hschneTT

I didn't have a chance to fully respond earlier this week on the discussion on ducting the ICs and the effect of the bigger Forge. Here's what I think:

First, I think the recent hoopla about larger intercoolers is unwarranted. The numbers posted on the Forge site don't add up. The inlet temp change they publish w/ the Forge IC is about 10C which based on their chart translates into about 3-5 hp gain. Hardly enough to change 0-60 from 5.7 to 5.3 like they claim. Without more/better data I would be very skeptical.

With all due respect to Pete, I think the $1500 price tag for the Forge setup is outrageous. The effect of 3-5hp can be had other ways for a lot less $. But, as a capitalist, whatever the market will bear, so be it.

With respect to the ducting AutoUnionJack, smallTTs and I were discussing, a couple thoughts. First, I am not an expert in compressible flow, but I do have some background in aerodynamics FWIW. Having looked at my 225TTR ducting, I noticed two things. A large obstruction (part of the headlight washer system) sits smack in the middle of the inlet to the ICs on both sides. This will have a negative effect on airflow IMO. Also important is the outlet behind the IC into the wheel well. This is the louvered plastic section at the front of the wheel well. To me this looks woefully inadequate. The combination of these two things significantly limits the flow over the OEM ICs. How significant? I don't have a number, but a WAG of 50% increased flow does not seem unreasonable to me.

What do you think?

smallTTs

Here's my take:

Sometimes that which is intuitively correct...isn't necessarily so. For example: intuitively putting a bigger inlet and outlet to the IC should make more air flow across it. That's not necessarily true nor the most efficient (packaging, aero-drag and cooling performance all factored in) way to do it.


Remember Bernoulli's discovery that static pressure varies INVERSLY to the square of the airspeed: Lower speed = higher pressure, and vice versa. Hey that's why the TT has lift at high speeds!

Air entering the duct needs to slow down before it reaches the IC to increase it's static pressure. A diffuser shaped area, sorta like the the divergent section of a (DeLaval) rocket engine nozzle, works well. Maybe the headlamp housing acts as part of the diffuser? As the volume behind the "throat" of the diffuser increases, we get lower speed, higher static pressure air building up at the face of and therefore flowing thru the IC. Remember too, the IC presents a restriction to the airstream, so the air needs help.

We also would like a lower than ambient pressure area for the air to exit after it passes thru the IC. Conveniently, the wheelwell fits that requirement. The little air dam at the leading edge of the wheel well helps a couple of things: this is one of them.

Now, if the exit area of the IC is also decreased a little, the exiting air velocity increases and the pressure drops, "sucking" the air thru the IC. It appears to me that Audi did just about that, not only on all their race cars but on the TT too.

Another very important advantage to the relatively small opening/diffuser ducting for the IC airflow is that it contributes less to total aero drag of the vehicle than would a large opening, say the area of the IC.

What do you think, Hans?

I strongly agree with your comments on the cost/benefit for the large Forge IC. I am fundamentally opposed to adding more weight farther forward on the TT. IMO, engine cooling may also suffer if that large IC is stuck in front.

My $.02.

joe@vwvortex

unless your going for major hp - all the intercooler is just extra weight and of little benefit. I wouldn't mine looking into an upgraded single side intercooler - and there are some available. APR's Stage III uses the stock intercooler and still can put out 280hp. If I were going for 300hp plus - I could see the need for the intercooler. I also think the $1500 price tag is high - however, this is an imported unit which increases it's cost.

As I've said before - with all the new development happening on the 1.8T aftermarket - I expect there to be several quality choices forthcoming. That will be good for everyone.

hschneTT

I am thinking more of the flow as it approaches the IC.

I think your point about the exit flow makes sense as a higher flow rate should produce reduced pressure on the back side if the IC, increasing draw through it. I think that is the real key - the draw through the IC rather than over the surface and around it. That is why I'm not sure I agree with your discussion of Bernoulli. In my mind that applies more to the flow around a body, such as an air foil. Like a wing where lift is generated by higher flow rate over the top (low pressure) than the bottom (high pressure).

Conversely, with a "buff" body or flat plate who's face is perpendicular to the flow direction, a low pressure zone occurs at the center of the plate, and a suction occurs on the downstream side of the plate at the center. An overall increase in the flow rate under these conditions increases this pressure difference with the square of flowrate. Two things complicate this situation, restriction of the flow around the body and the amount of flow through the body. I'm not sure how much room there is for flow around the IC, but I would guess the amount of flow through the IC is relatively small. In my experience with framed structures, a frame that is only 30-40% full acts like a complete wall, even though flow through the structure does occur.

I guess I'm disagreeing with you on the flow aspect. I do agree with you on the overall drag aspect of the vehicle. Opening up larger ports to increase flow through the front of the grill could adversely affect drag, though I'm not sure this would be enough to make a difference unless you were running at high speeds. Understanding all of this would take lots of wind tunnel testing!!

A funny thing about drag and autos, you always hear people touting the low drag coefficient of their vehicles, but they neglect to talk about the frontal area. I always laugh when I hear somebody like Ford brag about the drag coefficient of the Expedition. Who cares how low it is, the frontal area is so much bigger than any car the drag coefficient is relatively meaningless!

hschneTT

Oh, I forgot to mention, I think the louver causes a restriction that reduces flow rate behind IC

smallTTs

Primarily they keep the junk thrown off the tires out of the IC! This is very evident here driving on salted and cindered snowy roads. While the back face of the louvers are now covered with that road yuk, the openings are free.

The actual open area of the louvers is fairly generous @ about 33in^2 by my quick measurements. If you removed the louvers, you'd only pick up 6-7% area. Additionally, they help direct the flow. I can't agree that they cause excessive restriction, as they appear to be sized well for the whole IC system.

As for Bernoulli, did you say that lift is generated on a wing "by higher flow rate over the top (low pressure) than the bottom (high pressure)" ? By "Flow rate" you meant velocity, not quantity, as in CFM, right?

It's the air getting to the face of the IC that flows thru varying area ducting and therefore undergoes the - delta V and + delta P, so that it arrives at the face of the IC at higher than ambient static pressure (Ps). The same thing occurs with engine cooling, but without the fancy ducting. Right behind the small grille opening, the area increases significantly, the velocity of the air drops and the pressure in front of the radiator goes up to force the air thru the fins.

Ideally we want all the air getting to the IC to flow thru it. Anything that flows around it is wasted. With reasonable shrouding, as on the TT, we now do not have a flat plate in a free airstream at all. We have a bunch of air stacking up against this restriction (the IC) with only one place to go. Now with the lower Ps behind the IC, we have flow thru. Of course this is dependant upon vehicle velocity, but usually the engine isn't producing much power (and compresor outlet heat) unless the vehicle is moving. Chassis dyno testing is an exception. Fans blowing air across the IC could affect the power.
Something to watch during dyno pulls.

I absolutely agree that any changes to the IC system would have to be tested, not only in a tunnel for airflow/drag, but on the road with measurements of charge temps. Interestingly enough, that is precisely how these systems are developed by OEM engineers.

You are correct that total aero drag is the product of the drag coefficient times the frontal area. It is interesting that modern road vehicle drags vary from a Cd of maybe .29 to .39 or almost 35%. Car frontal areas from a Civic (18.5 ft^2)to a large sedan(BMW 7 series @23.0 ft^2) vary about 25%. Sports cars, however only vary about 12-13% from the smaller to the larger, while their Cd varies almost the whole 35%.

I couldn't find any frontal areas for the Excursion, but I'd estimate about 40-50% more than a large sedan, based on height/width. Lower Cd's always help...maybe with the exception of aerobraking from very high speeds. Hey, 18 mpg is a lot better than 14 mpg on a maxi-SUV.

BTW, what's your job? "Rocket Scientist" is sorta broad.

hschneTT

though as I said before compressible flow is not my specialty and I'm in over my head a bit. I am having trouble with the analogy to Bernoulli and the perpendicularity issue. (Yes, I meant velocity relative to the wing) I agree that the velocity of flow across a plate has an effect on the pressure perpendicular to the surface, but is this the same as flow perpendicular to the surface of the plate? Particularly when some/or all of the flow goes through that surface. I don't know, you make a good case however. What is your background in this area?

The "rocket scientist" is what my brothers call me. I am a structural dynamics engineer on the Atlas rocket program. Part of what I do is analyze the wind loads induced on the various Atlas vehicles as they stand on the launch pad awaiting launch. This includes estimating the effects of wind loading on the umbilical tower (framed structure) and the loads "dumped" into the rocket via the various connections, etc. Lots of things make this problem interesting, drag, vortex shedding, etc.

Flow through tubes is a much different phenomenon.

smallTTs

automotive engineer a while ago. Gearhead since about '57.