Drive Pressure, I don't get it!

[MAXLLY]

ok, the charger is pushing 40 PSI (pressure between the compressor and the intake valve?) and my drive pressure (pressure between the exhaust valve and the turbine?) is 80 lbs.

Whats wrong? When is it wrong? How do we make it better? Can it be to low? say 40 on each side? It relative to EGT's right?

Whats the deal, i have read its a great tuning tool but i don't get how to use it or understand it.

BTW- I don't have a VVT.

[Diesel Tech]

A properly sized charger will make more boost pressure than drive pressure, that is called positive crossover when you get it that way. If your making 40 psi boost and 42 psi drive pressure and then you lower the drive pressure to say 38 psi you will pickup about 50 Hp and get lower EGTS, so yes that is what you want to do.

Now the downside to getting things like this is slow turbo response and most will not live with it for a street application. To give you an idea our race package makes 50 - 51 psi with 45 psi drive pressure for Bonneville.

[RickDLance]

Where is your gauge hooked up at? Does it spool good? What size are your up pipes?

[MAXLLY]

My drive pressure could be on either exhaust manifold, currently its in the pass side exhaust between the last and second to last cylinder. I have 1 3/4" up pipes IIRC maybe 2" crap i forget it was a year ago when i built them.

I want to learn about this for tuning with or without nitrous, i haven't driven the truck down the street yet. But i need to know what its telling me.

If my boost is 40 and my drive is 80 this means i have excess fuel or a turbine thats to tight or pipes to small for the operating range i am driving in?

Combo of different things?

[MAXLLY]

A properly sized charger will make more boost pressure than drive pressure, that is called positive crossover when you get it that way. If your making 40 psi boost and 42 psi drive pressure and then you lower the drive pressure to say 38 psi you will pickup about 50 Hp and get lower EGTS, so yes that is what you want to do.

Now the downside to getting things like this is slow turbo response and most will not live with it for a street application. To give you an idea our race package makes 50 - 51 psi with 45 psi drive pressure for Bonneville.

This equates in my mind as, "get a bigger exhaust housing" and it'll have lower drive pressure but it'll spool like poo unti i hit a higher RPM.

Can i think a 1 to 1 ratio is good? Is it good for a street/strip application? Or do we need some drive pressure for spooling on the street?

[RickDLance]

The easiest way to check the size of the pipes is to move your drive pressure gauge to the pedestal and see if your drive pressure changes. I doubt it will be the pipes, but eliminating them will help you find the culprit.

[MAXLLY]

good idea (moving the sensor).

Maybe i need to be forward with the question.

How much is to much drive pressure?

[Nick]

James,

How did you test drive pressure? What load did you have on the motor? RPM/gear ect.

Also, What size is the exhaust size on your turbo? Wheel and housing? You can pm me if you don't want to post it public. I've done some DP testing with the S400.

Nick

[NWTDIESEL]

James,

How did you test drive pressure? What load did you have on the motor? RPM/gear ect.

Also, What size is the exhaust size on your turbo? Wheel and housing? You can pm me if you don't want to post it public. I've done some DP testing with the S400.

Nick

What is the proper load, rpm and gear?

[Nick]

There's isn't a 'proper' load necessarily, but different loading and RPM will affect drive pressure significantly. I know he just got the truck running, so I was curious under what conditions he was getting 80psi.

[Diesel Tech]

Matching the turbine Wheel and turbine Housing is all you can do to adjust drive pressure and yes the closer you can get to positive crossover the better the power will be and the slower the turbo will spool. You have to find the spot that best matches your combination. It maybe that less overall boost gets you a better match between drive pressure and boost. Airflow through the motor is your limit. Since boost is nothing more than the extra air the motor cannot use at any given time It lets you know where the compressor is working. The trick is to figure the same thing out for the turbine side. Since there are limits on what you can buy you either live with it or get into custom machining of the turbine wheel/housing.

[MAXLLY]

I wish i knew how to lump all the responses together...

Yes, the truck is running however i do not have 80lbs of DP in fact i do not have 40 lbs of boost. I am trying to understand the impacts of DP and how to utilize it as a tuning tool/information. What does DP tell me when tuning?

i have no problem telling anybody what i have or sending pictures, IMO its the tunes that make the truck run hard, not the parts.

As for parts: I have 2 chargers and 3 housings, S475 and S480 (75/80mm), I have 3 exhaust housings 90/110/132. I have 3 stages of N2O no stages are being considered at this point.

Currently on the truck is a S475 with a 90 exh housing.

[MAXLLY]

Matching the turbine Wheel and turbine Housing is all you can do to adjust drive pressure and yes the closer you can get to positive crossover the better the power will be and the slower the turbo will spool. You have to find the spot that best matches your combination. It maybe that less overall boost gets you a better match between drive pressure and boost. Airflow through the motor is your limit. Since boost is nothing more than the extra air the motor cannot use at any given time It lets you know where the compressor is working. The trick is to figure the same thing out for the turbine side. Since there are limits on what you can buy you either live with it or get into custom machining of the turbine wheel/housing.

If i don't understand DP then I'd never know if i needed a custom charger?

This trick is drive pressure monitoring against boost, right?

Whats your name, please?

BTW- Gents thanks for helping me out. Really i need it like a 6 year old, what is drive pressure telling me when compared to boost?

[Diesel Tech]

Remember what pressure is............ The amount of air that cannot be used, so pressure builds. In the exhaust side of things it's the amount of air that cannot get out and is being restricted by the turbine wheel and housing. The only way tuning will help this is to lower fuel delivery so not as much air is feed to the compressor side. Less fuel gets less boost so less air is coming out of the motor. It's really that simple, so if your after power you can only work with housing size and wheel size/shape. For a NOS application a wastegate can be used to bleed extra airflow off around the turbine to help control drive pressure while the NOS is adding extra air into the engine. You need to remember, what goes into the engine has to come out and any tuning you do to limit the air coming in will drop power right with it.

Steve Cole is the name.

[MAXLLY]

I got it now... Hey Steve. :Handshake:

let me regurgitate it back real simple like to be sure.

Comp wheel spins and forces air into the motor (boost/PSI). After its ignited and pushed out the exhaust its energy is used to move the turbine wheel side. When the pressure on the turbine side exceeds X amount its no longer productive and holds the motor/production back. We measure this as drive pressure. The amount of DP is not a fixed number. Every set up is different based on the motors abilty to spit/flow it out and the turbine side of the charger.

The only way to increase past this point (without VVT) is change the exhaust housing, trade off is increasing the housing size decreses low RPM response (lag). So we can make it breath easier but we get laggy.

I wanna ask what my DP will be on my charger but can't! Don't know how much boost it will make or how its gonna run.

Need a dyno and a map of my charger to figure out where she wants to work.

OT- how large a wastegate do we use? 40, 50mm? everything i found is for gassers and utilize springs for 10-15 lbs (example) we should be looking around 40-60lbs?

Took little time for the light to go on... but i got here! bringing up the rear.

[NWTDIESEL]

There's isn't a 'proper' load necessarily, but different loading and RPM will affect drive pressure significantly. I know he just got the truck running, so I was curious under what conditions he was getting 80psi.

Oh, i was just curious on any info you had to help me check my DP out......

[James Hess]

Im sure BD sells wastegates for our trucks(i think) along with Banks(dont flame me for saying that name please i know alot of people hate them but they actually make some good products) and im sure there are others.

By the way this is a good discussion its kind of clearing up some things for me.

[Mike]

Remember what pressure is............ The amount of air that cannot be used, so pressure builds. In the exhaust side of things it's the amount of air that cannot get out and is being restricted by the turbine wheel and housing. The only way tuning will help this is to lower fuel delivery so not as much air is feed to the compressor side. Less fuel gets less boost so less air is coming out of the motor. It's really that simple, so if your after power you can only work with housing size and wheel size/shape. For a NOS application a wastegate can be used to bleed extra airflow off around the turbine to help control drive pressure while the NOS is adding extra air into the engine. You need to remember, what goes into the engine has to come out and any tuning you do to limit the air coming in will drop power right with it.

Steve Cole is the name.


Howdy Steve Cole ( and I hope you are who you say you are? ).

I'm confused here.

I am thinking energy is stored with pressure and can be used? Pressure is required for air movement or is it? Does a larger A/R housing change the efficiency of a charger? Can a wastegate be added to an exhaust system to help with back pressure ( small a/r housing ) without too much effect on the compressors charging ability? And I'm with you on the tuning requirements based on the specific charger.

Thanks Steve.

[Mike]

I got it now... Hey Steve. :Handshake:

let me regurgitate it back real simple like to be sure.

Comp wheel spins and forces air into the motor (boost/PSI). After its ignited and pushed out the exhaust its energy is used to move the turbine wheel side. When the pressure on the turbine side exceeds X amount its no longer productive and holds the motor/production back. We measure this as drive pressure. The amount of DP is not a fixed number. Every set up is different based on the motors abilty to spit/flow it out and the turbine side of the charger.

The only way to increase past this point (without VVT) is change the exhaust housing, trade off is increasing the housing size decreses low RPM response (lag). So we can make it breath easier but we get laggy.

I wanna ask what my DP will be on my charger but can't! Don't know how much boost it will make or how its gonna run.

Need a dyno and a map of my charger to figure out where she wants to work.

OT- how large a wastegate do we use? 40, 50mm? everything i found is for gassers and utilize springs for 10-15 lbs (example) we should be looking around 40-60lbs?

Took little time for the light to go on... but i got here! bringing up the rear.

Mick used a 60mm blow off valve with some kind of modification done by a local man to work with 60 or 70 pounds. I remember that bov being electronic but just can't remember the specifics. I'll ask him if I can think about it while on the next call. I'll tell you though, it was way too big even for the amount of air through his engine. A blow off at anytime would cause a problem with lag just after the forth shift and the converter locking up. So, smaller would be better, maybe a 40mm or even less. We found it doesn't take much blow off because of the volume passed with increased pressure. But, if 10 pounds of back pressure is all that is required then a larger bov may be in order.

[MAXLLY]

Mick used a 60mm blow off valve with some kind of modification done by a local man to work with 60 or 70 pounds. I remember that bov being electronic but just can't remember the specifics. I'll ask him if I can think about it while on the next call. I'll tell you though, it was way too big even for the amount of air through his engine. A blow off at anytime would cause a problem with lag just after the forth shift and the converter locking up. So, smaller would be better, maybe a 40mm or even less. We found it doesn't take much blow off because of the volume passed with increased pressure. But, if 10 pounds of back pressure is all that is required then a larger bov may be in order.

THX Mike.

just saw you called, let me get out the door and i'll call you back in transit.

[Diesel Tech]

Howdy Steve Cole ( and I hope you are who you say you are? ).

I'm confused here.

I am thinking energy is stored with pressure and can be used? Pressure is required for air movement or is it? Does a larger A/R housing change the efficiency of a charger? Can a wastegate be added to an exhaust system to help with back pressure ( small a/r housing ) without too much effect on the compressors charging ability? And I'm with you on the tuning requirements based on the specific charger.

Thanks Steve.

Well let's start with, as with everything you do it all about a balance of what your looking for. If you lower drive pressure too low, the turbine will not see enough energy to get up to speed, so slow turbo response occurs. Yes, there is energy stored in the pressure but that same energy comes into play when the engine tries to get the burned mixture out of the cylinder. Let's think of what happens when the valves are in the overlap period of time. What this means is both vales are open at the same time. If you have 40 psi in the intake system and you have 80 psi drive pressure which way is the air in the cylinder going to go?

It is going to push the exhaust back up through the intake valve and into the intake until the pressure is the same between the cylinder and the intake manifold. At that point air flow will stop until the intake air pressure becomes greater than the cylinder pressure. Then what happens? All that burn mixture you shoved back up into the intake comes back into the cylinder followed by the new fresh air until the intake valve closes. So what you have is called internal EGR and all motors have it! When you start running the boost up you make it worse due to the turbine restriction in the exhaust side.

So now we need to balance how much EGR the motor can take Vs the power we are trying to make and that is limited by the turbocharger you pick for your application and the camshaft design you have in the motor. Adding a wastegate will let exhaust pressure bypass the turbine wheel but you have also tossed the energy stored in it go to waste. You need to be careful as many wastegates will hold closed then pop open to far all at once. This causes a drop in pressure too far and the turbine slows down. So the trick is to get a wastegate that opens smoothly and at a rate that matches what you need. It's easier said than done but with some time and experimenting you can balance things up.

[James Hess]

NOW im learning something :thumb: :thumb:

[Mike]

Well let's start with, as with everything you do it all about a balance of what your looking for. If you lower drive pressure too low, the turbine will not see enough energy to get up to speed, so slow turbo response occurs. Yes, there is energy stored in the pressure but that same energy comes into play when the engine tries to get the burned mixture out of the cylinder. Let's think of what happens when the valves are in the overlap period of time. What this means is both vales are open at the same time. If you have 40 psi in the intake system and you have 80 psi drive pressure which way is the air in the cylinder going to go?

It is going to push the exhaust back up through the intake valve and into the intake until the pressure is the same between the cylinder and the intake manifold. At that point air flow will stop until the intake air pressure becomes greater than the cylinder pressure. Then what happens? All that burn mixture you shoved back up into the intake comes back into the cylinder followed by the new fresh air until the intake valve closes. So what you have is called internal EGR and all motors have it! When you start running the boost up you make it worse due to the turbine restriction in the exhaust side.

So now we need to balance how much EGR the motor can take Vs the power we are trying to make and that is limited by the turbocharger you pick for your application and the camshaft design you have in the motor. Adding a wastegate will let exhaust pressure bypass the turbine wheel but you have also tossed the energy stored in it go to waste. You need to be careful as many wastegates will hold closed then pop open to far all at once. This causes a drop in pressure too far and the turbine slows down. So the trick is to get a wastegate that opens smoothly and at a rate that matches what you need. It's easier said than done but with some time and experimenting you can balance things up.

Thanks Steve. A post or two up explains my experience with wastegates and another thread also goes into boost pressure I think the thread is named " Let's talk Turbo's ".