Why doesn't it build boost if reved??

[skatershawn]

This Might be concerted a noob question.. but i always wondered why when we rev our cars it doesn't build boost!
I understand you will say... "it has to be under load"
Well why is it only our cars??
What causes this to happen?
The way i think of it is, as the exhaust spins the turbine it compresses the intake... the more you rev the more exhaust, (just like driving, im just in Neutral.) then it sould compress more intake air.
There is no bypass for it to exhaust to pass through besides the wastegate.. but that opens when the intake pressure rises so high to the set psi!
Im pretty knowledged on how turbos work.. but this has always blown my mind!

If theres a thread on it some where please link me... i searched and couldn't find anything but people saying, "it need to be under load"
i just want to understand my motor more!


thanks everyone haha

 

[DSM1G90]

Simple: when you rev the motor, yes there is jsut light exhaust pressure heading out the pipe, but when the motor has no work against it, there is no extra pressure coming from the motor's exhaust. Thus, without any work, there is no torque on the output end of the motor.

When you put something under a load, then the need for torque is required and the motor with additional fuel then delivers the power to produce the required torque for the needed work. Thus, the exhaust is putting this extra pressure out to work the turbo and the chain reaction begins.

Also, when you freeway drive a turbo and go at a constant speed, the turbo is just spinning in lag mode since, once again, there is no added work needed. It's just when you cram a bit on the gas is when the torque is needed to speed up the car and the turbo spools up by the extra force from the exhaust.

You might say, that any gas motor is just like a big air pump but in reverse since it's dependent of input to make it run. You open the throttle on it without any load, yet it will spin up but the output will be light due to no work on it. But you put a load on the output shaft, then it requires a lot more input just to work the load and that extra input then heads out the other end in a lot higher pressure..and turbos loves this higher pressure.

I'm sure you've mowed lawns before: you have the motor at a set RPM cutting the grass and suddenly, you hit a patch of grass that is taller, or thicker, and you suddenly hear the motor power up on occasion. This is the added torque produced by the motor to cut through that section of grass since the motor is pulling in a bit more fuel for the needed torque, thus the exhaust is louder.

Isn't WORK fascinating? -DSM

 

[skatershawn]

already... thats a very good answer. thanks!

But why is it only our cars? Maybe a few more out there!

NOT THAT IM TRYING TO DO THIS! just curious!
But i seen at car shows, for instance, a mazda 3 mazdaspeed rev and build boost quit quickly!
Seen a 240sx with a sr20det swap do the same!

i guess the question now is how their cars do it?
haha

i know this a somewhat a pointless thread to some... but please, im curious and like to know as much correct info as i can!
thanks again.

 

[DSM1G90]

a mazda 3 mazdaspeed rev and build boost quite quickly!
Seen a 240sx with a sr20det swap do the same!

Those are usually caused by longer stroke motors - which are designed for rally racing where ours are more for drag. Longer stroke sucks in more air and pushes it out in larger volumes and pressure which will spool the turbo much quicker meaning stronger takeoff pulls at the light. But there is a tradeoff in these designs: Longer strokes mean they run out of power at a lower rpm range than the drag motors like what we have.

You oughta see a turbo out of Germany: Unit has adjustable vanes inside the exhaust housing to direct the exhaust flow to either hit the vanes directly for quicker boost under lower RPM's, or more wide open for high powered boost pulls - a rather wicked turbo.

-DSM

 

[91talon27]

Actually the reason that our cars don't build boost is due to the ecu, to prevent boosting agressively and spinning on a normal takeoff. You don't have that on a 240 as the engine is a swap and usually you don't use the factory ecu. That is also the reason why you build boost when using a 2-step but don't build boost when you don't.

 

[airmail96]

I always wondered why a SRT-4 blows off just thinking about moving.

 

[ukdave]

i have a 97 talon and when i rev that up i can hit 15psi of boost is that not normal?

 

[dsmfa9nutter]

An automatic car will build boost because it has a load on the engine due to the torque converter. The converter starts to "lock up" aka, transfer power to the the transmission the faster it spins, putting a resistance (engine load) on the engine. A manual car free reving has no load until a clutch is applied and resistance (load) is put through to the engine.

Long story short, all of these cars that you saw building boost while free reving were probably automatics.

 

[EclipticalGS]

wheelspin on fwd cars can to the slightest degree affect spool, ### if youre tires are spinning, less load on engine whilst spinning, then it will affect the spool, but not enough ive noticed.

I also noticed when its icy and my wheels spin, i dont build good boost, but when my tires grip, its all over.

 

[xhackerekx]

u need just Ceddy mod

 

[synthetek]

I always wondered why a SRT-4 blows off just thinking about moving.

They are the only only cars I have seen that blow off while in reverse backing out of a parking space at 1 MPH too!

 

[jrohner]

But i seen at car shows, for instance, a mazda 3 mazdaspeed rev and build boost quit quickly!

Have you seen how small a mazdaspeed 3 turbo is? Or an SRT4 for that matter? They're smaller than a hairdryer. Those turbos should only be put on lawnmowers.


I don't see what the BOV opening has to do with anything, mine will push out LOTS of air just gunning the car up a little bit in neutral (not getting any positive pressure). Smallish turbos need the BOV to open easily as the wheels are so light and small that they slow down extremely fast. Revving up my Holset powered car (when I didn't have a BOV on it yet), made it seem like it didn't even need a BOV.

 

[skatershawn]

Originally Posted by DSM1G90
Those are usually caused by longer stroke motors - which are designed for rally racing where ours are more for drag. Longer stroke sucks in more air and pushes it out in larger volumes and pressure which will spool the turbo much quicker meaning stronger takeoff pulls at the light. But there is a tradeoff in these designs: Longer strokes mean they run out of power at a lower rpm range than the drag motors like what we have.

You oughta see a turbo out of Germany: Unit has adjustable vanes inside the exhaust housing to direct the exhaust flow to either hit the vanes directly for quicker boost under lower RPM's, or more wide open for high powered boost pulls - a rather wicked turbo.

-DSM

makes sense thanks

Originally Posted by jrohner
Have you seen how small a mazdaspeed 3 turbo is? Or an SRT4 for that matter? They're smaller than a hairdryer. Those turbos should only be put on lawnmowers.

haha ya funny and true!


well then ONE more quick question!!
How does a Studder box work??
The launch control on dsmlink!

i was told he keeps you turbo spooled.. it doesn't really get your system pressurized??
talked to someone and they said with it they launch out of the hole already spooled at 20psi with a hx35!


And i thank everyone for the helpful knowledge... except for the tell me im out of boost juice...
i know its sad... haha jk and im not even close to wanting to put NOS in the mix! thanks tho!
and thanks for not giving me shit like most other threads about people wanting to hear the BOV, like i stated before, that wasnt my intensions of posting this!

good day!

 

[tkelly27]

The way I've understood it to be is that load is a resistance to motion. Any pressure that would build up on top of the pistons without a load to resist it becomes a driving force for the pistons. It would be the difference between blowing straight out into the air, and blowing into a balloon. The air doesn't provide resistance and thusly no pressure, the balloon provides a resistance and there will be pressure in the balloon.

When you introduce the 2-step you are letting the engine become a load by shutting off injectors/ignition to given cylinders such that the engine isn't turning itself over and the ones that are being fired are generally timed such that it is putting more energy into the exhaust to spool the turbocharger.

It would seem that something about the exhaust would play in as well, but I just can't wrap my head around it. There is nothing to cause extra pressure in the exhaust, it's still the same cylinder evacuating into the same exhaust restrictions.

Blowing off after gunning it in neutral, or after a small number of rev's in reverse doesn't mean they can make boost without a load.

 

[cabbageman97]

hahaha! boost fluid
clever
i am also making 15psi sitting still, and its a 5spd not an automatic
could it have anything to do with having the wastegate solenoid being bypassed?

 

[skatershawn]

The way I've understood it to be is that load is a resistance to motion. Any pressure that would build up on top of the pistons without a load to resist it becomes a driving force for the pistons. It would be the difference between blowing straight out into the air, and blowing into a balloon. The air doesn't provide resistance and thusly no pressure, the balloon provides a resistance and there will be pressure in the balloon.

When you introduce the 2-step you are letting the engine become a load by shutting off injectors/ignition to given cylinders such that the engine isn't turning itself over and the ones that are being fired are generally timed such that it is putting more energy into the exhaust to spool the turbocharger.

It would seem that something about the exhaust would play in as well, but I just can't wrap my head around it. There is nothing to cause extra pressure in the exhaust, it's still the same cylinder evacuating into the same exhaust restrictions.

Blowing off after gunning it in neutral, or after a small number of rev's in reverse doesn't mean they can make boost without a load.

i like your answer too... thanks!

thats pretty much wrapped it up for me haha!!
i was just a curious cat, wondering how things worked!
and i thank everyone who used their time to give me their input!!

 

[DSM1G90]

and those SRT-4, "Skittle" turbos, even though built by Mitsu, are just proprietory turbos - only built for that car being constructed within the exhaust manifold .. and they spin counter-clockwise.

 

[ziggo0]

I always wondered why a SRT-4 blows off just thinking about moving.

It's due to it being a VERY tiny turbo and a very efficient manifold for the turbo...on top of that, an over sized engine for the under sized turbo, doesn't take much to hit 1psi on those.

 

[dsmfa9nutter]

A stutter box and antilag are two different things.

A "stutter box" is a "rev-limiter" that uses huge timing retard to limit the revs with the clutch in and the stutter box engaged. It basically does post-ignition, while the exhaust valve is open, and spools the turbo.

Anti-lag basically cuts ignition and lets raw gas fly into the turbo and explode, spinning the turbine and giving you boost while moving. Not great for turbo life. Perfectly fine for rally cars that have their turbo swapped for a brand new one after 20 minutes of driving.

 

[ruben2566]

To get back on track and answer the OP question. A turbo is a mechanical device that converts heat energy into mechanical energy, so the more heat you apply to the turbine wheel the more pressure it can produce at the compressor.
When you rev the engine without a load, there's not enough heat being produced at the exhaust to drive the compressor into boost.

 

[quiksilver20004]

To get back on track and answer the OP question. A turbo is a mechanical device that converts heat energy into mechanical energy, so the more heat you apply to the turbine wheel the more pressure it can produce at the compressor.
When you rev the engine without a load, there's not enough heat being produced at the exhaust to drive the compressor into boost.

Really they are run by heat??????? Does a motor run off of heat energy too?? No it runs off of combustion but it produces a lot of heat which is actually energy that is lost during the reaction, that is why combustion motors are considered inefficient. They lose a majority of their potential energy through heat and friction.. so if i take an acetylene torch (thats really hot!) to my exducer it will spin really really fast??? or should i take high pressure air and force it through the exducer? I vote air!
Seriously do you have any mechanical knowledge??? Turbos are run off of gas pressure whether it be hot or cold. Why do drag diesels use a turbo on turbo system? Take a compressor and air nozzel and put it in the exhaust side of your turbo and see if it spins, and see if there is air being forced out the compressor side. Heat is the enemy in any motor! It causes materials to degrade, reduces performance, and reduces engine life.

I cant believe i actually had to make this post.

 

[DSM1G90]

Ya - You can take a shop compressor and blow cold air into a turbo to make it spin ...

 

[skatershawn]

I cant believe i actually had to make this post.

Sorry you had to too!
I think he read through some other threads about people and big turbos and somestuff, cause i remember reading a post saying something that was in that area of blah! but it had to do with something totally different.. like the compression maps of the hx35 or something!!

He might have read to fast to understand correctly!!

haha

 

[dsmfa9nutter]

This forum needs a facepalm smiley so bad. (The head smack one doesn't count.)

 

[quiksilver20004]

I mean i will drop down and say that heat does play a part with a turbo and how much it spins. If you take a volume of cold air and heat it, it expands. But then again the exhaust gases that are coming out of the combustion chamber are already heated so yes heat plays a small part just due to the expansion and contraction of gasses. It all still comes down to the amount of pressure that is exerted on the exducer.