# How much power do you lose with carb heat?

### Help Support HomeBuiltAirplanes.com:

#### Tiger Tim

##### Well-Known Member
I know there is a power loss when carb heat is applied and have experienced it many times but is there a rule of thumb for roughly how much is lost?

#### Dana

##### Super Moderator
Staff member
How many rpm does it drop? For a fixed pitch prop power is proportional to the cube of the rpm, so take the cube of the percentage, for example if adding carb heat reduces rpm from 2500 to 2400, that's 2400/2500 = 96%, .96^3=.88, so 12% power loss.

#### Dan Thomas

##### Well-Known Member
Depends on how much the air is heated, and that varies widely. Lycomings typically need less carb heat since their carbs are mounted to the bottom of a hot oil sump, but other need more. The older Cessna 180s and 182 had too much of it and you'd have to lean the mixture some if you pulled it full on on downwind. The engine would stumble and barf otherwise.

Historically, the FARs required a 90°F rise for carb heat. I don't know how Lycoming met that with their tiny, open heat muffs. FAR Part 23 Sec. 23.1093 effective as of 03/11/1996

#### TFF

##### Well-Known Member
Two basic things happen. Density altitude goes up that the engine sees which causes the engine to go rich. Potential detonation can happen if you try and lean to regain some lost power. Summer time the engine might see 150 deg air where as cold winter might be less than regular summer.

#### Vigilant1

##### Well-Known Member
And we know how much power we could lose without carb heat.

#### Dan Thomas

##### Well-Known Member
Two basic things happen. Density altitude goes up that the engine sees which causes the engine to go rich. Potential detonation can happen if you try and lean to regain some lost power. Summer time the engine might see 150 deg air where as cold winter might be less than regular summer.
It typically takes higher manifold pressures along with the hot induction air to do that. I wouldn't take off with carb heat on. On downwind, leaning to get smoother operation with carb heat, should be no problem. Better than loading up the induction manifolding with puddling fuel that causes hesitation or failure in an overshoot. We had a Citabria that would do that.

#### Tiger Tim

##### Well-Known Member
I wouldn't take off with carb heat on.
I was surprised to learn that 65hp Luscombes have an AD insisting on using full carb heat for takeoff and landing.

#### Tiger Tim

##### Well-Known Member
How many rpm does it drop?
That’s a darn good question. The engine in question is a modified Gipsy Major and as designed they have carb heat linked to throttle which only comes on at or near idle power. Weird, I know, but it works. What may be even weirder is that it takes heat not from the exhaust but by drawing air in from alongside the crank case, which in practice has shown for nearly a century now to be sufficient.

So my basic plan is to have it draw warm air from beside the case all the time. Others have done this and it’s worked out fine, I’m just curious if there’s approximate guidance on how much power I’m leaving on the table. At the density altitudes I’ll see it doesn’t really matter much anyways.

#### Dana

##### Super Moderator
Staff member
...they have carb heat linked to throttle which only comes on at or near idle power. Weird, I know, but it works....my basic plan is to have it draw warm air from beside the case all the time.
If it works, why do you want to change it?

#### TFF

##### Well-Known Member
Your not in Florida which is making a difference. Hot Rodders go with 1% for every 2F deg. Most low performance engines have excess octane available with 100LL so detonation is at bay.
As for not taking off with carb heat just means you haven’t pretend to be a student in a long time.

#### Dan Thomas

##### Well-Known Member
That’s a darn good question. The engine in question is a modified Gipsy Major and as designed they have carb heat linked to throttle which only comes on at or near idle power. Weird, I know, but it works. What may be even weirder is that it takes heat not from the exhaust but by drawing air in from alongside the crank case, which in practice has shown for nearly a century now to be sufficient.

So my basic plan is to have it draw warm air from beside the case all the time. Others have done this and it’s worked out fine, I’m just curious if there’s approximate guidance on how much power I’m leaving on the table. At the density altitudes I’ll see it doesn’t really matter much anyways.
My Auster had that same throttle-linked carb heat, and I had carb ice once or twice. Maybe that old cowling was leaking a bit. The heat is taken from the side away from the cooling air inlet, and it should be plenty warm in there with minimal airflow.

No air filtration, either. Just the flame arrestor against the crankcase.

#### Tiger Tim

##### Well-Known Member
If it works, why do you want to change it?
Mods done to the engine prevent the stock system from being installed, the biggest being that in switching the whole engine from inverted to upright it needed a change from the stock downdraft to a different updraft carb. The stock valve/hot intake/flame arrestor assembly no longer fits and OTOH may be an integral casting with the stock carb body anyways. Perhaps something can be fabricobbled to function the same but I’d just as soon get it flying this summer and make that next winter’s problem. This project has gone on long enough and really needs to fly before the unintentional tontine of partners is down to a single owner... and it’s close.

#### Chilton

##### Well-Known Member
The GM carb air selector / flame arrestor is a seperate part from the carb, held on with 4 bolts.

If you have access to the Moth club magazines there were a series of tech pieces about 2001/20002 about converting to selectable carb heat on upright Gipsy 2 due to less effective deice on the upright engine. I think Mike Souch of Aero Antiques in the UK had a lot of involvement in this and in some conversions of GM to upright so maybe contact him through the DH 60 Facebook group, he is a good guy.

#### thjakits

##### Well-Known Member
"How much power do you lose with carb heat?"

As usual - "...it depends"!

Were you at you full power when carb heat was applied? Lean or up the rpm?
Unless you are at full power (and at full power you mostly do not need/use carb heat) you don't need to lose power with carb heat - re-adjust stuff (or have a govenor do it for you, ....whatever you got!)....

Do you have a carburetor temp gauge?

If you are on cruise power a little carb heat adn mixture adjustment might just get you better performance...
Cold air is no good unless you need to get that last bit of full/max power. Up to that point (...usually way beyond your 70-75% cruise) warmer air will help to vaporize your fuel better, which means better, more efficient burn.....
Obviously every engine is different and you will need some experimenting to find what's good for your's....

Someone mentioned "Florida temps" - I run across that argument quite often (...in Panama we usually have Florida temps and then some more humidity!). Watching the carb-temp gauges on the company Robinsons you will be quite surprised to find temps near 0°C with OATs around 27-30°C......
The high humidity makes it even more prone to ice up.... Mind you, that temp sensor is not at the really lowest pressure spot, so it easily could be colder there...
Robinson POH keeps it simple - "When in possible icing conditions keep the needle out of the yellow arc" - ...so usually take off with the carb heat all the way closed, but keep the look open - when you lower the collective, a "carb heat assist" linkage will apply carb heat when the power is low and the butterfly closing/closed. It also pulls it closed again when power is applied....

I understand when it goes below -5°C icing is pretty much not possible anymore, for the lack of moisture in the air.... - still I WOULD keep the needle out of the yellow....
[In reality - I would try to avoid to have to fly in sub-zero! Keep it tropical!!]

Cheers,
thjakits

#### TFF

##### Well-Known Member
You may be making Robinsons work, but for the uninitiated, having the auto carb heat linkage that will crash your chopper if you test it at idle is always the kind of fix you avoid.

#### Dan Thomas

##### Well-Known Member
I understand when it goes below -5°C icing is pretty much not possible anymore, for the lack of moisture in the air.... - still I WOULD keep the needle out of the yellow....
[In reality - I would try to avoid to have to fly in sub-zero! Keep it tropical!!]
In a Lycoming, maybe. Water can exist in liquid form down to -20°C and causes airframe icing. Carb ice is one of those things you have to to watch for all the time, and when checking the METAR before you fly, look at the temperature and dewpoint to see how likely it will be for your flight. This is something not taught nearly often enough or aggressively enough in flight schools, and the appalling accident rate due to carb icing proves it.

Carb ice charts like this vary somewhat and are sometimes taken from POHs/AFMs. Carb ice probability is airframe-specific, with some models not having the same risk as others. But the fact remains that it's a risk across a much wider ambient temperature range than most pilots think. For instance, this chart show the risk reaching 30°C, while others will show it to almost 40°C. 38°C is 100°F.

Last edited:

#### thjakits

##### Well-Known Member
You may be making Robinsons work, but for the uninitiated, having the auto carb heat linkage that will crash your chopper if you test it at idle is always the kind of fix you avoid.
...are you referring to the "collective up" if the carb-heat-assist linkage-friction is too tight?

thjakits

#### D Hillberg

##### Well-Known Member
Had a flight school that had issues with rough running engines at take off with zero clouds and no visible moisture in the sky... Ground runs and tests found nothing.

Chief Instructor took me up for a ride - rough on take off as usual, It got smooth as silk after I added a little carb heat...

Take off at this air port took everyone over a lake . . . . ."Visible Moisture"

With an old Hiller you add carb heat she'll suck the tanks dry in double time

#### Tiger Tim

##### Well-Known Member
If you have access to the Moth club magazines there were a series of tech pieces about 2001/20002 about converting to selectable carb heat on upright Gipsy 2 due to less effective deice on the upright engine. I think Mike Souch of Aero Antiques in the UK had a lot of involvement in this and in some conversions of GM to upright so maybe contact him through the DH 60 Facebook group, he is a good guy.
I’ll have to seek him out, thanks.

My basic reasoning on ‘always hot’ was to remove a variable in the proving of this engine installation. As far as I know the details of this conversion are currently one of a kind, though there ought to be enough parts in the world to do at least three of them. Unfortunately the two fellows who planned it all out have both passed in the last year and I’m left to pick up the pieces, which I can do, but it’s taking a lot of weighing out options. Safety comes first on this bird and I’m expecting a long process of sorting out the mechanicals. If it’s possible to run it always hot to get it flying then sort out a selectable air box later then I think that’s worthwhile... as long as it doesn’t lose an enormous amount of power in doing so.

#### TiPi

##### Well-Known Member
Log Member
If you go for permanent carb heat, make some ground-adjustable flap and install an intake air temp sensor. Then adjust that flap so the the temp increase is minimal at WOT (10-20deg C) and increases to maybe 30-50deg C at less than 1/2 throttle. Warm air at lower throttle settings helps with fuel evaporation and evens out the mixture distribution (might be useful on an in-line 4). It will be a bit of trial & error to get the heat transfer area right. I'm planning on using the hot air outlet of the oil cooler as the warm air intake (more stable and lower level heat than exhaust muffs).