Gas Mileage Issues

[Iowa4Runner]

Years ago on motorcycles, to compensate for the added air flow from modifications (air filter & exhaust) the carbs had to be re-jetted (larger jets), why?

Think about this; if you have increased the air flow through an engine by a significant amount, do you think you can use the same amount of fuel and maintain the same air/fuel ratio? Modern engines are tuned on the lean side from the factory, to run cleaner.

Koz

I agree with most of what you're saying. The concern I have is with the idea that anytime a person modifies their engine, they should expect to see a drop in MPG simply because their engine can flow more air / fuel. If the person keeps the same diving habits they'll just use a little less throttle to get the same result as before and the flow / fuel consumption should be about the same.

Implicit in your answer "You have increased the flow through the engine. More flow, more fuel" is that the person is driving differently than before. If you're sending more air through your engine then you're making more power, accelerating harder, and driving faster than you used to.

 

[Koz]

Re: Re: Re: Re: Re: Re: Re: Wait a while

No way? Really? 180HP doesn't seem impossible for a well set up 'busa. So you've got a 12R eh? cool. I've been considering trading in my 9R for a 12R, but I'm waiting and hoping Kawasaki will update the design. If the 2006 model is lighter and styled a little better I'll be getting one. Although with rumors of a new large displacement sport tourer, and reports of yet more updates for the 10R, I'm wondering if the 12R has much of a future.

The 12, stock, makes more HP then the busa and responds better to the common mods (filters, pipe, PC). Therefore the 12 will make more HP as long as you don't go in the motor. Allot has to do with the state of tune from the factory. The 12 has higher compression and more aggressive cams. The busa has 100cc more and produces more torque. The busa has more potential because of the extra CCs and there is allot more "go fast" aftermarket parts available. To get 180HP out of a busa you would have to change the cams. In stock form the 12 and the busa are pretty much equal, it all depends on who is riding them. The busa has the low end grunt and the 12 has the top end HP.

As far as the liter bikes; they make great track bikes but are too twitchy on the street, especially at high speed. Plus they are not very comfortable for people over 150lbs or 6' tall.

Koz

 

[Koz]

I agree with most of what you're saying. The concern I have is with the idea that anytime a person modifies their engine, they should expect to see a drop in MPG simply because their engine can flow more air / fuel. If the person keeps the same diving habits they'll just use a little less throttle to get the same result as before and the flow / fuel consumption should be about the same.

Implicit in your answer "You have increased the flow through the engine. More flow, more fuel" is that the person is driving differently than before. If you're sending more air through your engine then you're making more power, accelerating harder, and driving faster than you used to.

Here's the deal, if you putt around, yes, you will not see too much of a loss, because you are using less throttle. If you ride the same as you did before the mods there will be a percentage of lost mileage close to the percentage of increased air flow. If you only add a slip-on exhaust and don't have to re-map you may actually gain mileage. You also are not gaining the same percentage of HP. You could also re-map a stock bike and gain some efficiency. The bottom line is the more power you gain, the more fuel you will use. You have to maintain the same air/fuel ratio. You can't have your cake and eat it too, sorry!

I have a blown Taco and I can get anywhere from 21 MPG to 12, depending on my right foot.

Koz

Koz

 

[Iowa4Runner]

Here's the deal, if you putt around, yes, you will not see too much of a loss, because you are using less throttle. If you ride the same as you did before the mods there will be a percentage of lost mileage close to the percentage of increased air flow. If you only add a slip-on exhaust and don't have to re-map you may actually gain mileage. You also are not gaining the same percentage of HP. You could also re-map a stock bike and gain some efficiency. The bottom line is the more power you gain, the more fuel you will use. You have to maintain the same air/fuel ratio. You can't have your cake and eat it too, sorry!

I have a blown Taco and I can get anywhere from 21 MPG to 12, depending on my right foot.

Koz

Koz

You've still got some issues here. For one, if you're flowing more fuel / air as you ride, then you're not riding the same as before. I can have my cake and eat it too, just not simultaneously.

 

[Koz]

You've still got some issues here. For one, if you're flowing more fuel / air as you ride, then you're not riding the same as before. I can have my cake and eat it too, just not simultaneously.

Once you change/increase the flow of air through the engine by changing the intake and exhaust the engine WILL require more fuel, even at Idle. You may even have to raise the idle speed. This has nothing to do with your riding style.

Koz

 

[M6HTZ]

I know the ECU will tend to run pig rich to be on the safe side. I can see how mileage would be decreased by change in air flow if the computer dumped extra fuel as to not run too lean, but that should be short lived. I still have a hard time with less restriction decreasing fuel mileage.

It seems to me like the engine would have to work harder, burn more fuel, to push the exhaust gasses out. I guess I should put a restrictive air filter in and a restrictive exhaust and enjoy the increased fuel economy.

 

[Iowa4Runner]

Once you change/increase the flow of air through the engine by changing the intake and exhaust the engine WILL require more fuel, even at Idle. You may even have to raise the idle speed. This has nothing to do with your riding style.

Koz

Why would you have to raise the idle speed (increase butterfly valve opening) if the engine was flowing better? Wouldn't it be the other way around? The reason the idle is low is because the fuel/air ratio has been altered.

Riding style (throttle operation) has everything to do with the flow though your engine. The fact that it can now flow more air doesn't mean it has to. The throttle has an amazing ability to choke off the flow into an engine.

If 50 CFM of air/fuel mixture was enough to propel you down the road at 60 mph before the mods, then 50 CFM should still do the after. That is unless you've done something to decrease your engine's efficiency. The difference is, to get that 50 CFM now you may not have to open the throttle quite as much as you did in the past.

Your flow mods simply changed the airflow vs. throttle opening relationship.

 

[Koz]

Why would you have to raise the idle speed (increase butterfly valve opening) if the engine was flowing better?

Sometimes the added flow (less back pressure) can play tricks on the ECU (when idling), especially with aggressive cams. It can cause looping/surging; this is when the ECU compensates for less back pressure/more flow and as soon as it detects the adjustment it then re-compensates. (looping) Raising the idle will help smooth it out. My bike idles at approx 1000 to 1100RPM. Without raising the idle, it would loop from 700/800 to 1100 RPM. It actually sounds pretty cool but makes it a PITA when putting around (parking, maneuvering at very low speeds).

Koz

 

[Iowa4Runner]

Sometimes the added flow (less back pressure) can play tricks on the ECU (when idling), especially with aggressive cams. It can cause looping/surging; this is when the ECU compensates for less back pressure/more flow and as soon as it detects the adjustment it then re-compensates. (looping) Raising the idle will help smooth it out. My bike idles at approx 1000 to 1100RPM. Without raising the idle, it would loop from 700/800 to 1100 RPM. It actually sounds pretty cool but makes it a PITA when putting around (parking, maneuvering at very low speeds).

Koz

Yikes. I'd imagine making tight turns would be hairy in a situation like that.