-12 or -32? was Carb Differences

[Steve Pierce]

Just overhauled and installed a 160 hp O-320 with a -12 carb. Haven't seen a need to lean it as of yet as the fuel burns are low.

[sophistical]

The -12 sounds ideal in most conditions, but what about cold climates? Is this an option for those operating in cold climates?

[taildraggerpilot]

Steve:
What is your fuel burn on the -12 carb? My engine runs much smoother with the -12, but I still burn right at 10gph vs 11 with the -32 carb. However, I operate 75 degrees ROP. If I lean to 50 ROP, I typically see 9.5. I also typically cruise between 2500 and 2550 rpm. If I slow it down, it doesn't seem to impact the fuel burn by much.

[Steve Pierce]

Was seeing 8.5 gallons an hour at 2550 rim full rich on this 160 he Pacer. The prop is a 52" pitch which is getting re pitched to 58" today.

[taildraggerpilot]

This puzzles me. I would love to get 8.5 with my carb. I wish I understood why mine is running so rich compared to yours. I have a 60 pitch prop, so maybe with a 52 running at 2550 it isn't using as much power. I'd be interested to see what it gets at 2550 with the 58 pitch.

[Steve Pierce]

Ben, I am curious as well. I think you are correct in that it is not working the engine with such a low pitch. Will post some numbers with the 58" prop.

[Gilbert Pierce]

I started using the attached Lycoming Power Chart.

When leaning to about 1490 degrees EGT on #4 cylinder I was at about peak power with CHT never above 360. My fuel flows were always about 0.2 to 0.3 gph below what the chart said I should have for the given altitude and rpm setting.

Since installing the Sutton Exhaust my I have not sorted out the EGT issue as you cannot put each probe the the same distance from the flange. I now just lean to the chart fuel flow and my CHT's are running the same as before, Example; at 5500' and 2550 rpm the chart shows 9 GPH and I would see 8.8 with #4 leaned just below 1500- degrees so I now lean to 9GPH.

I used 1500 degrees as the limit because that is the limit EI said you should not exceed. I now question that logic as EGT is just a relative number depending on where your probes are.

I also lean at all altitudes. If cruising at 1000' msl I lean and I lean during climb but keep the EGT's to what they are at sea level full rich; about 1350 degrees. If you do that when you get to altitude and level off and set cruise power you need to push the mixture in a little because as Brian Neal said, full throttle you have an enrichment valve. Come off full throttle while leaned your EGT's will shoot up as that enrichment valve closes.

[59pacer]

A little bit 'off thread' but---
Pitch can have a big effect on fuel flow. Running a 69x54 and a 69x48 prop on my O-200 Continental, using 2550 rpm at 4000' will give 7.1 and 5.8 USG/hr respectively. The props were both 'approved' props, different ages and manufacturers, but that's about 20% difference! What would most probably tell the full story would be to see what the MAP was in each case, as that should tell the actual power output. The change in speed was about 10%.
After 500hrs on my current Pacer with a 160hp O-320, 61" pitch prop, Leading Edge exhaust, slightly different nose bowl to original, mainly long distance, heavy weight, high temperature operations, diligently watching CHTs and EGTs, I have never had a CHT anywhere within a bull's roar of the limit. I used the POH guide for 75% (2400rpm at sea level, plus 25rpm per 1000'), ran it slightly under that, and leaned for smooth running. I consistently get 8.7 to 9USG/hr average (includes climb and descent).
My concern was that I could be accidentally running over 75% and leaning too much, so a while back I ripped out the CHT/EGT system, and put a MAP gauge in the hole.
With a bit of fiddling I was able to blow up the appropriate power charts from the POH so I can read them, and had them plastic laminated so I could draw lines all over them. Now I can enter with RPM, MAP, altitude and temperature, get the %power, fuel flow etc.
The POH rule of thumb method is always at or below 75% (mostly just below), and the leaning method is fine. I'd have no trouble with pulling the MAP gauge out and forgetting all the complication, but I'd have to fill the hole with something!
The only change to my operating technique when the aux is full, and I'm a 'little bit' overweight, is to stay about 1500' and empty the aux as quickly as possible, then climb higher. Avoiding the early long heavy climb tends the average burn towards the lower end. If I have to climb early, the average burn is towards the higher end.
Got another trip 'outback' in September--started out as a week trip, but its gradually getting longer as destinations get added. Great machines these short wings!

[cozzmo81]

I am in the process of overhauling a O320-A2A for use on my Pacer. While I'm waiting to purchase new cylinders I thought I would go over my engine accessories. My O320 had a

10-3678-32 carburetor installed and the oil sump has the tapered intake port. Before I spend money on overhauling the carb. I want to make sure I am going with the correct one. It is my understanding that If I am going to use the -32 carb I would need to install the sleeve in the sump inlet per SB 258? If I were to switch to the -12 carb I would need to use my sump as is with the taper? I see that people generally get better fuel burn with the -12 carb. What carb would you recommend? Also does anyone know of any overhaul shops that will take a -32 core for a -12 carb? Thanks in advance for your suggestions.

[Steve Pierce]

Just merged your thread with another on the subject. Should be lots of information.