Vannin' Home Forums General Discussion Performance - non brand specific 3inch vs 2.5inch exhaust for a Dodge '95 318 5.2?

I have to admit to a moment of stupidity here. I was otherwise preoccupied when it hit me like a ton of bricks how I might have misread this statement. Where I assumed you were implying that Flowmaster was using "Bob's Shop-Made Recycled-Tin Mufflers" to re-brand as their own, or perhaps throwing Magna's in a Flow' case (being a parts guy, I knew that wasn't right), it just occurred to me that you probably meant something like when a shop has a Flowmaster banner hanging and a customer arrives to inquire, and the customer is often swayed after price/product comparison - which happens, I know it does, but I shot off at the keyboard without even thinking of it. Sorry for having to make this post, but I thought it only correct that I admit to my dumb@$$ery in public where I had this attack of heads-up disease. Humbly bowing out, Cotton

I wish I could find the video again, but awhile back, I saw a video on this very question and it was very informative.

They tested a mild 350 carb motor, edelbrock carb, intake and matched cams daily street driven pick up with 3.73 gears (if I recall right) and tried a 2.5” dual exhaust, a 3” dual exhaust and for giggles a 3” single exhaust, all with Flow Master 40 series mufflers. I was surprised when it showed that the 3” SINGLE exhaust was the best for torque. They did a shoot transition like a Y where the two pipes met up front instead of the normal T looking transition and mounted the muffler before the “axle hump”.

Everyone wants big HP numbers from their vans, but remember, torque is what gets these beasts moving, HP keeps them moving. I always try to build for torque myself.

Just my $0.02 on this long standing debate.

Reed, why did I read that to the tune of BARNEY?!? LOL

Well, that was the idea!

The thing to remember about our vans is that the motors will almost never turn faster than 3200-3500 unless you use it for racing. This operating range is below where you see any real-world gains from any of the trick aftermarket exhaust "upgrades" like big pipes or headers. Free flowing mufflers and quality free flowing cataytic converters are fine, but there is no (practical) need to go nuts and get dual 3 inch pipes.

On a 318 I say stick with 2.5 inch single or 2.25 inch dual, a good free flowing muffler like a Walker SoundFX or Quiet Flow, and a good free flowing cat. More gains are had through ignition tuning, camshaft selection, and head selection than through oversize exhaust. If you are lucky enough to have a Magnum 318, you are pretty dang good right out of the factory door.

Any modifications you do should be geared towards improving low RPM torque, not high RPM horsepower. Think cams, compression, and ignition.

If you want to cut torque, HP and MPG's put a 3 inch system on it.

Exhaust Pipe Sizes For Street Performance
Pipe OD (in) Power (hp) Engine Size (CI)
Single Dual
2.0 2.0 100 150–200
2.25 2.0 150
2.5 2.0 200
2.25 2.0 150 200–250
2.5 2.25 200
2.5 2.0 250
2.5 2.0 200 250–300
2.5 2.25 250
3.0 2.5 300
3.0 2.5 250 300–350
3.0 2.5 300
3.5 2.5 350
3.0 2.5 300 350–400
3.5 2.5 350
4.0 3.0 400
3.5 2.5 350 400–450
4.0 3.0 400
4.0 3.0 450
4.5 3.5 400 450–500
4.5 3.5 450
4.5 3.5 500-and-up

Quick rip from Hot rod magazine, sorry could have neatened it up a bit. You want gas velocity, yes it's been shown removing a restrictive exhaust improves power but that's not the whole story. I have a big block Beau" it's said to have 235 Hp and 375 Ft torque from the factory. From the numbers you can see the factory tune is a miss, torque is 375 at 1600 rpm. Not enough Fuel, low compression and the camshaft sucks. I can fix the fuel and camshaft, but compression I'm stuck with. My manifolds leaked and I switched to headers, I went with 2 3" pipes to one 4" tail pipe. From the chart you can see my future target. I went with 2 and one exhaust because it's a better set up, It's a street van not a track van. Yes the dual's are sexier, it just sounds better! Putting an oversized exhaust will leave you down on power.

Gas temp leaving the head is around 1400 degrees. By the time it hits the muffler it is down to 400 degrees.Larger tail pipes just changes the "note" a little , but offers no advantages.

I have a 70' 340 with a little work, around 375 HP, runs high 15's, fully loaded Maxi (B-300) 1972, including AC. I use a appliance header, w/ 2.5" collectors and a 2.5" pipe into Walker Turbo mufflers coming out in front of the rear tires. I have bottom and top end (over 120 mph) Not sure how much over, speedo only goes to 120, but you better be hanging on to the steering wheel. Had this set up sense 1978. Note: on a stock 318, you could get away with 2.25" pipes, but I would suggest to go with headers if your looking for better performance and sound.