How fast does drive pressure come up?

[SRadke]

I'm waiting on my drive pressure gauge to arrive so for the time being here is my question... ...



How fast does drive pressure come up relative to boost pressure? Does it immediately start to climb when you drop the hammer as oppose to the boost which has a small delay to get the compressor(s) wound up?



I know it's an odd question but bear with me, it my ideas work out I'll share



-Scott

 

[lmills]

usually on a good set up the drive pressure should stay fairly close to boost pressure. In fact, the closer you can keep the boost to the drive pressure, the more efficient the system is.

 

[Graphic man]

there is an article in the TDR mag. about drive pressures

i didn't read the whole thing but is was good



Scott

 

[Hohn]

I'm waiting on my drive pressure gauge to arrive so for the time being here is my question... ...



How fast does drive pressure come up relative to boost pressure? Does it immediately start to climb when you drop the hammer as oppose to the boost which has a small delay to get the compressor(s) wound up?



I know it's an odd question but bear with me, it my ideas work out I'll share



-Scott

Yes, dp comes up immediately. DP must come before boost-- as there is no boost without dp.



Your testing should verify this. If not, I want to know! Usually you will see DP come up fast, then level off at a given level relative to a boost level.

 

[lmills]

most people find that drive pressure runs fairly equal to boost till the point when the exhaust housing becomes to restrictive. Then the drive pressures spike and climb higher than boost pressures. Not too many people will see drive pressures level out unless they are running low amounts of boost or have a large external wastegate plumbing the excess exhaust around the turbine housing.

 

[SRadke]

Yes, dp comes up immediately. DP must come before boost-- as there is no boost without dp.



Your testing should verify this. If not, I want to know! Usually you will see DP come up fast, then level off at a given level relative to a boost level.

This is exactly what I was thinking. You need drive pressure before you can begin to build boost... that is where the power comes from.



I'll have to do a little experimenting but I suspect as Lloyd sugests I should be able to hold drive pressures down a bit using my external wastegate. With the fuel I'm running right now I'm sure my big turbo is doing the bulk of the work having it wastegated back to 50 psi.



Thanks for everyones imput.



-Scott

 

[Vaughn MacKenzie]

Drive pressure ratios vary considerably between turbos, it's one of those things that a turbo map can tell you.



Yes turbos with restrictive exhaust housings will have a high drive pressure ratio especially if the wastegate is small, but exhaust housing size isn't the only thing. . . the compressor plays a part to. As soon as it starts going out of its efficiency map, it takes more energy to drive it compared to how much boost it kicks out, which of course increases drive pressure.



Turbos with good ratios at higher boost are your big ones that are laggy at low boost. Some of the best turbos have less drive pressure than boost generated as well along some portion of the entire boost range.



Vaughn

 

[Devan Manis]

Thank you SRadke you asked the same question i had, and hohn had the same theory as me!