Planning an exhaust for 1982 351w ≈300hp at the crank. I see that pretty much everyone with headers is running a dual exhaust system. I've watched many YouTube videos of trucks with duals of many kinds but can't find any videos or sound clips of any with headers, a y-pipe, and a single exhaust (say 4"). Why does no one use this? I know I could use duals with an H or X pipe to get more scavenging over true duals but that costs a little more money and makes them sound more like a racecar rather than a truck in my opinion. Wouldn't a y-pipe help with more scavenging than true duals? I also want the deepest sound possible from mine when utilizing the electric exhaust cutout(s) I have planned. I feel like duals with cutouts would sound too raspy compared to a single 4 inch with a cutout. Thoughts?

 

If you notice in your research, dual exhaust does not fit very well, long tube headers fit ok in the engine compartment, but do not lend themselves to easy hook up underneath. Everyone still buys them and tortures themselves trying to get them hooked up, and then have no room to service the automatic trans or get the starter off.

You certainly could run a y pipe and a single exhaust. I think 4 inch is ridiculous as far as size goes, but that can be some people's theme. I would look at a later model truck with fuel injection, some places offer shortie headers that hookup to the original y-pipe. They won't list them as fitting the early trucks but those later trucks are basically the same and I do not see why they would not fit. You will probably have to plug one or two oxygen sensor holes, but there should be a solution to that somehow.

 

Every truck I have seen has dual 2-2.5" so don't tell me they are hard to fit or else a redneck in their backyard like me couldn't do it themselves.

 

I have headers on Big Blue. L&L headers with a 1/2" flange, and they cost $600. And they do not fit. I'm with Dave, who knows what he's talking about. For 300 HP stay with either stock manifolds or shorty headers. And 2 1/2" pipes are all you need.

 

Short headers were always the plan, I never said long tube. And I know that 2 1/2 is fine for duals for any HP I'd ever have money to achieve but the question is about the single exhaust with a y pipe and an electric cutout. Information about duals is everywhere but really none about the single. Do y'all know anything but that?

 

I'm not familiar with cutouts, other than the sound is awful. Loud and raspy. So I don't understand why you want a cutout.

From what I've read an X-pipe is better than a Y-pipe. But it is really hard to do and not enough better to warrant the effort/cost. So I'd run a Y-pipe and duals back to a Magnaflow 2-into-1 muffler if you don't care for the dual sound. A Magnaflow will give you a deep sound without being obnoxious.

 

Maybe I'm missing someone but wouldn't a y-pipe that goes back into duals for a 2 into 1 muffler just be an elaborate but less efficient x pipe? And the point of the cutout isn't for daily driving. I understand that it's way too loud for cruising but it's for when I'm showing the truck or just feeling frisky. I was thinking that a single exhaust with a cutout going to a larger dump/tip would take out some of the rasp as well as give a deeper sound when using the muffler compared to duals. Is this single exhaust system going to be detrimental though compared to duals? The truck may be modest now but AFR heads might be in the budget if the transmission plays nice so I thought a single 3.5 inch would be good for it.

 

The engine for Dad's truck dyno'd at 500 ft-lbs. And it will exhaust via either stock manifolds or shorty headers into either a 2-into-2 Magnaflow or dual Magnaflows. That'll be all the exhaust it needs.

As for X vs Y, I was wrong with what I said. If you run a Y-pipe you are automatically down to a single exhaust. But an X-pipe leaves you with dials and allows either bank to use both exhausts.

 

Thanks for the info, I don't plan on running a 5 inch straight pipe like on the diesel haha. But will a larger single exhaust give a deeper note with the cutout than cutouts on duals? I know that with mufflers I can make it sound however deep I want but I'm talking about when it's basically a dump 12" after the headers with the cutout.

 

At 4" I think you are into an echoing sound that will be like a little engine into a sewer pipe. And 5" is just a bigger sewer pipe.

As for the cutout, I don't see the need. It isn't going to sound good and it won't help the performance if your exhaust system is good. You may think it is running better because it is noisy, but the numbers won't back it up.

 

It's not for performance at all, only sound when I want it. I see duals with cutouts as basically a muffler dump with a foot of exhaust to save some low end torque. From the videos and sounds clips I can find it doesn't sound like crap to me like you say.

 

Flow in pipe roughly squares with an increase in diameter all things being equal.
Example the square inch area inlet of a 2.5" pipe is about 5" Jump that pipe to 3.5" A 40% increase in diameter dose not yield a 40% increase in square area.
3.5" pipe has an inlet area of 9.6 square inches almost double with just 40% increase in diameter. A single 3.5" exhaust will flow about the same volume as 2.5" Duals.

It may actually flow more as the radii of bends will have to be larger on single the 3.5" and this will affect total flow numbers, in high pressure applications the reduction in friction loss's along the wall of the pipe will also factor in as less fluid/air will be in contact with the walls of the pipe as there is less pipe wall over all in a single 3.5" compared to dual 2.5".

There are some arguments for large singles over dual's and I know they can be set up top provide equal or even better scavenging as you get to work with the exhaust pulses from all cylinders. I'm not expert in that but I do know Radial engines take advantage of this when fitted with full exhaust manifolds.

 

Exhaust cut-outs in my area are illegal. Probably in your area also, you may want to do some research on that.

About not fitting. Look at all the long tube header duals and even some of the manifold dual systems. If you got rid of the side gas tank you would be fine. But most people keep that, and have to run the driver's side exhaust pipe underneath the frame and get it over to the pass side, and try and run them side by side over there. I did one where I ran them under the frame and then on the outside of the frame. If you look at the truck from the side, you can see these pipes hanging down and they do not look good, they do look redneck.

Look at the stock exhaust. How it's tucked up nice and neat above the frame, and a nice spot where it crosses over under the engine up front. That's all I am saying, you can't run duals and make it look nice and neat without taking out the side gas tank on these trucks. Some people do, and run a large 38 gallon tank in the rear.

 

Sound is a preference. I like the sound of a large Magnaflow muffler over the flatulance of a small Flowmaster - and that's exactly how visitors to my shop described the sound Rusty made with the Flowmaster. And I do not like the sound of an unmuffled engine. To me it is loud, raw, and raspy. But you appear to be much younger than I, and what I consider loud, raw, and raspy may sound good to you. It is your truck, so do it your way.

 

Why an excessively large exhaust pipe/system is bad for street driven vehicles....

I copied/pasted this from another site - as the info is useful here. TTO summarize the info below, too large of an exhaust pipe will hurt low end torque. And torque is much more important in a street vehicle than HP.

Start of post:

Since many people get confused about backpressure, scavenging, exhaust sizing, etc., I wrote this up for another board. I just did a quick cut and paste here, so enjoy...

There is a common misconception that engines need backpressure in order to run properly, generate low end torque, etc. That is simply untrue. Backpressure is a bad thing. Always. Take a look at a top fuel dragster...how much backpressure do you think those zoomie headers make? Very little, and those engines produce 6500 hp.

So, what is backpressure? Any fluid flowing through a pipe experiences drag on the walls of the pipe. This depends on a number of factors, including the diameter of the pipe, the smoothness of the inside of the pipe, the viscosity of the fluid, and the velocity of the fluid. This drag results in a pressure drop through the pipe. In order for the fluid to flow at all, the pressure on one end of the pipe must be higher than at the other. In an exhaust system, that pressure drop is what we refer to as backpressure. It's pretty obvious that the engine has to produce this pressure differential, so the less power it has to spend making pressure to push the exhaust out, the more power it can send to the wheels.

Given that exhaust pipes are pretty smooth, and that we can't change the viscosity (thickness) of the waste gas being forced through the pipes, we are left with basically 2 parameters we can have any control over: The pipe diameter and the gas velocity.

Unfortunately, the pipe diameter controls the gas velocity since the volume of gas is prescribed by the engine. So, we really only have one thing we can change. So, bigger pipes allow less pressure drop for a given volume of gas because the velocity is lower. The pressure drop (backpressure increase) is proportional the gas velocity squared, so if I double the gas velocity (by reducing the cross sectional area of the exhaust pipe by half) then I quadruple the pressure drop.

Well, there's an easy solution for that: Just make the exhaust pipe bigger. Bigger pipe, lower gas velocity, less pressure drop, so less backpressure. Wow, that was easy. After all, this is the way it's done for basically any type of commercial plumbing system. Need less pressure drop on a chilled water pipe or a natural gas line? Just make the pipe bigger.

But wait, there's a problem....Having a huge exhaust pipe has killed my low end torque!!! What's different? Oh, there's no backpressure!! Therefore backpressure makes torque!

Wrong.

An exhaust system is different than just about any other plumbing situation. How? Because the flow is pulsed, and this turns out to be a big deal. Every time a pulse of exhaust gas runs through the pipe, a strange thing happens: it as it passes, it has a little area of vacuum behind it. Just like a NASCAR stocker running around the track, the pulse generates a little bit of a vacuum behind it. In NASCAR, a driver can take advantage of another driver's vacuum by getting right behind him and driving in it. The wind resistance is drastically reduced. This is called drafting.

Well, how big the vacuum behind each pules is depends on the gas velocity. The higher the velocity, the bigger the vacuum the pulse has behind it.

Now, this means that I can "draft" the next pulse, just like in NASCAR. In NASCAR, it's called drafting, in an exhaust system, it's called scavenging. You've probably seen this term used when talking about headers, but the same concept applies in the pipe.

I get the maximum scavenging effect if the gas velocity is high, so the pipe needs to be small. By maximizing the scavenging effect, I help to pull pulses out of the combustion chamber, which means the engine doesn't have to work as hard to do that.

This has the most effect when there's a bunch of time between pulses...in other words, at low rpm. As the revs rise, the pulsed flow becomes more and more like constant flow, and the scavenging effect is diminished.

So, at low rpm I need a small pipe to maximize scavenging, and at high rpm I need a big pipe to minimize pressure drop. My exhaust pipe can only be one size, so it's a compromise. For a given engine, one pipe diameter will make the most overall power (i.e., have the largest area under the curve on a dyno chart).

So, the loss of torque has nothing to do with backpressure, and everything to do with gas velocity. So you need exhaust components that are not restrictive (manifolds/headers, mufflers) and that are sized correctly for your application.

The one exception to sizing an exhaust is for turbo cars. Since the turbo is in the exhaust stream, the gas flow spinning the impeller tends to come out of the turbo with the pulses greatly diminished. In this case, you can get away with running a larger pipe than on an equivalent HP N/A engine because you can't take as much advantage of the scavenging effect.