I can only tell you what my engineering buddies tell me.

 

Ok, I'll chime in a here with the few tidbits I carried away with me from my thermodynamics class......

1. Gasoline engines and diesel engines are different animals. Gasoline burns much faster, to the point that when you model the engine mathematically, you assume combustion takes place at constant volume while the temperature and pressure increase greatly. Diesel, on the other hand is modeled as a constant pressure during combustion, i.e. combustion takes place over the length of the power stroke with volume and temperature increase.

2. Compressing gas adds energy-->raises temperature. That's how diesel engines run and what causes detonation in gasoline engines (or advancing the timing too far). The fuel-air mixture reaches the flash-point temperature by compression and ignites before it reaches the top of the stroke. High-compression gasoline engines use octane to retard the flash-point temperature.

3. Combustion of the fuel-air mixture produces energy in 2 forms---heat and work. Some of the work is converted to heat as well by friction. Compressing the mixture prior to combustion increases the amount of work (directly related to the change in pressure). The more you can cram in there, the more efficient the burn will be (I'm not sure whether there's a theoretical break-over point where it goes the other direction). Cooler intake temps (bak's & such) allow more air (and fuel) into the cylinder since cool air is more dense.

4. A higher efficiency engine does not necessarily mean a cooler running engine. It only means that you put more horsepower to the crankshaft (and less heat) for a given amount of fuel.

5. So, it works like this: If your fuel consumption stays about the same, but your engine makes 15-20% more horsepower, that means it is generating a good bit less heat.....right?

 

Oh yeah, if your fuel consumption increases as well as your horsepower, you can count on generating more heat. I know that doesn't answer the question posed, but if somebody has the hc pistons and can compare gas mileage before & after, that should answer the question.

 

I want a turbo so that I never have to worry about overheating in traffic...

 

Whoops, I missed the last few posts while I was writing this, but;

This discussion has the makings of a thermodynamics class and an internal combustion engine class. Fun though… really makes the brain work. [/emoticons/emotion-1.gif] We've got a few discussions going here now… one regarding compression ratio, the other regarding burn rate. The CR affects how much heat we make, the burn rate helps determines where the heat ends up going (either into work driving the piston, or into the metal of the engine, or out the exhaust pipe.) Here is more of my humble ramblings:

The heat produced from combustion is a function of how much fuel you burn completely. It doesn't matter how fast or how slow it burns. 1 gallon of gas has about 125,000 BTU's of heat energy… doesn't matter if you burn it one drop at a time for an hour, or burn it all up in 1 minute… it will still generate the same total amount of heat. This assumes complete combustion. If you don't burn the fuel completely, you will generate less heat, regardless of how fast or slow it burns. The higher the compression ratio, the more the piston compacts the fuel/air mixture as it reaches the top of its path. The denser the fuel/air mixture is, the more completely you burn the fuel, and the more heat you generate, even if it burns faster.

Heat is not merely a by-product of combustion… heat is the MAIN product of combustion. Burning fuel produces heat, not pressure. The rapidly heated gases in the combustion chamber want to expand… it is this expansion of hot gasses that create the pressure to push on the piston. More heat builds more pressure, which makes more power. But some of this extra heat will go into the cylinder head.

Burn Rate: The expansion of the hot gasses creates pressure… there is more pressure if the piston is at the top because there is less room for the gasses to expand. There is less pressure if the piston is further down because now you have more room for the hot gasses to go. So, you want the hot gas expansion to occur as rapidly as possible, ideally while the piston is just past TDC. Faster burn means more of the heat is converted into pressure on the piston. If the burn takes longer, the piston is further on it's way down… you still have the heat, but you have less pressure, and you have exposed more surface area of cylinder to the heat. So you get less push on the piston and you get more heat going to the cylinder walls and out the exhaust pipe when the ex valve opens.

So, a higher compression ratio will generate more overall combustion heat. It will also increase the burn rate, (but the burn rate will be slowed somewhat by a higher octane fuel which may be necessary to prevent detonation). Ideally, much of that extra heat will go into mechanical work (more power), while a smaller amount will go into the heads/cylinders, and none of the extra heat will go out the exhaust.

Sorry for being long-winded. [/emoticons/emotion-4.gif]

 

thanks bunker for a better explanation/correction of how the work is generated.

 

hi, guys, i have been reserching for the last couple hours. most of the scientific discriptions i have found, come to no conclusion. the only one i found stated, "if you have two like engines, the one with higher compression delivers more of the energy of fuel as power, and less for heat, therefore, a cooler running engine.
thanks, all, gary i love this s$#$

 

I still stand behind #5......if you find that you burn more fuel, you most likely have a hotter engine. After all's said & done though, for the question of your engine overheating, the only thing that matters much is how much fuel is burned when you're at idle. When you're moving, the engine doesn't have much trouble keeping cool.

 

If your PCIII is mapped properly and you run the lowest octane necessary to prevent knock, by raising the CR in the stock bore, I humbly predict that at part throttle, you will get slightly better fuel efficiency and generate a bit less waste heat. At full throttle, you will make more overall power, burn more fuel, and generate more waste heat though the head, but not out the exhaust pipe. At idle in traffic, I wouldn't want to hazzard a guess. (How's that for straddling the fence? [/emoticons/emotion-5.gif])

 

This is great. I think that I get out of this less exhaust heat, more power and a headache. I think that is what Rob said at the begining of this, less exhaust heat with higher compression pistons. Or did I miss something.

 

http://discovery.infopop.net/1/OpenTopic?a=tpc&s=6941912904&f=9701967776&m=4731901596&r=4731901596#4731901596

posted this topic on the mythbusters website under "show ideas"

 

good idea audio. Watch them hire some consultant expert who doesn't know the difference between a piston engine and an electric motor......

 

For crying out loud. Would Rob Hughes or somebody please install an oil temp gauge and an airhead temp sensor on a stock Warrior, take readings after a ride and do the same with the bike after installing the HC pistons. Granted, conditions should be the same on both rides, but if there is a significant difference it should be apparent. A test could be done with an infa-red heat gun, but you would have to be very careful as how you took the readings. Other factors also influence the temp. Camshafts overlap, according to Ford R and D do lower idling temps, and unrestricted exhaust definately can be a factor. I have had to install higher thermostats on cars I have replaced headers on to get the heater to put out. I am very interested in the results as I am contemplating the HC pistons.

 

Just culled up some more info on this subject from an Engineeres forum page...I forgot any of this math way back when, but that's what these guys get paid to know.

____________________________________________________________________

Actually, its not increasing the compression ratio that increases power output and decreases exhaust temperature. Its increasing the expansion ratio that does the trick. Now for most engines the expansion ratio is the inverse of the compression ratio -- an engine with a 10:1 compression ratio will have a 1:10 expansion ratio.

However -- with an engine with variable valve timing its possible to have, for instance, a 5:1 compression stroke and a 1:10 expansion (combustion) stroke.

How much of a difference do the different expansion ratios make?

Assuming a combustion temperature of 3500 deg F, here are some brief simplified calculations from the standard gas tables that are not completely accurate (neglects efficiencies, friction, doesn't model combustion right, etc) but should give you a rough idea of what's going on:

For a 10:1 compression/expansion ratio: Compression work: 137.9 BTU/lb gas: Output 441.09 BTU/lb, Net output: 303.18 BTU/lb, Fuel energy: 756.05 BTU/lb, Efficiency 40.1%, Exhaust gas temp 1520 deg F

for a 12:1 compression/expansion ratio: Compression work 155.4 BTU/lb gas, Output 465.1 BTU/lb, Net output 309.7 BTU/lb, Fuel energy 732.8 BTU/lb, Efficiency 42.26%, exhaust gas temp 1403 deg F

____________________________________________________________________________________________________

There ya go...

 

I'll throw a little curve into this theory. I am as confused as you Rob. In the late seventies, car manufacturers lowered compression in car engines. It wasn't so much as to reduce the use of higher leaded gas, cause they were taking the lead out. It was to reduce the combustion temperatures. Lower combustion temps produce less nitrous oxide pollutants. Lower compression produced lower combustion temps. Its also always been a given (so I have read) that when you raise compression by pistons, turbo charging supercharging etc, you really raise combustion temps. Thats why you have to go to better valves and seats as they can get really hot. Still would be nice if someone with a temp gauge could compare before and after. I plan on gettting the pistons, does anyone know of any temp gauges available for the Warrior? I'm sure an oil cooler would be a good addition anyway. Remember when Lockhart used to make the oil coolers with the optional thermostats. I had really good results from those.Lowered my '82 GS1100 Suzuki's temp by 25 or so degrees. Noticed a difference on the hot days. KC.

 

You know , no one has mentioned gas mileage before or after piston swap. Theoretically the hc pistons should be more efficent. Higher compression only improves efficiency. Hope no one out there still believes that higher octane fuel delivers more power. Octane only keeps fuel from igniting sooner. It allows engines with higher compression to deliver more power. The lower octane you can get away with, usually the snappier your engine will feel. Two more cents worth,KC.

 

Thanks KC...I'm just telling you what a couple of engineers said...higher compression equals more efficiency and lower exhaust gas temperatures.

Here's what another said:

the 10.5:1 CR engine will have "Lower EGTs"
than 8.5:1 CR

the lower the CR the higher the EGT

the higher the CR the lower the EGT

Larry Meaux
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
________________________________

Go figure it goes against simple commom (yet wrong) ideas...like the guys who think by just adding race gas to their car or bike will make it faster....

 

Yes, the mystery continues.....Here's another food for thought. Exhaust gas recirculation into your intake manifold lowers combustion temperature, therefore reducing nitrous oxide pollutants. You would think that the exhaust gas in the intake would heat up the combustion chamber. It is actually an inert gas(non-combustable) that under combustion reduces the temp. But you know, you can only believe 10 percent of what you read and sometimes I wonder about that last 9 percent. Isn't it nice living in a world of continuous blunder? KC.

 

I'm glad I ordered the 108 kit with the 12:1 pistons[/emoticons/emotion-2.gif]

 

Found a little info on the net that talks about compression ratio, combustion temps and pollutants.Its at,http://www.castle-comply.com/Pages/articles/RSGDAN.html More confussion. KC.

 

At least the guys I quoted actually have some math to back up the claim...but who knows?

 

Hey Rob. Can you ask these guys to respond to this info? Something doesn't jive. KC.

 

I will tonight.