Q about exhaust pipe size

[300rag]

OK guys, here's the conundrum. I went to the exhaust guru today. When we discussed what I wanted and he politely asked me why. I had been wanting to put 2 1/2" duals with an H pipe. He suggested that with heavy cruisers like ours it might be better to go 2 1\4" if not even 2". Now I am second guessing my original choice. His reasoning is that the larger pipe size will lower the torque and that is what we need to motivate our barges. I have looked on line and have seen a few articles saying the same thing, but none with any explanation as to why the pipe size would do this. I don't want to do something to hinder low end grunt for the sake of bigger pipes. What say ye??

 

[tbm3fan]

I could go through all the math on this pointing out how much air each pipe size can flow. Then compare that with how much air your heads can flow and at different RPMs. Long and short is that larger pipes flow more air and if you run high RPMs then it is what you need. If you go too big for your engine and RPM then that flow capacity is useless and wasted. Are you racing?

However, our engines are usually cruising at 2400-2700 on the freeway. That doesn't require large pipes. So what you want is a smaller pipe to create some back pressure, whereby the exiting exhaust can actually pull exhaust out with it besides being pushed out by the piston. This benefits low end torque that your exhaust guru is talking about. So large venturis, large ports and large pipes benefit high RPM and horsepower. Small venturis, smaller ports and smaller pipes benefit low end torque for get up and go. So I'm with the guy saying 2 1/4" max.

 

[300rag]

Only reason I had 2 1/2 in my head was because I had been toying with the idea of the TTI system and it is a 2 1/2. The car will not be raced and it still has the stock cast iron heads for 65 and I am running an Eddy 750. Rear is a 3.23.

 

[polara71]

The HP manifolds use a 2 1/2 "dump while the standard logs use 2 1/4" dumps

 

[Fratzog]

Only reason I had 2 1/2 in my head was because I had been toying with the idea of the TTI system and it is a 2 1/2. The car will not be raced and it still has the stock cast iron heads for 65 and I am running an Eddy 750. Rear is a 3.23.

That`s the same set up I have with a mild 440 in a 68 Monaco 500. Nothing wrong with low end torque but in fairness I have nothing to compare to. The TTI system is an easy DYI set up at a reasonable cost. It sounds great to with 40 series mufflers.

 

[saylor]

good discussion I always heard running open headers can scorch your valves - i.e. no backpressure, then thats letting air come in the wrong way into the motor...old wives tale? from racing quads,,we wanted the motor to breathe as easy as possible both inbound and outbound - the freer flowin the better... but this was wide open racing too...not stop and go street driving

 

[Red63440]

I have the TTI system on my Dodge but I think I have 3" from the manifolds to the mufflers and then 2.5 inches from the mufflers to the rear of the car.

 

[tbm3fan]

Take a look at this site that does a quick calculation for exhaust size. Didn't see it two years ago when doing the calculations for my cars. Here those calculations are already done and he gives a chart that is easy to follow.

http://www.exhaustvideos.com/faq/how-to-calculate-muffler-size-pipe-diameter/

 

[Bryan]

I don't know all the math either, but the exhaust guy I took mine to said 2 1/4 were what I needed. He said an "H" pipe was cool, but not really necessary for my standard 2 bbl 383 in a "whale" (He was humming "The Love Shack" while looking at the car). I went with what he said and haven't noticed any loss in torque, maybe a fraction more in fact.

 

[moper]

If you're building a cruiser than to me, the biggest concern aside from cost is sound. I bent custom pipes for a couple years and I'm particular about sound. In terms of performance in a naturally aspirated engine - there is never a "loss" of power when less backpressure is introduced. Not ever. But - if your engine isn't moving enough air to over-fill it, or the manifolds are more of a restriction (any upstream issue that causes more restriction than the pipe size means the pipe size becomes irrelevant), you may not ever notice a gain. As an example - a 2bbl 383 with a Y pipe and 2" tail pipe has virtually no exhaust backpressure at idle. Bring up rpms and it begins to overfill the exh system starting with the Y pipe and you get backpressure. Replace the Y and single pipe with 2" duals and you will get a gain in power that will be slightly offset by additional weight of the system. Replace the camshaft and you will get more of a gain just because the better exhaust lets the camshaft work. Right up until the manifolds become the restriction. Replace the log manifolds with HPs or headers, and it will realize a smaller gain because now it's the pipes and mufflers.
So - in terms of a recommendation for you - 2 1/4 will work and if you have log manifolds should be fine. If you have HPs or headers I'd go 2 1/2". The H or X pipes will even out and quiet down the tone and trick the gassses into thinking the system is bigger than it really is by harnessing the physics of pressure ways moving down an enclosed pipe. They are labor intensive if it's a custom shops and they add weight (although not much). IMO it's not needed but if you want it, or want to run the 2 1/4 with HPs or headers, go for it. Lastly - if you want sound - a larger diameter pipe will be louder at a higher rpm than a smaller one, all else being equal. I used to like the sound of 2 1/4 head pipes going into high flowing mufflers and a 2 1/2" tail pipes on a mild big block.

 

[commando1]

Stock cast iron manifolds. 2 1/4" - 2 1/2" pipes. No crossover. My 2 cents. It would take a treatise the size of the Manhattan phone directory to explain....

 

[peterlucas]

my new yorker with a 440 had a single exhaust, i have done a holley 750, edlebrock intake manifold, electronic ignition and dual 2 1/2" with no H pipe right from the standard manifolds with a flowmaster 40 on each pipe (doing TTi headers soon) and i got a definite improvement over the whole rev range and sounds awesome

 

[commando1]

Except for the use of the word "awesome", you heard it right there from the horses mouth. That's all C-yachts need.

 

[300rag]

Well, you know the old saying "**** or get off the pot"? Today was the day. I broke down and ordered my 2 1/2" TTI complete with super turbos. They say 7-10 business days plus shipping time. That means by about mid-month it should be at my door.

 

[70bigblockdodge]

You will be good easier breathing means more power and more gas milage, done. Go back to the exhaust shop and tell them they are lazy. Using there logic if I put a /six exhaust on a 440 I shouldn't be able to pull out without blowing the tires off. The only reason for them wanting to use 2 in is because it leaves more room and they done have to work or think as much I.e. lazy

 

[BIGBARNEYCARS]

Breathin' good and I can still see no gain. The 440 is .030 over, 188s and 214s pullin' and pushin' the mix from two 600CFM carters, 65lb cast iron headers with 2 1/2" dump tubes. 2 1/2"s streight back W/ no H pipe or Cross over to twin reverse flow 2 1/2"s in and out mufflers (Sans AAR Cudas). The AM, AM FM CD CASSETTE, AND XM were a waste of $ but 18 wheelers even give me a thumbs up when they hear me approachin', and gawd I love that sound, Jer

 

[300rag]

Well, done and running. I attached the camera to the fuel filler door and away I went.

 

[commando1]

I got a hardon listening it upshift three times!!

 

[rexus31]

I'm running 2.25" with an H pipe.

 

[Supershafts]

What you REALLY want is a single system.

Not the factory crap, what you want is a system that merges the both banks correctly and smoothly, that is worth way more performance all around then a dual system joined.
Done correctly you'll pick up mpg and perf and it will not sound like crap.

You would be better off with say 2" into a merge pipe and bumped to 2.75 and into a chamber muff that is placed correctly in the exhaust.
Back pressure isn't what you're after, you don't really want back pressure you want constant scavenging, and you can't have constant scavenging from a dual, now the issue is getting the merge in the right location and then the muffler
.