exhaust Size

[turbolegacy1991]

i have a 91 leg turbo going to do the exhause from turbo back is this ok. i know alot of the guys are talking that it can creat too much back pressure which is what i want to avoid but i want the good flow for the turbo. if anyone has any ideas let me know

[turbolegacy1991]

oops forgot to mention above that it is going to be 3 inch fron turbo back!!!!

[evolutionmovement]

Back pressure is from resistance to flow. A bigger exhaust will reduce back pressure.

Steve

[legacy92ej22t]

A 3" TBE is a good size.

[jake15]

i agree a 3" TBE is a good size..... thats why i have one

[BAC5.2]

If your stock, you might find it better to stick with a 2.5" exhaust.

3" without sufficient flow can reduce exhaust velocity and harm more than it hurts.

[SC00BS]

I don't want to step on anyones toes as I'm new but I have to throw in my 2 cents.

Turbo = forced induction therefore the flow is forced thru the exhaust by the turbo. A 3 inch will not harm your motor but in fact be better as it will offer more air flow. Turbo cars don't create back pressure as the exhaust gasses are pushed out your exhaust by the turbo itself.

A commonly used guideline in Australia in relation to turbos are 1.8l motor - 2.5-3 inch
2.0l motor - 3inch and sometimes bigger.
Some performance workshops will tell you the bigger the better.

With that said, go the 3 inch

[BAC5.2]

To large an exhaust WILL reduce exhaust velocity if not properly flowed.

Stock turbo, stock boost, 3" exhaust, the car will have less exhaust velocity.

Think of it like a straw.

Blow through a stirring straw, notice how hard it is to blow a significant amount of air through that straw? That's backpressure.

Blow through one of those HUGE 3/4" slurpie straws. Notice how hard it is to blow the same amount of air? That's a reduction in velocity.

There is a perfect exhaust size. For a stock car, 3" isn't it.

And for clarity, air is being pushed INTO the engine by the turbo creating more dense air. It's being pushed OUT of the engine by the rotating assembly inside (Since the intake valves are closed, the exhaust stroke relies on the power stroke of the opposing piston). The turbo itself, is a blockage in the exhaust, not a jet that speeds flow, it restricts it.

There is such a thing as to big.

Big exhausts don't offer more air flow. They offer more air flow potential. You need to create the volume of air, in order to take advantage of the larger exhaust. If you don't make the volume, velocity is reduced. The goal of a good exhaust is to maximize velocity, while minimizing backpressure.

[Kelly]

I think theres a sweet spot for turbo exhaust tuning. Kinda like bandpass boxes for speakers. Ideal would be infinate baffle, or no exhaust at all IMO.

[TDGKORN]

I was always been told that the turbo causes all the backpressure you need. so the perfect exhaust on a turbo car is a short pipe that increases in size as it goes out.
Think of turboed drag cars there exhaust is dumped out a short distance from the turbo.

Just my thoughts on it.

Troy

[tris91ricer]

Turbo = forced induction therefore the flow is forced thru the exhaust by the turbo. A 3 inch will not harm your motor but in fact be better as it will offer more air flow. Turbo cars don't create back pressure as the exhaust gasses are pushed out your exhaust by the turbo itself.

I thought you Aussies were better educated than us Dumb Americans. Did you miss turbo101, or what?
Please read the "..forced induction.." part again. If a turbo just pushed dense air out your exhaust, well.. ...that'd be stupid. 'Might as well swap out your flat four for a vectored-propulsion setup, where a turbo might actually do what you think it does.
I'm sorry. It was too funny not to, ya know?

[evolutionmovement]

You do not want no exhaust at all. You have no scavenging effect without an exhaust and the cylinders would not evacuate efficiently.

Steve

[Brat4by4]

Can exhaust pulses scavenge through a turbo?

[Legacy777]

Your exhaust flow characteristics are changing quite a bit by going through the turbine on the turbo.

They are undergoing a volumetric expansion and temperature decrease.

I don't know for sure whether the actual scavanging pulses make it through the turbo, but if I'd have to guess, I'd say no....or if they do, they are very weak.

I think the main driver for turbo cars is velocity. You need to keep your velocity up enough so that the exhaust evacuates the pipe fast enough not to cause excessive back pressure.

The drivers behind velocity are expansion & heat. If you put too large an exhaust on, the gasses will expand and cool too much, which will slow the velocity.

It's definitely a balancing act.

[TDGKORN]

It makes sense then that you would only have a short pipe after the turbo.
rather than going all the way through a system of pipes. (turbo-back)