True Dual Exhaust

JCedar · May 28, 2004

Hey everyone I have a question for ya,

I have a 99 GMC Sierra with the 5.3. If I run 2.5 inch True dual exhaust with NO mufflers, just off the cats am I going to gain or lose gas mileage. Everyone gave me a different answer so I don't know who to believe!! Should I put some mufflers to help with the back pressure OR a Y-Pipe for better for fuel economy. ALL I want is LOUD, THROATY exhaust with same as now or better fuel economy!!

Draphty · May 28, 2004

My 03 Z71 has a single, dual in dual out magnaflow muffler. I consistently get around 16-17 mpg around town, with the loud pedal hitting the floor very often. On the highway I get 18-19 running 75-80. My understanding is that if you don't have an "X" pipe in the system, gas mileage and power will suffer. I'm very happy with my setup.

Tucson_02Silverado · May 28, 2004

I"ve chose not to comment for a while about the term "TRUE DUALS" being constantly used to describe the most common of dual pipes out on this and other forums.

Today I guess I'm using this topic to help me vent from work...sorry no offense to anyone is meant.....

Referencing NBS, I guess OBS as well since they both have inframe tanks.

In order to have "TRUE DUALS" the pipes also need to be of equal length on both sides. Commonly ran duals describe in this and other forums are the stock pipe to the muffler then an X to two mufflers on the right side then the left side exit pipe ran over to the left rear.

What do you really have....a long pipe on the left side to the X and a long pipe on the left side after the mufflers, yea you have dual exit pipes but the exhaust is still taking unequal paths.

JCedar · May 28, 2004

I know there are a million topics dealing with dual exhaust but none really touch on the fact of running just two pipes from the cats back with NO mufflers. (2.5 inch pipe) I just want to know if I would hurt power, fuel economy or both together with out having mufflers on. I have a buddy that just put 2.5" dual exhaust with NO h or x pipe from the cats back with no mufflers, sounds great, but I am concerned if he lost power or fuel economy, too early for him to know about the fuel economy part otherwise I would just ask him! I am mostlikely going to go with an x-pipe or h-pipe with the duals, so I would think that would help out the loss of power aspect of it?????? Sorry if I didn't use the correct term when saying True Duals.

Draphty · May 28, 2004

Ok..now we're splittin' hairs....Laughing. 1 pipe per bank of cylinders.

AZ-Z71 · May 28, 2004

I'll tell you this, true duals are not as some people think two pipes out of one muffler.

LikeaRock14 · May 28, 2004

There's prolly a few people who knew I'd reply

Well, for starters, you were WAY off on the 2.5" pipe diameter. WAAAAY to big for ANY stock small block when you go true duals, especially with no mufflers. 2.25" is the biggest you want to go, that's what I reccomend for every system, even if it's a single/dual system. You'll retain more low end and control sound. If a straight pipe system is NOT DONE CORRECTLY, then yes, you're definately going to lose power and mileage. Most people assume the freer the flowing, the better. Yes, the lesser amount of backpressure, the more power you usually make, although you TOTALLY altert the power curve w/ no backpressure. I have heard of instances when people did go true ual and lost power, not horribly, but it was pretty noticeable. I assume the 99's still came with dual cats, then y'd into single to the muffler, so yes, you are capable of running true duals. I'm not a fan of really loud exhausts, but as long as you do it correctly, it will probably sound great. BUT KEEP IN MIND you WILL lose some low end torque, but you'll notice a difference up top. That's the main thing to remember. Although it may feel like you've lost wpower, the power curve's been altered.

Here's what you should run:

After Cat 2.25" X pipe

2.25" pipe to your choice of exit location w/ your choice of tips

If you run this system and still lose too much power, try a Dynomax 12" Bullet or a Borla XR1, you'll still be pleased.

Tucson_02Silverado · May 29, 2004

JCedar I wasn't slamin' on you or others really for that matter.

And I'm sorry for jumpin' your legitimate question with my venting rant.

It just seem to be the TRUE and DUALS were getting to be like the past in reference to engine building; building an engine or the ultimate is building a blueprinted engine.

'yep she's a 1967 Vette with a "blue printed" 427......."

All's blueprinted meant was that a bit more measurements were done and calculated for fit and clearance beyond just the usual line the marks up.

Technically and by emissions law in most states two pipes from the muffler is still dual pipe exhaust. True duals just went down to the base definition that if there are left and right pipes (regardless of muffler/cats, Y, H, X) providing flow from the heads then it's a true dual exhaust.

I wasn't entirely correct about True Duals means equal length per side exhaust.

True duals are a tuned item for a certain (desired) RPM range to manipulate the exhaust pulses from each cylinder for a certain engine behaviour +/- HP or +/-torque. Yes there is exhaust brakes (mainly diesels) to aid braking but put that aside for now. The characteristics of the each pulse tends to slow during time (equate time kind of to length of pipe) two types of actions need to happen first the next pulse needs to accelerate the previous pulse in a way as not to exceed nor over compress it, second each pulse should also be able to maintain a bit of vaccumm to pull what is behind it (at the heads cylinder exit gases and again not too much as to start pulling intake mixture as well)

So basically with a standard dual pipe back (on a V8) you have 4 pulses each almost equally timed apart. Adding Y, X, H pipes helps to take the "almost" equally time apart to a lot more equal. But the placement of the crossover depends on the RPM range (remember the pulses tendency is to slow), that's the tuning part. This is where my orignal statement of equal length pipe wasn't entirelly correct. Some performance still is had by placing the crossover at a "tuned" spot that complements the pulses. Still the drawback is that 3 feet of extra pipe on the left side before the crossover. And add in the 3 feet on the left side for exit, looks cool but just defeated the purpose a bit.

A good exhaust for most would be replacing the stock muffler with a high flow with dual out 2.25" pipe both to the right side exit or a single out 3". The general consenses is that is good for up to 325HP engines. Dual 2.5" or single 3.5" for up to 450HP engines.

My what a long A$$if puff puff no give that was.

maroonsilverado · May 29, 2004

well he would have backpressure if he kept the cats. but i dont think it would sound that good. my dad has a single flowmaster muffler running through single 3" pipe from the y-pipe back......it sounds great and he gets about 20 in town and about 25 on the highway....and that's driving the piss out of it. he's also got 3.73 gears in the rearend and running 305s on his 99 Z71.

i have true duals on my 91, complete with no cat and i dont like it as much as i thought. im about to put some hi-flow cats on it to quiten it down and give it a little more backpressure and bottom end torque

JCedar · May 29, 2004

No hard feelings Tucson, I know where your commin from, I am on midnights right now and that's NO fun at all. Plus I even read this thing sometimes and think to myselft, "THAT WAS JUST ON HERE"! haha

As far as exhuast goes, I am gonna go with 2.5" duals commin off the (two cats) and then have an x pipe with two 40 series Delta flowmaster mufflers! I figure if there would have been a loss using the too big of pipe that the mufflers would make up for it. Thanks for all the replies.

LikeaRock14 · May 29, 2004

The mufflers aren't going to help retain as much backpressure as you think when you run 2.5" pipe. For any small block true dual, 2.25" is the best all the way up to even 350 hp. engines. It's the most efficient. Bigger might seem cooler, and it might seem you're going to make more power, but remember, you're altering thee power curve a lot, espeially w/ true duals. 2.5" will work, yes, but it's not your best bet. The two chamber Flowmaster fl=ow significantly more than you sem to be thinking. Flow bench results aren't that much different when yo measure Flows vs. a straight through like Magnaflow. Companies space the numbers out a lot on the graphs so it looks like they seem to flow more. The baffles actually help scavenge gasses out easier than some straight rhough designs.

I'm not saying you shouldn't go with 2.5" pipe, I just higly reccomend you stay w/ 2.25" pipe. You'll be much happier in the long run

bigdogs · May 31, 2004

I ran the "true dual exhaust" on my 71 Camaro with a 350. It ran 2.5" pipes out to a glasspack on each side. No cat's no y-pipes, etc.. I didn't see any loss of power with it, just a nice rumble.

snoman · May 31, 2004

I think duals are overrated anyway. A high flow muffler with 2ea 2.5 inlets and a single 3 to 3.5 inch exhaust will work just fine up to 400hp or more. I like to see the 2.5 inch down pipes off of engine as it helps the greatly expanded hot gases get out of cylinder banks better and by the time is reaches the outlet of muffler it has cooled some and not expanded as much and the reduced volume will flow well through a 3 or 3.5 inch tail pipe. My 89 suburban has a single 3" in/ 3" out Flowmaster and it breaths real well. My 2000 K3500 has 2ea 2.75" 'inlets and a single 2 7/8" outlet. I plan to switch it over to a flowmaster with a single 3.5 outlet sometime.