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Old 05-16-2006, 08:57 AM   #1
JoeTX
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Question Low Compression/High Boost vs High Compression/Low Boost?

What are the pros and cons of both?

It seems most people go the bigger turbo/higher boost route, even those who build up the engine internals. And I do realize there’s a lot more work involved in upping the compression than installing external bolt-ons.

The one thing I hate about my WRX is the lack of low end torque. Higher compression would help that but at what expense? Could you run higher compression and the stock turbo (quick spooling) and get better results than just slapping on a bigger turbo/injectors?
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Old 05-16-2006, 10:10 AM   #2
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You know, I've been wondering the same thing for a couple weeks now. Why run 7.8:1 and 20 psi and have nothing until the boost comes on, when you can have 8.7:1 and, say, 12 psi? It would seem that as long as the chamber pressure is the same by both routes, it should have the same power up top, right? And you're already running premium fuel, so it's not like the higher compression off boost would require a fuel change... But I would think that if this were true, it would be a far more popular option when doing a build, so maybe I'm missing something here?
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Old 05-16-2006, 10:31 AM   #3
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Reliability. You don't want a high comp/low boost engine running around everywhere. If you hit a 2 psi spike, it could destroy your engine. Tuning for 200,000 miles as opposed to tuning for the ragged edge is a way for car makers to get more customers.
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Old 05-16-2006, 10:33 AM   #4
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....I'm diggin the 8.4:1 with the TR
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Old 05-16-2006, 10:34 AM   #5
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Interesting. I was JUST thinking about this on the way to work.

I'm getting a used RS motor and plan on rebuilding it with STi internals. Mainly for the reliability as I dont plan on adding boost at the moment. Later on tho, I wanted to add 5-8 psi just for a little push. Then I started thinking "how in the hell am I going to do that and still run 87 octane"? The NA motors take el-cheapo fuel by default, and I'm trying to get the best of each world...performance but staying on the "cheap" (like you can call it that now) gasoline.

I assume (because I know NOTHING about engines) that the higher the compression, the higher grade fuel you have to use. I'm starting to think thats false...and at this point I honestly don't knwo WHAT would change the octane requirements for the motor.

CN: I'd like to know myself as someday I want to run a SMALL amt of boost and remain NA for 95% of the cars usage.
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Old 05-16-2006, 10:40 AM   #6
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Compression ratio and vacuum/boost both alter the Fuel you need to use.
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Old 05-16-2006, 10:43 AM   #7
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hey we have the same cars

I read in a repair manual that the 2.2 OBS is 10:1..which sounds kinda high to me. If that is true, I assume the absense (speling) of boost is why we can run 87?

PS thank you for the clarification. It was driving me nuts (but not nuts enough to fire up google yet lol)
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Old 05-16-2006, 11:26 AM   #8
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Quote:
Originally Posted by Vashed
Reliability. You don't want a high comp/low boost engine running around everywhere. If you hit a 2 psi spike, it could destroy your engine. Tuning for 200,000 miles as opposed to tuning for the ragged edge is a way for car makers to get more customers.
How many people on here who build an engine build it for 200,000 miles? A boost spike on a higher-compression engine would gause a greater spike in chamber pressure, but that can be gotten around by a) using a better boost control system than a factory WRX and b) not tuning for the ragged edge. I'm not saying run higher compression AND higher boost, but to run a slightly higher CR and factory-ish boost. This would result in higher low-RPM power and off boost, and should result in about equal power up top. With all the money spent on things to decrease lag (IC hoses, inlet hose, ported turbo, TBE) this seems like something to think about if/when an engine build comes around. The higher compression should also create more exhaust energy to spool the turbo quicker. I'm thinking an EJ207 with a Deadbolt VF34, running about 9.2:1 CR, and Stg4 mods, would be fast as hell! Mmmmm, full boost by 2000 rpms!
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Old 05-16-2006, 11:46 AM   #9
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It depends on what you want. With a Higher Compression/Low boost setup you could use a smaller turbo and the throttle response would be much better on and off boost. Less Lag, more torque without boost. However you will not make as much power as the low compression/high boost people, especially in the top.

Boost increases air flow, air flow is really what you need in the top revs. Same principle as a N/A engine. High compression/Low boost can be just as safe as the opposite. I think most people here are interested in having a high peak horsepower number though. Either way, I believe a high comp/low boost engine will be more fun in the turns. It'll have a better torque curve and much better response.

There is one of those Japanese videos where they test a supra and a skyline on a track. The supra was tuned for max power, but the skyline was tuned for engine response. It made a huge difference on the track.
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Old 05-16-2006, 12:08 PM   #10
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Quote:
Originally Posted by Master2192
It depends on what you want. With a Higher Compression/Low boost setup you could use a smaller turbo and the throttle response would be much better on and off boost. Less Lag, more torque without boost. However you will not make as much power as the low compression/high boost people, especially in the top.

Boost increases air flow, air flow is really what you need in the top revs. Same principle as a N/A engine. High compression/Low boost can be just as safe as the opposite. I think most people here are interested in having a high peak horsepower number though. Either way, I believe a high comp/low boost engine will be more fun in the turns. It'll have a better torque curve and much better response.

There is one of those Japanese videos where they test a supra and a skyline on a track. The supra was tuned for max power, but the skyline was tuned for engine response. It made a huge difference on the track.

Interesting. Then I prob want the high comp/low boost because I'm a low-end and torque fiend...i couldnt care less about peak hp. 100 mph is fast enough for me
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Old 05-16-2006, 12:26 PM   #11
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The Chrysler PT Cruser offers a low pressure turbo verson and a high pressure turbo. I don't have the specs. but I am told the low pressure system, while lower on peak hp does run on lower octane fuel and has better low end grunt.
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Old 05-16-2006, 01:45 PM   #12
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any turbo running lower than premium is not a smart thing to do, IMO.
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Old 05-16-2006, 11:02 PM   #13
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Quote:
Originally Posted by firebox40dash5
You know, I've been wondering the same thing for a couple weeks now. Why run 7.8:1 and 20 psi and have nothing until the boost comes on, when you can have 8.7:1 and, say, 12 psi? It would seem that as long as the chamber pressure is the same by both routes, it should have the same power up top, right? And you're already running premium fuel, so it's not like the higher compression off boost would require a fuel change... But I would think that if this were true, it would be a far more popular option when doing a build, so maybe I'm missing something here?
well i've never had a problem with knock at all running stock boost, cobb AP and hte UTEC will run up to 18psi of boost on their stock maps keeping hte same compression ratio. i'm willing to bet you could increase compression by a pretty decent amount if you were running stock boost. high compression will also help the turbo spool up quicker too
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Old 05-16-2006, 11:33 PM   #14
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low comp and higher boost will yield more power overall than a high comp/low boost setup

I do believe that a high comp would be more responsive, on a stock setup it would be dangerous most likly but tuning for it could probably fix it. I remember being told that 9:1 is about the highest you would want to go with any type of forced induction setup, and just to point out that mazdaspeed mazda 6 runs 9:1 on direct injection which is better than the setup we run of course mazda didn't make it all out performance.
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Old 05-17-2006, 07:56 AM   #15
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I guess what would be good to compare is two motors with equal cylinder pressures, but with different compression and boost. What I mean by this is say you have a 10:1 n/a motor and an 8:1 turbocharged motor running like 17.5 psi absolute (is that how they measure boost, or should I just say ~3psi?). The pressure in the cylinders in each motor are all going to be around 147 psi, but the low compression motor is going to have more volume in the combustion chamber. That means more air and fuel can be crammed in there, so it will make more power. Maybe.

Disclaimer: It's very possible that I have no idea what I'm talking about. I kind of slacked off when I took thermodynamics a few years ago. Also I'm more interested in the handling department at the moment.

Last edited by jamal; 05-17-2006 at 08:05 AM.
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Old 05-17-2006, 11:17 AM   #16
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Quote:
Originally Posted by kaisersouse
I'm getting a used RS motor and plan on rebuilding it with STi internals. Mainly for the reliability as I dont plan on adding boost at the moment. Later on tho, I wanted to add 5-8 psi just for a little push.
Boosting that block will throw reliability right out the window, Kaiser. The RS block is open-deck.. meaning that there's no support for the cylinders near the head. When you start adding boost, even if the pistons hold together just fine, the cylinder can shift, causing head gasket leaks.

If you want to turbo, use the STi internals inside an STi shortblock... prefferably factory assembled from your dealership parts counter.
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Old 05-17-2006, 11:31 AM   #17
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Quote:
Originally Posted by Beaverboy
Boosting that block will throw reliability right out the window, Kaiser. The RS block is open-deck.. meaning that there's no support for the cylinders near the head. When you start adding boost, even if the pistons hold together just fine, the cylinder can shift, causing head gasket leaks.

If you want to turbo, use the STi internals inside an STi shortblock... prefferably factory assembled from your dealership parts counter.
I'd like to point out that the WRX engines have been Open Deck for years, with no problems from cylinder walk. Last I checked, they were boosting them up fine without blowing head gaskets left and right.

It is a fact that lower compression plus high boost will make more power, you've got a larger combustion chamber with more air/fuel. However a high compression low boost will have more torque and better response.

You could always put a small turbo on the low compression motor and have quick response too, but you would be limiting your top end power considerably. It really depends on if you want big power at the top, or a big torque curve in the middle. Properly tuned, both types will be as reliable as factory.
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Old 05-17-2006, 11:44 AM   #18
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I thought open deck only became an issue past 10psi. And the point of me getting a new motor is so I can learn engines by rebuilding it. Kinda defeats the purpose of having someone else build it.

That and I didnt want to go boost right off. Maybe in another year or even two.
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Old 05-17-2006, 11:58 AM   #19
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open deck is not the issue people are making it out to be, hell look at how people are tuning turbo hondas on non-sleeved stock blocks
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Old 05-17-2006, 01:29 PM   #20
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IF you keep your intake temps = and effective compression = then power should vary only slightyl

Example:
8:1 CR with 14.7 PSI = 16:1 effective compression ratio
10:1 with 9psi = 16.2:1 effectice compression ratio

Thus those two cars should make pretty close to equivalent power barring spool time variances which should be minor as CR will not help spool BUT it will take longer to spin up 14 psi Vs 9 psi. High CR low SI will also net better throttle response, better gas milage, and better low end grunt at the cost of a much more precise tune and much less leeway with a bad tank of gass or uncontrolable factors.

However as stated, for absolute max power low CR hgih boost is the best route simply because at absurd numbers there is simply more AIR AND FUEL in the motor, now it takes some pretty insain boost levels for this to become evident most likely. It is truly hard to d oa true comparison keeping all things equal, a low boost motor is going to like a different cam and port set up than a high boost motor, differtent piston designe is a must so there will be alot of factors that vary between a low/high CR motor.

Good article by a very smart guy on this very subject:
http://forums.probetalk.com/showthre...ighlight=boost
Quote:
The only way to make more power is to increase cylinder pressure and burn more fuel. The main purpose of the supercharger is to supply the motor with a more dense air charge, which allows for the ability to burn the additional fuel. By adding a supercharger, additional air should no longer be a problem. Ensuring that there will be enough additional fuel to maintain the proper air to fuel ratio will be the key to using the maximum effective compression.

All motors have a static compression ratio. This is the amount that the air inside the cylinder is compressed. It is a ratio of the cylinder volume at BDC to the volume at TDC. When a supercharger is added, additional air is forced into the cylinder effectively raising the compression ratio. The result of this is called effective compression. The formula for finding the effective compression is very easy:

((boost psi / 14.7) + 1) x motor compression = effective compression.

The effective compression allows a supercharged motor to be compared to a normally aspirated motor. For the most part, a supercharged motor with the same effective compression as a (similar) normally aspirated motor with the same static compression should have about the same overall power.

This may bring up the question that if the overall power should be about the same, why go with a supercharger? The main advantage of the supercharger is that it allows for a moderate compression level during normal driving while allowing for very high compression levels when needed. Obviously a high compression motor of about 14:1 makes a lot of power, but it would never survive daily driving. A lower compression motor is great for daily driving, but greatly reduces the potential for power. The supercharger allows for higher compression levels than could be used without a supercharger, while still offering the benifits of a standard compression motor. Many street supercharged systems will go beyond 18:1 effective compression under boost. Under race conditions, many supercharged race motors will go well beyond 22:1 effective compression. Both of these levels are far beyond what could be done reliably or cost effectively without a supercharger.

This brings us back to the question of just how much boost or compression can be run. Obviously there can't be a simple number that could be used for every application. This is why it's so critical to chose the proper components. It's not necessary to build a low compression motor to use a supercharger, but the correct parts are still necessary. The biggest factors will be in things like head bolts (or preferably studs), gaskets, and the strength of the other engine components. It goes without saying that the incredible power that a supercharger can add, can easily start breaking things. It is very important that as the boost levels rise, the need for a stronger crank, rods, pistons, etc... becomes very critical. Many people forget this as the motor itself is relatively mild, while the supercharger pushes it well beyond the practical limits it was intended for.

Now, back to the compression issue. Anyone who has looked into supercharging has heard that you need a low (static) compression motor. This may have been true once upon a time, when roots type (positive displacement) superchargers ruled the land, but it's not so necessary now. The problem with a low compression motor is that it relies heavily on the supercharger for its power. An 8:1 motor is definitely not going to be a power house. Sure, you can throw 18 lbs of boost on it and get some real power, but why? A higher compression motor of 9.5:1 will have much more power without the blower. Then, with less boost you could easily have the same overall power - only it would be much more usable. Both of the motors (8:1 with 18 lbs boost and 9.5:1 with 12 lbs boost) will have almost the same effective compression and about the same overall power. The big difference will be where you see the power, and how much of a demand will be placed on the supercharger. Obviously, the 9.5:1 motor is going to have far greater torque and low end power as the boost is only starting to come in. It is also going to be much easier to find a blower to survive only 12 lbs of boost -vs- one that would have to put out 18 lbs. It is now very easy to see why a higher compression motor with lower boost is becoming so popular.

Please understand that when I say higher compression and lower boost, there are limits to each. Going over about 10:1 will make the amount of boost that is usable drop quickly to the point that the supercharger is somewhat wasted. In my opinion, anything less than 8 lbs of boost is a waste of a supercharger. Going over 10:1 will also make daily driving with pump gas much more difficult. In this same way, compression levels much under 9:1 will require substantial boost levels to make massive power gains. This would require boost levels that are very demanding of a supercharger. This is truly unnecessary. This isn't to say that the lower compression / higher boost set-up doesn't have a slightly higher potential for power, because it does. A lower compression motor has the ability to contain more volume. This can be an advantage, but is such a minor one that it's not necessarily worth the effort - unless it's for an all out race motor. Even then there are limits for the same reasons as the street / strip motor.

Once again, the compression -vs- boost issue. For a car that will see the streets (actually for most applications), the best thing to do is start with a motor compression that is high enough to make the horsepower you want for normal driving. Don't rely on your supercharger to make all your horsepower. With a good motor compression, add as much boost as is safe for your particular application. Decide on a final effective compression, and work your way back through the formula to find your maximum boost level: ((effective compression / motor compression) - 1) x 14.7 = boost. With the proper fuel system and related engine components, an effective compression of 16:1 to 18:1 should be more than workable. For heavily modified cars, effective compressions over 20:1 should be very carefully considered. Remember, even Indy cars only run about 18 Lbs of boost and reasonable static compression levels. Technology has come a long way and modern day supercharging should take full advantage of this.

While these opinions are not exactly the most popular, they are based on facts and real world performance. While there will always be those who continue with tradition and stick with what was done in the past, it is those who reach for something more that are winning races. Often times, some of the best advice can be found from those who have done what you want to do. All too often it is those who know the least that offer the most advice. After having been involved in supercharging for many years, I have heard it all. Most of it was worthless. It was often the least mentioned things and trail and error that have been the most rewarding. Hopefully this information will help to explain some of the often misunderstood aspects of supercharging.
Read the entier thread though as there is alot of math and good discussion of max power Vs. usable rpm power.
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Old 05-17-2006, 01:46 PM   #21
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So while a high CR/Low FI motor will NEED a really good tune and be suseptible to failure for outside reasons...if someone were using a supercharger part time (Eaton has an SC w/ electronic clutch) ie during track runs or passing people...it wouldnt be that bad I wouldnt think.
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Old 05-17-2006, 02:40 PM   #22
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For any boost in a daily driver with pump gas and realatively simple mods without compromise (say uder 400 hp) high CR is ok... for max power and no regard for daily drivability or low end then certainly lowER CR and highER boost is the way to go!
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Old 05-17-2006, 02:40 PM   #23
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Quote:
Originally Posted by kaisersouse
So while a high CR/Low FI motor will NEED a really good tune and be suseptible to failure for outside reasons...if someone were using a supercharger part time (Eaton has an SC w/ electronic clutch) ie during track runs or passing people...it wouldnt be that bad I wouldnt think.
Only problem I see there is that Eaton Superchargers create alot of heat, so unless you intercool it properly it will make things more likely to go wrong.

I think a high compression engine with a supercharger would be Ideal for the street. 10:1 compression with 10-12 psi of boost would have awesome torque in the whole power band.

It won't have as high horsepower as a 8:1 engine running 22 psi of boost with a turbo, but imagine how much boost lag this setup would have.

The biggest problem I see is that it is much harder to intercool with the supercharger, unless you make custom intake manifolds. And price really hurts unless you are using a used Supercharger.

Now for a post I found in the AFI area, I believe these ECR calculations are better.

Quote:
I'm sorry, but I am going to have to chime in here. Although your calculations are correct, the fact that you looked over HP vs. Effective Compression Ratio vs. MAP has really confused your findings. Instead of explaining myself, here is a post from the TEC-II board:

I'm sure I will have a different take on this than most of you because I have a Turbo Eclipse AWD. But a month or so ago on a DSM board we went through this discussion. And here is what it came down to.
Higher static compression creates more power throughout the rpm band, but it'll lower your maximum allowed boost before the onset of detonation. Boost is worth way more power than compression, because boost raises your compression and your total air flow at the same time. With the down side of, when you're not on the boost, you have slightly less power.

Effective Compression Ratio = static compression ratio x (1 + boost/14.7)^1/2

For a car running 8.5:1 pistons and 18psi(~max on pump gas)
8.5 x (1 + 18/14.7)^1/2 = 12.67 ECR

If you run 9.0:1 pistons and want to maintain the same 12.67 ECR (~max on pump gas), you'll have to lower your boost to: 14.4psi
[(12.67/ 9.0)^2 - 1] x 14.7 = 14.4 psi

So you have to run 3.6 psi less boost to maybe pick up a tinny bit of bottom end. Or to take it even further for 9.5:1 you can only run 11.4psi. I'll tell you right now that the difference between 11.4psi and 18psi is huge. And at some point (12.67:1 in this case) you can run no boost and be maxed out on ECR for pump gas. And how fast is a N/A car with 12.67:1 compression, ask the Honda boys running 15's.

Of course you get to a point where you don't want to go the other way too much ether. A 6.0:1 car isn't too much fun on the street, but it can run 50.8 psi boost on pump gas. But the kind of turbo that could support that much boost would never spool up. You have to find what's right for you, but 8.5:1 is what most of the DSM guys like. My friend just built a motor for his DSM with 9.0:1 JE's and he is quite upset to find that he's getting lots of knock running 16psi. While lately in the cold weather, I've been running 20psi (8.5:1) on pump gas w/o any knock. And I drove his car and the difference in bottom end grunt isn't even noticeable. And his top end performance is sad compared to mine.
I believe the Effective Compression Ratio calculations in this are better. If you think about it, I haven't heard of anyone being able to run a 16:1 compression on pump gas. This calculation follows what i've seen here, 22 psi on 8:1 compression is possible, higher only if you get race gas.
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Old 05-17-2006, 02:46 PM   #24
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Those numbers are wrong!

8.5 CR with 18 psi of boost nets an 18.9 ECR
9.0 CR with 14.4 PSI nets a 17.8 ECR

My numbers were off this site: http://www.rbracing-rsr.com/compression.htm

The ^.5 is whats messing thisngs up, why take it to eh 1/2 power?!?!? makes no sense what so ever.... 18 pis is over double atmospheric pressure, how can it only raise CR 4 points?!?!? It HAS to at least double it by laws of physics.
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Old 05-17-2006, 02:51 PM   #25
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Location: Roy, Washington
Vehicle:
98 2.5 RS
04 WRX

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Quote:
Originally Posted by NattiRex
Those numbers are wrong!

8.5 CR with 18 psi of boost nets an 18.9 ECR
9.0 CR with 14.4 PSI nets a 17.8 ECR

My numbers were off this site: http://www.rbracing-rsr.com/compression.htm

The ^.5 is whats messing thisngs up, why take it to eh 1/2 power?!?!? makes no sense what so ever.... 18 pis is over double atmospheric pressure, how can it only raise CR 4 points?!?!? It HAS to at least double it by laws of physics.
I'm not totally sure about that. I'm going to get my Supercharger book when I get home. I believe its because they are both ratios of volume.
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