4.6-4.8 How much back pressure is enough back pressure?

[bcredliner]

The length of headers, especially the collectors are calculated to capitalize on the torque and/or horsepower of a particular engine.

Using the factory exhaust system as the baseline--does anyone know the optimum length that would govern placement of exhaust cutoff with goal of increasing torque while at least not losing any horsepower. Or, is there an equation to make that calculation?

[X5SND]

Did you mean a cutout? (electric operated valve to go to open headers). Just want to make sure were talking about the same thing here...

If you're after optimizing the header design to incorporate a cutout that's a massive undertaking undoubtedly going to include tons of compressible flow problems. On the other hand, if you just want to splice in a cutout to the existing system; I would think relatively close to, but after the collector would be appropriate....I'm no expert though.

[bcredliner]

Yup, I am talking about electric cutoffs. My thought is to weld in a pipe just prior to the point the exhaust turns toward the back of the X. There is no reason to do that if the result is that engine loses torque or horsepower. I would be fine if I was down on one as long as the other more than offset the loss. Since the X is so heavy, I would like to have increase in both but more benefit in torque. Too much back pressure reduces both, I assume I have too much back pressure now but I am concerned that cutoffs will be worse than where I am.

[JCL]

Just to make sure we are using common language, you have two exhaust manifolds/headers, going into two separate catalytic converters, coming together in a common silencer/collector, then running rearwards through two separate rear silencers and out two tailpipes (however many tips are involved). Correct me if I am mistaken.

It isn't just about backpressure, it is about utilizing the engine firing order to create balanced flow via exhaust scavenging. There are pressure waves travelling down the exhuast from each cylinder firing, and you want to maximize the benefit of those pressure waves, relative to each other.

So, you need the first centre silencer, or an x pipe, to manage those pressure waves, on a V8. The location of that pipe (distance from the engine) and the pipe diameter up to the crossover point (not too large or too small) needs to all work together as a system.

After the centre silencer, feel free to dump to atmosphere. It will be loud, and I wouldn't expect much if any performance improvement.

If you want to optimize the pipe diameter up front, expect to do lots of trial and error to find a sweet spot. You will be trading off power/torque numbers at one rpm range for another rpm range, in effect. Automakers will try for a balance, with a wider range. That is why lots of tuners can show more power at a single rpm point, but they don't talk about how it is worse elsewhere.

Is the above answering the original question?

[bcredliner]

Your description of my exhaust system is correct though I do have Dinan exhaust. The exits have double pipes on both sides.

I am familiar with scavenging and tradeoffs of HP or TQ depending on the exhaust system.

I am not interested in louder just for louder, I love the Dinan sound.

If I go forward I will install the cutoffs to extend the 'vertical' exhaust pipes near the engine. At that point I would think I would improve horsepower. At what RPM or how far up the RPM range I have no idea. Nor do I have any idea what would happen to torque. As you know there are many reasons that that could be false but that is my current thinking.

Usually it is fairly easy to determine if an engine design is torque or horsepower 'based'. I haven't found the answer to that question for the 4.6. Based on HP and TQ ratings and dyno runs my guess is that the best exhaust alteration, decreasing the time from point A to point B, will be torque improvement based. I can also accept the opposite scenario. Certainly BMW made a well informed exhaust decision based on engine design but a tuner certainly tries to improve the exhaust from the heads to the exhaust exit anyway.

I am also concerned that I could add cutoffs and initiate engine fault codes, so, right now I am looking for experienced input. I have gone through this many times with other vehicles but there was always the benefit of lots of technical info and verified results of others that had been down the same path before, and several brands that could get one where they wanted to go. It not like that with a 4.6 engine.

Another question I would like input--what percentage would you attach to slowing the time from point A to Point B--say 0 to 60 by adding cutoffs at the place mentioned above.

[JCL]

I would ensure there was still a cross over (x-pipe) in there, for the power and torque increases.

Some background reading.

Auto Exhaust Science

[bcredliner]

Thanks for the article, very informative.

I am not going to change the exhaust system front to back. I like the Dinan exhaust as is. If I install the cutoffs it will be only when I use nitrous, yes I am the fool with 150 shot of nitrous, so the best place to potentially get that amount of flow out should be at the point the primary pipe turns toward the back of the X though I will check and will shoot for 17" from where the 'collector' starts. I am on vacation so I can't look at my exhaust. If I were to guess I don't think I have an X pipe. If I don't I will add it as per the article it won't result in a decrease and 60% of the time it will be an increase.

Sound like the best plan based on the parameters or do you think I am I way off base?

[JCL]

Suspect you have the front silencer, two pipes in and two pipes out, which functions the same as an x-pipe.

[bcredliner]

Yes, I think that is the case but haven't been under there in a while, should take pictures and keep them on my phone I guess.

No comment on my so called plan?

[J.Belknap]

Without nuking the $@#% out of the problem, I'll just say that magnaflow offers a DIDO resonator with internal X-pipe. This won't help though, because the runners to the x-pipe resonator aren't equal.

K.I.S.S. and dump it after the cats. For our practical purposes, any backpressure is too much backpressure.