So you removed the large system, put back on your small system. and your power loss returned then... correct? Your kinda vague, or I am missing it... you used a Y pipe...

 

Well actually it was the other way around. I had a system on the truck that a muffler shop had created for me, they used a hydraulic pipe bender to shape it and that compresses the pipe and reduces the diameter when it makes curves. This was getting old so I cut it all off and build my own system using mendrel bent tubing for the Y and a high flow cat and muffler on a 3" catback versus the more restrictive stock type muffler. After the change the motor was a bit more powerful at high rpm but it lost TQ down low. Nothing else changed, same longtube headers and the motor was not touched.

 

I know one thing, when big tubes are used, the fuel economy tanks... Was it the same with yours?? Of course maybe it is that way with any improved exhaust...

 

It stands to reason fuel milage would suffer some if something is done to decrease engine TQ, it has to rev more to accelerate and downshift more when cruising. It's also likely those of us with an exhaust that actually makes some noise just like hearing the thing growl, you're definitely gonna burn more fuel if you have your foot planted on the firewall everywhere you go.

 

When the pipe is smaller, you have less tubing surface and the hot exhaust is moving faster. Because of the smaller area per foot and the high velocity, the exhaust has little opportunity to lose energy. When the tubing becomes oversized, the gases will expand to fill that area, and in the process the exhaust slows down, and that causes it to get in the way of the next exhaust wave. Also, you have more heat loss per foot because the remaining exhaust is moving slower and you have more surface area per foot. So as it travels down the pipe, it cools and slows even more.

Have doubts about this? Wrapping your exhaust system can be good for up to 5 hp without any other mods. A stainless steel system can also be good for performance, because stainless is a poor conductor, so you get less heat loss through the pipe. That difference is less noticeable however.

 

Sorting thru all of this discussion...

On a stock 5.0L, with shorty headers, would you run 2.5" pipe to a "Y" to a single cat and a 2.5" Inlet/Outlet "Performance" muffler and a turn down?

Or would you run 2.25" the whole way?

Or some combination of the above?

Thanks.

-cmc-

 

This is a bit confusing... But I started this thread, to see if the way I am thinking is off, and an opertunity to either learn something, or stay with what I have done for years... That beeing said

Shorty headers are minimal improvement over stock manifolds, but use when area is simply to tight... My opinion is they are worthless. but watever... I personaly would not run anything smaller the 3 inch duals on you 5.0... Thats me, But other People are saying, its to big, and you loose tq. I have yet to see that.

While there arguments, I apply to intake runners, when machining, and venturi size of carburator, or throttle boddies. I disagree, and think the fine tuneing on the intake is way more critical, and on the exhaust, as long as the tubes on the headers are matched to the port, the rest of the system, if there is one, simply needs to flow, and flow fast, not restricted, by small pipe. That being said, every engine will have its own definition, to the pipe size it uses.. A 460 will need a bigger pipe, then a 4 cylinder. The disagreement comes in, with how much bigger?

I have used 4 inch on a 302 street car. It turned mid 10's at the track, but was street driven... I ran open headers at the truck pulls with a 400... I won 3 years in a row, in the amature class, in Bowling green Ohio... But if If can squeeze more out I am open to it... But I have tried many combo's, and bigger is better, so far for me, when it comes to exhaust...

 

That is a valid point, and the reason why my radio isnt usually on LOL

For some reason, when I pull to a light, when some wann be has his stereo turned up, My engine mysteriouslt tends to load, and then needs cleared out, Just to the point of watching ther faces smirk becaus they cant hear there thump..., Or ther 4 banger with the dryer vent exhaust sytem...LOL

 

I did exhaust for many years in a shop. My shop sponsered a gentleman that ran dirt oval. He periodically came in and requested certain changes to his exhaust every other week or so. One being that we ran 3" dual off stock manifolds (stock manifolds were part of the requirements in the class he was running) with custom made adaptors over to the passenger side where he had a little 3" y pipe that turned to a side shooter out the passenger side. He swore up and down that it gained not only torque but horsepower at top end by adding this y. This was a SBC 350 but in all honesty was not tweeked hardcore by anymeans. However, on the flip side, I had a 4x4 with a mean little SBD 360 with small diameter shorty headers and dual 2 1/2 inch pipes. After I did the upgrade the horsepower gain at high rpm wasn't great, but the torque gains were awesome. I agree that large pipes have their place but from experience in the shop and all the experiments that I have seen guys do, SB anything with any mods, max size pipe is 3", anything larger is overkill. BB's I beleive do need that little extra however. I have a 400 BB in my dodge that runs 325 horse. I have dual 2 1/2 inch pipes with glass packs and it runs beautiful from top to bottom. Just opinions from a experienced exhaust bender.

 

If anything I think it's safe to say that in terms of exhaust there is no one size fits all, every motor is different and ever driver is different and both will determine what is the best exhaust system for them. Without a doubt if maximum peak HP is your only goal then the closer you can get to open headers the better. If however you want the broadest powerband possible then you'll want to be a little more conservative.. but only upto a point because if the system is too small it'll hurt the whole powerband. As I have said many times the system should be sized to the engine airflow capacity, and that means the specific engine in question not just it's displacement, a 302 with big heads and cam is going to require a much bigger system than a stock F150 5.0. The MAC system I had on my old '89 reg cab worked really well and it used 2.5" pipe from the longtubes back. This truck could not even chirp a tire with the stock exhaust, but the throttle response off idle with the MAC system was incredible.. no problem to spin both tires on dry pavement. That system wasn't really much bigger than the stock exhaust but it did flow a lot better, headers vs manifolds, mendrel bent pipe vs crimp bent tubing, a high flow cat and muffler vs the whisper quiet stock pieces.

 

Don't have dyno numbers but I had a 68 mustang in high school with a K code 289 and try Y headers with cut outs. Went to the high school drags and it ran a mid 15 second 1/4, so then I opened up the dumps to see what it would do that way. Well it wouldn't even do a burn out and ran dang close to an 18. So yeah I would be one of the bigger isn't always better crowd. The exhaust was just 2 1/2" duals off the headers back to the mufflers that dumped right in fort of the rear axle. So it had pretty free flowing exhaust, but there sure is such a thing as too free flowing for that car.
-Johnboy

 

I just stumbles across some related info over on the Corral forum, one of the guys mentioned he did some modeling with a program called PipeMax for a 280 rwhp Explorer motor and the "optimum" header tube size was 1.38" diameter and 19-23" long.

Thier comment was... "People oversize exhaust components all the time. A 1.5" primary long tube will support more HP than most folks make with streetable 302 combos -- and that will optimize low end torque if the rest of the combo is matched."

 

I run 1 5/8 on my Mustang...

Ironey, I had a 68... with a K code to... It was that obnouxious mint green... I painted it Fire engine rd LOL... She ran 14.10 with open exhaust... stock... I tore it down after that weekend.

I know where there is a Kcode sitting, and keep saying I am going to buy it. I want another 68 real bad...

My son and I went to Texas, brought back a Custom 500 with a 289...not a K code though.

 

For street use I think they are right. Seems for the most part the aftermarket is favoring high end horsepower. I've been looking for 1.5" primary with 2.5" collector long tubes for my stock 302 (150hp 2bbl) can't find them. I have Doug Thorley tri-y long tubes on my 6.8l 2V and they have 1 5/8" primaries and 2.5" collectors and the torque increase was awsome.

Too many variables in the whole exhaust thing to say X engine is to use X size exhaust. On a bone stock 460 that barely revs over 4k rpm I'd say a single 3" out the back is about perfect if not a tad large. I'm running a full mandrel bent single 3" system on my 6.8l.

I remember reading a mag article a while back on the Jeep Liberty with the diesel in it. They replaced the stock air filter with an S&B drop in and lost over 1hp and 1tq at the wheels. Then they replaced the stock 2.75" exhaust with a 3" MBRP exhaust (catback) lost a little more power but not over 2hp. The stock hp and tq curve was also better than the other two pulls. Goes to show popular mods don't always perform.

 

Doesn't this support what I was saying??? I don't know if you were correcting me or just adding to what I said with some more info.

No, I don't doubt what you said about wrapping the exhaust to keep the heat in the system. I think I proved it in my post when I mentioned about the converter raising the gas temp which enables the gas to move faster.
I know hotter gas temps and maximizing velocity compliment each other.