big turbos and little turbos: real or myth?
 

i know u mods have prob heard this come up a million times but i'm gonna ask it anyways cause i cant find a real answer anywhere.. i've looked:

how does a (generally speaking) bigger turbo put more air in a rotary engine compared with the same engine using a smaller turbo at the same psi and rpm?

two identical rotary engines displace the same rate of volume at identical rpm, so why should 10psi from larger compressor make more power than a small compressor at 10psi and the same intake manifold temperature?

is it because in real life bigger compressors never heat the intake air as much as smaller compressors at the same psi? or is it because the engine has to give power to higher back pressure of smaller turbine of a smaller turbo? even if so, how could a smaller turbine account for that much back pressure and power loss?

i know it's a complicated question, and i understand why larger turbos can sustain high psi at high rpm better than small turbos and that isn't the question. compressor maps calculate the efficiency (and yes i can read compressor and turbine maps) but they can't explain why there's a difference in efficiency.


the reason i think this is relevant to this forum is because i'm going to build a single turbo setup on my fd... and i want a certain power figure at the lowest possible intake manifold pressure. for example i would rather make 350rwhp on 11psi with a t04 and sacrifice 500 rpm boost threshold than make 350rwhp on a gt35 at a higher int man psi and work the turbo and engine harder...

but i really don't know if a for example if a larger turbo like a t04r just isn't meant to run lower int man psi... like all other things control (same), will 350@7k rpm@11psi get much worse gas mileage than 350@7k rpm@15psi??

see, i'm not power hungry (i know what muscle feels like), i'm longevity/reliability/endurance hungry right now for my set up.

i'm reading a forced induction textbook right now and it hasn't even answered my question yet so i'm asking you guys.

lol i'd be lucky if anyone was willing to answer this monster but i'm posting it anyways. thanks for your input!

Larger turbines rob less energy, however sacrifice it for a later spooling

Larger compressors are more efficient at a higher pressure ratio than a smaller ones (generally).

I see it as the bigger the compressor will shove more volume at the same pressure. vs smaller turbo having less volume at same pressure.. its not just pressure, its the volume with the pressure... thats where the power is...

The larger turbo flows more air at a given pressure then a smaller turbo at the same pressure. It just moves more air. Kinda like an air compressor with a 1 in hose will flow more air than an air compressor with a 1/4 in hose.

But not really, Intake manifold PSI is dependant on the motors ability to swallow the air. While a larger compressor will flow more air at a given turbo RPM a stock port 13b can only swallow so much of that air before the PSI climbs.

Ive always understood the problem as a combination of exhaust back pressure being lower on the larger turbo and the compressor being more efficient. Putting yourself in a better part of the compressor map and letting the motor breath the exhaust out better. Although i have wondered the same question as the OP because there are some scenarios that you wouldnt think should be as different as they are, ie a 60-1 and a t04r both with the same a/r exhaust housing and wheel. substancial power differences at boost levels that are in the efficiency range. About all i can come up with that is thr t04r is turning slower and lets more exhaust out the wastegate.

Good question, check out this thread.

https://www.rx7club.com/3rd-gen-archives-73/why-bigger-turbos-make-more-hp-same-psi-645551/

you really need to understand how compressor maps work for this to make sense.

Follow CarbonR1's links,(there will be two) it answers what your after.

I had the same question.. with engine characteristics, psi, rpm all fixed, what is the difference in turbo airflow. Airflow = power. You would think the airflow on the intake side would be fixed at a fixed psi, due to engine characteristics (porting, ect).

A lot of people in those other threads simply say the bigger turbo is higher efficiency.. well no kidding, of course its more efficient, its making more power at same rpm/boost. What you are asking, is how are bigger turbos more efficient at higher power levels..

It really all comes down to backpressure. A smaller turbo will have a smaller exhaust turbine, which limits airflow. but this makes them more efficient at lower power levels. This is why on hybrids, usually the exhaust turbine is clipped, to reduce this backpressure. If the turbine was kept like stock, the added airflow of the bigger compressor would be choked by the unchanged exhaust side, not gaining much in airflow overall

CarbonR1, thanks for the link! renns, thanks for the insight! the link led me to a reply post by forum user renns. the reply post answered my question:

i am a physics major and based off of thermo and mechanical theory i've learned the guys (i wont say who) saying air velocity at the intake port of the motor is higher at a certain psi than a small turbo making the same psi is not true.

the air velocity at the intake port depends on the engine. assuming intake temp and pressure are the same and exhaust back pressures are the same (and something else not the motor is driving the turbo) it doesnt matter what compressor is being used.
as long as the psi and temperature at the intake port are the same (and the compressor exducer is not right next to the port) the air velocity at the intake port depends on the motor and port size.

but renns answered my question hands down and i suspect this is the real reason for hp differences (assuming the intake plenum psi and temperature is the same on larger and smaller turbo environments):

in a rotary engine (and piston engine) not all exhaust gases leave the chamber between the exhaust port and intake port (or between exhaust valve lift and intake valve lift in piston)

so, if there is higher exhaust back pressure then there will remain more exhaust gases in this transition point and therefore more exhaust gas remains in the chamber when comes time to intake air from the intake port.

sooo, with higher exhaust back pressure the motor cannot intake as much air and the port air velocity is lower compared to a motor with lower exhaust back pressure.

sooooo, IF a larger compressor is more efficient (less energy needed) at pumping air and a larger turbine is more efficient at converting exhaust gases to rotating energy at high flow (high rpm) AND the motor has less exhaust back pressure at high rpm because of a more efficient turbine THEN air velocity at the intake port is higher at a certain psi and rpm (more powerful) than the same motor on a small turbo at the same psi and engine rpm!

a motor with a big turbo makes more power at 7k rpm and 10psi than a motor with a small turbo at 7k n 10psi!
BUT a motor with a small turbo will make positive psi with less air flow and therefore will have a more usable powerband at lower engine rpm!

all things considered, a small turbo motor will be more gas efficient at real life everyday driving but a large turbo motor will be more efficient at higher power and engine rpms.

Zero R is right that u need to know exactly what type of application you plan on using your motor for when choosing a turbo!

from my findings here are some examples (price is not considered):

Rx7 owners that are,

1. gas misers, everyday drivers, once in a while spirited drive to the lake:
use a "gorilla nut" RX6

2. short course road racer, stock vette n mustang hunter, highly illegal spirited drives to the lake more often, touge:
use GT35R or hybrid, 500R, TD06, T04s or hybrid

3. long course road racer (drag racing to each corner), street drag racer, pump gas dyno queen:
use T-88, GT40xx, T04R with big A/R

i knew before HOW to read a compressor map, but now i understand WHY certain areas are more efficient than others depending on pressure ratio and airflow!

thanks guys for the clarification and input! i think i have a much better idea now of what turbo i may be shopping for :)

You keep using the word "efficient". I do not think this word means what you think it means.

Efficiency, as regards to turbos, is how much the air is heated up as compared to how much heat it would create from compression alone. Or, another way, how much of the pressure increase is due to the air being denser, and how much of it is just due to being heated up because the turbo is beating the air.

10psi air at X degrees is the same no matter if it's from a crappy turbo and good intercooling or a good turbo and poor/no intercooling.

However the engine will have to work more on the exhaust side with the crappy turbo... so power potential is affected by how much the turbo restricts the exhaust.

Unfortunately, part of that exhaust restriction is what we use to spin the turbo up, so everything's a tradeoff.

The answer has been aswered numerous times now. I'll go ahead and reinforce it though. Bigger turbo compressors generaly have better efficiency verses a smaller one at the same boost pressure AND flow. So it heats the air up less thus having greater density charge. You also have the higher flowing turbine which creates less back pressure making the engine work less then if it were a smaller turbine spinning itself to death trying to keep up for the boost pressure you are asking from it were a larger turbo can pump those volumes and pressures more easily. The T04E 60 trim, 60-1, 62-1, GT35R, and T04R all make roughly the same power on a 13b at around 1 bar and below (10~14 psi). Its when you get above 1 bar the larger turbo's wake up and make more power then the smaller ones. You also generally need auxillery injection or higher octane to go over 1 bar to have a conservative tune (YES, there are MANY running more then 1 bar without problems running pump and no aux injection, but unless your an excellent tuner and have many safe guards I'd stict to 1 bar and less on pump).

~Mike................

i'd say, bigger turbo = more mass of air at the same pressure = higher velocity in runners = more gas/air mixture going into the combustion chamber during open ports.

volumetric efficiency = power

also, big turbo = less exhaust restriction

I like how you put that. I'm actually looking to upgrade to the "gorilla nut" just to simplify the engine bay and have the car be streetable in bumper to bumper driving. The side by side comparo started to make me think that maybe it is too small. TD06 is just a 20g so I figured it would be comparable to the RX7.