There She Blows

[bob58o]

If I’m going to blow it up, I should probably sitting on top of it.

 

[65ShelbyClone]

Ignore my comment about blower rotation. I got mixed-up looking at the blower being upside-down and backwards. Original post edited. Carry on.

I can't explain how he's getting 15psi with that combination of parts.

Is this a reasonable location to monitor vacuum / boost ?

Somewhere between the blower and the intake port ought to be fine.

 

[bob58o]

Ignore my comment about blower rotation. I got mixed-up looking at the blower being upside-down and backwards. Original post edited. Carry on.

I can't explain how he's getting 15psi with that combination of parts.


Somewhere between the blower and the intake port ought to be fine.

I was hoping the spigot of the blower because there is a bit of meat there to drill and tap. I was just wondering if the pressure in the 2.5” spigot is equal to the pressure n the 1” port.

I guess it has to be equal?
Fluid /thermodynamics makes my brain hurt.

Maybe pressure is equal in the 2.5” tube and the 1” tube because the air velocity speeds up with the smaller cross-sectional area of the 1” tube?

It’s either time for bed, or more coffee. I’m tired and can’t think straight. Maybe my gabapentin is showing.

 

[bob58o]

I wonder if I could also put something like this in the spigot of the compressed side? Think it’d work?

 

[Denny]

Since it is a 440cc engine your cylinder volume should be 440cc per revolution.

 

[65ShelbyClone]

A four-cycle engine has one intake stroke every two revolutions though.

 

[Denny]

Still 440ccs

 

[65ShelbyClone]

Right, so ½ of engine displacement per revolution which is 220cc.

 

[Denny]

 

[bob58o]

The crank shaft turns twice. During the intake stroke of the 4-stroke cycle the engine breathes 440cc of air.

440cc of air, but the crank spun twice.


440cc / 2 revolutions =
220cc / revolution

One revolution it gulps 440cc but the next it gulps nothing,

It averages to 220cc per rev.

I chose coffee.

 

[bob58o]

So if the crank spins twice (720*)

And the intake valve is open for around 220* (back to my engine)

720-200 = 500* of crankshaft rotation with the intake valve closed.

500/360 = 1.39 crankshaft revolutions with the intake valve closed (or close to closed, 220 duration is probably 0.050” lift, I don’t remember)

The 300cc blower is under driven (my engine)

so 1.39 crank revolutions is 0.855 blower revolutions.

300 cc per blower revolution =
256 cc per 0.855 blower rev

So maybe this means while the intake valve is closed, the blower turns enough to push 256cc into the intake manifold ready for the intake to open again?

 

[65ShelbyClone]

We usually assume the intake stroke pulls in the engine's whole displacement for simplicity equating to 100% volumetric efficiency. In reality the VE of a mower engine is going to be quite a bit lower than that, maybe 70-75%. That may be where some "bonus" boost came from.

 

[bob58o]

Bonus Boost sounds like a button I want somewhere on the dash board. I think I’m going to try installing that 15psi pop off valve. I’m not really sure how it works. Will it will beed off boost how I want, or will it act like an air leak and make me run lean on deceleration if I close the throttle after it pops?

Will it stay popped, or does the spring push it back once pressure is reduced?

I drilled out a few more jets to give a couple more options.

These are the sizes of my cheap jets measured with cheap tools and drilled by cheap bits.

I prefer to just use size in mm to prevent confusion in my head. I’d rather call my 1.60 mm jet a 160 rather than a 150 (which is how it’s labeled).

140 jet
#54 bit
1.40 mm

145 jet
#53 bit
1.50 mm

150 jet
#52 bit
1.60mm

#51 bit
1.70 mm

#52 bit
1.80 mm

 

[Denny]

Explosion valve yes, blow off valve no! Blowers do not continue to build boost when the throttle valve is closed. Turbochargers do! Besides you do not want a bunch of fuel vapors floating around the air if that thing decides to open. Unless you like being on fire?

 

[bob58o]

Explosion valve yes, blow off valve no! Blowers do not continue to build boost when the throttle valve is closed. Turbochargers do! Besides you do not want a bunch of fuel vapors floating around the air if that thing decides to open. Unless you like being on fire?

I was thinking if it made 15 psi, it would pop, I would then let off the throttle. I’m not worried about building boost at that point. I was thinking about air leak after the carb. Only fuel from the idle circuit, but then extra air coming in from the leak (popped valve). Or maybe the blower just pushes mixture out the leak and not in. I get confused thinking about the pumping of the thumper mixed with constant turning of the blower. Single cylinder boosted engineers aren’t that common.

I believe you mentioned something before. Maybe a burst plate or something? Like a disc of foil?

 

[bob58o]

I believe this line of thinking is how I got to the design I had before with the blow off valve being connected back to the piping between the carb and the blower to be used as a recirculating bypass valve.

I wanted the valve in the other design to be controlled by vacuum to somewhat operate with throttle position.

But now I guess I see how it would be used with boost pressure instead of vacuum.

Im not going back to that design now, so I guess there is no boost control.

 

[ezcome-ezgo]

I had forgotten was GAB is like. Thanks for the reminder.

 

[Denny]

With a blower you do not need boost control, that’s what different sized pulleys are for. What you do need is a burst panel in case of backfire so you don’t end up wearing the blower. You keep confusing blowers with turbochargers.

 

[bob58o]

With a blower you do not need boost control, that’s what different sized pulleys are for. What you do need is a burst panel in case of backfire so you don’t end up wearing the blower. You keep confusing blowers with turbochargers.

I’m not confusing superchargers with turbochargers. I don’t know about either. Lol

The heart of GAB is learning through brute force, trauma, and repetition.

If I don’t feel like I have a good understanding of something, I keep banging my head against it until it fits within the framework of my brain.

 

[bob58o]

So what’s the difference between a rupture disc and a spring operated pop off valve?

I assume the disc is made for higher pressures and probably has a larger hole to allow an explosion to escape quickly.

I can’t see the difference between it being spring operated or a foil that bursts.

Isn’t a blowoff valve just a better version of a foil disc. Except a spring valve doesn’t leave a huge hole in the intake that needs to be replaced?