Back pressure and exhaust techie Qs

[AnArKey]

Anyone know if there is some direct coorelation or at least good estimation of the effect of a given amount of backpressure on a turbo motor? I'm talking about back pressure AFTER the turbo.

Something like 2.5psi of back pressure equals 5% loss of HP over 0psi of back pressure (open turbo).

I ask because I found this great program where you can calculate the exact amount of back pressure in a tube. You can't count the muffler, but all the tubing for a given flow. You can even input the bends, as even a mandrel 90 degree bend will have more resistance than a straight pipe.

It's here: http://www.pressure-drop.com/

Also, I was wondering if anyone knew the conditions of air as it enters the downpipe? Aprox temp, pressure, density? If the car is at 20psi, will the air be ~1400F and 20psi, or atmospheric pressure and 1400F? It makes a big difference on the CFM out of the turbo, which is the most important variable.

 

[TurboDave]

Hmmm, this could turn into and interest thread if you can get the right/correct numbers.
I don't have any "actual" data, but knowing what I've learned about turbo's over the last 16 years has taught me a lot!
Post turbo backpressure is deffinately the turbo's enemy!!

1. I have what should be an extremely free flowing low low backpressure exhaust system, but can still substantially increase my performance at the track by unhooking it and running straight out the end of the downpipe!

2. Exhaust cutouts do very little to help performance, just make more noise, Take a look at them and 90% of them are in the area where the cat would normally be, hung off the pipe at a 45degree angle. At that point the velocity of the exhaust gasses carries most of it back into the exhaust system any way, and it can be only a marginal help in relieving pressure. Noting more than a huge leak.

3. Post turbo EGT's at 20# of boost would probably be closer to 1,000. I think mine runs pre turbo EGT's of around 450 or so at only 20# of boost, and since there's a huge (yet inconsistent) temperature drop across the turbine, it's hard to predict what post turbo EGT's would be in any given situation. After all, that's how turbo's make work, they extract heat. More heat, more work.

4. Reiterating, post turbo back pressure is bad, any backpressure will sap power. Take a look at a full on race car, Indy, GT, or whatever. The only pipe they have coming off the turbine housing is just long enough to direct the gasses away from expensive parts. Most aren't more than 6" to 10" long.
As you're aware, piping of any kind creates resistance to flow (backpressure). Minimizing it should be the ultimate goal
This is where the N/A (normally aspirated) hotrodders have a lot of trouble figuring out why we run the exhaust systems we do/or lack of.
To them, backpressure is an aid in making power. NOT HERE.

 

[lazaris]

Dave,

I have to disagree with you here.
" 2. Exhaust cutouts do very little to help performance, just make more noise, Take a look at them and 90% of them are in the area where the cat would normally be, hung off the pipe at a 45degree angle. At that point the velocity of the exhaust gasses carries most of it back into the exhaust system any way, and it can be only a marginal help in relieving pressure. Noting more than a huge leak. "
When i was using a 2 1/2 dual exh with ultra flo's I always picked up 2-3 mph in the low 20 psi range, uncapping.
I've since gone to a dual 2 3/4 and still pick up 1-2 mph uncapping.
I do agree with your last statement
"As you're aware, piping of any kind creates resistance to flow (backpressure). Minimizing it should be the ultimate goal"
Not just the piping but the bends in the piping even on mandrel bends create resistance.