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On 9 Jan 2002, at 17:22, jason_r.ford wrote: > The heads on a Type 3 run hotter than a Type 1 due to the higher oil > temperatures and the airflow design. Part of the job of the heat exchangers is > to "remove heat" away from the engine, not just to remove the exhaust fumes. > This design is accomplished partially through the metallurgy and density of the > metal. Since J-Tubes are less dense, thinner, and use metal compounds that > conduct heat faster than do the heat exchangers, it is certain that you will > increase the temperature below your cylinders. Not to mention, the heat > exchangers on a T3 are wrapped with a heatshield mesh, so by using J-tubes, you > are also sacrificing this heat-protecting mesh. There's probably a small kernal of truth here, in that the standard heat exchangers won't heat the heads by either radiation or convection because the outside of the HE, the part that the head can "see" is always cooled by internal airflow. But, I don't think this is ever likely to be significant, unless you spend a lot of time standing still with the engine running. > If the bottom of the cylinders is protected by the larger cooling tin (as > opposed to the smaller design found on earlier type 1's), this may not be as > much of a problem. But regardless of cooling tins used, cylinder #3 is going to > run even hotter, running a greater risk of cooking your engine. Don't think the size of the cooling tin here will really matter, because there will always be a big gap between it and the J-pipe. I also don't see why #3 would be any different from #1. I would expect the biggest effect to be in how the larger diameter pipe affects the breathing from those cylinders and what kind of effect that has on heat transfer from the exhaust gases to the head. I'm over my head here, but my GUESS is that allowing the exhaust gases to exhaust more freely would let them expand more quickly and this lowers their temp. This should lower the amount of heat available to flow into the head. We also need to allow for the fact that better exhaust breathing tends to make the engine run more lean, which makes it run hot. I think it is universally understood that anything you do to your exhaust will require that you reconsider your jetting to get your combustion back where it belongs. - ******************************* Jim Adney, jadney@vwtype3.org Madison, Wisconsin, USA ******************************* ------------------------------------------------------------------- List info at http://www.vwtype3.org/list or mailto:help@vwtype3.org