Mazda 6 Forums banner

1 - 20 of 83 Posts

·
Registered
Joined
·
10 Posts
Discussion Starter #1
I've been lurking around here for a while, so here goes:

This is why it is a bad idea to bypass your throttle body coolant line:

First, we need to make some assumptions. The first set of these assumptions deals with the operating condition of the engine. Let’s assume that we are running the engine at full throttle at 4,000 RPM. We have a 3.0L engine that is efficient and has about an 85% volumetric efficiency. Our effective engine volume is 0.85*3.0L or 2.55L. Since we have a four stroke motor, we are pulling in 2.55L of air 2000 times a minute. Therefore we are ingesting 3.00 cubic feet of air per second, after unit conversions. Our throttle body’s inside diameter is 2.5 inches (D) and its total length is 2 inches (L).

The second set of assumptions deals with the air and coolant flowing through our throttle body. Let’s assume that we are pulling in air from outside the engine bay on a warm day. Our intake air is 80 degrees, Fahrenheit. At 80F and 1 atmosphere, air has the following properties:

Density (p): 0.0735 lbm/ft^3
Thermal conductivity (k): 0.01516 BTU/hr*ft*F
Kinematic Viscosity (v): 16.88*(10^-5) ft^2/s
Specific heat (Cp): 0.24 BTU/lbm*F
Prandtl Number (Pr): 0.708 Unitless

Let us also assume that our coolant is 280F and that our throttle body is not cooled significantly by the incoming air. That is, the surface temperature of the inside of the throttle body is always 280F.

Now we will figure out how much and how fast the air is entering the engine. Through simple calculations, knowing the throttle body dimensions and volumetric flow rate and the density of the air, we can find out the mass flow rate and velocity of the air entering the engine. These values are found to be 794 lbm/hr (m) and 88 ft/s (V).

We now need to know if the flow of the air is turbulent or laminar. This will allow us to determine what appropriate equations to use later. First, we need to find the Reynolds Number (a unitless number that allows one to know if the flow is turbulent or not). This is found by the equation:

Re = V*D/v

We find our Reynolds Number to be 110,000. This is definitely turbulent flow! (Anything over 10,000 is defined as fully turbulent flow)

We need to find our entry length, or the length of tubing needed for the flow to become fully turbulent. This value is defined as Lh = 10*D. This value is found to be 2.08 ft. This is acceptable, since there is, most likely, two feet of piping between the throttle body and the air filter.

Since our entry length is less than our actual piping length, we can use Dittus-Boulter equation to determine the Nusselt Number (Nu):. (Sorry about all of this name dropping)

Nu = 0.023*Re^.8*Pr^.4 = h*D/k

We find our Nusselt Number to be 214. The ‘h’ value above is the average heat transfer coefficient. Now, we can actually find the temperature of the air coming out of our throttle body. Solving for h in the above equation yields h equal to 15.58 BTU/hr*ft^2*F.

By using Newton’s law of cooling, where the rate of heat transfer (Q) is determined to be:

Q=h*(area of heat transfer)*(Surface temperature-Medium Temperature)

By using differential equations, natural logs and some other hocus pocus, we get the following equation:

Texit=Tsurface-(Tsurface-Tinlet)*exp(-h*A/m*Cp)

Finally, by using the above equation, the outlet temperature can be determined to be 81.3 degrees Fahrenheit.

Now, using the SAE J1349 correction factor, you lose ~1% of your total power for each 10 degree increase in inlet air temperature. With this 1.3 degree increase, due to the throttle body coolant, you are losing 0.13% of your power. Or, on a 200hp car, you are losing 0.26hp. By overriding the coolant flowing through your throttle body, you are risking having your throttle body freeze open in cold weather (the whole purpose of running coolant through the throttle body in the first place). Hope this clears up any confusion.
 

·
Registered
Joined
·
5,757 Posts
Replying to Topic 'Why not to do the throttle body coolant bypass'

Great post! :thumbup

I've been wanting to calculate this, but don't know the equations.

Perhaps I'm missing it, but what did you use for your area of heat transfer?

I believe one of the reasons this mod can be effective is not just from the heat passing through the actual throttle body, but from the immese heat reduction throughout the entire engine bay. The heat radiates into the plastic air intake tubing and increases the overall engine bay temperature quite a bit, whereas the heat does not seem to be absorbed as much when cooled through the radiator.
 

·
Registered
Joined
·
10 Posts
Discussion Starter #3
Replying to Topic 'Why not to do the throttle body coolant bypass'

Area of heat transfer = only the inside of the TB

I was ignoring any heat transfer from underhood heat, so

A=Pi*D*L
 

·
Registered
Joined
·
5,757 Posts
Replying to Topic 'Why not to do the throttle body coolant bypass'

This confirms one thing I've long suspected- that air moves to quickly to change temperature due to such a small area radiating heat.

Put these numbers in for 6000rpm and the temperature increase would become even smaller.

Once again, good work :)

Anything else you care to school us on? I sent you a PM.
 

·
Registered
Joined
·
3,161 Posts
Reading Topic: Why not to do the throttle body coolant bypass

Yeah, that's what some people and myself have thought about as well. Of course I don't have the scientific knowledge to explain it, but it does make sense.

Now if the temperature of the TB is like 1500 degrees (won't happen but I'm saying IF), then it might affect it.
 

·
Registered
Joined
·
5,757 Posts
Replying to Topic 'Why not to do the throttle body coolant bypass'

The coolant does not normally exceed 195 degrees, but it does hit the T/B after the engine and before the radiator.
 

·
Registered
Joined
·
1,781 Posts
Reading Topic: Why not to do the throttle body coolant bypass

You should get this guy to work on all the crack brain theorys on here. HEHE I bet in a week he could have all the mods figured out, and be our own myth buster.
 

·
Registered
Joined
·
10 Posts
Discussion Starter #8
Replying to Topic 'Why not to do the throttle body coolant bypass'

Originally posted by dawei213Now if the temperature of the TB is like 1500 degrees (won't happen but I'm saying IF), then it might affect it.
Well, most aluminum melts at about 1220F, so I would be more concerned with injesting molten aluminum than inlet air temps if my throttle body was that hot!
 

·
Registered
Joined
·
135 Posts
Replying to Topic 'Why not to do the throttle body coolant bypass'

So, you're saying "don't do the TB coolant bypass because at best you'd bet a 1/4 HP improvement and that would be a waste of time", correct?

How do your figures square with the fact that I did notice an improvement in engine performance over time? That is, my engine used to lose power on long car trips but now it doesn't?
 

·
Registered
Joined
·
3,161 Posts
Replying to Topic 'Why not to do the throttle body coolant bypass'

Originally posted by Doctor Wicked


           
Originally posted by dawei213Now if the temperature of the TB is like 1500 degrees (won't happen but I'm saying IF), then it might affect it.
Well, most aluminum melts at about 1220F, so I would be more concerned with injesting molten aluminum than inlet air temps if my throttle body was that hot!

Yes yes, of course. I guess what I really meant was that if the same size and dimension of an object that can support temperature of 1500 F or so, and if air was to go in one end and out the other, then it might affect the temperature of the air.
 

·
Registered
Joined
·
10 Posts
Discussion Starter #12
Replying to Topic 'Why not to do the throttle body coolant bypass'

Yes I agree with what you are saying, I was just giving you a hard time. :)
 

·
Premium Member
Joined
·
3,997 Posts
Reading Topic: Why not to do the throttle body coolant bypass

I hardly consider a 10 minute job, which can be done on a $7 budget, which gains maybe 1/4 HP time wasted. Hell people buy intakes for $220 for maybe 10-12 HP gained. You got more horsepower for your money out of that $7 modification than you did out of that $220 intake.
 

·
Registered
Joined
·
1,781 Posts
Reading Topic: Why not to do the throttle body coolant bypass

It seems based on all this, the coolant running to the TB is only to keep it warm so it does not freeze. Which is normally not a problem on most cars because the TB is not surounded by pastic. And that the surface area of the TB is so small, that there is no noticable effect of it heating or cooling the air moving through it. Case closed.


" How do your figures square with the fact that I did notice an improvement in engine performance over time? That is, my engine used to lose power on long car trips but now it doesn't? "

Placebo effect.
 

·
Registered
Joined
·
3,161 Posts
Replying to Topic 'Why not to do the throttle body coolant bypass'

Originally posted by Doctor Wicked


            Yes I agree with what you are saying, I was just giving you a hard time.  :)
You suck! :p hehe...

Originally posted by rrhunt28

" How do your figures square with the fact that I did notice an improvement in engine performance over time? That is, my engine used to lose power on long car trips but now it doesn't? "

Placebo effect.  

I was thinking the same thing...thought didn't want to say anything since I've never perform the mod.
 

·
Registered
Joined
·
2,151 Posts
Replying to Topic 'Why not to do the throttle body coolant bypass'

A reason to do it while drag racing:

HEAT! My Contour (Duratec) ran 15 flat after a 1 hour cool off time and it did not have coolent running through the TB. It ran high 15's when I took it right off the highway (like 15.8), and dropped 2 MPH <- more important. HP = Speed at the end of the track. So in turn if you let you car cool down, bypass the coolent, preventing the engine from sucking hot air into it, you should run better times. BUT LET IT COOL OFF! After you leave the track, re-attach it, well unless you have someone talking crap... :D

To add, the heat soak is tramendous, of course the 2.5 Contour Duratec had an aluminum UIM, running probably at coolent temp ~ 195 degreese or better, so that equates to about a ~15 HP loss in good conditions. The 3.0 in the 6 has a plastic UIM, but the heads are still real hot, and you still have a small amout of heat soak through all your non metal parts.

PS Doctor, no engine runs at near 100% efficancy.
 

·
Registered
Joined
·
135 Posts
Replying to Topic 'Why not to do the throttle body coolant bypass'

What???

No way, it must be placebo!

You saw all that math, didn't you? I mean, he's got equations and numbers and stuff!! ;)
 

·
Registered
Joined
·
10 Posts
Discussion Starter #18
Replying to Topic 'Why not to do the throttle body coolant bypass'

Originally posted by mmarfan


             
PS Doctor, no engine runs at near 100% efficancy.
I said volumetric efficiency, not thermal efficiency. 85% volumetric efficiency is not unheard of in a modern, normally aspirated engine.

http://www.canadiandriver.com/articles/jk/020529.htm
 

·
Registered
Joined
·
1,781 Posts
Reading Topic: Replying to Topic 'Why not to do the throttle body coolant bypass'

If you have so much heat soak, wouldnt it stand to reason that the coolant running through your TB might actually be cooling it? It does sit at the top of the engine, where the temps are going to be the warmest in theory(heat rises). And you answered the biggest difference between the cars contour(who races those anyways?) v. 6. The 6 has plastic, which is a insulator(for the most part) instead of metal, which is a very good conductor.
 
1 - 20 of 83 Posts
Top