Exhaust evacuator

EmeraldFalcon · 11-13-06, 12:32

This is kind of a strange thread, be warned. I expect to get flamed for this. I don't even know if this is the right forum.

It seems a lot of money is spent on expensive exhaust systems, but for the system to truly be effective, you have to drop 400+ on just a cat-back. What's keeping people from just installing a high-performance fan like a leaf blower (Heartland America: Cordless Leaf Blower SC845 ) 30 bucks and 120 miles per hour, to push the exhaust. Depending on backpressure needs, you could install the tube closer to the headers to push the exhaust past the headers, rather than sucking it out. Leaf blowers also probably don't have enough torque to suck anything out of the engine. It seems like this is too obvious, why don't people do this?

the other pc · 11-13-06, 09:50

Now there’s an interesting concept. Instead of forced induction, aka supercharging, you’d have forced extraction. I guess you’d call that supersucking.

Well, the leaf blower is out for a number of reasons, the most significant of which is that you can’t blow into an exhaust system without blowing exhaust back up into the motor. Sort of defeats the whole purpose.

In theory you could set up a venturi to suction out exhaust. Just how much air it would take, how much power it would take to drive that air and whether or not you’d break even, gain performance or lose performance would be a substantial exercise in thermodynamics.

Fundamentally, active extraction isn’t a crazy idea. In fact, it’s a great idea. It’s why we have multi-stage gas turbine engines.

PC.

EmeraldFalcon · 11-13-06, 10:36

Quote:

Originally Posted by the other pc

Now there’s an interesting concept. Instead of forced induction, aka supercharging, you’d have forced extraction. I guess you’d call that supersucking.

Well, the leaf blower is out for a number of reasons, the most significant of which is that you can’t blow into an exhaust system without blowing exhaust back up into the motor. Sort of defeats the whole purpose.

In theory you could set up a venturi to suction out exhaust. Just how much air it would take, how much power it would take to drive that air and whether or not you’d break even, gain performance or lose performance would be a substantial exercise in thermodynamics.

Fundamentally, active extraction isn’t a crazy idea. In fact, it’s a great idea. It’s why we have multi-stage gas turbine engines.

PC.

I believe that might be engineering semantics in the end. In the event that aligning and angling the leaf blower's exhaust near the headers, as not to build up a large amount of pressure, but push it coming straight out of the engine created a problem; you could just hook up the intake of the blower to the exhaust system and make a bypass so it was actually pulled through the blower. Creating only negative pressure in the exhaust system.

It seems if that blower that I linked to could run well of a 12v lead acid battery, a car battery would be no sweat. That's the nice thing about cordless, small power restrictions, light, and small. The bad thing is battery life, which you'd bypass anyway for the car battery.

chu · 11-13-06, 10:53

Of course you want to lower back pressure, but it will actually hurt performance to have zero back pressure. I wish I knew a scientific way to explain this!

carn00bie · 11-13-06, 03:30

Quote:

Originally Posted by chu

Of course you want to lower back pressure, but it will actually hurt performance to have zero back pressure. I wish I knew a scientific way to explain this!

Backpressure is ALWAYS, ALWAYS bad in a 4 cycle engine! Its a misconception, to large of an exhaust system pipe will slow exhaust gas velocity to an unacceptible level and in turn, actualy create more back pressure than a smaller or properly sized pipe.
now to answer your question. its a good idea, BUT... its generaly thought its easier to empty a combustion chamber than it is to fill it, thats why some Europian cars have 3 intake and only 2 exhaust valves.
With all the time and effort put into making a "supersucker"

you could use a supercharger (or turbocharger) to force air into the engin and ultimatly make more power

butchdave · 11-13-06, 03:56

Hmmm my limited knowledge of tuning tells me that backpressure timing is essential in four stroke engines. And I think three intake valves is more to do with getting the mixture into the cylinder in the right places for optimum ignition rather than simply getting it in there. One intake valve will allow enough air and mixture in, but I'm no engineer

carn00bie · 11-13-06, 04:27

Quote:

Originally Posted by butchdave

Hmmm my limited knowledge of tuning tells me that backpressure timing is essential in four stroke engines. And I think three intake valves is more to do with getting the mixture into the cylinder in the right places for optimum ignition rather than simply getting it in there. One intake valve will allow enough air and mixture in, but I'm no engineer

By "backpressure timing" I assume your talking about exhaust scavaging effect, and your right, it is essential for maximum power. HOWEVER, now you getting into how exhaust pulses travel in the system, a very complicated subject. Scavenging is very diffrent from backpressure. backpressure is bad for maximum power.
If you really want to learn about the dynamics of exhaust gases I sugjest "Scientific Design Of Exhaust and Intake Systems" by Dr. Philip Smith and Dr. John Morrison. A warning though, is very long and complex.

Kanchou · 11-13-06, 05:49

umm...

the idea that a fan would somehow help blow out exaust more quickly than the exhaust is already pushing itself doesn't seem sound.

wouldn't the the fan need to be ridiculously strong to even surpass the power/flow of the exhaust itself?

carn00bie · 11-13-06, 07:22

Quote:

Originally Posted by Kanchou

wouldn't the the fan need to be ridiculously strong to even surpass the power/flow of the exhaust itself?

yes, it would. if I had to take a compleatly wild guess I would say ~30% of the engines crank HP

EmeraldFalcon · 11-13-06, 07:52

Yes, I've discovered that's the main error in my design. One high performance leaf blower can't handle exhaust from a 3.1 liter engine, and it probably wouldn't be worth the effort to use it for gaining negative pressure. I also have a Buick, not exactly a car that's worth tuning with weird, untested upgrades. Maybe I'll tool around with it some day.