Hi, I recently purchased a flying Dream Tundra on Montana 2500 Anphibs. I am unable to climb very high due to High CHT's, and must leave the mixture at full rich to keep the CHTs around 400F when level around 1000' with OAT's around 70F. I am equipped with an O-360 and a fixed pitch climb prop, which gives me about 85KTAS at 2500RPM and 21"MP. Baffle's are the typical GA style with the rubberized strips sealing against the top of the cowl, lower cowl exit is quite large and flat.

Some Tundra's have a plenum cowl. Someone suggested I add a lip which protrudes into the airflow at the cowl exit to increase flow, I would also like to install a constant speed prop at some point, and wonder if the higher speed of this in cruise may help with cooling.

Anyone have any suggestions?

Thanks

Do you have an external oil cooler? If so how is it mounted?

Could you take some pictures of the baffles?

I just spent a ton of time working on in-cowl airflow in order to get my oil temps down. Learned a lot of lessons in the process.

The first thing I'd do is check your mixture. You should be able to richen something like 250 degrees rich of peak at full throttle, which might identify a problem with the fuel side.

After that, I'd start working on ensuring there aren't any leaks between the upper and lower part of the cowling. A good way to do this is (with the cowling installed) to put a drop light in the lower cowl and peek through the cowl inlets. You shouldn't see any light making its way past your baffles. If you do, rework them. Any air that's allowed to pass from the top to the bottom of the cowl without having to go through the cylinder fins or oil cooler is wasted air.

As has been mentioned, there aren't many causes of high CHT. Culprit will likely be:

• Oil cooler


• Baffling


• Oil cooler + baffling combo, because rear mount oil coolers will rely on airflow directed at the cooler via baffling design.

In the absence of cowl flaps, the lower exit lip can have a huge effect on airflow rate, or so I have read.

A picture of the exposed engine and lower cowling would be informative. Fixed or adjustable cowl flaps and lower cowling lips are common modifications for seaplanes and others with high engine temps. Flat pitch propellers and those with little airfoil near their hub tend to not move as much air in cruise. Make sure your flexible engine baffling isn't too tall and doesn't roll back towards the engine. If there's any significant lip protruding into the airflow it can make them deflect down and open an air gap in flight instead of sealing due to upward air pressure.

Gary

Thanks guys.

I have an external oil cooler, it is fire wall mounted on the left side, fed from a scat hose attached to the upper side of the left engine baffles, The left CHT's are usually hotter, but the Right side ones are not exactly cool. I will post some pics in a few weeks as I'm away from my plane right now.

Somewhere here or there is a pic of a similar setup....remote SCAT fed oil cooler. The poster built a deflector plate at the bottom of the baffle outlet and angled forward up some in front to direct airflow over the rear cylinder. Pretty good idea to separate the relatively cool air inside the top of the cowling from warmer air next to the cylinders.

Gary

Also double check the mag timing to make sure it's not advanced.

PA1195 wrote:Somewhere here or there is a pic of a similar setup....remote SCAT fed oil cooler. The poster built a deflector plate at the bottom of the baffle outlet and angled forward up some in front to direct airflow over the rear cylinder. Pretty good idea to separate the relatively cool air inside the top of the cowling from warmer air next to the cylinders.

Gary

This one?

Yes that's the interesting mod. Thanks for the pics again.

Gary

Finally got back to my aircraft, here are some pics of the baffelling and cowl exit. Any suggestions would be greatly appreciated.

Thanks.

A lip on the lower cowl will help. All the above suggestions are good.

What are your oil temps like?

MTV

Oil cooler is obstructed by the looks? SCAT tube looks a little smaller in diameter too. Wouldn't be surprised if you are also seeing high oil temps.

Cowl exit area is very small and streamlined, this will be a major contributing factor. That will be vastly improved by adding a cowl lip and rounding the sharp edge where the firewall meets the exit area to promote smooth airflow out of the cowl.

I also think a CS prop will help a lot. You can run it at lower RPM (less friction, more effective timing advance) and higher MAP to get the same power settings. You will probably cruise faster also (forcing more air into the cowl). This makes a lot of difference to the CHT.

Pull carbs heat on during your climb and report back with the results.

Oil temps are 180-195F, any ideas on the dimensions of a lower lip?
Thanks.

What is the hole for on the RH side? Looks like maybe some scat tube close to it? If you have high pressure air entering there it wont help your cooling issues.
For the lip, an inch or 1.5" would make quite a difference.

The hole is just an inspection plate I had removed to have a look at something. Should the lip be about 90 degrees to the airflow?

anpbtundra wrote:The hole is just an inspection plate I had removed to have a look at something. Should the lip be about 90 degrees to the airflow?

About 45 degrees to the oncoming airflow.

Something like this.

I had the same problem with my Tundra. It had a Lasar electronic timing processor that went bad over a couple of years. I replaced it with Bendix mag’s and it completely solved the problem. Indeed, check you timing.