Does anyone have any real world experience of running dual 2.5 exhaust into a single Y and 3" outlet Thinking of running this configuration rather than trying to get both exhausts over the rear axle on drivers side.
I ran dual 2.5" pipes into a single 2.5" on my Model A with a Cadillac 390. My setup was more or less a pipe going from the shorty header on the passenger's side, in a "U" in front of the engine (had more space in front). A downpipe from driver's side header into the "U" and a flange immediately after that joint. Then a pipe all the way back on the driver's side, with a single muffler. Saved a lot of space, and I really liked the sound - more like a sports car than a farm tractor. It might not be a great solution for big horsepower applications, but for a 345 hp LOOOW rpm engine it worked fine. I actually think some early hemi cars had this setup - a pipe going in front of the engine.
The manufacturers have been doing it this way since the beginning. And the first thing some hoodlum would do is split them up . It's going to effect the sound and your ultimate HP but if you're good with the sound and not looking for that last 10th it might make packaging a lot easier. Phil
I guess those are the two main fconcerns. Impact to sound (how) and impact to performance (much?) I would think not much on a fun DD 300HP panel truck ....... but ....... never ran one setup like that
A good exhaust man can do it, mine made his own Y in just a few minutes on a ot truck that belongs to my son. We deleted the muffler that was toward the front of the truck on his and put the Y in and duals on out. Im sure flares/adapters to a bigger size or whatever would be no problem. I have put a few jinky exhaust systems on but have found out its better to figure out a way to pay the pros than figure out a way to do it myself
Not really...but they have for the past 20ish years, on pickup trucks, etc. Used to be they just ran two 2" or 2.25" pipes to one pipe the same size. And then charged extra money for a 4bbl carb and dual exhaust. Since the single exhaust was used on low perf engines, it worked fine, and it was one of the first things you'd change when you started hot rodding the car. Modern times are quite different...they are playing the horsepower wars with every engine, even the little ones.
2.5" can be a bit harder to bend tight over rear axle, 2.25 works a bit easier - adding 3" anywhere in system will not help anything unless maybe a turbo charged motor
Had a OT 90 pickup with a stock 350 that picked up HP by putting a y pipe and a 3.5 flowmaster. Had dual 2.25 pipes and glass packs.
Real world experience here. 1941 Chevy Coupe with a 355hp 350 / th350 / 10 bolt. No dyno test or timing slips to prove anything. Had long twin 2.5" steel pipes that merged under the back seat into a 3" y and straight into a turbo type muffler and dumped ahead of the axle. Sounded awesome outside. Ran fine. Droned/ resonated like crazy. Tiring after 30 minutes or so. Leaked some at the sleeved joints. Fell apart as it was only tacked together - typical muffler shop job for previous owner). There were other issues that required it to be redone. New system done by me. Stainless 2.5". Driver side runs around the front of engine and merges (swept) to the passenger pipe and then runs as a single 2.5" to the back seat area. Flowmaster huspower or something muffler. Dumps before axle as before. Did the full Dynamat Extreme and Dyanapad - previous insulation was poor. Sounds very different, not as aggressive but still purposeful. Drone/ resonance massively reduced but despite the new insulation is still moderately noisy inside (disappointed), but conversations are much easier. Seat of pants dyno can't tell the difference. It kind of feels more powerful but I suspect that's because I'm happier to put my foot deeper in as there's less noise! My intention is to now run it over the axle (as single and then split for two tailpipes) in the quest for some less noise - this is a big coupe we use for travelling / towing travel trailer / carrying grandkids. I have other cars to be a noisy hoodlum in Chris
Here is a bit of easy mental calisthenics I use when playing around with any plans to change or join pipes. I substitute a square shape for the round shape because the "formula" is super basic and I don't have to remember the more advanced formula for round shapes. For this exercise, I only want to know the proportions between 2.5" and 3", not the actual, "real" cross-sections. So, 2.5" x 2.5" = 6.25" x two pipes = 12.5" total cross-section. Next, I do the same for a single 3" square; 3" x 3" = 9" total cross-section for the single. You would be stepping down from 12.5" total to 9" total, which is a little more than 25% more constricted area or 3/4 smaller. This also means the exhaust flow has to speed up when it hits the 3" section. The constriction and the pushing for the speed change are 2 negative factors. But there's also some good news to offset the negs. A single 3" has less surface boundary area to cause drag on the exhaust flow. So, with the same pencil and scrap of paper, I'll calculate the sides of the two smaller pipes, 2.5" x 4 sides = 10" x 2 pipes = 20" total boundary layer. Compare that to the 3" x 4 sides = 12" total boundary layer of the single 3" pipe. This results in a larger area of cleaner flow, which is a big positive to offset a bunch of the negatives in the previous paragraph. Of course, a square is not a circle and this won't tell you actual cross-sections. But I think comparing just the ratios or proportions is fairly telling and accurate to determine if you are upgrading or downgrading. If you're a stickler for the"real" numbers, you can look up the more involved formulas that use radius, circumference, pi, etc.
You can't forget that exhaust cools the farther away from the motor it gets so it's volume is less. It would be nice if you can find a muffler that is two 2 1/2" inlets and one 3" outlet and only have one source of restriction.
Flowmaster shows a bunch of 2 inlet one outlet mufflers. I'm thinking that a local muffler shop puts those on a lot of late model pickups around here as you see trucks with one pipe that sound pretty healthy. Examples here https://www.summitracing.com/search/part-type/mufflers/inlet-quantity/two/outlet-quantity/one Depending on the muffler shop I don't think getting 2-1/2 or 3 inch pipe bent is the big deal it was a few years as so many guys are running it on trucks now. At the shop I go to it's just more money but not a big deal.
Found some ancient build Pictures. I was lucky enough to be able to route the pipe underneath my rear axle - saved a lot of job (and curse Words).
pretty easy....take your "squared" area number, and multiply by 3/4 (that's 0.75 for you decimal guys) and you'll get it really close.
You Sir, are a fountain of information and an asset to our community. I didn't know that. (I knew about you, I just didn't know about what you said.) I, on the other hand, am educated beyond my intelligence and have enough experience to have made many mistakes but still await the opportunity to apply the lessons learned from them.
As someone who's worked in the exhaust industry for about 30 years now, I think some of you guys are making things way too complicated. Ignoring circular cross sections vs rectangular but figuring in boundry layer effects? But no compensation for bends? No biggie though, it doesn't really matter anyway. A couple other things to consider. Rough numbers. Restriction rises with the square of velocity. At 3000 rpm you have 1/4 the restriction that you do at 6000rpm, and that's assuming WOT. If you aren't at WOT you aren't fighting the restriction of the exhaust. How much restriction does a 3" pipe add to the system at 2000rpm WOT? Not much. How much time do you spend at max rated RPM and WOT anyway? Also, the cooler the exhaust gets, the less viscous it is. Unlike oil, the exhaust behaves like it's thicker the hotter it gets. So the further back you are the less things matter. But how much does restriction really matter anyway? Flow DYNAMICS can make a difference, sure. But restriction by itself? Not worth worrying about. Back in the '90s I took a stock muffler off a current sportbike. 900cc, 100+hp. I put that muffler on a flowbench, then flowed an aftermarket muffler for comparison. The aftermarket muffler (essentially a glass pack) had 1/10 the restriction of the stock muffler. The stock muffler literally had ten times the restriction at the flow rates the bike made power. Common knowledge was, if you put the aftermarket muffler on the bike, and rejetted to boot, you might gain 3% on the top end. 1000% more restriction equaled 3% less power. If you aren't making a living beating the next guy to the finish line it's not worth worrying about. I'd bet there are other areas of your set up that are a lot more critical to your performance.
Twice pipes are nice but some rigs don't have room on both sides of the driveshaft for one down each side. The OP's rig being one of them. I'm real limited for room for duals down the left side of my 48 and I have cast headers for the 292 six. Luckily I can build room but it may be a bit of a challenge before all is said and done.
Thank you for explaining all that information. Now I know the reason I always drive at wide open throttle is simply because I don't have to think about idle to part throttle conditions and get mentally bogged down and distracted from driving. This may also be good to share with the jury if the judge lets me use it as a defense.
When I was running a pure stock dirt track car. We had to run manifolds , not headers. I experimented with late model Corvette center dumps with 2 1/2 exhaust pipes running into a single 3 inch pipe exiting the side of the car in front of the back tire on the passenger side so the fans in the stands could hear it. Not only did it pull harder coming up off the corner than when it just had 2 pipes , but it had a totally different sound than the other cars. In fact the following season there was a new exhaust rule outlawing my set up because our car won 23 out 26 races. the new rules was dual 2 1/4 inch pipes no more than 36 inches past the manifolds. I found a cross over pipe from an International school bus that was at an auction and it had the dual 2 1/2 into 3 inch pipe stock. I cut it up and that's what I made my cross over out of.
Had an OT mid-70s Malibu that I transplanted a 1970 Buick 350 4V engine into. 2 1/4" pipes back to a Y and then 3" back to a glasspack and turndown at the rear axle. Sounded great, no restriction issues, the pipe was cool enough to touch maybe a foot after the Y. Also idled fairly quietly. With 3.73 rear axle, it was great for stoplight racing and parking lot smokeshows.
Sounds reasonable to me. My OT LS3 Vette has the dual mode exhaust where the mufflers are essentially bypassed at either some engine speed or vacuum level, not sure which. Anyway, in the normal mode its as quiet as any sedan, but in bypass mode its seriously loud as hell, especially if you're on the throttle much at all. GM's published specs show that system is worth 6 hp and 4 ft-lbs at 5900 rpm where the engine hp peaks. Personally, the only thing it has taught me is that noise is greatly over rated when it comes to actually making measurable horsepower.
So could any of you guys provide a rough rule of thumb? In my case I have an issue with emergency brake/exhaust clearance after a gearbox swap. Car has a stock 2 barrel Ford 289 and a stock dual exhaust with 2" pipes. That system will be ditched when the engine gets rodded (e.g., Doug's Headers or Sandersons etc.) but that's in the future still, and I have no idea how the exhaust system will look until I decide the build specs. In the meantime I want to drive the car and trying to modify the system I have now will be a pain in the behind, hence thinking about a Y-piece and a single exhaust - what diameter should I chose for "A"? 2.5" or 3"?
The inside area of the 2" pipes are 3.14" each and the area of the 2.5 pipe is 9.62 so unless the are a lot of elbows in the pipe 2.5 should be fine. Phil
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