd motor. So at say you take the same two motors as I mentioned above and set boost at 8 psi (which is what both can take without blowing up), the SR20DE with the T25 will make a bit more power than the SR20DET with a T25.

Now do you understand?


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## FLIP (May 18, 2004)

kaptainkrollio said:


> I know what it is, but could somebody explain to me the pros/cons of high/low compression and what you can do to change it?


Lower CR: Fatter head gasket or double/triple head gasket
Higher CR: opposite of above

Pistons can control the CR
there are many ways of doing it...

You can even shave the head to reduce the volume between the head and block in the cylinder, this is popular for choppers


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## kaptainkrollio (Jul 29, 2003)

so then is it true that with higher compression and tuning that bolt ons will become more effective?


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## FLIP (May 18, 2004)

kaptainkrollio said:


> so then is it true that with higher compression and tuning that bolt ons will become more effective?


Higher compression is one of the main aspects to making power N/A.


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## James (Apr 29, 2002)

FLIP said:


> Higher compression is one of the main aspects to making power N/A.


Higher compression is one of the main aspects to making power.


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## kaptainkrollio (Jul 29, 2003)

thats good to know. how much is too much though?


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## 93blackSER (Feb 1, 2003)

kaptainkrollio said:


> thats good to know. how much is too much though?


depends on the engine, how you tune it, the type of gas you're running, etc....


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## AlexAtMyNismo (Apr 16, 2004)

Basically, to restate what most of the guys said very well: 

Higher compression setup will allow for a better, more powerful NA engine setup... But just as all things under more pressure do, high compression causes a lot more wear and tear on the engine, making it age a lot faster. 

Lower compression setup will allow for a higher amount of boost to be used with forced induction, whether it be supercharger or turbocharger, of course meaning more boost with less concern lying around detonation. But the major downside is for NA, low compression makes the car a pain in the A$$ to drive daily because of all the loss in power due to low compression within the engine. It will be sluggish until boost, and you would have to compensate with high boost.

The Ideal setup is a well tuned mildly high compression engine with mild to upper boost levels. This allows for good low-end power and mid-range, along with good turbo spool and power band range. Just my opionion as well as many others. 

-Alex B.


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## dburone (Apr 2, 2004)

AlexAtMyNismo said:


> Basically, to restate what most of the guys said very well:
> 
> Higher compression setup will allow for a better, more powerful NA engine setup... But just as all things under more pressure do, high compression causes a lot more wear and tear on the engine, making it age a lot faster.
> 
> ...


This explanation is great, but could anybody throw some numbers???
will a 9.0:1 CR 97 fuel and 7psi, no intercooler setup be safe??


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## kaptainkrollio (Jul 29, 2003)

that sounds like a lot of psi for no ic.


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## dburone (Apr 2, 2004)

kaptainkrollio said:


> that sounds like a lot of psi for no ic.


Here i´ve seen lots of cars going well without an IC, but im not sure of the CR with that condition, y will change the Carb for one with bigger jets and ready for turbo


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## ronaldo (Feb 23, 2004)

i know they sell turbo kits for cars like the toyota mr-s and shit, running similar psi without an intercooler...


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## Joel (Jun 11, 2003)

depends on the size (flow rate) of the turbo.
7psi from a T2 is nowhere near the amount of air from 7psi on a GT40 for example.


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## brainscratch (Jun 7, 2004)

a good little rule of thumb i learned dealing with compression and fuel octane is you take the first number of the ratio and multiply it by 10 and that is your optimal octane. its not a hardcore rule, but a good gauge to start from. so if you have 9.1:1 then shoot for 91 octane fuel


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## dburone (Apr 2, 2004)

brainscratch said:


> a good little rule of thumb i learned dealing with compression and fuel octane is you take the first number of the ratio and multiply it by 10 and that is your optimal octane. its not a hardcore rule, but a good gauge to start from. so if you have 9.1:1 then shoot for 91 octane fuel


I have 98 octane fuel with no problem, but in that rule you dont consider the pressure frome the turbo.
Y will use a t25 with 7psi.


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## brainscratch (Jun 7, 2004)

yeah, thats for n/a cars, turbo is a different matter


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## Drift Machine (Aug 22, 2004)

dburone said:


> This explanation is great, but could anybody throw some numbers???
> will a 9.0:1 CR 97 fuel and 7psi, no intercooler setup be safe??


Depends on a lot more then just CR. As stated what size turbo, along with other things.


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## dburone (Apr 2, 2004)

Drift Machine said:


> Depends on a lot more then just CR. As stated what size turbo, along with other things.


Well, with the size of my turbo, and my CR without an IC and putting in 7 psi what do you think?


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## Drift Machine (Aug 22, 2004)

What engine? What turbo? Where do you live? 

Personally I wouldn't go without an IC, though in SOME low boost applications it's possible. But installing an IC isn't really that hard and you can get small sidemount ones for pretty cheap.


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## lshadoff (Nov 26, 2002)

When air is compressed, it heats up. For example, if you start with 80*F air and compress it to 7psi, the temperature will rise to 143*F. 

For any given engine setup there is a pressure (Indicated Mean Effective Pressure or IMEP) above which detonation will occur. This pressure changes with the temperature of the intake air. Below is a graph published in the Journal of the Society of Automotive Engineering showing this effect for an aircraft engine running at a constant rpm. It is typical for any engine and serves as an example.










There are three lines indicating operating at different A/F ratios. Best Economy is the leanest and Full Rich is the richest. Under each condition, any pressure above the line will cause detonation. 

Examine the Best Power line. If the intake air is heated you move along the line to the right and the pressure above which detonation occurs drops. If the actual pressure in the cylinder is above this, detonation will occur. 

Notice that if you make the mixture richer, the detonation pressure goes up, so running rich allows running higher intake air temperatures. 

Depending on how the ECU runs your engine (A/F and spark timing), and the physical setup of the engine (CR, cylinder head design, ...) this rise in intake temperature either will or will not detonate under 7psi of boost. Perhaps on a cool day it will not but on a hot day the higher intake air temperature will move it over the detonation limit. Without an intercooler, you're rolling the dice. Consider that the factory SR20DET uses an intercooler, so that engine is probably near the detonation limit at the stock boost levels and running without the intercooler is risky.

Lew


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## dburone (Apr 2, 2004)

Drift Machine said:


> What engine? What turbo? Where do you live?
> 
> Personally I wouldn't go without an IC, though in SOME low boost applications it's possible. But installing an IC isn't really that hard and you can get small sidemount ones for pretty cheap.


I cant get an IC, the engine is an E16s 9.0:1 CR carburated with a bigger carb. The turbo a garret t25 ar40 giving 7 psi. I am worried about the CR maybe its to high, but searching the web i saw that it is posible to avoid detonation with that config. If i hade problems i would retard timming. Will this work?


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## lshadoff (Nov 26, 2002)

dburone said:


> I cant get an IC, the engine is an E16s 9.0:1 CR carburated with a bigger carb. The turbo a garret t25 ar40 giving 7 psi. I am worried about the CR maybe its to high, but searching the web i saw that it is posible to avoid detonation with that config. If i hade problems i would retard timming. Will this work?


Yes, retarding the timing will help. Also, re-jetting the carburetor to run richer will help. Under normal driving conditions, the car will be down in power and gas mileage, though.

Lew


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## TheNose247 (Jun 19, 2004)

lshadoff said:


> Yes, retarding the timing will help. Also, re-jetting the carburetor to run richer will help. Under normal driving conditions, the car will be down in power and gas mileage, though.
> 
> Lew



a price to pay not to blow teh motor


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## kaptainkrollio (Jul 29, 2003)

ahh, you are using PV=nRT


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## lshadoff (Nov 26, 2002)

kaptainkrollio said:


> ahh, you are using PV=nRT


No, I'm lazy and used the calculator from this page. It does the conversions for temperature in *Kelvin to *Fahrenheit and pressure from atmospheres to psi. 

Lew


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