Triumphrat

mikeinva · 11-01-2012, 11:49 AM

Quote:

Originally Posted by norton74

You're right. Raising the CR does improve thermal efficiency a little and as a result, torque and horsepower. What I haven't figured out is how fuel consumption goes down. At a given engine speed, the amount of fuel being burned with each combustion cycle would have to be pretty much constant. The only way you could burn less fuel is for the air fuel mixture to become leaner. Running lean presents a whole new set of problems so let's assume that the air fuel ratio stays constant no matter what the CR is. I guess that it's time to hit the books.

if you inprove thermal efficency you burn less full to make same amount of power.And its not like its just alittle.But i give up you know everything there is no need for any of us to tell you anything.

mikeinva · 11-01-2012, 11:55 AM

Quote:

Originally Posted by JustPete

Norton74, in simple terms raising the compression can result in more power using less fuel. In reality it's more then that, keeping ALL things the same and raising just the compression will most likely result in knock, lowered performance and MPG along with a lunched engine.This topic can go long and deep so I think if you research thermal efficiency of an internal combustion engine you will get the information you're seeking. BOL Pete

That is true but as long as you dont rase it to much on these motors you wont have any knock.I have run them up to 11.5 to 1 on 93 octane pump gas on the street for many miles.If you took a other wise stock motor you could prob run 10.5to1 on reg gas because stock bikes the timing is way slow to start with.These bikes are way under tuned to pass epa reg.

norton74 · 11-01-2012, 12:18 PM

Quote:

Originally Posted by mikeinva

if you inprove thermal efficency you burn less full to make same amount of power.And its not like its just alittle.But i give up you know everything there is no need for any of us to tell you anything.

Sorry, I didn't mean to put you out of sorts. I was simply asking a question that hasn't been answered.

The question is moot to me as I don't intend to change the CR of my bike and I doubt that anyone riding a motorcycle is going to change the CR of his bike just to improve (debatable) his mpg.

This discussion is quickly sounding like a debate over how many angels can dance on the head of a pin.

rodhotter · 11-01-2012, 01:26 PM

although a totally different animal look at diesel torque at low rpms, WOW!! their high compression leading to hi cylinder pressure is one of many reasons for it

Grubscrew · 11-02-2012, 06:24 AM

Quote:

Originally Posted by norton74

What I haven't figured out is how fuel consumption goes down. At a given engine speed, the amount of fuel being burned with each combustion cycle would have to be pretty much constant. The only way you could burn less fuel is for the air fuel mixture to become leaner.

Norton, the reason you get stuck on this is because you assume an engine has a fixed volumetric efficiency, which it doesn't. The cylinders of a NA internal combustion engine do not fill completely to the swept displacement, usually not even at WOT. Air is springy, a little bit can stretch to fill the cylinders, but it's still just a little bit of air carrying an equivalent little bit of fuel. Think of the throttle as a way of controlling volumetric efficiency by restricting the air going into an engine, and it that way it controls the power. If you are only using 5% of an engine's power at a given RPM, it's because you've decreased the volumetric efficiency to about 5% of maximum by using the throttle. The throttle butterflies are nothing more than variable air restrictors, they control power by controlling volumetric efficiency.

So, if a bike is making more power due to more compression and hence better burn efficiency, you will be using less throttle under the same conditions. In other words, you will be letting less air into the engine, and because the air/fuel ratio remains unchanged, you are also letting less fuel in.

norton74 · 11-02-2012, 07:02 AM

Quote:

Originally Posted by Grubscrew

Norton, the reason you get stuck on this is because you assume an engine has a fixed volumetric efficiency, which it doesn't. The cylinders of a NA internal combustion engine do not fill completely to the swept displacement, usually not even at WOT. Air is springy, a little bit can stretch to fill the cylinders, but it's still just a little bit of air carrying an equivalent little bit of fuel. Think of the throttle as a way of controlling volumetric efficiency by restricting the air going into an engine, and it that way it controls the power. If you are only using 5% of an engine's power at a given RPM, it's because you've decreased the volumetric efficiency to about 5% of maximum by using the throttle. The throttle butterflies are nothing more than variable air restrictors, they control power by controlling volumetric efficiency.

So, if a bike is making more power due to more compression and hence better burn efficiency, you will be using less throttle under the same conditions. In other words, you will be letting less air into the engine, and because the air/fuel ratio remains unchanged, you are also letting less fuel in.

Yours is an explaination that makes sense to me. Thanks

Still, few will go to the expense and bother of increasing the CR just to get a small increase in mpg. The more likely scenario is that a range of changes are performed to increase power and; ultimately, decreasing mpg.

mikeinva · 11-02-2012, 06:57 PM

Quote:

Originally Posted by Grubscrew

Norton, the reason you get stuck on this is because you assume an engine has a fixed volumetric efficiency, which it doesn't. The cylinders of a NA internal combustion engine do not fill completely to the swept displacement, usually not even at WOT. Air is springy, a little bit can stretch to fill the cylinders, but it's still just a little bit of air carrying an equivalent little bit of fuel. Think of the throttle as a way of controlling volumetric efficiency by restricting the air going into an engine, and it that way it controls the power. If you are only using 5% of an engine's power at a given RPM, it's because you've decreased the volumetric efficiency to about 5% of maximum by using the throttle. The throttle butterflies are nothing more than variable air restrictors, they control power by controlling volumetric efficiency.

So, if a bike is making more power due to more compression and hence better burn efficiency, you will be using less throttle under the same conditions. In other words, you will be letting less air into the engine, and because the air/fuel ratio remains unchanged, you are also letting less fuel in.

lol can you beleave it for once we both agree.

mikeinva · 11-02-2012, 07:10 PM

As far as doing mods hurting gas milage(on these bikes)If they are tuned right the only 2 things that can hurt milage are the bigger cams (not talking 813 class cams) and 2 useing the right hand more ,that is hard not to do when you have power because it feels so good.Even going to the big bore stroker motor does not kill mpg much.I get upper 40s mpg on the 1087 with full race cams .I got around 50 stock.the 1087 makes 2 x the hp of a stock bike and around 50% more torque.going a steady 65 mph on the open road you barely have the throttle open on the 1087.It does not slow down on hills as much as a stock bike does so you dont have to open the throttle to keep the same 65mph speed.

norton74 · 11-04-2012, 06:17 PM

Quote:

Originally Posted by mikeinva

As far as doing mods hurting gas milage(on these bikes)If they are tuned right the only 2 things that can hurt milage are the bigger cams (not talking 813 class cams) and 2 useing the right hand more ,that is hard not to do when you have power because it feels so good.Even going to the big bore stroker motor does not kill mpg much.I get upper 40s mpg on the 1087 with full race cams .I got around 50 stock.the 1087 makes 2 x the hp of a stock bike and around 50% more torque.going a steady 65 mph on the open road you barely have the throttle open on the 1087.It does not slow down on hills as much as a stock bike does so you dont have to open the throttle to keep the same 65mph speed.

You just said that your mileage was better with the stock configuration than when you modified it.

Do you have to use premium fuel? If you do, then your cost of operation is higher.

The average tourer would rather get better mileage and use less expensive fuel. If you are racing, then mileage is rarely of much concern.

leftarmlogan · 11-04-2012, 06:34 PM

Quote:

Originally Posted by norton74

You just said that your mileage was better with the stock configuration than when you modified it.

That's really splitting hairs, no?

If Mike got 50 mpg average on a bone-stock bike and now gets "upper 40s" (let's say 47-49 mpg average) while getting twice the bhp and 50% more torque, I'd say that's pretty good.

I get about 50 mpg out of my very mildly modded Bonneville. If I could have power like a 1087 and only lose one-to-three, even five-to-seven, mpg I'd be bouncing off the walls.