High CHT - Some thoughts

[walter]

I've been living with hight CHTs since my initial flight. Last weekend it
was pretty warm around here (70's) [don't say it, even if you do
live in WI] and I can *easily* get to 450 in a 500fpm climb at 6000'.

Several causes come to mind; one of them is the LASAR ignition which
aggressively advances the timing. But another, and I'm really starting
to suspect that *this* is the real culprit is the size of the exhaust
area of the cowl and all the **** in this area which includes the
nose gear attachment, exhaust hangers, vertical cowl attachment
(with it's flat top and bottom), and the lower firewall lip (RV8s have
a nice rounded lip dookickey. All of this junk is in the way of the
air escaping (don't tell me I should have built a TD) and, all of this
**** is either perfectly round (read: totally un-aerodynamic) or
perpendicular to the flow of escaping air.

So how to fix and test? Well I can't just add an aerodynamic leading and
trailing edge to all the round stuff. I could enhance the shape of the
aluminum parts that are perpendicular to the flow, but that would
probably not have any significant effects (cowl hanger). So I need
something else and this morning I read a post on the lycoming group
about Ron Lee adding louvers to the bottom of his cowl. He had a link
to http://www.attawayair.com/rv6a.htm and reading that link I read
that Robbie was able to drip his CHTs by 25 degrees **Celsius**,
dude that's 77 degrees F! That puts me in the high 300's, excellent.

Who else has added louvers? What was your experience with CHT and
was your speed affected?

I found this site which has two sizes of louvers, I might start with the
small ones and see what effect they have. http://www.raceace.com/


Thanks for your input.

[zilik]

Walter,

Several causes come to mind; one of them is the LASAR ignition which aggressively advances the timing.

From Unision Web site

When the engine is operated between 85% and 100% of its sea level rated power, the LASAR? System provides the same ignition timing as conventional magnetos

I seriously doubt that the Lasar is advancing the timing much past 25 degrees when at 6000' and full throttle. The spark may be hotter and the dwell time longer but that is about it. At low MP settings it will start to advance the spark past 25 degrees. Remember Lasar stands for Limited Authority Spark Advance Regulator. I would hesitate to use "Aggresive" in the same sentence as LASAR

Anyway, I digress. Your CHT's seem hot for a 6000' climb. Airspeed and fuel mixture have a lot to do with CHT's when in climb. I find that a good airspeed for sustained climbing is around 120 knots. Slower than this and there is just not enough air flow through the cowl. Mixture plays an important role too. Don't lean to aggresivly during climb and leave it a little rich once leveled off to allow the CHT's to come back down. Then you can lean as you please. Climb at 24 squared increasing the throttle to keep the MP up until your at full throtle.

Some things to check before resorting to louvers. If the cowl exit is cut so that it slopes towards the aft it is restricting the exit size. Make sure the cowl exit is cut parralel with the firewall. You could also slope it forward a little which would expand the exit a little. The old Cessnas had a lip on the bottom of the cowl exit to help scavenge air out of the cowl. You might try that. But remember more cooling airflow = more drag.

Baffles, make sure they are good and tight. No leaks!

Good luck on your CHT's

Gary

[walter]

Baffles are as good as I've seen anywhere. Doesn't matter if you
run my engine rich as **** or LOP, doesn't affect the CHT even 5 degrees
either way. Cowl is cut parallel to the firewall. Don't want to cut
any more of the cowl away, that would create more drag by allowing
the heated air to exit more perpendicular into the airstream.

More cooling airflow does not necessarily mean more drag. If the exit
airflow is exited parallel to the aircraft and it's sped up that will
minimize the drag. Exiting perpendicular and slow are the culprits, in
a nutshell (big nutshell).

Oh, and the LASAR is good for at least 10 degrees from what I've read/heard.
That's why I don't assume it's the #1 culprit.

I'm still thinking the louvers are the way to go. 25 Deg. C? Wow.

[rv8ch]

I'm sorry...what were you talking about?

[N1691M]

I built an aluminum plenum for my O-360 6A. Engine was used, not new. I've not seen CHT's over 400 degrees yet after 9 hours of test flights. Below 40 degrees OAT, I had to completely seal off the airflow to the oil cooler to keep the oil temp's up. I'd say the problem is in the baffle seals.

[walter]

I'm using essentially a one piece baffle made of red silicone baffle material.
No gaps = no loss. I've got a clear indication of scuffing on the top cowl
all the way around and on the baffles themselves with no gaps.
I've got red silicone on absolutely every possible gap larger than a hair
width. In short, my baffles are NOT the problem. I don't have any
flashing between the cooling fins either. And all my CHT's are within
15 degrees of each other.

Have you seen how much **** is *directly* in front of the lower cowl
exit on an A model? Have a look, and this is not a good picture, some
of the **** is not in the picture or missing.

http://rv7-a.com/Img_5103.jpg

Please, is there anybody out there that can comment on the CHT drop
(or not) that they experienced using louvers? Maybe I need to pose
this question to the Rocket folks...

[redbeardmark]

Someone recently posted regarding some metal residual that plugged the space between some of the cooling fins on Lycomings. After the excess metal was removed, the problem was solved. Worth checking.

What type engine, prop, pistons? Lots of horsepower means lots of heat to dump. My O-360/catto/std compression pistons doesn't have a problem with overheating.

New engine?

have you checked your CHT calibration?

Climbing at 500fpm at 6000' with OAT 70F and you're overheating? Sounds odd. Is that 500fpm with nose pointed straight up or just above horizon in cruise-climb?... just asking.

[RV7Guy]

Hi Walter,

I have a new ECI IO360 with 19 hours on the new 7. The first couple of flights I had high oil temps but the CHT's were excellent. I cleaned up the baffling and double checked to insure a good seal and no holes in the system. I found a couple and plugged them.

Robbie Attaway has been my guidance on this endeavor and he recommended that I cut the tunnel forward to increase the exit air. This dropped the oil temps into acceptable levels. I cut another 1" off and also built a small dam on the bottom of the tunnel. The temps came down even more. It is raked forward and looks kind of cool.

I suspect as the engine gets more broken in and I get the pants and fairings on the temps will improve more.

I will post a couple of pictures later. I would look at the exit air for sure could be the same issue.

Darwin N. Barrie
Chandler AZ
RV-7 N717EE

[walter]

I don't have the flashing problem.

New, Injected O-360, standard lycoming stuff, nothing weird.

All CHT's are within 15 degrees, by this measurement
I'm assuming that they are either all wrong or all right.

Mid power or high power or low power climb at 500fpm
all keep the CHT's well into the 400's. Higher power =
higher CHT.

I'm pretty darn sure it's an exhaust size issue
or rather an exhaust blockage issue.

For the sake of arguments, let's assume that is the
issue, nothing else, let's just talk louvers. Anybody have
any experience with louvers?

btw, cafefoundation.org is non-responsive and they
have some info I seek. Anybody have a copy of
localflow1.pdf, localflow2.pdf, localflow3.pdf handy???

[walter]

Originally Posted by RV7Guy

Robbie Attaway has been my guidance on this endeavor and he recommended that I cut the tunnel forward to increase the exit air. This dropped the oil temps into acceptable levels. I cut another 1" off and also built a small dam on the bottom of the tunnel. The temps came down even more. It is raked forward and looks kind of cool.

Hey Darwin,

I sent an email to Robbie earlier, perhaps he'll have some time to respond.
I thought about cutting it off more forward too. If the louvers, or
perhaps even a reverse scoop or pressure lip doesn't seem appealing from
an aerodynamic perspective I will have to cut the tunnel back.

How's that baby flying?