One approach to lower CHTs

Ever since first flight, I've been struggling with high CHTs on my back two cylinders (#5 & #6). With WOT and full rich, I'd see #6 climb through 400 degrees pretty quick if it was any hotter than 65 OAT. By the time I would notice, and react, it'd usually make it to 405-410 range. Usually, I'd pull the throttle back to about 23" or so, sometimes lower.

The first thing I found that helped was the washer behind the back cylinders (between the baffle and the cylinder). That helped, but only by slowing down the progression to 400+ during climb out. It also helped in cruise.

Last week, I went to Spartanburg to visit AirFlow Performance for injector tuning. After the first flight with Don, the analysis showed that #3 & #4 were running rich and #5 & #6 were lean.

Cue to me smacking my head and wishing I had a V8 (the drink, not the engine). Why didn't I make that connection before? Anyway, Don makes some injector adjustments and we go fly again, climbing up to 8500'. I was able to leave it WOT all the way up, and even was able to start leaning a bit in the climb too, and never did I get close to 400CHT.

After one last injector adjustment (#1 was showing slightly rich), I left for home. Oh boy, what a difference. Again, WOT climb out to 8000' (OAT on the ground was 84F). Highest CHT observed was 387! This is easily 20 degrees cooler than the previous setup.

This may or may not be a solution for everyone, but wanted to throw it out there as another potential solution for high CHTs. Don at Airflow Performance was fantastic to deal with and has a wealth of knowledge he's very willing to share. Well worth the trip up there to have this done.

Oh, for the record, my engine is a stock IO-540 from Van's with two mags and Silverhawk throttle body.

Yay!

Atta boy, Aaron!

Carry on!
Mark

Aaron,
What kind of tests did you and Don run - something like the Gami lean test? Also what size injectors did you start with on 5 & 6 and what did you end up with?

I'm having basically the same issues you had and plan to run the Gami test this weekend to gather some data, but any other suggestions would be greatly appreciated.

Yes, it was the GAMI lean test. My engine came with stock .028 injectors. When I got there, Don swapped them all out with .025 injectors and we went flying to do the lean tests. The tests were done at 3500' and 8500' and at different MP settings. I think we wound up going to .0245 on 1, 5 & 6, and .0255 on 3 & 4 when we were all done.

As close to South Carolina as you are, I'd really consider just taking your plane down to Spartanburg and have Don work his magic. That way, you're done in just one day. It's $450, and that includes the injectors.

Thanks for the advice Aaron!

Looking for cause and effect

GTWreck said:

(from earlier post) After the first flight with Don, the analysis showed that #3 & #4 were running rich and #5 & #6 were lean? (from second post) ?My engine came with stock .028 injectors. When I got there, Don swapped them all out with .025 injectors ? I think we wound up going to .0245 on 1, 5 & 6, and .0255 on 3 & 4 when we were all done. ..

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This is a counterintuitive. The two cylinders that were lean are two of the three with the smallest injectors and the two that were rich now have the largest.

#2 .0250 #1 .0245
#4 .0255 #3 .0255
#6 .0245 #5 .0245

I?m not any kind of expert on fuel injection. I just automatically look for cause and effect and I expected either that #5 and #6 injectors would have gotten bigger or that the injectors for one or more of the other cylinders would have gotten smaller so they would be less rich and more fuel would be driven to #5 and #6.

Maybe I don?t understand how injectors are numbered. Are the numbers (.0250, .0255 etc.) the orifice size or the flow rate at some standard conditions?

Nozzle set

On this installation the injector sizes ended up .0245 on cylinder 1, 3, & 4. .025 on cylinder 2 and .0255 on 5 & 6. That got the total spread to within 0.1 GPH or so. Couldn't really tell with the flow meter mounted in the tunnel the fuel flow readings were not really stable. Used the "bump" method on the vernier control.

Just as a caution. This set probably won't work on a different plane. We find they are all different. And a lot depends on the mission the plane is used for.

Don

Don,
Can you elaborate as to how a lower nozzle number (.028 to .0255) led to lower CHT's on #5 & 6 for Aaron?

Bubblehead said:

This is a counterintuitive. The two cylinders that were lean are two of the three with the smallest injectors and the two that were rich now have the largest.

#2 .0250 #1 .0245
#4 .0255 #3 .0255
#6 .0245 #5 .0245

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Yep, my bad. I was going from memory, which isn't what it used to be. Don's post has the right numbers.

Auburntsts said:

Don,
Can you elaborate as to how a lower nozzle number (.028 to .0255) led to lower CHT's on #5 & 6 for Aaron?

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No expert here, but I i would think that it was a balancing act. The absolute orifice sizes would be less important than the relative change in size between the richer and leaner cylinders while maintaining total flow at a constant pressure drop.

I would be interested in understanding the tradeoffs involved with using .028 vs. .025 as a baseline size.

Lower CHT

From the feedback I have received having 400F plus on climb is totally normal, but there is always a desire to lower my CHT being is a rather warm area 12 months out of the year (aka Southern California) I too wonder what effect having a smaller orifice has on CHT? all I can think is less fuel, less combustion, less heat (aka lean of Peak) I am at .3 spread and the engine runs wonderfully, just hot on climb.
Anxiously awaiting feedback from Don!

1001001 said:

No expert here, but I i would think that it was a balancing act. The absolute orifice sizes would be less important than the relative change in size between the richer and leaner cylinders while maintaining total flow at a constant pressure drop.

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Very much agree. Diesel orifices are much the same. For example, an abrasive slurry, (looks and acts like silly putty) is pushed through the nozzles to radius the inlet to each orifice. Delivery by each orifice is then stabilized to it's diameter.

Although our systems are much lower pressure, the same principles apply. Flow variations due to micron sized manufacturing differences certainly exist, and overpower the direct link to diameter unless a similar (expensive) process was/is used.

Disclaimer: I have no personal knowledge of the manufacturing processing of the orifices for our aircraft engines. If someone does, please add to our knowledge.

ReidVaitor said:

I too wonder what effect having a smaller orifice has on CHT? all I can think is less fuel, less combustion, less heat (aka lean of Peak) I am at .3 spread and the engine runs wonderfully, just hot on climb.

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The flow -------and thus the CHT------is set with the red knob.

The balance is set with the individual injector nozzle insert. This is what equalized the CHTs.

A complex calculation

To understand why we choose what size nozzles orifice or how we manufacture our injector nozzles is a discussion left for our FI-101 class. You have to understand the theory of operation of the fuel control unit, the operation of the flow divider and the interaction of the fuel control, flow divider and injector nozzles to determine all this. Its pretty in depth to explain this is a few paragraphs.

Don

The secret to getting reasonable CHT's is not to monkey with the injectors unless you know what you are doing. Basically there is Don Rivera at Airflow Performance, Andrew Denyer at Riverina Airmotive(for the Aussies) and of course GAMI in Ada OK.

Once you get the Fuel/Air ratio's balanced, then and only then can you start looking at the baffles etc.

As Don mentioned above it is all about getting consistent F/A ratios first. Then get the average cooling right and do not sweat a 20dF or there about variation in CHT.