Xcentric

Guys, what is the window of acceptable boost to backpressure ratio?

mwatson184

What turbo/exhaust housing? What ports? What power level?

Your question is a little too general. Give some more info and you will find some help.

FB II

5" downpipe to 4.5" back

aahhhaha, nah it's like watson said... it all depends on the setup.

MakoRacing

iTrader: (12)

Pfft! Real men run 5" dp to stock a stock cat and exhaust!

Ottoman

5" downpipe exiting below the passenger tire/fender area

Xcentric

How'd you know my setup? Ok, streetport 20psi boost. What would be a good
backpressure number? How do you know if you need a bigger a/r on
turbine without actually changing it?

mwatson184

It really depends on if you want faster spoolup, or higher HP. You don't know if you need bigger, you decide that you want bigger (unless it is ridiculously small of course).

OMGWTHDRIFTGUY

How the **** does it depend on the steup? He's talking aobut an ideal ratio. You change you setup to hit your goal.

Mist will say anything above 1.5:1 is a complete **** setup.

Xcentric

Finally, someone who gets it. Let me get this right. It is the boost
pressure vs manifold pre turbo, not post turbo[downpipe]right?

Turblown

iTrader: (12)

Yes, intake manifold pressure versus exhaust manifold pressure( pre turbo, post motor)

Back in the late 70s, and up to the late 80s when turbo's were allowed in F1, those cars ran near 1:1 and sometimes even less pressure on the exhaust side vs intake using T6 frame turbos. However those cars were turning in my opinion unstreetable rpms.

Ideal to me is the same ratio the F1 cars used. I think of it this way, you can't shove 10psi of **** in a 5 psi bag. However the closer you get to this ratio the more unstreetable the car becomes, the powerband becomes smaller and smaller.

BMike

It's always been my impression that anything over 2:1 is sacrificing power, but thats also a piston engine guideline. It will somewhat depend on how high you're willing to rev the car out to, the higher you go the lower the ratio can be and still make usable power.

Howard Coleman

iTrader: (6)

i have 6500 miles of logged exhaust manifold pressure... EMP.

i run a Power FC w a Datalogit so i have potentially 400 discrete cells (rpm & boost) where i have pre turbo exhaust manifold psi readings... high low average. at the same time i have turbo boost readings in each of the cells to compare w EMP.

i run two TO4s. each turbo has a separate manifold and downpipe. so i am exploring relatively new areas.

if you don't monitor EMP you are missing alot of important data.

by switching hotside components i lowered my EMP/BOOST ratio from more than 3 to 1 to 1.2 to one.

lots of good things happen.

the rotary is especially in need of low EMP v a piston engine.

part of the answer is hot v cold wheel sizing. the other is hot housing A/R.

the rotary needs a cold to hot wheel sizing near 1 to 1.

i always look at wheels in the following manner:

using the R85, or TEC HT300 as an example..

cold wheel is 3.44 X 2.42
hot wheel is 3.14 X 2.789

solve for the area of each diminsion....

cold wheel area is 9.29 X 4.59
hot wheel area is 7.74 X 6.109

take the average for each above wheel

cold wheel average area 6.94 sq inches
hot wheel average area 6.92 sq inches

that's a really nice ( 1-1) ratio for the rotary. combined w a large-ish A/R hot housing and you will generate low EMP.

the rotary will generate approx 300 degrees more EGT than a piston engine apples to apples and when you combine a restrictive EMP really bad things happen. firstly, alot of exhaust ends up polluting the next intake charge. the exhaust is hot and it warps apex seals and promotes detonation. high EMP also robs energy that drives the turbine. EGTs skyrocket.

i learned this in 05 running the turbine package recommended by my turbo vendor. i split both hot housings and was seeing 60 pounds of EMP at one bar.

my cold/hot ratio wasn't that out of wack ( 1.15 to 1) but my housings were too small. again, my setup is unique and there is a ton of info on the single setup but it is important to run the right setup..... especially on a rotary.

howard coleman

Xcentric

Wow,lots of great info. This confirms my fears that my detonation on a not
so lean tune could be from too small hotside. I am running dual egts, and
am going to do a pressure reading through those couplings. Thanks for
the great info. guys.

JZ 97 SS 1500

Ideal is 1:1, but most common is 2:1. Will depend mostly on the turbine wheel size, trim, pitch and A/R. Good size downpipe is also a must. Downpipe rule is turbine inducer X 1.25.