Professional Answer to Exhaust Size Question 2.5" vs 3"

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Comicly the simple chart is pretty accurate for a baseline. Most people go overkill on header tube diameter, merge collector throat size and obviously exhaust size as well. The key is to pick the correct size complete exhaust system from the header flange to the bumper that will scavenge the engine correctly. As I said there are so many variables it will make your head spin, which is probably why everyone just makes everything on the big side. If you exhaust is built and sized correctly you will not have backpressure, the exhaust system will act as a venturi and draw air into the engine on overlap. This is how race engines make well over 110% VE, the air is drawn into the engine. X pipes are a great tool especially for you street guys, but dont go overkill on your tube diameter. Try to go on the small side especailly when you have a street driven car as well, it will be quieter, and accelerate faster. The larger exhaust might help peak HP and TQ at hight RPM but that dosent usually make the car faster on the racetrack. The car will pick up the most ET at the racetrack in the first 330 ft and at each gear change and not at high RPM. Big HP #s from large headers and exhaust are just that, Big #s, there not always helpful when accelerating a race car.

Mark Lelchook
Performance Welding
Racing Headers
 
Here's my .02 real world combo.
2" tti headers and 3" pipes
Hooker areo mufflers
Car runs 10.71 off the juice and the exhaust shown
93 decibles at 3500 rpm's-needed to know for track we run at.

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Well, I ordered the 3" TTI this week. Once I get it installed I will post some video. Went with the recommended Dynomax Turbos and X pipe. I always have my 2.5" x-pipe system if I want to go with that and run ultraflow's later. (which I planned on using for Coronet but who knows) Thanks for all the input Guys, it really helps.
 
That chart can't be accurate. In that case a slant six would use a 1 1/4" or 1 1/2" exhaust.
 
That chart is terrible! I think alot of people go overkill on exhaust but that chart is just underkill. my personal thouhts are 2-2 1/4 for 300 hp 2 1/4-2 1/2 for 400 and 2 1/2- 3 for 400+. for turbos i might step up a size in pipe since I hear they spool quicker with bigger pipes. I might run 3 inch if your looking for that loud echo sound ( on a lower hp engine).
 
SirDan said:
That chart is terrible! I think alot of people go overkill on exhaust but that chart is just underkill. my personal thouhts are 2-2 1/4 for 300 hp 2 1/4-2 1/2 for 400 and 2 1/2- 3 for 400+. for turbos i might step up a size in pipe since I hear they spool quicker with bigger pipes. I might run 3 inch if your looking for that loud echo sound ( on a lower hp engine).
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I like your numbers and being that camquest puts my build at 512 HP, and 565 HP if I decide to change to my tunnel ram, I have alot of room for error.
 
HotLines said:
I'm so happy I read what was said here as there is much information for my future builds. On my end I personally have 2 1/2 inch pipes from the fenderwell headers straight back under the rear going to 5 inch supertrapps which are adjustable for sound as well as low, mid and top end. I've run these on this car on 3 motors so far, without a clearance problem or any problem for that matter that I have noticed and the car is lowered. For my next build which I won't discuss here because its not a mopar, chevy or a ford and have considered going with headers like the old Lola and GT40's exhaust which are sometimes called the bundle of snakes, sorta hard to explain but you can search for it on Google. I believe they are great for low rpm motors like strokers but will cost you big dollars if you haven't welding experience.

The info given here is so intense and truthful that I have been very impressed with the FABO members who replied to the poster and where I too have learned something.
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Now that is the true intention of this forum....:thumblef::thumblef: and why it exists in the first place...
 
Exhaust Pipe Diameter is comparable to Soda Straws.

You ever notice how when you go to Taco Villa....you get a Monster 44oz cup with a 6" wide lid, and a tiny .25" diameter 12" long straw?

You go over to McDonalds, you get a 32oz cup, a 4" lid and a .375" (3/8") 10" straw.

Which one can you empty the cup FASTER with?

Exactly. The .375" High Flow McDonalds Straw.

Interior Volume of a given size of pipe HAS a Maximum Flow for a given pressure/vacuum.

(CID X RPM) / 3456 = CFM

So for a 360 to breathe IN and Exhale OUT 7000 rpm worth of air....the exhaust system would need to be able to flow 730cfm WITHOUT RESTRICTION....if you choose to small of pipe diameter, the engine will see flow restriction and there will be BACK PRESSURE which will hinder high rpm FLOW.

Choose too big and......the engine may be a little soggy on the bottom end.


By messing around with turbochargers for the last couple of years, I have learned how the diameter and interior VOLUME of a stick of pipe can translate to FLOW.


The chart posted is pretty accurate....if you read the fine print....it said CRANK HP recommendation.

My general rule of thumb is this:

Single Exhaust:
320hp WILL FLOW THROUGH 2.5" pipe
550hp WILL FLOW THROUGH 3"
700hp WILL FLOW THROUGH 3.5"

Dual Exhaust:
450hp WILL FLOW THROUGH 2.25"
600hp WILL FLOW THROUGH 2.5"
750+ WILL FLOW THROUGH 3"

Each one of those listed HP's is the limit for the size of pipe.

You could always put a Pressure Gauge in the rear of your exhaust system.
When the (PSI) pressure of the exhaust system reaches 1.5 - 2 psi the pipe has reached its flow limit and is starting to cause excessive BackPressure.

IMO....you want less than .5 psi of pressure at Max RPM....equalling ALMOST ZERO BACK PRESSURE.

MY Naturally Aspirated 360 will have a 2.5" Dual System, and when I Turbocharge it later on down the road...It will have a single 3.5" system.

The same interior volume/flow application is relevant to Pre-Carb, or Throttlebody Intake Piping as well.


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So based on this information, 2.5" duals is too much for my stock 5.7 Hemi. 2" or 2.25" max.
 
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