TTAWD

Exhaust backpressure plays a big roll in a turbo car’s performance,
Has any one took the time to monitor or record this compared to boost
RPM and power curve?

engineermike

Yes, quite a few of us have.

Smearin

Id like to see some numbers on the smaller turbos... like

70- 76 on 346... boost #'s from 7- 15psi

TTAWD

If there is a thread out there with some info that you know about
please post a link.

engineermike

Here's 1:

http://www.turbomustangs.com/smf/ind...?topic=63459.0

And there are plenty more out there if you search.

Mike

corvettetimmy

After looking at the mustang thread, It leads me to believe that turbos arent free horsepower like many people believe. I would bet that the hp loss due to the 35-40 psi of exhaust backpressure is comparable to the parasitic losses of a belt driven supercharger. I know that some variables like friction and rotating mass will cause the SC to have a higer loss but I dont think that a comparable turbo car will be getting that much "free power" as compared to a SC setup. How much extra horsepower does a turbo engine need to make to work against that 35-40 psi pushing against those pistons on the exhaust stroke?

Roadie

That is precisely why I say that the base STS system is very comparable to centrifugal superchargers.

The way I look at it, the only "free" horsepower you can get is from heat. With the centrifugal setup, all the heat energy coming out the exhaust is lost. In a front-mount turbo setup, that energy is used to help spool the turbine.

Wrapping the exhaust of an STS setup helps use some of that lost heat and has proven to make a lot of extra horsepower with nothing more than exhaust wrap.

koolaid_kid

iTrader: (21)

No, they can't be free horsepower. It takes power (energy) to spin the turbine blades. As noted above, the heat energy can also be used, making them slightly more efficient than a s/c. On the rear mount, wrapping is required to recover a percentage of the heat energy so the turbine can use it. But in a sense, they are both blowers; one is driven off the crankshaft, the other off the exhaust.

engineermike

I've gone through this exercise quite a few times. Basically, a poorly designed turbo system will create alot of backpressure, to the point of performing worse than a comparable centrifugal blower. However, a well-designed turbo system will have low backpressure and produce more power than a comparable blower.

I ran some quick calc's based purely on pumping losses, assuming a 4" bore, 3.5" stroke, and 6000 rpm.

50 psi backpressure = 133 hp
34 psi backpressure = 90 hp
18 psi backpressure - 48 hp

At somewhat equivalent power levels, a centrifugal blower will take 150 - 200 hp to drive. A well designed turbo can gain up to 150 hp over a supercharger at best. But, on a typical installation, a turbo will be worth only 20 - 50 hp over a blower (as was in my case).

Mike

02SSLE

iTrader: (26)

I checked my backpreasure on my old system at 15psi of boost I made the power below but little did I know I had 65 psi of backpreasure. That's why I am installing a complete new turbo system.

Smearin

Do you happen to have a pic of where and how you tapped and what you used to measure the back pressure?

engineermike

I just installed a 1/8" pipe bung just ahead of my turbine. I screwed a 6" long piece is pipe into it (to isolate the heat), then ran a small plastic tube to a 0-60 psi gauge.

Mike

02SSLE

iTrader: (26)

that's pretty much how I done mine.