Sonnick

As some of you may know, or not know, I am planning on doing a 3" exhaust. I also had plans of making a 3" collector to keep the volume the same after the Jpipe. However, I was steered away from this from a fellow Vee Six Pee member (not sure if he has a name here).

Basically, he told me that a 3" straight collector would certainly push more air through, however at a cost of air velocity. Having the primaries of the Jpipe come together into a straight 3" collector would surely slow down the air.

What he pointed out is that there is something called a "Venturi exhaust tube." Basically, all this is, is an hour glass shaped piece of tubing that is used at the collector.

http://www.carcraft.com/techarticles/0304_exhaust_system/index.html

That is an article that explains it pretty well and is the link he provided me.

http://www.performanceparts4less.com/gdcicat.html?partid=674434

That is the link to the actual part, or one of them that is available. They are pretty cheap and should speed up the exhaust flow. He explained to me that reversion (having air sucked back into the cylinders) is BAD and slows down the flow of exhaust. Backpressure actually does this, hence why it is a MYTH that we need backpressure! It actually hurts us!

I'll just copy the email into here to make it easier. Enjoy

I saw you say that you want a 3" collector. A 3" collector will kill power on our car because if you have the collector that big it will certainly allow MORE AIR to go through the pipe since it has more volume, but the velocity of the exhaust will slow down. When the exhaust slows down there is an increase in pressure at this area, and it causes the exhaust from the block to the collector to be less freeflowing. This will cause reversion, and that means that the gasses are sticking in the cylinder and this will kill power in the low end/midrange. If you put a Venturi (hourglass shape) at the collector, (often found in very expensive headers with lots of R&D put into them). You essentially have what is similar to a pinched hose. You increase the velocity, because the exhaust gases are forced to speed up. The higher the velocity of the exhaust gasses, the lower the pressure in that area.

If something has low pressure compared to its close component it will draw the air toward it, and this literally sucks the air from the header through the primaries, which is most important to reduce reversion.

I just figured I would give you my two cents since you are trying to build a custom Jpipe.

http://www.burnsstainless.com/mergecollectorsmain.aspx

I know that is for a v8 collector, but notice how it slowly decreases. It increases velocity in that area so that it creates a virtual vacuum.

I wish I could find the article that shows the difference between a header collector with the hourglass shaped dyno compared to the same header with straight collector.

It sounds really counterintuitive to exhaust flow design, but really the goal of the exhaust is to get the gasses away from the engine away as quickly as possible.

phee

iTrader: (5)

this is why i use crush bends

Inaccurate

I have not read the links yet so apologizes if this is covered already therein.

I might add that a good exhaust will expel the exhaust with minimal back pressure (pumping loss) and will help draw in the fresh charge (scavenging) during the valve overlap duration (camshaft dependant).