OddUnit

Understanding Mass air flow with emphasis on Throttle bodies.

Go down

Understanding Mass air flow with emphasis on Throttle bodies.

Post by ctwg60 on Thu Apr 02, 2009 1:52 pm

I've just closed my eyes for about five minutes and thought about whether the TB on my car could be a restriction to the new head and I've come to the conclusion that it is not. I've done no maths but with the engine being a 4 pot only one cylinder is ever on the induction stroke granted there will be other cylinder/s with a part opened intake valve but there is no reason that a the TB could possibly be a restriction no matter what revs the engine reaches it will only ever require flow through one port so a 52mm (82sqmm) intake elbow will easily supply air to a single 35.5sqmm port with other cylinder overlap and cam restriction.

Any arguments for and against? Have I somehow missed something obvious.

Just realised if this is the case then FT is unecessary until max rpm. Hmmmm! Suspect Is it even necessary then! Neutral

ctwg60
.:Stroked:.
.:Stroked:.

Number of posts : 770

View user profile

Back to top Go down

Re: Understanding Mass air flow with emphasis on Throttle bodies.

Post by mrbeige on Thu Apr 02, 2009 2:07 pm

I think it might be worth considering the actual air flow itself. It's all well and good determining the area of the aperture that the air flows through, but you have to consider air velocity as well as volume. You could do some maths to determine the volume consumed at different RPMs to then determine whether a certain sized aperture would affect the total flow. For example; take a 2litre carbonated drinks bottle and cut the bottom off it, then take the bottle top and put a small hole in it. Fill a bucket with water and push the bottle into the water with the top on, the bottle will fill relatively slowly. Then do the same with the top off, and it will fill much more quickly, due to the larger aperture. Surely the same logic applies when sucking the air through the aperture? Hope all that makes sense...

_________________
Stu
"Take one old performance car, modify it so that it is as good, if not better, than a new performance car"

£300 Corrado 1.8 16v
The good lady wife's Golf VR
avatar
mrbeige
.:Admin:.

Male Number of posts : 1384
Age : 41
Location : Near Ely, Cambs.......but another planet according to my wife....

View user profile http://www.oddunit.com

Back to top Go down

Re: Understanding Mass air flow with emphasis on Throttle bodies.

Post by ctwg60 on Thu Apr 02, 2009 2:21 pm

I understand what your saying but it's really hard to do that though as there are so many factors that affect the amount of air that gets into the cylinder. I need a simple calc tool to work it out. I really can't see it being a restriction in my application but still, interested in the proof.

Even if we could calculate the CFM choke point of a 52mm tube would be a start. How do you do that??

ctwg60
.:Stroked:.
.:Stroked:.

Number of posts : 770

View user profile

Back to top Go down

Re: Understanding Mass air flow with emphasis on Throttle bodies.

Post by JNLRacing on Thu Apr 02, 2009 2:25 pm

This is a bit of a difficult one as theory and practice does not quite work like you want it to as it is all to do with the volumetric efficiency of the engine.

You see at a given rpm, stroke/bore of an engine it will create a theoretical airflow based on piston speed. The limitation of the flow now becomes the cylinderhead/intake that feeds the bottom end.
So now you can take the actual measured flow and work out the volumetric efficiency of an engine at a given rpm.

At a certain point in the rpm range the volumetric efficiency will become less than a 100% and carry on decreasing higher up the rpm range.

Based on the theory of volumetric effiency you then only need a throttle body that has enough surface area that is physically capable of flowing enough air to feed the cylinderhead.

This means yes depending on the size of the throttle body you may infact never have to go WOT. I know at the moment on my S6 I can get to 160mph ( top of the rev range in 6th) just as easily on part throttle as I can on WOT neither getting there particularly faster than the other when timing from 100mph. (All done on a private track ofcourse Wink ).

JNLRacing
.:Cammed:.
.:Cammed:.

Number of posts : 205

View user profile

Back to top Go down

Re: Understanding Mass air flow with emphasis on Throttle bodies.

Post by boost panda on Thu Apr 02, 2009 2:27 pm

*enters*

Well hello, good gentlemen. Someone said something about calculating flow rates? Wink
avatar
boost panda
.:Bored:.
.:Bored:.

Number of posts : 307

View user profile

Back to top Go down

Re: Understanding Mass air flow with emphasis on Throttle bodies.

Post by JNLRacing on Thu Apr 02, 2009 2:32 pm

ctwg60 wrote:
Even if we could calculate the CFM choke point of a 52mm tube would be a start. How do you do that??

Measured at a 10" pressure differential equivalent to your head flow figs. A 52mm tube is capable of flowing 289.78cfm taking a general assumption of 75% efficiency once a butterfly is put in the way 217.34cfm. Then keeping in mind you have a twin butterfly setup with turbulance being introduced 65% 188.36 is your total flow through the 52mm butterfly. Which is equivalent to more than two intakes being open on full lift at the same time.

Long and short of it if you can eliminate the turbulance during the transition through the TB you don't have to worry about getting enough air though the standard one.

JNLRacing
.:Cammed:.
.:Cammed:.

Number of posts : 205

View user profile

Back to top Go down

Re: Understanding Mass air flow with emphasis on Throttle bodies.

Post by mrbeige on Thu Apr 02, 2009 2:36 pm

So, to determine the surface area of the throttle, you need the flow capability of the cylinder head and then obviously what the bottom end can 'consume'?

panda, do you have some lovely equations that we could have a play with for calculating flow rates?

_________________
Stu
"Take one old performance car, modify it so that it is as good, if not better, than a new performance car"

£300 Corrado 1.8 16v
The good lady wife's Golf VR
avatar
mrbeige
.:Admin:.

Male Number of posts : 1384
Age : 41
Location : Near Ely, Cambs.......but another planet according to my wife....

View user profile http://www.oddunit.com

Back to top Go down

Re: Understanding Mass air flow with emphasis on Throttle bodies.

Post by ctwg60 on Thu Apr 02, 2009 2:39 pm

Thank you once again JP for another excellent post! Good work Come on boost_mammal hit those books and give us some toys!

ctwg60
.:Stroked:.
.:Stroked:.

Number of posts : 770

View user profile

Back to top Go down

Re: Understanding Mass air flow with emphasis on Throttle bodies.

Post by junkie on Fri Apr 03, 2009 1:51 pm

JNLRacing wrote:

Long and short of it if you can eliminate the turbulance during the transition through the TB you don't have to worry about getting enough air though the standard one.

I think thats what has been done to mine, its had a little work as it can be seen by looking at it but its not one of those mega worked on reshaped jobbies and on part throttle it revs just as easily as full throttle. Just not as viscous.
avatar
junkie
.:Bored:.
.:Bored:.

Male Number of posts : 376
Age : 38
Location : Rotherham

View user profile

Back to top Go down

Re: Understanding Mass air flow with emphasis on Throttle bodies.

Post by boost panda on Sat Apr 04, 2009 2:27 am

Reynolds number is used to calculate turbulence (or lack of turbulence) in pipe flow.

Re = (Density x fluid velocity x pipe diameter)/ fluid viscosity.

All SI units, so:

Density in kg/m^3,
velocity in m/s,
diameter in m,
viscosity in Pa.s,

Anything above 4000 is turbulent
Between 2k and 4k is transitional,
Below 2k is laminar flow.
avatar
boost panda
.:Bored:.
.:Bored:.

Number of posts : 307

View user profile

Back to top Go down

Re: Understanding Mass air flow with emphasis on Throttle bodies.

Post by ctwg60 on Sun Apr 05, 2009 12:16 pm

Ahh Reynolds Number.. forgot all about this from my university days. study

ctwg60
.:Stroked:.
.:Stroked:.

Number of posts : 770

View user profile

Back to top Go down

Re: Understanding Mass air flow with emphasis on Throttle bodies.

Post by Sponsored content


Sponsored content


Back to top Go down

Back to top

- Similar topics

 
Permissions in this forum:
You cannot reply to topics in this forum