Engine/Transmission (1998.5 - 2002) Max boost from Hx35 in twin set up

[KKunz]

From what I understand 30-35 psi of boost is the max a stock hx35 can handle then Why when the hx35 is used in a twin set up one can see upwards of 50psi of boost how is this not harmful to the hx35? Is it because the hx35 is not making all the pressure? What should the drive pressure be on a twin setup with hx35/12 and bht3b/26 making about 50psi of boost?

 

[fox]

I am not an expert at this but will give an answer!!!

35 psi is plenty for a HX35, after that it is heating air.

When a twin set-up reaches its settings ( say 25 for the top turbo) the wastegate on that turbo opens and bypasses to the bottom turbo. It then spools it to a higher number.

As I understand the drive pressure thing, you want the boost to be higher than drive pressure. Each turbo will have its own drive pressure.

 

[Devan Manis]

as i understand it turbo's compound the air pressure they are fed, so if the hx35 is fed 25lbs of air pressure from a larger turbo it then compounds that pressure to a pressure ratio of 2 for about 50psi total. or where ever you set the wastegate. Where if you only feed the hx35 atmospheric pressure of 14. 7 for a gauge pressure of 35, thats a 2. 38 pressure ratio, and if you had a compressor map a 2. 38 pressure ratio would probably be at its peak.

 

[WestTN]

The pressure differential from inlet to outlet is what is important to the turbo.

 

[NoSeeUm]

Probably more than you wanted to know, but I already typed most of this in another thread. Read through it if you are interested. Pay more attention to the part about Pressure Ratios for a change in altitude or compound turbo's. That part should answer your question.



The whole thread is here, its a good discussion.

PR means Pressure Ratio. PR is the way that compression pressure is described for an air compressor. Roughly equate Compressor Pressure to Boost Pressure.



PR formula:

PR = ( Inlet Pressure + Compressor Pressure ) / Inlet Pressure



Single Turbo:

For sea level PR = ( 14. 25 psia + 30 psia ) / 14. 25 psia = 3. 1

For 4000' PR = ( 12. 5 psia + 30 psia ) / 12. 5 psia = 3. 4



Compare the higher PR for the same pressure. That means the air will be hotter.



Compound turbos:

Primary PR = ( 14. 25 psia + 20 psia ) / 14. 25 psia = 2. 4

Secondary PR = ( 20 psia + 30 psia ) / 20 psia = 2. 5



So with twins the air temperature ends up being quite a bit cooler for the same boost pressure. Both compressors are running at the more efficient sections of their maps.

One thing I did not really bring up all that well is that PR roughly means turbo RPM. So at sea level and 35 psi the HX35 is at its maximum RPM.



For discussion assume that at 35 psi discharge pressure the HX25 is running at a 3. 0 PR. Also notice that as you go up in altitude the corresponding discharge pressure for the same PR drops. So as you increase in altitude, to reach the same 35 psi discharge pressure the PR must increase, Lets say to a value of 3. 4 for the PR. A higher PR means the turbo's RPM is even higher. So realistically, the maximum discharge pressure should be reduced from 35 psi for the HX35 as you increase in altitude.



This problem is solved by twins, because the secondary turbo (HX35) never sees atmospheric pressure. It only sees the discharge pressure of the primary turbo.



Jim