Boost pressure questions

I've just been having random thoughts tonight due to boredom and i've came across something i can't get my head round without knowing where the reading is taken from.

If its taken from the intake manifold surely 1 bar of boost is 1 bar of boost regardless of whether you are using a single or stock? Its just that i've read figures, for example 500bhp @ 1 Bar, which are less than what i run my sequential setup at.

Now, obviously i know my train of thought on this is wrong so i would just like to have it clear.

I like to understand things

But at 1 bar a single will be flowing a lot more air than stock twins

But at 1 bar a single will be flowing a lot more air than stock twins

This is the part that i know, but i can't understand. If the boost pressure was measured at the turbo i could understand this but at the manifold i can't get my head round it.

The volume of the manifold (if stock) is constant regardless of what turbo(s) are being used. If the pressure is 1 bar and the volume is constant surely the flow is in direct relation to the pressure?

Hi Scott, this was discussed a little while back, have a read of this thread, it will explain all

http://www.mkivsupra.net/vbb/showthread.php?t=127950

In summary, it's all about air density.

Volume of air moved is not totally dependant on pressure.

just trying to think of an easy to understand example.

Back shortly:)

Edit, beaten to it!

Just read all that, i still don't have my answer

The only thing i can see in that entire thread that would make a difference would be the temperature.

Volume of air moved is not totally dependant on pressure.

Its dependant on pressure and temperature though.

Edit: Just going through the links on that other thread.

Ok so reading those links brings me to the conclusion

1bar of cold air has far more oxygen molecules than 1bar of hot air.

Is that really the only difference in turbo's? A larger turbo can flow more air at a slower speed therefor the temperature of the air at any given rev range is lower than a smaller turbo?

If thats the case then the stock turbo's must be pretty rubbish as both of them flowing 1bar of air create more heat than a single larger turbo flowing 1 bar of air.

 

The only thing i can see in that entire thread that would make a difference would be the temperature.

Tadaaaa.

With my limited understanding, as a high level view thats more or less it Scott.

If thats the case then the stock turbo's must be pretty rubbish as both of them flowing 1bar of air create more heat than a single larger turbo flowing 1 bar of air.

Again, more or less correct, remember that stock turbo's were designed to be at their optimum at 0.76bar. If we go beyond that we're outside of their design specification.

Tadaaaa.

I understand it but i can't fathom how it makes THAT much of a difference. Is there any info on the intake temps of a single at 1bar in relation to the intake temps of a TT at 1 bar?

Had another thought, if you change the manifold then you change the volume of the manifold. If thats the case then you will see a pressure increase or decrease depending whether its bigger or smaller.

Say you have a turbo rated to 1.6bar max. What is it rated to 1.6bar max in? If you take a massive intercooler with 4" piping and a large intake manifold then it will take a lot more flow from the turbo to get up to 1.6 bar.

Sorry for all the questions, i just like to get things clear in my head. Once i understand it all i won't forget it

With my limited understanding, as a high level view thats more or less it Scott.

 

 

 

Again, more or less correct, remember that stock turbo's were designed to be at their optimum at 0.76bar. If we go beyond that we're outside of their design specification.

Eyeopening to say the least lol.

As per my above post, would that mean that us adding a 3" FMIC takes them outwith their optimum specification? The turbo will have to flow far more air to reach 0.76bar in relation to the SMIC?

I understand it but i can't fathom how it makes THAT much of a difference. Is there any info on the intake temps of a single at 1bar in relation to the intake temps of a TT at 1 bar?

It makes a very large difference to the number of oxygen molecules in a given volume. Hoter air means less molecules, therefore less potential explosive force when combined with a matched level of fuel. (It's a lot more complex than that of course, but enough for this discussion)

Had another thought, if you change the manifold then you change the volume of the manifold. If thats the case then you will see a pressure increase or decrease depending whether its bigger or smaller.

A larger manifold will simply mean that the turbo takes slightly longer to produce the required boost pressure. Remember that pressure is measured at the intake manifold so the wastegate only starts operating once that target boost level is acheived. Changing the volume of the manifold (or pipework) only changes the time it takes for a set level of pressure to be reached.

Its dependant on pressure and temperature though.

 

Edit: Just going through the links on that other thread.

No thats not quite it, as i said its not totally down to pressure and heat,

If you think about two different sized fans, one thats say 6" in dia and one thats 3" dia, otherwise equal, same speed,blade number and shape, fix each to a 6" dia tube, its then pretty much a foregone conclusion which is moving more air, no heat involved to complicate things.

Now take a 10" fan and seal it to the 6" tube, it will still shift more air than the 6" fan, but now the constraints of the tube size come into play, so you can now equate this to a turbo, the larger one will shift more air independent of temperature, up to the point where the tubing starts to become a restricting factor.

Long winded i know and may not be 100% mathematically correct but i hope it serves to illustrate.

There are a myriad of small factors once you get past the "compressing the air heats it up" issue that can affect airflow, velocity, pressure, temperature etc., but the first 90% or so is the fact that compressing it heats up the air.

No hard and fast numbers, but I think the Corky Bell book said the air could get over 100degC pre-intercooler. Toasty.

It makes a very large difference to the number of oxygen molecules in a given volume. Hoter air means less molecules, therefore less potential explosive force when combined with a matched level of fuel. (It's a lot more complex than that of course, but enough for this discussion)

 

 

 

A larger manifold will simply mean that the turbo takes slightly longer to produce the required boost pressure. Remember that pressure is measured at the intake manifold so the wastegate only starts operating once that target boost level is acheived. Changing the volume of the manifold (or pipework) only changes the time it takes for a set level of pressure to be reached.

That makes sense but what about when it comes to the higher end of the rev range and the maximum efficiency of the turbo itself.

Example...

At maximum revs, and with an intercooler/manifold system that equals 1m cubed, the turbine spins at 20000rpm to flow enough air for 1.6bar.

Given the rule above if you change the intercooler and manifold for a much larger system equalling 1.5m cubed the turbine would need to rotate at 30000rpm to flow enough air for 1.6bar.

Say the maximum efficiency of the turbine was 20000rpm..... it would probably pop with the 2nd system in place.

I can't understand where turbo manufacturers get their figures from, unless they mean maximum pressure 1.6 from the actual turbo itself regardless of intercooler and manifold. If that is the case though a lot of people MAY be using their turbo's well past their specification.

No thats not quite it, as i said its not totally down to pressure and heat,

If you think about two different sized fans, one thats say 6" in dia and one thats 3" dia, otherwise equal, same speed,blade number and shape, fix each to a 6" dia tube, its then pretty much a foregone conclusion which is moving more air, no heat involved to complicate things.

 

Now take a 10" fan and seal it to the 6" tube, it will still shift more air than the 6" fan, but now the constraints of the tube size come into play, so you can now equate this to a turbo, the larger one will shift more air independent of temperature, up to the point where the tubing starts to become a restricting factor.

 

Long winded i know and may not be 100% mathematically correct but i hope it serves to illustrate.

The pressure from the 10" fan in the 6" tube would be far far higher though.

There are a myriad of small factors once you get past the "compressing the air heats it up" issue that can affect airflow, velocity, pressure, temperature etc., but the first 90% or so is the fact that compressing it heats up the air.

 

No hard and fast numbers, but I think the Corky Bell book said the air could get over 100degC pre-intercooler. Toasty.

Its sad i know but i find that fascinating lol.

Query 1 sorted thanks guys

Wait... i've just answered my own question.

Nevermind

Edit: for those that want to know.. once the pressure has been built up in the system the airflow won't increase or decrease as the pressure will remain constant(ish). It just takes longer to build up the pressure and more air in the beginning, once you are at pressure the system is filled and the turbo only needs to supply the amount of air that the engine can handle at 1.6 bar which is always constant.

Strange explanation but thats how i have it in my head lol.

That makes sense but what about when it comes to the higher end of the rev range and the maximum efficiency of the turbo itself.

 

Example...

 

At maximum revs, and with an intercooler/manifold system that equals 1m cubed, the turbine spins at 20000rpm to flow enough air for 1.6bar.

 

Given the rule above if you change the intercooler and manifold for a much larger system equalling 1.5m cubed the turbine would need to rotate at 30000rpm to flow enough air for 1.6bar.

 

Say the maximum efficiency of the turbine was 20000rpm..... it would probably pop with the 2nd system in place.

 

I can't understand where turbo manufacturers get their figures from, unless they mean maximum pressure 1.6 from the actual turbo itself regardless of intercooler and manifold. If that is the case though a lot of people MAY be using their turbo's well past their specification.

 

 

 

The pressure from the 10" fan in the 6" tube would be far far higher though.

I should of added that its an open tube;) once you add pressure by restricting the end of the tube, the larger fan will still shift more air if the pressures are equal for all fans.

Have a look at buying this book Scott, it's very enlightening, but not exactly bedtime reading

http://www.amazon.com/Maximum-Boost-Turbocharger-Engineering-Performance/dp/0837601606/ref=sr_1_1?ie=UTF8&s=books&qid=1238106076&sr=1-1

Edit - bah, beaten to it by Ian (again!)

I should of added that it an open tube;) once you add pressure by restricting the end of the tube, the larger fan will still shift more air if the pressures are equal for all fans.

Even with the tube being open there will still be a difference in pressure when the tube is open ended

Have a look at buying this book Scott, it's very enlightening, but not exactly bedtime reading

 

http://www.amazon.com/Maximum-Boost-Turbocharger-Engineering-Performance/dp/0837601606/ref=sr_1_1?ie=UTF8&s=books&qid=1238106076&sr=1-1

 

Edit - bah, beaten to it by Ian (again!)

Forget i asked

compresser design an sizes also make a difference to the way the turbo behaves. the characteristics of the air flow vary greatly dependant upon the no of vanes ,angle an differential of size between the compressor wheel and the driven side, thus giving different flow and boost characteristics whilst pressure remains constant, you have to remember that flow is not directly linked to pressure they are two entirely different things

compresser design an sizes also make a difference to the way the turbo behaves. the characteristics of the air flow vary greatly dependant upon the no of vanes ,angle an differential of size between the compressor wheel and the driven side, thus giving different flow and boost characteristics whilst pressure remains constant, you have to remember that flow is not directly linked to pressure they are two entirely different things

Your confusing me because that is totally different to what i have been taught in physics. Flow IS directly related to pressure. If temperature of a fluid/gas remains constant, and obvioulsy the volume, and you increase the pressure, you increase the flow.

I'm not sure if people are reading into things too much and coming up with these theory's or whether or not i was just told a simplistic equation with regards to pressure.

The difference in vanes only changes the efficiency and the efficiency range of the turbo as far as i know. Thats a completely different point though.

Flow is DIRECLY proportionate to volume, temperature and pressure. If you can show me an equation that says otherwise then i would be greatfull.