Ok, I didn’t realize this was about timing belts :smack: I didn’t read the post carefully that originally mentioned belts, clearly toothless belts have never been used. Timing belts would be fine.
Um, 100K miles would be a fantastic warranty - in 1975.
With engines now lasting 200-300K miles, the obscene complexity of many belt replacements makes me want to look at alternatives.
Like chains or, at the very least, an easy way to replace the belt.
In 1987 (last new car), it cost $800 (small shop - don’t know what the dealership would have charged) to replace a belt. That was required at 80K miles.
100K isn’t all that impressive.
Q: How do you know if an engine is or is not an interference* design? Are there warning signs on the MSRP sticker? Owner’s manual? Shop manual?
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- an ‘interference’ engine is one is which the valves and piston occupy the same space - it is only the cam which determines which is where. If the timing belt/chain fails, the valves and pistons attempt to occupy the same space at the same time. Bad news - engine is now a large paperweight.
The information might be stated clearly in the owners manual in referencing the change interval for the timing belt. It should be in the shop manual or found on the less squishy automotive fact sites like Edmonds.com.
I believe most current engines are interference designs.
Timing belts are one of the worst ideas Detroit ever came up with. They were born out of one thing: Saving money during the tough economic times of the late 70s. & early 80s. IOW a way to save a few dollars per unit on mass-produced engines. Whether it’s 60K, 80K or 100K a timing belt will eventually break. And always long before the engine comes even close to wearing out. And combined with the fact that the vast majority of engines became interference around the same time (for better performance, fuel economy, emissions etc.) it was a perfect recipe of planned obsolescence. Most people were not going to spend the fairly large amount of money it cost to replace the timing belt when scheduled because the car would run absolutely perfectly, right up until it broke. But once it broke, the engine was so badly damaged as to be unaffordably fixable in most cases. IOW the car went from completely normal to worthless junk in literally a heartbeat with no sign of eminent failure whatsoever. It was essentially an engine self-destruct countdown timer.
Timing belts were not an advantage in any way other than short-term profit for the manufacturer. Noise, lubrication, adjustment etc. none of these are issues with chains. Saying timing belts are no worse than chains is like saying donut spare tires are no worse than real ones. Thankfully, because of their guaranteed failure rate, manufacturers have finally moved away from timing belts in all but the most inexpensive of cars.
Maserati MC12 https://s-media-cache-ak0.pinimg.com/236x/a9/70/a3/a970a3112fc106a1af90354b0830fc3a.jpg
The upthread posts about the Norton and other shaft driven cams were cool. In the “Nothing New Under the Sun” department, Aero Engines have used shaft driven OHC’s forever. The American designed Liberty engine from 1916 was a V12 OHC, shaft driven. The successor Allison V12 and the Rolls Royce Merlin and Griffon were too. German engines from Daimler & Maybach had them. I’m sure a bit of research would show just about any inline or V aircraft engine eventually went that direction.
Since so many aircraft ideas made it into auto racing and then to general automotive engineering it does seem surprising that more automotive companies didn’t use that setup. I would imagine it comes down to build cost and complexity.
And if you want that gear drive sound in your small block Chevy, check this out.
From the Overview tab: "Proform high-performance gear drive sets offer high-precision timing, eliminating chain stretch. They are **machined for noise
**I’ll stick with my good old double row chain.
That’s not totally true. Ford runs it’s belt through the oil as it rotates around the sprockets because the oil reduces friction and increases reliability. As an example, see the new 1 liter Eco engine.
Studebaker V8s all had gear driven cams, but were not overhead cams.
This strikes me as a lot of blather. Timing belts originated in Europe, not Detroit, and were introduced well before the 70’s. Timing belt replacement on many/most early designs was fairly simple and inexpensive. Noise, lubrication, and wear [adjustment] are indeed issues with chains. Plenty of higher end cars still use timing belts. Thanks for your opinions, but please don’t present them as facts.
What I have learnt from this thread is that an awful lot of modern petrol engines are piston/valve interference design. That is news to me.
AFAIK that model had “desmodromic” valvetrain, where each valve had two cam followers. one to open the valve, and one to close the valve instead of relying on a valve spring.
It is a ton of blather. I think Hail Ants and usedtobe have convinced themselves that every single automotive thing they don’t like came from “Detroit.” Why were British Leyland’s cars so bad in the '70s? Detroit. Why did early Japanese cars rust away rapidly? Detroit. Why did Audi decide to put their timing chainsets on the back of their vee engine, necessitating an engine pull when the failure-prone tensioner eventually goes? Detroit.
How about doing away with chains, belts, camshafts forever? using hydraulic or electric valve activation, we could eliminate valve float and all the belt/chain-related issues.
How about slide valves? years ago, a guy in England was experimenting with a rotating sleeve at the top of the head-it would rotate to alternately open and close holes in the sleeve, providing the valve action. anybody know more about this “Heron” valve engine?
Porsche’s flat boxer engines have used shafts/gears for years, known as the intermediate shaft (or IMS in Porsche-speak). Mostly trouble-free, except for a run from about 2001-2005 when the shaft spun in a somewhat undersized, under-lubricated bearing. Many Porsches are driving around with cams but no belt or chain driving the cams.
I don’t have a good cite, but I’d bet other than the few remaining 1960s legacy designs like small-block Chevy V-8s, damn near every modern engine is interference.
To the point that it’s no longer a meaningful distinction. Kind of like proudly announcing that your latest model is fuel injected. Whoop do doo; they all are now.
we’re already straining 12 volt electrical systems as it is, and things like EPAS (electronic power assisted steering) are just making things worse. As it is, right now car makers are employing “charging strategies” such as trying to delay energizing the alternator’s field to times when the car is coasting. Hydraulic pumps take engine power to run; likely more than a simple belt/gear/chain driving the cams directly.
and electric valve actuation has one more ball-ache than you might think- the (presumable) solenoids have to be energized to both open and close the valves. With a conventional camshaft, even though the valve is closed by a spring, it still has to follow the cam profile and soft-land on the valve seat. You can’t just let poppet valves slam closed or they’ll stretch and break in short order. So you need the electric acutators to consume power gently closing the valves, power which comes from the alternator and thus the engine.
lubrication/wear is and continues to be the problem with such things.
Sleeve valves have some history in large IC engines in the WWII era. See Sleeve valve - Wikipedia for more.
Are these gear driven systems still using cams and springs? How about a gear driven system that magically uses positive action to move the valves that wouldn’t destroy an interference engine as a result of a bad spring or lever. What I do hear a lot about with interference engines is broken valves, obviously nothing will help that if the valve is inside the cylinder, don’t know if it’s that common or that’s all I heard about, or if that’s just a generic description for any valve problem with an interference engine.
Gears need some slop (backlash) to operate without binding and excessive wear, and this results in some vagueness of the cam timing. Big gears have less effect, so it works OK with pushrod engines. To drive an overhead cam, bevel gears are the usual solution, and they tend to be fairly small, because they are in places where big gears would get in the way, and weight, etc. To minimize backlash, the gears have to be very precisely mated, usually requiring a number of shimming and measuring iterations, which is not acceptable for high volume production. And yes Ducati built thousands of bikes, but not 100’s of thousands, and they tend to be among the more expensive/exotic.
Crankshaft position sensors are a fairly high failure rate item, but at least the engine would stop with the valves closed, and no damage. The engine oil can be run through a high pressure hydraulic pump and used to actuate the valves…an example would be the unit injectors used on many modern diesels.
Sleeve and rotary valves are nothing new. They all suffer from differential expansion of the valve parts due to having to pass hot exhaust gases. They either bind up, or are so loose as to be hopelessly leaky, until they fill with carbon and bind. Poppet valves have the advantage that high combustion pressure actually pushes them more tightly closed, and also the close contact with the head permits a high degree of cooling, positioned exactly at the area of the exhaust valve that experiences the greatest heating. Hydraulic tappets/adjusters cure the one downside of needing regular adjustment of the valve train lash.
the only thing the modern “small block” shares with the classic SBC is bore spacing and an OHV setup. The newest ones are direct-injected with 10:1 and higher compression ratios, so the likelihood of them being interference is high.
This is a bit of a mess and you’ve glued a few concepts together.
The gear-driven systems we’re discussing are about carrying crankshaft rotation up to the head area to drive a traditional cam. IOW, an “overhead cam”. And in most cases the cams are used to depress / open the valves, while springs are used to retract /close the valves.
We’ve also mentioned desmodromic valve actuation. This uses the a cam to depress the valve and a different cam/follower arrangement to retract the valve. So no springs are used. But this feature is independent of the camshaft drive details. Desmodromic valve - Wikipedia
Levers", more properly called “rocker arms” are not part of any overhead cam system. The whole point of the additional complexity of moving the camshaft(s) up to the head(s) is to eliminate extra moving parts such as lifters, pushrods and rocker arms. As well, the failure of a lifter, pushrod, or rocker arm usually results in the valve staying closed, not open.
The severe failures in interference engines happen when the whole valve drive system becomes de-synced from the crankshaft. Such as when the timing chain/belt/gear fails and the camshaft stops turning. At least some valves are held at least somewhat open no matter where in the rotation the camshaft is. So in no more than one crankshaft revolution the piston(s) will collide with those open valve(s) creating mayhem. Note that this can happen whether the camshaft is overhead or the more traditional down-in-the-block-with-pushrods design.
The way to stop this problem is to have each valve independently actuated with a mechanism that can only fail in the valve-closed position. Some combo of electrical solenoid plus return spring is probably the closest to that. And even then if the computer goes stupid and pokes the valve open at the wrong time or holds it open too long you’re still going to have a valve/piston collision within one crank revolution.
As the esteemed jz78817 has said, crank position sensors are troublesome parts and would constitute a single point of failure for such a system. As well as the other objections raised in earlier posts.