Hey if we know that the eaton blower is not a very efficient unit then why do we insist on overdriving the blower ? Why not just rebuild the motor with a relatively high static compression ratio and underdrive the blower ? If we underdrove the blower wouldnt the blower suck less power from the motor, and at the same time stay within its limits as far as rpm, and flow capability ?
Compression VS. Boost ?
- Thread starter blksc
- Start date
T-bird4vr
Registered User
In a word yes. But you are forgetting about the intercooler. It allows us to have a higher effective compression ratio and still run on pump gas. Dropping the boost pressure and increasing the motors CR will reduce the power gained at the intercooler. Lower boost would mean the IC would give off less heat (since there is less to start with) and more would be made internally in the motor, which may cause detionation. There is an ideal point, but a lot of variables effect it.
I hope I am thinking about this correctly.
I hope I am thinking about this correctly.
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XxSlowpokexX
Registered User
I think 9.5 would be obtainable..And a frontmount...SOmeones has had to have treid it
David Neibert
SCCoA Admin
Carlos,
It sounds good on paper, but it doesn't work very well on an SC motor. Several guys have tried raising the base compression and can't run as much boost or timing on pump gas. The lower compression motor with more boost and more timing advance makes more power on pump gas. On race gas it doesn't really matter. I would not go any higher than 8.5:1.
Tim K. just built a high compression motor for his SC (9.8:1 I think) and is not happy with the results. Chris Wise got away with 9.3 only because he had a huge liquid IC.
David
It sounds good on paper, but it doesn't work very well on an SC motor. Several guys have tried raising the base compression and can't run as much boost or timing on pump gas. The lower compression motor with more boost and more timing advance makes more power on pump gas. On race gas it doesn't really matter. I would not go any higher than 8.5:1.
Tim K. just built a high compression motor for his SC (9.8:1 I think) and is not happy with the results. Chris Wise got away with 9.3 only because he had a huge liquid IC.
David
XR7 Dave
Registered User
The reason that the results are not satisfactory is that boost not only creates higher compression but it also increases displacement. At 15psi your 3.8L motor is displacing 7.6L as well as creating 14:1 compression. Therefore it is not an even trade off boost vrs. compression.
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14 psi at sea level
68COUGAR
Easy! At sea level there is 14.1 psi of air pushing down on the earth. That's what a NA engine sees. If you add another 14.1 psi via supercharging (or turbo charging) you are putting twice as much air into the engine. Therefore the engine acts like a 7.6 Litre engine at sea level. Simple, No?dms825 said:
68COUGAR
David Neibert
SCCoA Admin
No not simple..I don't understand the increased displacement thing either (please explain Dave). I get the part about the high effective compression ratio, so to my way of thinking it's just a high compression 3.8.
David
007_SuperCoupe
Registered User
Think of it in terms of atmospheric pressure. First you have to assume that the measure of boost pressure is accurate to the amount of air entering the combustion chamber, and not the measure of exhaust back pressure and inlet restrictions. Once you have that figured, at 15 psi you are basically forcing 2x the air into the combustion chamber than a normally aspriated engine. The 3.8 L in naturally aspirated form is now receiving enough air for a 7.6 L engine. Now with a primarily stock engine, the boost that you see is more a measure of backpressure and inlet restriction than it is of airflow into the engine. So, like in my case where I see 15lbs of boost, I'm not actually pumping 7.6 L of air into my engine. That's just what it is trying to pump in. Until I get the inlet restrictions and heads done, that 15lbs is really maybe like 10 lbs which would make the displacement volume around 5.8 L or so. Once I remove all my restrictions, then I don't think that I'm going to see more than 10lbs of boost...With the current M90 driven at the same speed...
XR7 Dave
Registered User
Pretty much what he said. It's not *exactly* double the displacement but it's close. The actual size of the cylinders is pretty much irrellevant when you start altering the pressure of the gas in them. It's kind of like fitting 1,000,000 cubic feet of propane into a bottle. Remember, it's not just air volume that is increased, fuel is also increased linearly with boost. However, the compression volume (chamber volume) is the same regardless so all that extra air is still compressed into a little chamber under compression hence the high "theoretic" compression ratio.
When rating engines we actually measure the swept volume of the cylinders and that becomes the *advertised* displacement. However, the real displacement should be an accurate measurement of the amount of air drawn in under load. An inefficient 7.6L N/A motor may only be able to draw in 6.9L of air per cycle under load whereas a very efficient supercharged 3.8L may be able to draw in 8.0L of air per cycle under boost.
This also begins the discussion on volumetric efficiency. Most N/A motors will achieve about 95% VE at torque peak. Some really efficient and highly tuned NA motors will generate as much as 115% VE at certain speeds. The Honda S2000 motor is an example of one such motor. In boosted applications it is common to see 150%-180% VE and race motors see much more than that.
Clear as mud?
When rating engines we actually measure the swept volume of the cylinders and that becomes the *advertised* displacement. However, the real displacement should be an accurate measurement of the amount of air drawn in under load. An inefficient 7.6L N/A motor may only be able to draw in 6.9L of air per cycle under load whereas a very efficient supercharged 3.8L may be able to draw in 8.0L of air per cycle under boost.
This also begins the discussion on volumetric efficiency. Most N/A motors will achieve about 95% VE at torque peak. Some really efficient and highly tuned NA motors will generate as much as 115% VE at certain speeds. The Honda S2000 motor is an example of one such motor. In boosted applications it is common to see 150%-180% VE and race motors see much more than that.
Clear as mud?
XxSlowpokexX
Registered User
Ever drive one of those things though..Not fun in my book..No low end power at all
XR7 Dave
Registered User
Correct, but to achieve more than 100% cylinder filling without the aid of an artificial boost source, careful tuning of both intake and exhaust is necessary. The higher the rpm you can create the tune at, the more "additional filling" can be achieved. Imagine a 4.6L that makes 500hp naturally aspirated and passes emmisions.DamonSlowpokeBaumann said:
XxSlowpokexX
Registered User
XR7 Dave said:Correct, but to achieve more than 100% cylinder filling without the aid of an artificial boost source, careful tuning of both intake and exhaust is necessary. The higher the rpm you can create the tune at, the more "additional filling" can be achieved. Imagine a 4.6L that makes 500hp naturally aspirated and passes emmisions.
It would have to rev to 10,000 RPM's and feel slow as crapolla downstairs...Which is how the Honda feels...Looks good on paper though..But drive one and it just plain bites..Seriously