Technical Article

Attacking Vapor Lock from a Different Angle – A Mini Study of Methods in Radiation Heat Reduction

By Michael Anderson

A condensed version of this study was published in the September/October 2024 issue of The Restorer, the magazine of the Model A Ford Club of America.

Abstract

A series of tests was conducted to collect temperature measurements of the fuel system and engine compartment components, with the aim of assessing the impacts of CERamic-METallic (CERMET) coatings on the exhaust and intake manifolds, along with the utilization of a heatshield on the upper exhaust pipe. These tests involved examining the treated manifolds and heatshield both individually and in combination.

Data collection was facilitated with a series of nine temperature probes positioned on the fuel line, carburetor, and various other locations within the engine compartment. Each test involved recording temperature measurements at a two-second interval over a 40-minute period. Following this, the data was analyzed and compared for each test.

Relative to ambient temperature, employing the combined heat reduction methods resulted in a substantial reduction in temperatures: a 24% decrease in carburetor bowl, a 17% decrease in the fuel line, and an overall 12% decrease in the engine compartment temperatures was observed.

Introduction

Some of us are cursed by a phenomenon called vapor lock, while others swear it must be a figment of one's imagination. This wicked phenomenon only occurs at the worst possible time. Climbing a narrow mountain road with traffic backed up behind you and a blind curve just ahead, suddenly experiencing a loss of power followed by an engine stall; sitting in traffic at red lights, when the light turns green, you push on the accelerator, and there is hardly any power, followed by a sputtering engine stall in the middle of the intersection; or, after topping off the gas tank during a long drive, getting back on the road and about half a block down, experiencing that loss of power. These are all uncomfortable situations that we don’t enjoy, and they are all symptoms of the phenomenon called vapor lock.

After talking to fellow club members and looking at various forums, I learned there are numerous ways to try and prevent vapor lock. One thing I noticed that was common across all these methods was that they were methods to mitigate the excessive heat. There was no mention on how to reduce the heat to avoid having vapor lock conditions. I decided to go and study this wicked phenomenon in more detail and gain understanding of how and why we have this heat and how we can reduce the heat to avoid vapor lock conditions.

Before I go into the study in detail, I’ll provide background information on terminology, concepts, and design aspects of the Ford Model ‘A’. I will go over the following concepts:

Vaporization, evaporation, and boiling
How vaporization takes place in the carburetor
Vapor lock vs. percolation
The boiling point of gasoline
The forms of heat transfer: conductive, radiant, and convection
Factoring in heat transfer in the design of the ‘A’

Background

Vaporization, Evaporation, and Boiling

Vaporization is a general term that applies to evaporation and boiling. Vaporization is any process that describes the conversion of liquid to a gas.

Evaporation takes place below the boiling point where particles at the surface of the liquid transitions to the vapor state.

Boiling is where a liquid transitions to the vapor state at the temperature where the vapor pressure is equal to the ambient pressure. Vaporization can occur anywhere within the body of the liquid, not just the surface.

How vaporization takes place in the carburetor

Vaporization of fuel is designed to take place in the carburetor just prior to being drawn into the engine’s cylinders where it is ignited by the spark plug.

Figure 2 shows the break down of the carburetor and the process of the fuel vaporization.

Air Intake: The carburetor draws in air from the atmosphere as the engine operates.
Fuel Supply: The float bowl
Venturi Effect: As air flows through a narrow section called the venturi, it speeds up. This creates a partial vacuum.
Atomization: The partial vacuum pulls fuel from the float bowl and is mixed with the fast-moving air. This process atomizes the liquid fuel, breaking it into tiny droplets.
Vaporization: The high-speed air flow in the venturi causes further mixing and vaporization of the fuel, turning it into a fine mist of fuel vapor.
Air-Fuel Mixture: The resulting air-fuel mixture is drawn into the engine’s cylinders, where it is ignited by a spark plug.

Figure 2: Vaporization in the carburetor

Vapor lock and percolation

Vapor lock occurs when the liquid fuel in the fuel line or carburetor bowl turns into vapor due to high temperatures. When this takes place, a disruption of normal flow of fuel to the engine takes place. Symptoms of vapor lock include sputtering, stalling, or rough idling.

Percolation takes place after the engine has been turned off. Vaporization of fuel in the carburetor bowl or fuel line occur due to residual heat. After starting of the engine, the carburetor may draw in vaporized fuel instead of liquid fuel. Symptoms of percolation include difficulties starting the engine, rough idling, or stalling.

Boiling point of gasoline

The boiling point of gasoline is a wild card. If one would do a search on the internet for the boiling point of gasoline, you would likely find the answer for the boiling point to be within the range of approximately -40°F to 401°F. There are many variables that determine the boiling point of gasoline. The following are some of the items that impact the gasoline boiling point:

Winter and summer blends: seasonal blends are formulated for proper volatility based on seasonal temperatures. Winter blends have a lower volatility while summer blends have a higher volatility.
Ethanol based blends: Ethanol based blends have lower boiling temperature compared to nonethanol-based blends. Higher the percentage of ethanol, lower the boiling point.
Geography based blends: Blends may vary based where the gasoline was refined. For example, California has its own unique gasoline blend specifications.
Atmospheric pressure: High atmospheric pressure, sea level, liquids need to reach higher temperature to boil. Low atmospheric pressure, mountainous regions, liquids can reach its boiling point at lower temperatures.

Driving your ‘A’ on a warm winter day up the mountain will have a higher chance of vapor locking than driving your ‘A’ on a cool summer day along the coast.

Forms of heat transfer

There are three forms of heat transfer, conductive, radiant, and convection.

Conductive heat is the transfer of heat through direct contact between two objects that are touching. An ‘A’ example of this would be where the exhaust manifold touches the intake manifold. The intake manifold becomes hot.

Radiant heat travels in waves and doesn’t need a direct connection. It can heat things even if there’s empty space between. An ‘A’ example of this would be the front passenger’s feet getting hot from the muffler under the floorboard.

Convection heat is all about the movement of heat through a fluid caused by the changes in temperature and density. An ‘A’ example would be the radiator fluid.

Heat transfer and the design of the Ford Model A

Conductive and radiant heat transfer are clear design considerations of the ‘A’. With the manufacturing and utilization of the ‘A’ in the north region of the country, the cold environment is factored into the design of the exhaust and intake manifolds along with the carburetor placement. Heat is a requirement for proper fuel vaporization which is fundamental for ideal engine performance. Heat is our friend and not our foe.

If you separate the exhaust manifold from the intake manifold, you’ll notice a flat area, approximately 3”x3”, on both pieces. This is to purposely transfer heat from the exhaust manifold to the intake manifold through conductive heat transfer.

With conductive heat transfer from the exhaust manifold to the intake manifold, radiant heat will take place inside the intake passage of the intake manifold. The radiant heat improves fuel vaporization, reduce cold start issues, and maximizes fuel combustion.

There is conductive heat transfer between the intake manifold and the carburetor. This will generate radiant heat within the upper part of the carburetor with vaporization is to take place. The radiant heat improves the fuel atomization and vaporization. It also reduces risk of carburetor icing, stabilizes idle and part-throttle operation, and enhances cold start performance.

Intake to carburetor heat transfer — Figure 5: Intake to carburetor conductive heat transfer

To further improve air-fuel mixture, the air intake to the carburetor is only 4” away from the exhaust manifold and exhaust pipe. With this close distance, the air is preheated prior to entering the carburetor. This improves better air-fuel mixture for vaporization and combustion and contributes to stable and consistent engine performance.

Figure 6: Radiant heat transfer to carburetor and intake manifold

Heat mitigation

Like anything else, too much of a good thing can be bad for you. Too much heat can lead to problems like vapor lock and percolation. This is where fuel vaporization in the fuel line or carburetor bowl can take place. Too much radiant heat can also impact nearby components, i.e., distributor and condenser.

There are plenty of articles and opinions on how to best mitigate the impacts of too much heat around the fuel line, carburetor, and other nearby components. These methods can be broken down into five different areas. I won’t go into detail here on each method since it can easily be found elsewhere.

Fuel treatment: this is done to increase the boiling point of the fuel. Ethanol free gas, Marvel Mystery Oil, diesel fuel, and mineral spirits have all known to be used to increase the boiling point.
Fuel line: high temperature insulating sleeve/wrap/tape, rubber fuel line around the metal fuel line or in place of, a larger fuel line, avoiding glass/metal inline fuel filters, clothes pins and tin foil are all different methods to either insulate or prevent fuel vaporization to take place within the fuel line.
Heat shields: there are several products available to shield heat from the exhaust manifold.
Carburetor: non-cast-iron carburetor which will retain less heat and phenolic insulator spacer to reduce heat transfer
Venting: opening the hood at fuel/rest stops to let out residual heat. Airflow control though the use of engine splash pans, or even removing hood side panel can be done. The 1930-31 A’s have improved hood venting compared to the 1928-29.

Selection of Radiant Heat Reduction Methods

CERamic-METallic (CERMET) coatings

Now that I understand how things work, I needed to determine what I could do to reduce the radiant heat around the fuel line and carburetor bowl to try and avoid vaporization in the fuel line and carburetor bowl. Standing around the water cooler at the office, I was discussing my predicament with a colleague that is into muscle cars. My colleague suggested that I investigate ceramic-metallic (CERMET) coatings for the exhaust and intake manifolds. CERMET coatings are often utilized in hot rods, muscle cars, and motorcycles. Exhaust headers and pipes can become very hot in these vehicles and essentially cook anything nearby. The benefit of CERMET coatings is their ability to reduce radiant heat transfer. This helps prevent surrounding components and materials from absorbing excessive heat.

Figure 7 is an example of two coats of CERMAT then polished for maximum reflection. This process provides the maximum heat reduction performance.

I had a lengthy discussion with a reputable shop owner, Michael Tomlin, that specializes in CERMAT coatings in the DFW area. I showed Michael pictures of the engine layout with the manifolds, upper exhaust pipe, fuel line, and carburetor. I also indicated that I want to maintain a stock look (don’t want that hot-rod look). For the exhaust manifold, Michael recommended to apply a CERMAT coat inside and outside, followed by a Cerakote cast iron gray coating on the outside. This would give the exhaust manifold a nice clean stock look as shown in Figure 8.

For the intake manifold, Michael recommended to apply a CERMAT coat inside and outside follow by a light blasting on the outside to allow for painting. This would allow me to paint the intake manifold Ford Green.

Figure 9: Treated intake manifold prior to painting

With Michael’s recommendation, on both manifolds, I would receive benefits, but not the maximum possible benefits. I thought this would be acceptable since I would be able to maintain a stock look.

One thing to note is that CERMAT coating is not the same thing as a ceramic based paint. The ceramic coated exhaust manifolds that you'll see from our part suppliers do not have the heat reduction properties.

Upper exhaust pipe heat shield

With a solution in mind for the exhaust and intake manifolds, there is still approximately 5” of upper exhaust pipe in the compartment that needs to be addressed. It was not feasible to try and do CERMAT coating on this. Best option is to put a heat shield around the upper part of the exhaust pipe as shown in Figure 10.

The Study

With possible methods of radiation heat reduction identified, the question of how effective these methods are is raised. How do I know the benefits of just having the CERMAT treated manifolds versus of just applying the heat shield on the exhaust pipe? How much heat reduction takes place in the critical areas such as the fuel line and carburetor bowl? Is this even worth the investment? I was set to go and find out.

I needed a repeatable approach to the testing. This included a reliable way to repeatably take temperatures on a regular basis without interruption of the test and reproducible test conditions. This includes same ambient temperature and most importantly engine running conditions.

Tests would be done with untreated manifolds and no heatshield. This would be the baseline for comparison. Testing would be done with the treated manifolds. Testing of the heatshield on the exhaust pipe with the untreated manifolds. Then lastly test the treated manifolds in conjunction with the heatshield.

Test Equipment and Methodology

Temperature collection

Test method utilized a series of temperature probes connected to a small computer called a Raspberry PI as shown in Figure 11. During each test, temperature readings are recorded every two seconds from each of the probe.

Raspberry Pi and probes — Figure 11: Raspberry Pi and temperature probes

As shown in Figure 12, temperature probes were placed in various locations which included two on the fuel line, three on the carburetor bowl, radiator outlet hose, upper engine compartment, and intake manifold. Hindsight, I should have had a probe on the condenser and air intake to the carburetor. Perhaps next time.

Probe placements — Figure 12: Temperature probe placements

Additional temperature readings are manually collected. This includes water and ambient temperatures every five minutes. Every ten minutes, the lower exhaust pipe temperature is recorded with an IR temperature gun, Figure 14. The reason for recording the lower exhaust pipe is to observe possible increase in temperature with the different heat reduction methods. The heat reduction methods do not make the heat magically disappear, the methods are preventing the heat to dissipate and go further downstream. At the end of the test duration, the hood is opened, and recordings of the exhaust manifold and upper exhaust pipe surface temperatures are recorded with an IR temperature gun, Figure 15.

Additional measuring devices — Figure 13: Additional temperature measuring devices

Lower exhaust measurement location — Figure 14: Lower exhaust pipe measurement location

Figure 15: Exhaust manifold and upper exhaust pipe measurement locations

Ambient temperature comparison

The term ambient temperature has been mentioned multiple times so far. The ambient temperature is the surrounding temperature where the test is being performed. In our situation, it is the outside temperature away from any heat source from the car. The ambient temperature plays an important factor when doing a study where you are doing a comparison of temperature differences. Measured temperatures are in relation to ambient temperature. This creates a temperature floor as well as neutralizes scaling, i.e., Fahrenheit vs. Celsius.

Percentage Difference = [((First Object Temperature - Ambient Temperature) - (Second Object Temperature - Ambient Temperature)) / (First Object Temperature - Ambient Temperature)] * 100

Engine operating conditions

Engine running conditions for each test needed to be the same. This ruled out driving since there would be too many variables that would be introduced. I ended up doing a stationary test at a medium idle with spark advanced to cruising position. It would have been ideal to have been able to put the car on a dyno machine and put under load conditions, but that wasn’t available.

Test steps

Check ambient temperature for desired test temperature
Start temperature readings, then start car (cold)
Medium idle for 40 minutes
Record ambient temperature and water temperature every 5 minutes
Record lower exhaust pipe temperature every 10 minutes with IR temperature gun
After 40 minutes – open hood and quickly measure exhaust manifold @ #1 and #4 and upper exhaust pipe with IR temperature gun – do 2 times and average results

Test Results

Ambient temperature comparison - exploratory

I performed multiple exploratory tests to get a feel of the numbers and to refine the test process. Here is a comparison of two different ambient temperatures, 100°F and 83°F. This is with the untreated manifolds and no heat shield.

First observation in this comparison test is that it does take longer for temperatures to stabilize with the lower ambient temperature. I think we would have all expected this. An interesting observation is that the temperatures eventually converge. If we think about this, the exhaust temperature generated by the engine is going to be constant and it is matter of time when the surrounding environment reaches a constant temperature as well. This observation led me to decide on using a 40-minute test interval.

Ambient comparison carburetor bowl — Figure 16: Ambient temperature comparison - Carburetor Bowl

Ambient comparison lower fuel line — Figure 17: Ambient temperature comparison - Lower Fuel Line

An artifact can be observed with the intake manifold temperature. With the venturi effect taking place in the carburetor causing the air to speed up, we can see a drop in temperature taking place.

Ambient comparison intake manifold — Figure 18: Ambient temperature comparison - Intake Manifold

Result comparisons

When comparing test results, the 30 samples from the 40th minute of each test are utilized. During this time, temperatures are stable with minimal variance. We will view the results for four key areas that are impacted by radiant heat, lower fuel line, carburetor bowl, upper engine compartment, and estimated water temperature.

The estimated water temperature was determined by the 5-minute manual recordings of the water temperature gauge in the vehicle and the temperature of the upper radiator hose collected electronically. Once temperatures stabilized, the two temperature readings demonstrated a strong correlation, thus the electronic data collected could be calibrated and utilized as an estimated water temperature.

Surface temperature results for the manifolds and exhaust pipe are included as well.

Lower fuel line temperature

Reduction in radiant heat from the treated manifolds have a bigger impact than the heatshield to the lower fuel line. The two methods together provide a cumulative effect of 16.6% reduction in heat.

Carburetor bowl temperature

Both the heatshield and the treated manifolds have substantial impacts to the carburetor bowl temperature. The heatshield by itself provides a slightly greater benefit than the treated manifolds. The two methods combined extend the benefits further with a 23.8% reduction in heat.

Upper engine compartment temperature

The heatshield and treated manifolds have comparable impacts to the upper engine compartment. The two methods together provide a cumulative effect provide a 12.1% reduction in heat.

Estimated water temperatures

Due to proximity, reduction in radiant heat from the manifolds have a bigger impact than the heatshield to the water/engine temperature. The two methods together provide a cumulative effect of 8.1% reduction in heat.

Manifold surface temperatures

There is no clear impact in the surface temperatures of the intake and exhaust manifolds. This was expected since the CERMAC coating focuses on radiant heat reduction.

Exhaust manifold surface temperatures — Figure 24: Exhaust Manifold Surface Temperatures

Exhaust pipe surface temperatures

Without the heatshield, the upper exhaust pipe showed a 6% increase in temperature with the treated manifolds. This is likely contributed to the passing of the heat downstream since there is less heat dissipation taking place at the exhaust manifold. Surface temperature of the heat shield is 50% lower than the upper exhaust pipe without the heat shield.

The lower exhaust pipe showed an increase of 15% with the treated manifolds. There is no clear impact to the lower exhaust pipe with the heatshield.

Upper exhaust pipe temperatures — Figure 25: Upper Exhaust Pipe Temperatures

Lower exhaust pipe temperatures — Figure 26: Lower Exhaust Pipe Temperatures

Conclusion

The combined usage of the heatshield on the upper exhaust pipe and CERMET coated manifolds has a cumulative effect on the reduction of radiant heat. Each method has its own target areas of impact. The heatshield provided higher impact to the lower fuel line while the CERMET coated manifolds had a slightly larger impact to the carburetor. The two methods combined provide the maximum impact.

With the combined methods, the carburetor bowl and lower fuel line had reduction of 24% and 17% respectively. Additional benefits were observed in the reduction of engine compartment temperature as well as the engine operating (water) temperature.

There is a 15% increase in the lower exhaust pipe temperature due to the heat being passed down from the exhaust manifold and shielded exhaust pipe. An increase is expected since heat is not able to dissipate from the manifold surfaces.

There are limitations to this study such as only medium idle conditions were utilized and limited test samples collected. I do not know yet the outcome in challenging touring conditions, time will tell. After a year’s worth of touring, I should have a good idea if my problems are gone.

If you are cursed with vapor locking and/or percolation and your vehicle is timed properly and no fuel flow issues, the heat shield on the upper exhaust pipe may be enough address the issue. This is a cheap and easy option. If you need to go further to address excessive radiant heat, CERMET coating on the manifolds is a viable option.

Originally published on the Dallas Model A Ford Club website and now included in the Workshop Notes technical library.