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Heat Generated by a V8 Gasoline Engine?

Heat Generated by a V8 Gasoline Engine?

Heat Generated by a V8 Gasoline Engine?

Does anyone out there have some rule of thumb numbers or better yet technical information for how much heat a 300 cubic inch V8 will produce at an iddle.  I want to determine how much heat the radiator or an equivalent heat exchanger would need to remove from the coolant loop to maintain a somewhat constant engine temperature when the engine is iddling.  The engine in question is a 1978 AMC 304 with a 2 barrel carburator and dual exhaust.

RE: Heat Generated by a V8 Gasoline Engine?

Isn't that what the engine thermostat is for?  Your post is not very specific, but a in general radiator just large enough to remove idle heat would be insufficient when the engine is loaded.


RE: Heat Generated by a V8 Gasoline Engine?

I understand what the engine thermostat is for.  It will not allow water to circulate in the block until the water temperature in the block is up to the thermostat setting.  Then it will allow the water to circulate and will open or close depending on the water temperature.  What I am looking for is the amount of heat the engine will produce at idle.  I want to use this heat for another purpose and I want to know, at worst case (at idle, when the minimum amount of heat is produced) how much heat I can rely on to always be produced by the engine.  When the engine is at load, more heat will be produced and that can be removed by the main radiator.  I am not worried about the loaded condition, I need to know the condition at idle.

RE: Heat Generated by a V8 Gasoline Engine?


One way to figure out the amount of heat is to actually measure the amount of fuel used at idle-for your engine.

The efficiency is very low at idle-no load, probably less than 10%.  That leaves ~90% of the heating value being converted to heat, and I'd guess that ~20-30% of the 90% goes out the exhaust when unloaded.

So, if you use ~70% of the total heat in the fuel, you should be fairly close.  You might try to recover some of the exhaust heat too.

RE: Heat Generated by a V8 Gasoline Engine?

Thanks for the info.  I have located some information but I wanted some feedback to see if I was at least in the ballpark.  I think you are right about the idle efficiency and the fact that 30% or so goes out the exhaust.  I am assuming that there would probably be another 10% or so radiated from the block with the remainder available to be removed from the coolant loop.  The best information I could find indicates that an engine like this may use around 2 gallons per hour at an idle.  That means about 50% of that input is being extracted through the coolant loop.  So I might have around 144,000 btu/hr to use.  What do you think.

RE: Heat Generated by a V8 Gasoline Engine?

I thought the BTU content of "common" gasoline is around 114,000/gal-still a lot of heat.

If you intend to heat a house this way, why not add a big generator/alternator and make your own electricity?

RE: Heat Generated by a V8 Gasoline Engine?


Typical pump gas will produce about 19,000-21,000 BTU per pound. Figuring a weight of 5.8-6.5 lbs per gallon, gives you anywhere between 110,200 and 136,500 BTUs per gallon.

How many BTUs you have available to power your alternate device, is largely dependent on how effectively this device can remove heat from the engine. The more energy your device can safely remove, the lower the requirements of the cooling system will need to be.

Bryan Carter

RE: Heat Generated by a V8 Gasoline Engine?

OOPs,  Never trust your memory.  I somehow had 144 stuck in my head, not 114.  Thanks for the correction.  I am using this for a mobile operation so the generator/house heating thing wouldn't apply and the heat exchanger I am using in parallel with the radiator is rejecting the heat to a fairly low temperature source and the heat exchanger is sized with excess capacity for the temperature differential I will see even considering up to a 50% glycol solution.  Thanks for your help!

RE: Heat Generated by a V8 Gasoline Engine?

Coolant always circulates thru the block. If it did not, there would be severe localized overheating long before the thermostat got slightly warm. So, I'd say the thermostat's job is to decide how much of the constantly circulating water should be fed thru the radiator.


RE: Heat Generated by a V8 Gasoline Engine?

If you are going to run this engine at idle to generate some form of heat, you might want to consider reducing the throttling losses by modifying the engine to only use four of the eight cylinders.

If you want to reject heat to some other device, you may want to replace the thermostat with a different flow control device so that under a steady state idle rpm that you always circulate warmed coolant to the radiator.  A thermostat replacement used by many racers is a disk with a center hole pierced in it.  You could experiment with different diameter holes.  You could plumb in a flow control valve like a plumber might use as well.

RE: Heat Generated by a V8 Gasoline Engine?

BTDT, but if you are trying to get the engine to run efficiently don't do that. Overcooling the heads really stuffs the fuel consumption and the emissions.

For extra brave points - lag the engine, or at least run it in an enclosed box. You might want to recover the heat in the exhaust (easy) and the cooling air (tricky).


Greg Locock

RE: Heat Generated by a V8 Gasoline Engine?

Thanks for the info guys,

What I am actually doing is using the 304 V8 from my 1978 CJ-7 Jeep to provide hot water for a shower while I am camping or in hunting camp.  I have a small plate and frame heat exchanger that I will mount under the hood.  I will then mount a 12V self priming RV pump beside it to pull around 2gpm of water from the creek and pump it through the heat exchanger to a shower head outlet.  The other side of the heat exchanger will be plumbed into the coolant loop just like the heater core and will have a cable operated adjustable valve to control the flow.  What I wanted to find out was if the engine could produce enough waste heat at idle to heat the water for my shower.  My gut feeling said it would and the ballpark numbers also indicate it should.  I guess we will see.  The engine may produce around 114 mbh, the heat exchanger is rated at 90 mbh with an inlet of 180 deg which should give me around a 90 deg delta T on the water from the creek assuming a 2 gpm flow.  This arrangement should be able to continually heat water from 35 to 125 deg for the shower.  Not the most efficient use of gasoline but it would be worth if for a nice long hot shower in the middle of the winter in the middle of the wild.  I thought about using a control valve with a sensing bulb to regulate the flow from the coolant loop but this arrangement just seemed simpler.  The temperature should be fairly stable once the engine is up to temp and additional temperature control while showering will be available by increasing or decreasing the flow at the shower head.
Once again, Thanks for the input.

RE: Heat Generated by a V8 Gasoline Engine?

I wish the details had been posted in the first post.

If you are plumbing the secondary heat exchanger in the bypass circuit of the engine, you will have more luck maintaining the shower water temperature.  If you plumb the secondary loop into the main heat exchanger hoses, you'll have to deal with the continual thermostat cycling.  This cycling is approximately 10 deg F.  If the thermostat is set to open at 190 F, it will be fully open at 200 F.  I'd say with cold winter air across the idle that the inlet water back to the engine would be below 150 and that there may be a bit of thermostatic cycling.

RE: Heat Generated by a V8 Gasoline Engine?


Sorry I didn't include all the details in the first posting, but I wasn't sure what I would do with the "design/idea"(other people have done this also).  I even toyed with the idea at one time of building a compact little package that could be sold.  I have since decided to just build one for my Jeep and probably another for my brother.  I think these things will be too expensive for many people to be interested.  I will let you know how it works if anyone is thinking of building one.

The bypass from the water pump goes around the thermostat and it appears that the loop for the heater/defroster coil does also.  I am planning on connecting into the heater loop right off the pump.  I think this may be a higher pressure tap from the pump that would maintain flow even if the thermostat for the main loop opens.  If I cannot get enough heat out because the thermostat cycles and the radiator dissipates the heat, I may need to look into temporarily restricting the radiator flow, or installing a diverter valve (expensive) that would divert water through the heat exchanger vs the radiator(need to be carefull though).  I am hoping the heat exchanger will get as much heat as it needs and any extra would be removed by the radiator once the thermostat opens.  I may even put in a higher temperature thermostat to help things out, but I don't want to do that unless I have to. I probably only need 105 - 110 deg for a shower so it may work with a lower inlet temp.

RE: Heat Generated by a V8 Gasoline Engine?

Seems like too much messing around under the hood.
'Think outside the box!'

Get some flexible exhaust duct hose & run your tailpipe exhaust through the HXer.

If you didn't already have the HXer, you could just bubble the exhaust up through a drum of water (make sure to remove exhaust hose before shutting off engine).  The basic principle is used in Vertical Tubeless hot water heaters from Fulton

RE: Heat Generated by a V8 Gasoline Engine?

You will also catch the water vapour in the exhaust gas and save just a tad of water


RE: Heat Generated by a V8 Gasoline Engine?

Good thinking, Pat.

Burning 1 gallon of gasoline would produce ~1 gallon of condensate (+ additional BTUs).

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