Alright im so confused...some people say you can run 7's with no ehxaust and some say you need back pressure...Does backpressure give you more spin in the rotor when in low rpm's and when in higher rpms its better b/c there is nothing restricking the flow of everything..someone plz clear this up for me im in the process of replacing my pipes and would like the advice

Stepdaddy

 

i always thought the idea was to balance the intake pressure with the exhaust pressure for an even flow. so if you have a ported intake and/or bigger carb jets (decreasing intake pressure), then its best for a decrease in exhaust pressure.. so basically each car is different depending on your mods im pretty sure. thats always what i thought... i dont know if its right..? nothing was really accomplished in that other thread about backpressure.. its was basically 8 opinions colliding lol.

--eric

 

Stepdaddy,

I'll say this. Backpressure by definition will ALWAYS opposes the rotation of the rotor.

Specrx7,

A carb engine ALWAYS works on intake vacuum.

If you are trying to say that an increase in available intake flow should be matched with an equal increase in exhaust flow, that is generally correct.

 

TUNED backpressure creates a pressure wave that helps scavenge out the exhaust from the chamber (the pulse from exhaust leaving rotor 'a' creates a sort of vacuum that helps pull the exhaust from rotor 'b' and so forth)

EXCESS backpressure (restrictions in exhaust piping) rob the engine of power.

Just as air/fuel mix being too lean or too rich is bad, but just slightly richer than stoichiometric is good, backpressure has its ideal. Too much (high restrictions) is bad, not enough (straight pipe or no exhaust at all) is also below optimum performance. That's why a tuned exhaust is called tuned - it creates the 'pressure wave vacuum' for ideal scavenging and thus more efficient engine operation.

EDIT: Carbs work on venturi effect (pressure differentials) based on bernuli's principle, the same principle that enables airplanes to fly. The faster a fluid is moving, the lower its relative pressure. The faster flowing airstream over a plane's wing creates a lower pressure, enabling the aircraft to plane along on the higher pressure air underneath.

Intake and exhaust tuning are both based on fluid pressure management.

 

thats exactly what i was trying to say, thank you Stripgear

--eric

 

You for sure need back pressure for a GSL-SE in order to actuate your secondary ports. Otherwise you gotta use some other means of providing pressure to the actuators (like the air pump).

And Manntis is right: TUNED exhaust backpressure is a good thing.

 

I had to drive my GSL-SE with NO exhaust for a day and did not notice anything differnt, except that it was ungoddly loud.

 

were you just casually street driving? or were you pushing it a little. because if you were just putting around town you probably wouldnt notice anything.

--eric

 

Oh, I was DRIVING it alright, it was hard not to have fun with the exhaust uncorked.

 

Many times, I can actually feel a slight surge when my secondaries actuate (yeah, yeah, I know - *** dyno again) at around 4k rpm.

Mostly, I can tell when they don't open because the car feels sluggish and won't redline. I've had that happen more than once.