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<x-charset iso-8859-1>Hello- I *might* be writing a tech article on this soon... it is oft misunderstood... Debating the quantity of maximum airflow through the different engines is quite misleading. Ultimately, the maximum amount of air that can flow past the heads and cyls is defined by the geometry of the fins, not the fan! (this has to do with the choking area of a compressible flow... fluid dynamics stuff... let me know if you want more details as I've done the math myself and it does check out against the VW specs). The reason why early T1 and late T1 cooling systems are rated at different cooling air amounts is because the later ones had "extra air" that went by the oil cooler. The T3's have a similar situation, which is why their spec also looks like that. But, ALL of these flow the same maximum amount through the head and cylinder fins, and by about 4000-4500rpm, they've hit this maximum airflow and cannot flow more (this is mistakingly referred to as fan stalling or fan cavitation by some, but it is not... it's just choked flow!). So, now that we see that the AMOUNT of cooling air can't get better, what else should we optimize? Well, we want that bit of air that cools to be as cold as possible. May sound like a "duh" statement, but there's more to it than you think. First, of course, the fan inlet should have nice, cold air. In this respect, the T3 and T4 are ahead as there is no engine compartment ahead of the fan to heat up the air like a T1. Second, the same temp of air should get to each cylinder. The early T1 cooling system is the worst off here - the left bank gets hot air from the oil cooler. The T3 and late T1 systems tackle this, but both still have a problem. Look at the direction of airflow. We know that ultimately, the amount of air exiting each cylinder and head will be essentially identical thanks to the laws of physics. But, on the upper (i.e. vertically on top) fins, where the flow isn't as rigidly governed by the choking flow phenomenon, the shrouding can have a great effect on how each cylinder is cooled. The T3 is way behind here, especially when below 4000rpm (when the choked flow bit no longer applies). If you take a look at a T4 cooling system, you'll see that there are large "plenums" of air above the banks... even larger than the T3 ones. This is a great improvement. The late T1 system is pretty good, however the late T1 still isn't ideal as cylinders 1 and 4 get cooler air than 2 and 3. On the later T3's, they tried to make the plenums above the cylinder banks more effective by installing the "cool tins" under the cylinders. This does help at lower rpms. On late T1 systems, though, the gain is questionable since the flow is quite a bit more even than the T3 at lower rpms. On these systems, where all of the cooling air is dumped between the cylinders, it'd be most beneficial to force the air all the way to the outside of the cylinder banks, just like the factory did on the T1. Alas, the T3 "cool tin" on T1 idea is a bad one. For a better cooling system, the late Joe Locicero designed a DTM (down-the-middle) shroud for the T4 in upright form. It kicks butt, big time. Although little known, he also developed a DTM for the T1, but very few people ever purchased it. It is the perfect T1-style cooling system. Also, his systems allowed the use of a modified T4 fan... a bigger fan he could spin slower to get the same airflow for less horsepower. Dyno tests have been done... it works like a charm. His son will be restarting production on these items shortly, I hope. There are also a lot of 911 cooling systems out there on VWs. Guys look at the factory specs for how much a 911 fan pushes though a 911 engine and think it's a perfect idea. What they don't realize is that a 911 fan on a VW engine still can't push more air than a VW fan!!! While true that it takes less power to spin the fan, comparing it to the modified T4 fan on a DTM is difficult, especially on a bang-for-buck scale. And, many of the shrouds aren't designed well-enough to distribute the air evenly at low rpm. Take care, Shad Laws LN Engineering - Aircooled Precision Performance http://www.LNengineering.com ------------------------------------------------ To unsubscribe, E-mail to: <type3-off@vwtype3.org> For more help, see http://vwtype3.org/list/ </x-charset>