Valve /Piston interference vs. non interference engines in Motorcycle engines

[Bob Nixon]

I was following a thread over in Reeky that posed this question in
terms of MC engines. The thread got all convoluted as they usually do
but it has been my experience that non-interference engines (cages)
are the general rule "IF" the engine has a rubber belted cam drive and
engines with a cam chains were more prone to have interference engines
like most high performance Motorcycle engines save Ducati.

I had an old 84 MB 300TD with ~300,000 miles and the cam chain that
was severely stretched causing a general gutlessness due to late valve
timing and was told by my the local MB non factory mechanic that this
was common with that many miles on the engine. He also said that if
I'd let it go much further that the valves would start hitting the
pistons. Now gasoline powered cage engines rarely see 300,000 miles
without something else going south so I wondered if at lower CR's if
typical cage engines with ~4" bore's had mostly non interference
engines these days. I also had a 1991 Toyota with DOHC & 10:1 CR with
a belt drive that was advertised as a non-interference engine. IOW,
even with the valves open 1/2" it wouldn't hit the piston. RL on this
under square engine was just over 6000RPM so I doubt the cam was very
radical with 4 valves/ cylinder & only 75HP/liter. What about most
sport bikes specifically the 600SS's & rubber belted Ducati's and the
former with over 15,000RPM RL's? Interference or not???

TIA
Bob Nixon..

[BryanUT]

I always thought Ducs were interference engines, a real concern.

Also I am of the belief that regardless of belt or chain, some engines
are interfernce and some aren't.

Thank goodness I've never had experienced the results of throwing a
belt/chain on an interference engine.

[phil scott]

you need the valve timing overlap, to enable and work with the tuned
intake and exhaust,, as you know but some do not, it is the
compression/ vacuum shock waves generated by the tuned exhaust and
intake tracts that create a natural super charging effect and longer
and more efficient intake and exhaust cycles.. the only way to max HP
per liter.. high rpm is the blood brother of these, producing more
power strokes per second,

Phil scott

[Bob Nixon]

I figured either you, Krusty or both would chime in on this one sonner
or later. Obviously a two valve engine with >300 degrees valve
duration would be the hardest to be a candidate for a non interference
engine. Four or five valve engines may do the job of less lift easier
but a single or a twin would have the hardest time no matter what.
Ducati's have an ace in the hole because their desmo systems can
control the ramps better with lower overall peak valve opening.On your
comment about natural intake and exhaust super charging in the four
stroke it's tougher than two strokes with a limit of not more than
120% VE (two strokes are =>175%VE) and that's with at least a four
cylinder collector. intake ramming is a tougher game with the angles
being straight in to the intake valve and a well design plenum or air-
box + of course optimal opening and closing of the valves. But this
would definitely be an "interference engine".

Bob Nixon..

[saddlebag]

Few will ever ride one far enough to ever find out, so my guess is
unless it's cheaper to make them non-interference (doubt it) they
probably are interference types.

Where did you hear that Ducs weren't? If I'd have known the worst
that could happen by not replacing a belt was to have to replace the
belt (and maybe call for a tow) I'd have saved a fortune on belts!

[Bob Nixon]

Replacing belts is overrated Saddle. My 1991 Toyota with well over
100K miles and a more complex turning of angles never did break and it
had 60,000 mile replacement schedules. Particularly the old air cooled
two valve Ducs with a simple oval cam belts are less likely to break
that their water cooled high performance DOHC engines. I wonder though
how much more mechanical loss it would have been to continue with the
old bevel drive positive no maintenance system on the really old
Ducs.

Bob..

[tomorrowat...@yahoo.com]

They didn't switch from towershaft and bevel drive gears to toothed
rubber belt because of mechanical inefficiencies in the towershaft
engines. They changed because every engine required hand shimming of
the cams to properly mate the driven bevel gear faces with the drive
bevel gear faces on the towershaft when the engine was being
assembled, and that was hugely expensive.

Tim

P.S. I know lots of people who have had their cam belts fray and start
to come apart well before the 7,500 mile recommended change interval
on 2v air-cooled Ducatis. Age and heat play a much larger role than
miles. I have seen 50,000 mile Ducatis which have only had their
belts changed once (but the bike itself is only three years old) that
looked better than the 6,000 mile oem belts on the 2000 750SS that I
just picked up, and that bike is perfect - just almost 10 years old.

[saddlebag]

On Mar 20, 7:12 pm, "tomor...@erols.com"

I always had them replaced at 12k intervals (per the manual) and they
always looked new. Still, a lot cheaper than replacing an engine head
or two. Unless Bob is right and they aren't interference motors. Any
idea?

[tomorrowe...@yahoo.com]

Ducati twins are, to the best of my knowledge, all interference
motors.

I think changing the cam drive belts at the manufacturer's recommended
interval is a good idea.

Ymmv, and all that.