Date: Wed, 26 Apr 2000 14:27:51 EDT
Reply-To: FrankGRUN@AOL.COM
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Frank Grunthaner <FrankGRUN@AOL.COM>
Subject: Re: Tire Carriers Again (cooling system comments)
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I have been following this thread with some amusement. In many of these
discussions it is assumed that the Vanagon cooling system is a weak link.
From an engineering perspective, this is simply not true. Stuart is dead on!
As I prepared for my Diesel to GTi conversion (note that my analysis is only
correct for the diesel cooling system - didn't consider any details of the
waterboxer system), I analyzed the thermal capacity, coolant volume, radiator
surface area, fan-induced air flow, and coolant flow from the engine
compartment to the radiator and back. the numbers took into account the power
(heat) generated by the gasoline engine, the peak thermal inputs of both
diesel and gas engine, etc, etc. The bottom line is that the Vanagon system
is seriously overcooled. The thermal capacity is capable of handling the
thermal load generated by a 400 hp V8 Chevy small block. The peak thermal
loads were higher for the diesel (I was considering both NA and turbo
applications) than for the 1.8 L gasoline engine. This analysis included the
additional thermal load introduced by the addition of Air Conditioning. I
also used the capacity recommendations for high output V8 engines in low
relative humidity conditions and desert ambients (105 F air temperatures).
After the conversion, I have monitored the cooling system under load. The
coolant temperature in the return line from the radiator was more than 20
degrees cooler than the engineering tables suggested as nominal (don't have
my notes here, so I can't be more quantitative). These tests were done on a
100 F day romping around the Angeles Crest (LA) in first and third with AC on
full bore! As some of you might suspect, I tried to optimize the system. I
quickly found two limitations:1) air in the system and 2) water pump flow
capacity.
The first of these is the most important real time variable. Air in the
cooling system seems to segregate to pockets in the cylinder head.
Substantial temperature variations between cylinder #1 and #4. I currently
monitor the head temperature with a Westach gauge with senders under the
spark plug for #1 and #4. With air in the system I can get temperature
differences of up to 35 C. Bleed the system thoroughly! I have designed a
bleed screw modification of the head coolant outlet, but haven't implemented
this yet. Trying to follow the system used by Porsche for the 944 Turbo which
also has bleeding problems. With the coolant pressure gauge, I can readily
detect the slow creep of air into the system (small heater core leaks, air
from coolant overflow tank, small hose leak, etc.). When sealed, the system
has to be bleed at several week intervals to remove trapped air. When fully
bleed, no further air introduction happens, and the temperature differential
between cylinders is less that a few degrees (probably system measurement
resolution).
The second issue is water pump flow rate. In the case of the I-4 engines, the
most accessible variable is the pulley diameter, and the radial belt contact
area. From my parts stock, I looked at 4 different pulley diameters (all used
on various implementations of the 1.8 L 8V engine - Golf, Cab, Fox,
Dasher/Quantum). In the end, I equated highest flow with maximum pressure at
the radiator inlet at 6000 rpm. This was achieved with an intermediate size
pulley (not the largest or smallest diameter). It was important to use the
pulley from the three pulley belt design (crank, water pump and AC compressor
on one belt, AC compressor and alternator on the other). I found no pressure
difference in the pumps below 3000 rpm, and concluded that the smaller pulley
lead to cavitation at the pump impeller reducing the net flow.
As to closing off the radiator flow with the spare tire in place, the swept
area feeding the front of the radiator (top and bottom grills) is actually
quite large. The radiator is placed rather forward compared to an American
Iron design minimizing the plenum effect. I found that the area was poorly
sealed at the bottom and the sides. Consequently, I sealed all the area
around the radiator (including the bottom) with thick neoprene sheet (about
1/8 inch thick). With a similar precaution, I would assume there would be no
problem with the coolant system.
Would never do it myself though, don't like the looks, and it has to increase
the drag (more power, more fuel) to maintain over 50 mph.
Sorry for the length, hope this helps someone,
Frank Grunthaner
After the conversion,
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