Intake Resonator

[YetiWoodz]

I don't want to make another thread for this question. How do high compression heads add power, like how are they different than regular heads? Do I have to shave down the cylinder for it to work? And what's the best kind of high compression head for a flying horse 66cc or best head in general for power?

 

[Chainlube]

You're not going to gain a lot of power if you're a Punkin' Head.

 

[Chainlube]

You did mention something about the shape of your head?

 

[Chainlube]

No, I was just being an ass.

 

[YetiWoodz]

I think my new intake has air leaks. The idle rpm is weird. It idles really high, then when I turn the idle adjuster counterclockwise it doesn't really do anything, it wants to stay above 3200 rpm. and then at some point it just drops off and my engine bogs down and shuts off. I had a 44T sprocket on with my original carb. I reached 6100 peak rpm. Now with my new runtong carb, (its the same exact carb,) I have a 38T sprocket and I'm only reaching 5700 peak rpm. What should I do? I heard these carbs often have air leaks and can easily be fixed but how?

 

[Steve Best]

Yeti, I did a lot of experimentation with intake shape and boost bottles (helmholtz resonators)
Best result for me was a short intake runner that was actually a waterbottle: (sorry for bad photo)

I tried many different shapes and lengths. Used oiled open cell foam as an air filter.

The bottles are easy to cut and even shape with hot water. My carb adapter is 3/4" copper pipe and epoxy putty.
This engine was running at 8000-9500 rpm (hence the short intake) but still pulled strong off idle.
Could be clutched away from a stop and yet did 40mph with stock gears.

The boost bottle experiment was not so much of a success.
They definitely work and have been used on several Yamaha bikes and even KTM exhausts:

Notice the strange bump over the exhaust port on the right?

So, I soldered a 1/2" junction into one of our intake tracts:

Then expanded it up to 3/4" and clamped on some vinyl hose:

Another view:

And then I geared up a piston so I could vary the volume to anything I desired.

By changing the volume I could get tractor like power at certain low RPMs, but only in a narrow range, and then it took from high rpm.
After several days of tinkering I was not able to gain any true advantage over stock.

They work, sort of, but I could not gain any advantage.

 

[YetiWoodz]

Yeah, from what I researched, boost bottles work best with an expansion chamber and reed valve because those upgrades cause power dips. And when you tune the boost bottle to the rpm of the power dip, it will eliminate that power dip. The power dip is also richer than other rpms, so the boost bottle will not affect other rpms by making them leaner. Regardless of having power dips or not, the intake resonator improves gas mileage and quiets the intake noise. If tuned to around idle rpm, you'll be able to keep your engine running without holding in the clutch when stopped. I'm not sure if an engine without power dips can have an advantage speed wise with a boost bottle because it makes other rpms leaner. Maybe have the engine run a little on the richer side? Idk. The band is 2000 rpm btw. The farther it gets from the middle, the less it does.
Source

 

[YetiWoodz]

Correction- the boost bottle makes the 2000rpm band leaner, not the other rpms. Not sure if this is even true though. The boost bottle operates after the carb where the gasoline and air are already mixed. I don't know why it would be leaner

 

[FurryOnTheInside]

Correction- the boost bottle makes the 2000rpm band leaner, not the other rpms. Not sure if this is even true though. The boost bottle operates after the carb where the gasoline and air are already mixed. I don't know why it would be leaner

Compressibility of air, versus gasoline and oil?

 

[YetiWoodz]

Yeah I think you're probably right