Hank - what's that got to do with anything we're talking about here? The article reports PEAK power at relatively high RPMs, and it reports on the effect of increasing backpressure on an overly large exhaust system. I have never, and will never, say that backpressure is good. It's not. Backpressure is never good. Don't confuse having properly tuned pipes for adding backpressure. The article states that in testing there isn't any gain caused by increasing backpressure.

What it doesn't tell you is what happens to low RPM power when you increase or decrease the exhaust pipe diameter. We're not talking about adding backpressure, we're talking about tuning the pipes for the best compromise between low-rpm velocity to assist with exhaust scavenging and high-rpm flow.

 

That graph where the Austrailan checked torque versus rpm at full throttle went down to lower rpms...2000 or so if memory serves me.

It is a common 'internet myth'
repeated over and over
that a low restriction or large diameter exhaust system
robs low end torque.
IT IS JUST NOT TRUE AND NOT FOUND ON DYNO'S OR TIMED ACCELERATIONS.

At lot of the confusion is where people have read about optimum exhaust port sizes and primary header internal diameters...then mistakenly confused that to mean exhaust pipe sizes.

You can 'tune' an exhaust port size, valve lift, or header primary diameter to move the torque peak around. For example see this excellent series of articles by old timer Jim McFarland, who is now technical director of Superchips:

http://www.n2performance.com/archives.shtml

Particularly lecture 5

The thing to note is that
when the exhaust gas is moving about 260-360 mph
the 'momentum' is optimum for a port or primary pipe
but that should not be mistaken
for meaning that your final exhaust pipe downstream of whatever is acting like the collector
should restrict the speed inside to 260 mph.

For engines with cast iron exhaust manifolds this matters ziltch.

 

Maybe the solution to my loss of low end torque would be to just buythe complete exhaust set up for my truck from flowmaster, and remove the crap that the muffler shop put on there. They sized up my stock 2.5" up to 3" right before the muffler (i guess to make the muffler fit)and then back down to 2.75" after the muffler. Or maybe i should just go back to the stock pipe set up, scrap the dual exhaust, and just put a performace muffler on there.

I dont know much about exhaust, but i guess there is a whole lot more involved than just sticking big 3" pipes on there and expecting it to go fast. If only i knew that about $500 ago...

 

The australian fitted a huge exhaust and then increased backpressure, from your post. His findings still don't apply because he was measuring something completely different.

 

I was doing some tech research on the 4.7l yesterday. Come to find out, the intake, throttlebody, and cam should be done before the exhaust on this particular engine. If the exhaust is done first, it will only hurt performance. The key is to build the intake side first. Get as much air into the engine first, and then build a nice fre-flowing exhaust. The heads actually flow really well on this engine, and unless running turbo or supercharger, stock heads should work really well. Wish i would have known that about two weeks ago.

 

musta ran outa reducers that day, and the bender wasn't working,
i have 3" on mine cat back no muffler, its got plenty of powa...

 

What else have you done to your 4.7??? Intake, cam or TB??

I lost all low end torque when they added the exhaust. I also heard that you shouldn't use anything bigger than 2.75". But yours runs good eh??

 

3" duals are way too much. now a 3" single exhaustafter the catwould be good, but if you're running true duals go with 2.25 inch and 3 inch tips. by the way, stock intake on 4.7 is pretty damn good, dont bother with it unless you're getting a bigger TB. if you're not running TRUE DUALS then you wont see a power gain, true duals meaning you get rid of the Y pipe because that's the real bottleneck in the exhaust. i didnt read your whole post because it was too much rambling so im not sure what you already have. Also, dont bother with headers, the stock manifolds were built to flow within 2% of headers.

 

Yeah wink. i read that about the stock exhaust manifolds also. No, i didn't run true duals. Just stock 2.5" to the muffler. Muffler has a 3" inlet so they sized it up, and then 2.75 duals on the back side of the muffler. From what i can gather on different sites is that this is too big. Thats why i have lost all low end torque. I dunno.....there also seems to be people who have use this same set up and had good results. I dont know what my problem is with this application, but it has actually hurt my performance, and i cant really get a good reason as to why. Could it be because of the Y pipe?? And wouldn't you have to run true duals to get rid of the Y pipe??

I am at a loss as far as finding the problem for my loss of performance, but from everything i read it keeps pointing back to the exhaust. Do you have any ideas??? Would be appreciated.

Oh....the only other mod i have applied is a K&N intake. Relocated the filter to the front pass. side to get some fresh air.

Thanks for the help everyone!!

 

just k&n drop in 3 inch cat back not duals, whats the point...? not running a diesel, dumped out the back behind the tire... run 89 octane gas w/ ethnol...i live in Iowa [:-]