I recently purchased a straight pipe and downpipe for my 87 N/A (not from racing beat) and i want to put a tube branching off the straight pipe for backpressure, much like racing beat's presilencer set up. I found out the diameter of the tube they use, but I am afraid that the presilencer might generate more backpressure than a regulard straight pipe. What do you guys think? Will a small tube branching from the straight pipe the same diameter as the one on the RB presilencer work? Or should i go a bit bigger?

 

and why do you want backpressure again? I think there's some good threads on backpressure for N/A on this forum, you can search for them. The thing is, it's not the backpressure that makes the power. It's the size of the exhaust.

As much as I can remember, if you have an exhaust sized for peak power rpm, you'll make crappy power in the low end, but good power in the top end. IF you have an exhaust slightly smaller than that, you'll make better power in the low end, and lose a bit in the top end. Overall, though, you'll make more power under the curve. It's not because the small exhaust generates more backpressure. YOu can find a better explanation if you search.

 

well thats noit entirely true. if you dont have enough back pressure to open the aux exhaust ports, then your almost counter acting the exhaust. at least thats how my fc behaves.

 

that's true for S4's, for S5, it's driven off the air pump instead.

 

ya its an s4, and i need backpressure to open the ports. thats why i need the right amount of backpressure. sorry for not making that clear.

 

if u want to weld a pipe for back pressure for your s4 aux ports, then you have to make a U tube (heheh you tube... lol ok im dumb) the U shape having the opening pointed towards the engine in the pipe and then out and back towards the engine so the exhaust gases actually do travel through that pipe, since if you have free flowing (straight through) mufflers, you probably dont have enough bp to push the exhaust gases back through the little tiny restrictive pipe, then again i dunno **** about this so i might be making this stuff up.

 

Any other suggestions? Is the U-shape necessary?

 

If I remember correctly on the RB presilencer I intalled, the split air connection for the presilencer did not just branch off the outside edge of the casing . Rather, the tube extended into the center of housing in order to collect and channel the exhaust velocity back to the aux port actuators.

 

oh very interesting. was it place at an angle towards the engine? i would like to get this as exact as possible. thanks!

 

I don't think some of you guys realise that the backpressure you're trying to get to open the auxiliary ports is also the same same backpressure that's hurting power. You can't add exhaust restrictions that only opens the ports.

The only time the 6PI system was working properly was when the entire stock exhaust was in place. Any reduction is exhaust backpressure from a better-flowing exhaust system will result in the ports opening later than they should, and any time they're closed when they should be open you're losing performance. Even if they do eventaully open, your mid-range (where most driving is done) has suffered.

A system that mimics the S5's electronically-controlled system with pressure from an air pump is the only way to have an exhaust that doesn't compromise power.

 

Any other suggestions?

 

May be NOT what you are loookin for,But My set-up is A simple one.It is an extra Piece incorporated in the Outlet portion of the Header.A tube stemming from the Outlet flange is used to Activate the Ports..(welded into it as a Integrated part of the TWO pipes at the outlet flange,before the Presilencer.It works Great,and I have no problem opening up the Ports.This tube,is off the Back of the Header..and Not off the Presilencer.I hope you can understand What I am tryin to Explain..Sometimes I Make it more complex than it really is.OH..and this Is on an S4.

 

Ya thats plausible, except that I would have to go through the troblem of taking off the headers to do that modification. Well it sounds like Im just gunna have to guess and go for the modification of the straight pipe, and hopefully it will work like Racing Beat's version.

 

Heres a great suggestion:


Look at the FAQ and Archives to figure out how to rig the Aux ports electronically. Then you dont have to do this guessing when **** is going to work/open crap anymore. You will know when it will open, and yo can experiment on a dyno to find the optimum opening time for your setup.


BC

 

Well, I've gone through many of the options, tested theories such as the air pump actuation, and came to the conclusion that the backpressure actuation came from the factory as it did for a reason - it works, and its consistent. I'd really like it to work just like stock.

 

It came from the factory like that because it was a simple failsafe system designed for the engine as it left the factory.


Your car is no longer the car that left the factory, so why stick to a method of activation that is a compromise to begin with? there IS a reason why they went to the airpump for the S5....

the point is that by the time you get a tube hooked up you will have spent as much time, and probably close to as much effort, as it would have taken to do the electronic activation. It would be really pointless, and annoying, to take the time to make the split airtube for your exhaust just to find out that the aux ports only open at 6000 rpms... Witht he electronic actuation you can be CERTAIN it is opening when you want it to, AND you can vary the actuation point so as to open the ports at the most optimal time.


BC

 

In case you missed it...

 

Is it your contention that Racing Beat did not consider or factor for these assumptions in the design and manufacture of their exhaust components? I find that very hard to believe. Now if you are only talking about shade tree mods, I can see the logic.

 

More like the aftermarket had to design an allowance for the stock system. They assumed that you have a stock engine and stock actuation, and they have to design a system that supports it. Because of that, they have a very defined exhaust size to choose from. It doesn't mean that it's the most efficient, it's the most efficient given stock constraints. If you want the better flowing exhaust system that exceeds stock constraints, you probably open your ports later and lose power. The only way to make that power efficiently is to port out the motor and match the exhaust.

 

Sorry if I wasn't clear in the post. My post assumed stock engine, stock actuation, and street driven application for the application of the Racing Beat exhaust components. Obviously, just slamming various mods together without considering the overall interaction and impact could render a net loss.

 

What you're suggesting is that RB's exhaust improves performance by being less restrictive and creates the same amount of backpressure as the stock system to open the ports at the correct time. This is not physically possible. The backpressure created is a direct result of how restrictive an exhasut is. I'm sure RB considered this in their design, but I'm also sure they would've had to look for for the best compromise between improving flow and not opening the ports too late.

 

I believe that perhaps by using a funnel shaped tube for backpressure activation, you could increase the pressure while maintaining better than stock exhaust flow. I don't know if Racing Beat uses that method, but I may try it.

 

And you don't think a funnel hanging into the exhaust will be a restriction?

 

No, I never suggested or said the less restrictive RB presilencer creates the same amount of back pressure as a stock cat system. I said and am saying that RB was obviously aware of the potential velocity and back pressure difference and compensated for the difference in the design so the ports would open at or near to the correct time, which I believe is what your last sentence states.

I believe that using a RB presilencer would require either a stock cat back or RB cat back to keep the the ports opening at or near to the correct time.

 

If you have your airpump, drill a hole in the exhaust tube off the air control valve. then run a vacuum line from your actuator to the air pump exhaust tube and glue it in with glue that melts plastic/vinyl/rubber. I have this done to my 88 n/a with full stock exhaust and it feels exactly the same whether hooked up to the cat or the acv. I used this setup with a header back free flow setup and was happy.

if you don't have a airpump, take off your header and drill a hole in the rear rotor tube facing the engine. weld a fitting and run copper tube up to your actuator. it will work just as good as the stock setup.