Airflow velocity for different intercooler piping diameters (2.0 2.25 2.5 2.75 3.0 inches)

Hi guys,

In the last few weeks I've seen several posts on whether it is ok to use 2.5" or higher piping, or if somebody could use their ITR AEM CAI as a charge pipe. I tried to help all of them best I could, but jus a short while ago I helped another H-T member out with a similar question. I searched online and used a flow calculator to help me figure out the airflow velocity for a given area and cfm. The site I used is THIS ONE. As you guys can see it has the area in length times width instead of PiRsquare which is the area of a circle. I took the area of each pipie diameter and usd the equivalent area of a square and used those as my length and width. I made it simple and just took the area of the circle and divided by 2. The resulting number was usd as my length and 2 was used as my width.

Your actual results may vary (ie when you add an IC) so just use my numbers as sort of a bench mark to compare different piping sizes. One thing you guys will notice is that none of the velocites goes above 304 MPH or 0.4 mach. According to Corky Bell, Maximum Boost pg 61, 304 MPH or 0.4 mach is the point at which airflow meets increased resistance (drag) and flow losses are experienced.

Anyways here are the numbers I came up with. The velocities are in miles per hour and mach, and the flow rates are in cfm. Measurements for the piping are in inches.

Re: Airflow velocity for different intercooler piping diameters (BlueShadow)

Oh yah, I believe that an average velocity would be the 2.25" pipe with 350-400 cfm. I believe that where your average drag or Revhard kit at 7 psi would be. For 15 psi it would be around 500-550 cfm. (That's for a B18C1)

Re: Airflow velocity for different intercooler piping diameters (BlueShadow)

Very cool, I was hoping someone was goign to do some research on this topic. I hope to dyno tune with differnect combos (difference size before and after intercooler) and see if we can get some real numbers to work with also.

__________________
-Dustin EJ8 Squad #033 Gt28RS powered D16Y8 (9psi - stock internals) 1/8 mile:8.90 @ 83.94 on street tires

Re: Airflow velocity for different intercooler piping diameters (Civicman86)

Yah I was thinking about this too. After watching Tony the Tigers video of the ITB GSR I wanted to look into building a really fast spooling quick response turbo too. I was looking at stuff like ITB's, built head, titanium rods and pistons (I think someone on H-T is making em) no intercooler, 2" diameter piping and the GT28RS. hehehe

btw..i'm in the process of putting together a parts list for a 393" SBF build.....and am going to be using a 72 or 74mm turbo (simple street car..only looking for 600whp on pump and maybe 900 or so on race gas) and i was just trying to use this to maybe help me figure the pipe diameter aspect out (i allready made a thread about it..but i figured i'd post this here instead..since i didn't really get any TECHNICAL answers before..just good advice directed toward a honda application)

Back when I wrote this thread there were about 3-4 threads about people wanting to use their 3" CAI as a charge pipe. I made this thread to show why it was a bad idea to use pipe that big in dia.

The numbers can be used as sort of a rough guide for selecting piping diameter. The numbers above are the different velocities for the different amount of air going through a given size pipe. The more air or CFM you have going through a given area, the faster it is moving. A smaller pipe will pressurize faster so there is less lag and the response is better. But the faster the air is moving the hotter it will become.

I guess one eay I could use the numbers is to determine what size pipe to go with from turbo to IC inlet. Say I have a GSR and I want to run 18 psi. The esiest way to find my CFM is multiply the CFM of the engine by the pressure ratio. Just for this example lets say the B18C's CFM is 275 with no boost, but under 18 psi of boost it will flow 610 CFM. Normally people use 2" piping from the turbo to the IC inlet, but if you look at the chart you'll see that with 600+ CFM and a 2" pipe, the air is moving so fast that there is a lot of resistance. So you might be better off using 2.25" pipe or even 2.5" pipe.

That's one way to use it...but like I always say, dont get too caught up in all this number crunching. It is always a good idea to get the opinion of somebody who has a lot of experience with turbo cars.

You guys are all missing that fluid density changes when you pressurize the air. As in the charge pipes will have a much lower volumetric flowrate than compared to the compressor inlet because the pressure is higher, and the same mass flowrate requires a much lower volumetric flowrate.

All the CFM numbers you guys are posting are pre compressor inlet CFMs, and they don't apply to post compressor CFMs.

This is exactly why OEM turbo cars have like 2" and 2.25" charge pipes right off the turbo and for most of the way to the throttle body.

Just to clarify for those that think I'm saying there is a math error - there is not, you're just calculating the CFM for the PRE-COMPRESSOR inlet pipe. These numbers are not applicable to your charge pipe sizing.

This leads me to ask something.
Lag and response aside, does this mean that no matter how much cfm ur running the larger the pipe means the lower the speed which means the lower the temp.
This would create better PEAK #'s correct ?
Remember this is lag and response aside.

You guys are all missing that fluid density changes when you pressurize the air. As in the charge pipes will have a much lower volumetric flowrate than compared to the compressor inlet because the pressure is higher, and the same mass flowrate requires a much lower volumetric flowrate.

All the CFM numbers you guys are posting are pre compressor inlet CFMs, and they don't apply to post compressor CFMs.

This is exactly why OEM turbo cars have like 2" and 2.25" charge pipes right off the turbo and for most of the way to the throttle body.

Just to clarify for those that think I'm saying there is a math error - there is not, you're just calculating the CFM for the PRE-COMPRESSOR inlet pipe. These numbers are not applicable to your charge pipe sizing.

So rather then just pointing out an error, why dont you post some corrected numbers.

It took me long enough to come up with just these numbers. Factoring in real world conditions like temperature, intercooler pressure drop, and stuff like that would have made it a lot more work.

This leads me to ask something.
Lag and response aside, does this mean that no matter how much cfm ur running the larger the pipe means the lower the speed which means the lower the temp.
This would create better PEAK #'s correct ?
Remember this is lag and response aside.

Yes, the fast the air is moving the hotter it will become. That is the "resistance" that Corky Bell was talking about when you increase the air speed. That's without taking into account intercooler cooling efficiency, lag and response.

So rather then just pointing out an error, why dont you post some corrected numbers.

It took me long enough to come up with just these numbers. Factoring in real world conditions like temperature, intercooler pressure drop, and stuff like that would have made it a lot more work.

It is true that air viscosity changes when pressure changes. It is also true that temperature changes, and as a result, the Mach number changes.

However in most cases these changes are small.

Someone stated earlier that the above calculations were wrong because the actual charge piping has pressureized air flow through it. This isn't true. CFM's are CFM's regardless of the pressure, and in the case of internal combustion engines, the only things that effect CFM's are displacement, RPM, and volumetric efficiency:

CFM flow = displacement (liters) / 28.32 (liters/ft³) x RPM x VE

*note this is a special volumetric effiency: The amount of air that enters the cylinders divided by the amount of air that could enter if the flow occured at very slow speeds. Not the usual VE, which is related only to N/A flow

For example, a Honda 1.8 liter engine at 13,000 RPM and 100% VE would flow only 826 CFM. (And would need only 2.5" charge pipes, regardless of how much boost they were pushing)

CFM's are CFM's regardless of the pressure, and in the case of internal combustion engines, the only things that effect CFM's are displacement, RPM, and volumetric efficiency:

I was just thinking about that. What difference does it make if it's pre or post compressor. If there is 300 CFM leaving the compressor, then what difference does the temp make. I think it would be different if we were measuring the airflow in terms of LB/MIN. Cause in that cause air temperature affects the density of the air. But with CFM we are only measuring how much space the air takes up, not it's density.

Quote:

Originally Posted by beepy

CFM flow = displacement (liters) / 28.32 (liters/ft³) x RPM x VE

*note this is a special volumetric effiency: The amount of air that enters the cylinders divided by the amount of air that could enter if the flow occured at very slow speeds. Not the usual VE, which is related only to N/A flow

For example, a Honda 1.8 liter engine at 13,000 RPM and 100% VE would flow only 826 CFM. (And would need only 2.5" charge pipes, regardless of how much boost they were pushing)

Another example you can use is 7000 RPM's and 104% VE for a GSR. I found this info a few years back when I was trying to figure out VE% for an H22. H22's have peak VE% at peak horsepower and it is 102%. S2000's have it happenening at peak horespower as well but it is 112%. So using your calculations a 1.8L GSR would have 462 CFM.

But I dont know why you would calculate charge pipe size without taking into account the amount of boost. Using out same 1.8L GSR with say an SC61. At 8 psi and 18 psi the compressor is putting out very different amounts of air right? Isn't this the airflow that you are trying to match up to your piping size? using my other example a few posts up for 8 psi I calculated 425 CFM and for 18 psi I calculated 610 CFM.

So if we were to run a GSR with an SC61 at 18-20 PSI which flow rate do we use? 600+ or the one from your calculations which is around 460 CFM?

It is true that air viscosity changes when pressure changes. It is also true that temperature changes, and as a result, the Mach number changes.

However in most cases these changes are small.

Someone stated earlier that the above calculations were wrong because the actual charge piping has pressureized air flow through it. This isn't true. CFM's are CFM's regardless of the pressure, and in the case of internal combustion engines, the only things that effect CFM's are displacement, RPM, and volumetric efficiency:

CFM flow = displacement (liters) / 28.32 (liters/ft³) x RPM x VE

*note this is a special volumetric effiency: The amount of air that enters the cylinders divided by the amount of air that could enter if the flow occured at very slow speeds. Not the usual VE, which is related only to N/A flow

For example, a Honda 1.8 liter engine at 13,000 RPM and 100% VE would flow only 826 CFM. (And would need only 2.5" charge pipes, regardless of how much boost they were pushing)

What I posted was correct for the original post. I never said the math was wrong(in fact I said it was right), but the original post had something about power output and a rough relationship to the CFM flowing through the charge pipes. While this relationship might be "sort of right" for pre-compressor inlet flow, the original poster was trying to apply this to the volumetric flowrate through the charge pipes which doesn't work due to the pressure difference and subsequent massflow difference.

...and the reason why I didn't post up any "corrected" numbers is that an excel spreadsheet would be much more useful in this circumstance since the volumetric flowrate is going to change for a given inlet vdot(i'm getting lazy here) depending on the pressure. I also think these calculations are ok if you want to prove your "gut feeling" to yourself, but they're not something to really stress about. The negative side effect to too small charge piping is not heat, but head loss. This will be seen as an excessive amount of pressure drop through the pipes as the frictional losses require more energy to overcome due to the high velocities. Too large of charge piping has the disadvantage of a longer transient boost response since you're increasing the volume of space you need to pressurize.

BTW - 2.75" charge piping at the TB should be large enough for almost anybody here except for maybe a select few who are pushing huge power.

Sorry to bring this back. Does anybody know when 2 inches diameter (pre intercooler) becomes a restriction? Is there a horsepower number (like 400whp) that would require a larger pipe?

Is the downside that the air will be hotter in the smaller pipe? I think that's what I read above.

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