steve's dog

im just wondering why back pressure helps. i dont have any aux. ports anymore so they're basically always open. what other purpose would i need backpressure for???

Kenteth

the scavenge effect, other than that... you don't really... At least to my knowledge. A turbo may require velocity of exhuast though, and velocity is hard to come by with out accompaning backpressure.

Center of The Universe

From what I have learned, In N/A type engines, backpressure helps with throttle response.

Drifter101

I always thought backpressure helped open the 5th and 6th actuators and **** if you hadnt air pumped them or wired them open

Generic

OK, on NA engines backpressure affects your HP/TQ curve. Not in the sense of making it peak higher, but just shifting the peak around.
If you have two cars with stock exhaust, both have peak HP around say 6000RPM
You now replace the stock exhaust on car A with a more restrictive type, and on car B with an more open type.
The peak HP on car A would now be say 5000RPM
and on car B it would move up to say 7000 RPM
Its all about tuning for what you need/where you wanna make power. Want low in torque, dont' care about suffering up high? Stick with a high back pressure restrictive exhaust. Wanna be able to run really well in the upper RPM ranges, get a more open lower back pressure exhaust, but your low-end torque is going to suffer.

On a Turbo car you want it open as little restrictions as you can get.

And yes, on the 6-port cars, they open based on exhaust back pressure, which you say you disabled, so no worries.

Murdock

mazdaspeed7

You never want backpressure. People confuse velocity with backpressure. Smaller pipes have a higher velocity, as well as mroe backpressure. Backpressure is always bad, and ALWAYS hurts power. Veloctiy is what makes power. If the pipe is too big, you lose velocity, and lose hp. So its a compromise. You need a pipe small enough to have good velocity, but large enough to not create much back pressure.

Hot_Dog

On the early (i.e., 86-88) NA 2nd gen, exhaust backpressure was used to open the auxiliary ports. On the late (i.e., 89-91) NA 2nd gen, the auxiliary ports are open by air pressure from the air pump. The late NA 2ng gen also had VDI, which is actuated by air pressure from the air pump. The early NA 2nd gen did not have VDI. Of course, the TII had neither auxiliary ports or VDI.

Some backpressure aids in increasing low end torque. The exhaust backpressure effect is probably most noticeable when the stock exhaust system is replaced with a high-flow exhaust system. The result being a more HP and a higher top end with some torque lost on the low end. Basically, it's a compromise and depends on what type of performance you're looking for.

Hot_Dog
90 RX7 GXL
02 RSX-S

Kyrasis6

Without an automotive Physicist we could all bat this around to no end and never come up with a fully accurate answer. I will take a stab at it from what I have read in my dads old hot rod books and what I've read from my engineering and physics books. Take note that most of this is in reference to a piston engine but the real pysics should be different between pistons and rotary.

First of all scavaging is caused by a combination of pressure and vacuum there may be other factors but those are the 2 I read most about. In a N/A piston engine the larger the valve the more pressure behind it. There for you intake valve is always larger than your exhaust valve otherwise the flow would tend to reverse and you would get reversion, i.e. getting exhaust in your intake charge. This pressure is also why you pulse tune your inake manifold. As a valve opens it creates a pulse in the direction it is traveling, there for when the valve opens you want the intake charge to pulse into the port (posative pulse flows in negative flows out) and when it closes you want it to pulse out of the port. This is done by using several mathmatical formulas to determin intake runner length and diameter. In terms of a piston engine the advantage of having a valve is that it aids in the flow of the pulse. You still want to do this in a rotary engine in terms of getting the intake charge to pulse in as the ports open up. This same concept applies to exhaust. Being since rotaries don't have valves and I'm not a rotary expert I don't know how much of this applies to a rotary.

Ok so taking what was mentioned above into fact you want a negative pulse on the exhaust port when it opens. This creates vacuum that not only helps pull exhaust gas out but also pulls intake charge in further aiding the pulse from the intake charge. Knowing the statics of how a gas flows through a confined area you can determine what diameter exhaust to use and how long down the line to constrict it to time the pulse. Smaller diameters have faster velocities as mentioned which you can tune to get the pulse to move faster or slower. Headers are the primary factor in doing this. The runner diameter and length determines your power band, and the collector pipe is used to help tune the pulse. If I remember right the longer the runner length the higher your torque/power band is and the shorter the runners the lower the torque band. Diameter also effects this, typically the smaller the diameter runner the higher the power band because the more flow the less inertia. Larger diameters more or less fizzle out the exhaust charge flow as it expands in the larger pipe which creates inertia and lowers your power band. The lower your power band the more low end throttle response you get. This is why my B2200 can launch most hondas out of the hole and I amost always loose my *** at the end of third gear, the B2200 being a truck is built entirely for low end torque.

So accordance to that you need vacuum not back pressure to promote scavaging. Of course I don't know how this applies to 6 ports, I don't know how they work. This all deals with intake/exhaust charge and scavaging which should apply to both piston engines and rotaries in one form or another. I don't think I exactly answered the question but I did explain part of the puzzle.

LilNARxDrifter01

yea...

rotarygod

iTrader: (1)

That was pretty good.

If backpressure was responsible for low end power then I could theoetically put a cork with a small hole in it in the end of my muffler to tune for powerband. (Wait, some riceburner muflers do this for noise reasons! LOL) This doesn't work though. Less backpressure is best. It is a host of other variables that effect where the useful powerband is.

Kyrasis6

There was a typo in that, the laws of physics would be no different for a rotary engine. You just have to learn how to tailor the systems in an engine to take utilize the best combination of physics. As mentioned before smaller pipes have higher velocities of flow where larger diameters have a more stop and go motion. So for a low reving stroker motor like my truck you want large ports to get that pulsing motion of the stop and go of the charge. On a rotary because it operates at such a high rev you want smaller ports to lower the inertia and get the charge in as fast as possable. This is because depending on the motor after a certain rpm the engine is pulling in so much air that you need a constant non stop flow. All of this applies to port diameters too. If a port is too small to handle higher rpm demand for air then consiquently you will have a loss of power, hence porting but if you inlarge the port too much you will lower your power band and increase torque. But with a rotary you can do something that you would never be able to do to a piston engine, bridge porting. If I understand the procedure right it is drilling a small auxilary port in the rotor housing?? Well how ever it is done this smaller port has a higher velocity charge than the standard port. So what happens is when the engine is running at high rpms your getting more of a pulsing charge from the original port and a flowing charge from the drilled port. If I am right in that bridge porting raises your power band this would be why. That information is sketchy at best though because I am not completly familiar with everything about bridge porting, but I think you got the idea.

813KR$

Back pressure bad, flow good ! ! !

9.8 in the 1/8th mile with back pressure, 9.5 in the 1/8th with headers only!

Blake_91

Agreed! That is exactly what happened with my car. Which I knew would happen, because its a proven fact.

-Blake
P.S. A high flow exhaust is prob one of the best things that you can do for your quck and easy 3rd and 4th gear top end's... And if aided by and aluminum flywheel, you wont even notice the loss of torque, unless you do the flywheel first and wait a few months for the exhaust... Which is what I did.

-Blake

White_FC

Exhaust backpressure after the tuned section of the exhaust is Baaad.. mmmmkay.

...but like Mazdaspeed7 said earlier, and no one seemed to take any notice really, if you make the size of the header pipe too small you could quite possibly make a bit more power 'low down'.. But more importantly it will seriously hamper your power 'up-top' which will make it Appear like you have lots more 'low down' torque... Do due the same reasons as Mazdaspeed7 already stated..

truespin88

I thought I should throw in what I have noticed about headers for piston engines... probably applies to rotaries too but I'm not sure.

With a piston engine's header or exhaust manifold, length and pulse timing has alot to do with how beneficial/sacrificial it is. A longer, smoother flowing header will produce more hp, while a shorter header will produce more torque... But also valve timing and ignition timing have great effects. You can cam an engine to desire high rpm, or to hate it, a la porting a rotary.