View Single Post
Old 01-11-2013, 03:48 PM   #4
downforce22's Avatar
Join Date: Aug 2009
Location: Colorado Springs, CO
Posts: 2,996
Nando nailed it. VE is amount the engine can flow, so by definition the VE of a FI Spark Ignition engine is going to at some point be higher than 100% because the compressed air is moving more air than the engine can theoretically move at ambient pressure (14.7 psi). So for the N/A m20b25 say it can flow 250 cfm at sea level at 85% VE. (Doing the math that is at 6660 RPM) With a VE increase to 110% we get 323.5 CF. So we are flowing more because the engine is matched well with exhaust, intake, valves, cam etc. You probably wouldn't see these big of gains on any engines without forced induction, nonetheless a 2 valve per cylinder m20. The Mclaren F1 has 105% VE as a natural aspirated V12 (Built by BMW ), so it is possible to achieve over 100% VE on an NA engine.

CFM = L x RPM x VE x Pr
This m20 example is just to get an idea of how much the volumetric efficiency affects the output of the engine. But remember, this is just making the engine more efficient. A byproduct of boost is increasing the pressure ratio, so on a FI engine the numbers change a bit. Since Displacement, RPM, and VE are multiplied by the pressure ratio the increase in air flow is also multiplied meaning there is more flow due to the higher the pressure ratio. This is why most people think cranking up the boost will help, because it will increase flow if the turbo can handle flowing more. But when PR goes up, I believe so does VE to an extent. The purpose of this thread is to shed some light onto where a forced induction engine increases in VE, at what point it crosses over 100% or ideal volumetric efficiency and beyond, as well as how far it goes, (150% VE, or 200% VE?).

The main reason I brought this up is for turbo selection. If you calculate a certain flow from the engine, but it is actually flowing more because an increase in VE has not been taken into account in the calculations, then the turbo you choose will be too small.

Refer to the attached pictures, but I converted the CFM rating into lb/min numbers by dividing by 14.5.

85% VE at 3500 and at 6000 RPM, PR= 1.68

220.7 CFM = 15.2 lb/min
378.5 CFM = 26.1 lb/min

100% VE at 3500 and 110% VE at 6000 RPM, PR= 1.68

259.7 CFM = 17.9 lb/min
489.8 CFM = 33.8 lb/min

Comparing the graphs below for the same exact turbo at the same exact pressure ratio; At the usually suggested 85% VE for 2 valve engines you can see not only does the engine flow less, but it is in a low efficiency at 3500 RPM, meaning the turbo is adding more than optimum heat. It is also just getting into the most efficient zone near redline around 6000 RPM. The second map shows the engine being more efficient at 3500 RPM and just starting to run out of steam at 6000 RPM. It is important to note the efficiency of the turbo is right in the powerband, about 4000-5500 RPM,

Using a higher VE for the same engine at the same conditions produces the dilemma of what value to use. Because one formula shows the turbo being just right for the turbo, right in the rev range, pulling to redline and spooling quickly while the other shows some lag and being a bit big. Probably not much fun unless in the higher revs, like on a race track. So what VE are we at during FI and during spooling? You could see how this could make somebody believe their turbo will surge, when it really is incresing VE and not going to be a problem.

Attached Images
File Type: jpg 85ve.jpg (85.0 KB, 2787 views)
File Type: jpg 100ve.jpg (86.0 KB, 2586 views)
downforce22 is offline   Reply With Quote