I spent all day today sifting through the DML archives reading about
throttle body mods. I was able to do this because the boss was out and the
nuclear missile buisiness is kinda slow since the end of the cold war.
After reading through many, many posts (and a raging flame war), I came to
a few conclusions:
1) People who have modified throttle bodys often are incredibly happy with
the results.
2) No one has actually documented any substantial performace increase from
throttle body mods.
3) There are some things you can do by yourself for free.
There were some other conclusions that I came to, but I won't post them
because I have no desire to get into a flame war.
Observation:
There's a restriction in the throttle body above the butterfly valves.
People who have reduced or removed this restriction have reported
tremendous gains, but no one managed to quantify those gains on a dyno or
at the strip. The butterfly valves are 52mm diameter. The restriction is
48mm. I looked at my throttle body, and on initial inspection, this
restriction doesn't appear to have any effect on the operation of the
butterfly valves at all. Therefore, it seems reasonable to remove the
entire restriction.
Theory:
Removing the restriction should result in a noticable increase in
performance.
<geek mode ON>
A 52mm bore has 1.17 times more cross sectional area than a 48mm bore. So
what's the expected increase in flow capacity?
To start with, the general equation governing flow capability is:
f= -(DP/pg)/((l/d)*((v^2)/(2g))
where
f= friction factor
DP= Pressure drop
p= air density
g= acceleration due to gravity
l= pipe length
d= pipe diameter
v= average velocity of the air flowing through the pipe
What we're really interrested in is volume flow rate of air through the
throttle body (Q).
The eqaution to figure out Q is:
v= Q/a
where a is the cross sectional area of the pipe. a=(pi*(d^2))/4
Substituting Q into the first equation and solving for Q, we get:
Q=-((DP*d^5*pi^2*g)/(p*g*f*l*2))^(1/2)
If we assume that the friction factor (f), air density (p), pressure drop
(DP) and length of the throttle body (l) are constants, we can simplify the
equation to:
Q= (c*d^5)^(1/2)
where c= (-DP*g*pi^2)/(p*l*f*g*2)
Now, I'll calculate the ratio of the flow rate of the larger bore to the
flow rate of the smaller bore:
Q1/Q2 = ((c*d1^5)/(c*d2^5))^(1/2)
where
Q1 = the flow rate of the larger bore
Q2 = the flow rate of the smaller bore
d1 = the diameter of the larger bore (52mm)
d2= the diameter of the smaller bore (48mm)
When we solve this equation, we find that
Q1/Q2 = 1.22
<geek mode OFF>
You're probably asking, "so what does this mean to me?" Well, lets suppose
that the current throttle body found on a 5.9L R/T flows 500 cfm. If you
get rid of the restriction, the throttle body will flow 1.22 times as much
air, or slightly above 600 cfm. Note however, that my analysis is a gross
simplification. Even so, if I get only half the expected increase in
airfow, the difference should be measurable at the dragstrip.
I posit that a measurable improvement of .2-.3 seconds in 1/4 mile ET is
sufficient to prove my theory correct.
The Experiment:
I have a 1998 Dak regular cab R/T with only an Edelbrock 1208 air cleaner
as a modification (no K&N element). I'm going to take this to the dragstip
tomorrow and see what it runs to establish a baseline. I'll be going to NE
Dragway in Epping, NH. The strip is 100 feet ASL, and the weather is
expected to be 80-90 degrees F and high humidity. I'll run five or six
passes and report the results. Then, I'll use a small engine cylinder hone
I happen to have to bore the throttle body out to 52mm. Then I'll run the
truck again (maybe next weekend), report the results and confirm or deny
the validity of my theory. Stay tuned.
-Mike Miller, the guy with WAY too much time on his hands
This archive was generated by hypermail 2b29 : Fri Jun 20 2003 - 12:09:07 EDT