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Author Topic: boost is boost??  (Read 18038 times)
Mean88
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« on: August 13, 2009, 08:36:24 pm »

Got this off of the VW forums.  Debate is about a bigger vs a smaller turbo.  Is this true???

ship that bad boy over to me for a few days, let me see how it works on my 5000. i want to build one, but if you say its not worth it, then its not worth it. the biggest thing with the K26 tho, it flows alot better than the K24, it may not make more boost, but it has alot more air going through it at a set boost pressure than the K24 does.

I've heard similar before and it's never made sense to me.  I would welcome anyone giving a good explanation of how that's possible.  The pressure differential is what causes any substance (in this case, air) to flow.  Pressure is pressure.  A k03 pushing 15 psi of boost will flow the same amount of air into an engine as a k26 at 15 psi of boost provided the intake tract from turbo to cylinder is the same and the rpms are the same.  There might be added hp robbing back pressure on the K03, but the same amount of air is flowing in and out of the cylinders.  Boost pressure and the flow of the intake AFTER the turbo determines the amount of air entering an engine for a given rpm.  That's my story and I'm sticking to it unless someone can give a logical and scientific reason otherwise.
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347HO
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« Reply #1 on: August 13, 2009, 09:00:29 pm »

Interesting Wonder, Ponder

My opinion with a little bit of experience since I worked on some damn small to ludicrously large turbines.

If EVERYTHING being the same on the engine minus the turbos...   15psi is 15psi no matter how big or small the plenum is.
The only difference will be the spool up and spool down time plus the amount of cycles the blow off or bypass valve turns.
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... it was REALLY revving at like 4K...
If I saw that thing in my rear view Im pulling over to let you by, I be scared of that thing Huh? i dont know wich car is uglier ur or mine?
Javier
Ugly?  Easy now -- that's my baby and it's got lots of unique character!  When I drop that built 445" stroker in it you won't have any time to "pull over to let me pass" because I'll have already blown past you when you figure out what the hell was that loud noise behind you . . .  
SteveL
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« Reply #2 on: August 13, 2009, 09:09:11 pm »

Interesting Wonder, Ponder

My opinion with a little bit of experience since I worked on some damn small to ludicrously large turbines.

If EVERYTHING being the same on the engine minus the turbos...   15psi is 15psi no matter how big or small the plenum is.
The only difference will be the spool up and spool down time plus the amount of cycles the blow off or bypass valve turns.

Correct, 15psi is 15psi, so it's not a matter of how much boost, it's a matter of when
you get 15psi...

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« Reply #3 on: August 13, 2009, 09:36:53 pm »

Well, boost is just one unit of measurement, not an actual reading of the condition and thus mass of air. The temperature of the air charge is about as important as the boost reading.

If you compare two turbos in a manner that has them differing by other restrictions, the temperatures are going to be a lot different at the same boost. The lower temp boost is going to be more air, more air flow actually, and more power.
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Don

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347HO
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« Reply #4 on: August 13, 2009, 09:53:31 pm »

Well, boost is just one unit of measurement, not an actual reading of the condition and thus mass of air. The temperature of the air charge is about as important as the boost reading.

If you compare two turbos in a manner that has them differing by other restrictions, the temperatures are going to be a lot different at the same boost. The lower temp boost is going to be more air, more air flow actually, and more power.
If the air charge enters the engine directly from the turbo plenum, I would agree...  but today's technology now incorporates the pre cooler and after cooler.  Because it's common knowledge that cooler air is denser, thus "more" it's a "no brainer".  The boost valve is then placed in the cool air duct and is vented back into the front side of the turbo or to atmosphere.  As you can see...  one method is better than the other.

So again, 15psi is still 15psi.
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... it was REALLY revving at like 4K...
If I saw that thing in my rear view Im pulling over to let you by, I be scared of that thing Huh? i dont know wich car is uglier ur or mine?
Javier
Ugly?  Easy now -- that's my baby and it's got lots of unique character!  When I drop that built 445" stroker in it you won't have any time to "pull over to let me pass" because I'll have already blown past you when you figure out what the hell was that loud noise behind you . . .  
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« Reply #5 on: August 13, 2009, 10:57:21 pm »

Yes it's a complicated subject. I just tried to point that out and say that the temperature changes are important.

The things related to the turbo specifically are out of my league, I learned that in a conversation with Jay last year. He pointed out two big things that affect the rpm band, and the initial boost rpm points. I've forgotten those details, but I understood that those two factors are the most important about setting up a turbo system. I differ to experts who know the affects of the various turbo components upon rpm's for a given engine. This is like picking a camshaft, few people know how to do it well. Night,
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Don

1991 Mark VII LSC Special Edition, soon to be OBDII and 4R70W, then GTC body kit and 347.
1998 Mountaineer, plans for A4WD and KB 2200 blower later, XP8 parts, paint and a 332.
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« Reply #6 on: August 13, 2009, 11:03:07 pm »

It would depend to me on where you were measuring this "boost".......

I can guarantee that I change valve events and the boost gauge reads another number.
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Jay Allen
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« Reply #7 on: August 13, 2009, 11:52:33 pm »

Yes it's a complicated subject. I just tried to point that out and say that the temperature changes are important.
No argument, and your point should be taken as fact.

It would depend to me on where you were measuring this "boost".......

I can guarantee that I change valve events and the boost gauge reads another number.
No argument again.
All these factors being put on the table is why I suggested we go the "if all things are equal/same and only the size of the tubo has changed.".
15psi is still 15 psi.

I can only imagine if we were to throw in camshaft events, intake and exhaust valve diameter and placement (angle and distance from bore wall), cylinder psi, tube length from exhaust port to turbine just to throw a few out there would only raise eyebrows and curiosity of the average boost enthusiast...  I'm including those in the original thread posted by Mean88.
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... it was REALLY revving at like 4K...
If I saw that thing in my rear view Im pulling over to let you by, I be scared of that thing Huh? i dont know wich car is uglier ur or mine?
Javier
Ugly?  Easy now -- that's my baby and it's got lots of unique character!  When I drop that built 445" stroker in it you won't have any time to "pull over to let me pass" because I'll have already blown past you when you figure out what the hell was that loud noise behind you . . .  
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« Reply #8 on: August 14, 2009, 01:11:51 am »

Wouldnt a smaller turbo need to spin faster to move the same amount of air as the larger one, thus creating more heat.
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Mean88
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« Reply #9 on: August 14, 2009, 07:33:21 am »

One of the big things over  is larger cams.  I was always under the impression that for a diesel, especially a turbo diesel would be hard to make more power by changing the cam.  I thought I might go to a larger turbo on this car, but I am afraid of losing MPG by doing so.   Same with a cam, many on that site are going with different "stage" cams.  I dont see how it can be much benefit for either.......

I guess I just thought that a larger turbine could flow more air at the same psi given its size.  I guess I was wrong......
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« Reply #10 on: August 14, 2009, 08:44:51 am »

Wouldnt a smaller turbo need to spin faster to move the same amount of air as the larger one, thus creating more heat.

This was where I was going to go with part #2 of the discussion.

Boost in fact is not boost.  15 psi is not 15 psi.

Indy car back when they had turbos on them used a pop off valve at 25 psi.  At first there was 2 cars that seemingly had 100 more HP than everyone else.  Hmmm.......Then whatever the name of the sanctioning body was stepped in and imposed a "maximum turbo size" as the fast guys were using a GINORMOUS turbo and it was barely working.  The air was being bleed off so no one "really" thought about it.

Do you guys remember who this was?  They could not keep engines together at Indy to save their life.

An air charge temperature sensor points this out, instantly.  Boost is heat and hence why intercoolers kick ass.  A smaller unit with a much higher impeller speed is going to create far more heat.  So while "15 psi is 15 psi" in your mind and on the "boost" gauge, there is a significant amount of power to be had with a bigger unit that is not working near as hard.

Where you measure matters as well.  Check boost at the turbo, in the intercooler (if there is one), at the blade of the throttle body, and then in the plenum.

Hope This Helps.
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Jay Allen
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UNTIL FURTHER NOTICE, ALL PM's ARE TURNED OFF.  I AM NOT IGNORING ANYONE, E-MAIL ME IF YOU NEED SOMETHING.
347HO
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« Reply #11 on: August 14, 2009, 09:59:34 am »

I know where you're going with this...  but still, the engine can only take what it's limitations are.
In a perfect world, with a perfect engine, that engine could take as much boost just at the point of equal delta P.  You would then have a very explosive stall.
I agree a larger turbo will make more power, but 15psi is still 15psi.

A jet engine is the perfect example.  The compressor section 1 to 2 per engine (turbo) compresses the air however many times...  could be anywhere from 1 to 21 times, diffuser section (intake on a car engine) turns airspeed into increased air pressure and less speed, combustion section (combustion chamber) where the high pressure air and fuel are mixed and ignited, turbine section (exhaust) where the escaping high pressure gases are used to spin the compressors, tail pipe (collector) high pressure gases are turned back into high speed, low pressure.

In my example, there are no valves or pistons to impact the incoming air charge.  The turbine is the limiting factor.  Whether I put a 5 foot compressor section on this jet engine or a 20 foot compressor, the compressed air must be limited to what the turbine and exhaust section can escape.  In our example we are using 15psi.

A jet engine is in fact a 4 cycle engine and by design is more simple than a gas piston engine.
A jet engine used a "bleed valve" system which blows excess air pressure from the compressor out into atmosphere or bypass air.  The bleed valve system keeps the turbine from over-spinning, and in a car engine...  to keep the combustion chamber from blowing up.

So...  am I totally missing the point here?
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... it was REALLY revving at like 4K...
If I saw that thing in my rear view Im pulling over to let you by, I be scared of that thing Huh? i dont know wich car is uglier ur or mine?
Javier
Ugly?  Easy now -- that's my baby and it's got lots of unique character!  When I drop that built 445" stroker in it you won't have any time to "pull over to let me pass" because I'll have already blown past you when you figure out what the hell was that loud noise behind you . . .  
mxracer652
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« Reply #12 on: August 14, 2009, 10:16:45 am »

Combined gas law:  P=nRT/V

Not all pressures are created equal.   Whistling
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347HO
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« Reply #13 on: August 14, 2009, 10:28:51 am »

Combined gas law:  P=nRT/V

Not all pressures are created equal.   Whistling
How do we use that?

So, using the formula you posted, equate it out.
Ambient is 29.92" @ 74*F.
Turbine #1 is 5" dia. w/8" scroll
Turbine #2 is 10" dia. w/13" scroll

not given is rpm per 1psi because we don't know what the pitch of the blades are.
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... it was REALLY revving at like 4K...
If I saw that thing in my rear view Im pulling over to let you by, I be scared of that thing Huh? i dont know wich car is uglier ur or mine?
Javier
Ugly?  Easy now -- that's my baby and it's got lots of unique character!  When I drop that built 445" stroker in it you won't have any time to "pull over to let me pass" because I'll have already blown past you when you figure out what the hell was that loud noise behind you . . .  
mxracer652
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« Reply #14 on: August 14, 2009, 11:05:11 am »

Damn, a pop quiz.   
I was just saying that you can 15psi on the same setup w/ 2 different turbos and one could very well be much faster than the other due to the variables involved.

Just sayin'  Wink

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86 LTD wagon, stock longblock 302 & 70mm turbo
95 Ranger + 302/362 + 4 link
http://sbftech.com/index.php/topic,26347.0.html
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