Compression release for slowing down

[bruosb1105]

I’ve started back riding trails after many years of street motorcycles. Back when I rode trails I had an old Yamaha 360 with a compression release that I used for going down hills to control the rear tire from locking up. It worked great. Now I have a 92 XR600 with a compression release and out of old habit I would use this to slow me down going down a steep hill. Being that this is a four stroker, could this harm the engine? I’m wondering if this would burn a valve or cause and other damage. Any advise out there? Thanks ?

[BWB63]

Pulling the compression release opens one of the exhaust valve'. If you pull it while the engine is running this release the copression stroke letting the engine move faster, not slower. The compression release is for releasing the compression by opening that exhaust valve to make it easier to kick the bike over to start it. Not for being used while riding. It does make it easier to bump start down a slippery hill though.

[roadcam]

yes, we always used to use compression releases as an engine-retarder for downhills, slowing, etc..... (just like in a big truck) ... and , no, you won't hurt anything, ... when I bought my XR650L, the first thing I looked for was room to install a compression release, but, won't fit ... they did come in handy

[captb]

I’ve started back riding trails after many years of street motorcycles. Back when I rode trails I had an old Yamaha 360 with a compression release that I used for going down hills to control the rear tire from locking up. It worked great. Now I have a 92 XR600 with a compression release and out of old habit I would use this to slow me down going down a steep hill. Being that this is a four stroker, could this harm the engine? I’m wondering if this would burn a valve or cause and other damage. Any advise out there? Thanks ?

I had a 360 Yamaha then my friend bought it, he gave it a half hearted kick one day and it kicked back and broke the case, it started ok compared to the Suzuki TM400.

[qadsan]

yes, we always used to use compression releases as an engine-retarder for downhills, slowing, etc..... (just like in a big truck)...

roadcam, I remember the days of aftermarket compression release kits being the rage for four strokes in the 70's and they were commonly used for braking and they worked great, especially on downhills! The compression release kit I bought for my TT500 screwed into a hole on the head (2nd spark plug hole) and you could open this valve to slow the bike down without any issues, but the decomp mechansim on the 650r and other modern 4 strokes physically opens an exhaust valve and if you try to pull your decomp lever while the engine is running, you'll feel the lever pusling with the valve movement. It's probably not a good idea to use the decomp lever like the compression release kits of yesteryear as they operately differently, but I do remember the old style compression release and especially the farting sounds they made.

The reason the compression release worked as a brake is because it converted the resistance encountered by the piston in the compression stroke into brake pressure and it was very effective at braking. If you remember compression release kits, then you've obviously been riding a while ?

captb, you too must have been riding a while. Gosh, a TM400 ? . Those things would sprain an ankle if they kicked back . I also had a Yamaha 360MX and a Suzuki TM400 . In fact, I still have my 74 MX250A .

[roadcam]

BWB63 .... just for your information ... a Jacobs Brake (ala "Jake Brake" on a diesel) is a compression release .... the mechanism holds the exhaust valves open to use the engine as a brake, but releasing enough of the compression, so as to not blow head gaskets, bend rods, etc ..... I have 31 years of diesels behind me, I know what I'm talking about (this time anyway ? ) ... I loved the release on my old 2-strokes for downhills ..

[BWB63]

BWB63 .... just for your information ... a Jacobs Brake (ala "Jake Brake" on a diesel) is a compression release .... the mechanism holds the exhaust valves open to use the engine as a brake, but releasing enough of the compression, so as to not blow head gaskets, bend rods, etc ..... I have 31 years of diesels behind me, I know what I'm talking about (this time anyway ? ) ... I loved the release on my old 2-strokes for downhills ..

It's in the timing of the compression release. Jacobs Engine Brake is a "compression release" engine brake used by large diesel trucks, especially on steep downgrades. To understand how it works, remember that a diesel engine has much higher compression than a gasoline engine, typically 15:1. The jake brake slightly opens the exhaust valves when the piston is near top dead center (where ignition normally occurs). On the upstroke, the piston compresses the air in the cylinder to 1/15th its original volume. This creates a lot of drag on the engine. The Jacobs Engine Brake then releases the compressed air, and the energy stored in it, before it can push back on the piston during the downstroke. In addition, releasing the compression prevents any fuel in the cylinder from igniting. (Remember, diesels don't have spark plugs like gasoline engines - they rely on compression alone to ignite the fuel.) So, you've got drag on the upstroke, no power on the downstroke. This makes more compression and no power. It only release the compression for a short time to stop detination. By doing this you get full compression without combustion.

In short, the jake brake turns a power-producing engine into a power-absorbing air compressor, thus slowing the truck. The brake sits in a box over the engine. The trucker has a switch in the cab where she can choose how many cylinders to cut out; the more cylinders, the more powerful the slowing of the truck. The compression release on the XR650 is a full manuel release. It opens the exhaust valve and release all the compression letting none biuld.

[Miker]

It's in the timing of the compression release. Jacobs Engine Brake is a "compression release" engine brake used by large diesel trucks, especially on steep downgrades. To understand how it works, remember that a diesel engine has much higher compression than a gasoline engine, typically 15:1. The jake brake slightly opens the exhaust valves when the piston is near top dead center (where ignition normally occurs). On the upstroke, the piston compresses the air in the cylinder to 1/15th its original volume. This creates a lot of drag on the engine. The Jacobs Engine Brake then releases the compressed air, and the energy stored in it, before it can push back on the piston during the downstroke. In addition, releasing the compression prevents any fuel in the cylinder from igniting. (Remember, diesels don't have spark plugs like gasoline engines - they rely on compression alone to ignite the fuel.) So, you've got drag on the upstroke, no power on the downstroke. This makes more compression and no power. It only release the compression for a short time to stop detination.

In short, the jake brake turns a power-producing engine into a power-absorbing air compressor, thus slowing the truck. The brake sits in a box over the engine. The trucker has a switch in the cab where she can choose how many cylinders to cut out; the more cylinders, the more powerful the slowing of the truck. I was a state certified mechanic. The compression release on the XR650 is a full manuel release. It opens the exhaust valve and release all the compression letting none biuld.

Did you by any chance stay in a Holiday Inn last night? ?

[morkys]

Engine braking. People do it in cars. Off road on a hill, engine braking apparantly causes the tire to lock up. Makes sense.

How can you release compression on a two-stroke? They already have a measly 6.2:1 in some cases (pardon the pun).

[justicedone]

Did BWB say he was State Certified...

hmmmmmm

that explains A LOT !!!

lol

?

[my98mugen]

BWB63 .... just for your information ... a Jacobs Brake (ala "Jake Brake" on a diesel) is a compression release .... the mechanism holds the exhaust valves open to use the engine as a brake, but releasing enough of the compression, so as to not blow head gaskets, bend rods, etc ..... I have 31 years of diesels behind me, I know what I'm talking about (this time anyway ? ) ... I loved the release on my old 2-strokes for downhills ..

If you wanna slow down, you put it in first gear and let the compression build up. If you pull in the compression release, you might as well pull in the clutch.

Its not a diesel.

And its not a jake brake.

[BWB63]

Did BWB say he was State Certified...

hmmmmmm

that explains A LOT !!!

lol

? : ?

[LotsOfBikes]

I use the compression release on my modified ATK605 all the time. Frank White (CEO of ATK), thinks it is a bad idea, but it hasn't killed or effected the engine yet, after 20,000 miles. I use it frequently in conjunction with the brakes.

I've been in many diesel trucks.. They don't slow down at all when you let your foot off the gas. The compression release is quicker-acting so much less effort to use than picking your foot up off of the floor to mash down on the brakes.

Diesels are always at full throttle. May people don't know this, and have a hard time accepting this fact. The only thing that changes is how much fuel is injected (under very high pressure) into the cylinder at TDC. Diesel injection.

Try finding a throttle body on any modern diesel engine. It is usually near to the blinker fluid reservoir and the muffler bearing assembly.

GM tried to make one of their standard 350 gas V8's into a diesel back in the 1970's. Since they didn't want to put stronger crank bearings in (cheap gomers), they installed a throttle body so that there wouldn't be so much stressful compression all of the time... It didn't work well at all. The engines ran horribly, and still ruined their crank bearings. Losers! They wondered why people bought Japanese ?

[BWB63]

Well we are getting closer to putting this thing to rest. Like it was said, a diesel will always fire on the compression stroke and the intake does not close. The from idle to full rpm is only about 2500rpm. Take your foot off the peddle and you are still under power. Just a lot less. The compression release is turning off and on at just the right time to stop the compression stroke from firing and still leave the full compression of that stroke. That is why you hear the hammering sound when you turn the Jacob's brake switch on in the truck and let your foot of the peddle. Try turning the key off on the truck and see how fast it slows down.

On our Four stroke engines when you pull the compression release you are releasing all the compression for the full stroke. You are allowing the engine to move un-restricked by the compression when the compression lever is pulled.

They are not even close to the samething. One is a timed mechanism the other is a full manual release. On the diesel it slows the engine down, on the gas engine the manual release lets the engine free wheel.

[miguelitro]

no se frene con motor

[LotsOfBikes]

I've turned the key off on a dodge 3500 Cummins manual while falling into California on I-80. It felt no different. It felt the same as when the ignition was on. Compression helps to push the piston down as well. It sort of cancelles out the upward compression strokes on any other cylinders. The compresson energy just doesn't go away, even though there may be no fuel injected at TDC. The only energy that is lost is due to heat, blowby and normal engine drag from friction.

When I pull the compression release on either one of my ATK605's (one is bone-stock), or when I did it on my XR400, XR600, or a buddy's YZ400, the engines produce a significant slowing of the rear wheel. With the trottle chopped, pulling the compression release in every single four stroke I have ever ridden, increased the drag that the engines had on the rear wheel. Every time, without exception.

Go try it. Then pin the throttle WO and pull the compression release. You will notice a difference there too. It may backfire. When going downhill on my modified 605, pulling the compression release lever will almost put you over the bars if you aren't prepared.

In any case, in my experiences, I have never had an engine problem related to using the compression release. I've brutally beat the 'fornicatin' out of every dirtbike I've owned. I've raced every one of them hard.

So Compression releases, desert races, monkey punches, verbal abuse, physical abuse, filfthy Sanches's, taco salads, Cleveland steamers, Alaskan Pipelines: my bikes have done it all. They all still run. ?