What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Hot CHTs?

CHT

Dan do you ever feel that you are beating a dead horse when it comes to high CHT's and Hot Oil ? You have a lot of patience 😩
 
Fell like he's beating a dead horse ???

Maybe if he had a God Complex.


Good Guys value themselves on how they can help others be the best that they can be. I see this in Dan by the effort he expends and the intellect he lends to those tasks.

Way to go, DanH !!
 
CHT

Dan definitely knows how cooling works . And he definetly cringes at bandaid cooling fixes like louvers.
I'm just surprised he doesn't get tired of trying to educate builders that do the same stuff over and over .
 
Last edited by a moderator:
Thanks for the friendly words guys! Sorry if I have offended the purists!
CHT temps were up around 410-415 F oil up to 210 in climb.
Have Showplanes cowl, 10:1 Pistons on a Superior IO360, James plenum chamber (modified to fit and seal properly), standard Vans oil cooler.
OAT was around 70F at the time of testing.
P-Mags were set to 25c.
Everything looked to be set up right, but the temps were too high.
I added the louvres to get the problem solved during flight testing to get the permit to fly.
It worked. CHTs now steady and balanced at 370-380 in climb, oil no higher than 180.
I saw no noticeable reduction in speeds, so drag penalty seems low. Why not keep these on permenantly? They don't look too bad (pretty good actually).
 
...P-Mags were set to 25c.
...
With the jumper in, the P-mags are set to 26.2* BTC when timed to TDC.
If you programmed them, with a -1.4 offset, then you are at 24.8* BTC.

On an engine with a tight cowl (Show planes or Sam James) and high compression pistons, dropping the timing that 1.4 degrees can make a big difference without a lose of power. In fact, as you raise your compression ratio, you should move the timing event closer to TDC.
 
CHT temps were up around 410-415 F oil up to 210 in climb...

Everything looked to be set up right, but the temps were too high...

I saw no noticeable reduction in speeds, so drag penalty seems low...

What did the temps do in cruise? A transient temp of 415 is not out of line, but at 70 OAT should be better, admittedly.

Do you have the bypass ducts on #3 and #1 to address the well known "zero fin depth issue"? Also, did you tune up your intercylinder baffles to seal ALL air leaks, or are they just installed staight out of the box?

Ignition timing is also a big factor.

Concerning the drag penalty, adding a massive increase in outlet area certainly hurts you. There's no way around that. Or more to the point, if you missed some of the details mentioned above, you have a lot of air running through the cowl that is not contributing to cooling, and THAT is where the drag is.
 
Thanks for the friendly words guys! Sorry if I have offended the purists!
CHT temps were up around 410-415 F oil up to 210 in climb.
Have Showplanes cowl, 10:1 Pistons on a Superior IO360, James plenum chamber (modified to fit and seal properly), standard Vans oil cooler.
OAT was around 70F at the time of testing.
P-Mags were set to 25c.
Everything looked to be set up right, but the temps were too high.
I added the louvres to get the problem solved during flight testing to get the permit to fly.
It worked. CHTs now steady and balanced at 370-380 in climb, oil no higher than 180.
I saw no noticeable reduction in speeds, so drag penalty seems low. Why not keep these on permenantly? They don't look too bad (pretty good actually).

Thank you David.

You're a serial builder with past RV-8 experience. A parallel valve with p-mags and 10:1 pistons is a difficult case, but it's not greatly unlike your previous RV-8.

Did the previous -8 present a cooling problem? If not, what individual factors do you feel made this one different?

BTW, you said "to get the permit to fly". Is there a temperature limit of some kind in the UK?
 
High CHT

I apologize if my post offended anyone , but 3 times a week someone has this same problem . As far as purist goes ,I'm pretty far from it , Sam James Cowl , Plenum ,10-1 pistons , 17 row oil cooler ,4:1 exhaust ,Bendix Mags 25*

Now some more info on yours , how did you match your plenum to the cowl (photos) . I had an issue with mine . Exhaust ? How many pipes out the cowl ? Prop to cowl seal ?
 
Temps

22 or even 20 degrees timing can help during break in. Some of the angle valve 360's in certificated aircraft REQUIRE 20 degree timing.
I personally hate the look of the louvers. A cowl flap is a much more elegant solution.
 
Do you have the bypass ducts on #3 and #1 to address the well known "zero fin depth issue"? Also, did you tune up your intercylinder baffles to seal ALL air leaks, or are they just installed staight out of the box?

sorry for the thread drift, but where could I learn more about zero fin depth and bypass ducts. First I have heard of it.

Agree that there is nothing magic about louvers. Opening the outlet a bit more, or fairing it a bit better (bottom of the firewall) could do the same thing. I have the vetterman straight pipe RV4 system and it calls for a 1/8" by 3/4" aluminum brace bar going right across the air outlet. I will find a different way to stabilize the exhaust as that seems like it will act like a big dam.
 
Dan has eloquently illustrated the issue in several threads and building off that, several of us have documented our successes in our own threads.

A search for "Hot #3" or similar will turn up multiple results. You can search for my or Dan's screen names as authors to narrow the search.

Here is one of mine to get you started down the wormhole of info.
 
Last edited:
Dan,
Our Permit to Fly is granted after satisfactory flight test schedule and sign off. It's the Certificate of Airworthiness, essentially.
The differences to my last 8 (which had no cooling issues at all) are:
High compression pistons
Showplanes cowl
James plenum
That's about it. On my last 8 I never once opened the oil cooler grills. The oil struggled to get up to temp with them closed. The new one has to have the oil cooler grills open constantly. Same cooler and installation on both.

When I first experienced this overheating on the first test flight, I was really shocked. The flight had to be cut short to avoid overheating the engine. I had the standard vans plenum arrangement at that point. I then fitted he James cowl, modified it and made sure no leaks. I wasn't pretty, but it was sealed. I put spacers at the back of 3 & 4 cyls. Still not enough cooling. I was running out of time to get the testing done in my allotted schedule, so I added the louvres to solve the immediate problem, with the view to re-addressing the issue once testing done and Permit issued.
Hence my original post. I am now thinking "it works well as is, good temps, not great drag penalty, and I think they look ok". Why change it?
I have read all the historical posts on this subject, and see no single magic cure to the cooling/drag issue.
 
Last edited:
There is no single path for efficient cooling of a Lycoming. It is a game of inches. EVERY SINGLE area for air to get through without a cooling fin in the way needs to be sealed. Those intercylinder baffles are a major job to seal all by themselves.
 
The differences to my last 8 (which had no cooling issues at all) are:
High compression pistons
Showplanes cowl
James plenum

Interesting. Upper and lower pressures, as well as exit temperature, would tell a lot.

I am now thinking "it works well as is, good temps, not great drag penalty, and I think they look ok". Why change it?

Then don't change it. A few knots won't make much difference, given English distances ;)

I'm kidding.

Seriously, not knowing the root problem(s) would bother me. And there is a drag penalty. I'd guess two, maybe three knots. Sound about right?
 
Last edited:
Probably about right.
As you say, 2-3 knots over a 2-3 hour trip no great penalty for healthy temps.
So without measuring top/bottom pressures, outlet temps and tinkering with a myriad of other factors (I've read all the threads on this topic), I think I'll take the easy option and go flying with what I've got! I was seeing 173 knots IAS (178 TAS from Skyview) flat out at 1,000ft at close to ISA, so that'll do me!
 
I solved a cooling issue on my high compression Pmag'd lyc by cleaning out the flashing between the fins that are located near the spark plug. You can buy needle files for a few bucks at harbor freight. I polished each fin until there was absolutely no evidence of flashing casting flaws. The results were a 20F drop in CHT during extended climb. Plan on spending 3-4 hours on this job.
 
I don't understand why people continue to post successful modifications when "God like" posters question anything that doesn't fit into their box. Good for you Tiger74 for doing what you think is best in your scenario. BTW, I did the just about the same thing as you with the same results. Move on and fly!
 
I don't understand why people continue to post successful modifications when "God like" posters question anything that doesn't fit into their box. Good for you Tiger74 for doing what you think is best in your scenario. BTW, I did the just about the same thing as you with the same results. Move on and fly!

I think the issue is that people might think that these will solve all cooling problems without digging in to the real issue i.e. is it ignition timing, baffle leakage, poor ducting to the oil cooler etc etc. The OP did not provide any details in the first post, so there was no way to put this mod in context with the details of the problem I.e. what is the setup and what else had been tried. That is why Dan asked for more info.

Louvers won't solve all cooling problems but in some cases they might work very well. It depends what the problem is. I would want to know the source of the problem before I opened up the cowling exit area, which is what the louvers are doing.

The tone of the negative replies is a separate issue. I wouldn't lose sleep over that personally.
 
Probably about right.
As you say, 2-3 knots over a 2-3 hour trip no great penalty for healthy temps.
So without measuring top/bottom pressures, outlet temps and tinkering with a myriad of other factors (I've read all the threads on this topic), I think I'll take the easy option and go flying with what I've got! I was seeing 173 knots IAS (178 TAS from Skyview) flat out at 1,000ft at close to ISA, so that'll do me!

Ummmm.....178 KTAS at WOT, 1000 ft, standard day? Dave, assuming good airspeed calibration, that is really slow. A Showplanes fastback on a clean -8, with 185~195 HP from a 10:1 IO-360 parallel valve should be good for 190 knots plus. Are you sure of engine health? Perhaps ignition timing?
 
Last edited:
I don't understand why people continue to post successful modifications when "God like" posters question anything that doesn't fit into their box. Good for you Tiger74 for doing what you think is best in your scenario. BTW, I did the just about the same thing as you with the same results. Move on and fly!

I just moved you from my "Nice" list to the "Naughty" list.

Oops, sorry, wrong deity ;)
 
I solved a cooling issue on my high compression Pmag'd lyc by cleaning out the flashing between the fins that are located near the spark plug. You can buy needle files for a few bucks at harbor freight. I polished each fin until there was absolutely no evidence of flashing casting flaws. The results were a 20F drop in CHT during extended climb. Plan on spending 3-4 hours on this job.

Can anyone show me what "flashing" looks like? None of my fins appear clogged, but the paint is textured making it pretty bumpy between the fins. Is that robbing my cylinders of cooling air?
 
If I was building per plans and specs, build and go fly. (that is what I did)
If you're going to go through the expense and trouble to put "after market" higher performance parts and pieces in your build, I sure would want to get what I paid for and do some extra digging....

Dan - way too close to Christmas man :) For an engineer, you're pretty funny. Ha!
 
I solved a cooling issue on my high compression Pmag'd lyc by cleaning out the flashing between the fins that are located near the spark plug. You can buy needle files for a few bucks at harbor freight. I polished each fin until there was absolutely no evidence of flashing casting flaws. The results were a 20F drop in CHT during extended climb. Plan on spending 3-4 hours on this job.

I like to learn more about this as I had thought about it without any good/practical way of doing it. Over the years, I have followed and learned much about improving cooling specially from Dan. I have improved the baffle seal to a near @nal point with little appreciable improvements in temps. Of course my temps are OK or better than OK during cruise but in steady climb out, I have to reduce power to keep it under 400F and 400F is my own upper limit. My oil temp on the other hand runs on the cool side even in the CA temp.
 
Can anyone show me what "flashing" looks like? None of my fins appear clogged, but the paint is textured making it pretty bumpy between the fins. Is that robbing my cylinders of cooling air?

No. Bumpy's are good; more surface area.
Flashing is very obvious. It looks like a thin bridge of material between the fins. I don't have any pictures as mine don't have any.
 
No. Bumpy's are good; more surface area.
Flashing is very obvious. It looks like a thin bridge of material between the fins. I don't have any pictures as mine don't have any.
Thanks Jon for clarifying as mine also has bumps but not bridges
 
Thanks Jon for clarifying as mine also has bumps but not bridges

Ok, I need to be careful here, it is a "rough" finish, tiny bumps. Looks like it was peened or sandblasted. Just not sure what the definition of a "bump" is.
;)
 
I should also add, it isn't just the surface area, there are other factors affecting the airflow between rough finishes and smooth. Dan and others have documented them, but it would take a bit of searching for the "complete" scientific explanation.
I suspect your cylinder outer surfaces are "normal" and as designed.
I am scared of over simplification les I be smote by the Cooling Gods!
 
Last edited:
Dan
"
Ummmm.....178 KTAS at WOT, 1000 ft, standard day? Dave, assuming good airspeed calibration, that is really slow. A Showplanes fastback on a clean -8, with 185~195 HP from a 10:1 IO-360 parallel valve should be good for 190 knots plus. Are you sure of engine health? Perhaps ignition timing?
Really? Maybe I'm loosing more speed than I thought. I do have an MT 3 blade prop. That takes another 2-3 knots right?
Engine seems healthy, no signs of anything wrong. Ignition timing was checked. No flashing between fins. Oil cooler standard set up, without ducting to the rear. Plenum well sealed.
Maybe I'll have another look.
I knew I shouldn't have posted this thread!!!! There goes any free time over Christmas!
 
Dan
"
Really? Maybe I'm loosing more speed than I thought. I do have an MT 3 blade prop. That takes another 2-3 knots right?
Engine seems healthy, no signs of anything wrong. Ignition timing was checked. No flashing between fins. Oil cooler standard set up, without ducting to the rear. Plenum well sealed.
Maybe I'll have another look.
I knew I shouldn't have posted this thread!!!! There goes any free time over Christmas!

Not sure about the MT.

Double check me on the speed. Vans says top speed is 214 mph for 180 hp and 222 mph for 200 hp...186 knots or 193 knots.

P-mags are easy to time...just blow in the tube. Want to conduct a simple experiment? Go fly a top speed run, record TAS and CHTs. Now retard timing 5 degrees and repeat the flight. Just put the crank on 5 degrees ATDC (rather than TDC), do the timing set procedure, and go fly again. If I recall correctly, you'll be at 21.6 with the jumper in.
 
...P-mags are easy to time...just blow in the tube. Want to conduct a simple experiment? Go fly a top speed run, record TAS and CHTs. Now retard timing 5 degrees and repeat the flight. Just put the crank on 5 degrees ATDC (rather than TDC), do the timing set procedure, and go fly again. If I recall correctly, you'll be at 21.6 with the jumper in.

Assuming this flight is best power mixture and low altitude, my prediction is no change in speed and the CHT drops 5-10 degrees across the board.
 
Speed Comparisons

Double check me on the speed. Vans says top speed is 214 mph for 180 hp and 222 mph for 200 hp...186 knots or 193 knots.

Just for comparison, I have a stock airframe -8, stock factory Lycoming IO-360-A1B6 (650 TTSN), slick mags, and 74" Hartzell BA prop.

At Reno this year, I qualified at 222.681 mph, slightly better than Van's numbers but on a closed course.

I have cooling drag to lose because my CHT's never go above 380, and during a hot Reno race (95+ degrees) the max CHT I've seen is 365!

Skylor
 
Just for comparison, I have a stock airframe -8, stock factory Lycoming IO-360-A1B6 (650 TTSN), slick mags, and 74" Hartzell BA prop.

At Reno this year, I qualified at 222.681 mph, slightly better than Van's numbers but on a closed course.

I have cooling drag to lose because my CHT's never go above 380, and during a hot Reno race (95+ degrees) the max CHT I've seen is 365!

Skylor

Impressive.
 
Respectable.

Impressive.

It's respectable. Jason Rovey was impressive in 2013 when he qualified his stock -8 in the 225 mph range! My point is, the original poster may have something going that that his both hurting his performance as well as causing high CHT's. The louvers are a band-aid to get temps under control but he still has either poor engine performance or high (cooling) drag, or both...
 
Last edited:
It's respectable. Jason Rovey was impressive in 2013 when he qualified his stock -8 in the 225 mph range!

Both darn respectable, given Stead's elevation and temperature. The 6000~7000 ft density altitude means they go even faster near sea level.
 
Both darn respectable, given Stead's elevation and temperature. The 6000~7000 ft density altitude means they go even faster near sea level.

Conditions for my qual run this year:

Temp: 73F
Baro: 29.88
Wind: 10G19

That works out to be about 7300' DA "on the course"
 
Last edited:
Where did you buy those vents for the lower cowling?

I have the same problem and want to buy a set and get them installed this week. Any guidance is appreciated!
 
Back
Top