Boost from bottom turbo in twins.

[RonA]

I know it depends on fueling, but in twins where does the bottom turbo start working. Is it after the wastegate opens on the top turbo at around 25 to 30 lbs. Or does boost with twins build faster than a single after it gets up over 10 to 15 lbs?(PDR towing twins).

RonA

 

[Huff N Puff]

Its Always Working

At about 10psi of boost at the intake I have about 1psi between the twins. At 65psi at the intake I have 30psi in the middle. It gets more efficient after the waste gate blows open. I have a solenoid operated pneumatic valve that holds the waste gate shut to 40psi then blows the waste gate open after that,to get the big turbo spinning. Merv

 

[Jim Fulmer]

It really depends on how it's setup, there are Twin setups that just have the turbo to turbo pipe, meaning everything that comes out of the motor has to go through the little turbo and that is fine if you have some huge top turbo with a huge housing... ... ... Sled Pulling Twins, on what we want for the street it will never push the big charger hard enough.



A well setup set of Twins will have some sort of by-pass wastegate so once the wastegate is open allot of the air by-passes to drive the big charger hard... ... you know, the one with the good map. As a wide guess I'd say that the bottom charger should be putting out 25-35 psi.



The only real data I personally have is from my current setup and with it the bottom charger makes 36-37 psi.



Jim

 

[Forrest Nearing]

Jim,



when your bottom charger is making 36-37psi, how much are you seeing at the manifold? I wanna figure out what the HX40 is doing.



thanks,



Forrest

 

[Jim Fulmer]

Ok, figure I'm bleeding off air too, the pops crack at 50 psi and are full open at 60 with shop air... ..... now the real story... . dyno!



On the road I have the pops set at 80 psi, now on the dyno I ran at 73 psi in Vegas and 75 at D/FW, One time and one time only I capped the pops here in OKC and made 80 psi on a 248, So that tells me at the track I should make much more as the B/B makes 32 psi on the dyno repeatedly..... here is were that theory stuff comes in. Theory says 95 at full load, the thing is by then I'll have a Custom Intercooler, fresh motor with a cam, you name it from there. I will do what my knowledge and budget will allow!



Forrest, let me know the number you crunch... . I'm curious!



Jim

 

[Forrest Nearing]

it's tough to say with the pops bleeding pressure, but I'll crunch a little and get back w/ ya

 

[Forrest Nearing]

Jim,



as near as I can figure, running 95psi of manifold pressure with 36psi on the inlet side of the HX40 is like running the HX40 at 38. 66psi by itself. at 37 on the inlet, it's like running at 37. 63...



so those should be your worst case scenarios for the top turbo with a theoretical 95psi max.



Forrest

 

[Jim Fulmer]

Time to cap it off at the track and see



Jim

 

[Forrest Nearing]

just don't stay in it long enough to cook anything... I want to talk w/ ya about taking your old motor off your hands



Forrest

 

[Strick-9]

Forrest,



I think your numbers are a bit high. When dealing with twin applications, you multiply the Pressure ratios of each turbo.



If the big turbo makes 36 psi of boost that is a pressure ratio of 3. 45.



For a total of 95 psi, the HX-40 would only need to operate at a 2. 15 pressure ratio which is equivalent to 17 psi of boost.



In reality there is a pretty substantial psi drop through the factory intercooler though. I've measured about a 10 psi drop through the intercooler at 90 psi, but even after accounting for the pressure drop the HX-40 is still operating at only 20 psi.



Hope that helps,

Chris

 

[Forrest Nearing]

Chris,



thanks for the help. I thought you took the inlet vs. the outlet on the small turbo, but the way you explained it as far as total pressure ratio over atmospheric helped a lot.



Forrest

 

[Cooker]

Originally posted by Strick-9

In reality there is a pretty substantial psi drop through the factory intercooler though. I've measured about a 10 psi drop through the intercooler at 90 psi,

Is this on a 12v or 24v cooler, or doesn't it matter.



Is the pressure drop a static number or does it increase as boost increases?



Thanks for the info,

Ryan

 

[RonA]

I have heard that there is a book available that gives the formulas for calculating boost on twins. Does anyone happen to know the name of it ?

RonA

 

[J.R. Adkins]

I have a truck making 40 psi on the low pressure charger. Wish we could get more.



Ron let me see if I can find that book if it is the same one.



J. R.

 

[RonA]

If you turbo guys would design a turbo that would spool and tow like my 35/40 hybrid and run 1200 deg egt at 40 to 50 lbs of boost like the towing twins I'd have more room in my engine compartment and more money in my bank account. So what's the holdup?

Is either of these the book you're thinking of. Turbochargers by Hugh MacInnes, or Maximum boost by Corky Bell?

RonA

 

[Jim Fulmer]

Who woke Chris up... good to see you post again!



Jim

 

[Forrest Nearing]

ok, I'll admit, I was kinda lying when I said I understood why I was wrong... I understood THAT I was wrong and why my method didn't make sense, but I didn't quite understand why Chris's method DID. but I got it now...



I was calculating by simply taking the 95psi manifold pressure and dividing it by the 36 to get a 2. 64 multiplier... then multiplying that by 14. 7 to get ~38. 8psi but that doesn't take into consideration the atmosphere on the other side of the boost gauge.



Chris said to take pressure ratio of the big turbo out of the total pressure ratio (here 7. 46:1)



if you subtract the 3. 45:1 pressure ratio you get roughly 4:1 which is around 45psi. that's clearly not right...



dividing the pressure ratio makes more sense as the top turbo is COMPOUNDING (multiplying) the pressure of the bottom turbo... so to take it out of the equation, you divide it out... 7. 46 total divided by 3. 45 of the bottom = 2. 16 a 2. 16:1 pressure ratio is only 17. 05psi doesn't seem like much!



but I finally sat down and thought it out, and the best way I was able to wrap my brain around it is as follows:



the gauge is telling us the pressure OVER atmospheric, right? so let's talk ACTUAL numbers... for the sake of simplicity, let's talk about 14. 7 as atmospheric.



ok, so when the gauge tells us the bottom is making 36, it's making 36 OVER the 14. 7 that's pushing on the other side of the gauge, so that's 50. 7psi absolute pressure.



now, when the gauge is reading 95psi, add 14. 7 for 109. 7psi absolute.



so the small turbo is multiplying that 50. 7psi 2. 16 times to yield 109. 7 there's that 2. 16 again! must be on to something here



well... at 0psi on the gauge, you're at 14. 7psi absolute... multiply that by 2. 16 times, and you've got 31. 75 absolute... subtract the 14. 7 for atmospheric, and you're left with 17. 05 psi.



damn, no wonder twins are so efficient!!!



I'm so used to calculating for a single, I had to really sit and think through the whole thing... OUCH! LOL!



thanks again Chris for taking the time to answer my questions on the phone earlier!



Forrest

 

[Forrest Nearing]

obviously this doesn't take into account pressure drops, temps, etc. but it gives a ballpark at least.



g'nite, I'm gonna go dream about twins,



Forrest

 

[FreeBMW]

good thread!!! lots of good info!Oo.

 

[Mark_Kendrick]

To get accurate calculations you would have to place a vacume gauge in front of the big turbo's inlet.



the air filter ads a restriction that reduces the input pressure to the large turbo. I would't be suprised to see 2-4 psi of vacume at WOT and 50+psi boost... the higher the boost the higher the vacume.



so if you use 13psi as your input pressure you will find that you move 'up the map' a ways.



FWIW,

Mark

Originally posted by Forrest Nearing

ok, I'll admit, I was kinda lying when I said I understood why I was wrong... I understood THAT I was wrong and why my method didn't make sense, but I didn't quite understand why Chris's method DID. but I got it now...



I was calculating by simply taking the 95psi manifold pressure and dividing it by the 36 to get a 2. 64 multiplier... then multiplying that by 14. 7 to get ~38. 8psi but that doesn't take into consideration the atmosphere on the other side of the boost gauge.



Chris said to take pressure ratio of the big turbo out of the total pressure ratio (here 7. 46:1)



if you subtract the 3. 45:1 pressure ratio you get roughly 4:1 which is around 45psi. that's clearly not right...



dividing the pressure ratio makes more sense as the top turbo is COMPOUNDING (multiplying) the pressure of the bottom turbo... so to take it out of the equation, you divide it out... 7. 46 total divided by 3. 45 of the bottom = 2. 16 a 2. 16:1 pressure ratio is only 17. 05psi doesn't seem like much!



but I finally sat down and thought it out, and the best way I was able to wrap my brain around it is as follows:



the gauge is telling us the pressure OVER atmospheric, right? so let's talk ACTUAL numbers... for the sake of simplicity, let's talk about 14. 7 as atmospheric.



ok, so when the gauge tells us the bottom is making 36, it's making 36 OVER the 14. 7 that's pushing on the other side of the gauge, so that's 50. 7psi absolute pressure.



now, when the gauge is reading 95psi, add 14. 7 for 109. 7psi absolute.



so the small turbo is multiplying that 50. 7psi 2. 16 times to yield 109. 7 there's that 2. 16 again! must be on to something here



well... at 0psi on the gauge, you're at 14. 7psi absolute... multiply that by 2. 16 times, and you've got 31. 75 absolute... subtract the 14. 7 for atmospheric, and you're left with 17. 05 psi.



damn, no wonder twins are so efficient!!!



I'm so used to calculating for a single, I had to really sit and think through the whole thing... OUCH! LOL!



thanks again Chris for taking the time to answer my questions on the phone earlier!



Forrest