Is this an interference engine? - BMW Luxury Touring Community
 
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post #1 of 17 Old Mar 7th, 2012, 10:23 pm Thread Starter
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Is this an interference engine?

It's been long enough that, well, I feel comfortable talking about it now.

A few weeks into ownership, I was just starting to get comfortable with the LT and figured I'd 'give er the beans'. WOT in 3rd, Preparing to grab 4th when my stupid learned reflexes from the last bike had me select 2nd*. Lord only knows how high it revved, but it didn't do it for very long. I'm not even sure I let the clutch fully out before catching it.

Didn't seem to cause any lasting damage as I've put another 1000 miles on it since then and it runs like a champ.

So...what happens when you accidently overrev a Brickmotor?

*= last bike had a heel and toe shifter...brain said 'Grab another gear', foot said 'stomp'.

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post #2 of 17 Old Mar 7th, 2012, 10:33 pm
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Re: Is this an interference engine?

Wouldn't it just hit the rev limiter and start cutting out?

Howard Schisler
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post #3 of 17 Old Mar 7th, 2012, 10:37 pm
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Re: Is this an interference engine?

Quote:
Originally Posted by hschisler
Wouldn't it just hit the rev limiter and start cutting out?
No rev limiter will work on a mechanical over rev.

Dave Selvig
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post #4 of 17 Old Mar 7th, 2012, 10:43 pm
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Re: Is this an interference engine?

Quote:
Originally Posted by saddleman
No rev limiter will work on a mechanical over rev.
OK. Then that means I didn't understand the original question, or what a mechanical over-rev is. Thanks, Dave.

Howard Schisler
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post #5 of 17 Old Mar 8th, 2012, 12:20 am
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Re: Is this an interference engine?

A mechanical over rev occurs when you down shift and the revs exceed the rated rpm. This can happen closed throttle. The rev's are all driven by the rear wheel through the gear box into the engine. No amount of fuel reduction will change the rpm's. The rev limiter protects against the riders right wrist.

Isn't this what a slipper clutch protects against?

Loren

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post #6 of 17 Old Mar 8th, 2012, 5:07 am
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Re: Is this an interference engine?

To answer the thread title, the valves will hit the pistons if it jumps time.
I don't know of any 4 valve head that won't, they may be out there but I have not seen one.

I doubt you would have over rev'd it, the rear wheel would have locked before that happened. I believe you are correct in thinking the clutch was not fully engaged.

Last edited by New2rt; Mar 8th, 2012 at 5:13 am.
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post #7 of 17 Old Mar 8th, 2012, 5:24 am
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Re: Is this an interference engine?

Quote:
Originally Posted by Rocketsled
So...what happens when you accidently overrev a Brickmotor?
You hit the rev limiter. Nothing bad happens.

To answer the question in your post title:

My understanding is the term "interference engine" refers to what happens when the valve timing is lost due to breakage of a timing chain or belt. In a non-interfering engine, the engine stops running but no internal damage is likely to occur.

In an interference engine, when the timing chain or belt is broken (or removed) rotation of the crankshaft will cause a piston to hit an open valve. This results in bending of the valve, breakage of the valve, cracking or holing of the piston. The "interference" is between valves and pistons.

My understanding is that the K1200LT "brick" engine is an interfering engine. And the significance of this is when adjusting the valves. With the cam chain removed but the camshafts still in place, you don't want to rotate the crank or you risk piston and/or valve damage. This is why the BMW Service Manual states to position cylinder #1 at half way before TDC before starting the valve adjustment procedure.

The term "mechanical over rev" is a new one to me. I understand the concept as described. It seems unlikely to me that damage to the brick engine would occur from the bike's momentum over reving the motor. I suspect the rear tire would break traction first, but I'm just speculating.
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post #8 of 17 Old Mar 8th, 2012, 6:51 am Thread Starter
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Re: Is this an interference engine?

Wow, serves me right for going to bed! Thanks for the replies guys!

(this is all theory, learned from different motors)
If you're near redline in 3rd gear, the motor is spinning at 7500-8000 rpm. If you then select 2nd, the forward motion of the motorcycle if all factors allow will attempt to spin the motor at a much higher rpm, regardless of whether fuel is being delivered to the motor or not...I'd have to find some numbers and do some math...but it wouldn't be double redline in 3rd gear....say 12,000 rpm or so...again, assuming the wheel maintains traction, the clutch is fully out, and the motor can overcome the deceleration due to engine breaking.

Motors have a redline for a number of reasons, in some designs, the valves float, in others weaknesses in the valvetrain can cause the motor to lose it's timing...if the latter happens, bad things can occur. Floating valves occur when the mass of the valve cannot be overcome by the valve spring and the valve fails to close in time. In that case, it's _possible_ for the valve and piston to be in the same place at the same time. If the valvetrain loses it's timing by breaking a belt or jumping a tooth, the same thing can happen. It's why Ducatis are desmodromic, they pull the valve UP as well as pushing the valve DOWN.


In a big meaty stock V-8 (without really expensive reciprocating parts), going much past redline can be a very bad thing...But it's got great big slugs of metal for pistons, and it's redline is up close to the mechanical limit of it's parts.

I had a dentist once who picked the wrong gear in his porsche...944...with a timing belt. Very expensive lesson.

Since the K motor is a smaller, lighter, motor, with direct application of the valves, and a timing chain...and if the clutch was not fully locked up...then the motor might have seen 9000-10,000 rpm. Death to a big bore motor, but I guess not to this one, based in-part on the overdesign of the motor. (Knocking on wood) Needless to say, I wouldn't want to do it again.

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post #9 of 17 Old Mar 8th, 2012, 7:08 am
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Re: Is this an interference engine?

If you shift in to the wrong gear as you said in 2nd instead of the 4th and dont release the clutch just let her slip you wont have any damage done to the engine even if the rev's came up quite high. If you would have done some damage to the engine you would have no running engine anymore. It will stop working and stall immediately.

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post #10 of 17 Old Mar 8th, 2012, 7:23 am Thread Starter
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Re: Is this an interference engine?

Quote:
Originally Posted by mreuter
If you shift in to the wrong gear as you said in 2nd instead of the 4th and dont release the clutch just let her slip you wont have any damage done to the engine even if the rev's came up quite high. If you would have done some damage to the engine you would have no running engine anymore. It will stop working and stall immediately.

Manfred
Yeah, I'm fairly sure I exceeded redline, but I have no idea by how much, and the motor ran fine after that.

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post #11 of 17 Old Mar 8th, 2012, 9:42 am
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Re: Is this an interference engine?

Quote:
Originally Posted by Rocketsled
Yeah, I'm fairly sure I exceeded redline, but I have no idea by how much, and the motor ran fine after that.
I think each gear is under 1K per shift, have to check tach again, a friend of mine accidently put his Porche 924 in 1st at 100kph/60mph dropped the clutch and broke a motor mount which they fixed and apologised for it happening

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post #12 of 17 Old Mar 8th, 2012, 11:13 am
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Re: Is this an interference engine?

Quote:
Originally Posted by New2rt
I don't know of any 4 valve head that won't, they may be out there but I have not seen one.
The Toyota 22RE is a non-invasive engine. It only has 2 valves per cylinder but the dynamics of poping a timing chain at 70 mph did cause 5 valves to bend ever so slightly so as to not fully close. No piston damage. So even a non-invasive engine is vulnerable.

I guess the reason 4 valve designs are invasive is that the point of 4 valves is better breathing on a high compression cylinder.

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post #13 of 17 Old Mar 8th, 2012, 11:17 am
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Re: Is this an interference engine?

I'd guess (and I am guessing) that the valve won't start to float until around 12,000 rpm. I think it'd be pretty tough to mechanically over-rev the K1200.

I have a '78 kz1000 that doesn't have a rev limiter (other than valve float and the damage that often follows) Redline is 9000, and I've seen the needle past 12000 on a few shifts that was initiated at about 10,000 and found a false neutral.


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post #14 of 17 Old Mar 8th, 2012, 11:18 am
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Re: Is this an interference engine?

Of course, that bike is a two-valve design, so there could be more clearance than on a 4-valve....


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post #15 of 17 Old Mar 8th, 2012, 11:21 am Thread Starter
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Re: Is this an interference engine?

Quote:
Originally Posted by JoeTerry
I'd guess (and I am guessing) that the valve won't start to float until around 12,000 rpm. I think it'd be pretty tough to mechanically over-rev the K1200.

I have a '78 kz1000 that doesn't have a rev limiter (other than valve float and the damage that often follows) Redline is 9000, and I've seen the needle past 12000 on a few shifts that was initiated at about 10,000 and found a false neutral.
That always amazed me with motorcycles...Redline? What redline? If you need one your testicles are too big.

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post #16 of 17 Old Mar 8th, 2012, 11:27 am
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Re: Is this an interference engine?

Just don't do it again Rocket S. You'll sleep better at night for it.

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post #17 of 17 Old Mar 8th, 2012, 2:03 pm
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Re: Is this an interference engine?

Once upon a time I was in a 1965 Galaxie 500. The driver thought he was in 2nd gear and popped the shifter up two slots to rev the engine up in neutral. Turns out he was in drive when he started and hit reverse while flooring the gas at 45 MPH. WOW!!!!!!! Turned the rear tires backwards at 45 MPH. Newer cars won't allow the transmission to engage reverse at that speed. Probably a good thing based upon my recollections of that day.


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