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  #1  
Old 07-07-2005, 04:15 PM
Mister Byrnzoil's Avatar
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Why a timing chain instead of timing gears?

seems like gears might last longer, stay in spec longer, and be less likely to break than a chain.

I have seen timing gear replacements for gasser engines...

someone please 'edumacate' me.

thx,
Robert
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  #2  
Old 07-07-2005, 04:20 PM
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It all has to do with th edistance of the centerline of tha cam and the crank.....you need multiple gears....more than three adn likely 5 to bridge the distance...and with the lash (play or clearance) with gears you would find them being far less accurate than a chain.
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  #3  
Old 07-07-2005, 04:43 PM
LarryBible
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The doc is correct. All MB engines for the last fifty years or so have been overhead cam engines. In this configuration it would take four or five gears to transfer the motion all the way from the crankshaft to the camshaft(s.) With all those gears each requiring a small amount of lash, would add up to lots of slop even when new.

On OHV engines with the cam right next to the crankshaft, a couple of gears is a good choice as was done on early six cylinder Chevies and the 240 and 300 Ford sixes of the sixties, seventies and even used in the nineties.

Another consideration is noise. Timing gears can be pretty noisy, not something desirable in a luxury car. MB uses the optimum timing system for their application, a true roller chain. Some of the engines require more service than others, but even the worst of them beat the heck out of a stupid rubber belt as is found in the vast majority of engines in recent years.

Have a great day,
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  #4  
Old 07-07-2005, 05:19 PM
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Yep. The previous posts are right....

In addition to the slop and durability issues, gear drives are VERY noisy too.


Obviously not something you're worried about too much, if you already own a diesel, but.....

Mike
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  #5  
Old 07-07-2005, 06:36 PM
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Cool, thanks to all, makes some sense now, and I had not considered space or sound factors, straight cut gears are noisy.

I puzzled over that one for a while now.

thanks,
Robert
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  #6  
Old 07-07-2005, 06:40 PM
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with regard to overhead valve engines

If you used straight cut gears it would be very noisy.... these are available for Race Cars...
but I don't know that ' normal ' angled gears would be very noisy... as evidenced by the fact that they were used in normal passenger car engines.

Why would racers ever choose noisy straight cut gears ? It eliminates side thrust.

The option, but way more expensive , for no side thrust ( requireing a thrust bearing ) would be heringbone cut gears. They combine angled interaction with no side thrust.
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  #7  
Old 07-07-2005, 08:20 PM
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....and some of those older fiber timing gears used to lose a few teeth once in a while. Can be just as bad as a worn timing chain. My '05 Tacoma V6 engine has gone back to timing chains for the four cams and I feel better with it than I did with the '95 Tacoma V6 with the timing belt.

I'm for chains period. (Although some of those straight-cut metal timing gears can sure make a nice sound.)

Wes
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  #8  
Old 07-07-2005, 08:29 PM
LarryBible
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Quote:
Originally Posted by Wes Bender
....and some of those older fiber timing gears used to lose a few teeth once in a while. Can be just as bad as a worn timing chain. My '05 Tacoma V6 engine has gone back to timing chains for the four cams and I feel better with it than I did with the '95 Tacoma V6 with the timing belt.

I'm for chains period. (Although some of those straight-cut metal timing gears can sure make a nice sound.)

Wes
That's good news on the Toyota V6 going back to a chain. My wife had a 98 4Runner with a 3.4 that had a belt. It was a really tough one to change too. You had to strip most everything off the front of the engine. As far as I was concerned that stupid belt was the only thing that kept the engine from being an OUTSTANDING engine.

Have a great day,
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  #9  
Old 07-07-2005, 09:02 PM
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The VW V10 tdi engine also does away with timing belts and even a timing chain, this is from the site about the engine:

"A torsional vibration damper on the crankshaft is of a viscous pattern and occupies a space only 24 millimeters—less than an inch—deep. Adding to the compact engine design is a helical-cut spur gear train at the flywheel, replacing the typical timing chain or toothed belt. Keeping everything adequately lubricated is a duplex oil delivery pump and two scavenger pumps that operate reliably even at the extreme angles sometimes encountered during off-road maneuvers."

I want one.
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  #10  
Old 07-07-2005, 09:57 PM
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options

to a chain include bevel gears with a vertical shaft. this is used on a italian motor cycle commonly known as the two wheeled ferrari. the name escapes me. i have also seen it on automotive engines... all low production.
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  #11  
Old 07-07-2005, 11:27 PM
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Not positive but I think the bike you're thinking of is a Ducati. Not only do they have gear driven cams but the valves have two set of rockers, one that opens and one that closes the valve. No conventional valve springs. As the Brits say, it's a neat bit of kit. In that case though the engine, cams, etc. are quite a bit smaller than on a car so I think it's easier to get away with. Also, bikes don't have to worry about the emissions constraints quite so much.
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  #12  
Old 07-08-2005, 10:43 AM
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ducati

yes. i forgot about the ducati valves...the dual cam valve system is called desdemonic i believe and it was first used to my knowledge by mercedes in their incredible grand prix engines in the thirties. they also were used in the awesome 300slr race sports car that moss won the mille miglia in 1955 with.
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  #13  
Old 07-08-2005, 12:39 PM
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Quote:
Originally Posted by t walgamuth
to a chain include bevel gears with a vertical shaft. this is used on a italian motor cycle commonly known as the two wheeled ferrari. the name escapes me. i have also seen it on automotive engines... all low production.
I really, really miss my Ducati...

Scott
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  #14  
Old 07-08-2005, 12:41 PM
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Quote:
Originally Posted by lrg
Not positive but I think the bike you're thinking of is a Ducati. Not only do they have gear driven cams but the valves have two set of rockers, one that opens and one that closes the valve. No conventional valve springs. As the Brits say, it's a neat bit of kit. In that case though the engine, cams, etc. are quite a bit smaller than on a car so I think it's easier to get away with. Also, bikes don't have to worry about the emissions constraints quite so much.
It completely eliminates the chance for valve float. Makes the valves difficult to adjust though. I tried it once and couldn't figure it out. It's very easy on my Mercedes however..

Scott
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  #15  
Old 07-09-2005, 01:37 AM
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difficult

everything has its cost. ferraris are harder to work on too.
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Old 07-09-2005, 01:37 AM
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