SOHC Gear Drive

[Autoholic]

I'm working on a design for a spring loaded piston / ram, that fits on the tension bolt. The backside of the spring would be held in place with a nut and washer. At the tip of bolt, would be the piston / ram that encloses the spring. The idea would be that the spring would exert enough force against the piston / ram, to take up any slack in the tensioner. If this idea could work, it wouldn't require any new parts to be made, other than a simple metal ram with a hole in the middle to fit the spring and the bolt. Looking at McMaster-Carr, there are springs of a suitable size and spring pressure. This idea would mean that the tensioner would have to be free enough to move on its own.

Thoughts? It wouldn't be a hydraulic solution, but it would take up slack in the system. McMaster-Carr has some springs that could put well over 100 lbs of force per inch on the tensioner, a few even over 1,000 lbs/in. It's a cheap solution, keeping everything stock. We're talking less than $50.

[WConley]

Joe -

Using just a spring will cause significant resonance problems with that chain drive.  You need an oil damper in the system. 

Even with hundreds of pounds of tension on that SOHC chain, it has still been shown to jump around quite a bit.  Big-time damping is a must if you are not cinching it up fairly tight.

Look at all of the production OHC chain tensioners out there.  You'll see oil-fed pistons (often backed with springs) for a good reason.

- Bill

[Autoholic]

Thanks for the feedback Bill. I'll have to make some changes. Besides needing to incorporate oil pressure, is there any particular reason why the stock tension mechanism can't be used? Bore the mechanism out a little bit and put needle bearings around the bolt? Or redesign the whole thing to ride on an off the shelf bearing?

[wsu0702]

Another belt drive Cammer.

[WConley]

Joe -

A few years ago I was thinking of a simple solution that would provide spring tension and oil damping.  It involves redesigning the stock tensioning screw to allow for a spring-backed piston on the end.  You'd probably only need 0.5 inch of travel or less for the piston.

Inside the piston would be a strong die spring (oval profile coils) and it would be fed from a flexible line plumbed to the main oil gallery.  Getting the diameters and forces right would take a bit of doing, but I think it would work well to dampen the chain harmonics.  You'd use the existing screw to tighten the tensioner to a given force, and let the spring / oil-damped piston take up small movements.

For the arm pivot, I would recommend an oilite bronze bushing.  There's enough oil splash in there and the movements are minimal.

[Autoholic]

Bill, that's pretty much what I'm trying to design. I want to retain the stock tension bolt, which would be used to tension the chain normally. The problem I'm seeing with this however, is making it oil pressure driven By coming up with a new bolt of sorts, the inside could be hallow which would be feed with oil.

[machoneman]

Very cool guys! You too may have the best or perhaps optimal fix in mind. Once again, Jay's foresight to create this website, allowing a lot of FE thoughts to come together, may pay some nice dividends!

 

[wsu0702]

So many issues with the chain drive.  And no issues at all with the belt drive. 

[Heo]

This is the way Mercedes solved it on their SOHC V8s
Works flawless year after year
http://www.ecklersmbzparts.com/diagram/view/index/id/110

[jayb]

So many issues with the chain drive.  And no issues at all with the belt drive.

No issues with the belt drive?  How about cutting up the SOHC valve covers so the belt drive will fit?  Same with the modifications to the front cover?  Sealing the shafts that come through the valve covers?  Fitting all the other components on the front of the engine?  The chain drive isn't perfect, but it works, and is a better solution than that big ugly belt drive, in my opinion...

[Autoholic]

This is the way Mercedes solved it on their SOHC V8s
Works flawless year after year
http://www.ecklersmbzparts.com/diagram/view/index/id/110

That design is typical of OHC timing chain tensioners. They use a chain guide that pivots about one end, with some sort of spring loaded tensioner on the other end. It pushes the guide into the chain, taking the slack out of the chain. Many modern systems that do this, use oil pressure in the tensioner mechanism that pushes against one end of a chain guide. Eventually, the chain guides have to be replaced as they are sacrificial pieces.

So many issues with the chain drive.  And no issues at all with the belt drive. 

As Jay pointed out, there are a lot of known problems with a gilmer belt drive that have to be accounted for. Also, most people who want a 427 SOHC under the hood of their ride, don't wan't to take a "blue tip wrench" to it. A properly designed timing belt system, has some sort of spring loaded tension mechanism. While you can take most of the slack out of a timing belt in static, you don't want any slippage here. Chances are the cogs on the gears would all stay in the right place, but the belt would stretch just enough to put some harmonics in the valve timing, as we see with the stock chain drive. If your goal is to eliminate the harmonics within the 427 SOHC's timing, you either need to go with a gear drive or have a dynamic chain tension mechanism created.

Jay likes this within his engine, however I believe that this is an uncontrolled variable that is unpredictable, engine to engine. If I had a crap ton of money right now. I'd build one of these engines and study it. But the fact that you have to study it within your engine shows that there is a problem. Not everyone who owns one of these engines wants to have to come up with some sort of testing apparatus in order to understand how it is impacting their engine. Also, not everyone wants the timing to have any variation in it. From a failure mode standpoint, allowing the timing chain to experience a harmonic wave that effects the amount of tension on the chain, is putting the chain through a cyclical loading instead of a constant load. A dynamic tensioner helps diminish fluctuations in loading on a chain, which helps a little to extend the life of the chain. This might not be a serious problem on the 427 SOHC, because an owner likely wouldn't put enough miles on the engine to ever need to worry about a timing chain's service life. 

[Heo]

Well i had mercedeses as dailydrivers since the early 80s. And i
don't had to replace any chainguide yet.So i don't want to call them
sacrificial any more than any other parts of the engine.I have rebuilt
many engines that was totally worn out (300000-500000 km)
 but the chainguides was OK to reuse. The tensioner is hydraulic
operated,  on some very high milage engines you can hear a slight
knock from the tensioner when cold starting  till the oilpressure
builds up. Some times i had rolled in a new chain just for "better
safe than sorry" on some high milage ones i had
So i would say a good enginered chaindrive is more or less trouble free

[Autoholic]

They are designed to last, but it depends on what the chain guides are made of. Today, I'm pretty sure they use PTFE but I could be mistaken. They are sacrificial because when they push into the chain, you have essentially an extremely dull chain saw. Normally these chain guides have a pretty generous curve to them, so that the chain isn't grinding against a corner. A well used chain guide should have a noticeable groove, like below... For preventative maintenance, it would most likely be in the engine's best interest to replace these when you replace a timing chain. Unless we're talking Ferrari, these aren't that expensive.