Understanding compound boost numbers

[thunder550]

I've been a little confused about compound boost numbers and contribution by each turbo...I think I'm starting to understand it more but wanted to check with you guys too.

I'm seeing a max of 60 psi compounded, and only about 18 psi on my low pressure S475. I was initially concerned that my boost gauge was messed up, didn't see how it was possible for the stock turbo to add the other 42 psi (saw a max of 35 psi out of it before the twins). Gauge tested good, which got me to thinking about pressure ratios rather that straight PSI numbers.

If I am feeding the stock turbo with 18psi from the S475, that would be about 32 psi absolute pressure (18+14). My 60 psi total pressure would be 74 psi absolute (60+14). So if I take 74/32 I get about 2.3:1 pressure ratio for the stock turbo. For the S475, it's being fed 14 and producing 32, so that would be 32/14 = 2.3:1 pressure ratio as well.

Am I thinking about this the right way? That seems reasonable to me, much more so than imagining the stock turbo making 42 psi by itself.I couldn't quite wrap my head around that one.

 

[countrycorey]

Yes sir, that sounds right to me


Corey

 

[Chevy1925]

Yup sounds right! Now plot it on the compressor maps and see how they look

 

[minerigger]

Holy head scratcher. I'm confused as f

 

[PureHybrid]

Holy head scratcher. I'm confused as f

He is dividing the outlet and inlet pressures, to find the ratio.

If he takes those number and corresponds them to a compressor map, he can find out where in the efficiency range its in. And depending on how his s475 is set up, might be able to push it harder.

And if his s475 still made 18 psi, but his valley turbo could push a 3:1, a total of close to 80psi is possible. Not ideal as both should be working equally like his already is.

 

[minerigger]

Gotcha

 

[thunder550]

Thanks for the confirmation. I haven't been able to find a compressor map for the stock turbo. Also, how do you guys estimate airflow with a maxed out maf sensor?

 

[chevyburnout1]

Thanks for the confirmation. I haven't been able to find a compressor map for the stock turbo. Also, how do you guys estimate airflow with a maxed out maf sensor?

I usually base it off of hp it puts down vs smoke output. Garrett's website has a couple good equations to roughly find your LB/MIN of airflow.

 

[Chevy1925]

I usually base it off of hp it puts down vs smoke output. Garrett's website has a couple good equations to roughly find your LB/MIN of airflow.

bout the closest i could get as well. 60psi total and 18 out the big charger doesnt sound too bad but id have to see a stock charger map for yours vs mine. i know at the 52-54 i hit with 17 out my big charger im right in a good zone and can push it to 60 without hurting it. im 90% sure im overpowering my stock gate though cause i have my boost reg shut off and thats all she does on the tow tune. ill hit 60 on the race tune but drive pressure is also much higher to get it there lol

 

[Dynospeed]

I know I'm bringing up a old post but I have been thinking about compounds a lot. We do a lot of turbo gas engines but I have very little diesel experience. Wouldn't it be better to have a good size wastegate bypassing the high pressure turbo at a point and feeding more exhaust energy into the larger low pressure. I'll just pull some boost numbers out of my butt for a example.
low pressure - 17psi high pressure - 40psi
vs
low pressure - 26psi high pressure - 28-30psi

Again I just pulled these #'s out of my ass. I'm ready to change my cp3 (I need a regulator & I have 290k miles on it) and I would like to do a turbo at the same time. I like the idea of compounds but I don't like the idea of making 55-60psi on the stock pistons, rods, and headgaskets when I can make the same power on 15psi less with a single. I don't tow over 12-14k lbs. Does anyone make a cast t4 pedestal with a wastegate port for the lb7?

 

[IdahoRob]

I've been a little confused about compound boost numbers and contribution by each turbo...I think I'm starting to understand it more but wanted to check with you guys too.

I'm seeing a max of 60 psi compounded, and only about 18 psi on my low pressure S475. I was initially concerned that my boost gauge was messed up, didn't see how it was possible for the stock turbo to add the other 42 psi (saw a max of 35 psi out of it before the twins). Gauge tested good, which got me to thinking about pressure ratios rather that straight PSI numbers.

If I am feeding the stock turbo with 18psi from the S475, that would be about 32 psi absolute pressure (18+14). My 60 psi total pressure would be 74 psi absolute (60+14). So if I take 74/32 I get about 2.3:1 pressure ratio for the stock turbo. For the S475, it's being fed 14 and producing 32, so that would be 32/14 = 2.3:1 pressure ratio as well.

Am I thinking about this the right way? That seems reasonable to me, much more so than imagining the stock turbo making 42 psi by itself.I couldn't quite wrap my head around that one.

Total boost is 60psi, low pressure turbo boost is 18psi.

Your overall P/R is 5.08
Primary (low pressure turbo) P/R is 2.22
Secondary (high pressure turbo) P/r is 2.28

Your high pressure turbo is making approx. 19 psi of boost (2.28 x 14.7) minus 14.7 = 18.8psi (all based on if baro is 14.7)

Equal P/R between the turbos is very good. I bet your drive pressure and egts are happy.

You might try to find a way to run the low pressure turbo a bit harder(tighter turbine housing maybe) for testing if looking for something to play with. If you don't want to run that kind of boost (over 60psi) the high pressure charger boost could be reduced a bit (waste gate or VVT adjustment)