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Heater Hose Takeoff

flickroll

flickroll

Well Known Member
The #3 CHT on my RV-8 runs high on t/o. If I don't reduce power to 24" - 25" it will go to 420 or higher. T/O EGT's are great...~1200. In cruise the #3 CHT is just fine. The builder of this airplane installed the heater hose hole and flange at the intersection of the #3 cylinder and the baffle, which allows a LOT of air to be dumped overboard without the benefit of cooling the cylinder. Today as a test I placed a piece of aluminum tape over the heater hose hole, and the CHT on t/o is now fine. So it looks like I need to relocate the heater hose hole.

What is the most common place to have the hole for the heater hose air?

Thanks
 
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Hi Jim,
Mine is in front of #1 cylinder at cowl opening entrance. Never have CHT or EGT issues in 200 hrs. My baffling is very neat and tight with very little pressure escape possible. Don't know if location from #1 to #3 makes much difference as I have friends with it at #3 with no problems. I would be sure baffling is installed correctly with all pressure escape points sealed as best as possible before I moved it from it's current location. YMMV :)

P.S. I'm glad you didn't buy my plane when you were in the market for one. :D I couldn't imagine missing the past 200 hrs. with my 8.
 
decathlon737 said:
Hi Jim,
Mine is in front of #1 cylinder at cowl opening entrance. Never have CHT or EGT issues in 200 hrs. My baffling is very neat and tight with very little pressure escape possible.
Click to expand...

Bill ain't kidding....best conventional baffle seal job I've ever seen, period.
 
Here is a photo of the takeoff for my heater hose, behind #3. For a test I taped over it with aluminum tape, plus added an AN970 washer between the cylinder head and the rear baffle to provide a gap between the rear baffle and the cylinder head. Those two things dramatically improved the #3 CHT. So for a permanent fix I am planning to relocate the heater hose hole. Suggestions for a better location appreciated. Thanks

web.jpg
 
Jim,

I have no direct experience (so big grain of salt here), but maybe locating it further down the baffle would help. Still get some cooling air across the fins, and might even preheat the air going into the heater.

greg
 
decathlon737 said:
Hi Jim,
My baffling is very neat and tight with very little pressure escape possible.
Click to expand...

If Dan says so.... :D
Then I sure would like to see a picture of that, Bill.
I'm working on the baffle seals now & would like to see a good example.

Thanks,
 
Your location is similar to mine and I have no problems with CHT for #3. I think this is a common location. What may be causing the problem is that honking big 2" hole! Although the duct is 2", you don't need near that for heating. I originally drilled a 1" hole through the baffle, with the 2" adapter mounted as you have it. It still blasted the heated air into the cabin. To reduce the loss, I now have a 3/4" hole and the flow rate of heated air into the cabin (-6A) seems just right. I just screwed an aluminum plate over the inlet with a 3/4" hole drilled in it to reduce the flow. You might give this a try first as it will be easier than relocating the whole assembly.
 
trib said:
Your location is similar to mine and I have no problems with CHT for #3. I think this is a common location. What may be causing the problem is that honking big 2" hole! Although the duct is 2", you don't need near that for heating. I originally drilled a 1" hole through the baffle, with the 2" adapter mounted as you have it. It still blasted the heated air into the cabin. To reduce the loss, I now have a 3/4" hole and the flow rate of heated air into the cabin (-6A) seems just right. I just screwed an aluminum plate over the inlet with a 3/4" hole drilled in it to reduce the flow. You might give this a try first as it will be easier than relocating the whole assembly.
Click to expand...

Thanks for the reply Tim. I talked to someone else who also mentioned the 2" hole is way too big and suggested a 1 incher. Looks like I have some experimenting to do!

I also talked to Vans today who said the pressures in the cowl are relatively the same everywhere and therefore if I wanted I could relocate the hole most anywhere that is convenient. I'll try your idea first however as that is the easiest solution. It looks like no matter where it is mounted, 2" is way too big, and in my case it has to be robbing a lot of air from #3. Thanks again
 
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Now here is a thought...

When the air goes through the heatbox it picks up heat from the exhaust and also cools the exhaust. Could that cooling action have the effect of increasing back pressure and thus increasing EGT?

I have my carb heat box and cabin heat box on the same exhaust that deals with Cyl 3&4 - both 3&4 have high EGT and high CHT, whilst 1&2 have low CHT and EGT. This is partly due to airflow, but I am beginning to wonder if having the boxes on the same exhaust is not overcooling it in two patches to create back pressure...
 
The airflow with the reduced hole is still quite adequate. Similar to your car fan set on medium. With the 2" hole it was like the outlet from a blowdrier on high!
 
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