I will be needing new exhaust on my truck in another year or so, I can't decide if I should install single or dual exhaust on it. I currently have EFI manifolds with homemade down pipes into a single in dual outlet muffler. I am trying to decide if I should run 2.25" true duals from the manifolds back. Or install a Walker down pipe and run 2.5" single exhaust out the back. I plan to use a 40 series Flowmaster muffler(s) so I know it will sound good. Which do you guys think would be best for making more low end torque, but still have decent mid-top end power. All suggestions are appreciated, thanx fellas!

 

Go with the 2.25" duals and make sure you have an X or H pipe to prevent scavenging. What motor do you have?

 

if you are on a tight budget then go with the single pipe cause flows are expensive. Something you might want to look into if you are going to go the dual route would be to check out the flowsound line of mufflers. They are exact copys of flowmaster 40 series, only they are made in china instead of the usa. We sell them where i work and they are 20 dollars cheaper then flowmasters. As a mater of fact i hear they got sewed by flowmaster because the design is so close.

 

I run a single 2.5" on my 300 with a Dynomax ultra flow muffler. It has a nice rumble at idle and sounds great accelerating under load.

 

Just a minor correction. You actually want to encourage "scavenging", which is the purpose of a cross-over, or "x" pipe.

What happens when people run true duals, but do not install the cross-over, is that each "bank" (in this case group of 3 cylinders) will only scavenge for themselves.

Scavenging occurs when one cylinder is on exhaust stroke. In effect, the exhaust being pushed from that cylinder helps pull left-over exhaust from the other cylinders that are not firing, or on compression stroke. This in turn allows for the other cyliders to be free of spent exhaust, and make more room for fresh fuel/air mixture, providing a more efficient burn, and more power.

That being said, the best system for any street truck is actually a single exhaust, of larger diameter. You only have one pipe, and each cylinder contributes to clearing the others. Cross over pipes can actually contribute to anti-scavenging at low rpm's and do not always help. They're usually mounted to far rearward to be of any use in everyday street driving appplications.

Being as the truck in question is an I6, a header with equal length down-tubes running into a 2.5" or 3" collector, and running the equivelant sized pipe and muffler would produce the best results for low end and mid-range power in the everyday driving application that our trucks see. Sound is merely a by-product.

If this truck is a fuel injected rig, installing single exhaust will allow for retention of the O2 sensor, which is very important in regards to how well the truck runs, fuel economy, and not having a bunch of wierd codes come up on your computer.

 

I am sorry I guess I wasn't clear on my description on my running gear. It's an 84 F150 4wd 4 speed with the 300 inline six. The motor is punched .040 over, mild cam, Holley 350 2bbl carb mounted on a Cliffords adapter plate, EFI manifolds, K&N air filter in an open element housing, MSD Blaster TFI coil. The downpipes I got from the bone yard didn't fit...I think because they came from a E150 van with a 300 in it. Being pressed for time I decided to cut the flanges off and make my own, I built it with pre-bent 2" diameter 90 degree pipes and straight pipes. I then made a Y-pipe to connect them so I could tie it into my existing exhaust system. I assembled it in the truck and tack welded it, then removed it to finish weld all the seams. So basically my exhaust consists of 2" duals off the manifolds into a single 2.5" pipe, into a 40 series Flowmaster with 2.25" duals out the rear with 3.5" tips. It sounds sweet at idle and sounds like a straight piped Peterbilt when you get on it a little. The truck has great low end, but I think I can still get more out of it. My buddy's dad told me I should run 2" duals all the way back with 40 series Flowmaster mufflers. He said it would really make my six sound sweet, and it would give me more power throughout the rpm range. I guess he would know, he used to build hot rod sixes all the time when he was younger. Thanks for the help guys, any other suggestions are appreciated!

 

I have always liked the sound of an I6 with duals. I would run duals with both exiting on the pass. side in front of the rear tire to keep the pipes out of the way but that all depends on personal preference and might not help with low end as much.

 

I was thinking about doing that, but I am concerned if I exit in front of the rear tires on the PS; the exhaust might harm the finish on my wheels. I was going to have them run straight out the back, but everyone is doing that nowadays. I am thinking I will run duals out the side behind the rear wheels on both sides. This would give me plenty of protection from pinching the pipes shut if I bottom out in the rear when off roading. I wonder if I should go 2" or 2.25" either way I will have 3.5" tips on the tailpipes. I know 2" would be cheaper than 2.25, I think 2" would be adequate for running 3 cylinders through ( I am running EFI manifolds).

 

One single 3" pipe flows more than 2x2.5" pipes will. Food for thought.

Dual exhaust, in actual fact, only really shows any power on a dyno in the mid to upper rpm band for most automotive applications.

Given the mods you've already done for power improvements, at least run 2.5" pipe if going dual. You also have another option. You could run a header with 3" collector, 3" pipe rearward, and install a 3" in, dual 2.5" out muffler and have the dual tips, or run 3" pipe rearward, and split it with a y adapter into 2 2.25 or 2.5" mufflers.

The cost of dual 2.25 plus two mufflers is slightly more than a single 3" system, and you'd get better gains all around with the 3".

 

Wow, I learned something, thanks for clearing that up for me

so pkupman82, did u decide what kind of system ur getting?

 

I think I am going to go with the Walker downpipe with 2.5" single into a dual outlet Flowmaster muffler, with 2.25" duals and 3.5" tips. The down pipe I made would be perfect for running true duals, but I doesn't allow me to remove the starter from underneath the truck. I would some day like to install a set of headers with the OFFY intake and true duals...that will have to wait because funds do not permit right now.

 

stick with single exhaust thats no larger than 2.5". duals just eat your torque with little to gain in horse power. 3" is ok but sometimes causes idle problems.

 

Its been preoven that 2.25 inch single exhaust on a straight six is best for tourqe and Hp.with a stock motor, of coarse. Your motor, being modded, may be different, 2.5 might be for you, not 100% sure on that.

Thats my $.02
Brent

 

When I first rebuilt my 300, of course I had to add a few num nums to it .040 over, RV cam, etc.. it still had the old one piece single outlet manifold with stock exhaust cats and all. While the truck ran much better than it did with the no. 5 rod bearing knock, it didn't feel any quicker off the line and the torque was not noticibly better. First I cut both cats off it, and that helped quite a bit... better acceleration more hp and tq. I didn't like how it sounded with the stock muffler, so I hacked the entire system from the down pipe back. I then stepped up to 2.5" pipe into a Flowmaster 40 series 2.5 in/2.25 dual outlet muffler and ran 2.25 duals out the back. I felt a pretty big difference, plus it sounded sweet. Then I found out about the benefits of EFI manifolds over stock, so I had to do that too. The get-up-and-go really came out from this mod, but it still suffered another breathing problem...carburetion. The truck would carry it's self down the road great but if you were towing something big and came to any kind of a hill, she was workin really hard and would still lose speed with the pedal to the floor. I added a Clifford's adapter plate and a Holley 350 2bbl carb, along with and MSD Blaster TFI coil. Now my 300 is running really sweet, especially since I dumped the stock 3.08 cogs for 4.10s. She's a rocket ship out of the hole until I get up to about 75 mph, if I push it any harder I might float a lifter or break valve springs. I don't like to beat on it too much, as tough as the 300 is it won't last as long if I do. Thanks fellas!