Factory exhaust

On the OE exhaust layout for these trucks the 2 1/2" Y pipe to a single round muffler to a 2 1/4" tail pipe.  I wanted to ask what does everyone here think the capability of this exhaust layout is power wise?

I had intended to keep the OE layout minus converters and running shorty headers but I have many telling me that this OE layout will not support over 250 hp even though I remember reading years ago it was capable of handling up to 400 hp up to 5,500 rpm for a 302 displacement.  Now I am second guessing myself if I just spent a bunch of money on exhaust components that I probably shouldnt use now as it wont meet my 300 - 350 hp goals.

Rusty_S85 wrote

On the OE exhaust layout for these trucks the 2 1/2" Y pipe to a single round muffler to a 2 1/4" tail pipe.  I wanted to ask what does everyone here think the capability of this exhaust layout is power wise?

I had intended to keep the OE layout minus converters and running shorty headers but I have many telling me that this OE layout will not support over 250 hp even though I remember reading years ago it was capable of handling up to 400 hp up to 5,500 rpm for a 302 displacement.  Now I am second guessing myself if I just spent a bunch of money on exhaust components that I probably shouldnt use now as it wont meet my 300 - 350 hp goals.

It'll be just fine. Pipe size is really relative...the effects of scavenging and velocity are far more important. Obviously you don't want to strangle a 500hp engine with a 1.5" pipe and a 3" pipe is mega overkill on a 150hp engine.


The way I look at it is this, and this comes from a friend who has 30+ years of engine building and dyno experience. 150-250hp, 2" pipe. 300-375hp, 2 1/4", 400-600hp 2 1/2". Anything past that point run a 3" and really  you don't need much more than 3" pipe for almost any horsepower level after that.

Thats what I figured but I saw it mentioned and I did some reading and its like for single exhaust 2 1/2" is only good for 245hp at most and for 300hp you need 3" single and I am like I dont believe that cause single 3" is equal to dual exhaust in 2 1/2" and I know many muscle cars making 350 - 400 hp with 2 1/4" dual exhaust which is smaller than single 3" exhaust.

But with everything online it seems like its all stating I should be running 3" and it has me a little concerned I might be choking my engine down when I throw this on as Im going to be running hedman shorty street headers with OE ball flange with OE 2 1/2" Y pipe with a straight 2 1/2" pipe ending in a 2 1/2" to 2 1/4" reducer to a magnaflow XL 3 chamber 6" round muffler and then the OE mandrel bent tail pipe in 2 1/4".

If need be I could always opt out of the 2 1/4" and have someone mandrel bend me the tail pipe using what I have as a pattern in 2 1/2" and then buy a new magnaflow 6" round muffler in 2 1/2" and run it all in 2 1/2" but I didnt think the 2 1/4" tail pipe would be that big of a restriction that far after the motor in the exhaust system.

How often do you plan to be running at 5500 RPM? That's where your 300-350HP is.

I remember going through all of these thoughts and calculations, and had originally planned on 2 1/2" duals for my 300HP 302. I ended up with BBK shorty headers, 2 1/4" full duals, with Magnflow mufflers with an "H" pipe just in front of the mufflers. When I talked to the muffler shop that built this exhaust for me, I remember him saying that 2 1/2" exhaust was overkill and that there were trucks out there more powerful than mine running 2 1/4" exhaust.

I know a lot of guys don't like shorty headers, but I really didn't want to screw around with long tubes on this truck. My exhaust is all run in the stock locations with the exception of a driver's side exit for one of the duals. The BBK's are chrome (and still look good thousands of miles later) and they have 1 5/8" primaries (I think?).

Thats my thing most of my driving will be freeway driving and if I need to pass someone at 75 mph the rpm will shoot up there when I nail the throttle.  Thats why I am wanting to make sure I am going the right way about this cause I am going single exhaust, I dont want the added expense of dual exhaust and then having to route exhaust from driverside to passengerside the back to driverside as I would want the tail pipes to come out behind the tires on each side if I had dual exhaust.

I will be using my hedman headers which are 89470 part number which is a 1 1/2" primary tube shorty street header with a 3" OE flange collector.  From here my plan can change cause Bassani makes a offroad Y pipe that I have to get confirmation on but I believe it will fit the OE flange as this Y pipe is for 86-95 and my header is 86 - 96.  But if its mandrel bent with no joints on the Y pipe except from after the Y then I can consider this as an option as from the way I understand the description it is 2 1/4" tubes before the Y then when the left and right banks merge at the Y it comes out of the Y as a 3" pipe.  I also found that Magnaflow makes the same muffler  I bought but slightly larger for a 3" inlet/outlet pipe.  This would mean I would have to source a local shop that can do mandrel bending of exhaust that can use my 2 1/2" replacement tail pipe as a pattern to bend me a 3" pipe and remove the mount from the 2 1/2" and attach it to the 3" then I can simply assemble the exhaust myself.

From the charts I found it appears 3" single exhaust is good for 300hp.  But my motor based off other similar builds and DD2000, I should be in the neighborhood of 375 - 385 hp which I am hopeful 3" single exhaust with the large 7" round muffler would flow enough to support it.

I do still need to source the Y pipe as I want a new mandrel bent Y pipe and I am still on the hunt for a Magnaflow one before they started adding precats to them.  Since I havent found and bought a Y pipe yet I can easily spend $350 on the Bassani Y pipe and run 3" single exhaust all the way to the back of the truck.

Rusty, I wouldn't go crazy looking for mandrel bent tubes especially if you end up at 3".

Gary got a beautiful press bent exhaust made up at a local shop.
If you haven't seen it (and the commentary leading up to it) it's worth going back in Big Blue's thread to have a look.

And, I'm 100% with Cory.
If you don't drive it at 5,000 rpm all the time you're WAY better off to keep volume down and velocity up than to lose all that heat and scavenging under 85% of your driving.

If it was a strip car that you could drive to the track I would feel the other way.

If it makes you feel better about it Rusty, I went to my exhaust guy wanting a single 3 inch exhaust to make sure I wasn’t wasting any power and he told me it wouldn’t do any good. He said the only way he’d recommend that is if I had a turbo. I’d have no issue running a 400 horse motor with a single 2.5 inch exhaust. 600 horse? Sure I’d get a 3 inch

I’ve got 2 inch pipes from each manifold to a y. Then a single 2.5 to an xcelerator (magnaflow offbrand) muffler and 2.5 inch tail pipe. It’s not mandrel bent either. Got real good torque and I’m not missing out on anything up high. I get much more excited about low and midrange torque than I do about total hp. Torque is a lot more fun when you’ve got 33s and stock gears

Ill take a look at the exhaust, I dont want to have the local shops here do it cause Ive seen how they do the hangers and I just know the shop is going to attach the hanger to the bed instead of my frame.  If I go with 3" I figured why not go mandrel bent but as of now I am not sure.  I am on the fence and trying to figure what to do.  I just dont want to choke the engine down because I will be having the truck dynotuned once I get the engine finished.

Seems like online all the charts list 3" exhaust for 300 - 400 hp but then reading forums there are posts like what was presented here about 2 1/2" being perfectly fine for single exhaust for a 302 that is stock or mildly built.

I've got original stock headpipes (2 1/2") going to a welded muffler 8" dia X 40" long, single 3" out behind the right hand tire.

There's no restriction in my system and the stock sizes seem to work fine.
It would be interesting to compare CFM between a 460 and a 351 or 302.
I may just do that across the rev range.

So you have 2 1/2" single all the way out back, I wonder how it would work with mine as my muffler is 2 1/4" to fit my 2 1/4" tail pipe.  My plan was to do it like Ford did it with 2 1/2" to the muffler then 2 1/4" from the muffler back.  I could toss the magnaflow 2 1/4" muffler and buy a 2 1/2" and find a shop that can bend me a 2 1/2" tail pipe using my 2 1/4" as a pattern and to rob the hanger off of.

For me most of my driving will be idle to 3,000 rpm.  Highway driving will be a lot of my driving and I will be between 2,500 and 3,000 rpm for highway driving.  The only reason I was thinking on this is cause I dont want to choke my 306 down when I nail the throttle at 2,750 rpm for example at highway speed to pass and the rpms shoot up to 4,000 and I just lack power.

Currently I have a 2 1/4" tail pipe that is not mandrel bent I thought it was but when I looked closer noticed its just a really nice traditional bent pipe without the ribs on the inside bend but a smooth radius.

Muffler I currently have is the 13645 Magnaflow XL 3chamber 6" round offset/offset in 2 1/4".  I can go up to 2 1/2" or even 3" but would require going a different route with my tailpipe.

Don't be afraid of press bends either.
Go back and look at Big Blue's exhaust.

I was real surprised!

Im not the tail pipe I have is a press bend and I thought it was mandrel bent cause the Y pipes I first got from walker through rockauto was press bent but it had all these creases in the bend area and I thought that was going to really disrupt the flow.  Thats why I was looking for the Magnaflow Y pipe but trying to find a old release before they added the precat to it.

This is the Y pipe they have listed 93307 number but it doesnt look like that anymore instead of the mini H pipe between the two pipes before the Y it is now a triangular merger with the merger being a precat which I dont want.



And a photo of how they look now as posted on FTE.



Really dont want one with a cat or a pre cat since I am going to run a sniper and they clearly state to not use emission systems with it.

Walker's crinkle bends suck 🐎.
I can't get a pipe to last more than two winters up here.

I'm not sure why the pre-cat is a restriction, but I would much rather the lower pipe with it than the rusted junk above.
How much difference are you talking?
Ceramic is easy enough to bust out, and the Y is surely stainless if it has to see that kind of heat.

I dont like walker either the tail pipe for my truck with the correct 82 only hanger mount is a freaking 2" which was never offered for a 302 truck they were all 2 1/4" just like the 351W trucks.  The Y pipe is correct but the walker one has crinkles in it which I dont like.  My OE one on my '82 truck doesnt even have crinkles in it, it just has a slight radius indentation like my tail pipe I picked up for a 83-86 F150 which has the rubber doughnut hanger.

For me it was a restriction as I dont like the look of it compared to the stock photo that to this day they still show it without the pre cat.  I was informed the pre cat and the cat supplied from Magnaflow is not as restrictive as the OE units but still I am afraid of them creating issues with my fuel injection system.  Like wise I thought about gutting the pre cat and even possibly gutting the cat itself out to have the look of a cat system without but I was partially concerned with the disruption to exhaust flow having large open chambers where the cats are located at.

The Y pipe from Magnaflow is stainless steel and the price isnt bad at $386.  The Bassani Y pipe with the 3" pipe without cat is $322.14 and it is stainless steel as well.  The description for the Bassani Y pipe is as follows

"Bassani 501502 - Bassani Ypipe for F-150 302; 1986-1995.

2 1/4 diameter tube at Y and 3 diameter tube at leg. Legal in California only for racing vehicles which may never be used upon a highway. It is illegal to remove a properly functioning catalytic converter.

BASSANI Xhaust is recognized for unsurpassed quality and fit; starting with superior stainless steel materials for durability, mandrel-bent and welded by experienced craftsmen. Quality stainless steel products by BASSANI Xhaust are backed by a LIFETIME WARRANTY."

So for me the Bassani would be cheaper than the Magnaflow setup but I cant find photos of the Bassani exhaust to know how it looks and if its mandrel bent like the Magnaflow one and if its in one piece or if it is like the Jegs Offroad Y pipe I bought that is mandrel bent but has these splits in it that requires clamping under the oil pan which I could probably weld it up but I just dont like the thought of a weld being visible in the exhaust directly under my oil pan.  But I think the Bassani would cost me more cause I would have to get a 3" tail pipe bent up, then I would have to spend $111 for a Magnaflow 13749 7" round XL 3 chamber muffler vs what I already have that cost me $88 which is the 13645 6" round magnaflow XL 3 chamber which is almost OE dimensions at 27" case length and 33" overall length.  But if I make a change I would still have to buy a 2 1/2" version of this Magnaflow XL 3 chamber for $98 and I would still have to have a new tail pipe made in 2 1/2".

So I guess either way I go it is fairly close to the same price.

I'm not hating on Walker.
I have a Walker medium duty school bus muffler with Walker flanges welded on the front to meet my stock headpipes.

But (in my opinion) their crinkle bent aluminized pipes are the lowest common denominator for people who "don't want to pay a lot for this muffler" (yeah, I'm old.. get over it)

But I think you'll be leaving a lot on the table if you go to 3".

I would try to find a shop to bend up your Y-pipe the way you want and keep the rest of your system.
If they can't bend and weld a Y for less than $300 I'd offer to go smoke crack out behind the shop with them.

I am perfectly happy with my dual 2.5" pipes from the manifolds to the muffler (still the original 1986 one) then the 3" tailpipe that goes out to 4" where the two spark screens used to be (no longer needed as no air pumps). The only part that has been replaced are the dual 2.5" many years ago when I couldn't get the front joints apart and then later modified when I installed the EFI system.

I will buy Walker and Monroe products, but in my case it is because one of my pension checks comes from them (dates to when we were owned by Tenneco before Northrop Grumman).

BTW Jim, to some of us you're still young.

You’re not gonna be missing out on anything with a 2.5 inch pipe. My 351w breathes well enough it’s still pulling at 5k rpm with the 2.5 inch exhaust. And I also spend most of my time around 2k-3k rpm, it’s got awesome power in the midrange and when you tip into the throttle that magnaflow muffler really barks in that range.

I did the whole exhaust thing about a year ago. I went through trying to find y pipes, exhaust systems, hangers, reducers, etc. Trying to get exactly what I wanted and to do it myself. It was a disaster. I tried to mesh two kits, nothing worked and parts would not fit right. I finally got to a point where I just wanted everything welded up and leak proof but I was embarrassed by all the clamps, reducers, Florida man look to it. It looked chaotic. I found an exhaust shop to weld it all up. The shop owner saw what I wanted and basically cut me a deal. He saw I worked hard on it and was impressed I pulled it off but if put his tools to it, it has his name on it. So he built me a new, professional, smooth sounding exhaust from the y pipe back at a great price. We were both happy and he got a lifetime customer.

For the amount of money companies charge for a mandrel bent y pipe you can have a shop build you an entire exhaust. The going rate around here for a dual exhaust is $400. It may not be mandrel bent but we don’t have 700 horse motors where mandrel bends may net us a decent gain, rather we might see a 2-5 horse gain, if we’re lucky. They sound a little smoother mandrel bent but the extra $$$ ain’t worth it for me. There’s a larger difference in power when dry and cool air moves in than in the way the exhaust pipes are bent.

Also, if you’re wanting to make it look like Ford did it a good shop can do that. The guy that did mine tucked the entire exhaust, all you see is the tail pipe but he flared the end of the tailpipe like the factory did. Other than the 60 degree bent tail pipe and better sound, I promise no one thinks it’s not stock.

But I also see your concern with the workmanship of the shops around you. I understand that. I got burned by one so I get it. Maybe there’s a place just a little further out from you that would work. Make a day of it, a good shop should be able to build a full exhaust in a matter of a couple hours or thereabouts if they’re good.

Whatever you decide to do I hope it works out, sounds great, and performs great

Jim- I’d also like to see the cfm differences, that would be interesting to know

I thought of this thread when I saw this: https://www.ebay.com/itm/Ford-Truck-Pipes-E2TZ5246E/201392578965?hash=item2ee3eee195:g:p34AAOxyqKVRhgeo

I was asked why I am going to dolly my Lebaron convertible to a shop an hour and a half or more away to have an exhaust built for it (2.5" pipe, high flow cat and low restriction muffler). I have been dealing with them through now 4 generations they do some of the nicest custom work I have ever seen. I had a 1977 F150, originally a 300 with a C4. I built a nice Camper Special 390 and had a design I wanted for the dual exhaust. They built it exactly as I wanted, nice mellow sound until you stood on it, then it would "crack". The truck would get 16-17 mpg on the highway and had lots of power.

I had bought a pair of Corvair turbo mufflers as they are a nice free flowing 2.5" in and out oval muffler. They put them both on the right side, H pipe before them and left side muffler in front with the oval vertical, back over the axle and out exactly where the original tailpipe had been. Even used the original hangers.

David, I've been watching those pipes, but doubt they'd be the right width for a 302.
Maybe someone with the decoder ring (MPC) can tell us what engine they fit?  

I had a detailed and consise reply typed up for Jacob and hit send.
The damned connection to the Nabble servers flaked out and.... Poof!  😖

The crux of my reply was that thermodynamics is complex and I enjoyed the mental exercise, but even with a lot of assumptions it comes down to throttle position and how much work is being extracted by the engine.

So, a 5 litre engine at 2,500 rpm (taking this as "average") at 1/4 throttle (an assumption from my driving, but I have no TPS) is still only 25% of the exhaust volume I'd have if it was wide open with 0" manifold vacuum at the same rpm's

I want to say that I think I spend driving time above and below 25% and 2,500, but I spend "average" at this.