I have a 97 f150 4.6 2wd. I wanted a little nicer sound so I went to the local muffler shop for a Flowmaster. Well after learning that the Dynomax was cheaper and seems to flow better (according to the box it came in) I went for the Dynomax and had the guy put a very short turndown pipe (about 8in.) in order to get some sound that missed out on by not getting the Flowmaster. My question is will this basically non existent exhaust pipe ruin the amount of back pressure necessary for top performance? Prior to this, the pipe was only about 3-4 feet and came out in front of the right wheel

 

I don't really think it's going to matter much in the way of performance, but you should check your local and state laws regarding tailpipe length. Some areas have a minimum length allowed.

 

I don't think it will matter on top-end, but bottom end and on the hwy in overdrive it may, but you should already know atleast about the bottom end. if it takes more peddle to get up the hill, oops!
I am still trying to figure out what to do with my 2000 v6 that I put a single in dual out with 2.5" pipes and it sounds good but I lost my bottom end power.

------------------
2000 F-150 4x2 XLT Supercab Flairside Toreador Red, 4.2 Auto 3.55 LS axle 275/60R17 Lariat Wheels" Factory CD, Keyless entry, sliding rear window. Slim-Top hard tonneau, K&N, Lund Interceptor

92 4x2 4dr XLT Explorer 121K, 4.0 v6, PaceSetter Exhaust, Wayback seat, Running boards



[This message has been edited by caddpro (edited 07-05-2000).]

 

Laws of physics seem to have changed from when I was a young man playing with engines to now as an aging man merely enjoying engines.

Would someone care to explain why reducing back pressure has any negative effect on low-end (or at any speed, for that matter) performance?

I always went under the operational theory that we were supposed to:

1) Cram as much air/gas in as fast as possible; and

2) Get out the spent stuff as fast as you can.

I think I understand the operational cautions for changing the scavenging a two-cycle engine -- and think I understand the concept of adding an expansion chamber before the exhaust pipes to lessen the pulse-peaks (motorcycyle) -- but...

...for the life of me, I cannot understand why reducing back pressure on our venerable 5.4 liter engines can hurt low end performance.

Somebody please try to teach this old dog whuuuzaa!

?



------------------
Y2K™

Toreador Red, Keyless XLT SC SB 5.4L E4x4 4wDisc/ABS, 3.73LS, Skid, HD 7700# Towing, LT-245's on Chrome, Tube-Steps, Captain's, 6CD, Tonneau, named: "Nick"

 

Roadrage,
I highly doubt you will lose anything. You still have catalytic converters and mufflers. These should provide plenty of backpressure for your setup. I have heard you can lose some low end torque with extremely high flowing exhaust, but I have done that and haven't lost a thing. Also remember, race cars and tucks in tractor pulls run open headers.

[This message has been edited by primetime (edited 07-05-2000).]

 

Y2k I believe that the theory is that with todays low compression engines the amount of backpressure that is created actually begins to act as a vacuum to help extract the exhaust. Other wise the engine actually has to push the exhaust until it is completely exited from the pipe. Older ( back in yours and my dads time -no offense) high horsepower large compression engines need the shortest distance possible to exit exhaust gases because these engines had so much power they could peel a potato just by setting it in front of an exhaust pipe after a throttle stomp. Well maybe not but you get my drift. This why those "older" style of exhaust routing is done on race cars that have custom built (non emission compliant) high hp/compression engines. By the way thanks for the response guys. I actually put on the Dynomax about a year ago and have since raised truck 6in. with 33's. I haven't done gears yet the truck is pretty sluggish and I was re thinking my exhaust. I think I'm gonna go with a flowmaster ( I want moare sound) with a little longer pipe. sorry for the long post but I know you guys love em. Later Rage

 

Y2k I believe that the theory is that with todays low compression engines the amount of backpressure that is created actually begins to act as a vacuum to help extract the exhaust. Other wise the engine actually has to push the exhaust until it is completely exited from the pipe. Older ( back in yours and my dads time -no offense) high horsepower large compression engines need the shortest distance possible to exit exhaust gases because these engines had so much power they could peel a potato just by setting it in front of an exhaust pipe after a throttle stomp. Well maybe not but you get my drift. This why those "older" style of exhaust routing is done on race cars that have custom built (non emission compliant) high hp/compression engines. By the way thanks for the response guys. I actually put on the Dynomax about a year ago and have since raised truck 6in. with 33's. I haven't done gears yet the truck is pretty sluggish and I was re thinking my exhaust. I think I'm gonna go with a flowmaster ( I want moare sound) with a little longer pipe. sorry for the long post but I know you guys love em. Later Rage

 

The 4)pre and reg cats provide the necessary back pressure. The large rear pipes people use do nothing if you don't replace the Y-pipe; it's single 2.5" exit is the most forward limit in the factory system. Increases in pipe/muffler/& flow do matter more for the larger & more powerful 99-00 5.4s. I lost no low end torque or towing ability, and got more response at all speeds. My y-pipe is a 3" exit Flowmaster, my cat-back has 3" muffler and pipe. Flowmaster's mufflers don't flow enough for this application (with one muffler) 400cfm's if compared to others: Dynomax, Borla, or Edelbrock. I've tried to help people several times with this and got slammed every time. Go look and measure your y-pipe. Edelbrocks Honda Civic cat-back for 1.5L engine is this big. Don't slam me if you have 2 FM mufflers like a Mustang because that is twice the flow (800cfms). Also, did you all notice that the 6 cyl, and the 4.6 all have the same size pipe as the 5.4? Y2K is not off; Different engines and HP also have different needs. Look at a pipe size chart from any manufacturer: 300-350ci @ 250HP needs 2.5" pipe/300-350ci @ 300HP needs 3". We are talking about 260HP stock and approx. 300HP with mods. And remember we are talking one pipe.

------------------
2000 F-150 XL,RC,LB,5.4,4R70W,3.55LS,
Class III tow/Payload #3/Convenience pkgs.,
4-wheel disc/ABS,Chestnut/Parchment 40/60,
Ford bedliner & gas/wheel/spare locks,
3" cold air box modification,Superchip,
Dynomax ultra-flow welded 3" cat-back.

 

Y2K,

This is part of a message I posted a while back on the board:

Exhaust backpressure is NOT the reason for the loss in low-end torque, but is caused by a resonance tuning effect. The IC engine is an air pump, which has distinct cycles. As the piston pushes the exhaust gases out through the exhaust valve, it creates a high pressure zone in the exhaust port. This high pressure "zone" traverses through the entire exhaust stream (at the speed of sound) in a pressure wave. This pressure wave then "bounces" back when it reaches a "wall" or an opening, such as the cat or muffler. (Note that these pulses travel at MUCH HIGHER velocities than the average exahust velocity.) This pressure wave will travel through the exhaust system, from the manifold to the limiting component (which could be the cat, muffler, or even the tailpipe opening). A simple correlation is an organ pipe. Resonance is whay makes the sound. And sound is nothing more than air pressure waves.

If you can "time" the reflected pressure pulse so that the low pressure "portion" correlates to right before the exhaust valve closes, you create a natural scavenging effect (negative pressure). This tuning is determined primarily by the diameters and lengths of the exhaust piping (with a slight contribution from geometry). Note that this tuning effect is only valid for a very small RPM band, and that there is an offset exhaust valve positive pressure at a different RPM band. This is exactly the same scenario in the intake system, except that you tune to harness the positive pressure at the intake valve.

Lower back pressure is always good, but in the process you may effect the tuning of the engine, which is why there may be a loss in low-end. So, anything to reduce backpressure is GOOD - backpressure does nothing but harm performance. However, some people complain of a loss in low end torque when increasing the intermediate pipe diameter. This is attributed to TUNING not BACKPRESSURE. You DON'T need backpressure, as some may believe.

I hope this clears up some ambiguity...



------------------
VENOM 99
2000 F-150 SuperCab 4x4
Off-Road package
Extang BlackMax tonneau cover
BedRug
www.students.uiuc.edu/~cellariu