Dual Exhaust=Torque Loss

[Tundratow]

I read in a post that said torque goes down after installing a dual exhaust with free flowing mufflers.
Can someone explain why? Does this also hold true for a single exhaust with free-flowing muffler?
I tow and need all the torque I can get out of the 4.7.
Thanks

[peid]

I read in a post that said torque goes down after installing a dual exhaust with free flowing mufflers.
Can someone explain why? Does this also hold true for a single exhaust with free-flowing muffler?
I tow and need all the torque I can get out of the 4.7.
Thanks

Back Pressure.

I have the dual exhaust and lost quite a bit of throttle response, like when getting on it from a stop light. A stock Tundra will blow me a way I think.

But man it sounds cool.

The sound means more to me than the 0-60 times.

I don't think you will lose as much with just a single, but the stock is about as good as it gets, except it is quieter than most cars!

[mike.elmes]

Is the torque really lost? Perhaps it's just relocated somewhere higher in the RPM range. True for towing the torque is better to have low in the rpm but where does it re-appear? If it's just a 1000-2000 rpm higher it's still on tap but will require more foot.

[RockyMtnRay]

I read in a post that said torque goes down after installing a dual exhaust with free flowing mufflers.
Can someone explain why? Does this also hold true for a single exhaust with free-flowing muffler?
I tow and need all the torque I can get out of the 4.7.
Thanks

I'm going to be towing as well...over Colorado's high passes so torque is also supremely important to me. I've done hours and hours of research on this issue to see how I can optimize the torque on my Tundra in that all-important-for-towing 2500 to 3500 RPM range.

From my research, I've concluded the key to getting maximum mid range torque is to maximize exhaust energy (heat) and velocity all the way to the end of the tailpipe...at the exhaust volumes and flow rates the engine produces in the mid range.

Furthermore, if possible, the pulses (from the exhaust valve openings) in the exhaust flow should be maintained...the low pressure on the back edge of one pulse helps pull along the high pressure on the leading edge of the next pulse. This is the principal of a scavenging exhaust.

The reason that dual exhausts...or overly large (like 2.5 inch and larger) single exhausts reduce low to mid range torque is they allow the exhaust gasses to expand (which cools them) and to lose velocity. The cooled air is denser/heavier...and the exhaust pulses more or less are merged together further losing their velocity and scavenging action. The net result is the larger exhaust pipes become essentially long tanks of semi-stagnant gas which has to be pushed out by the incoming exhaust...it's no longer self-scavenging. Or, in other words, a larger or dual exhaust has higher effective backpressure than a smaller, single exhaust at mid-range RPM . Yes, the larger/dual pipes have less wall friction on the gasses than the smaller single pipes but this effect is nullified by the loss of energy/velocity in the larger or dual pipes. It's very important to note this situation applies to the mid-RPM ranges...at very high RPM (like near redline), there's enough energy, volume, and velocity in the exhaust gasses to maintain scavenging all the way to the end of the pipes (even duals) and the lower wall friction of the larger pipes does come into play.

Bottom line: for mid RPM ranges, intuition is wrong...there's more effective back pressure (or less scavenging) in large or dual exhausts; for top end RPM, intuition is right...large/dual exhausts have lower backpressure. And at any RPM, backpressure is just plain bad for torque. There's a myth out there that engines need backpressure to make more torque. This is wrong. Backpressure is resistance and you want the least resistance. If you look at the dyno results and comments posted by V8toilet about his various systems you'll see that he got noticeably better mid range torque than stock by using a single 2.25 pipe running through a fairly low restriction Spintech muffler...and much, much better mid range torque than with either 2.5 inch single pipe or with dual exhausts.

I'm personally going to leave the truck's stock 2.25 inch pipe alone...I totally agree with V8toilet that it's really the right diameter for maximum torque from a 4.7 L engine. I am, however, going to try replacing the stock muffler with an allegedly lower restriction unit from Gibson. Interestingly, the Gibson, which is a baffled straight through design, is just slightly louder (about 3 db) than the stock muffler. IOW, louder does not equate at all to more power. I've considered using a Spintech muffler instead of Gibson but there are too many comments on this board and others that indicate the Spintech (even the quietest one) is just too loud for my tastes.

[Imdone]

Also the tundra's stock exhaust system is not exactly set up for scanvenging.

The stock system is really pushing the exhaust out of the pipe rather than it being pulled out via scavenging vacuum. If you change the stock muffler and tailpipe to 3" without changing the rest of the system you will notice a drop in lower end torque.

If you change the rest of the system to something that promotes scavenging you will find that a larger diameter pipe will work but you can still over-size the pipes and have the same problem.

[dsconstructs]

This seemed like a good discussion concerning the dual exhaust setup. Anyone know what kind of a difference a blower makes with this. First off, it is on a V6 and if some other header makers don't come through soon I am considering going dual catbacks or something along those lines to open the exhaust up some more. I may be misunderstanding some of the many exhaust topics I've read on, but, I am thinking I may get a happy medium (well happy enough) by going dual catback. Any thoughts?

[Agent WD-40]

What would happen if you put a vacuum like device on the tail pipe? Would it help pull the exhaust gas out better and make more power? It of course would have to make more power than it used to run itself. But in theroy, would that work?

[RockyMtnRay]

Also the tundra's stock exhaust system is not exactly set up for scanvenging.

The stock system is really pushing the exhaust out of the pipe rather than it being pulled out via scavenging vacuum. If you change the stock muffler and tailpipe to 3" without changing the rest of the system you will notice a drop in lower end torque.

If you change the rest of the system to something that promotes scavenging you will find that a larger diameter pipe will work but you can still over-size the pipes and have the same problem.

Definitely. Especially the exhaust manifold. That "log", as you've conclusively proven (to me anyway ) with your tests of the long tube S&S headers, significantly kills scavenging and produces a lot of backpressure...at both mid range or top end RPM. This thread was basically about cat-back improvements but you're totally right...incorrectly mucking with tailpipe and muffler (like duals or overly large pipes) can really hurt torque but in actuality there's not a lot of torque gain available over stock (maybe 10 to 15 lbs-feet). As others have noted, the stock muffler isn't all that bad for flow. OTOH, a new properly sized torque-increasing muffler (specifically Gibson or Spintech) at around $150 in stainless is a heckuva lot more cost effective than headers that provide 3 times the torque gain at 6 times the price.

But if those of us who tow really want to boost mid range torque and are willing to pay the price, then headers tuned for low to mid range are definitely the answer...in particular S&S or JBA. With even a stock muffler/tailpipe, I'm estimating the torque gain at the rear wheels will be around 20 lbs-feet at about 3000 RPM (roughly 10% over rear wheel stock torque).

[RockyMtnRay]

This seemed like a good discussion concerning the dual exhaust setup. Anyone know what kind of a difference a blower makes with this. First off, it is on a V6 and if some other header makers don't come through soon I am considering going dual catbacks or something along those lines to open the exhaust up some more. I may be misunderstanding some of the many exhaust topics I've read on, but, I am thinking I may get a happy medium (well happy enough) by going dual catback. Any thoughts?

If you dig deep enough on this forum with your searches, you'll find several posts that indicate the 3.4L V6 with blower is effectively a 4.7L engine from a volumetric viewpoint. IOW, you've now sorta got the V8.

With regard to opening up the exhaust system, the first question has to be where do you want more power? Do you want it in the mid range (as in for towing)...or do you want it more toward the top end (as in for drag racing, highway passing, etc.?). If the top end, then duals make some sense...but don't go much over 2 inches on your pipe sizes. If mid-range, then the points about tuning the V8 for mid range power apply: a proper header with smallish primaries (1.5 inches), single exhaust of no more than 2.25 inches.

Lots and lots of people on this forum think that "opening up the exhaust" is going to really boost their power. In most cases, they are thinking big pipes and/or duals when they use that phrase. The reality is that such "opening" will actually really hurt their power output at most RPMs because the "opened" exhaust is just a big, fat tube of almost stagnant gas at anything much below redline RPM.

[RockyMtnRay]

What would happen if you put a vacuum like device on the tail pipe? Would it help pull the exhaust gas out better and make more power? It of course would have to make more power than it used to run itself. But in theroy, would that work?

Good question and from my knowledge of fluid dynamics, yes it would somewhat...but only somewhat...pull out exhaust faster to make for more power. As for providing a net power gain after subtracting the pumping losses, I'm not so sure and, to be honest, pretty doubtful.

The real point of my post on this topic is that "Theory of Exhaust 101" isn't a simple nor particularly intuitive knowlege area. There are really a lot of hard to sort out things happening inside that exhaust system...not only do you have the relatively long-wave (acoustically speaking) pulses coming from exhaust valves opening and closing but there's a lot of strong higher frequency (short wavelength) sonic pressure waves bouncing around as well. Those wave interactions can, if properly timed in suitably sized resonant chambers cancel themselves out...or...produce standing waves that really block up gas flow.

But as a general principal...and acoustics notwithstanding...the best flow is going to happen if both exhaust velocity and energy (heat) is maintained. Higher or lower pressure at the exit of the tailpipe are secondary to these in-pipe fluid dynamics concerns...mainly because the gas pressure at the beginning of the exhaust tubing (e.g. at the exhaust valve) is very, very high compared to atmospheric pressure.

[KLS]

There are only two advantages of true dual exhaust...smaller diameter mufflers for more ground clearance and that great V8 sound.

Assuming correct sizing (probably 2" for duals on our small V8) duals with an H-pipe crossover will produce more power that individual duals. An X-pipe crossover will produce even more power, about the same as a properly sized single.
http://www.drgas.com/art-syncronicity.asp


Ken

[RockyMtnRay]

There are only two advantages of true dual exhaust...smaller diameter mufflers for more ground clearance and that great V8 sound.

Assuming correct sizing (probably 2" for duals on our small V8) duals with an H-pipe crossover will produce more power that individual duals. An X-pipe crossover will produce even more power, about the same as a properly sized single.
http://www.drgas.com/art-syncronicity.asp


Ken

Ahh...excellent source there Ken...kinda "Exhaust Theory 102"

But sorta doubt the X-pipe crossover is going to ever be real popular with the sound-is-more-important-than-performance crowd given this comment in the link "The deep growl common to V-8s is replaced by a smoother, higher-pitched sound, almost like a high-RPM Japanese motorcycle engine or the shriek of an Indy car".

And then there's the issue of what works best for racing optimizations (usually high RPM operations) isn't always...or perhaps not even often...what's best for maximizing the low to mid-range RPM torque peak for towing. Towing (especially mountain towing with long (5 to 10 mile) demands) is a special case that not many worry about. I've researched the RV.NET forums (particularly the the "Tow Vehicle" forum) very carefully for any mentions of mufflers or exhausts or catback systems and the only regular positive mention/recommendation is for Gibson mufflers/single sideswept catback systems. Also found a fair number of negative mentions about Flowmasters, almost nothing about the other systems commonly mentioned here (Borla, Bassani, etc.)...and that says something to me.

[v8Toilet]

I agree with what RockyMtnRay said. I have found though that the stock exhaust pipe diameter is actually about 2-3/8" in diameter and not 2-1/4" diameter. Just wanted to let you guys know if you are going to order a muffler to get the 2.5" inlet outlet muffler and use 2-3/8" exhaust pipe not 2-1/2" pipe. I say that because in the case with Spintech you can't get a 2-3/8" inlet outlet muffler. You have to get either 2-1/4" or 2-1/2".

[RockyMtnRay]

I agree with what RockyMtnRay said. I have found though that the stock exhaust pipe diameter is actually about 2-3/8" in diameter and not 2-1/4" diameter. Just wanted to let you guys know if you are going to order a muffler to get the 2.5" inlet outlet muffler and use 2-3/8" exhaust pipe not 2-1/2" pipe. I say that because in the case with Spintech you can't get a 2-3/8" inlet outlet muffler. You have to get either 2-1/4" or 2-1/2".

Appreciate the agreement...especially since your numerous and solid posts on the topic were the basis for a good part of my conclusions.

But I am a bit confused that you're now recommending a 2.5 inch inlet/outlet muffler with the stock pipe. In many previous posts on the topic...
such as this one...you've been pretty explicit about recommending the Spintech 3222XL muffler which is 2.25 inch inlet/outlet. You've also commented that 2.5 inch piping (as in the JBA Evol system you tried) slightly reduced torque in the lower RPM ranges. Soooo, any particular reason why you're now recommending the slightly larger muffler/pipe? It seems to me that a 2.25 inch muffler with the stock pipe would be better for towing where torque between 2500 and 3500 RPM is most important.

[v8Toilet]

Appreciate the agreement...especially since your numerous and solid posts on the topic were the basis for a good part of my conclusions.

But I am a bit confused that you're now recommending a 2.5 inch inlet/outlet muffler with the stock pipe. In many previous posts on the topic...
such as this one...you've been pretty explicit about recommending the Spintech 3222XL muffler which is 2.25 inch inlet/outlet. You've also commented that 2.5 inch piping (as in the JBA Evol system you tried) slightly reduced torque in the lower RPM ranges. Soooo, any particular reason why you're now recommending the slightly larger muffler/pipe? It seems to me that a 2.25 inch muffler with the stock pipe would be better for towing where torque between 2500 and 3500 RPM is most important.

I’m only changing my recommendation on the muffler inlet outlet size not the exhaust pipe diameter for the rest of the system. The stock exhaust pipe diameter is a bit larger than 2-1/4” and it’s actually more like 2-3/8” in diameter. I’m recommending that anyone who installs a muffler get a 2-1/2” inlet outlet muffler and reuse the stock 2-3/8” exhaust pipe. If you get the 2-1/4” muffler you’ll just have a short section of pipe that has a slightly smaller inside diameter than the stock pipe. It’s not a big issue because were only talking about a few inches of pipe. I did my testing with a 2-1/2” inlet outlet Spintech muffler welded to the stock 2-3/8” exhaust pipe.

I still do not recommend 2-1/2” diameter exhaust pipe based on my own experiences. Sorry for the confusion! I used to think that the stock pipe was 2-1/4” because I used a tape measure to measure it but later on I used a venire caliper and got 2-3/8” OD diameter.