Quote:
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