Carburetor air intake manifold

[Jim Meeker]

At the recent Columbia Taylorcraft Rendezvous an issue came up in the general discussion of improving performance for the A65. It was suggested that the carburetor air intake manifold interior is rough cast and if you grind it down so that it is smooth and increase the diameter it would improve performance and reduce the likelihood of carburetor icing. It was also suggested that these improvements have been made for this part of the engine for the C85. Does any one out there have any thoughts on the matter? Is it possible to substitute the C85 inake manifold part 35145-A1 for the A65 4780-A1. Your collective wisdom would be appreciated.
Cheers
Jim

[flyguy]

Re: Carburetor air intake manifold

A intake passage that is too big for the operating rpm of the engine will reduce the intake velocity and kill the mid-range torque. I know what that means for a small block chevy, but I have no idea what happens on an engine with a prop on it. I assume the condensed gas drops will just fly out the exhaust and the power will be down unless you run the engine at higher rpms.
Smooth on the inside is good, especially if the passage curves.
Darryl

[lktiller]

Re: Carburetor air intake manifold

More air = more fuel = more power= more rpm.
Cleaning up the ports won't change the fuel/air mix or increase rpm. The prop is the limiting factor. Want more power? turn the engine faster by cutting or untwisting the prop. Brings on other problems,tho.....
On slow turning engines the fuel doesn't accelerate fast enough to benefit from reduced manifold drag.

Can't compare with a hi-reving chevy small-block. Chevy didn't put a fixed pitched prop on the output shaft to limit the rpm !!!!!

[flyguy]

Re: Carburetor air intake manifold

Generally on air-cooled engines the restriction is in the ports due to casting/cooling fin requirements, but it was implied that the 85 uses a different intake spider in the thread starter. Is that so? Is it bigger? Engineers usually change things for a reason.
D.

[lktiller]

Re: Carburetor air intake manifold

someone with experience on the 85 hp will have to chime in here.
I do know the ORIGINAL 65hp intake spider has a slightly smaller throat than the 0-200 one.
On my '41 bc12-65, someone had installed a spider from an 0-200. Worked fine up until I took the engine off in '97. That means they will interchange mechanically. What advantage there might be in the larger one installed on the 65 hp I don't know.
My current t-craft, '46 bc12-d, 65 hp has been built up to a 75 hp. It has the larger opening manifold but whether it's from an 85 or an 0-200 I don't know.

[flyguy]

Re: Carburetor air intake manifold

Just a second thought: .."chevy didn't put a prop...ect."
How about a heavy suv going uphill 50 mph, full throttle, manual transmission in overdrive?
My point was that a (larger) intake spider would not do any good unless the 65 intake was actually restricting the airflow at whatever RPM you are using. Sometimes I put things too simply, leading to it being assumed that I am either stupid of ignorant.
On the other hand, you can wind an engine to as high as you want, but if the intake system is restricted and your pumping losses go out of sight, the torque drops off and the horse power will head for the bottom of the chart.
Darryl

[lktiller]

Re: Carburetor air intake manifold

don't think for a moment that i'm assuming ignorance on anyone's part!!!! I wouldn't hurt anyone's feelings if at all possible. Problem is all mine. Just can't convey emotions via email. And can't convey knowledge in a few short lines.
Note on the lugging SUV --- difference is it won't stay in that mode as long as an airplane engine. either you reach the top or you shift gears. Also, difference in rpm at peak HP.
With the aircraft engine you either accept the climb rate (full power) or you change props.
I guess my whole point is: modifying the aircraft intake manifold will gain little or nothing in output.
Back to the original thought: some way to get a bit more pep out of the 65 hp engine.
best concept is to mod it to 75 hp factory specs, then untwist the prop to gain 100 rpm at full power. Gotta get the rpm up, otherwise it's still a 65 hp.
I just made a 20 hr journey in my t-craft this past week (75 hp engine) That extra 10 hp made a difference when the density altitude reached 8000 ft @ 110 deg F.

Now if you want to compare it with my '29 Ford A-model ?!!! Won't climb that hill very fast but I betcha it will outlug that SUV in high gear .......

[Jim Meeker]

Re: Carburetor air intake manifold

Darryl and Iktiller
Thanks for the response, it is helpful. Iktiller so I take it you have the larger spider on your A75 but not sure it is from an 85 or 0200? Also could you tell me what prop you have and what static and full power in the air RPM you get. I have a 74 45 metal prop and only get about 2K static on the ground and 2.2K full power.
Jim

[lktiller]

Re: Carburetor air intake manifold

continental A75-8 engine. modified according to mfg bulletin from an A65-8. Cont. specs says the engine develops 75 HP @ 2600 rpm.
prop is a McCauley 1B90, Yellow tag says it is a 7138; reduced dia.& pitch from 7442. Essentially a climb prop for a 65 hp engine.
static gnd is 2000 rpm. On climb out I keep rpm under 2350 and let the airspeed fall where it may, usually 70-75 mph.
Continental says 2250 rpm in cruise. I run 2300 continuously @ 90-95 mph
I've never let it run full throttle to red line (2650) but it easily reaches 2500 rpm in level flight.
more of my engine specs: chrome cyls, comp 78/80 all cyls, smoh 1200, 1qt oil in 10 hrs

thoughts:
put my prop on any Cont. 65 engine and it will perform as a 75 hp engine. However, mfg says the 65 was not engineered to run continuously at that power. Hence the bulletin (and reason) to upgrade to 75 hp parts.

more thoughts on pepping up the little engine:
to get more HP in any manner requires more rpm. Shoving more fuel at the cyl in the form of mods (such as porting) will not increase HP unless more air is included. The engine will pump only so much air at any RPM. limiting factor is the prop.

more thoughts to keep this discussion alive!!!!!
Now if we're talking increasing power at full throttle then mods will make a small difference. but again, to realize that gain the prop must be reduced in dia or pitch. Then you get into the red line region. even there, these engines will perform great; just more wear and tear: (and uncertainty as to when it will fail !!!!!!!!!!!!!!!!!!!)

Hope somebody chimes in to prove me wrong. everybody's dream is more HP

Larry Tillery

[flyguy]

Re: Carburetor air intake manifold

Larry,
Give me your input on this one, I haven't quite got it figured out yet.
If I take my 85 hp up to 8k ft and run full throttle I can get 110mph (gps 2 direction average), but though I haven't measured it accurately I suspect that I am burning fuel at the full throttle rate as measured at sea level. I have an old type carb with the mixture control wired to rich.
Now I'm thinking that the amount of fuel being sucked into the venturi might be somewhat less because the pressure differential (outside to cylinder) is less and therefore the carb air velocity is less. I don't mean correct-mixture less, but rather just, I-am-courious less.
Whatever that is, I have considered getting a replacement carb with a good mixute control on it so I can lean for peak stuff at 75% power at altitude.
Does anyone in the group have an 85 with a good working mixture control? Read "modern" carb. What kind of results do you get at altitude with full throttle? Anyone ever get down to 3 or 4 gals per hour at Wide open throttle at altitude?
Darryl

[taylorcraftbc65]

Re: Carburetor air intake manifold

O.K. guys, let me see if I can help. I have a BC65 with an A-65-8 that has had the 75 H.P. mods done to it PLUS it has the large spyder that has been polished, And it has the carb off the 90 H.P. Aeronca L-bird on it. It also has a square tip metal Mc Cauly on it. Several months ago, VERY "healthy" 250 pound friend of mine and I took off from El Paso, Tx. with full tanks (18 gal. total), and took off in 450-500 feet. This was at 4,000 msl, and 98 degrees. Climbout was at 50 miles an hour, and it outclimbed Any other Taylorcraft that anyone at that airport had ever seen.

[TimHicks]

Re: Carburetor air intake manifold

Dang......

[lktiller]

Re: Carburetor air intake manifold

taylorcraftbc65, what pitch & diameter prop do you have?

flyguy, I do have a functional mixture control on an M-S carb. I do use the mixture control because I think my carb is set too rich. Does save fuel.
One recent flight was 250 mi. @ 7500 ft.MSL. Avg 4 gal/hr.
Cross-country last week, 2000 mi. round trip. All at 1000 ft agl or less! Still avg 4 gal/hr fuel burn

[flyguy]

Re: Carburetor air intake manifold

Guess I'll have to spring for a new carb. one of these days. Let's see. $1000 carb. Save maybe 1 gal an hour at 65% altitude. Fuel here is 3.09/gal. Take me about 323 hours to get my money back in fuel savings. Not to mention the additional range, better climb rate at altitude, higher ceiling (have I forgotten anything?)
Such a deal. How can I resist?
Darryl

[lktiller]

Re: Carburetor air intake manifold

Maybe something good is to be said about rising gas prices.......If you'd bought the carb two years ago, payback would have been six hundred hours!!!