Why 2" is better than 3", Intake diameter wars...

[David Coleman]

I busted out some flow calculations from <I>Maximum Boost</I> and did some number crunching last night. The results were interesting to say the least.... <P>First I calculated a few Air flow rates.<BR>(cid x rpm x .5 x Ev)/ 1728 = Airflow rate in CFM<P><B>2.5L @ 7000RPM, 95% Volumetric Efficiency</B><BR>(152 x 7000 x .5 x .95)/ 1728 = 292.47 CFM<BR><B>2.5L @ 15000RPM, 95% Ev</B> [you’ll see why in a minute <BR>(152 x 15000 x .5 x .95)/ 1728 = 626.74 CFM<P>This next equation was in the section specifically dealing with Throttle Body size, but one can assume that pre-TB values would adhere to the same rules. Bell said that 300ft/sec is about the limit for a street car. Methinks that Ev must be velocity dependent [as in jet engines maybe?]<P>Velocity = airflow rate / area of section<P><B>3” pipe, 2.5L @ 7000RPM</B><BR>[292.47/(3.14159 x 1.5^2)] x (144/60) = 99.304 feet/sec<P><B>3” pipe, 2.5L @ 15000RPM</B><BR>[626.74/(3.14159 x 1.5^2)] x (144/60) = 212.79 feet/sec<P><B>2” pipe, 2.5L @ 7000RPM</B><BR>[292.47/(3.14159 x 1^2)] x (144/60) = 223.44 feet/sec<P>Now the flow dynamics state that <B>mass flow = ro x A x V</B><BR>Ro is generally constant, so as A is decreased, V increases. The only thing I can think of is that Volumetric Effeciency is velocity dependent, because otherwise mass flow remains constant. Or perhaps, using laminar flow instead of slug flow, there is something funky that goes on there. I’m not entirely sure. So basically these equations say that 2” pipe is better for intakes than 3” for a 2.5 @ 7000 while a 3" pipe is tuned for a KL running 15000RPM. With the velocity high, methinks the engine has to work less hard to get it in the cylinder. <P>I havent taken the time to do the calulations for a K8, but needless to say 3" is <B><I>WAY</B></I> too big if its a smidge too big for a 2.5. I'm redoing my intake with 2" to see what happens...<BR>Now fire away, oh disbelievers <BR><P>------------------<BR>David Coleman- <B><I>Moderator</B></I><BR><A HREF="http://www.PinkMX-3.com" TARGET=_blank>1993 Mazda MX-3 GS Special Edition</A><BR><B><I>Why get clear tail lights when only Geo Metros are gonna see them?</I></B>

[white fish]

what about starting at 4" then step down to a 3" then to a 2" then to the TB?????????

[David Coleman]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by white fish:<BR><B>what about starting at 4" then step down to a 3" then to a 2" then to the TB?????????</B><HR></BLOCKQUOTE><P>That would be a velocity stack...sorta. I doubt without breaking out the resin and fiber that you could construct a smooth transition from 4" to 2". Turbulence is bad, except for when it's tumbling into the chamber [which is about 3 feet further down the line than where this is addressing]<P><P>------------------<BR>David Coleman- <B><I>Moderator</B></I><BR><A HREF="http://www.PinkMX-3.com" TARGET=_blank>1993 Mazda MX-3 GS Special Edition</A><BR><B><I>Why get clear tail lights when only Geo Metros are gonna see them?</I></B>

[Falstaff]

so what exactly ever came of this experiment? has anyone constructed a 2" cold air intake with some sort of 3" to 2" adapter from the VAF? <P>just curious.. i have all the parts for a 3" intake for my 1.8l V6 and am putting the project on hold after hearing protege and david coleman suggesting 3" is too large.. just curious.. but what is the inside diameter of the PRM intake piping?<P>secondly, the section of piping after the VAF is 3" in diameter.. wouldn't the air entering that section lose any velocity / increased volume rate gains through this section and into the TB? <BR> <BR>thx in advance<p>[ May 31, 2002: Message edited by: Falstaff ]

[pelado]

Ya gotta remember that what goes in, goes in!<P>The mass of air in the intake piping prior to the throttle body will be the same mass of air in the intake manifold, right? This mass of air may slow down in a large pipe prior to the TB but it'll speed up again in the smaller intake passages, it has to. Ever see an engine using a MAP sensor (vs. MAF) run with nothing on the upstream side of the TB? It runs just fine.<P>While a 2" pipe upstream of the TB may create high air velocities, it will ultimately limit the amount of horsepower you can make by robbing the engine of mass air flow. Mass flow rate of a compressible fluid is dependent upon many things, the diameter of the pipe, the smoothness of the inner surface, the temperature of the material flowing through the pipe, and most importantly, the differential pressure that is driving the flow in the first place. Which is why you use turbos and superchargers to crank up the delta P and increase the volumetric efficiency by cramming more stuff into the same space. Where is the differential pressure in your calculations? In my mind, that equation is only useful for calculating TB size and little else.<P>There are books with tables giving the flow rate capabilities and they usually consider the type of fluid (water, steam, natural gas, DRY AIR), temperature of the fluid, and differential pressure. If you raise the temperature of the fluid (and keep everything else constant) you'll be flowing less mass at the same speed per unit volume. There's friction losses to consider, also.<P>Granted that there is an ideal size intake pipe for a specific size engine but using too small of an intake pipe just means you'll exceed the mass flow rate capability of the pipe faster than a larger pipe. I don't have my paper brain with me so I can't prove it with numbers but I'll bet a 2" intake upstream of the TB will choke the s--- out of a 2.5L V-6.

[pelado]

My long winded post could have been boiled down to this: all that calculation is telling you is that for a 2.5L engine, a 2" TB will give excellent throttle response while a 3" TB will cause the engine to bog until the RPM's get way up there.

[David Coleman]

2" chokes the crap out of a 1.8 around redline. My velocity goals were apparently on the high side for pre TB intake piping.

[fuzzyboy]

technaly i'm still a little confused on wether the 3" is still usable,with a ractive filter and running the pvc to the bottom of the bumper. [img]shrug.gif"%20border="0[/img]

[David Coleman]

3" is better than 2"

[ZUNE MX-3 Gs]

The Vaf proximity to the throttle body won´t produce an air speed loss? Won´t you consider to relocate it further on the intake tube to let air recover it´s speed? what about this plus a venturi placed post Vaf?

[fuzzyboy]

thanks dave

[David Coleman]

the KLZE uses the same VAF, so I think it flows fine for the 1.8L<P>Just a thought...

[fuzzyboy]

where would the best place to locate the vaf. [img]shrug.gif"%20border="0[/img]

[David Coleman]

Where the wiring harness can reach. :darin:<P>I don't think it makes a huge difference. Right now mine is ~2" from the TB with no problems. I know Jeff Abrams runs his like that as well. You have to keep it horizontal, so that really limits you on where you can put it.

[fuzzyboy]

thanks for the reply dave <P>the only reason i was asking is because i'm going to leave the stock tubing up to the vaf sensor and put the ractive filter right after it and run piping down to the bumper. [img]shrug.gif"%20border="0[/img]