To Plenum, Or Not........

[hydroguy2]

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.

[rv6rick]

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

Originally Posted by hydroguy2

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

[hydroguy2]

Originally Posted by rv6rick

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

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.

[Jamie Aust]

Ok, flame suit on:
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 !

[rzbill]

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.

[tim2542]

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

[Lars]

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.

[Tony Rome]

Originally Posted by RV10inOz

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.

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

[hydroguy2]

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!"

[RV8RIVETER]

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.