if you nevermind the shaft and butterfly on a round throttlebody or the resistor wires on a MAF, how do you figure out the airflow through a round pipe like this by just knowing the inside diameter?

AND, could somebody explain the pressure this type of thing is rated at? I know it's different than the pressure that headflow CFM is rated at.

 

If it was a simple tube, it would be very easy to calculate the pressure loss vs. flow curve. But a TB or an MAF has obstructions, and that makes it easier to measure the flow on a bench, as a function of pressure loss. In order to measure flow, you have to pick a pressure loss number. 28" of H2O seems to be popular for heads, and that number represensts about 1 psi. I don't think there is a "standard" for things like TB's or MAF's, so everyone can pretty much use whatever pressure differential they want for a test, and then tell you any number they want, since they don't bother to tell you the pressure differential they used for the test.

In general, you can think in terms of "open flow area"... the area of the tube. But that doesn't produce accurate results for significant changes in diameter. Since area is a function of the square of the diameter, you could start with that premise. A pipe that is 10% larger (1.10) would flow 21% more air (1.10 X 1.10 = 1.21). But just looking at open area ignores the issue of wall friction. And wall friction is not a function of the square of the diameter, but of the diameter itself (perimeter = pi X D). In practice, when sizing pipes, my mental calculator tends to use the 2.5 power for comparing flow capacity of pipes - just a simple "rule of thumb" and not 100% accurate.... that would indicate a 58mm bore would flow flow 31% more at the same pressure drop than a 52mm bore. If you just based it on area, the 58 would only flow 24% more.

But all that is "approximation". In the real world, the pressure loss through the MAF or TB is a function of how the fluid approaches the entry to the device, what the exit looks like, and what all the obstructions are in between. Add to that the affects of things like surface roughness, and whether you are looking at laminar or turbulent flow, and it gets all complicated.

What we really need is for someone to build a mockup of the typical inlet air flow path, from the air cleaner to the plenum, and measure the pressure loss for each component over a wide range of flows representing the air demands of a variety of engines. Might make an interesting project for a school science project, or even a good "project" for a college paper. If you have access to a flow bench, you could do it yourself, and probably sell the article to one of the popular auto rags.... then it would be quoted as gospel by everyone posting on "Advanced Tech", "LT1 Tech"... etc.....

 

I didn't realize it would be so complicated. I was just trying to figure out the CFM through different size throttle bodies.

I might just do that when I get back to school

 

here is a good link to testing some intake system parts on a LS1.

http://www.installuniversity.com/ins...st3.132001.htm

TB's and MAF's should be flowed at 20.3"of H2O or 1.5"Hg

There is only one really good way to see what a TB flows, put it on a flow bench. I know because I've tested alot of LS1 stuff out. They flow about 700cfm stock and a well ported stock one flows about 900cfm. That's thru the same size 75mm throttle plate too. It has alot to with the shape of the TB or MAF being tested too, a MAF is not all about flow, because bad MAF readings can be worse for the engine than more airflow thru it. The TB's like carbs aren't all about size on the other hand. 80mm BBK TB's don't do a very good job on a LS1/6, but a ported stock unit does.

Bret

 

Bret, why don't the 80mm TBs work well?

I've got a stock Z06 MAF on my impala that will probably stay there until I go to a FAST system or something later, and I was thinking of doing some work to my intake manifold that would give more plenum volume and would let me use a bigger TB from another application. I've seen some pretty big diameter TBs available for the Buick 3.8L Turbo

 

You know what, I don't know why the BBK's don't work well on a LS1. It's odd how a ported stock one does better, but part of it has to do with the 75.5mm hole in the intake that is not going to get any bigger since it can't be ported.

The problem on a LT1 intake is that there is no way to get a big circular TB on that intake. The 1300cfm Monoblade is much more than enough.

Bret

 

Correction, there is no way to get a big circular TB on a STOCK LT1 intake Like I said, I'll be modifying it for more plenum volume which will mean cutting the top off Remember, anything's possible

The problem with the monoblade is the lack of IAT. I think that is a good thing to have on a daily driver.

 

Bret, I found that link to be pretty cool. I'd like to try that stuff with the LT1 intakes.

On the B-bodies the intake tract is pretty crappy IMHO. First the air goes through a small scoop behind the headlight, then up through the air filter, then a 90* bend to the 3" MAF, then that curved and squished "1st base" pipe with a resonator built in, the another 90* through the elbow with a "Homeplate -shaped" resonator on top, to the TB. There are good gains from "Swiss-cheesing" the intake box pre-filter, K&N drop in filter, putting a straight pipe in to replace the 1st base, then a hockey puck in place of the homeplate for around $50. On my Simpy I've just got a 3.5" K&N cone filter on the Z06 85mm MAF, then a Fernco boot to the TB. There is a heatsheild/scoop behind the filter that seems to help at higher speeds.