Manifold Temperature & Proper Carburetor Adjustment

Ideally, engine temperature should be 180 to 190, but on a summer day 195 to 200 wouldn’t be a major cause for concern. Radiator temperature should be around that on inlet tank, and obviously lower on discharge from radiator. The temperature differential is dependent on ambient temperature, condition of radiator, and other factors.

An exhaust manifold can be anywhere from 400 to 900 degrees, depending on load, rpm, speed, ambient air, and so forth.

I’m no expert, but this is my experience.

David is understating his capabilities, for sure. He is dead on with his wide temperature range for the exhaust manifold readings. The range is so wide based on a number of factors; ambient temperature, engine condition, carb settings, speed of the car, octane of the fuel, wind speed, radiator condition, and a host of other factors. While it would be fantastic to be able to correlate exhaust temp with a carb adjustment, there are way too many factors external to the “experiment” that enter into the picture. It is safe to say an exhaust gas check would probably be a better indicator of proper carb adjustment. In later cars, we adjust by vacuum and RPM as much as temperature, as these are easier to determine and “hear or feel”. Robert Brown can write a book on how to tune a Pierce engine by now.

There was a thread a year or two ago where someone mentioned a cheap but decent infrared thermometer on Amazon. I do expert witness work as a professional electrical engineer, and bought a very expensive infrared thermometer that isn’t worth squat, then bought the one recommended by one of the club members (help me remember who suggested this, guys!), and have been using it (until my kids lost it playing here in the house) on cases and at our house to tune up the new geothermal heat pumps. I believe the unit was under 20 bucks, and works beautifully.

Hi John, I have a Craftsman meter from Sears I bought for under $100. It measures to 1400F-it works dandy and isn’t delicate.It uses a 9V battery

John

John,

For all that you will use it, go with the price.

For the $17.00 on Amazon you cannot go wrong, or at least not more than $17 wrong.

Peter

The exhaust manifold will not reach it’s maximum temperature except when under full load, like climbing a hill at full throttle, with all the seats full and a headwind. Under conditions like described, the exhaust manifold will be close to glowing red.

But, there is no way for you to put the engine and car into these conditions and measure the temperature of the manifold under these conditions unless you put the car on a chassis dynamometer.

I do not think I’d want the mixture adjusted by temperature on one of my cars. I have no Idea if for a car, it would be correct to adjust mixture for maximum temperature, or find maximum, then richen the mixture to lower the temp by say 50-100 degrees. [this is how piston powered airplanes are adjusted in flight]

I much prefer to adjust the idle mixture for rpm and smoothness. I rarely use a vacuum gauge. For the main jet size, I usually increase the jet size a step or two to compensate for ethanol in the fuel.

Greg Long

John, I suspect you have some airplane experience since measuring and controlling induction temp is commonly done, particularly for supercharged aircraft engines. I don’t think it would work too well based on the outside surface temp of the intake manifold of a Pierce, since there is so much thermal mass in the cast iron manifold (wouldn’t respond quick enough to changing AF ratio) and its temperature is going to be highly influenced by the heat conduction from the block and the heat from the exhaust manifold. I think you would need a probe in the air flow itself downstream of the carb, and even then it probably wouldn’t tell you too much.

A big problem for long engines such as straight 8’s is the uneven distribution of the mixture from cylinder to cylinder. The fuel is still liquid mist when atomized at the carburetor and being heavier than air the centrifugal and gravitational forces on the drops as they fly around corners and along the length of the manifold effects the AF ratio at each cylinder. The heating of the intake manifold by the exhaust manifold and block help to vaporize the fuel along the way, but a huge challenge for these engines was trying to minimize the variation in AF ratio cylinder to cylinder.

Measuring the exhaust chemistry downstream of the exhaust manifold would only give an average of all cylinders, and one cylinder is probably running leaner than the others. As noted above, running leaner than stoichiometric is the most fuel efficient, but increases probability of lean misfire, maximizes exhaust temp stressing the valves and also provides the excess oxygen that promotes valve seat recession. Because of the cylinder to cylinder fuel distribution problem these engines were set up to run richer than stoichiometric to avoid lean misfire for the leanest cylinder at basic highway cruise power. Maximum power occurs richer than stoichiometric, and typically these engines were set up to run significantly richer than max power at wide open throttle to reduce knock and provide internal cooling. It also produces a lot of carbon from incomplete combustion that fouls the engine and was part of the reason for rapid wear leading to replacing piston rings and removing the head to “decarbonize” at ~15000 mile intervals.

The peak temperature one would expect in the exhaust manifold is not at wide open throttle but probably closer to the highway cruise position that coincides with the throttle position just before the carburetor “economizer” or “booster” valve starts to open up, dramatically richening the mixture for maximum power by dumping excess fuel to cool and prevent knock. Years ago I measured the AF ratio on my ’36 Packard EE-23 with a machine that you could put in the car and monitor while driving. I was a bit shocked to find the “booster” valve opened up at about 60 mph on a level road, and the slightest increase in throttle to go up a grade would peg the AF ratio to less than eight, the same territory one would expect of an aircraft engine at maximum takeoff power. I adjusted the “booster” to open a little later. Even so, I spend so much time at wide open throttle driving up hills at high altitude that the rich mixture always leaves carbon soot on the back of the car. I only worry about valve and exhaust manifold temps cruising down I-5, not climbing hills at wide open throttle.

Bottom line is I don’t think there is a proper AF ratio for these cars regardless of instrumentation beyond a bit of trial and error to keep them a little richer than the lean misfire limit.

I am reminded of my father who flew P-38’s in WWII on long missions where minimizing fuel burn to make it home was an obvious concern. They would slow the RPM way down, go to maximum cruise manifold pressure (throttle), and lean the mixture down to barely above the point of detonation (knock).

Jim

Hi James, you mention some interesting insights into the mixture ratio and the load on the engine you observed in your Packard. I’m not surprised that roughly 60mph is the speed that enough throttle opening to open the ‘power valve’ or economizer valve. About 60mph is where air resistance becomes greater than rolling resistance in most vehicles.

I have to disagree with when the maximum exhaust manifold temperature would be realized. Even though the carburetor would have the ‘power valve’ open at wide open throttle, and the rich mixture would cool the combustion temperature, nevertheless, the exhaust gasses are still very hot, in excess of 1200* usually and the exhaust manifold would be hottest at full power output, not at a moderate cruise speed.

The reasons are several: the main one is that with full throttle, the amount of air and fuel being burned is significantly greater than at partial throttle. The amount of hot exhaust gasses being pushed through the exhaust manifold is greatest at full throttle. The outside of the exhaust manifold is in the relatively cool engine compartment, where the heated air coming through the radiator will keep the temps around 180*.

The difference in EGT [exhaust gas temperature] in the many piston powered aircraft I’ve flown varies from about 1200″ to 1650*. This is at a given cruise power setting.

These numbers vary with a naturally-aspirated engine and a turbo-charged engine. The turbo’d engines intake air gets very hot when compressed, and even with an air-to-air intercooler, the combustion air entering the cylinder was a lot hotter than ambient. The result is a hotter combustion temperature.

I could run several different power settings for climb and cruise, and a low power setting would always have a cooler EGT, both full rich and at peak temp when leaned. We always ran 75-100* rich of peak EGT for power and engine longevity. For climb power, where airspeed was lower, and therefore available cooling air for the engines was less, we ran 200* rich of peak for cooling and exhaust valve life.

In our old flathead engines the much richer mixture at full throttle would result in cooler combustion temperatures, less knock, and longer valve life like you mentioned. But the volume of say 1200* exhaust is so much greater at full throttle, the much lower volume of hotter, leaner mixture combustion air at lower throttle openings would not be able to raise the manifold temperatures as high as the full throttle volume of exhaust.

The Stromberg carbs on our Pierces and on the Packards were state of the art in the ’30’s, but are rather crude compared to carburetor technology that was developed and used in the ’50’s and later.

It would be interesting to put an egt probe into the exhaust of a Pierce engine or a Packard, and see if the actual EGT’s are anyway near the numbers I saw when flying piston engine aircraft. I may have an EGT gauge left over from some diesel projects, and I think there is a pipe plug in the exhaust manifold in most of the 8 cylinder engines. I’ll have to look and see if I can put in an EGT gauge to see what it reads.

Your comments on fuel air distribution in long intake manifolds is right on. This problem is why Pierce used a ‘duplex’ type manifold in the straight eights. The intake manifold is set up to have one throat of the carb feed the front two and the rear two cylinders. The other throat in the carb feeds the center four cylinders. The result is that the mixture is much more equal than a single tube with a port at each cylinder.

Greg Long

Hi Jim, I think we are both thinking along the same lines: The higher EGT from the leaner part-throttle mixture is countered by the lower mass of gases flowing through the exhaust manifold. Your thinking that the difference between the mass flow at a steady state 60mph and a full throttle 60mph, such as when climbing a hill, you stated a doubling of the mass air-flow. Now THAT would be an interesting engine parameter to test !! I’m familiar with mass air flow metering in the intakes of some modern fuel injected engines. but a similar device to measure mass air flow of 1200* to 1600* exhaust gasses I have not seen, maybe some engine dynamometer setups have such a device ?

My experiences of watching the exhaust manifolds glow red through the cooling louvers of an aircraft engine, as well as seeing the exhaust manifolds glow red on an engine on a dynamometer are why I believe that full throttle or very high power output will result in the hottest exhaust manifold external temperatures.

I believe that the actual external heat loss from surface cooling of the exhaust manifold will result in a lower external temperature of the iron exhaust manifold with the lower volume of part throttle exhaust gas mass air flow. It is my untested belief that the much greater [2x greater? ] exhaust gas air flow with full throttle would increase the heating of the iron manifold, even if the exhaust gas temperature is a bit lower. Since the sources of external heat loss do not change if the road speed does not change. [ a variable would be a headwind increasing air flow through the radiator and over the engine]. With identical heat loss, the increased mass of hot exhaust gases should cause the surface temperature of the manifold to increase.

I guess that if the exact volume of exhaust gases, and their temperature could be used to calculate an actual BTU content for both part and full throttle exhaust gases, and then a heat loss per square inch of exhaust manifold could be used to calculate what the external temperature of the exhaust manifold would be. But the math and instrumentation needed make this a rather daunting task !!

On that straight 8 engine you had some mixture info on, it brings up something I’ve thought of several times: the Stromberg carbs, both the UU updrafts and the EE downdrafts, do not have any difference in the jetting for the duplex type intake manifolds. At least I have never seen or found different jets or Venturi sizes between the two carb throats. Maybe the drilled hole for the air-correction is a different diameter? I might try different drill sizes in that drilled hole to see if it differs from one side to the other. But the brass jets are always identical. I would have thought that there would be some noticeable different jetting to make the half of the carb on the very long 1-2 and 7-8 intake have a slightly richer mixture to correct for the fuel drop out under part throttle and cool running temperatures. Maybe the only difference would be the idle needle valve adjustments that result in a smooth idling engine, maybe they are slightly different, but I’ve never been able to prove to myself that gently seating the needle then counting the turns out to the best/smoothest idle setting was any different on the inner or outer throat of the carburetor. With a cold or not fully warmed up engine I can get a smoother idle with a richer setting for the long manifold half of the carb. But I don’t leave the mixture that way for regular use of the car.

A great discussion, thanks !

Greg Long

Greg, I hadn’t thought about it but you are right, the jets are the same for both branches, so it doesn’t seem they were intentionally compensating for the difference in length with the jets. Reading a little further in the text where I found the distribution data (“Internal Combustion Engines” by Polson 1942) the main factor for the duplex manifold was to minimize the tendency of the heavy fuel drops to not reverse course as the different intake valves opened and closed. With an outside intake valve open it establishes flow down the length, then when it closes and an inside valve opens it forces the flow to reverse direction and the heavy fuel isn’t going to reverse with it, thus the outside cylinders tend to run richer than the inside cylinders if they aren’t split up.

Trying do all the inter-related calcs for airflow, AF ratio, heat loss would be very challenging. Better to just measure it. A thermocouple in the exhaust would be ideal, but just slapping one on the outside of the manifold and driving at a fixed condition long enough for the temp to stabilize would probably answer the basic question. Come spring I may do that on the Packard.

The change in airflow is simple to approximate without measuring for our purposes, at a fixed RPM it is going to be pretty close to proportional to manifold pressure. I tend to think in terms of fixed RPM, because I spend a lot of time trying to stay at 65 mph as I go up and down hills, so the speeds and cooling airflows are constant. Manifold pressure in turn will be pretty close to proportional to indicated horsepower =brake horsepower + engine friction. With sea level pressure at 29.92″, an engine cruising at 15″ vacuum has an absolute manifold pressure of 14.92, or approximately 1/2 of sea level pressure and 1/2 of mass flow.

Approximating the performance of the Pierce 385 Eight from the 323 ci inch Eight in the text book, at 2400 rpm and 60 mph (with an overdrive or tall rear end gear), the max indicated power will be about 158, friction hp 38, for a net max brake hp of 120. At 60 mph level road power will be about 45 bhp+38 engine friction hp = 83 ihp, and that should be around 14″ vacuum and is in the vicinity of a net 1/2 net airflow compared to wide open throttle.

Jim