Baffle Gaps and leaks
For one thing you have a new engine which will run hot and it is summer, but anything over 400F CHT not normally a good thing. I would shoot for something well below 400F in cruise, and my goal is never exceed 400F except for a short time. Of course with ot air temps and new engine this may be normal. Here is some tips I posted in the Lycoming engine yahoo group.
All Lycoming Cylinders are the same, no matter where they are mounted, left or right, front back, there is no difference one interchangeable cylinder is used. However they are not symmetric front to back/top to bottom. There are shorter fins and longer fins on the cylinder heads. On the Cyl 1 and 3 (aircraft right) the short fins are on the aft side of the Jug. On Cylinders 2 and 4 the short fins are facing forward. Lets take each cyl separate:
Cyl #3: (right rear) tends to be the hottest in part due to the airflow going into the cowl is asymmetric, meaning more air is coming in the left side (cyl 2 and 4) than the right side, influenced by the prop wash and angle of attack of the air going into the inlets. Less air goes in the right side (cyl #1 and #3). This has been verified by NASA and flight test. However it tends to even out by the time you get to the back cylinders.
The real reasons for the high #3 temps is the aft facing fins, slightly less cowl inlet airflow and wrapping the baffle too tight. The easiest thing to do is modify the baffle gap.
CLARIFICATION: What do I mean by baffle gap. Most people build their baffle tight around the cylinders, tight on the aft side of Cyl #3 & #4 and tight around the front side of Cyl #1 & #2, so that the aluminum baffle touches and sits directly on the Cyl fins. You know what I mean. Look at a use baffle. You see the cylinder fins have worn grooves into the lower baffle that wraps around the lower part of the jug and head. They are touching. As I mentioned cyl #3 has the aft facing short fins against the baffle. By giving a small gap between fins and baffle you will increase airflow around this cyl. How much gap and how to assure a gap? Place 2 washers under the baffle mounting screw to open the baffle up slightly around the rear of #3. It does not take much. You may see a
30-40F temp drop. It works.
Cyl #1 runs cool because it is near the front cowl inlet and the long fins are fwd. The baffle adjustment here is block #1 cyl to increase temp, which may block the whole front of the head. You must experiment and what you do here affects #3 to an extent: (Note: blocking is a ramp not a flat vertical plate)
http://img330.imageshack.us/my.php?image=frontbaffle34wd.jpg
Number #2 does not run as cool as the other fwd Cyl #1. Although the right side (#1 cyl) has less flow it has the fwd facing long fins against the baffle. Number #2 (left front) may be OK or run hot just like #3 and for the same reason, the baffle wraps around the short fins. If it is too hot (relative to #2 or #4) a small gap between the lower wrap around baffle and cyl fins may help. This is done like Cyl #3 explained above but less gap may be required. If for some reason you need to increase Cyl #2 temp, smaller block plate like cyl #1 may be in order. It takes trial and error of baffles spacing and blocking plates. Use metal tape for temp fit of blocking plates to figure out what works
best.
Cyl #4 should be ok should be the nominal temp you are shooting for. #4 is not real adjustable, but assure the other 3 cylinders, #1, #2 and #3 are as balanced as possible. By blocking #2 a little (if it is too cool) you may drop #4 a little.
If you have soft seals to the cowl they may be leaking. Make sure they fit perfect and not gaping in flight. That is why the sealed HARD TOP plenums are better, they leak way less. Also calibrate your CHT probes to make sure they are accurate. The Well type is the kind Lycoming uses to base their limits on. The ring spark plug type suck and tend to be inaccurate in my opinion.
The stock cowls usually have plenty of inlet area, just make sure you have no leaks and the exit is smooth. Install a sheet or lip at the bottom of the firewall adjacent to the exit. You don?t want that sharp corner at the top of the cowl air exit. Kent Paser, Speed with economy, spells this out in his book. He also discusses the baffle mods I discuses to an extent. If you do all this you will get cooler cylinders and less drag. Builders/pilots take their baffles for granted. They are an important part of the engine and airframe.
Also make sure there is no casting flashing in the fins that are between the valves. These are the inner fins that are right in the center of the head.
Good Luck Cheers George.
PS: How much gap is enough. 2 washers worked 3/16" for one builder, but I have read over 2 inches in Kent's book, which seems excessive. Also the washer trick is crude and there are probably better ways to add a space or gap between the fins and baffle. Also as you add gap to say the #3 cyl you will see that you will have a gap at the edge of the baffle that air can escape from , with out cooling the cylinder, so a edge flange may help control or channel the air flow down and around the cylinder vs. just out the sides. I used high temp silicon to make baffle spacers and edge seals.
(using the 'embarassed' smiley).
