exhasut flow question?

[95LT1Z]

hey guys im new to the forum. i have a 95 z28 LT1 engine, automatic trans,i have all the typical boltons, my exhaust is longtube headers w/ 3" collectors, 3" pacesetter y pipe, no cats. i have a cutout in the i pipe and after that it goes into a spintech crossflow muffler. i am suspecting that this muffler is very restrictive b/c i lose high rpm performance and gas mileage. i have a 2.5" dynomax bullet muffler at my house. i was wondering if i could come off of the 3" i pipe and put in the 2.5" bullet and put a turndown tip on it before the axle. i know that the size of the muffler doesnt matter just as long as the flow of it is acceptable. using the 2.2cfm/hp rule. i figure my engine needs about 750cfm of flow. on dynomax website Dynomax :: Mufflers :: Ultra Flo Welded
all of the straight throuh style mufflers flow from 1000-1133cfm. this is with a 2.5" muffler flowed at 20.3" H2O

will using the 2.5" bullet muffler off of my 3" single exhaust be a restriction?

[FlowSpecialist]

Originally Posted by 95LT1Z

hey guys im new to the forum. i have a 95 z28 LT1 engine, automatic trans,i have all the typical boltons, my exhaust is longtube headers w/ 3" collectors, 3" pacesetter y pipe, no cats. i have a cutout in the i pipe and after that it goes into a spintech crossflow muffler. i am suspecting that this muffler is very restrictive b/c i lose high rpm performance and gas mileage. i have a 2.5" dynomax bullet muffler at my house. i was wondering if i could come off of the 3" i pipe and put in the 2.5" bullet and put a turndown tip on it before the axle. i know that the size of the muffler doesnt matter just as long as the flow of it is acceptable. using the 2.2cfm/hp rule. i figure my engine needs about 750cfm of flow. on dynomax website Dynomax :: Mufflers :: Ultra Flo Welded
all of the straight throuh style mufflers flow from 1000-1133cfm. this is with a 2.5" muffler flowed at 20.3" H2O

will using the 2.5" bullet muffler off of my 3" single exhaust be a restriction?

First of all a CFM figure is only meaningful when considered at a given pressure drop. I have no idea what pressure drop the 2.2 CFM per bhp rule was devised at but I suspect 25" of water. The website you refer to tests at 20.3" of water so that is going to produce lower CFM numbers for a given item than a test at 25".

So let's look at the possible validity of their flow claims.

The maximum that a straight piece of tube with a nicely tapered inlet and outlet (a venturi tube) can flow at 25" pressure drop is 138 CFM per square inch of cross sectional area. That's not open to debate it's part of the basic flow equations. Flow follows a square root law so at 20.3" pressure drop that maximum will be 138 x sq rt (20.3 / 25) = 124 CFM per square inch.

A 2.5" diameter tube has a cross sectional area of 2.5 x 2.5 x pi / 4 = 4.9 square inches. The most it can therefore flow at 20.3" pressure drop is 4.9 x 124 = 608 CFM.

You say, and I've looked myself even though I can't quite read the small print it's so tiny, that they are claiming over 1000 CFM from a 2.5" muffler. That has to be the biggest load of bullshit I've ever seen on a website because it's physically impossible by a factor of nearly 2.

Those mufflers have never been anywhere near a flowbench, or at least an accurate flowbench. They've just made the numbers up. You can take that to the bank.

Someone needs to take those guys out someplace quiet and give them a good lesson in fluid mechanics - with a 2 x 4!

Now assuming a proper test at 25" of water then the most any 2.5" muffler could flow, even if 100% efficient, is 138 x 4.9 = 676 CFM. A perfectly efficient 3" muffler could flow 138 x 3 x 3 x pi / 4 = 975 CFM.

So you do need a 3" muffler.

Dave

Addendum:

Looking back through DV's Mini book the flowtests of exhausts in which the 2.2 CFM per bhp rule was used were indeed at 25" of water. In fact the rule as stated in that book was you need somewhere between 2.2 and 2.5 CFM per bhp.

Now as I've shown it's easy to calculate the maximum possible flow of a given pipe size at 100% efficiency but at what efficiency do actual silencers flow? It's hard to tell from the book because he doesn't give the pipe diameter for most of the flow tests but reading between the lines it seems 75% is pretty good going. That means you should just about get away with a 3" muffler if it's a well designed item but you are close to the point of needing a bigger exhaust system than 3". A 3.5" system at 75% efficency would be about 1000 CFM and at 2.2 to 2.5 CFM per bhp would be good for 400 to 450 bhp.

A 3" system at 75% efficiency would be 730 CFM and 290 to 330 bhp.

Dave

[MilesTugoh]

A minor typo error shows the pipe diameter, 2.5", being squared times pi for a result of 4.9 sq in. I am sure you intended pi(1.25 x 1.25) = 4.9 sq in. the cross sectional area of the 2.5" dia pipe.

More importantly for me is the clarification and comparison you so clearly establish to allow accurate comparison of data from different sources. Thank you .

Now I must return to my calculations and use the knowledge you just gave me to make a better choice for my 496ci Caddy BB exhaust system.

http://www.dynomax.com/ecat/pdfs/ultraflowelded.pdf

http://www.magnaflow.com/02product/universalsatinxl.asp

[FlowSpecialist]

Originally Posted by MilesTugoh

A minor typo error shows the pipe diameter, 2.5", being squared times pi for a result of 4.9 sq in. I am sure you intended pi(1.25 x 1.25) = 4.9 sq in. the cross sectional area of the 2.5" dia pipe.

Moi, typo?? Never made one in me life matey You'll see if you look again that there's a "divide by 4" factor you've missed.

More importantly for me is the clarification and comparison you so clearly establish to allow accurate comparison of data from different sources. Thank you .

Now I must return to my calculations and use the knowledge you just gave me to make a better choice for my 496ci Caddy BB exhaust system.

You're welcome

Dave

[MilesTugoh]

Dave,
You beat me to it !!

I just saw the /4 and knew you were correct . That will teach me to not so quickly doubt the master ..

Miles

[1989GTA]

Dave, can you use the same calculations for sizing the pipe diameter of a cold air intake? My throttle body is 1300cfm and I would think a 3.5" pipe would suffice. Over here they use 28" of water for head flow. Thank You, Allen

[FlowSpecialist]

Something else has just occured to me. Are the pipe diameters referred to in the states o/ds or i/ds? If it's o/ds then of course the flow potential is lower than I've stated because the i/d is the diameter the calculations need to use.

Dave

[FlowSpecialist]

Originally Posted by 1989GTA

Dave, can you use the same calculations for sizing the pipe diameter of a cold air intake? My throttle body is 1300cfm and I would think a 3.5" pipe would suffice. Over here they use 28" of water for head flow. Thank You, Allen

Yes you can. The 100% efficiency figure for 28" is 138 x sq rt (28/25) = 146 CFM per sq inch.

That means you need 1300 / 146 = 8.9 sq inches at 100% efficiency which equates to 3.36" i/d. However very few things flow at 100% efficiency but smooth round pipe without abrupt bends won't be far off. You should be ok at 3.5" i/d.

Dave

[95LT1Z]

Originally Posted by FlowSpecialist

First of all a CFM figure is only meaningful when considered at a given pressure drop. I have no idea what pressure drop the 2.2 CFM per bhp rule was devised at but I suspect 25" of water. The website you refer to tests at 20.3" of water so that is going to produce lower CFM numbers for a given item than a test at 25".

So let's look at the possible validity of their flow claims.

The maximum that a straight piece of tube with a nicely tapered inlet and outlet (a venturi tube) can flow at 25" pressure drop is 138 CFM per square inch of cross sectional area. That's not open to debate it's part of the basic flow equations. Flow follows a square root law so at 20.3" pressure drop that maximum will be 138 x sq rt (20.3 / 25) = 124 CFM per square inch.

A 2.5" diameter tube has a cross sectional area of 2.5 x 2.5 x pi / 4 = 4.9 square inches. The most it can therefore flow at 20.3" pressure drop is 4.9 x 124 = 608 CFM.

You say, and I've looked myself even though I can't quite read the small print it's so tiny, that they are claiming over 1000 CFM from a 2.5" muffler. That has to be the biggest load of bullshit I've ever seen on a website because it's physically impossible by a factor of nearly 2.

Those mufflers have never been anywhere near a flowbench, or at least an accurate flowbench. They've just made the numbers up. You can take that to the bank.

Someone needs to take those guys out someplace quiet and give them a good lesson in fluid mechanics - with a 2 x 4!

Now assuming a proper test at 25" of water then the most any 2.5" muffler could flow, even if 100% efficient, is 138 x 4.9 = 676 CFM. A perfectly efficient 3" muffler could flow 138 x 3 x 3 x pi / 4 = 975 CFM.

So you do need a 3" muffler.

Dave

Addendum:

Looking back through DV's Mini book the flowtests of exhausts in which the 2.2 CFM per bhp rule was used were indeed at 25" of water. In fact the rule as stated in that book was you need somewhere between 2.2 and 2.5 CFM per bhp.

Now as I've shown it's easy to calculate the maximum possible flow of a given pipe size at 100% efficiency but at what efficiency do actual silencers flow? It's hard to tell from the book because he doesn't give the pipe diameter for most of the flow tests but reading between the lines it seems 75% is pretty good going. That means you should just about get away with a 3" muffler if it's a well designed item but you are close to the point of needing a bigger exhaust system than 3". A 3.5" system at 75% efficency would be about 1000 CFM and at 2.2 to 2.5 CFM per bhp would be good for 400 to 450 bhp.

A 3" system at 75% efficiency would be 730 CFM and 290 to 330 bhp.

Dave

wow thanks for all the info man! i've been waiting to get a straight answer on this and can't seem to find one til now. one more question... on spintechs website for the crossflow type muffler such as the one i have on my car. they advertise that it flows 175cfm @5" H2O. why do they use such a low pressure drop. and is it true? the muffler in/out specs are 3" in/ dual 2.5" outlets.

[FlowSpecialist]

Originally Posted by 95LT1Z

wow thanks for all the info man! i've been waiting to get a straight answer on this and can't seem to find one til now. one more question... on spintechs website for the crossflow type muffler such as the one i have on my car. they advertise that it flows 175cfm @5" H2O. why do they use such a low pressure drop. and is it true? the muffler in/out specs are 3" in/ dual 2.5" outlets.

You want me to do even more calculations for you when I've been kind enough to give you the equations to use? Ho hum, ok one more.

Flow at 25" would be sq rt (25 / 5) = 2.23 times as much as at 5" so 175 CFM at 5" would be 391 CFM at 25".

Clearly that's abundantly possible from a 3" pipe and also drastically restrictive for your engine. Barely good enough for 175 bhp.

Why are they testing at 5"? I haven't the faintest idea. Are their figures even correct? Same answer.

Dave

Edit: They're probably testing at 5" because they don't have a big enough flow bench to pull a higher pressure drop across things as big as mufflers.