How many CFM?????
 

So, How many CFM would a 3.6L push at 6000 rpm ?????

The answer will lead on to a muffler discussion.

Re: How many CFM?????
 

3.6L = 762.8 CFM @ 6000 rpm in total.....

BUT, each piston is only pushing exhaust every forth storke.
So 762.8 / 4 = 190.7 CFM

Dan

CFM = [CID x RPM x VE] / [2 x 1728]

Re: Re: How many CFM?????
 

every 4 strokes is 2 rpm, so it takes 2 full revolutions to expel the gas..so, wouldn't it be 762.8 / 2 = 381.4 ?????

Re: Re: Re: How many CFM?????
 

You're right. I hastily confused 4 strokes with 4 revs, but obviously 4 strokes means 2 revs.

Dan

It's not clear to me that the flow numbers for a muffler in CFM is really going to tell you anything meaningful about the muffler since it ignore the following critical aspects:

Accoustics
Primary lengths (prior to muffler)
Secondary lengths (prior to muffler)
Collector design

I suspect that as long as the muffler outlet(s) have about the same cross sectional area (or slightly larger) then the secondaries going in to the muffler you'll be fine. As example, you see all of these kids with coffee cans on their mufflers which have more then ample flow, but often still hurt the HP.

I agree - it would be useful for a 1st approximation -- like a flowbench for heads. After that, you have more complex issues to deal with.

Another important point is that the combustion will heat up the air / fuel mixture, and thus increase its volume. Therefore the volume of air drawn into the engine is not the same as the volume expelled through the exhaust. The mass will be more or less the same, but because the volume depends on the temperature, which varies throught the entire length of the exhaust system, it requires a lot of math to calculate the volume of exhaust passing through a muffler over a certain period of time.

/Peter

The other major factor your overlooking is the chemistry of burning fuel and the way intenal combustion engines and all other fuel burning propulsion systems work in the first place, that been that the products of the combustion have a larger volume than the fuel and air burnt in the first place hence what drives the piston back down the cylinder. So to work this out you would have to start looking at molar values of whats put in to what comes out. Anyone got a periodic table and a few hours?

I mole of any gas = 23 liters at room temperature IIRC

Steve

I guess I was just looking at the CFM question and didn't notice it was related to a muffler. I agree with what the others noted here. This calc is for intake sizing, not exhaust.

So a muffler that can pass a similar amount of CFM as the engine wont cut it as the gasses are super heated and volume has increased.

John is right on target here,....:)

All four factors are just as critical as the muffler internals and dimensions.

A note about race mufflers,......we did some testing a few years ago between a Flowmaster Series 80 muffler and a similar Magnaflow. On smaller engines, there was an 18 HP difference from 3K to 7K with a FAR smoother air/fuel curve. Outwardly, they looked almost identical except for materials. :) :)

IMHO, there are more bad mufflers out there than good ones so one should do some dyno tests with the specific engine being used since that too, is a big variable. In many cases, its not a "one-size-fits all" sort of thing.

So, who had better performance magnaflow or flowmaster?

It depended on displacement, cams, and compression ratio. :)

Re: How many CFM?????
 

Don't forget the normally aspirated cars have about 80% volumetric efficiency, so multiply a theoretical calculation by 0.8 for an approximation of CFM flow.

As I recall from the books I've read exhaust sizing is more a function of maximum HP that it is engine displacement. I used to have a calcultion around somewhere for it. On a street car as well as some lower reving track cars you'll find a little back pressure is benefitial. In an all out drag race engine I've used 4" pipes into 4" two chamber mufflers vs. open headers and seen little difference in ET. I did note that with mufflers the 60' time was a touch less so there's agood chance there was more HP with open headers (4.5") but more traction with the mufflers (about 60lbs near the wheels). When I built the X-pipe for my 911 I went with what I know works well in the 200-250 range for a V8. With two 2.5" Walker Dynomax mufflers and 2.5" pipes onto shortened heat exchangers my 3.0 made roughly 210 HP at the flywheel after converting from rear wheel HP. The downside is the muffler is designed to be used with a tail pipe and I only had room for about 6" after the mufflers so it's loud, I like it but others might think it's too loud. Maybe bell will chime in here as he's both driven as well as ridden in my car. Meanwhile I'll dig up my books and try to find those calculations.