Since I got my 3" exhaust and headers installed, my car hasn't become much faster. :/ Talked to a mechanic who makes most of his money street racing and he said that the car probably doesn't have any backpressure. That makes sense to me cause the cats are gone and it's a basically stock engine.

Next week I'm going back to a 2.25" dual setup with both the pipes going over the rear axle where the stock pipe originally was. I had the option of going with 2.50" but that was too much. There won't be any cats, but I want to retain (well, actually gain back) some backpressure. Now I like the sound of the 40 series flowmasters, but they are pretty much a free flow muffler, as many name brand mufflers are.

Now my question is how am I going to retain some backpressure with this system?

 

You dont want backpressure. It is not your friend. Also, if you are looking to keep more low end torqe I would go with a x or h pipe if you are running true duals in order to keep scavenging. You want scavenging, not backpressure.

Ben

 

Dont put duel 2.25s its basicly the same as 1 3 inch pipe. Get 2.5 inch duel if anything and just like jd13 said u dont want any back preasure. I dont knat ure mechanic was talking about ure 3 inch pipe with no cats is perfect. A catback system isnt gonna give u big seat in the pants difference probably .1 maybe .2 off ure quater.

 

I'm not even goin to go over this again. When will the backpressure myth be realized. Do some searching it was covered in some detail by five7kid withint a post.

 

Where's the pic of the "O" pipe?

 

LOL...

 

What size SLP's did you get? What other mods have you done?

If you went with the 1 3/4" like EVERYONE and their brother does on here, then you may not be getting much scavenging at all. That will really kill your low RPM torque. Your cam isn't letting the pistons push enough air out to really "fill" the large primaries. You need a lot of upper RPM HP to do that, and the stock cam ain't gonna do that.

If you went with the 1 5/8" and if you haven't done anything else to the motor (besides a high flow filter, and maybe a chip), you could be OVER scavenging the cylinders. That's were backpressure comes in.

"Back in the day" the hot rodders used backpressure to make their cars more streetable and get better gas milage until it was time to race. That's how cut-outs come about. The backpressure would decrease the amount of air and fuel in, so it was more drivable.

Your stock engine set up (cam, heads, ECM) is made to have some resistance. The fuel parameters are set up for max fuel econo and low emissions. When you start "pulling" more exhaust out, you will start to run lean. This will kill your upper RPM power more so than the low/midrange RPM since upper RPMs require more air and fuel.

By putting more backpressure in is just a crutch for what you should do. Increase fuel flow. By putting backpressure on, you are limiting the amount of air that can flow into the engine, so the ECM, fuel pump, and injectors can keep up.

So it's not more backpressure that you want, it's more fuel, or less scavenging. But as I said, increasing backpressure will increase your low RPM torque and the SOTP meter is MUCH more sensative to torque than it is HP. That's why you noticed it, and it "feels" like you lost power.

AJ

 

How is that. BP puts strain on an engine , thus creating worse mpg. That's why putting on a free flow exhaust should help mpg a little.

 

As proven, there was no need for the 1 3/4 headers until over 400 something hp, so I went with the 1 5/8.

Thank you for clearing this up to me. I'll be sticking with the 40 series flowmasters. Now should I go with the 2.5" pipe or stay with the 2.25"?

 

2.5

 

"Back in the day" all they knew was big cams, big carbs, big tires, and big hair. They also only new solid cams. Hyd. cams were for Waldo's car, the school geek.

Thank goodness for Waldo cause he's the one that figured out you don't need a solid cam to make 400+ HP!!!

They used to use cams with close LSA and huge duration which caused a lot of overlap. Couple that with heavy scavenging and the exhaust system starts pulling the intake charge right out the exhaust valve. Cylinder filling at low RPM is lost, as well as cylinder pressure. That results in low MPG. You don't see the gains until over ~4000RPM. Putting a muffler on there slows the flow/scavenging and keeps the intake charge from getting sucked out the ex. valve. Not 100%, but a little better than before.

This is not so bad anymore thanks to Waldo. He figured out how to increase LSA and lower overlap while still using more duration. But that also gets rid of that cool *** choppy idle that we all know and love.

Even so, he's probably causing a lean mixture and needs to increase FUEL FLOW!!! which he didn't seem to understand.

There is such a thing as too much. Like putting 4.56 gears on a stock 305. Or using a single plane intake and solid cam on a car with a 1500RPM TQ and 2:73 rear gears. Using the wrong size exhaust (read "over" or "under scavenging") can also have adverse effects.

AJ

 

Actually, that may not be too much, granted for most it would be. But if you're only interested in 0-60 then that would be perfect.