No, that's not what I said. It is impossible to reduce backpressure and have the ports open at the same time as they did with the stock exhaust. It cannot be done. The less restrictive the exhaust is, the later the ports will open. What I said was that RB's design is probably a compromise between flow and port opening, i.e. flow is increased but the ports open later.

 

The problem I have in accepting your position, is that it is an absolute. You are suggesting that RB presilencer renders less restriction, therefore less back pressure, thereby concluding the ports must open late. Further, based on this absolute, you are suggesting, unless stock, there is no other exhaust configurations utilizing pressure that will open the ports at the correct time. “Impossible” is the term you used.

Sorry not buying it. If you replace the pre-cats and the main cat with the RB presilencer, although different from a stock system, you still have exhaust velocity and pressure. I think Racing Beat, which engineered the 2nd Gen. that set a land speed record, has enough engineering know how to design a down pipe and presilencer combination that accounts for any difference or change in exhaust velocity and pressure so the auxiliary ports will open at the correct time.

 

I'm afraid RB's experience setting land-speed records (with an engine that didn't have 6PI) doesn't allow them to break very basic and very immovable laws of physics.

You can doubt me all you want, but for a given design (length, number and shape of bends, etc) the pressure created in an exhaust system is a direct result of the velocity of the gases flowing through it. The faster the flow, the higher the pressure. As an example, if you change from a 2" system to a 2.5" one, and get 10% more peak flow as a result, the velocity in the pipe will have dropped over 30%. What do you think that does to the exhaust backpressure? The whole point of a modified exhaust is to reduce backpressure. Since backpressure rises with flow (i.e. revs), the only possible result is that the pressure required to open the post occurs at a higher rpm.

So far all you've said is "I don't believe you", so prove to me I'm wrong. Explain to me (in the same way I've just explained to you) how it is possible to have an exhaust that has the same backpressure and greater flow. Get all technical on my ***...

 

The last time I had a RB header and presilencers on a n/a, the downpipe had a boss welded on the header with an approx 3/16" pipe that was about a foot long, that you connected to the aux port actuator inlet pipe, using a piece of hose.

The Boss was on the pipe coming from the front exaust port and about four inches back from the mating joint. It was one of those Dual header outfits with two pre-silencers and total seperation of the front and rear rotors exaust.

 

here is a picture of the header I have on my car with the actuator tube.. u will clearly see that the tube will have to enter the center of the silencer so that the exhaust will get pushed into the tube and out into your split ait pipe...



so yes a "u" shaped tube would be correct u have to have it in the middle of the silencer comming out and to your split air pipe..

Dave

 

To summarize, let's pick some arbitary numbers. Let's say that stock exhaust requires 50 units of pressure to open the aux ports. Let's also state that pressure increases at .5 units of pressure per 38 rpms. Do some quick math and you see that at 3800 rpms, you get your units of pressure required. Now, let's increase the exhaust size 25%. Pressure should decrease 25% across the board. Now pressure in your exhausts rises at .5 units per 47.5 rpms. Since the pressure requirement does not change when installing a new exhaust, and doing some quick math, you see that your ports will now open at 4750 rpms.

Now, in the real world, relationships aren't as linear as described, however the trends remain the same. Installing a bigger exhaust will lower pressure. If you've taken chemistry, this follows the example of PV = nRT. V increases, n, R, T stay the same, so P must decrease.

 

First, you’ve moved off point. The original point of discussion dealt with replacing the pre-cats and main cat on a stock system with a RB presilencer or similar set up. Nowhere did I see any mention of changing the size of the exhaust components. Obviously, if you enlarge the exhaust components pressure changes.

[/QUOTE] So far all you've said is "I don't believe you", so prove to me I'm wrong. Explain to me (in the same way I've just explained to you) how it is possible to have an exhaust that has the same backpressure and greater flow. Get all technical on my ***...[/QUOTE]

If you replace the pre-cats and main cat with a RB presilencer, you have changed exhaust velocity and pressure. However, although different, the exhaust system is still pressurized. You need 1.2 PSI to start the ports to open and 2.0 PSI to fully open the ports. On a stock system, with only a RB presilencer replacement, exhaust pressure under load at 3,800 RPM is certainly greater than 2.0 PSI. The exact pressure at 3,800 RPM under load I don’t not have the figure for, but I’m sure Racing Beat does. If you don’t believe the exhaust pressure is greater than 2.0 PSI, here is an old school game. Take a pair of gloves and block the muffler exhaust exits and trying to stop or stifle the exhaust at 3,800 RPM. Good Luck. There are other means to simply test the exhaust pressure, which I’m sure will come to you.

So, we now have certain givens that can be ascertained: 1: Exhaust Velocity and Pressure under load. 2. 1.2 PSI to 2.0 PSI required to operate the ports correctly. 3. Exhaust pressure at 3,800 under load most likely exceeds 2.0 PSI. 4. The pick up tube in a RB presilencer is a simple pitot tube. Based on these measurable and known factors, it seems the correct size (diameter and positioning), along with the location of the pick up tube on an RB presilencer could be calculated to open the ports at the proper time. Essentially, you are tapping or siphoning known pressure factors to achieve the needed 1.2 PSI to 2.0 PSI from a source that exceeds 2.0 PSI

The size of the pick up tube varies from stock cat to racing beat presilencer to bonez presilencer, where the stock cat is the largest and to bonez is the smallest. This leads me to believe that depending upon the design, the engineers considered pressure and the size of the pick up tube necessary to open the ports at the correct time in relation to their design

 

The effect is exactly the same. A bigger pipe is less restrictive, so pressure in the system will be lower. If you remove a restrictive cat and replace it with a free-flowing muffler, pressure in the system will be lower.

Correct, velocity will increase proportionally with the increase in flow caused by the reduced restriction, which will also result in less pressure in the system.

You seem to have missed an obvious point. If the pressure in this system is greater than 2psi at 3800rpm then the ports have already fully opened. This would imply that in a completely stock system the pressure will be greater than 2psi before 3800rpm, and hence the auxiliary ports would open before then. But we know this isn't true.

How do you know the pressure in this exhaust system (or the stock system) under these conditions? Have you measured it?

This test could not be any less relevant. Restricting or blocking exhaust flow will obviously increase the pressure in the system. It does not prove anything about what the pressure in the exhaust was before you blocked it. The fact that you would even suggest doing this shows a lack of basic understanding about flow and pressure in pipe systems.

So what you're saying is that the pressure in the 6PI actuators is actually lower than the exhaust pressure at the pick-up point. Where is this pressure being bled out? AFAIK it's a completely closed system. The pressure at the actuators can only be the same as the pressure at the beginning of the pick-up point. You'll notice that RB positioned the pick-up point on the header as close to the ports as possible, because that's the point of greatest pressure. If less pressure was required as you imply, they'd use a point further downstream because pressure drops the further along the system you go.

And all of this has nothing to do with "siphoning".

FYI, the velocity pressure (as opposed to static pressure) created in a pick-up pipe pointed at the exhaust port on like the RB header is easily calculated. Assuming 100% VE, and using the RB header's 2" diameter pipe with a 0.12" wall thickness:

654cc @ 3800rpm = 41.4L/s (~88cfm) = 26.4m/s (~87fps) = 418Pa (~0.06psi)

That's a tiny amount of pressure. Even at redline the effect is very small.

654cc @ 7000rpm = 76.3L/s (~162cfm) = 48.6m/s (~159fps) = 1418Pa (~0.21psi)

Compared to the factory pressure pick-up tapped into the side of the exhaust, this isn't going to increase pressure much. It's a small fraction of the 2.1psi the actuators need to fully open the ports.