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Old 10-13-2001, 04:46 PM   #1
Eric SS
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Exclamation CR vs. boost explained

I posted this in another thread but wanted to give it it's own thread because I think it is a question that a lot of people have and there are a lot of misnomers about how Compression ratio and boost affect your engine. So I don't have to write the whole thing out again I just cut and pasted it. I love that function! anyway:

This is a difficult question to answer because so many factors play into it.

Say a car maked 300hp with 8lbs. of boost with 9.5:1 compression. Now say that the person lowers the compression to 8.0:1 and runs 14lbs of boost and makes the same 300hp.

Boost and compression ratio combined is known as static compression ratio. On a street driven car on pump gas, your goal is to keep your static CR under 18.0:1 or you sill start getting detonation. Fort short periods of time, such as a full throttle 1/4 mile pass, you can go up to 22:1 but don't stay there long . And If you run a timing retard you can run a higher Static CR but I would not recomend going much higher. Here is a good chart to show what your static CR will be based on your CR and boost:



As you can see, an engine with a 9.0:1 CR running 4lbs. of boost nes you a static CR of 11.4:1. Similarly, an engine with CR of 8.0:1 running 6lbs. of boost has an almost identical static compression ratio! In theory (and it is what happens) these too engines should make the same hp. because all things else being equal, they have the same cylinder pressure.

The problem with a low compression motor is that it relies heavily on the turbo for its power. An 8:1 motor is definitely not going to to make monster power. A higher compression motor of 9.5:1 will have much more power without the turbo. 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 4lbs boost and 9.0:1 with 6 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 turbo. Obviously, the 9.0:1 motor is going to have far greater torque and low end power as the boost is only starting to come in.

The high compression engine will make more low end power, like stated, and have a flatter torque curve (As flat as one can be for a turbo car anyway) than a lower compression engine. Also, it will generally reach it's power peak at a lower rpm than a lower compression engine.

So, if you build the car right and it is for the street, go for the highest possible CR that is safe for the engine. Keep in mind also that the less boost you run, the cooler the charge will be and the less you will work your turbo. Not to mention, the car will be more drivable.

So why do automotive companies generally go with low compression and higher boost? One of the reasons is that the parts used in mass produced cars are not of the highest quality. The companies err on the safe side and use low compression and medium amounts of boost that are easily within range of a safe CR.

If you build the car right, in my mind the ideal compression ratio for our engine is 9.0:1. Most people will run from 8.0-8.5:1 to be safe I would guess though. But, for me, 9.0:1

Eric
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Last edited by Eric SS; 03-10-2002 at 01:10 AM.
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Old 10-13-2001, 05:44 PM   #2
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so whats the CR for the MY02 WRX and RS?
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Old 10-13-2001, 05:50 PM   #3
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Not exactly sure. do a search and it will come up.

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Old 10-13-2001, 06:15 PM   #4
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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'd be real interested to hear what you guys think of this stuff.

Kevin 90GSX



That basically covers all of it. Please make sure you have your information straight before posting such important stuff. There are many readers here that take everything posted as the complete truth.

Cheers,

MattC
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Old 10-13-2001, 06:20 PM   #5
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The RS uses 9.7:1 and the WRX uses 8.0:1 (though that might have been increased to 9.0:1 with the 2002 WRX).


Ben
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Old 10-13-2001, 07:39 PM   #6
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MattC, Keep in mind that all my coments work within a paramater. I woudn't run 11:1 and 15lbs. of boost.

My calculations and comments were based around staying between 8.0:1 and 9.5:1. I should have made that more clear.

Within those parameters, given the same forged internals (except CR), the higher temperatue of the charge from running more boost will pretty much equal the higher temp. produced from a higher compression ratio and lower boost. Also, your horsepower increases will be about the same.

This topic was designed for those people who are building up a high horsepower engine that will have top quality internals.

so again, my opinion is that I would run a higher compression ratio and a little lower boost.

Sorry for the misunderstanding. Unfortunetly, my area of expertise is in superchargers which work a little differnt that turbo's because you are going to be under pressure more often But I do know much about the dynamics of both

Eric

Last edited by Eric SS; 03-10-2002 at 01:13 AM.
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Old 10-13-2001, 08:35 PM   #7
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Eric,

The equations given in the above post are w/o parmeters. Meaning that they will work in all cases dealing with max HP.

Higher intake temperatures are handled by efficent intercoolers and properly sized turbos. Why do you think so much effort is given towards purchasing the proper turbo for a given setup?

Reread over my last post. All calculations and conclusions are for street cars. The strength of a motor is a constant and can be thrown out. Cylinder pressures are what we are after. We want to make max HP at a given cylinder pressure. DSM trial and error has yielded a strong motor with 8.5:1 compression to be, by popular opinion, the greatest compromise. This compromise being driveability vs. max HP.

Regards,

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Old 10-14-2001, 12:24 AM   #8
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Here, I just stumpbled accross this webpage. check it out. It does a good job of explaining what i am trying to explain. This article was written up for superchargers but the dynamics involved are the same http://www.motorsportsdigest.com/forced2.htm

Eric
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Old 10-14-2001, 01:48 AM   #9
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One of the flaws stated directly in that article is that it is specific to superchargers and superchargers aren't as effective in making a lot of boost from a small package as turbochargers. You can easily bolt on a turbo to push 30psi, but a supercharger to do it will be sticking out of your hood.


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Old 10-14-2001, 04:25 AM   #10
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OK, I've been doing Google searches for about an hour and can't find how to determine the needed octane other than trial and error.

MattC, how do you get that you can run 12.67 ECR on pump gas? By pump gas, what octane do you mean as it differs by area. 91 octane? 92? 93?

I'm working on pushing my stock 9.0 to 1 1.8L Impreza turbo to 200hp. This should get the 2730 pound car into the upper 13's and max out a set of RS injectors. Also, boost will have to be around 12psi which according to your equation would have an ECR of 12.13 which also provides a bit of a buffer zone to your 12.67 ECR and "pump gas". This is also assuming adequate fuel and an efficient intercooler to help battle detination.

Chad
93 1.8T
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Old 10-14-2001, 06:12 PM   #11
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Chad,

The 12.67 ECR was already in the equation when I stumbled across it. By no means is it the exact ECR for all motors on "pump gas". As you very well know, and have stated, there are great variances in octane throughout the country. On top of that different motors with different flow charactorisitics are also going to have different maximum safe ECR. What that means is this equation isn't going to give you the max boost you can run on your motor. It will just give you a very rough estimate. What it does show is that lower compression yields much higher "safe" boost levels, because the cylinder pressures will remain the same. Boost not only increases compression, it also increases air flow. Thus allowing more air into your motor at a given boost level.

In your case, with 9:1 compression 12psi does, however, seem very feasible. The 1.8L motors are known to be "strong". I believe this is partly thought because of there lower compression, however I have heard they do share some other internals with stronger EJ motors too.

Cheers,

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Old 10-14-2001, 07:50 PM   #12
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Quote:
Originally posted by bsquare
The RS uses 9.7:1 and the WRX uses 8.0:1 (though that might have been increased to 9.0:1 with the 2002 WRX).


Ben
MY02 wrx's are still 8.0:1 CR.
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Old 10-15-2001, 02:52 PM   #13
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The 98 and 99 RS have 9.7:1 CR, the 00 and 01 have 10:1
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Old 10-15-2001, 11:16 PM   #14
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I am using a MY99 shortblock with MY00 heads, but a very very thick head gasket from a phase I MY96 motor. The gasket seems about 3x more likely 4x thicker than normal.

How much drop in compression do you guys think I am seeing from the 9.7:1 we'd normally see with this? Do you think 9.5:1 is a real possibility?

Larry
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Old 10-15-2001, 11:20 PM   #15
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Larry
I have been trying to work that out
it seems those numbers are quite feasible
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Old 10-16-2001, 01:39 AM   #16
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i think it's quite feasible that you'll blow that composite head gasket, they seemed to do that quite often making stock power.

observation: Eric and Matt's formulas (well Matt's reposted material ) are the same except Matt's has a ^.5 on the outside. if you take that in context the numbers come out to be very similar.

of course, there's a lot more than effective CR that will determine the power and longevity of a motor. cam timing, ignition timing and compressor efficiency to name a few.
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Old 10-16-2001, 02:03 AM   #17
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Exclamation Larry's C/R

Larry,

If you're using the MY96 head gasket, it should be the same as mine which is MY98. I believe these were used from MY95-98. Anyways, I measured my just removed gasket and it is 0.060" The New MY99 is 0.025" That's 0.035" difference. As told to me by many machinist and engine builders this week that it will equate to about 0.3 C/R. So realistically, YES you're probably at 9.4-9.5:1 C/R.

I've been going through this all week. So this is just my $.02

-Adam
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Old 10-16-2001, 03:04 AM   #18
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Quote:
Originally posted by ColinL
i think it's quite feasible that you'll blow that composite head gasket, they seemed to do that quite often making stock power.

observation: Eric and Matt's formulas (well Matt's reposted material ) are the same except Matt's has a ^.5 on the outside. if you take that in context the numbers come out to be very similar.

of course, there's a lot more than effective CR that will determine the power and longevity of a motor. cam timing, ignition timing and compressor efficiency to name a few.
You are correct, much goes into building the engine, especially timing and compressor effeciency. I still stick by my formulas and understanding in my first post.

MattC, I still don;t understand what "moving more air through the engine" makes a differnce to CR vs. boost. The boost part of my table takes into account the amount of air flowing through the engine. ColinL makes a very good point that I purposly left out of the post in order to simplify things a bit and keep it on topic with what my post started out as but I will briefly go into it now.

A little differnt Scenario: one engine is running x amount of boost and x.x:1 CR. A second is running a differnt amount of boost and CR but the static CR is the same. With differnt cam timing, ignition timing, compressor maps, and a host of other things like I:E flow ratios, the engine will make the same power and be just as reliable! Granted, the power curves and driveablility will be completly different but the reliability will be the same.

However, this is meant to be obviously within reason of what people will do. I doubt anyone will run 11.5:1 with 4lbs. of boost when they can run 9.0:1 with 10lbs. of boost.

If you look at my map, you will notice on the outskirts of it that as your CR gets higher, the amount of boost you need to get the same efective CR decreases almost exponentially towars the end. The reason people generally tend to stick between 8.0:1 and 9.5:1 on a turbo car is for a couple reasons.
1) the price of the parts is cheaper because that is what most people do.
2) drivability is best because you still have enough low end torque but you don't have to run 115 octane gas when not under boost.
3) turbochargers are designed to work most effectivly within a certain amount of boost vs. engine size. Certain turbo's won't be as effecient at 2psi and 20psi. vs. 6-10psi or whatever. Therefore, engine CR is built around a specific turbo's compressor effeciency map.



Eric
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Old 03-09-2002, 11:55 PM   #19
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can we make this a sticky? i like it.....since my topic brought this up......long ago..........

Thanks!
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Old 03-10-2002, 01:03 AM   #20
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Uh oh. This again. hehe.

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Old 03-10-2002, 03:04 AM   #21
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Imprezinator:

Unless I've missed something in your posts, I don't think you've caught on to the primary advantage of a lower compression engine running more boost. Yes, to get the same effective CRs, the higher compression engine gets to run less boost. But, at top dead center, it has less volume in the cylinder. So it has less air in there for the same amount of compression. Thus, less air and fuel in the cylinder. See were this is going? The lower (static) compression engine is making more power, because it has more air and fuel in the cylinder.

In summary, if you have (for example) 20% more area inside your cylinder at top dead center, and you compensate for that by running a little more boost, you can (in theory) make about 20% more power. It's almost like you have a bigger engine.

Make sense?

Jeff
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Old 03-10-2002, 03:48 AM   #22
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Quote:
Originally posted by Portly
Imprezinator:

Unless I've missed something in your posts, I don't think you've caught on to the primary advantage of a lower compression engine running more boost. Yes, to get the same effective CRs, the higher compression engine gets to run less boost. But, at top dead center, it has less volume in the cylinder. So it has less air in there for the same amount of compression. Thus, less air and fuel in the cylinder. See were this is going? The lower (static) compression engine is making more power, because it has more air and fuel in the cylinder.

In summary, if you have (for example) 20% more area inside your cylinder at top dead center, and you compensate for that by running a little more boost, you can (in theory) make about 20% more power. It's almost like you have a bigger engine.

Make sense?

Jeff
no. If you look at the table I posted, effective compression ratio is the amount of air per cubic centimeter (or whatever volume measurement you want to use) of space. That's what makes the amount of cylinder pressure. The same amount of air is taking up the same volume in an engine running 4psi with a static CR of 9.0:1 as an engine running 6psi with a static CR of 8.0:1. That's my point. Same cylinder pressure = same amount of power (in a nutshell. Forget timing and a whole bunch of other variables)

Eric
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Old 03-10-2002, 04:06 AM   #23
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maybe im missing something...what is the target ECR we are trying to achieve? how can we be saying you should use X CR w/ X boost if we dont even have a number to base it on? is there a magical ECR number im missing thats for normal pump gas?
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Old 03-10-2002, 01:00 PM   #24
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Eric

Ok, since you're not really reading what I'm saying, I'll try again. Let's use your 9.0:1 @ 4psi and 8.0:1 @ 6psi example. All else being equal, why are the (unboosted) compression ratios of these engines different? Because the the 8.0:1 engine has a lower piston, thus creating a slightly larger combustion chamber at TDC. Without boost, they both end up with essentially the same amount of fuel/air mix in the combustion chamber. But, in your example, our engines have been boosted so they both have almost exactly the same cylinder pressure at TDC. Since pressure in the cylinder is the same, and the 8.0:1 engine contains greater volume, the 8.0:1 engine creates more power because it has more fuel and air at the same cylinder pressure. Again, this only works if the 8.0:1 engine is boosted more than the 9.0:1 to create the same internal pressure, like is done in your example.

Taken to an extreme, let's take identical engines, but one has a CR of 8.0:1 and 6 psi of boost. The other has a CR of 11.3:1 and 0 psi of boost. Per your chart, they have identical cylinder pressures at TDC. Your argument is that both engines will make identical power, as they have the same internal cylinder pressure. My position is that the 8.0:1 engine will make far more power, as it's combustion chamber contains a greater amount of fuel/air mix at the same pressure.

To quote you. "The same amount of air is taking up the same volume in an engine running 4psi with a static CR of 9.0:1 as an engine running 6psi with a static CR of 8.0:1." I agree. But the 8.0:1 engine has more volume, and thus has more air/fuel mix.

I hope this was clearer.

Jeff
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Old 03-10-2002, 08:39 PM   #25
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More volume + equal compression = more power

than

Less volume + equal compression = less power

I gte it now why people go with a low compression motor. I have been wondering about that myself.

Interesting.

RawCode
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