I originally put the factory exhaust that comes with the engine kit on my Velocity. Worked well, but after I had been flying it for awhile and was looking ways to make it go faster, it seemed like this exhaust might not be optimal.
The factory exhaust exits out of the cowl normal to the air flow. This creates an exhaust plume that has been thought to induce drag. Also, it disturbs the airflow going into the prop.
A lot of Cozy’s and LongEZ’s terminate their exhaust in the cowl and exit it out the rear of the cowl into the prop. The lore is that it eliminates that “plume drag,” gives a little additional thrust and may help with cooling by something known as exhaust augmentation where the exhaust gas pulls along cooling air flow. Those sounded like pretty good reasons to me to give it a try. So, I had Clinton at Custom Aircraft fabricate me a new exhaust. He made four separate exhausts that weave each exhaust to a point just aft of the heads. It was very easy to remove my stock exhaust, rivet a patch onto each exhaust hold in the cowling and attach the new custom exhaust.
I used this exhaust for about a year. One thing that troubled me was the exhaust gas hitting the prop. It always left soot on the prob blades, but that was relatively easy to wipe off. The more serious problem was that I was seeing some bubbling of the fiberglass of the prop on the one blade that gets a direct hit (with a 3-bladed prop, its not possible to clock it so no blades get a full blast of exhaust – one always will). It wasn’t severe, but it bothered me and didn’t seem optimal.
I had a set of factory exhaust fairings that were made so that the exhaust could vent out the rear, bottom of the cowling. I never used them when I was building the plane as it seemed like most were using the normal out the side of the cowling exhaust, so I thought I’d just stick to the plans. But, with this hot exhaust gas hitting right near the root of one of the prop blades, it seemed like it might be worth it to switch to venting the exhaust, still out the rear, but out of the bottom of the cowling. I figured that the exhaust gas would then be hitting the blade further out where it had a higher velocity so won’t be exposed for as long a period. Also, the hot exhaust gas would get diluted by all the airflow around the cowling.
I found that the original factory exhaust used to vent out the side of the cowling could be modified for this new configuration. If I swapped the sides the exhaust was mounted on and positioned them so the exhaust was venting aft, it looked like I just needed to cut off the down-turn elbow of the exhaust and get a new straight piece welded on, they’d work perfect. I had a local custom motorcycle shop who had a professional welder on staff and knew how to TIG weld this type of stainless modify the exhaust.
Once I had the modified exhaust headers, I mounted them, lined up the exhaust fairings on the cowl, tacked them in place with a few rivets, then glassed them in. I smoothed over the cowling with micro pretty good and gave it a coat of primer. I didn’t spend a huge amount of time getting it perfect for paint as I intend to make another lower cowl modification shortly (ram air duct).
Once I had it all together and checked out, I took it for a spin. First off, it looks great having those exhaust fairings in. Engine performed well, same static RPM. Seemed to fly at about the same speed, but I wasn’t too careful taking down all the numbers this time – I didn’t expect to seen any real speed difference. But, there were two big differences. As expected, less prop heating. Just a little carbon dust on the blades. Even the blade that was getting a direct hit did not seem to be getting much heat. That was great to see. But the big surprise was that my CHT’s were much, much better! I’ve taken it up a few times since putting in this new exhaust configuration and each time my CHT’s never get much past 400F for very aggressive climb outs. Even after a run to get fuel from a local airport where I climbed out with a hot engine they stayed under 400F. That’s huge.
Ok, why was I seeing better cooling? When I cut the exhaust pipes, I set them quite a ways back in the fairing. This provides some exhaust augmentation of the cooling airflow and sucks it out. I certainly wasn’t the first to experience this. I’d first learned about it in Ken Paser’s book, Speed with Economy. Also, Brett Ferrell used this configuration on his Velocity. I was very surprised to see how effective it is at reducing the pressure in the lower cowling. High CHT’s on climb out was the last issue I had been facing with my Velocity. I had always assumed that it was due to poor removal of the cooling air from the lower cowling, but had not really figured out how to improve it. Well, this did it.
All in all, if you are using the stock Velocity exhaust, I would highly recommend changing to rear exit from the lower cowling. You’ll see a nice speed boost and better engine cooling.
