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Isn't intake manifold pre-heating obselete?


ersatzS2
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post-71557-143139158886_thumb.jpgMy L-head mercer engine features a crossover pipe that routes hot exhaust gasses up and over the head, and into a jacket around the intake manifold, exiting via a down-pipe under the engine (even has a small muffler attached)

My '46 willys has a something similar, a flap regulated by a bi-metal spring that routes hot air around the intake until the engine is warm.

My question is this: heating the intake mixture is regarded today as a bad thing, since hot gas is less dense than cool gas and therefore contains less oxygen. They did it, I presume in 1915 due to the poor quality of the fuel, which ignited better if it was warmed up first. By the mid forties, fuel was better, but still benefited from a little pre heating while cold.

Hot rodders and race cars go to great lengths to cool the air as much as possible, and today's high octane fuel is far more combustible than the old days.

So can't I just substitute a blocking plate for a gasket on the crossover pipe? Are there any other issues at play here that I'm not considering?

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You might get away with it... depends on how warm your Mercer engine gets... does it have a thermostat ?

By the late 1930's, vehicles with exhaust heat risers had them automatically controlled by a thermostatic spring, which allowed exhaust pressure to force the flapper open as the things heated-up and the spring relaxed.

I have a 1941 De Soto with a flathead six and down-draft carb with automatic choke and an automatic heat riser that presently does not function (spring is broken). In cool to cold weather, there is a period between "cold-start" and "fully warmed-up to operating temperature (180*f)" where the choke has opened as it normally would, but the non-functioning heat riser is wide-open (no heat), and the result is a tremendous flat-spot in acceleration... I tied the heat riser closed as an experiment, and the flat-spot went away...

My point is, while the fuels may have improved, your Mercer engine might still be on the cold-blooded side, and need the help of the heated intake.

You certainly could experiment; if you find yourself having to use the choke more / longer during warm-up, that suggests the preheater is necessary, especially if the car sees a lot of short operating cycles where the engine doesn't get hot & stay hot long enough to "burn" / boil condensation / unburnt fuel blow-by out of the crankcase...

Let us know your results...

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The whole idea behind the "heat riser" mechanism is to put a "hot spot" or area in the intake manifold, modulated by a valve, to help the normal "warm engine" fuel mixture work better in cooler weather. This means less choke action, which means the engine gets the leaner "warm motor" mixture sooner. This decreases (or should decrease) fuel consumption in the "warm-up" mode and also prevent an over-rich fuel mixture from decreasing the lubrication of the piston rings and cylinder walls.

Certainly, in more modern engines, performance-oriented people have blocked the heat passages to the intake manifold to allegedly get better power, BUT that's with an already warmed-up engine . . . not a "dead stone cold" engine. So, just how far you can get away with it can greatly depend upon the typical ambient temperatures in which your vehicle is operated. What works in Houston, TX might not really work as well in NJ, for example. AND how willing you are to put up with deep flatspots in throttle response as the engine gets around it's new-found "cold naturedness".

In so many older vehicles, the heat riser valves were usually stuck. Either full closed or part-closed . . . and nobody really seemed to notice . . . unless the additional heat in the manifold caused the carburetor to boil over in the summer.

Another factor is what type of metals are involved in getting the exhaust heat? If it's sheet metal or aluminum, they'll heat sooner than cast iron.

The whole idea is to get the engine's operating fuel/air mixture to its leanest setting (normal running) as quickly as possible while still maintaining good driveability during this transition period. Whether the manifold heat is there or not. This might even mean leaning-out the automatic choke thermostat some from the stated factory specs. From my own experiences, the factory specs can work for the vehicle when it's new, but the choke thermostat spring can tighten-up with age, which means the end adjustment will be "Leaner" than the factory spec. Adjust it to work best with YOUR engine and the way YOU drive it. That means a good deal of time investment, but once it's done, it should work well.

And then there's the issue of "modern gasolines", too. I suspect they would work better with "less" choke than "more".

As for the "obsolete" part . . . fuel injection did that.

Just some thoughts,

NTX5467

Edited by NTX5467 (see edit history)
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Except for the very earliest, alll cars with carburetors had some form of heat. It started about the time your Mercer was built or a few years earlier.

The very first cars ran on gasoline that resembled naptha or Coleman fuel. One early motorist remembers filling the tank on his car (it held about a gallon) after returning from a journey and finding the tank empty a few days later - he forgot to put the cap on and the gas all evaporated.

Such volatile fuel was sold as gasoline because the low grade gas was being put into kerosene. There are many accounts of kerosene lamp explosions and of kerosene having a piercing odor, all before 1910. Later on, kerosene became safer and less smelly.

In the early days all the demand was for kerosene lamp oil. There was a little demand for gasoline for gasoline lanterns and stoves, practically none for cars as there were only a few in existence.

By 1910 - 1913 that all changed. There were more cars, more demand for gasoline, while kerosene lamps were being replaced by electricity and gas. The result was a drastic drop off in the quality of gasoline as the low grade gas, and kerosene as well, went into the gasoline.

Suddenly the gas became heavier, oilier, harder to vaporize and slower to burn. The first response was to lower compression ratios to prevent knocking. Then the designers had to add hot spots and vaporizers to get the gas to even burn. The next result was a change from square cylinder dimensions to oversquare.

Previous to 1910 a typical engine would have cylinders of 4 inch bore and 4 inch or 4 1/4 inch stroke. The new long stroke motors would be more like 3 inch bore by 5 inch stroke.

Some owners of old cars with long stroke, low compression motors add up to 25% kerosene to the fuel. They find it makes the car start easier, run smoother and cooler, and develop more power. They were just designed for a heavier fuel than modern gasoline.

Edited by Rusty_OToole (see edit history)
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I would expect that the intake manifold heating was to improve the vaporisation of the fuel & possibly to prevent fuel condensing on the inside of the manifold under extremely cold conditions?

I have a small brass era car that virtually freezes at the carburettor flange to the manifold when started even in warm weather. I have measured this temperature using a small hand held infra red temperature "gun" and it is only about 1-2 degrees above freezing when the car is started and remains fairly cold most of the time. The air intake to the carburettor originally was via a shroud around one cylinder to heat the incoming air a little but my car does not have that. I wonder what is happening to the fuel mixture inside the manifold at these very low temperatures.

I have a friend who had a 1920's car with a non original, reproduction intake manifold that had the original exhaust heated "hot spot" removed. The car was a dog to drive until he reinstated some intake manifold heating. I would have thought that with todays more volatile fuel, intake heating would be unnecessary but perhaps not.

The incoming fuel air mixture needs to be at the lowest temperature to increase the density of the charge and therefore the volumetric efficiency but it also needs to be properly atomised for good combustion.

Just my 2 cents worth.

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While driving my 1918 Buick the last 3 years or so i experienced problems where I would need to choke the engine at times to just keep it going. T he vacuum tank was working like a champ so this year I looked else where. Back when I assembled the engine I installed a flexible metal tube from the exhaust manifold to the carburetor as the book showed. This summer I removed that tube, plugged the exhaust manifold and I've experienced a 90 percent improvement in performance. I don't believe that today's fuel needs all that extra heat, a little but not to much. It has been a good runner all along but the other day my wife clocked me on a flat stretch of road cruising at a little over 50 MPH. In that thing it seemed a bit to fast to be going for me. I better slow down.

Edited by frazer51 (see edit history)
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It appears that folks here are missing the whole point of intake heat. On an engine with a carb or throttle body injection (as opposed to port injection), the liquid gasoline will separate from the airflow and condense in the intake runners, especially on a cold engine. This reduces power and mileage and increases emissions. Preheating the intake promotes fuel atomization and prevents this separation in the flow. Once the engine is at operating temperature, the normal heat of the engine is enough to maintain this atomization, so there is usually a butterfly valve with a bimetallic spring (or a computer-controlled vacuum actuator on newer cars) to prevent or reduce exhaust flow through the heat riser. GM throttle body injection on the 454 truck engines even had a coolant-heated riser under the throttle body in addition to the exhaust heat provision. Note that some cars (mid-1980s GMs with the carb version of the Buick V6, for example) even use an electric heating element under the carb for this purpose. Port fuel injection doesn't require mainfold heat because the liquid fuel is not introduced into the manifold but is injected in the intake ports at the heads and thus can't condense out.

For maximum performance on a race engine, yes you want to block the heat riser. On a highly tuned race engine where you're looking for the last few horsepower, the small difference in intake charge density due to the heat can cause the loss of a handful of horsepower. As noted, these engines are fully warmed up before any race so there really is no need for exhaust heat of the intake. Don't confuse what's required for a race engine that normally operates near wide open throttle and what's needed for a street engine that needs to operate over a wide range of temperatures and throttle positions.

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Ersatz,

Are you presently experiencing driveability issues with your Mercer ?

Looks like you've got a Carter BB-1 updraft carb, which is about as good and "modern" an updraft as you can get, so it's not like you're fighting a primitive "air-valve carb" or other pre-historic "mixer"...

Regards,

Frank McMullen

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It appears that folks here are missing the whole point of intake heat. On an engine with a carb or throttle body injection (as opposed to port injection), the liquid gasoline will separate from the airflow and condense in the intake runners, especially on a cold engine. This reduces power and mileage and increases emissions. Preheating the intake promotes fuel atomization and prevents this separation in the flow. Once the engine is at operating temperature, the normal heat of the engine is enough to maintain this atomization, so there is usually a butterfly valve with a bimetallic spring (or a computer-controlled vacuum actuator on newer cars) to prevent or reduce exhaust flow through the heat riser. GM throttle body injection on the 454 truck engines even had a coolant-heated riser under the throttle body in addition to the exhaust heat provision. Note that some cars (mid-1980s GMs with the carb version of the Buick V6, for example) even use an electric heating element under the carb for this purpose. Port fuel injection doesn't require mainfold heat because the liquid fuel is not introduced into the manifold but is injected in the intake ports at the heads and thus can't condense out.

For maximum performance on a race engine, yes you want to block the heat riser. On a highly tuned race engine where you're looking for the last few horsepower, the small difference in intake charge density due to the heat can cause the loss of a handful of horsepower. As noted, these engines are fully warmed up before any race so there really is no need for exhaust heat of the intake. Don't confuse what's required for a race engine that normally operates near wide open throttle and what's needed for a street engine that needs to operate over a wide range of temperatures and throttle positions.

Joe, the best answer so far. I would only add that Datsun/Nissan also had heat element's electronically controlled under throttle body and electronically controlled pulse jet carburetors in the 80's early 90's because of the distance between the throttle body carburetor was long enough when cold to cause the fuel to start to puddle.

Why don't cars have a heat riser today is because with multi port injection and direct injection the fuel is sprayed directly into the chamber or just before the combustion chamber so the fuel does not have a chance to puddle. Long runner intakes have the greatest chance of puddling the one car that we see this mostly is the VW aircooled engine. When the heat riser tube gets blocked ice will form on the neck of the intake manifold before it splits going to either cylinder head. Air cooled engines also warm up much slower than liquid cooled engines and that is the reason Porsche and VW had to give up the air cooled engine, Those engines could not warm up fast enough to pass a seven mode EPA certification test which takes a car from the cold room to the dyno for a complete driving cycle. In this picture shows how long the intake manifold runners are plus the smaller heat riser tube below and attached to it and you will also notice a hose to the air cleaner inlet to warm up the intake air when the engine is cold (this engine is 63-65 vintage); http://members.trainorders.com/android/VW08/40HPtin-fresh63.jpg Why do some engines have long intake runners, because in a low horsepower engine such as the VW they produce better torque than short runners. Short runners are better for peak high RPM horsepower. On some engines of today like Nissan/Infiniti the intake is a variable chamber that can go from short runner to long runner depending on load and RPM and work in conjunction with variable valve timing with intake and exhaust valves advancing and retarding separately for the best economy, power and emissions

Take a look at this early 60's Chrysler wedge ram manifold, can you imagine how the fuel would puddle without some kind of heat application?

http://upload.wikimedia.org/wikipedia/en/7/7a/Chrysler_300F_cross_ram.jpg

That 413 produced a lot of torque though.

D.

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The OTHER thing is that moving air is cooler than non-moving/ambient air. Add the vaporized fuel and the mixture becomes cooler, still. So, adding some heat will better vaporize the fuel droplets so they're more useable in the combustion chamber. Once the heat from the engine also heats up the intake manifold, the additional heat from the "hot spot" is not needed.

In some V-type motors, some are more critical for the need of exhaust heat than others. On engines, ala 1955 Chevy V-8, the intake manifold also forms the cover for the block. Engine oil will splash around in there, which will help heat the intake manifold, although it also had a shielded heat crossover passage under the carburetor. On my '77 Camaro, it now has an aluminum intake manifold and an open heat riser valve. In our more temperate N TX climate, it gets along just fine once it gets past that first mile of driving. I have some friends who blocked-off the heat passage with no really apparent driveability ills, as I recall . . . with an aluminum intake manifold.

On a Chrysler B/RB series V-8, and a variety of higher-carline GM V-8s, there is a steel plate which seals the top of the motor, with "airspace" between that plate and the bottom of the intake manifold. Only heat coming from the cylinder heads or heat from the radiator's airflow can heat the intake manifold, so the need for the heat crossover passage is more necessary for cold-start driveability in cold weather.

Spark plug gap design/modification can play a part in cold-start driveability, too, by observation and experience. PM me for details.

Just some thoughts,

NTX5467

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The OTHER thing is that moving air is cooler than non-moving/ambient air. Add the vaporized fuel and the mixture becomes cooler, still. So, adding some heat will better vaporize the fuel droplets so they're more useable in the combustion chamber. Once the heat from the engine also heats up the intake manifold, the additional heat from the "hot spot" is not needed.

In some V-type motors, some are more critical for the need of exhaust heat than others. On engines, ala 1955 Chevy V-8, the intake manifold also forms the cover for the block. Engine oil will splash around in there, which will help heat the intake manifold, although it also had a shielded heat crossover passage under the carburetor. On my '77 Camaro, it now has an aluminum intake manifold and an open heat riser valve. In our more temperate N TX climate, it gets along just fine once it gets past that first mile of driving. I have some friends who blocked-off the heat passage with no really apparent driveability ills, as I recall . . . with an aluminum intake manifold.

On a Chrysler B/RB series V-8, and a variety of higher-carline GM V-8s, there is a steel plate which seals the top of the motor, with "airspace" between that plate and the bottom of the intake manifold. Only heat coming from the cylinder heads or heat from the radiator's airflow can heat the intake manifold, so the need for the heat crossover passage is more necessary for cold-start driveability in cold weather.

7

Just some thoughts,

NTX5467

Real Pontiac V-8's as well as first generation Olds, Buick, Cadillac use a valley cover between the lifter/pushrod valley and the intake manifold. I like this because when installing the intake you can spend more time making sure the port aligning matching is accurate instead of trying keep track of sealing the valley at the same time. From a maintenance standpoint it's easier to change the intake, and you can leave the distributor in place. I don't have cold start up problems on any of my carbureted cars and all of those cars either don't have chokes or they are disconnected. My two Pontiac's have factory intakes without exhaust passages ( Ram Air 4 manifolds ) and have no problem starting even in seventeen degree weather with the chokes and their fast idle cams removed. Starting procedures is almost like procedures on the old Porsches that come from the factory without chokes. The only difference is I don't touch the throttle until the oil pressure comes up. Here is a picture of one of them;notice all the space in the center of the valley where the crossover would go. http://www.pismoderelicts.com/photogallery/new%20format%20937/images/p1020753.jpg Here is a standard manifold;http://www.pontiacparadise.com/images/parts-id/engine-parts/spc-intake-manifolds/spc-intake-mani20.jpg

Here is a Ram Air 4;http://t0.gstatic.com/images?q=tbn:ANd9GcQEWv_J1-Juy7R27gb1BQSwYBjuWiH79IObQogC_5h6AzIiSF6UEg&t=1

D.

Edited by helfen (see edit history)
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The OTHER thing is that moving air is cooler than non-moving/ambient air.

Uhhhh.... close, but not quite. Moving air by itself has no effect on air temperature. What you're thinking of is that as a gas expands (after moving through a carburetor venturi, for example), the EXPANSION of the gas causes the temperature to drop. Those who stayed awake in high school physics will recognize this as a function of the ideal gas law. This is also how automobile air conditioning systems work, by the way.

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That expansion can also cool the intake air enough to go below the dew point and moisture will form. If it started close to freezing the condensate may be cold enough to freeze which results in carb icing (more common in aircraft). This is why even port injected engines often have a coolant tube to the throttle body.

So two reasons:

- maintain atomization (have seen gas form a puddle under a carb, part of the reason a carb needs an accelleration pump).

- prevent carb icing

FI esp. port or direct injection can help with the first but the second can still be a problem particularly for a MAF controlled engine.

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Guys I'm in awe of the teaching and learning going on! Thanks again for the education, very interesting stuff.

Ersatz,

Are you presently experiencing driveability issues with your Mercer ?

Looks like you've got a Carter BB-1 updraft carb, which is about as good and "modern" an updraft as you can get, so it's not like you're fighting a primitive "air-valve carb" or other pre-historic "mixer"...

umm, somewhat embarrassing to admit, but NO, it's running great, carb is terrific, starts instantly even on the crank if I am feeling frisky. It was merely an academic question, it certainly 'ain't broke'. Now, I am planning to restore and install the original Penberthy Ball & Ball dual throat carb eventually, as well as return to a Stewart vacuum fuel pump so all that is subject to change...

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  • 2 years later...
I know this is an old thread and I'm not trying to restart the conversation, but I have to put this here just to add to the info. I'll probably link a few more places where people are discussing the issue. Its a good article to explain the carb freezing issue. http://dodgepilothouseclub.org/know/riser/riser.htm

Yes a generally good article, but there are inaccuracies in it.

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When the air goes through the venturi, it is sped up. Now remember the Bernoulli equation, which contains three terms: head (height above datum), pressure head and velocity head, all in compatible units. Increase the velocity and the pressure reduces. The venturi thus causes low pressure. The main jet comes in at the venturi, so the low pressure pulls fuel into the venturi. Just below the venturi, the chamber expands and the mixture slows. The fuel is atomised in the rushing and turbulent air created. The mixture expands to fill the extra space below the venturi. To expand it must absorb heat (one of those laws of thermodynamics), so the area is cooled. If the mixture temperature goes below the dew point, condensation forms and can freeze in the base of the carb. below the venturi. Humidity and ambient temperature thus affect carb. icing, as has been said.

I have had this problem a few times in my 1930 Dodge 8. As has been said, warm-up time fixes the problem. If immediate departure is needed, the heat riser (warming the base of the carb.) prevents carb. icing.

Someone said there was an accelerator pump to overcome the puddle forming below a carb. I think not. IMO The puddle forms because of poor atomisation and excess fuel. The accelerator pump is a simple solution to an extremely lean mixture when you open the throttle - you suddenly increase the air flow but the engine can't speed up yet due to lack of fuel caused by lag in the intake system, so there is a shortage of fuel momentarily. By simultaneously pumping a little (or a lot of) fuel in, the flat spot is overcome.

In injected systems, other methods are used to prevent this flat spot.

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[ATTACH=CONFIG]152374[/ATTACH]My L-head mercer engine features a crossover pipe that routes hot exhaust gasses up and over the head, and into a jacket around the intake manifold, exiting via a down-pipe under the engine (even has a small muffler attached)

My '46 willys has a something similar, a flap regulated by a bi-metal spring that routes hot air around the intake until the engine is warm.

My question is this: heating the intake mixture is regarded today as a bad thing, since hot gas is less dense than cool gas and therefore contains less oxygen. They did it, I presume in 1915 due to the poor quality of the fuel, which ignited better if it was warmed up first. By the mid forties, fuel was better, but still benefited from a little pre heating while cold.

Hot rodders and race cars go to great lengths to cool the air as much as possible, and today's high octane fuel is far more combustible than the old days.

So can't I just substitute a blocking plate for a gasket on the crossover pipe? Are there any other issues at play here that I'm not considering?

but you're not hotrodding or racing are you? My '55 V8 has carburetor heat. Exhaust goes through the intake manifold under the carburetor. It helps warm up the engine faster. Mine will stumble considerably if I drive it with out warming up.

I add marvel Mystery oil to the gasoline. It seems to help eliminate vapor lock. Dad's L head Cadillac had a Marvel Mystery oiler on the firewall feeding the carburetor.

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but you're not hotrodding or racing are you? My '55 V8 has carburetor heat. Exhaust goes through the intake manifold under the carburetor. It helps warm up the engine faster. Mine will stumble considerably if I drive it with out warming up.

I add marvel Mystery oil to the gasoline. It seems to help eliminate vapor lock. Dad's L head Cadillac had a Marvel Mystery oiler on the firewall feeding the carburetor.

The stumbling that your referring to is the reason the early Porsche cars had no chokes at all. It's is also the reason many VW owners remove chokes from their cars. I have done this as well on my VW's and also on my Pontiac's and Oldsmobile. The VW's don't get driven off until the oil temp is 120 degrees, the domestic's don't get driven off until 114 degrees of coolant temp. You can substitute Dexron for MMO, and it's a lot cheaper too.

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To put this info to practical use. Many early tractors had a lever/ manifold Valve to go from hot to cold that was meant to be used as the engine warmed up. From actual experience I can tell you of many up draft tractor/ stationary engines that just will not not start in cold weather because the fuel condenses before it get to the cylinders. warm the manifold and they will fire up and run. I have a propane touch I would do this with from time to time with some of the "cold blooded" machines and engines that had to be started in the winter. Dandy Dave!

Edited by Dandy Dave (see edit history)
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