Butler put the engine together, Quillen designed the turbo kit and intake from scratch. If there was any sort of edge to be had with larger piping or a larger TB they would have run it. I believe they run that piping because it's the optimal size.

How to calculate optimal size is beyond me. I sent an email a while back asking. Guessing they either thought it was a noob question, or it would take too long to answer.

 

Because CFM has no direct relationship with density. It is only a measurement of the volume, and not a measurement of the density or mass. Its 1800 CFM whether it is at standard pressure, or 5000 PSI. So a 3.5" flowing enough for 1200 HP is its N/A flow. At 14.7 PSI, it should flow enough for 2400 HP, at 29.4 PSI, it would flow enough for 3600 HP, etc. CFM doesn't change, but the lb/min doubles everytime boost is doubled.

So if your calculator is correct and his 2.5" pipe is able to flow 613 HP N/A, then it will be able to flow 1226 HP at 14.7 PSI, etc etc.

 

Cfm changes with pressure and temp. That is density. Cfm = air mass flow * specific volume which is inverse density. When you increase boost what you are trying to do is increase air mass. Not cfm. Air mass is what matters when discussing power production. Thats why turbos are rated in lb min flow

Converting it back to cfm may be helpful for velocity calculations in the pipe, to determine how close to the flow limit one can get, whatever that limit may be for the pipe system. 0.4 mach? 0.6 mach? That figure i am not sure but those figures seem way fast for intake piping. You can see those figures at the head port before the valve but in the induction piping i would think you'd want it slower. Air doesnt like to turn, fast air is hard to turn

 

So far it doesn't seem to be holding me back. I changed from a dual 2.5" exhaust with press bends to a 4" mandrel bend downpipe and it picked up 2.5mph in 1/8th. So far it has been 107mph in the 1/8th at 15psi 3,750 race weight. I think once I get a tighter converter in it it will pick up a lot more. I'm only getting 300rpms of drop on the shift. The new downpipe amazed me on spool time and increased my 9.5psi setting to 12.5psi. Before I could only get 2psi on the 2 step now it goes to 5.5psi within a second and was at full boost .6 seconds into the run.

 

That’s similar to what I was saying about sizing the charge pipe to the engines NA CFM requirements. Seems like there has to be more to it though. If that were the case then 2.5” piping should be more than enough for the majority of the “stockish” LS builds. Why aren’t the “big boys” running singles using 2.5” piping? And the twin guys running 2” or smaller piping?

Assuming charge temperatures were some how kept constant. How does the amount of boost run relate to the velocity in the pipe? For example, is the velocity the same at 1psi as it is at 40psi and 100% dependent on the pipe diameter?


We arent' talking about post turbo exhaust pipe diameter, that doesn't apply to the discussion here at all.

 

I originally asked the question so I realize we were talking cold side. I just wanted to update that I'm still gaining mph with increased boost and making it more efficient. My guess is that I will hit a wall at some point but I think my gen3 sbe will give up before I find it. If nothing else I guess I can be the test mule for this discussion.

 

I gotcha now, my bad. There’s another member with a similar name that’s a bit of a tool so I thought he was chiming in and spoke too soon!

Grab some adapter couplings run 2” piping on that sucker… If it doesn’t slow down, we’ll know there is at least some truth to the “NA pipe theory”. If it’s not a restriction it should drop system volume and speed up response. 2" pipe is dirt cheap.

 

I would try that if it wasn't so much pipe with a bunch of bends. Lol. So far my et and mph are in line with my other truck that has 3" pipe on it and is a front mount. At my current boost level it doesn't seem to be causing any inefficiencies. This is a pretty cool topic though when you start getting into all of the math behind it.

 

I'm going with 2.5 on my build, I figured it would help with lag since it's less volume to pressurize. Kind if like an air compressor with a 10 gal tank will reach full pressure faster than that same compressor with a 15 gal tank. That was my thought process.

 

If temp is held constant, and you increase pressure for a given mass flow, cfm goes down. Velocity goes down thru given pipe size.

But most of the time if compressor is not maxed out, increasing boost increases mass flow. So hold temp constant, you now have more pressure which drops cfm, but more mass flow which raises cfm back up. So net result could be same velocity or more depending how much mass flow is added to the system.

Q = m * v where q is cfm, m is mass flow lbs min, and v is specific volume or inverse density in ft^3/lbmass

 

I should not have dropped out of college. Lol

 

You want to explain this in more detail? Because your equation says that m times v is CFM. Which means that if either m or v get bigger, CFM also gets bigger. If m is mass flow, then increasing boost makes m bigger. If v is volume, then volume remains constant. Maybe you can use the equation in some examples? Because right now, according to the idea that "increasing pressure decreases CFM when we hold temperature constant" also says that "at some increased pressure CFM goes to zero" So I have a feeling you are on the right path but side tracked to a non-applicable set of unrelated variables, at least for this application.

This just... doesn't make any sense to me. Does it decrease or not? Your message is unclear. Your meaning is unclear.

 

V is specific volume. That is inverse density. I noted that above. That gets smaller with more pressure. Its not volume

 

Its hard to tell without real data. All depends on the system. How much mass flow does 2 psi more from a turbo give you? Well that depends on the turbo and motor combo. If your near max on the turbo. 5 more psi may only pick up few hp or give you basically a few lbs per min more mass flow.

If you are just getting into the turbo, then 5 psi more may give you alot of hp or alot more lbs per min air flow.

Density of the air changed the same amount, 5 psi. But one system didnt increase hp as much so mass flow didnt go up as much. Cfm may drop compared to lower boost. Other combo makes alot more power with a 5 psi change, then mass flow went up alot so cfm may go up compared to base boost in this example.


Heres my setup for example. 17 psi made something like 793 whp. On 24.5 it did 1009. Averages 31-33 hp per psi boost.
At 17 psi assuming 90 deg inlet temps, specific volume is 6.42. 793 whp is 991 flywheel hp assuming 80% drivetrain loss. 991 hp is approx 99.1 lbs min airflow. My cfm is 99.1 x 6.42 = 636 cfm.

At 24.5 psi also at 90 deg f inlet temps post cooler, specific volume is 5.19. 1009 whp is 1261 flywheel hp assuming 80% loss. Thats 126.1 lbs per min air flow. My cfm is now 654.

So a minor increase. Generally more boost means higher temps so expect to see abit more cfm over lower boost. But my turbos were gaining good hp per lb and never fell off. A combo maxing out or having back pressure issues would just increase boost and not see a gain in cfm necessarily

 

So if overall velocity decreases with an increase in pressure/temperature (Aka more boost)

If we assume .4 mach is the do not exceed speed for air in a charge pipe and we size the piping to the engines NA needs at ambient (say sea level) pressures/temps. Then the piping diameter requirements shouldn’t increase as boost increases? Meaning as long as the OP was clear of the .4 mach point by a healthy margin "NA", why would larger pipe be beneficial in any way "boosted"?

We are talking below .25 mach for a sub 400bhp LS engine.

 

Sounds like you just picked up on the reason for my frustration...

 

Cfm converted to cubic feet per sec diviced by cross sectional area of pipe gives velocity

As boost increases, as long as more mass flow is present with the boost increase, you'll have more cfm and more velocity as a result

Eventually you'll find a limit on piping size but i think most go bigger on piping than needed. Now this doesnt take into account any flow losses in the system. Pipe bends, surface finish, length etc. All of that can disturb flow enough to create turbulent areas, and high velocity areas

Much like a head port. You have to design it so the velocity profile inside the system is relatively even. CFD analysis can show this type of stuff.

So with that taken into account, just using the simple mass flow times air specific volume at that given pressure/temp to calc cfm and resulting velocity isnt quite enough information lol.
Actual velocity could be much higher due to losses and flow profiles in the pipe

 

I get bends and length of pipe come into play, but let’s leave that out for the moment. Let’s also leave out extreme examples… massive lengths of piping, hard angle bends, extreme boost levels, huge/tiny engines/turbos etc…

So the above statement in layman’s terms says… “If the turbo can keep up, CFM and Velocity increase with additional boost/pressure.” Is that true?

Above you said as pressure/boost increases velocity decreases due to additional friction form the added pressure, which also makes sense to me.

Don’t the two statements contradict each other? I suppose both could happening… My point is there should be a way to calculate the average rise/fall in velocity (at sea level) per added atmosphere of boost in a fixed pipe size. Without over complicating the matter with exact figures. This is the "formula" I've been after and can't seem to find.

 

Just worry about this part. Ignoring losses from bends and lengths etc. this part remains true

Just note the gain in velocity is not significant as long as inlet temps dont vary greatly. Like 1 bar boost you see 90 deg after cooler and 2 bar you see 200 deg or something like that. Say 10-20 deg spread is more typical. Velocity increases just alittle bit more because overall cfm is only increasing alittle bit more.

I guess there really is no general formula. Just need to compute the air properties at given temp and pressure. That specific volume is used to calculate cfm rate based on your air mass flow. Air mass flow is basically hp divided by 10. Thats rough estimate for mass flow in lbs per minute

Best i can simplify it is following assumptions.
10 psig of air at post cooler temp of 90 deg f which is a typical expected value most setups can see in a short run. Specific volume is 8.24. Use this for your cfm calc
15 psig of air at post cooler temp of 110 deg f, estimated typical post cooler temp, specific volume will be 7.103.
20 psig at 140 deg f lets say typical post cooler temp, specific volume is 6.40.

Use your estimated mass flow from turbo at that boost or your known/estimated hp level divided by 10 to get ur cfms.

2.5" pipe has area of 0.03409 ft square
3" pipe has area of 0.04909 ft sq
4" area of 0.08727 ft sq

Take cfm divide by 60 to get cfs and divide by pipe area to get velocity.

600 hp 5.3 on 10 psi would have 494 cfm. 167 feet per sec velocity in 3" pipe. Imo i see no reason to run more than 150-200 fps so 3" is perfect.

800 hp 5.3 on 20 psi would have cfm of 512 cfm. Velocity is now 174 fps in 3" pipe

See its not a significant gain in speed.

 

In the context of the OP's post....every single one of these apply lol.