Camshaft Gear Drives

[roverv8]

How much better is a gear drive going to be for accurate cam timing compared to a good true roller duplex replacement and what are the differences in the gear drives (ie) idlers etc.

 

[Windsor377]

Please don't run a gear set. They give more friction and scattered timing.

If you can get a belt set I'd recommend that. A good chain will work just fine.

 

[roverv8]

I am surprised at the advise not to run a gear set, although I have never run anything but duplex true roller, I thought the cam timing would be more accurate with a gear set, however I am not keen on the noise they make, but the car I would be putting this engine in will most likely have a straight cut gearbox, so there is going to be a fair amount of mechanical noise about the place!!!

 

[Metalguy]

I put a gear drive on a BBC (a Muldoon), which used a single idler gear mounted on a post. When set up right with very little backlash, there isn't any slop, especially compared with ANY chain drive I've ever run, and that includes a bunch of them--after 20-30,000 miles or so. Every chain had slop when checked/replaced, including full rollers and silent chains (which seemed to last longer than rollers).

I could easily hear the gears over the fairly loud side pipes (1969 Vette), but only at idle or below about 1,500 RPM.

People used to ask me if it was supercharged--it wasn't.

 

[spdingtkts]

Properly set up, a gear drive will give you more consistant cam timing.

The bad thing is that it will transmit all the harmonics (vibration) from the crankshaft to the valvetrain.

A belt is a good way to go. It will give you more accurate cam timing than a chain and it does not transmit any harmonics.

I would recomend a belt assembly that uses some type of roller tensioner.

 

[rd400guy]

This may be a bit off topic, but I'm using a belt-drive system (Kevlar reinforced, I believe), and was wondering about tensioning. The "factory spec" says you should be able to deflect the belt 4-5mm from the idler pulley.

My question is, how much force should be used to move the belt? And, how would you measure said force? The quick-and-dirty method taught to me was that I should be able to fit a 4mm and/or 5mm allen wrench between the belt and pulley.

I gained a reputation in college for "over-torquing" fasteners and other things that didn't have a torque spec. So I'm wondering if I'm applying too much force in deflecting my belt from the idler (fitting the allen wrenches in). Any advice would be appreciated.

 

[ShaunT]

The chains stretch on early non thrust blocks very badly if you don't run a button.
The Belt drive is definately the best of both worlds.

Gear drives like the Milodon, are good gear drives when installed properly, but they suck up horsepower, and they definately transmit harmonics.

For belt/pulley measurement, I reccomend calling JESEL.


Shaun TiedeULTRADYNE Arl,TX(stiede@ev1.net)

 

[Windsor377]

Well, I guess we all agree about belts.

But, I guess we've had some different experiences with gear sets.

My own personnal experience: I will only try another gear set in an OHV pushrod engine, IF and only IF it is without idlers and has a built in dampening system.

The gear sets I looked at beat chains in only one category: fastest to the trash bin.

The slop in a chain sits to the passive side of the cam. The driven side remains pretty consistent within the duty cycle of a race application. I agree, over time the chain does degrade. But that time is a long time in "race car years".

I agree too, at slow checking speeds the gear is more consistent. But at higher speeds contributes to more valve train violence.

Shaun, if you are reading this, I consider this analogous to Ultradyne's closing ramp as it transitions to the base circle. I used to run a cam from a company that has an opposite philosophy. Their profies tend to hit the base circle REAL fast and hard. Hmmm, should make more power? Right? Wrong. In my own personal track test of two VERY compatible profiles (I personnaly measured the lobes), the other brand was a full tenth and a half slower...and pounded my locks and retainers on a regular basis.

Violence in an engine, of any kind, isn't good. Violence = lost power + lost reliability.

Quick question: Before belts, how many Comp Eliminator, PS and NASCAR team used gears instead of chains?

 

[ShaunT]

Not too many, consistently.

Shaun TiedeULTRADYNE Arl,TX(stiede@ev1.net)

 

[spdingtkts]

The chain wips itself into and S at higher RPM. This will cause you valve timing to vary considerably.

I have gotten away from the car stuff pretty much and do almost all motorcycle stuff now. I know a few years ago that if you look at sprint cars in the WoO, alot of those guys are running gear drives.

For the motorcycles, there is a Superbike class which is based on a production motorcycle that is allowed to be modified very heavily.

Honda produces the bike they run with a gear drive.
Kawasaki replace the roller chain with a gear drive.
Suzuki replaces the hyvo style chain with a roller chain.
I am not sure what type of chain that Yamaha is using but they were running that and their were plans of a gear drive, but they pulled out of the class to concentrate on another class before they released anything.

Rob Muzzy used to head factory Kawasaki's racing program in both Europe and the US at one time. He is recognised in the motorcycle industry as one of the top technical minds in the field. He always recomends the gear drive for racing use. He said that the chain had less friction than the gear drive, but the gear drive will give you much more consistant valve timing.

 

[spdingtkts]

Another thing that just thought of, is/was a gear drive legal in Nascar?

I believe that all you could run was a chain for along time.

 

[Windsor377]

I wouldn't argue gear or chain for OHC applications. That seems to be a pendulum that swings back and forth over the years. Also, as facinated as I am with muliti valve DOHC combinations, I really don't spend time in that arena, other than visiting a friends shop that specializes in Bike and "Import Racer" cylinder head work.

However, I would never run a gear set in an OHV pushrod application. I understand the problems you identify with the chain, but consider them much more predictable and controlled than the chaos of a valve train experiencing violence. Excessive spring pressure can "manage" the violence, but the valve train is put under more stress than I am willing to accept (high maintenance issues).

You are right about the WoO guys running a lot of gear sets. Many of them run a lot of IR/NR/IF opening and closing from various "mind sets". In turn they run a lot of valve spring to keep the rollers on the lobes. So much so, that the pressure would probably break a chain and stretch a belt.

I've learned that agressive timing doesn't need to mean violent timing. If my favorite cam supplier is reading I'll let him finish the next sentence. If you need excessive spring pressure to control yout valve train, you are using the wrong _ _ _.

But, like I said I would try a gear set again IF it was sans the idler gears and had a built in dampening system.

Have you ever thought of some type of viscous coupler in the crank gear augmented by a "hard stop" to aid during start up? There has to be some cost effective and "mechanically" effective way to do it. I would try that.

Your question about the NASCAR guys running only chains has me scratching my head a bit. I know I've seen a gear set on one of their roller cam engines. The Cup guys have a unique set of rules alright.

 

[spdingtkts]

If I remember correctly it took alot of campaining by the teams to get NASCAR to let them run the belts as the only thing that was allowed was a chain.

 

[ShaunT]

With regards to crank harmonics, incidentaly, I use FluiDamper. It is all I use. Crank harmonics resonate through the cam drive and through the valve train. There are also Valve spring harmonics. The belt drives are best at dampening shock and harmonics. If we all build engines with attention to dampening harmonics in any and every way possible, things will be better in the long run. Remember:If you do what the rest of the racers do, you are not going to beat them. You will be luck if all you do is keep up with them.
Don't cut corners, pick your battles, and choose wisely.

Shaun TiedeULTRADYNE Arl,TX(stiede@ev1.net)

 

[spdingtkts]

Something that I was just thinking about.

Don't all/most Formula One engines use gear drives?

 

[matthews443]

Attention: Read this article from Pro Stock engine builder Warren Johnson on gears, belts, and gear drives.

http://www.nhra.com/dragster/1999/issue16/speed.html

 

[Windsor377]

Interesting. Thanks. So the helical cut will tend to undo he things I am worried about in a gear set?...hmmm...I still think a gear set upsets the valve train when an idler is used.

 

[bazeleur]

Slightly off topic... Can an OHC belt drive be used with multiple valves and high(ish) spring loads, or would that compromise durability too much? I was thinking of spring loads in the region of 330-380lb/in for a 24V engine...

 

[Windsor377]

How many cylinders and are those open or seat numbers?

 

[bazeleur]

This is for a boxer-6. The 330-380 was the spring rate, and the seat number would be in the region of 130lb, giving an open rate of say 360-400lb with 0.7" valve lift