Novo chat
Ctrl
K
Plugins Tarefas agendadas
Kimi Work Kimi Code
  • Obter aplicativo
  • Sobre nós
  • Idioma
  • Ajuda e suporte

E85 Carburetor Temperature Impact

If you take an engine (like lawn mower) with a carbureter that is jetted for gasoline, and you put E85 in it without changing the mixture, will the cylinder head / exhaust run: * Hotter * The same * Cooler ? Why?
Search for scenario to verify the correct answer
The user is asking about the thermal effects of running E85 (85% ethanol, 15% gasoline) in a small engine carbureted for gasoline without re-jetting. This is a thermodynamics/combustion engineering question.
Key factors to consider:
  1. Stoichiometry: Gasoline stoichiometric AFR is ~14.7:1. Ethanol is ~9:1. E85 is roughly 9.8:1.
  2. If the carb is jetted for gasoline, it delivers the same volume of fuel for a given air volume. But E85 has different density and energy content.
  3. Without re-jetting, the engine will run lean (less fuel relative to air than needed for E85).
  4. Ethanol has higher latent heat of vaporization than gasoline, which has a cooling effect.
  5. Ethanol has lower energy content per unit mass/volume.
  6. Flame speed and combustion characteristics differ.
  7. Lean mixtures generally run hotter (excess oxygen, complete combustion, less diluent).
  8. But ethanol's higher vaporization cooling might offset some of this.
I need to be careful here. The question is specifically about a lawn mower engine with a carb jetted for gasoline, putting E85 in without changing mixture.
If you just swap fuels without changing jets:
  • The carburetor meters by volume (or more precisely, by venturi vacuum and orifice size).
  • Gasoline specific gravity ~0.72-0.78, E85 ~0.78-0.79.
  • The carb doesn't know what fuel is in it. It will flow roughly the same volume of liquid fuel for a given airflow.
  • Gasoline stoich: 14.7 lb air / lb fuel. E85 stoich: ~9.8 lb air / lb fuel.
  • Since E85 is denser, same volume means more mass of fuel? Wait, E85 is slightly denser than gasoline (ethanol is ~0.789, gasoline ~0.745, so E85 is ~0.78 vs gas ~0.74). So same volume = slightly more mass.
  • But the stoich ratio is much richer for E85 (needs less air per unit fuel, or more fuel per unit air).
  • If the carb was delivering the correct amount of gasoline (say 14.7:1), and now delivers the same volume of E85, what AFR do we get?
  • Let's say gasoline: 1 volume unit = 0.74 mass. Air needed = 14.7 * 0.74 = 10.878 mass units.
  • E85: 1 volume unit = 0.78 mass. Air available = 10.878. AFR = 10.878 / 0.78 = 13.95.
  • Stoich for E85 is 9.8. So 13.95 is very lean (leaner than stoich).
  • Actually, wait. Let me recalculate. If the carb is properly jetted for gasoline, it flows enough gasoline to make 14.7:1. If you put E85 in, the same jetting flows roughly the same volume. Since E85 is slightly denser, you get slightly more mass. But stoich for E85 is much richer, so you end up lean.
  • Lean mixtures burn hotter because there's excess oxygen, combustion is more complete, and there's less excess fuel to act as a diluent/coolant.
  • However, E85 has a much higher latent heat of vaporization (about 2.5x gasoline). In a carburetor, this provides significant evaporative cooling of the intake charge. This could cool the cylinder head.
  • But in a small engine carb, especially a simple lawn mower, the fuel vaporization happens in the carb and intake, cooling those parts. The head and exhaust temperatures are more a function of combustion temperature and the overall energy balance.
  • Also, ethanol has a higher octane rating (~105 for E85), so it's more resistant to knock. A lean mixture with high octane fuel might not knock, but will it run hotter?
  • The energy content of E85 is lower (~25-30% less per gallon than gasoline). So even if it burns, there's less total energy going into the engine.
  • But the question is about temperature, not power. Temperature is a function of the thermodynamic cycle and heat transfer.
Let me think about this more carefully. There are competing effects:
  1. Lean mixture -> hotter combustion (generally true for slightly lean, though extremely lean can get cooler due to incomplete combustion or slower flame speed).
  2. Higher vaporization cooling -> cooler intake, potentially cooler combustion.
  3. Lower energy content -> less heat release per cycle.
  4. Different flame speed -> may affect combustion efficiency and timing.
  5. Ethanol carries oxygen (35% oxygen by mass), so it needs less air for complete combustion. If you run the same air/fuel ratio as gasoline, you're definitely lean.
For a lawn mower engine without any changes:
  • It will run lean. Very lean. Possibly too lean to run well or even start.
  • If it does run, the lean mixture will cause higher combustion temperatures.
  • But the higher vaporization cooling of ethanol is significant.
  • The exhaust gas temperature (EGT) and cylinder head temperature (CHT) are what matter.
I should search for authoritative sources on this, or I can reason through it. But I want to make sure I give the right answer. Let me search for this specific scenario.
Pesquisar na web
E85 in gasoline engine without rejetting cylinder head temperature hotter cooler lawn mower carburetor E85 lean mixture exhaust temperature effects
14 resultados
E85 Lean Mixture Raises Exhaust Temperature Despite Ethanol Cooling
Now I have good information. Let me analyze the specific scenario:
The user asks: Take an engine (like lawn mower) with a carburetor jetted for gasoline, put E85 in it without changing the mixture. Will the cylinder head / exhaust run hotter, the same, or cooler? Why?
Key facts from search:
  1. E85 stoichiometric AFR is ~9.8:1 to 10:1, vs gasoline 14.7:1. (web_search:1#1, web_search:1#3)
  2. If you run E85 in a carb jetted for gasoline without richening, the mixture will be very lean. (web_search:1#3: "If you attempted to operate a carbureted engine on E85 without richening the mixture, the air/fuel ratio would be very lean and the engine would misfire along with having very poor driveability.")
  3. E85 has ~25-30% less heat per pound of fuel, and lower BTU content (~82,000 vs 115,000 per gallon). (web_search:1#0, web_search:1#3)
  4. Ethanol has much higher latent heat of vaporization: gasoline ~150 BTU/lb, ethanol ~360 BTU/lb. (web_search:1#6) This provides significant charge cooling.
  5. Lean mixtures generally result in higher combustion temperatures and higher exhaust gas temperatures (though there are nuances). (web_search:1#2, web_search:1#9)
  6. However, in aircraft engines, max EGT is at stoichiometric, and both leaner and richer drop EGT. But that's for gasoline. For E85, the situation is different because the engine is running very lean relative to E85 stoich.
  7. The higher vaporization cooling of ethanol is significant - "ethanol sucks up 2140 BTU for gaseous state conversion" vs gasoline 700 BTU. (web_search:1#1)
Now, for the specific case of a lawn mower engine:
  • Carb jetted for gasoline means it's delivering fuel based on gasoline stoich (14.7:1).
  • With E85, same jetting means you get roughly the same volume flow. E85 is slightly denser, but the stoich ratio is much richer (needs ~50% more fuel by mass/volume).
  • So the engine will run very lean on E85 - perhaps 40-50% lean.
  • A very lean mixture on E85 with gasoline carb jetting:
    • Less total fuel mass being burned per cycle (because same volume but E85 has less energy, and the carb flows based on air demand)
    • Actually wait - the carb flows fuel based on air velocity through the venturi. For a given airflow, it flows a certain volume of fuel. If the fuel is denser, slightly more mass. But since E85 needs ~40-50% more volume to achieve stoich, and the carb is flowing the same volume as it did for gasoline, the mixture is very lean.
    • Very lean mixtures can cause:
      • Higher combustion temperatures (lean = hot is the common rule, though at extreme lean it can get cooler due to incomplete combustion)
      • Slower combustion
      • Possible misfire/hesitation (web_search:1#3 mentions misfire and poor driveability)
      • Detonation/pre-ignition risk? But E85 has 105 octane, so knock resistance is very high.
    • However, ethanol's high latent heat of vaporization cools the intake charge significantly. In a carbureted engine, this happens in the carb and intake manifold.
    • But if the mixture is very lean, there's less fuel to vaporize, so less total cooling effect.
Wait, I need to think about this more carefully. The question is specifically about cylinder head and exhaust temperature.
If the engine runs very lean:
  • Peak combustion temperature could be high (if it burns).
  • But there's less fuel, so less total energy released.
  • The exhaust gas temperature depends on how much energy is carried out in the exhaust vs transferred to the head/cooling system.
There's a nuanced answer here. Let me look at what happens when you run E85 lean.
From web_search:1#3 (MotorTrend/Chevy High Performance): "If you attempted to operate a carbureted engine on E85 without richening the mixture, the air/fuel ratio would be very lean and the engine would misfire along with having very poor driveability."
Misfiring means incomplete combustion, which means some fuel passes through unburned. This would actually lower exhaust temperatures because the energy isn't being released in the cylinder.
But if it does run (maybe not too lean to misfire completely, just very lean):
  • The mixture is leaner than stoich for E85.
  • For gasoline, slightly lean of stoich gives max combustion temp. Very lean gives lower temps due to incomplete combustion.
  • For E85, the same principle applies but shifted to the E85 stoich point.
  • Since the carb is delivering ~40-50% less fuel than E85 needs, the equivalence ratio would be around 0.6-0.7 (very lean).
  • At that lean, combustion may be incomplete or unstable.
However, the higher octane of E85 might allow it to burn even when lean, whereas gasoline might knock. But very lean mixtures burn slower.
Actually, I think the answer is nuanced but the most likely outcome for a lawn mower is:
  • It will run LEAN.
  • Lean mixtures generally run HOTTER in the cylinder head and exhaust, up to a point.
  • BUT E85 has much higher vaporization cooling.
  • AND the total energy content is lower.
  • AND if it's too lean, it may misfire/run poorly.
For a practical answer to this specific question, I think the dominant effect is: The engine will run LEAN, and lean mixtures run HOTTER. The high latent heat of vaporization helps cool the intake, but the combustion itself will be hotter due to the lean mixture. The exhaust will likely run hotter because:
  1. Lean combustion releases less heat, but the heat is concentrated in less mass (less fuel, same air).
  2. Slower combustion means more heat is released later in the cycle and into the exhaust.
  3. The high vaporization cooling affects the intake, not so much the exhaust.
Wait, I need to reconsider. The user is asking a specific question that seems to have a definitive answer expected. Let me think about what the most accurate engineering answer is.
Key thermodynamic facts:
  • Stoichiometric AFR for E85 is ~9.8:1.
  • Gasoline carb delivers ~14.7:1 (by mass) for gasoline.
  • With E85 in a gas carb, the actual AFR will be ~14.7:1 (air/fuel ratio by mass), because the carb meters fuel volume based on air volume, and E85 density is similar to gasoline.
  • 14.7:1 for E85 is extremely lean (stoich is 9.8:1). That's 50% lean.
  • At 50% lean, combustion will be very slow, possibly incomplete.
  • However, the total fuel mass is less than stoich, so the total chemical energy is lower.
  • The exhaust gas temperature: for a given fuel energy, if combustion is incomplete, EGT is lower. If combustion is complete but lean, EGT can be higher because less diluent (fuel) to absorb heat.
But wait - there's another factor. The user says "without changing the mixture." In carburetor terms, "changing the mixture" means changing the jetting. So yes, same jets.
I think the answer is: It will run LEAN, and therefore the exhaust will run HOTTER (initially/slightly lean), but if it's too lean, it may actually run COOLER due to incomplete combustion/misfire. However, for the typical range where the engine still runs, lean = hot.
But I need to be careful. The search results show conflicting info about lean vs temperature. The aircraft engine data shows max EGT at stoich, with EGT dropping on both rich and lean sides. But CHT (cylinder head temp) peaks slightly lean of stoich and then drops.
For E85 specifically:
  • If properly jetted, E85 runs COOLER than gasoline due to the high latent heat of vaporization and the fact that you can run richer mixtures for cooling. (web_search:1#6: "Why does E85 fuel run cooler and make more power?")
  • But that's when properly jetted and tuned. The user is asking about NOT changing the mixture (jets).
So the answer should be:
  • Without re-jetting, the engine runs VERY LEAN on E85.
  • A lean mixture generally produces higher combustion and exhaust temperatures (up to the point of misfire).
  • Therefore, the cylinder head and exhaust will likely run HOTTER.
  • However, if the mixture is so lean that the engine misfires or runs poorly, the temperatures could drop due to incomplete combustion.
  • The high latent heat of vaporization of ethanol provides some intake cooling, but this is outweighed by the lean combustion condition when the carb is not re-jetted.
Actually, let me reconsider the mass balance again. Gasoline stoich: 14.7 lb air / lb fuel. E85 stoich: ~9.8 lb air / lb fuel. Carb jetted for gasoline: for a given air mass, it delivers enough fuel for 14.7:1 with gasoline. If we put E85 in, the carb still delivers the same fuel volume (roughly) for that air mass. E85 density: ~0.78 kg/L. Gasoline: ~0.74 kg/L. So E85 is ~5% denser. So for the same air mass, we get ~5% more fuel mass with E85. But E85 needs ~50% more fuel mass (14.7/9.8 = 1.5). So the AFR with E85 in a gas carb = 14.7 / 1.05 ≈ 14.0:1. Wait, that's not right. Let me recalculate.
If the carb is properly jetted for gasoline, it flows X volume of gasoline per Y volume of air to achieve 14.7:1 by mass. Gasoline mass = X * 0.74. Air mass = Y * 1.225 (at STP). 14.7 = (Y * 1.225) / (X * 0.74) => Y/X = 14.7 * 0.74 / 1.225 = 8.88. So air volume per fuel volume ratio is ~8.88.
Now with E85: Same carb, same Y/X ratio (8.88). E85 mass = X * 0.78. Air mass = Y * 1.225. AFR = (Y * 1.225) / (X * 0.78) = 8.88 * 1.225 / 0.78 = 13.95.
Stoich for E85 is 9.8. So 13.95 is quite lean. The equivalence ratio φ = 9.8/13.95 = 0.70. That's 30% lean (or 70% of stoich? No, φ = actual AFR / stoich AFR? No. Equivalence ratio φ = (F/A)actual / (F/A)stoich = (AFRstoich / AFRactual). φ = 9.8 / 13.95 = 0.70. So it's running at 0.70 lambda (30% excess air, or rather 30% less fuel than needed). Actually, lambda = AFRactual / AFRstoich = 13.95 / 9.8 = 1.42. So 42% excess air.
Is 1.42 lambda runnable? For gasoline, 1.42 is very lean and likely to misfire. For E85, with its higher octane and oxygen content, it might still burn, but poorly.
At lambda 1.42, the mixture is very lean. Combustion will be slow and possibly incomplete. The exhaust temperature might actually be lower because:
  1. Less fuel burned = less total heat release.
  2. Incomplete combustion = unburned fuel carries energy out the exhaust.
  3. The high latent heat of vaporization cools the charge.
BUT, the common wisdom in the search results is that lean mixtures run hotter. However, that's usually for slightly lean (lambda 1.05-1.2), not extremely lean (1.4+).
For a lawn mower engine specifically:
  • It will likely run very poorly, if at all.
  • If it does run, it will likely run HOTTER in the exhaust because any combustion that does occur will be hot and slow, transferring more heat to the exhaust port.
  • But the cylinder head might not get as hot because there's less total heat being generated, and the high vaporization cooling in the intake.
Hmm, this is tricky. Let me look for more specific info on this exact scenario.
Actually, I recall from engine theory:
  • For a given fuel, max combustion temperature is slightly lean of stoich (lambda ~1.05-1.1).
  • As you go leaner, combustion temperature drops because there's too much dilution and incomplete combustion.
  • Exhaust gas temperature peaks slightly rich of stoich for gasoline (around lambda 0.9), then drops as you go leaner.
  • Wait, no - the search result from the aircraft forum shows max EGT at stoich, dropping on both sides.
But for E85 at lambda 1.42, it's way lean. The engine will likely:
  • Have hard starting (E85 needs more heat to vaporize)
  • Run rough/misfire
  • Have low power
  • Cylinder head might run cooler due to less heat release and high vaporization cooling
  • Exhaust might run cooler or hotter depending on whether afterburning happens
However, the user is asking a question that implies there's a clear answer. Let me think about what the most accurate answer is.
I think the most accurate answer is: The engine will run LEAN, and as a result, the cylinder head and exhaust will run HOTTER - but with important caveats. The lean mixture causes higher combustion temperatures (if it combusts), and the slower flame speed means more heat is released during the expansion and exhaust strokes, raising exhaust temperatures. The high latent heat of vaporization of ethanol cools the intake, but this doesn't compensate for the lean combustion condition in the cylinder.
Wait, but I need to verify this. Let me search more specifically for "E85 lean exhaust temperature" or similar.
Pesquisar na web
E85 lean mixture exhaust gas temperature hotter E85 without rejetting run hotter or cooler small engine
10 resultados
Using E85 in a gasoline carburetor causes cooler exhaust temperatures
Now I have more information. Let me synthesize:
From the search results, the most relevant points for this specific question:
  1. Without re-jetting, E85 runs very lean: "If you attempted to operate a carbureted engine on E85 without richening the mixture, the air/fuel ratio would be very lean and the engine would misfire along with having very poor driveability." (web_search:1#3)
  2. E85 stoichiometry: Gasoline stoich is 14.7:1, E85 is ~9.8:1 to 10:1. (web_search:1#1, web_search:1#3) This means E85 needs ~50% more fuel mass for the same air.
  3. Lean mixture temperature effects: There's conflicting information. The common lore is "lean = hot." However, the Engineering Explained video title says "Why Lean Engines Do NOT Run Hot - Myth Busted." And the aircraft data shows max EGT at stoich, dropping on both sides.
But the user's scenario is specific: E85 in a gas-jetted carb. This creates a VERY lean condition (lambda ~1.4+).
From web_search:2#0 (Quora): "Small amounts lean of stoichiometric often raise EGT; very lean conditions eventually lower EGT when combustion becomes too weak or incomplete and a lot of fuel/energy is unburned."
This is key. The user's scenario is "very lean conditions" - lambda ~1.4. At this point, combustion becomes weak/incomplete, and EGT drops.
Also from web_search:2#0: "Peak combustion temperature is at slightly leaner than 1, around 10% excess air." - This is for normal combustion. At 40% excess air (lambda 1.4), we're way past peak.
From web_search:2#2 (turbobricks): "Well, maybe temps drop leaner than stoich because the mixture isn't combustible. If it isn't burning, it isn't transferring heat to the cylinder."
From web_search:1#6 (Grassroots Motorsports): "E85 reduces inlet air temperatures which improves power by cooling the air entering the cylinder." - This is the latent heat of vaporization effect.
From web_search:1#1 (Third Gen): "Ethanol sucks up 2140 BTU for gaseous state conversion" vs gasoline 700 BTU. "Engine temperature would be critical but just opposite of what was accustomed." - This suggests ethanol cooling is significant.
From web_search:2#3 (HP Tuners): "It will, at proper mixtures lower EGT's by around 200 deg F" - but that's at PROPER mixtures, not lean.
So for the specific scenario:
  • Carb jetted for gasoline (14.7:1) with E85 (needs 9.8:1) = very lean, ~lambda 1.4+
  • At this lean condition, the engine will likely misfire/run poorly (web_search:1#3)
  • If it does burn, it's very weak combustion
  • The high latent heat of vaporization of ethanol provides significant cooling to the intake charge
  • Less total fuel being burned = less total heat release
  • Very lean = incomplete combustion = lower temperatures
BUT, there's a competing effect: if the engine does manage to burn some of it, the slow combustion can shift heat to the exhaust.
However, I think the dominant answer is: It will run COOLER, or at least not hotter, because:
  1. The mixture is so lean that combustion is weak/incomplete (misfire, poor driveability)
  2. Less total fuel mass = less total heat release
  3. Ethanol's very high latent heat of vaporization cools the intake significantly
  4. The engine will make less power, generating less heat
Wait, but I need to be careful. The user might be expecting "hotter" because of the "lean = hot" myth. But the evidence suggests that at the extreme lean condition created by this mismatch, it would actually run cooler or at least the head/exhaust wouldn't get hotter.
Let me reconsider. The question says "without changing the mixture." In a lawn mower, the mixture is often adjustable via a needle screw, but "jetted" implies the main jet. The user means without re-jetting.
If the engine does run (even poorly), what happens to head and exhaust temps?
  • Cylinder head temp: May be cooler because less heat is being transferred to the head. The high vaporization cooling and lower energy content dominate. Also, if it's misfiring, there's no combustion heat.
  • Exhaust temp: Could be cooler due to incomplete combustion, OR could have hot spots due to afterburning of unburned fuel in the exhaust.
But the most authoritative answer I can give is: The engine will run very lean - approximately 40-50% leaner than E85's stoichiometric ratio. At this extreme lean condition, combustion becomes weak and incomplete. The cylinder head and exhaust will likely run COOLER (or at least not hotter) because:
  1. Less total heat release: The carb delivers far less E85 mass than needed, and E85 has 25-30% less energy per pound anyway.
  2. Incomplete combustion/misfire: The very lean mixture leads to poor driveability and misfire, meaning much of the fuel's energy passes through unburned.
  3. High vaporization cooling: Ethanol has ~3x the latent heat of vaporization of gasoline, providing significant charge cooling even with the reduced fuel quantity.
However, I should mention that if the engine manages to run slightly lean but not extremely so (e.g., if the jets are already slightly rich for gasoline), it could run hotter. But the typical scenario is very lean.
Actually, let me reconsider once more. The user is asking a question that seems to test knowledge of engine theory. The "common lore" answer is "lean = hot." But the user is asking about a very specific scenario where the lean condition is extreme.
I think the most accurate and complete answer is: It will run LEAN, and in this specific extreme case, the head/exhaust will likely run COOLER or at least not reliably hotter, because the mixture is so lean that combustion is compromised. But I need to explain the nuance.
Wait, I found a key piece in web_search:2#0: "The maximum temperature for a combustion is for a stoichiometric mixture." And "You either moderate the combustion temperature with excess fuel or excess air."
At lambda 1.4 (40% excess air), we're using excess air to moderate temperature. The Quora answer explicitly says: "It’s just that air has less thermal capacity than liquid gasoline, so you need 20% excess air (~18:1 in AFR terms) to get the same burn temperature as stoichiometric (14.7:1 AFR)."
For E85 at 14.7:1 actual AFR, with stoich at 9.8:1, that's equivalent to having 50% excess air. That should definitely lower combustion temperature.
So the answer is: COOLER (or at least not hotter, and likely cooler due to incomplete combustion and high vaporization cooling).
But I want to double-check. Is there any chance it runs hotter?
  • If the engine has an extremely rich gasoline jet (e.g., designed to run rich for cooling), then putting E85 in might make it only moderately lean instead of very lean. But standard lawn mower carbs are usually jetted close to stoich.
  • The "lean = hot" rule applies to slightly lean conditions (lambda 1.05-1.2), not extremely lean.
I think the answer is COOLER, with the caveat that it will run poorly.
Let me formulate my response:
  1. State clearly: The cylinder head and exhaust will run COOLER (or at least not hotter; they won't run reliably hotter).
  2. Explain why: The carb, jetted for gasoline stoichiometry (14.7:1), will deliver only about two-thirds the fuel mass that E85 needs for its stoichiometry (~9.8:1). This creates an extremely lean condition (lambda ~1.4+).
  3. At this extreme lean condition, combustion becomes weak, slow, and incomplete. The engine will likely misfire and have poor driveability.
  4. Thermodynamically, peak combustion temperature occurs near stoichiometric. Moving significantly lean (40-50% excess air) lowers combustion temperature because the heat is diluted by excess air and combustion becomes incomplete.
  5. Additionally, ethanol has ~3x the latent heat of vaporization of gasoline, providing significant evaporative cooling to the intake charge.
  6. E85 also contains ~25-30% less energy per unit mass than gasoline, so even if burned completely, less heat is generated.
  7. The combination of extreme leanness, incomplete combustion, high vaporization cooling, and lower energy content means less heat is transferred to the cylinder head and exhaust.
I should also mention that if the engine somehow runs at only moderately lean (e.g., if the gasoline jets were already oversized), it could run hotter, but that's not the typical case.
Actually, I want to reconsider one more time. The user might have heard that E85 "runs cooler" when properly tuned, and wants to know if the same applies when just dumped in without changes. Or they might be testing the "lean = hot" concept.
Let me look at the exact wording: "If you take an engine (like lawn mower) with a carbureter that is jetted for gasoline, and you put E85 in it without changing the mixture, will the cylinder head / exhaust run: Hotter, The same, Cooler? Why?"
This is a specific thermodynamics question. I think the answer is COOLER, and here's my final reasoning:
The engine will be running at approximately 14.7:1 AFR with E85, while E85 wants ~9.8:1. That's ~50% too little fuel. At this extreme leanness:
  • Combustion is incomplete/weak (misfire)
  • Peak temps drop (stoich is hottest, extreme lean is cooler)
  • Less fuel mass = less total heat
  • Ethanol's high vaporization cooling
  • The engine will make less power and generate less waste heat
So: COOLER.
But I should mention that the exhaust might have hot spots from afterburning of unburned fuel, and the engine will run very poorly. But overall head/exhaust temps will be lower.
Let me write a clear, well-cited answer.
Cooler — and it will also run very poorly, if it runs at all.
Here is why:

1. The carburetor will deliver a very lean mixture

A carb jetted for gasoline is sized to deliver a stoichiometric ratio of about 14.7 : 1 (air-to-fuel by mass). E85’s stoichiometric ratio is roughly 9.8 : 1 — it needs about 40–50 % more fuel for the same amount of air.
If you simply swap fuels without re-jetting, the engine sees an extreme lean condition (lambda ≈ 1.4+). At that level the mixture is too weak to sustain complete combustion; the engine will misfire, hesitate, and have very poor driveability.

2. Extreme lean is colder, not hotter

The “lean = hot” rule only holds for slightly lean mixtures near stoichiometric. Peak combustion temperature actually occurs right around stoichiometric; as you move significantly leaner, the flame becomes weak and incomplete, and temperatures fall because there is simply not enough fuel to generate heat, and the excess air dilutes what little combustion does occur.

3. Ethanol’s evaporative cooling is much stronger than gasoline’s

Even with the reduced fuel flow, the ethanol in E85 has about three times the latent heat of vaporization of gasoline. As it evaporates in the carb and intake, it pulls a great deal of heat out of the incoming charge.

4. Less total energy is being released

E85 contains roughly 25–30 % less heat per pound (and per gallon) than gasoline. Because the carb is already delivering far less fuel mass than E85 needs, the total chemical energy entering the cylinder is substantially lower.

Bottom line

The combination of extreme leanness, incomplete combustion, ethanol’s high vaporization cooling, and lower overall energy content means the cylinder head and exhaust will run cooler — but the engine will also be down on power, rough-running, and hard to start. To run E85 properly you would need to richen the mixture by about 30–40 % (larger jets) so the engine can reach E85’s stoichiometric point and take advantage of its octane and cooling benefits.