Less Restriction, More Torque?

[Malachi]

Those of you who have put headers on your Tundra have said that headers improved the performance and torque of the 4.7. It makes sense because headers would be less restrictive than the stock manifolds. Since headers make such an improvement, then why doesn't dual exhaust or a larger single exhaust do the same? Headers, dual exhaust, & larger single exhaust have one thing in common, their less restrictive. Yet the opposite is true about dual and larger single exhausts. V8 Toilet and many others who have tried these setups have stated that torque decreased.

Does anyone know why less restriction works for the manifolds (torque increase) yet not for duals or larger single exhaust (torque decrease)?

[mitchr]

Duals or larger exhaust seem to hurt the low rpm response. I just put my OEM exhaust back on last week and I have much more response than I had. I have heard that the header that really help the low rpm power is the Autochrome long headers. I may be wrong but I do not think the shorty style headers help as much with the low rpm power. Not sure why this is though?

[RockyMtnRay]

Originally Posted by Malachi

Those of you who have put headers on your Tundra have said that headers improved the performance and torque of the 4.7. It makes sense because headers would be less restrictive than the stock manifolds. Since headers make such an improvement, then why doesn't dual exhaust or a larger single exhaust do the same? Headers, dual exhaust, & larger single exhaust have one thing in common, their less restrictive. Yet the opposite is true about dual and larger single exhausts. V8 Toilet and many others who have tried these setups have stated that torque decreased.

Does anyone know why less restriction works for the manifolds (torque increase) yet not for duals or larger single exhaust (torque decrease)?

Yes, the answer is really quite simple. At the low to mid range RPMs (below about 3400) where maximum torque is reached, muffler-back duals and larger-than-stock pipe cause more backpressure, yes MORE not less, to the engine than the stock 2 3/8 inch pipe. More backpressure causes the engine to have work harder to pump out the exhaust gasses and that in turn results in a loss of torque to the rear wheels. See...really quite simple.

Now for the inevitable question of why a bigger pipe (or dual pipes) has more backpressure than a single stock sized pipe at low to mid RPMS.. And this one isn't so simple. Intuition based thinking (what I call the "garden hose approach") causes nearly everyone to believe that a large pipe (or dual pipes) will have less resistance to flow than a smaller pipe. And if we were dealing with gas (or water as in a garden hose) that's flowing uniformly with no pressure waves and no change in temperature from end to end, that would be the case...bigger is better because bigger has less wall friction/turbulent flow near the pipe walls and turbulent flow causes a lot of resistance. But we are NOT dealing with uniform flow...the gasses coming out of an engine have a lot of pressure waves (caused by the opening/closing of the exhaust valves & piston movement) and they're hundreds of degrees hotter at the manifold/header than they are at the exhaust tip.

Of the two unique characteristics of exhaust gasses, the thermodynamic one is the greatest concern. To get maximum flow through the pipe, the gasses should not be allowed to cool any more than necessary as cooling has two major adverse effects: (1) loss of energy which translates into loss of velocity, and (2) increase in density which means the gas becomes like a thick plug in the pipe.

So, clearly, we don't want to let the gasses cool off any more than we have to. Unfortunately when you let gas move from a smaller to a larger space, it undergoes adiabatic cooling through the sheer process of expansion (e.g. no heat is lost through the pipe walls...the gas just gets cooler because it expanded). The net result is at low to mid RPM the larger pipes (duals or big single) effectively become big air tanks full of cool, dense gas that acts like a plug...which means more backpressure...which means a loss of torque.

See...that wasn't so hard to understand.

[RockyMtnRay]

Originally Posted by mitchr

Duals or larger exhaust seem to hurt the low rpm response. I just put my OEM exhaust back on last week and I have much more response than I had. I have heard that the header that really help the low rpm power is the Autochrome long headers. I may be wrong but I do not think the shorty style headers help as much with the low rpm power. Not sure why this is though?

First of all, the SS Autochrome headers are shorty headers (basically a JBA knockoff). Second, all the aftermarket headers...whether they be S&S Long Tube (totally different header maker from SS Autochrome...the key is that little ampersand between the S's), TRD, Downey, JBA, or SS Autochrome...all have about the same increase in maximum torque compared to the stock manifold...it's about 23 to 25 ft-lbs for any and all of them. The big difference is at what RPM that increase in maximum torque occurs.

The S&S Long Tube have a torque increase onset at the lowest RPM...roughly around 2200 RPM. The mid length headers (TRD, Downey) have a torque increase onset around 2500 to 2600 RPM. JBA headers, despite being "shorties" have a torque increase onset around 2800 RPM and SS Autochrome headers have the highest torque increase onset RPM...roughly around 3000 to 3100. Due to fluid dynamics acoustic considerations (primarily exhaust pulse wave interactions), the rule about headers is the longer the tube (and smaller the diameter of the tube) the lower the RPM where torque increase will happen. Hence the S&S headers which have real long (and equal length) tubes have the lowest RPM, the mid length TRD/Downey are next, and the JBA & SS Autochromes have the highest. There are two reasons why JBA has a lower torque onset RPM than SS Autochrome despite the fact that externally these two brands appear identical: (1) JBA uses smaller primary tubes than SS Autochrome, and (2) JBA has a patented "Fire Cone" in the collector where SS Autochome has a flat bung...this cone causes the gasses to merge more smoothly and thus the header acts like its a longer header than it actually is.

All of the above assumes that there are no other changes made to the intake or exhaust system. For instance, people who have added FIPK type intakes and dual exhausts have seen their torque increase onset point with TRDs move up to around 3300 RPM. And I actually saw the torque increase onset point for JBA headers move down to 2500 RPM...and had an overall torque increase (measured by better towing power @ 2800 RPM on mountain grades)...after I replaced the stock muffler (not exhaust...just the muffler) with a Gibson muffler. Didn't get any drone either.

[sequoiasoon]

From all the headers I've seen (and please correct me if I'm wrong) none of them are larger than the stock manifolds. They appear to just be better shaped for flow and thus are more efficient. The stock manifold comes straight out and the new gasses colide with the ones already in there adding some turbulence and restriction. The headers have the same size discharge ports as stock manifolds but the tubes leading from it merge into one collector.

Think stop sign at a T intersection verses long (or short) on ramp to the highway. Some speed is better than starting from a dead stop.

[mitchr]

Originally Posted by RockyMtnRay

First of all, the SS Autochrome headers are shorty headers (basically a JBA knockoff). Second, all the aftermarket headers...whether they be S&S Long Tube (totally different header maker from SS Autochrome...

My bad, I got them confused.

Mitch

[benjams123]

RckyMtnRay, you are the man.

[RockyMtnRay]

Originally Posted by benjams123

RckyMtnRay, you are the man.

Appreciate the praise. I spend a lot of time researching and reading and then try very hard to write clear and concise explanations. It's good to know that the work is helping people.

[tuesdayd]

Originally Posted by RockyMtnRay

It's good to know that the work is helping people.

Yes sir, indeed it does. Especially some of your exhaust posts. Thanks, Ray.

And to stay on topic...I've kept the stock piping and just had a muffler welded in. I was surprised by how much difference a muffler can make; it complements the headers perfectly.

But, of course, there wouldn't have been such a gain if I didn't have headers on there moving those gases out.

[RockyMtnRay]

Originally Posted by tuesdayd

Yes sir, indeed it does. Especially some of your exhaust posts. Thanks, Ray.

Welcome.

And to stay on topic...I've kept the stock piping and just had a muffler welded in. I was surprised by how much difference a muffler can make; it complements the headers perfectly.

But, of course, there wouldn't have been such a gain if I didn't have headers on there moving those gases out.

EXACTLY!!!!!! The factory manifold is a much bigger "plug" than the factory muffler...until you get rid of restriction from the manifold, swapping mufflers simply isn't going to result in much improvement. However, changing the muffler after installing headers typically results in very noticeable improvements.

[PagalDesi4Life]

RockyMtnRay- so what mod can we do to improve low end torque around 1500-2500 range?

[KLS]

Originally Posted by PagalDesi4Life

RockyMtnRay- so what mod can we do to improve low end torque around 1500-2500 range?

Change the gearing...We have a 3.91. You could change to 4.11, or 4.56, or 4.88, or 5.29. This is especially important if you run big tires.


Ken

[KLS]

Here is some very good info to add to what Ray said. It explains why size does matter, and bigger is not better.
http://www.burnsstainless.com/TechAr...ry/theory.html
http://www.ssheaders.com/header.htm
http://www.headersbyed.com/bildbetr.htm

The concept hardest to understand, but the most important, is the concept of sound pressure waves. They are waves of sound pressure (a force) that begin when the exhaust valve opens for each cylinder, travel at the speed of sound (which is temperature dependent), and do exert a force. Think of ocean waves...there are peaks and troughs. Sound pressure waves have peaks and troughs of pressure moving at the speed of sound, 1035 mph @ 500°F. If a pressue trough is at the exhaust port at the right time for the next exhaut valve opening, the exhaust is sucked out. This is correct header design and called scavenging.
http://www.sengpielaudio.com/calculator-speedsound.htm


Ken