Carb jet changes and fuel efficiency

[pac00t]

Hi, will increasing air flow and upping jet sizes drop your mileage? Because even if there's more possible flow due to the increases, won't you still get the same mileage at lower RPM's because the throttle plate is limiting everything?

[prometheus578]

Yup.
Increasing air flow leans the mixture, which would give better fuel economy, but will run the engine hot, so one must use larger jets to get the engine to run cooler, and then the fuel economy goes down.
"What... you want it both ways?"
Yes, at lower RPM the throttle plate controls... almost everything, but not quite.
If you're so inclined... run the engine with the air filters off. Look into the carbs(being careful that you don't get an eyeful of backfire)
It doesn't take much twisting of the throttle before you can see the mainjet needle start to shake and rise a little and fuel start spraying up out of the main jet hole.
And how often are you actually at "low RPM'? This is an XS, after all!

[Douglas Money]

About 30 years ago, Smokey Yunick was working on this same problem. A normal gasoline racing engine with all of its efficiency, burns 2500 HP of fuel to produce just 500HP. He was working with internally ceramic coated engines so they could stand the heat and try to raise the efficiency. Last I heard it could withstand the heat but not the vibration.

[Dennyz]

Yep. My 'F' has all sorts of things done to it to increase air flow with jetting to match. I get the exact same fuel milage as my dad's stock 'F'. IF, I drive it as he did.

[HobbyMan]

The only way to get both efficiency and power would to dump the carbs and go fuel injected. Hey, I had a couple grand sitting around, I would put something like a FJR1300 throttle body if it would fit. That with a high tech controll unit that adjusted the spark and fuel would deliver much more power and increase your fuel economy. Since most of are stuck with carbs, when you up the jet and air flow, you are getting more power but your fuel economy will be different. I don't know by how much though.

[Snow]

Even the world's most efficient internal combustion engine will never get anywhere near 1/2 efficiency.

[HobbyMan]

This is very true

[trbig]

"You know something, You can't polish a turd"

YES you can!!!!

[HobbyMan]

I know it ain't exactly true. In fact, I have done some turd polishing myself. You gotta love "Christine" though. And yes, it does look very cool

[Ljay40]

Nice looking motor trbig!!

[Crazcnuk]

"Even the world's most efficient internal combustion engine will never get anywhere near 1/2 efficiency."

This isn't exactly true. Most internal combustion engines are somewhere around 50-60% efficient, but not all.

In 1954 the engineer that designed the small block Chevy V-8 stated that any internal combustion gasoline engine SHOULD produce approximately 1hp/cuin. That is around 60% efficient.
He was, of course talking about mass produced standard engines.

If you look at the XS1100 it's closer to 1.5hp/cu in. Far more efficient than what he was talking about.

Efficiency is the ability for an engine to USE all the hp potential of whatever fuel it is using. In our modern engines a huge loss is in heat. Heat is in effect the measure of ineffiency.

When you start to 'hop up' an engine, you will get an increase in both HP and fuel economy. This is because the first part is simply 'blueprinting' or getting the design to work as it was supposed to. Up until this point you don't have to add any additional fuel to get the same work, in fact as the power is going up, you can regear to compensate, and run the engine at lower rpms for a given task.

Once you get to 90% efficiency or better, you now have to start adding more fuel to keep the increase in HP going. You have now entered into 'hotrodding'. By using Superchargers, Turbochargers or NOS, you can actually get engines well over 100% volumetric efficiency. However, fuel mileage wil usually suffer, although it depends on how you drive and what you are doing with your engine.

In north america we have become used to LARGE engines, with relatively small vehicles. We don't think a truck should come with any engine under 350cuin. A 350 is actually far larger than your average pickup should need, IF the factories built them with efficiency in mind. When our factories have been selling us the 'All new 235hp.... engine' for over 50 years, and it's usually 350cuin or bigger.

My 502cu in GM performance crate engine is rated at 502hp. It is NOT hopped up. In fact, GM sells it as a truck replacment motor, not a hot rod motor. With a change in camshaft it will put out over 640hp. B&M makes a bolt-on blower kit that advertises 840hp, with no other changes to the engine.

If the average p/u driver believes that 250hp is decent in a truck, then really he should be looking at a 265cuin (4.3L) v-8 or V6 that is built to produce 250+ hp. This should give you good driveability and much better fuel economy than we currently expect from a pickup. You could easily get the 250hp from an even smaller engine than this, using a super or turbocharger.

Back to the XS1100. Being already at 1.5hp/cu in, this engine is fairly efficient to start with. So, anything you do to get better power is going to be hotrodding. You could probably increase fuel mileage by adding a turbo, if you then regear to keep the rpms as low as possible while cruising, but crack the throttle and mileage will drop.

We increased the mileage in a friend's 1991 Dakota 4x4 from 14mpg (highway) to over 25mpg with an engine rebuild and camshaft change, coupled with regearing his diffs.

If I had the resources I would love to do nothing but research this further.

[pggg]

wow that bike/motor would be worth mint down here, a bikeshop would be wanting 4 or 5 grand at least, if it was mine I'd fill the yamaha nameplate with bog and paint it black, just the nameplate that is.

[HobbyMan]

I think that we are just getting at the inevitability of the fact that soo much energy is lost through heat and internal friction forces. In an ideal world, physically speaking, all engines would be 100% efficient. Actually, on straight roads, we would just need to accelerate ourselves and shut the engine off.(providing no elevation change or turns)

[Crazcnuk]

Most the energy is lost because the manufacturers like it that way.

The only reason we have to have all the emission controls on our cars is because there is so much unburnt or partially burnt fuel coming out the exhaust pipe.

A lot of hot rods out there pass emission testing w/o any emission controls, simply because the owners have made them far more efficient than the factory. Not because the factory couldn't, but because they don't want to.

How many shares in the oil industry do the car makers have, do you think?

The reason thatmost motorcycles don't have or need emission controls, is because they are usually way more efficient than car engines, but this is changing now with really big bike engines, because a big engine does not have to be efficient to pull a small vehicle around.

[HobbyMan]

Actually, there are even more efficient carb designs out there. The problem is that they are owned by the oil companies. I find it interesting that my 86 Suburban with 350 (4brrl carb) has cleaner emissions than most new cars. Yea, the vavle guids are worn so there is a little smoke on start up, but what is a little burnt oil? Every time we get it inspected, they ask where we get our gasoline from. It is one of the cleanest running engines around our area.