Turbo and back pressure

[sootygu]

What is the side affects of having too much turbo back pressure on a diesel. Can it cause long term problems.

I understand going to a small A/R will help with early spool up time but does this also create extra back pressure?

Am I right that back pressure can help with torque?

Just trying to understand the relation between the 2.

[Dzltec]

It will leave combustion residue in the chamber, not allowing for a full volume of air to enter before the next process. This will make egts hotter, can result in lag.

Yes it does, hence finding a happy medium between boost rise and ultimate power.

Not in a diesel.


Andy

[KiwiBacon]

The back pressure continually varies. It's at the highest when the turbo is cold and at it's lowest when the turbo is hot and the engine is turning the lowest revs necessary to get boost.

Turbine sizing is a compromise, smaller for more low end torque or bigger for more high end power. Unless you've got a VNT turbo which gives you the best of both worlds with only a small efficiency loss.

I'm running what most consider a turbo that's way too small (T25 with 0.49 A/R turbine on a 3.9L diesel). It can produce more boost than backpressure as long as the revs are below 2000 and the EGT's above 600C.

[coxy321]

What is the side affects of having too much turbo back pressure on a diesel. Can it cause long term problems.

I understand going to a small A/R will help with early spool up time but does this also create extra back pressure?

Am I right that back pressure can help with torque?

Just trying to understand the relation between the 2.

Are you talking about exhaust manifold to turbo, or from the turbo back through the exhaust pipe?

[sootygu]

What is the side affects of having too much turbo back pressure on a diesel. Can it cause long term problems.

I understand going to a small A/R will help with early spool up time but does this also create extra back pressure?

Am I right that back pressure can help with torque?

Just trying to understand the relation between the 2.

Are you talking about exhaust manifold to turbo, or from the turbo back through the exhaust pipe?

Not exhaust.

Cyl head to turbo.

[bogged]

What is the side affects of having too much turbo back pressure on a diesel. Can it cause long term problems.

I understand going to a small A/R will help with early spool up time but does this also create extra back pressure?

Am I right that back pressure can help with torque?

Just trying to understand the relation between the 2.

Are you talking about exhaust manifold to turbo, or from the turbo back through the exhaust pipe?

Not exhaust.

Cyl head to turbo.

this was the issue I believe with the first turbo tried on my truck.


Edit:
But it squealed like bitch when you sank the loud pedal..
This was due to too much pressure on the turbine side of the turbo Couldnt keep it as it was, so drove it for a week while we sorted out more bits.

[sootygu]

Just considering running a .48 A/R turbine housing.

Had this size on the 100 and was pleased with performance but there is conjecture re the back pressures with this size. So was wondering what the possible problems that may occur with an increase in back pressure.

[coxy321]

Just considering running a .48 A/R turbine housing.

Had this size on the 100 and was pleased with performance but there is conjecture re the back pressures with this size. So was wondering what the possible problems that may occur with an increase in back pressure.

Just at a guess, it will probably spool up at around 1000RPM, but will start to choke a bit over 3000RPM.

Probably not all bad if you dont ring your trucks neck...

[KiwiBacon]

Just considering running a .48 A/R turbine housing.

Had this size on the 100 and was pleased with performance but there is conjecture re the back pressures with this size. So was wondering what the possible problems that may occur with an increase in back pressure.

What is your current turbo and exhaust housing A/R?

Decrease in the rpm needed to deliver x amount of boost will be pretty much exactly the % difference in size between the two housings.

[guzzla]

Turbo and A/R choice will depend on engine size and the RPM it operates in. You want the turbo to be efficient across the entire rev range or you will greatly shorten the life of the turbo and possibly the engine.

Having a small A/R will give you boost lower in the RPM range but the turbo will be useless over 3000RPM or even earlier. (I think this has already been stated) Conversely, an oversize A/R will give you later boost but work perfectly at higher RPM. I know most diesels don't go much over this but by the same token you dont want to have the turbo working at its limit every time you do.

Turbo effeciency is parmount - the Garrett website explains this im much detail.

For maximum effeciency, economy and durability, you're much better off having a turbo properly suited to your engine and instead play with diff or transfer case ratios for improved acceleration or low speed boost for off road work.

[tweak'e]

J..... So was wondering what the possible problems that may occur with an increase in back pressure.

couple of things to add.....
forced induction motors hate back pressure. lower the better. but less back pressure = less boost, more back pressure = less power, bit of a trade off. the higher the pressure the more exhaust gas gets left in the cylinder for the next stroke. that removes some of the cooling so higher EGT's are expected.
a lot of it also depends on cam timing to.
some manufactures actually use back pressure + cam timing to increase the amount of exhaust gas left behind (otherwise know as internal EGR). they use that to reduce Nox just like a normal EGR system.

[Dzltec]

The pressure in the exhaust manifold, generally called turbine inlet pressure is anywhere from 1:1-2:1 of boost pressure developed.

The lower the better, but that will give lag, thats why garrett generally offer a couple of different a/r exhaust housings, these are a tuning device. There is a comprimise between how quickly boost rises and the maximum power made.



Andy

[coxy321]

J..... So was wondering what the possible problems that may occur with an increase in back pressure.

couple of things to add.....
forced induction motors hate back pressure. lower the better. but less back pressure = less boost, more back pressure = less power, bit of a trade off. the higher the pressure the more exhaust gas gets left in the cylinder for the next stroke. that removes some of the cooling so higher EGT's are expected.
a lot of it also depends on cam timing to.
some manufactures actually use back pressure + cam timing to increase the amount of exhaust gas left behind (otherwise know as internal EGR). they use that to reduce Nox just like a normal EGR system.

Thats in relation to the exhaust after the turbo - which is correct. Less back pressure is better for all turbocharged applications. Sooty is reffering to pre-turbo back pressure, which Andy has answered.

[KiwiBacon]

The pressure in the exhaust manifold, generally called turbine inlet pressure is anywhere from 1:1-2:1 of boost pressure developed.

I once fitted a really small turbine for an experiment.
Hit 4:1 under full throttle and full revs.

That's 60psi in the exhaust manifold for 15psi in the inlet manifold. Needless to say, it didn't go very well.

*edit* if anyone is googling for info, some people also call it "turbo drive pressure" */edit*

[tweak'e]

J..... So was wondering what the possible problems that may occur with an increase in back pressure.

couple of things to add.....
forced induction motors hate back pressure. lower the better. but less back pressure = less boost, more back pressure = less power, bit of a trade off. the higher the pressure the more exhaust gas gets left in the cylinder for the next stroke. that removes some of the cooling so higher EGT's are expected.
a lot of it also depends on cam timing to.
some manufactures actually use back pressure + cam timing to increase the amount of exhaust gas left behind (otherwise know as internal EGR). they use that to reduce Nox just like a normal EGR system.

Thats in relation to the exhaust after the turbo - which is correct. Less back pressure is better for all turbocharged applications. Sooty is reffering to pre-turbo back pressure, which Andy has answered.

i was actually referring to pre turbo pressure eg exhaust manifold pressure. i think you missed what i meant, i;m not the best at explaining things. i was talking about the motor outside of the turbo. ie also applies to supercharged motors. the lower the back pressure on the motor the better the motor performs. how well the turbo operates is different.
if you can get the same boost but lower back pressure you will get more power. for eg like using sequential turbo's.
thats proberly clear as mud

[coxy321]

J..... So was wondering what the possible problems that may occur with an increase in back pressure.

couple of things to add.....
forced induction motors hate back pressure. lower the better. but less back pressure = less boost, more back pressure = less power, bit of a trade off. the higher the pressure the more exhaust gas gets left in the cylinder for the next stroke. that removes some of the cooling so higher EGT's are expected.
a lot of it also depends on cam timing to.
some manufactures actually use back pressure + cam timing to increase the amount of exhaust gas left behind (otherwise know as internal EGR). they use that to reduce Nox just like a normal EGR system.

Thats in relation to the exhaust after the turbo - which is correct. Less back pressure is better for all turbocharged applications. Sooty is reffering to pre-turbo back pressure, which Andy has answered.

i was actually referring to pre turbo pressure eg exhaust manifold pressure. i think you missed what i meant, i;m not the best at explaining things. i was talking about the motor outside of the turbo. ie also applies to supercharged motors. the lower the back pressure on the motor the better the motor performs. how well the turbo operates is different.
if you can get the same boost but lower back pressure you will get more power. for eg like using sequential turbo's.
thats proberly clear as mud

Clear enough for me to understand where you were coming from though. That'll do donkey, that'll do.