Straight stack exhausts on VW?

[ProfJ]

Hi Folks,

Looking for generic VW aero wisdom here, apologies if this is too far off topic (I am sure someone will let me know :-)). I have a motorglider with a VW pusher installation (Revmaster 2100D). I am looking to improve the cooling, which is updraft from under-wing scoops. At present the exhaust setup is 4-into-1 as it was designed to feed a turbo (removed by a past owner) and then a 45-degree down pointing exit pipe. The exit pipe goes out the cold (upstream, intake) cooling side and definitely screws up cooling and airflow. So I want to re-route the exhaust and am having creative (yes, that means dumb and dangerous) thoughts.

So my stupid question is: If I fabricate 4 pipes that individually and separately take the exhaust and point it straight back - and particularly given that two of these pipes will be real short, maybe 4-6" - will it negatively affect the engine in terms of vibration or something else I haven't thought of?

I have attached a photo of one of the current pipes. Bearing in mind this is a pusher, you can see I could point this straight back and it would be real short (more like 4"). Yes it looks like it will probably burn the hell out of the prop, but the Vari-Eze and Long-Eze guys do this as standard and they're exhausting 4x IO-235 pipes at their props. So the prop is another issue; I may have to angle the pipe out to the side.

Looking forward to hearing some thoughts.

Thanks
Jon

[n502pd]

yes that appears to be a real problem. My experience in other areas with really short exhaust stacks is that the valve associated with a particular stack can warp because of too rapid of cooling from ambient air getting to the valve via really short stack. I may suggest a 180 deg curve fwd, then parrellel the fwd stack to exit either rearward or slightly downward as then, because of the much longer stack, cold air has much less chance to warp the really hot exh valve. and maybe that can remove some heat to the prop. Drag racers dont worry about short stacks because they rebuild the heads nightly! bad valves are therefore cought befor they can break off. also, if the four seperate stacks can be exited away from the upflow cooling, as in the Globe swift/o300 installations, that probably will elliminate that problem too.I am not sure if there would be a vibration problem. the exh note would be different thou! Just MHO, of course! have fun anyway!!

[ProfJ]

Thanks for that, I had not considered the possibility of valve warping but it sounds like a real concern. There are some other good reasons to loop the pipe as you suggest, although space is tight (as always...)

[lutorm]

A 4-1, if it's properly sized, will give you better peak power than short stacks. But if yours was made to feed a turbo it's probably not correctly sized to maximize NA torque, so that's probably not a concern.

[kmacht]

Maybe I'm not remembering correctly but didn't the very early sonex have straight individual exhausts that came out the side of the cowl? This was before they came out with the universal cowl and the 2 into 1 exhaust options.

[pfhoeycfi]

When I built my Sonerai IIL back in the 80s the straight stacks were common. Just dropped them out of the cowl a few inches, no turn back. Flew great, looked like hell. Eventually I did the 4 into one with little if any improvement in performance..but it looked much faster and the cowl stayed clean.

The valve issue makes sense...fortunately that didn't happen to mine.

good luck

peter
SNB0021
wings and tail done

[ProfJ]

A 4-1, if it's properly sized, will give you better peak power than short stacks. But if yours was made to feed a turbo it's probably not correctly sized to maximize NA torque, so that's probably not a concern.

Yeah, I thought that might be the case (both yes, 4-1 should be better; and yes, my 4-1 is not optimal). What is NA torque? Thanks for the input...

[lutorm]

A 4-1, if it's properly sized, will give you better peak power than short stacks. But if yours was made to feed a turbo it's probably not correctly sized to maximize NA torque, so that's probably not a concern.

Yeah, I thought that might be the case (both yes, 4-1 should be better; and yes, my 4-1 is not optimal). What is NA torque? Thanks for the input...

"normally aspirated", as opposed to turbocharged.