Boost pressure vs. Drive pressure?

[ASTAR]

Can someone explain this to me? I know what boost pressure is but I have no idea what drive pressure is and I have been told that it is a lot more important.

 

[Jim Fulmer]

That is the pressure that is on the manifold while at boost, the higher the drive pressure the less efficent the turbo/s are. Like an HY-9 is 2-2. 5 psi per pound of boost, 35/12's are about 1. 5, 40-16's are a little better and most properly build Twins are 1 to 1, on Twins typically when they get worse than 1 to 1 it's time to limit boost or get a better setup. My old 35/18. 5-3B/26 was good till about 65 then it started going south fast, drive pressure started going well past 1 to 1 and intake temp started going up fast also.



Jim

 

[ASTAR]

Thanks Jim. How can I measure drive pressure? Is there a standard I should expect with my charger?

 

[SRadke]

Pull your pyro and put a length of copper tube in the hole. Run it out a ways to dissipate heat then run into the cab with a gauge. It should read very close to what your boost gauge reads.



-Scott

 

[RCone]

Take a 0-60psi boost pressure gauge, attached to some small copper tubing coiled (it will get hot) (enough to reach inside of the cab) and remove your pyro probe and install your tubing with the same size fitting there. That way you will see boost and drive pressure simultaneously.

 

[Jim Fulmer]

I for one would like to hear what you are running for drive pressure at said amount of boost, if you can increase boost and drive pressure not rise over the boost amount gained you are in the game, one drive goes up fast and boost doesn't your done. Jet Pilot made a comment that the power gain on his Steriod B-1 from 37 to 50psi was on 10 HP, now if you added a good intercooler into that it would change more I'd bet.



Jim

 

[Vaughn MacKenzie]

I'm glad to see more people thinking about drive pressures because this is one topic that many are not aware of or concerned with but should be (primarily those running a lot of fuel with stock turbos).



Vaughn

 

[Hohn]

Agreed. Drive pressures are very important, and I think they are one of the single best indicators of turbo selection/performance/packaging.



On a related note, consider a turbo like J. R. 's '55 variants. In some configurations, they will support 500hp with only 38-39psi of boost and EGTs under control (whatever that means).



The rule of thumb of 1psi-per-10hp is so far off from being close that it makes you wonder how it became a rule of thumb!



Heck, you can spin an hy-9 to 40psi, and I PROMISE that it won't support 400hp!



Maybe we should come up with new "rules of thumb"--

1) Drive pressure should be as low as possible for a given amount of boost.

2) Boost should be as low as possible for a given EGT.



By these measures, it appears that the number of "good" turbos available for our trucks is very very low. Twins really are the only option given what we're trying to accomplish.



JMHO

 

[tractorface]

Trying to learn

Are we striving for lowest drive pressure possible or a 1:1 ratio?



Does Drive pressure "stress" the head gasket? Is that the problem with it? Makes sense to me, just want to clarify



I was under the impression that twins put added "stress" on the head gasket etc. Sounds like Twins wastegated at say 35psi would put LESS stress on the motor than a single wastegated at 35.



If we want low drive pressure why not plumb a belt driven super charger on our system? (drive pressure free boost?)Could this be plumbed into a "twin" setup easier? Before I up my fuel and purchase a turbo I want to understand so I can do it right.



Would be great to see a compilation list of typical charger setups with boost and drive pressure numbers. Do the manufacturers print that sort of data. My brain is burning up AHHHHH :-laf