To Plenum, Or Not........
 

Installed a plenum on our 7.

We still have issues with high CHT's and am not sure if they are related.

Cracking on with the 8 and wondering.....

Do we plenum or baffle.

Let me have some thoughts please.

Mike & Mark

Hello
I have a Plenum on my RV6A with a NEW Lycoming 0360 A1A and although I only have 6hrs on her the CHT are 350 -360 degrees runniing the engine at 2400rpm and full mixture because I'm running it in.
If you send me your email I will send you pic. The guy who made the Plenum is Wayne miller at HWO 305 298-2603 does a nice job.
Hope this helps.
aussieflyer654@gmail.com

Hi Mike how about both. I installed a Sam James cowling on my RV6A and it is built with a baffling system and a plenum.

Bill

I've done both and what I have realized is that they both work but you must seal every air leak, regardless if you have a plenum or not.

I have two plenums, one for each side and it works very well so far. I made composite cylinder wraps that keep the air inside the fins until it exits the bottom of the cylinders. My inlet area is just about 50% of the stock cowl and the highest CHT on the first few flights was 390, Texas, September, 2 hours on a rebuilt engine. They were all within 20-30 degree of each other.

Plenum
 

I have the James plenum and 90hrs on my 9:1 0-320. Its winter here with temps ranging 20-40*F. I am only getting 325* max in cruise, reducing to 250* when I throttle back for the circuit. I think the plenum cools too efficiently. Time to make a cooling air exit restriction.

NO PLENUMS
 

In 100% of cases with plenums on RV7's that I know of, they all have chronic CHT issues.

In 100% of cases of standard baffles (not the black rubber though the orange stuff) that I know of the CHT's are good, i.e. around 300 +/-10 when LOP and 330-360 ROP.

It is just my opinion......but based on the evidence so far the plenum is not working and while some folk with the will argue....I say show me the data.

It is ironic you are asking the question when I think you know the answer already, but just are not sure you want to accept it.

Just remember that no amount of science and data can overcome the warm fuzzy feeling of a closely held superstition.

Here are my RV8's with the plenum.



I can't say anything on the RV7 but at least on (my) RV8 it works pretty well. Not too hot/cold and CHTs are really close.
Never had any issues for the moment from 90F to 10F OAT.

I am sure baffles work well if you pay attention to the leaks (as you would also do with a plenum)

Note: Mike do you have the ECI cylinders with the tapered fins? If you have them and did not do any mod on the cylinder baffle, it could be an issue resulting in high CHTs.

The biggest and most often cooling issue I see is restricted airflow into the plenum/baffle caused by the air ramps in the upper cowling. Often, these are glassed into the upper cowling without trimming them back to open up the airflow. I've solved several CHT/Oil temp issues by cutting this back.

Stick your fist into your cowl opening on the right side. If your fist won't fit, your opening is too small. Fix that first and you may be surprised by the results.

Exactly my point. I would class that as a Plenum FAIL.

Sorry if you feel upset by that harsh comment but lets be real here folks, the OAT is -8 deg C and at 61% power and running LOP !!!!

That should be CHT of around 290 d F.

You have proven my point exactly. ;)

You are confirming exactly what I see here. Try that in Australia or the southern USA.

Here is +8degC OAT, and 67% power LOP


or how about +2deg C at 63% LOP


How about -2dC at 61% power and before I fixed a few things


and last one for tonight, +13dC, 68% power and check out the CHT


I understand my post above and this one will hurt some feelings, but you can have 100 opinions and beliefs, all it takes is one shot of real data to shoot them down.

All the properly baffled -7 and -10 installations I have seen have had good results.

Air inlets
 

Randy
Would you expand upon your experience with the cooling ramps, particularly on the right side? I've always felt like that was a problem if installed as Van's directs. I believe the instructions were "where they fit best". There is a significant difference between #1 and #2 in the room between the ramps and the cylinders and I have always wondered if that made a difference.

I'm about to go to paint and am seriously thinking about a redo on the right side.

You bring up a good point, though I completely disagree with your conclusion. The data is out there.

The point is there is nothing magic about a plenum, so just because you choose to use one does not mean it will work. Just as with a stock set-up you must carefully consider how you are controlling the air flow, from the inlet all the way to the exit. It must be sealed well so that the air only goes where you want it to. Simplified.

The only difference between a plenum and stock baffle set-up is the ability to form a completely pressurized seal of the incoming air.

A plenum is also easier to inspect for leaks as you can disregard the distortion of the cowl in flight.

I think there are some very neat plenums out there, and they can work very well if you put the time into them (which is, of course, experimenting, and the point for many in "experimental aviation").

On the other hand, with very few exceptions, if a person has cooling problems with their new RV, the first thing an experienced troubleshooter will ask is what did you do differently from the Van's design?" And almost invariably, the builder has done something original (maybe a plenum, maybe something else).

Let me state with emphasis - there is nothing wrong with experimenting and working to find something better! However, recognize that tinkering may be required to perfect your new ideas, and accept that.

BTW, I think enclosed plenums are kinda' cool.... :)

The right side is where cylinder one is. On this side, the cylinders are offset towards the front and the ramp on this side tends to get too close to the cylinder to allow enough cooling air into the pressure side. Cut the edge of the ramp back so that it makes the transition up to the top of the cowl more quickly. The ramps are too long in this area and need to be trimmed back prior to installation. Again, the test is if you can get your fist in between the ramp and the number one cylinder. If not, re-install the ramp.

Good info, this may be one of the problems I am fighting.

I need to go investigate ................

Great topic. I have long considered a plenum to be more effective than baffles. I would not go to the trouble of a Sam's Cowl and plenum but I did see another build where the builder made his own plenum.
I have seen certified LSA with very good looking dual plenums. I also remember seeing were someone made the comment that the plenum help with vibration and noise since there would no longer be the baffling rubbing on the bottom of the cowl. I would assume the toughest part of a homemade plenum would be the transition from the inlets. One advantage of the Sam's cowl, round inlets. I don't really want to add more time to the build but some aspect of a plenum seem nice.

I have a 7A and I experienced some cooling issues early on. The chts were not extremely high but I would like them to run around 325-350 I installed cooling fins on the bottom of the cowl and replaced the vans black baffling with the thicker orange. That did the trick ! I have a Vettermans 4 pipe exhaust and I think that the opening was not quite large enough. Also I have learned how to run lean of peak and that really drops your chts.

Your last line applies to you as well...

My -8 has a Sam James Plenum and rings grafted to a stock Van's cowl and I have zero cooling issues. I have tested in 40 deg C temps and have never seen a cylinder above 400F despite 80 knot climbs to 10,000 feet.

Here is a picture of my chronic CHT. ;)



I still have a problem with #3 in the climb, it will hit 400F, if I climb slower than 100kts, once I level off I can see 350 - 360 F on all CHTs.
I love my DIY ( experimental ) plenum, however I would be just as happy if I had made a baffle system.

I contemplated the Sam James plenum but in the end ended up going with the standard Van's baffle. I am running the standard Vans baffles with the Oil cooler mounted on the baffle behind the #4 Cylinder. Here are three different flight pics I have taken over the years flying LOP. Most of the time my CHT temps range anywhere in the 275 F to the 320 F range while flying LOP. The 4 cylinders usually are within 10 deg F of each other when doing so.

The numbers may be hard to see but this pic shows flight at 9500ft, OAT 29F, CHT's 261-273F:


And another flight at 11500ft, OAT 37F, CHT's 292-297F:


And one down low at 3000ft, OAT 42F, CHT's 268-275F:

Sam James plenum, IO-320 ...
 

My Sam James plenum has cooled wonderfully from Day 1. I spent a reasonable (i.e. not excessive) amount of time sealing up the front edges where it abuts the engine case, and have a soft gasket across the aft edge at the rear baffle(s). My oil cooler is behind Cyl#4, and despite my pre-flying concerns about high oil temps given that location, my problem is too much cooling when air temps are under 40 to 45F. In these winter months I have 80% of the oil cooler blocked off to get oil temps of 175 to 180; my CHTs running LOP are very close to 300 depending on OAT. Quite honestly, I couldn't be happier with the plenum arrangement.

Build the cowling the way Van asks you to and it will always work just fine, in my opinion a separte plenum is a total waste of money and building time and weight.

Waste of time and money is in the eye of the beholder....also in the eye of the fuel receipt holder.

I took off last of 4 RV's heading to OSH last year, but landed 15 minutes in front of everyone at the first fuel stop. I also took less fuel then anyone else(even the 320).

YMMV, but I like cruising fast. 8.5gph is 178kts TAS at 9500' CHTs 340-350.

Jamie, you should be saying if you climb slower than 120+ knots, you are only using 100 on initial climb out.

And at 29LPH, despite the Dynon being confused there about ROP, the CHT's should be down around 295-310, exactly as others report. I have had a few PM's on this thread, all agreeing but not having taken the time to photograph it. I hope they do.

Experiment for sure, but don't settle for something less than optimal is what I am saying.

Tim sure thing, but where was I holding the superstition? Please point it out if I have. We as humans are all guilty of it at times. We will justify the indefensible for hours despite the data, just because we have a pride issue with the purchasing decission we made. I am just as capable of this as anyone.

Now how about you define the "zero cooling issues" a little bit. You may well have a Sam James plenum that is set up well and works as effectively as the standard baffles. I would like to see the evidence, and I would be really pleased if you did achieve it. I would like to see others with plenums have success. But so far I have not.

Why go to all that trouble and expense and have higher CHT's? Unless you gained 10+ knots in cooling drag reduction I would argue it was not worth it.

They also make it harder to get to the top of the engine, plugs etc. Despite looking cool! ;)

Down here in OZ its pretty warm a lot of the time.
As David says, almost all of the RVs here with plenums that I know of, run significantly higher CHTs than those with stock setups. This seems counter-intuitive, I agree, but thats the data set we have.
I have a standard baffle setup and never see higher than 380 in climb on very hot (40c+) days Cruise ROP around 330-350 dependent on ambient, and 310 or thereabouts LOP.

One theory we have kicked around here is that the plenum removes a lot of the Al baffle material which undoubtedly acts as additional heat sink for cylinder heads in a stock setup. Im not sure how one would quantify that though.

Its also hard to make apples/apples comparisons of how much air is flowing throught the different setups. All things being equal, hotter = less air which should equal less drag - which is the selling point of aftermarket cowls and plenums in the first place. So one would expect a plenum to be hotter and quicker. Or if you talk to someone with a plenum cooler and quicker! ;)

my 2c

Cheers

Richard,

Funny your data set or experience is like mine.

The unfortunate part is I wonder if some plenums are not just hotter, but hotter and slower.

DanH is probably the VAF guru on this topic and I would suggest he has helped many folk succeed in getting cooling right.

So far I have never seen anyone with better cooling results and a faster plane than one with standard baffles using the better seal material.

Heck, mine have leaks in the corners and around the front near the crankshaft.

Tufting an engine and lots of camera's is the answer! I know folk who did this to STC Beech baffles, and they learned a whole heap of stuff that defied commonly held beliefs that even they had. The data changed their minds, even if it was hard to swallow at the time.

This is external testing http://www.youtube.com/watch?v=QbMQWPTzS3M

I looked for some pics, but can't locate many with enough info.

This was taken about a year ago during race testing. I was running about as hard as possible. 2600rpm, 150ROP, 24.7"mp. OAT 2C. CHTs under 350. 190KTAS


Then in April I ran the Hill Country 150 at similar power settings but OAT was about 21C. CHTs were under 395. I was making max power for the conditions and still below 400. :)

Coming back from OSH'12, we were at gross weight and ramp temp in South Dakota was 33*C. It was still 27C at 8500', I kept climbing looking for some relief of the bumps and heat. CHTs never got above 390. I like my plenum.

To be fair, I did have a cooling issue the first summer after phase 1(jan-feb11). When summer rolled around I had to step climb several times to keep oil temp and CHTs in check. I found many "leaks" in plenum and plugging them was the fix.

Not bad, but not as good as or better than the standard baffle results, but to be fair, you are operating at a different MO than the normal cruise. It would be interesting to see what you get operating at a DA of say 8-9K and WOT and 10-20LOP.
By the way your % power is wrong, some engine settings are not entered right. It should be showing about 79% power.

Just to thro the can among the pidgeons a bit, here is 29.6" MP (yes high QNH) and 2450RPM, about 80dF LOP and a genuine 79% power This was S&L in the cruise at 1000'AMSL. It is true that the denser air helps.

Going from memory here...but 8500' for me is 23"mp, WOT, 2450rpm and 8.5gph will give me 178Ktas and CHTs are about 330-340.

I know my %power is wrong, but can't seem to correct it. I always assumed it was off about 8%, thinking it something to do with running ROP fuel flow.

When you say "good but not as good as standard" I don't understand. What is the goal you are trying to achieve. As cool as possible? Fast? Even spread?

I could make mine run cooler but am balancing running fast (racing) and yet still climb during the summer at gross. So we all have parameters we are trying to achieve. I probably can't hit your benchmark...but show me a RV-7 with a parallel valve 360 that can run 190kts flat out for an hour, cruise 178-180kts on 8.5-9.0 gph AND meet your 310*CHTs.

If you like 'cruising fast' I think you may have the wrong aeroplane....but what do I know :rolleyes:.

:D I know that now...but I had to balance my 3 W's. Wants, Wife and Wallet. So I go as fast as I can with what I could afford.

Ok, flame suit on: :o
So what if my CHT are 50 - 60 F over what everyone else has. I can still run LOP, what damage can I do to my 0-360 its still within the lycoming CHT range for normal OPs.
Ahhhh I can remember the good old days when the C172 only had one EGT !
:D

To your point Jamie, if your engine is "within limits" why worry? I think it boils down to "What am I getting for this extra temperature?" That is the basis for Hydroguys posts about intentional higher temps being traded for higher speed at lower fuel consumption.

I'm verry happy with my "normal" baffles because I spent more than average time fabricating good nose seals. I don't have performance pic to share yet. Might get some tomorrow in a flight to see first grandson (kids at hospital today).

I would like to pose the following position with upfront apologies to some. My supposition is that well done engine cooling systems will perform similarly no matter whether they are std baffled or plenumed. With enough data (temp, weight, maint PITA), I could be convinced that a plenum could do better than std baffles. I have yet to see enough hard data to swing my internal "value meter" towards plenums.

**EDIT** Here are pics from a flight today. 70% power (both from RPM/MP Lyc chart and LOP 14.9 calc)
Sorry for the fuzzy pics. Poor field of focus on camera setting. Images were taken a few seconds apart.

LOP & Percet power
 

I think what may be missing in this discussion is what is true % power these CHT's are recorded at?
I believe % power calculations are based on ROP, at least on GRT equipment.
Try using Fuel flow / 14.9 (assuming 8.5 CR), you probably will find you get a much lower % power than your engine monitor, hence the lower CHTs
Tim

My RV-7 with a parallel valve IO-360, standard pink cowl and stock baffles is decidedly not the best-cooled example of its type. I can easily get CHTs above (well above) 400 degrees on a hot day slow climbout at full throttle. Or at least I can based on the numbers displayed on my EFIS. Cruising at, say, 5500 feet on a hot Sacramento Valley day where OAT might still be above 80 degrees, I struggle to get CHTs below 350 even running LOP.

Which makes me wonder: there is an implicit assumption in this discussion that indicated CHT values are accurate. Even assuming that every EFIS is dead-on, how much accuracy degradation can one expect to see due to differences in individual probes, installation (in the cylinder head), wiring? In the context of this discussion, 20 degrees fahrenheit difference is a lot, but it's only a 5% delta at 400 degrees.

Post #25

Hello

I hope I'm not hijacking this thread. I noticed in Jamie's picture one of his EGT's was higher in comparision to the others. His CHT's are nice and balanced. I also have this situation on my RV6A with ECI IO360. It has a plenum. Total hours on the engine are approximately 75 hours. My number 2cylinder is the coolest CHT of the 4, CHT's are within 10-15 degrees of each other (middle 350's in cruise). My number 2 EGT runs about 60-80 degrees above the others in cruise. EGT's run 1350's, while the number 2 will show 1430.
Looking for some advice.
Tony

When I was trying to increase cooling, I spoke with a very experienced A&P about all my numbers. I was concerned about oil temp going above 215 and chts above 400(during hot summer days w/extended climbs), after his review of the situation he said "it appears your problem is...too much information, Go fly your airplane!"

The original question was whether to use a plenum or not.

As with every other choice on aircraft it boils down to what you, as the builder want to do and what goals are to be achieved.
Do you want the engine to run the coldest that is possible?
Do you want the aircraft to be as fast as possible?
Do you want the most efficiency, highest speed but OK temps all year round?

Your choices will choose the route you will be happy with. There is no one answer that is a cure all. The cooling system like any other must be taken as a whole, it is the sum of all of it's parts, starting at the inlet and going all of the way to the exit. To optimize cooling you need to optimize each stage of the system.

---

As for those wanting data, it is out there. As has been posted many times Chris Zavatson's Lancair article is a very good source. Different airplane , same cooling system choices.
http://www.n91cz.net/cooling/webCowlrep.htm

The Miss St. study. Dave Anders RV-4 work. Reno Racing articles. If you want more efficiency a plenum is the best choice.

It is simple physics, the more air you take in, the more mass momentum you absorb and the slower you will go.

To say some plenum installations had issues, therefore they are a worse choice is an error in logic. A plenum install done badly is the same as a stock cooling system done badly.

I read many posts on here about a 200 hp motor in an RV-8 and how they had to put in louvers to get adequate cooling. I guess I should have thought that the stock set-up was a failure? I cut my inlet area in half, which I was told was crazy and would not work, but alas my CHTs are fine.