The Rabbit TDI Project
Part 8: Intercooler and Pipework
Or: do it once, do it right
The TDI engine is intercooled.
For those that don't know, air is heated as it is compressed. Intercoolers are
used to remove heat from the charge air to allow the charge air to increase in
density so you have more combustion air entering the combustion chamber.
More air allows for more fuel. More fuel means more power.
Or basically: Intercoolers are good.
There are lots of schools of thought on the methods of intercooling. To summarize
that, most people are wrong. Bigger is not always better, as it all depends on flow.
If you install an intercooler with a huge area, let's say it's 36" long in the
direction of flow, and 8" tall, but only 1" thick. All of your combustion air
must pass thru 4 square inches, and the intercooler will have a significant
pressure drop and will be a huge flow restriction. (8"x1" is 8 square inches,
but only 1/2 of that is for charge flow. the other half is atmospheric flow thru the IC).
If you read Corky Bell's book on turbocharging he states that a
good rule of thumb is 6 square inches of flow area for every 100 hp.
The second rule of thumb is 1.5xHP = airflow in CFM for gasoline engines.
The third rule of thumb is diesels require 25% more air than gas engines.
Analysis of the OEM TDI intercooler:
flow area: 3.5x7/2 = 12.25 sq in
A quick estimate on flowrate based on BellIntercoolers.com tables: Max flow = 240cfm.
Max horsepower for the stock IC: 240/(1.5x1.25)=~128hp.
flow area = 3x9.4/2 = 14.1 sq in
flowrate = 302 cfm (from the tables)
max hp: 302/(1.5x1.25) = 161 HP
I want the pickup to be a sleeper. I don't want big scoops or wings or anything
like that. Being a pickup, there are somethings that are easier on the cars, and
not so easy on the trucks. The TDI intercooler that came with the engine would've
worked fine, it's just that I had nowhere to put it. I could've put it where the
battery is, and then relocated the battery, but that would've been a bit of a pain.
I could've put it over the tranny, but then I'd need a hood scoop. I could've put a
front mount down below the bumper, but I couldn't figure out how to run the pipework.
I decided to locate the IC in front of the radiator. But to do that, I couldn't use
the TDI IC, I had to get a custom built one.
Could I have gone larger? Yes. I could've gone with one about 8" wider.
(basically, to fill the entire space between the headlights.
Could I afford a larger IC? Well, not really.
And with a max HP rating for the IC of about 160 hp, all should be fine.
By the time the TDI gets to that power level, I should have a larger budget.
I had to clearance the front support to mount the intercooler, and it's a very
tight fit with the radiator. A 2.25" thick core would've fit perfectly. Also,
I had to shorten the safety catch on the hood, as I couldn't close the hood
(the catch was hitting the IC)
The pipework is thinwall 2" stainless steel.
The elbows are all rad-hoses.
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