drive pressure

cm cows

cm cows

Here by the owl
Joined
Oct 10, 2006
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can some one shed alittle light on exactly what it is. i got an idea but not a 100% understanding.

Thanks
 
Basically it is the Pressure "driving" your turbo(s) It would be measured right off the exhaust manifold.

or how much backpressure your turbo(s) are creating for your engine
 
Its the amount of pressure between the engine and the turbo. From what little I understand, the problem is that when drive pressure is significantly higher than boost pressure, when the exhaust valve opens some of the exhaust is fed back into the cylinder causing a loss of power.
 
cm cows said:
how is the problem fixed
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Look for a turbo charger with as close to a 1 to 1 ratio. Drive pressure to boost pressure. The more efficient the turbo is, the less back pressure you will have. I am not a big fan of a larger waste gate. This is truly what it describes, "WASTE". Check out the ATS trubos. They don't have waste gates at all. This is because they are designed with a true 1 to 1 ratio. :cheer:
 
Wastgates do not lower drive pressure, they just keep it from going "too" much higher.

Drive pressure (turbine inlet pressure) is a necessary evil.

Even with a wastgate limiting boost presure to a constant, drive pressure still rises with RPM. 1:1 boost to drive is optimal, 1:1.2 is acceptable. Anything higher sucks

The only way to actually drop drive pressure is by changing the turbine and turbine housing geometry.

Wastgates are the best way to limit boost. It allows a smaller turbine section thus allowing better performance at lower power levels while still allowing the turbo to "swallow" enough energy to run well at higher power levels.
 
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Diesel Freak said:
Wastgates do not lower drive pressure, they just keep it from going "too" much higher.

Drive pressure (turbine inlet pressure) is a necessary evil.

Even with a wastgate limiting boost presure to a constant, drive pressure still rises with RPM. 1:1 boost to drive is optimal, 1:1.2 is acceptable. Anything higher sucks

The only way to actually drop drive pressure is by changing the turbine and turbine housing geometry.

Wastgates are the best way to limit boost. It allows a smaller turbine section thus allowing better performance at lower power levels while still allowing the turbo to "swallow" enough energy to run well at higher power levels.
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So if a turbo was not wastegated and had high drive pressures at WOT and you put a wastegated housing on it same size it would Lower the drive pressure
 
Yes it would lower drive pressure....

however, it would also lower boost pressure.

If the turbo has high drive pressure at the boost pressure you WANT to run it at, a wastgate will not solve the problem.

It's give and take. To get decent driveability/useability when running around town/using the truck as a truck at lower RPM (<1600) some sacrifices will have to be made at the top end (>3000).

This is very obvious when using a single turbo.

Not as much with twins, but the principle still applies.
 
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Ph4tty said:
Its the amount of pressure between the engine and the turbo. From what little I understand, the problem is that when drive pressure is significantly higher than boost pressure, when the exhaust valve opens some of the exhaust is fed back into the cylinder causing a loss of power.
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Not sure that the ISB has any valve overlap?

The problem really with a higher drive pressure has to do allot with pumping losses. Keep in mind that any internal combustion engine at its very soul is an air pump. This means that it simply moves air from point A to point B. It takes a significant amount of Hp just to rotate the engine with no fuel added at all. As the rpm increases the neccessary Hp increases at a much faster rate.

Basically the the amount of rotation Hp required is determined by the bore / stroke as well as the compression ratio, and of course the rpm. But in essence, you are compressing the air then decompressing the same air. You are sucking it in at one pressure (inlet) and exhausting at another pressure (outlet).

Now ask "The inlet and outlet pressures are equal right?" because the engine is on planet earth lets say at sea level. So one would think that it requires zero Hp to rotate the engine. This is because the Hp needed to compress the air is returned when you decompress the air, as the two forces are equal. All that being true it still requires significant Hp just to rotate the engine. The key thing to remember is that it is an air pump, moving air from point A to point B.

This rotational Hp is often called pumping losses. When the engine burns fuel to make a usable shaft Hp it must also make the rotational Hp required to equal the pumping losses. Basically the shaft Hp is the gross Hp minus the rotational Hp (pumping losses) minus the exhaust heat rejection Hp minus the coolant heat rejection Hp minus the friction Hp losses minus the mechanical Hp losses (valve train) minus the oil pump Hp requirements etc etc etc....

The actual amount of fuel burned is more directly related to gross Hp, not shaft Hp. This is the whole foundation for efficientcy of the engine.

Still speaking from an air oump stand point and thinking about raising or lowering the inlet / outlet pressures. Say if the engine were in a pressurized room that could raise and lower the pressure. The pumping losses would be the same for all pressures. This is true as long as the inlet and exhaust pressures were the same. This would also mean that the pumping losses were at their minimum. But if the outlet pressure was greater than the inlet pressure then the pumping losses would increase. This means the rotational Hp required to overcome the pumping losses also goes up.

This is all true for a turbocharged engine. Where the inlet pressure is boost and the outlet pressure is the turbine drive pressure. If inlet (boost) and outlet (drive) pressures are equal then pumping losses are the same for all pressures. If this is the case, then we are getting boost for free and the engines pumping losses are minimum. (As the "exhaust heat rejection Hp" of the engines gross Hp is powering the turbo)

If the outlet (drive) pressure is higher than the inlet (boost) pressure then the pumping losses will increase.

So lets say that the engine is already running at a maximum fuel rate or the maximum gross Hp. With increased pumping losses then the shaft Hp will decrease, because it now requires more Hp to rotate the engine.

On the flip side for the engine to make the same shaft Hp the gross Hp must increase. The only way to increase the gross Hp is to increase the fuel rate. The increased fuel rate drops the air fuel ratio which causes the EGT to increase.

In just about any case you end up with higher EGT and / or lower shaft Hp and a less efficient engine.

It is a vicous circle. *bdh*

Jim
 
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Jim interesting write up, you must work for a fluid power company?

High BP also holds the exhaust valves open and increases the chances of kissing a valve with the top of the piston, if runnning twins you should always put heavier spring rate springs on the valves.
 
Ph4tty said:
Its the amount of pressure between the engine and the turbo. From what little I understand, the problem is that when drive pressure is significantly higher than boost pressure, when the exhaust valve opens some of the exhaust is fed back into the cylinder causing a loss of power.
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these cams really don't have any overlap I don't believe, so reversion isn't huge, but I have seen evidence of reversion on a tear-down, but that truck had an aftermarket cam.

high backpressure = loss of power, that's all there is to it

contrary to popular myth, high drive pressure doesn't kill head gaskets
 
Forrest Nearing said:
contrary to popular myth, high drive pressure doesn't kill head gaskets
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so what exactly blows the gasket? is it high cylinder pressure or is it high boost pressure
 
cm cows said:
so what exactly blows the gasket? is it high cylinder pressure or is it high boost pressure
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cylinder pressure

high boost increases cylinder pressure. advanced timing increases cylinder pressure.

drive pressure IS a measurement of cylinder pressure, but it's a LOT lower than when that diesel starts to ignite BTDC!!!!!!
 
For measuring drive pressure Im guessing a boost gauge with metal tubing?
 
Noreaster said:
For measuring drive pressure Im guessing a boost gauge with metal tubing?
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I used copper up to about the brake booster, then used a union to nylon and ran the nylon into the cab. I did put a few coils on the copper coming off the manifold to dissipate heat and allow some flex (copper work hardens, keep an eye on it)
 
I can say that the heavy fueling of a smarty can be a mofo for drive pressure. I just ran the copper tubing in the passenger side window $.02 :hehe:
Forrest's advice might be better though LOL
 
Ph4tty said:
I can say that the heavy fueling of a smarty can be a mofo for drive pressure. I just ran the copper tubing in the passenger side window $.02 :hehe:
Forrest's advice might be better though LOL
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as long as you didn't plumb the copper into your mouth, I don't see a problem w/ that for dialing in a combo :hehe:
 
Forrest Nearing said:
I used copper up to about the brake booster, then used a union to nylon and ran the nylon into the cab. I did put a few coils on the copper coming off the manifold to dissipate heat and allow some flex (copper work hardens, keep an eye on it)
Click to expand...

Yeah same hear, I got mine from my partner C-hawk with a 100lb boost gauge. But I ended up cloging up the nylon line with soot gunk, and I couldn't even blow it out with a air nozzle.
 
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