Something to ponder when you consider exhaust pipe diameter...

[Hohn]

Generally, an open tube will flow about 115cfm per square inch of cross-sectional area.



The stock 3" pipe yields an area of 7. 07 inches, and therefore flows about 813CFM.



A 4" pipe yields an area of 12. 57 inches, and therefore flows about 1445 CFM.



A 5" pipe yields an area of 19. 63 inches, and therefore flows about 2258CFM.





So there you have it:

3"= 813 CFM

4"= 1445 CFM

5"= 2258 CFM



Also note how increasing the pipe diameter affects AREA. When we go from 3" to 4", we've increase diameter by only 33%. But we've increased flow by 78%!



Going from 4" pipe to 5" pipe is a 25% increase in pipe diameter. The flow increase is 56%.



Finally, note that the difference between 4" and 5" pipe is EXACTLY that capacity of the 3" pipe by itself. So stepping up from 4" to 5" is exactly like ADDING another 3" pipe worth of flow capability.



The numbers also tell us that many of us cannot get to the point where 4" is a restriction, so stepping up to 5" will yield no performance benefit-- exceptions being those with large twins.



Consider that with no boost, the 359 Cubic inches of your CTD will need 332 CFM at redline of 3200 rpm. The first bar of boost doubles that to 664 CFM. The next bar (29. 4 psi of boost) gets us to 996 CFM. Going up to 3 bars takes us to 44. 1 psi of boost and 1328 CFM.



So on paper, a 4" exhaust isn't even nearing it's flow limits at ~45psi of boost. In REALITY, though, the actual flow will be MUCH lower, further increasing the margin that a 4" exhaust has.



This is because our above illustration assumed that as intake pressure was raised, we would get a linear increase in airflow. NOT TRUE. With each succeeding bar of boost, we gain less and less actual airflow. Eventually, you get to the point of stagnation, where no matter how much higher you raise the boost, you will NOT see any more airflow through the engine.



So while the "paper" example shows that 4" is more than adequate up to 45psi of boost or so, in the real world, the flow limit of 4" is probably much higher-- in the mid 50s or so.



Just something that occurred to me last night laying in bed.



Justin

 

[JoshPeters]

Just laying in bed... ... Sounds like me!



Great stuff Hohn! Thanx for the information and the education!



Josh

 

[Gary - K7GLD]

The relationship between needed flow capability in relation to turbo boost is something I have stressed a few times too - especially where intake flow and air filter capacity is concerned.



One thing your musing above left out, is the typical added FUEL consumption/combustion as boost increases, and the resulting added heat expansion it creates over and above plain air-only boost levels...

 

[JGK]

To sort of expand on what Gary just referred to, don't you have to consider the extra gases produced by the burning of the fuel when considering the gas flow out the exhaust?



Your thoughts are interesting, but if I understand what you're saying, you're viewing the engine as simply an air pump. That is, at 45 lbs of boost the engine is simply pumping that pressurised air charge out each cylinder unmodified.



So, throw in the volume of heated gas produced by the fuel being burned at any particular moment and the temperature of the gas charge being pushed out of each cylinder at a given RPM and recalculate.



Beyond me, but there must be a program out there somewhere that could do it!



-Jay

 

[zstroken]

How did you determine what a given crosssectional area will flow? A 3 inch will flow as much as a 4 inch, but your frictional losses will be greater. I am just wandering what we deem acceptable pressure drop per foot should be

 

[4xquadrod]

I have a 5" on my truck but I think a 4" sounds better. I also have wonderd whats the point of my 5" system when the down pipe is a 4".

Steve

 

[Forrest Nearing]

justin, your CFM calculations are for intake air, correct? how about for heated exhaust gasses? we have a turbo to compress the intake air, but when that hot exhaust gas expands, there's a LOT of it!!!



you can raise boost all you want, and you will NEVER increase air flow through the engine... you will increase the CFM that the TURBO flows, but it compresses it into a little package that the engine can accept. your MASS increases, but your flow doesn't... same CFM, different lbs/min



when the exhaust leaves the cylinder it is met by the turbine which will create pretty substantial backpressure... backpressure is bad... heck, we shoot for 1:1 pressures boost:backpressure (often called drive pressure)



after the exhaust passes the turbine, it's swirling around the outside of the pipe... CFM kinda goes out the window when turbulence is introduced... think of water... it doesn't flow so well if it's swirling around the outside of the pipe... best way to increase flow is to go LARGER diameter... a lot of turbo gassers will run a larger pipe right off the turbo than they will on the rest of the car...



there's a reason the new trucks come with 4" from the factory...



there's a reason the old trucks picked up so much when you added an aftermarket 4" system...



there's also a point where a 5" system will help... you'll SOONER reach a point where a 5" downpipe will help...



most guys won't be able to quantify any losses by going to a 5" system, but many will see gains...



now, on new trucks, you've already got a 4" system, so just run it till you start making some more power... but on older trucks, if the guy is a hot rodder, I often suggest just going 5" right off the bat... I mean, who is ever "finished" modding their vehicle?!?!? :-laf



for older guys who just want to add a little power, I'll suggest a 4" system, but if the guy is drooling over twins, just do a 5" off the bat... it won't hurt you any, and hopefully you'll grow into it eventually



Forrest

 

[JBlock]

I think Forest is right, youneed take in to considereration the expanded volume of with fuel, combustion volume, and temperature. pv=nrt, as i real from college date some 30 years ago.

since your exhuast is now (460+1200) 1640 R (Rankin) inlet side (460=200) 920,



the temperature effect has double volume of exhaust gas, then saying the pressure expandin through the exhaust system say averabe pressure of 2 bar, you got another 1. 5

So not we at 1. 5 x 2 or 3 bar, so you can see where I going, the volume maybe significantly higher after combustion. I not an expert. but you might need a 4 to 5 inch sooneer than you think. Me I still stock, Taking Joe D advice it good to 350HP or maybe 400HP.



Just playing with numbers.

 

[RKerner]

More fun with flow!

How did you determine what a given crosssectional area will flow? A 3 inch will flow as much as a 4 inch, but your frictional losses will be greater. I am just wandering what we deem acceptable pressure drop per foot should be

A 3" pipe will only flow as much as a 4" until its pressure drop becomes the governor potential flow. Friction losses are part of the overall picture as you state. A rule of thumb pressure drop per foot is sort of possible but the boundary layer growth is non-linear, and the effective cross-sectional area is also non-linear. For a short length of 4" pipe, boundary layer growth should be negligible.



I'm thinking Hohn was stating info from tabular data or emprical data for flow across an orifice. Basically, a perfect pressure/volume reservoir with a short entrance that exits to atmospheric conditions. To answer to the combustion heated flow and its effects, one needs to play with a few other principles like Bernoulli's and then add friction to calculate the boundary layer growth for the given length of pipe. Basically, upon heating, the air's volume is increased and its density is decreased. Now the mass flow balance needs to occur between the cold side and hot side of the engine. So, the mass flow equation looks like:

rho(cold)*velocity(cold)*area = rho(hot)*velocity(hot)*area.

To help with determining the velocity,

1/2*rho*velocity^2 + rho*g*height + current pressure = new pressure + 1/2*rho(2) *v(2)^2 + rho(2)*g*h

Throw in Pv=nRT with the correct R's for air and exhaust and you have a simple model for determining the exhaust velocity just aft of the engine. (for compressible flow, the exponential relationships for Temp, Pressure, and Density should be used but this is just a simple example right now. ) Straighten out the exhaust into a simple straight pipe and then do some more ciphering for the boundary layer thickness at the exit. The boundary layer will grow to reduce the effective diameter of the pipe and this is the flow potential constraint at higher pressure differentials that Hohn alluded to.



One more detail would be to bring in fuel mass flow to the overall flow which would add finer resolution to the model. I'm not sure what the stoichiometric ratio between fuel and air is in a diesel but the difference will be somewhere around 11-13:1. This makes for fuel adding to about 6% to the total mass flow which can be neglected for basic exhaust pipe sizing. But then you could figger out the work needed to turn the turbo and subtract that from the total energy in the flow and then.....

 

[Forrest Nearing]

I not an expert. but you might need a 45 inch sooneer than you think.

not sure if you'd have to go THAT big... :-laf

 

[Hohn]

not sure if you'd have to go THAT big... :-laf

From what I recall, the flow spec of 115cfm per sq inch comes at 28in of water. I pulled that number out of a popular hot rodding article by David Vizard (whose name needs no explanation).



It is, in fact, EXHAUST, not intake related.



The raw numbers as far as pipe flow are concerned are valid, imo.



However, you all are correct in needing to account for the mass flow of the FUEL as well.



That said, the mass flow of the fuel is not that high. Or is it? We're talking about what-- 40-50gph? Thus, we get about 280-350 lb/hr?



Converting that from lb/hr to CFM would tell us just how significant the Mdot contribution is.



The easy part is knowing that all the mass into the engine comes out of the engine, so all we have to account for is air and fuel.



Yes, hot air is less dense and flows more easily. How does this affect pipe diameter?



I'll repeat my previous contention that the "perfect" exhaust pipe size only exists for a given set of conditions (RPM, boost, EGT, turbo). You change ANY of these things, and the optimum pipe size changes.



What we're hoping for is to find the pipe size that's best for our range of conditions.





In a lot of ways, we have it easy with a turbodiesel. Going too big on a turbodiesel won't hurt you much. Going too big on a gasser can kill power.



I think I may end up going 5" just to "plan ahead"-- ideally a system that's 4" DP to a muffler that's 4"in, 5" out, then full 5" from there back.



jmho, as always



jlh

(EDIT: corrected bonehead usage and spelling errors)

 

[Gary - K7GLD]

"I'll repeat my previous contention that the "perfect" exhaust pipe size only exists for a given set of conditions (RPM, boost, EGT, turbo). You change ANY of these things, and the optimum pipe size changes. "



THUS, the "tuned" exhaust systems on race bikes that hit a very audible resonance and power surge at their design point.



"What we're hoping for is to find the pipe size that's best for our range of conditions. "



And THAT'S the trick!



In my case and typical operation (towing), I'd look for a system that delivered maximum efficiency in the 1800-2200 RPM range at a boost level of 25-30 PSI...

 

[barbwire]

Can I ask a question of you guys that have the ability to figure these things out?

OK, here goes.

If I seldom ever run my RPMs over 2300,and average travel RPMs are 1900, and boost never exceeds factory settings of 22, would I see any benefit in a 4" over the stock system on my 04? I think it is 3. 5"

I tow constantly, if that makes any difference. I also put on about 80,000 mi a year. I would like to know if the benefit of 4"would outweigh the cost. Givin the cost of fuel, I need all the feasable edge I can get. TIA

 

[Forrest]

If I seldom ever run my RPMs over 2300,and average travel RPMs are 1900, and boost never exceeds factory settings of 22, would I see any benefit in a 4" over the stock system on my 04? I think it is 3. 5"

I don't know about mileage & performance, but IMO, there is a huge benefit ... stock = boring ... not stock = ... more exhaust sound in the campgrounds = :-laf

 

[R-N-R]

Aside from size increase, don't most aftermarket systems also offer mandrel bends, thus improving flow over stock systems?

 

[Hohn]

Can I ask a question of you guys that have the ability to figure these things out?

OK, here goes.

If I seldom ever run my RPMs over 2300,and average travel RPMs are 1900, and boost never exceeds factory settings of 22, would I see any benefit in a 4" over the stock system on my 04? I think it is 3. 5"

I tow constantly, if that makes any difference. I also put on about 80,000 mi a year. I would like to know if the benefit of 4"would outweigh the cost. Givin the cost of fuel, I need all the feasable edge I can get. TIA

I really don't think that you would benefit much at all from an aftermarket exhaust. If there WAS a benefit, it would certainly be small, and the cost/benefit would be unfavorable for sure.



In stock form, the exhaust is pretty good for stock fueling. I mean, if there was a lot of hidden MPG there, wouldn't the factory do it?



jlh

 

[barbwire]

Well Justin, in a way I wish you were wrong :-laf . If 10 people would have posted that they picked up 3 mpg, while towing, then I would have gone ahead with the expense of a 4" system. But, I have a tendency to agree with you. However, I would like to know why they went with a 4" system on the newer models. must be some benefit there.

 

[Gary - K7GLD]

Well Justin, in a way I wish you were wrong :-laf . If 10 people would have posted that they picked up 3 mpg, while towing, then I would have gone ahead with the expense of a 4" system. But, I have a tendency to agree with you. However, I would like to know why they went with a 4" system on the newer models. must be some benefit there.

I think HOHN is referring to purely STOCK trucks:



In stock form, the exhaust is pretty good for stock fueling.



- adding power mods certainly changes the picture!



Then:



I mean, if there was a lot of hidden MPG there, wouldn't the factory do it?



Well, maybe, maybe NOT! :-laf



After all, IF we could merely trust the experts and beancounters at DC to "do the right thing", we wouldn't have things like weak trannies, poor fuel distribution systems, 70 MPH vibrations etc. , etc. :-laf :-laf

 

[Hohn]

I can only surmise why they went to 4" exhaust on the newer trucks.



I'm guessing that they discovered a better pressure differential across the turbine side, enhancing spoolup and reducing emissions.



Obviously, the cost was not significantly higher for them to do 4", as the benefits don't seem to be significantly higher.



Either that, or they are trying to encourage people to BOMB their engines by giving them bomb-friendly exhaust from the showroom



jlh

 

[aaronisbad]

There is a pretty big debate on one of the Ford sites over 3. 5" or 4" downpipe. Most of the guys feel that using the 3. 5 downpipe keeps bottom end alive, where the 4" cause's lost bottom end with increased flow on top.