[url=https://flic.kr/p/PL2dqK][/url]

Brand?

Thorley. Only DT and AC make headers for my Nissan. Won't get much out of them until such a time as I pull the heads and port and polish them and grind the valves, and get a dyno tune, but at least I won't have any more broken exhaust manifolds, and it should sound cooler.

Unforutnately, a VG30DETT doesn't fit in the engine bay of an Xterra that I know of.

I wish I followed the auto board more closely. What kind of car are these going on?

An XTerra that he's bro-ing out.

to the last part.

Well, you could say I was bro-ing it out but mine actually goes off-road, tows trailers, gets used on my land to do work, has dents and scratches from wheeling, has been to 12,900 feet up on mountain trails....I like to think of it as my trail or expedition vehicle...it will get you there.

I really need to squeeze a bit more power out of that VG33E, and I don't want to resort to having to put on the VG33ER manifold and supercharger and ECU.

I would turbo it if anyone had bothered to make one designed for the VG33 or VG30 that wasn't HUGE. Other options include older nissan V8 engines like the 4.1 or 4.5, but that is a major major project.

I love the drivetrain on the Nissan. It is absolutely bulletproof, tough, reliable, and surprisingly off-road capable even without upgrades but the engine is a bit underpowered.

Eventually I am going to put in a 3.7 gear set to replace the factory 2.0 set in the transfer case, and my high and low range will barely overlap and I'll be able to crawl it up just about anything the tires will grip on.

Until then, I just want it to run a little better and quit puffing out manifold cracks.

So 3 exit front and 3 exit rear? What kinda straight six are folks using these days?!

They aren't mirrored, but it is a V6

Installed the new Gibson catback stainless exhaust I ordered to get the ball rolling. Pretty easy. I may go back and weld the joints instead of clamps, eventually. It doesn't have a resonator and it has a higher flow muffler, mandrel bends, and is a true 2.25" all the way through instead of 2", so I get a little more flow without dropping the back pressure too much.

I can already see I will need to order new intermediate pipes and flowmaster cats, and probably a new Y pipe. The hardest part is actually finding the right exhaust studs, as I need all short studs instead of a mix of short and long ones used by the manifolds.

Heh, I've got to stop reading car sites heavily populated with rednecks. Apparently the issue is exhaust Velocity, and as your exhaust Has no variable diameter, you have to pick the size exhaust system to optimize either torque with lower velocity or power with higher velocity, in simplistic terms. So I probably did good not upsizing much as I am more focused on torque.

MouthBQ98 said:

---

Heh, I've got to stop reading car sites heavily populated with rednecks. Apparently the issue is exhaust Velocity, and as your exhaust Has no variable diameter, you have to pick the size exhaust system to optimize either torque with lower velocity or power with higher velocity, in simplistic terms. So I probably did good not upsizing much as I am more focused on torque.

---

I am puzzled....


Higher backpressure = less net work available for use.

Increasing the size of the outlet pipe (exhaust), reduces the flow resistance, which lowers the overall pressure required to push the mass of exhaust out of the engine. Less pressure required (in a sense, back pressure), means more available for use.

Why would a diameter have any bearing on the choice of torque vs. power? I would think that is a function of gearing.

~egon

Your optimal velocity will change as you alter the restrictiveness of the system, changing the rpm range where you get the most power or torque. I want more torque at a lower rpm so I will want my velocity max to be reached sooner.

The effect that exhaust flow has on efficiency is not linear. The exhaust system can be designed to help efficiency in the high RPM power band or the low RPM torque band.

higher velocities = more low end torque

I guess I'm not getting it.

To me, you have X amount of material that needs to go through a pipe. At one RPM, the amount is Xx. At a larger RPM, the material is XXx. This will set your pressure drop, since length and diameter (and material) are set.


How does limiting flow from the combustion chamber or requiring more work to remove combustion products make you more power?

Not trying to be obtuse, just goes against all my Thermo thinking and learning.

~egon

Higher velocities help create a scavenging affect on the cylinder. If you want lower end torque, then you want those velocities at lower rpms.

If you are designing for maximum HP, then your exhaust will be larger to the point the higher velocities and scavenging affect happens on the upper end of the rpm range.

Its the same with head designs and valve events/timing (cam). high HP engines can be "de-tuned" to develop peak torque at a lower rpm. This is not only done with different cam profiles, but lower cc combustion chambers, smaller exhaust, etc.

Obviously, car manufactures and engine builders have to find the perfect balance between torque/hp, but it all depends on the requirement/use.

It comes out in pulses, and you want those pulses to form a constant fast flowing stream of air with the least turbulence and most velocity. At a certain rpm range, a certain exhaust confit will have the optimal flow characteristics. Out of that range, there is either too much turbulence to keep an optimal flow or too much pressure does build up that slows the velocity.

I think if you make the pipe big, the air flows are slow and turbulent at lower RPM, but smooth out as you reach higher rpm. If you make it way too big, it remains turbulent and slower at higher rpm. That's what I am reading, anyhow.

Dug this up to from the google:

Quote:

---

We've seen quiet a few "experienced" racers tell people that a bigger exhaust is a better exhaust. Hahaha NOT.

As discussed earlier, exhaust gas is hot. And we'd like to keep it hot throughout the exhaust system. Why? The answer is simple. Cold air is dense air, and dense air is heavy air. We don't want our engine to be pushing a heavy mass of exhaust gas out of the tailpipe. An extremely large exhaust pipe will cause a slow exhaust flow, which will in turn give the gas plenty of time to cool off en route. Overlarge piping will also allow our exhaust pulses to achieve a higher level of entropy, which will take all of our header tuning and throw it out the window, as pulses will not have the same tendency to line up as they would in a smaller pipe. Coating the entire exhaust system with an insulative material, such as header wrap or a ceramic thermal barrier coating reduces this effect somewhat, but unless you have lots of cash burning a hole in your pocket, is probably not worth the expense on a street driven car.

---

and this:

Quote:

---

To gain a more complete understanding of how mufflers and headers do their job, we must be familiar with the dynamics of the exhaust pulse itself. Exhaust gas does not come out of the engine in one continuous stream. Since exhaust valves open and close, exhaust gas will flow, then stop, and then flow again as the exhaust valve opens. The more cylinders you have, the closer together these pulses run.

Keep in mind that for a "pulse" to move, the leading edge must be of a higher pressure than the surrounding atmosphere. The "body" of a pulse is very close to ambient pressure, and the tail end of the pulse is lower than ambient. It is so low, in fact, that it is almost a complete vacuum! The pressure differential is what keeps a pulse moving. A good Mr. Wizard experiment to illustrate this is a coffee can with the metal ends cut out and replaced with the plastic lids. Cut a hole in one of the lids, point it toward a lit candle and thump on the other plastic lid. What happens? The candle flame jumps, then blows out! The "jump" is caused by the high-pressure bow of the pulse we just created, and the candle goes out because the trailing portion of the pulse doesn't have enough oxygen-containing air to support combustion. Neat, huh?

Ok, now that we know that exhaust gas is actually a series of pulses, we can use this knowledge to propagate the forward-motion to the tailpipe. How? Ah, more of the engineering tricks we are so fond of come in to play here.

Just as Paula Abdul will tell you that opposites attract, the low pressure tail end of an exhaust pulse will most definitely attract the high-pressure bow of the following pulse, effectively "sucking" it along. This is what's so cool about a header. The runners on a header are specifically tuned to allow our exhaust pulses to "line up" and "suck" each other along! Whoa, bet you didn't know that! This brings up a few more issues, since engines rev at various speeds, the exhaust pulses don't always exactly line up. Thus, the reason for the Try-Y header, a 4-into-1 header, etc. Most Honda headers are tuned to make the most horsepower in high RPM ranges; usually 4,500 to 6,500 RPM. A good 4-into-1 header, such as the ones sold by Gude, are optimal for that high winding horsepower you've always dreamed of. What are exhaust manifolds and stock exhaust systems good for? Besides a really cheap boat anchor? If you think about it, you'll realize that since stock exhausts are so good at restricting that they'll actually ram the exhaust pulses together and actually make pretty darn good low-end torque! Something to keep in mind, though, is that even though an OEM exhaust may make gobs of low-end torque, they are not the most efficient setup overall, since your engine has to work so hard to expel those exhaust gasses. Also, a header does a pretty good job of additionally "sucking" more exhaust from your combustion chamber, so on the next intake stroke there's lots more fresh air to burn. Think of it this way: At 8,000 RPM, your Integra GS-R is making 280 pulses per second. There's a lot more to be gained by minimizing pumping losses as this busy time than optimizing torque production during the slow season.

---

It doesn't so much "make more" as it just "robs less"

Thanks. Makes more sense now.

I guess I'm thinking of the end game with a near continuous stream. Not the batch steps at low end.

Cat back Gibson exhaust is on, the replacement intermediate pipes, new studs and bolts, seals and gaskets, and flowmaster cats to replace the factory cats that "suddenly rusted out" arrive tomorrow.

MouthBQ98 said:

---

Cat back Gibson exhaust is on, the replacement intermediate pipes, new studs and bolts, seals and gaskets, and flowmaster cats to replace the factory cats that "suddenly rusted out" arrive tomorrow.

---

Vid?

Grrr. They moved back delivery of my intermediate pipes to Monday.