"high compression motor has pressure release valves"

[HardDrive]

Good thing too. My experiments with an 'intake valve only' system (I was trying to cut down on emissions) didn't pan out.

2002 custom softail chopper

[Aurel]

Bwahahahaha! ...what else can I say?

[Noah930]

Isn't that what you get when your high compression motor puts a rod through the side of the case?

[A930Rocket]

I know it sounds goofy, but I had a Yamaha dirt bike or such that had a pressure release valve, that was operated by a thumb lever or such, that made the bike easier to start. Once started, you let go, and off you went.

[Laneco]

I've always referred to a rod through the block as "custom ventilation."

Regarding the motorbike, is it an air rear suspension bike? That might explain the "release valves."

angela

[wdfifteen]

Originally Posted by A930Rocket

I know it sounds goofy, but I had a Yamaha dirt bike or such that had a pressure release valve, that was operated by a thumb lever or such, that made the bike easier to start. Once started, you let go, and off you went.

It's a common way of making human powered starting easier. I don't know anything about Harleys, so I don't know why its so funny that this engine would have it.
There were times when I wished my Norton had it. Damn thing would kick back and try to throw me over the handlebars.

[dhoward]

Compression release. Common in high compression motors.
The Yamaha 360 enduro was a single-jug bike that had the bar-mounted release.

[bell]

my honda 350x/blaster had a pressure relieve valve.....it was operated by a cable attached to the kick start for easier starts.....it is simply a lever which bumps open the exhaust valves so it'll start easier.....
so yes.....LOL

[HardDrive]

I guess the real story here is my lack of mechanical knowledge. Looks like a common feature on larger displacement bikes that are kick started. You learn something new everyday.....

[EarlyPorsche]

Originally Posted by HardDrive

I guess the real story here is my lack of mechanical knowledge. Looks like a common feature on larger displacement bikes that are kick started. You learn something new everyday.....

Don't be too hard on yourself. Its not just for high-compression kick started but mainly high compression in ratio to what you use to turn the engine over. Lots of small diesels have compression release simply because the starter size is smaller. The big CC motor cycles have them even without high compression sometimes because they have to turn an engine and a primary (and sometimes a goofy 6" wide belt) with a tiny motorcycle starter and battery. Your 748S engine is high compression but doesn't need compression release most likely because its high compression with only 748cc's instead of high compression with 2100cc's.

[jwasbury]

Originally Posted by EarlyPorsche

Lots of small diesels have compression release

Yup.

My bro's 1964 Cheoy Lee sailboat is sporting the original Volvo MD-1, 1 cylinder diesel engine. We call it the one-lung'r.

The compression release is nothing more than a lever on the valve cover that forces one of the valves to remain open. You press the starter button, wait a few moments for the starter (doubles duty as the generator) to spin up the massive flywheel, flip down the compression release and she chug, chug chugs to life.

[Jeff Higgins]

Big inch Harleys get that way primarily from extremely long strokes. Try 4 3/4" or 5" strokes on a two cylinder motor with 11:1 compression. You just can't mount a big enough starter, or a big enough battery to drive that starter on a motorcycle, even if it is a Harley. No human alive could ever kick one over. Hence the compression release.

You guys who love to knock Harleys should try riding a big inch stroker Harley sometime, like something in the 120-140 cubic inch range. Granted, they don't live long at those displacements, but OMFG, do they have torque. And a sound and feel you will simply never experience anywhere else. When you whack that throttle open, one hell of a lot more than acceleration starts to happen - the whole damn bike shakes so hard it blurs your vision, it's damn near impossible to hang onto the grips or keep your feet on the pegs, much less your ass in the seat. And the noise will make your ears bleed...

I've ridden a couple that were honest-to-god superbike fast, where their owners claim 10 second quarter miles. Let me tell you, they accomplish that in a whole different way. Nothing the least bit refined or genteel about it. There is just so much noise, vibration, and all around commotion, plus the unsettling suspicion that something might just fly out of that rear jug at any second. You know, the one aimed right at your crotch.

[Hugh R]

Originally Posted by A930Rocket

I know it sounds goofy, but I had a Yamaha dirt bike or such that had a pressure release valve, that was operated by a thumb lever or such, that made the bike easier to start. Once started, you let go, and off you went.

My 1930 Rudge Whitworth motorcycle also has a compression release because it hasn't got a kick start, you pull in the compression release to slightly open the exhaust valves and roll it down a hill to get everything turning you dump the compression release and off it goes. Am I missing something?

[john70t]

A pressure release device would be a good application for cruising on just a few cylinders for good milage, or a hybrid start sequence.

[Hugh R]

Maybe, provided you can turn off the fuel injection to those "off" cylinders. I would imagine it would run rough. I think Caddie made something like that in the 90s and it was a disaster.

[Porsche-O-Phile]

Yeah, "D-O-D" (Displacement on Demand"). I wouldn't touch one of those cars with someone else's 10-foot-pole.

[yetibone]

My DT 360 had one, my XL 250 had one, and my DR 350 had one.

I don't know why a DT 360 had one though, being a two stroke with a long kick start lever and all.

[asphaltgambler]

My old school pro street bike = 93 CI - 3.5" bore X 4 13/16" stroke - 13:2 compression. I hve a manual coarse-thread bolt with a wing nut threaded into the rocker box on the front cylinder that just holds the exhaust valve open. I hit the starter, get the engine whirling over (if I'm lucky) pull the choke once, then open kill switch. If all goes as planned - which it sometimes does all H3ll breaks loose and I back off the threaded bolt by hand - then let it warm up

[wdfifteen]

Originally Posted by jwasbury

The compression release is nothing more than a lever on the valve cover that forces one of the valves to remain open. You press the starter button, wait a few moments for the starter (doubles duty as the generator) to spin up the massive flywheel, flip down the compression release and she chug, chug chugs to life.

The compression releases on my 1930 John Deere are two petcocks, one in each cylinder, that open into the cylinder about 3 inches from the top of the head. As the piston moves forward (it's a horizontal engine), the cylinder contents go out the petcocks until the piston covers the hole, the remaining contents are compressed or go out the exhaust valves if it's on the exhaust stroke. As you can imagine, once the engine fires, flames shoot out the petcocks several feet. John Deere decided that it was a good idea to have them point down at a 45 degree angle, so it's fairly common to set your field on fire when starting your tractor.

[john70t]

Originally Posted by Hugh R

Maybe, provided you can turn off the fuel injection to those "off" cylinders. I would imagine it would run rough. I think Caddie made something like that in the 90s and it was a disaster.

I think the caddy engine only killed the fuel injection on cylinders, and left it dead but still compressing/consuming power.

A free-riding open cylinder with an individual trottle should only remove some bangs per rotation from the flywheel, I'm guessing.