PSi Doesnt matter???

[spardikis]

Hey guys, just wondering why everyone asks this question? How much boost can i run?


Surely the amount of air being flowed is more important then the psi of the air??
I mean 10 pounds of boost from a TDO5 is not the same amount of air as 10 pounds out of a GT3540?? SUrely a better question would be.... How much power can i make....??

Thoughts??





P>S this is a genuine question as to why people ask this question, not a dig or to start a shiv fight

[Q_ball]

Its because people dont have a good understanding of the concepts behind turbo charging their vehicle.
Initial thoughts will be more boost = more power = faster car.
I dare say 99% of cases will start off in that fashion.

Once ppl start reading and learning more about the topic, then they'd see differently.

[ProECU]

If you can understand a turbo compressor map... you will then understand the answer to your initial question.

[kid_dynamite]

I'd say it's because ppl don't understand that boost is a measure of restriction compared to atmospheric pressure
As ProECU said reading comp maps are pretty handy. Here's a link FYI:
http://www.turbobygarrett.com/turbobygarrett/tech_center/turbo_tech103.html

[JasonGilholme]

Let have a chat about air pressure and how turbo's used to work on air planes (mostly back in the war days).

Air pressure at sea level is about 14.7 PSI. When planes take off they are sucking in are at 14.7 PSI (providing they're at sea level) once they get to a higher altitude the air pressure drops and the engine looses power. This is cause there is less PSI of air where they are flying at that point in time.

In order to allow planes to fly higher they needed some way of controlling the air pressure to keep it at or above 14.7 PSI. This s where the turbo charger came in.

So yes, psi does matter.

[trism]

^^^ im with him

they used either turbo, or superchargers.

some tho, back in WWII such as the P51 Mustang had twin superchargers. one was to keep constant air pressure, the other was to create boost, to increase power..

some tho even had Nitrous!!!

no joke, when the throttle was pushed to Emergency War Setting, the engine would gain more power.

1. throught the use of nitrous, although not all planes had thise, only the elite units
2. on turbo planes, if effectively shut the wastegate, forcing all exhaust thru the turbo, and spiking the boost thru the roof
3.on S/C planes, the 2nd charger would kick in, or it would work harder...

keep in mind that these engines are already up in the region of 27litre V16s!!!!

[JasonGilholme]

trism: you know your planes better then me man! Top stuff. I just know the used it to equilize pressure. Didn't realise they would actually use secondaries to boost power. And notrious! Hell yeah!!

BTW, plane engines are massive :thumbsup:

[RCS 20L]

YEs i found that out myself tooo.. I had my car running at 7psi and now running 10psi... Still having the same power after two different tunes?? I guess i should of told the tuner i wanted a specific power output instead of telling him to tune it to 10psi.:o

[string]

If you can understand a turbo compressor map... you will then understand the answer to your initial question.
No he won't. There's more to the power difference than simply charge temperature difference.


YEs i found that out myself tooo.. I had my car running at 7psi and now running 10psi... Still having the same power after two different tunes?? I guess i should of told the tuner i wanted a specific power output instead of telling him to tune it to 10psi.:o
Do you have a graph of boost pressure versus rpm (or speed)?

You may be making the same peak power, but chances are that you're making more torque than before in the mid-rpm range meaning you'll squeeze out a little more area under the curve.

Disregard numbers; does your car feel faster?

[spardikis]

yes more boost does equal more power but there is a difference between the same boost pressure coming out of different compressors.

ie 10 pound out of a tiny compressor is not as much air and therfore not as much power as 10 pound out of a huge compressor. I get turbine maps but dont agree when them completely when choosing a turbo for an engine.

[JasonGilholme]

manifold pressure is manifold pressure.

AFAIK the engine can't flow any more CFM then what it can at any given RPM.

[ek4-guy]

lol thats right if one turbo is filling a pipe with air compressed at 10psi and the other is doin the same thing then the output is the same psi is psi

thats like saying a 1.6L Civic doin 10kph cant be travelling the same speed as 6L Chevy truck doin 10kph

[string]

yes more boost does equal more power but there is a difference between the same boost pressure coming out of different compressors.

ie 10 pound out of a tiny compressor is not as much air and therfore not as much power as 10 pound out of a huge compressor. I get turbine maps but dont agree when them completely when choosing a turbo for an engine.
There is no difference. If you have two air mass loads such as an engine with perfect exhaust characteristics, each compressor will flow the exact same mass of air. Pressure is a result of a resistance to flow. For example 1800cc of cylinder volume at 10psi is the same no matter what was causing the compression. Power loss (of a small turbo versus large) is caused by a reduced number of air molecules in the charge.

Charge temperature is far from a major contributor when it comes to the magnitudes of power difference you see when going from a small turbo to a large one.

Major power differences obviously then come from a volume differential in the cylinder contributed by reversion of exhaust gases caused by the exhaust side inefficiencies. The misconception obviously arrises from the fact that your boost gauge reads the same number, but your effective engine capacity is decreasing the more air you flow, reducing output power.

The trade off between a small exhaust housing/turbine is a big one to say the least. A tiny stock skyline/silvia/wrx turbo spools in the 3,xxx rpm range, but runs out of legs up high, whereas a gt28/30/35+ might not hit boost until 4,xxx rpm+, but be efficient over a large rpm, and specifically where you want it. This means that pound for pound of boost pressure, you'll usually make far more peak power with a larger turbo, however peak torque won't always be too much behind. This is obviously not going to hold with a vtec motor (or other high revving large port/cam motor) which makes peak torque well into vtec, meaning that a smaller turbo will definately be reverting a lot of it's exhaust causing great torque losses.

Though inefficient charge cooling certainly can cause great power losses, but it's so common that it should be assumed that both cases have reasonably efficient methods in place.