New bikes have less compression braking?


19 replies to this topic
  • FZ1426

Posted August 08, 2005 - 08:09 AM

#1

So why do newer (than a 426) bikes have less compression braking?

  • dirtgood1

Posted August 08, 2005 - 12:03 PM

#2

The newer bikes (or a 400/426 w/the 03 cam mod) have less compression braking because they use an auto decompression exhaust cam. The auto decompession makes the bikes very easy to start, but also decreases the amount of compression braking.

  • FZ1426

Posted August 08, 2005 - 12:27 PM

#3

The newer bikes (or a 400/426 w/the 03 cam mod) have less compression braking because they use an auto decompression exhaust cam. The auto decompession makes the bikes very easy to start, but also decreases the amount of compression braking.

So these cams decrease compression during deceleration but not during acceleration? Please explain how this is possible.

  • revolucien

Posted August 08, 2005 - 12:37 PM

#4

The newer bikes (or a 400/426 w/the 03 cam mod) have less compression braking because they use an auto decompression exhaust cam. The auto decompession makes the bikes very easy to start, but also decreases the amount of compression braking.

I don't think that's right... but maybe I'm misunderstanding the concept


I have owned the '02 426 and still own an '04 450 and I don't feel that much difference... the only thing i can think of that might give a little of that feel is the heavier flywheel :D

  • the go-devil

Posted August 09, 2005 - 04:20 AM

#5

I think anybody would be able to tell if the decomp pin on the cam didn't disengage when the engines fires up. There would be some bad snap crackle pops! :D and maybe a bent valve. The decomp has nothing to do with engine braking.

  • MisterMan

Posted August 09, 2005 - 09:10 PM

#6

put an x-ring chain on and prepair to be thrown over the bars :D

  • Slowmotion426

Posted August 09, 2005 - 10:13 PM

#7

Take the sparkplug out. It'll relieve the compression.

  • ncmountainman

Posted August 10, 2005 - 07:12 AM

#8

So why do newer (than a 426) bikes have less compression braking?

the primary gear ratio's have been changing allowing the clutch to spin faster ,which in turn reduces percieved engine braking. they did a good job in reducing it in the 05's :D

  • grayracer513

Posted August 10, 2005 - 09:51 AM

#9

The newer bikes (or a 400/426 w/the 03 cam mod) have less compression braking because they use an auto decompression exhaust cam. The auto decompession makes the bikes very easy to start, but also decreases the amount of compression braking.

Bovine Scatology. The auto decomp mechanism deactivates entirely at speeds below idle, somewhere around 1000 rpm or less. It works only while at cranking speeds, and has not one thing to do with engine braking. As to the basic question of why the later ones have less compression braking, I don't know that they do. Mine has a bunch of it, and I like it like that, thank you.

  • Tubo

Posted August 10, 2005 - 10:13 AM

#10

Newer bikes are running higher compression which should translate to increased engine braking.

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  • mnmbb01

Posted August 10, 2005 - 10:26 AM

#11

Thanks Gray Racer for squashing that bit of mis-information.

  • revolucien

Posted August 10, 2005 - 10:50 AM

#12

Newer bikes are running higher compression which should translate to increased engine braking.



Actually the 426 had more compression, it was running 12.5:1 and the newer 450's are running 12.3:1 At that small of a change I don't think to many will notice a difference.

  • ncmountainman

Posted August 10, 2005 - 10:57 AM

#13

Actually the 426 had more compression, it was running 12.5:1 and the newer 450's are running 12.3:1 At that small of a change I don't think to many will notice a difference.

theres more to it than compression,they intentionally reduced it in 05 with the primary gear ratio :D my 05 has very little engine braking compared to other yz/wr's i've ridden :D

  • grayracer513

Posted August 10, 2005 - 01:24 PM

#14

But Bob, when they went higher on the primary to reduce the load on the clutch, they went lower on the rear gearing to make up for it. At any given speed, with the same overall ratio, the engine is spinning at the same RPM. So, that in itself should not effect it.

I have been told that the CRF had the wrist pin offset in the piston to lower engine braking, but whether it's true or it would accomplish that, I'm not sure. It would alter the rod angle, which would change piston acceleration, so it could, I suppose.

But here's some food for thought: We know that two-strokes have compression. Some of them have a lot of it. We also know that they don't have engine braking, in fact, some of them speed up when you shut the throttle. The same thing is true of two-cycle diesels. But four-strokes do. And what was done about this problem in big trucks and by some two-stroke motorcycle riders in the early 70's? The compression release, know to truckers as a Jacob's Brake. The reason given is that the energy that gets absorbed in compressing the air in the two-stroke cylinder while coasting is returned in the manner of a rebounding spring when the piston goes over the top. The compressed air drives the piston down, and the loss of speed is wiped out. The compression release stops this by making the engine pump air out of the combustion chamber, so that there is no residual pressure at TDC to push on the piston.

So assuming that's true, and there is no doubt that compression releases work, what's the difference in the compression stroke and the following downstroke of a two-stroke, and the the same two strokes on a four stroke? None that I can see. If a two-stroke gets a bounce from the compressed air at TDC, a four-stroke should, shouldn't it?

So what is different, then? Well, the four-stroke has two more strokes, doesn't it? And during a coast, it pumps a partial air charge out during the exhaust, and pulls a partial charge in past the closed throttle during the intake. Could it be that the "compression" braking in a four-stroke is actually pumping drag that happens during the exhaust and intake strokes? I wonder.

This, of course, doesn't really answer the original question, but I find it intriguing, myself.

  • FZ1426

Posted August 10, 2005 - 02:44 PM

#15

Thanks Gray,

I think another reason the two stroke differs is in that the exhaust port opens in the downstroke which it seems would alter the whole dynamic as compared to a four stroke. Then you can throw in different expansion chamber effects and change it more. The original question was actually in another thread, where I observed that a CRF450 seemed to glide by me into corners with (seemingly) less compression braking. My reference to the hotcam mod on my 426 spurred the magic exhaust cam declaration by some others. Thanks for the reminder on the offset rod thing, I knew that something was done on the CRF to reduce CB but had mistakenly thought they had a slipper clutch.

  • grayracer513

Posted August 10, 2005 - 03:20 PM

#16

I think another reason the two stroke differs is in that the exhaust port opens in the downstroke which it seems would alter the whole dynamic as compared to a four stroke. Then you can throw in different expansion chamber effects and change it more.

That's true, but it's open as the upstroke starts, too. In a four-stroke, the intake valve is open until nearly the same point it terms of crank degrees.

The expansion chamber operates on shockwaves generated by the exhaust port opening under power. Those shock waves are totally absent during coast, so I doubt it has much effect.

  • ncmountainman

Posted August 10, 2005 - 06:07 PM

#17

i remember reading an article that the primary drive gearing was changed specifically to reduce engine braking :D seems to me a faster spinning clutch would offer more centrifugal force much like flywheel weight,making it easier on the whole drivetrain when on and off the gas,regardless of secondary gearing?

  • CHEEZE13

Posted August 10, 2005 - 06:25 PM

#18

If you would simply not let off the gas you would not experience any effects of compression braking!

  • FZ1426

Posted August 10, 2005 - 06:49 PM

#19

If you would simply not let off the gas you would not experience any effects of compression braking!

I was wondering why someone hadn't played that yet... :D :D

  • mxaction

Posted August 10, 2005 - 06:59 PM

#20

The newer bikes (or a 400/426 w/the 03 cam mod) have less compression braking because they use an auto decompression exhaust cam. The auto decompession makes the bikes very easy to start, but also decreases the amount of compression braking.

thanks, now i understand why my friend's 400 has way more compression braking than my 250f.





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