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Cowl Mod for exit air

zcv7853

Member
I am interested in adding some vents or enlarging the air exit on my -6A cowl. I have seen rumors of a "boat tail" mod but am unable to find the post.

Anyone have suggestions of a vent or something to help facilitate more cooling air passing through the cowl? In FL. and my engine is getting toasty.

Thanks.
 
Some have done this, I didn’t see much change if any. Getting air out quickly in my reading isnt the answer to cooler CHT. Mine was issue in climb out, #1#2 cylinders. I reduced and removed air dams in front….that made a difference. Obviously check for good baffle sealing, ignition timing, lower the nose on climb.

If your issues are at cruise, baffling likely issue. Good luck, plenty here on heat in cowl, cht etc….search function is your friend.

https://store.vansaircraft.com/rv-6-6a-7-7a-9-9a-cowl-louver-kit-cowl-louver-kit.html
 
I am interested in adding some vents or enlarging the air exit on my -6A cowl. I have seen rumors of a "boat tail" mod but am unable to find the post.

Anyone have suggestions of a vent or something to help facilitate more cooling air passing through the cowl? In FL. and my engine is getting toasty.

Thanks.

Are you sure you need more air....or more fuel?
 
Are you sure you need more air....or more fuel?

Unless its less than 60 degress outside - my cht's are reading 450+ during climb out.

1. this is unacceptably hot - yes I know.
2. I have been reducing power and climbing as flat as possible
3. I have checked my rtv and seal around my shroud
4. I have completely replaced my baffling and this made zero difference
5. This is a recently overhauled o-320 with 9:1 pistons in it. Less than 15 hours on it.

There is one caveat that I have not tried yet and is next on my list. I recently installed a JPI EDM701 and reused my existing bayonet style probes from my old engine monitor. I assumed they were the same, however.....?
 
Uh oh

Newly overhauled with less than 15 hours? It is likely going to run hot and it likely needs to run hard…plenty of threads about glazed cylinders.

Of course follow the engine mfg recommendations for break in…
 
Unless its less than 60 degress outside - my cht's are reading 450+ during climb out.

Timing - get a second person to verify timing on both ignitions.
Carb - Is the engine running lean? How much EGT rise can you get in level flight?
Airflow - Have an experienced builder check your baffles, baffle seals, inlet ramps, etc.
Instrumentation - Validate what you have by either finding a single cylinder CHT on someone's shelf to install and test against, or find some other way to verify that the readings are correct.

Something is wrong with one or all of these if you're seeing 450 CHT's, even on a new engine. Most engine heat comes from combustion, not friction, and nothing about being new creates more combustion heat.
 
Unless its less than 60 degress outside - my cht's are reading 450+ during climb out.

1. this is unacceptably hot - yes I know.
2. I have been reducing power and climbing as flat as possible
3. I have checked my rtv and seal around my shroud
4. I have completely replaced my baffling and this made zero difference
5. This is a recently overhauled o-320 with 9:1 pistons in it. Less than 15 hours on it.

There is one caveat that I have not tried yet and is next on my list. I recently installed a JPI EDM701 and reused my existing bayonet style probes from my old engine monitor. I assumed they were the same, however.....?

I helped work a similar CHT issue on an RV-7. The problem was the dual Lightspeed ignitions were timed using the instructions. - BUT never verified with a timing light. It turns out both boxes were running ~40 degrees of advance, all the time. This issue existed since first engine start.

The boxes went back to LightSpeed for repair.

Check timing with a light.
Carl
 
RPM, MAP and fuel flow climbing at Vy best rate?

Did you increase the carb jet/available fuel IF 9:1 was new with the recent engine work?
 
Unless its less than 60 degress outside - my cht's are reading 450+ during climb out.

1. this is unacceptably hot - yes I know.
2. I have been reducing power and climbing as flat as possible
3. I have checked my rtv and seal around my shroud
4. I have completely replaced my baffling and this made zero difference
5. This is a recently overhauled o-320 with 9:1 pistons in it. Less than 15 hours on it.

There is one caveat that I have not tried yet and is next on my list. I recently installed a JPI EDM701 and reused my existing bayonet style probes from my old engine monitor. I assumed they were the same, however.....?

Do you have the back side of the prop cutouts filled in on the spinner?
Are you sure your cowl seals at the front edges?
Confirm that the way the cowl top 'ramps' are installed aren't actually a funnel to allow top of cowl air to go past the baffles.

And if any of this helps, let us know. A thank you is all the payment we get!
 
summarizing the most likely causes mentioned from the previous posts:

1) make sure your cooling intake ramps are closed off on the ends, so they are not just tunnels for the cooling flow to escape. I have found this on several RVs that had high CHTs after the owners SWORE their baffles were tight. 2 minutes with duct tape proved the point, more permanent fix followed.

2) ignition timing. Retarding timing will lower CHT. Advancing timing will raise CHT

3) mixture. The suggestion to make sure you can get EGT rise by leaning proves whether or not you are rich enough. If you start pulling the mixture and the EGTs fall, it means you are too lean -- near peak EGT
 
After reading everything here.....

Step #1 check probes and engine monitor - ill work on this and report back

EGT's raise about 100-150 during cruise when leaning. Takeoff at full rich to help as much as possible - plus field elevation in FL is about 5 feet MSL.

Ramps are tight - edges are sealed with new baffle and right against cowls.

P-Mag installed and timed 0% TDC as specified in manual on the right side
impulse coupled slick mag installed and timed 25 degrees left side

Vetterman crossover exhaust - 2 pipes in the outflow stream

**I have no idea if the carb was tuned properly. I doubt it. A/C used to have 7:1 pistons. Ill see if I can find some insight on tuning a marvel schleb carb on here.
 
After reading everything here.....

Step #1 check probes and engine monitor - ill work on this and report back

EGT's raise about 100-150 during cruise when leaning. Takeoff at full rich to help as much as possible - plus field elevation in FL is about 5 feet MSL.

Ramps are tight - edges are sealed with new baffle and right against cowls.

P-Mag installed and timed 0% TDC as specified in manual on the right side
impulse coupled slick mag installed and timed 25 degrees left side

Vetterman crossover exhaust - 2 pipes in the outflow stream

**I have no idea if the carb was tuned properly. I doubt it. A/C used to have 7:1 pistons. Ill see if I can find some insight on tuning a marvel schleb carb on here.

You might want to check the timing spec. Factory timing is 25 deg with lower compression pistons. I don't know the correct number, but thought it was more like 20 deg BTC for 9:1 Cr. Higher compression has best efficiency with less advance. 7 to 9 is quite the step.
 
Correct Probes make all the difference

So I uhhhhh.... figured out the problem!

It is IMPERITAVE that you use the correct CHT probes for your model EMS.

I swapped out my hottest cylinder (#3) probe for the one specified in the install manual and BAM lost 100 degrees right out of the gate.

Did a ground run up until my second hottest cylinder (#4) was reading 350 and my #3 cylinder barely passed 250 with the correct probe.

Will be flight testing it in a few days and will see what cruise temps are with all the correct probes.


ALSO - want to give a HUGE shoutout to the guy at Anti-Splat. I emailed him a few questions and he called that afternoon to discuss my issues. Absolute top notch customer service and I'm not even a customer .... yet! Thanks again!

Cheers
 
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An easy way to check is to place the probe in boiling water. An accurate and known temperature. Especially at sea level.
 
Unless its less than 60 degress outside - my cht's are reading 450+ during climb out.

1. this is unacceptably hot - yes I know.
2. I have been reducing power and climbing as flat as possible
3. I have checked my rtv and seal around my shroud
4. I have completely replaced my baffling and this made zero difference
5. This is a recently overhauled o-320 with 9:1 pistons in it. Less than 15 hours on it.

There is one caveat that I have not tried yet and is next on my list. I recently installed a JPI EDM701 and reused my existing bayonet style probes from my old engine monitor. I assumed they were the same, however.....?

I found my recently overhauled O-320 was running lean with the standard carb. Ended up drilling out the main jet to increase the fuel flow, plenty of threads here on this topic. Was still getting high temps occasionally so added louvres and dropped CHTs by 30 degrees, and OT by 15. Van's now offer a louvre kit. Was straight forward to add.
 
I helped work a similar CHT issue on an RV-7. The problem was the dual Lightspeed ignitions were timed using the instructions. - BUT never verified with a timing light. It turns out both boxes were running ~40 degrees of advance, all the time. This issue existed since first engine start. The boxes went back to LightSpeed for repair. Check timing with a light. Carl
Engine did not detonate. The engine was run at high power at 40 BTDC? :eek: Did you borescope piston tops. If you asked Lycoming my guess would be they'd say tear it down for inspection.
 
Something is wrong with one or all of these if you're seeing 450 CHT's, even on a new engine. Most engine heat comes from combustion, not friction, and nothing about being new creates more combustion heat.

In general, I would agree. However, friction heat during the first hour or two on a new engine is significant and can raise CHTs by a good 50* or more. As the rings and walls wear in, A LOT of heat is produced and is very noticeable on an air cooled engine . For most people, this takes place on the test stand and the user doesn't see it.

Larry
 
I found my recently overhauled O-320 was running lean with the standard carb. Ended up drilling out the main jet to increase the fuel flow, plenty of threads here on this topic. Was still getting high temps occasionally so added louvres and dropped CHTs by 30 degrees, and OT by 15. Van's now offer a louvre kit. Was straight forward to add.

Ya, will likely be checking out my carb in the near future - I doubt its been modified. Thanks!
 
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