As to valve reliefs, I still maintain it’s feasible and done. Just one example below.
Obviously, it is technically feasible and practiced on a limited basis. No one would dispute that. Your proposal is an across the board implementation and that is what I am addressing. While they can and do have models that are on the high end of both gas consumption and emissions, they cannot have ALL of their cars in the same class. ON AVERAGE, they need to be below a threshold and deploying valve reliefs on every single model would push their emissions average over the threshold.
I think the reason most people hate timing belts is because having any work of that magnitude done is an expensive and high-anxiety proposition. Even our forum is peppered with timing belt changes gone wrong. And the general public knows it. Nobody wants to be saddled with the inherant risk. Or the guaranteed cost.
George, there IS an optimumally reliable way to drive the camshaft(s) with the crank…gears. It has been occasionally (rarely) used on stock vehicles in the past, and aftermarket gear trains used to be available on the market for V8s for those building race engines. But geartrains are expensive and add rotating mass, which robs power at the bottom end, so I seriously doubt of we’ll ever see one again on a stock vehicle.
I prefer chains for a number of reasons. TSM mentioned a few. It seems anytime a service is performed, there is risk of collateral damage. The older I get, the less inclined I am to work on my cars. Therefore, I don’t like a major service I can virtually mark on my calendar as being required and looming.
Chains have the potential to be more reliable. I have only had two failures in my experience. One was stretched on a beat motor in a 60s boat of a car, the other failed due to the nylon tooth covering GM used back in the 60s to quiet them down (on a big block w/side pipes!). Like you care about timing chain noise in those applications. Heck people ran gears cut specifically FOR the noise.
Anyway, like anything today, the quest for saving a few dollars is what concerns me. They can make chains with very little design headroom and then shop the chain out to some questionable vendor in China to save a few bucks. The end result is they have an opportunity for massive problems and you’re stuck holding the bag…
I have 3 vehicles. One is 2000 Chevy S-10 4.3L with the cam run by a timing chain. It has a little over 100k and no engine issues at all. I bought it new and have always kept up on the maintenance including oil changes. I switched to synthetic oil in it at around 50k.
Another is a 1997 Ford F-250 Light Duty (basically an F-150) that I got on a trade with nearly 300k. The engine itself was solid with no rattles so I went for it. This is Ford 4.6 with a timing chain. It was a million small little things I had to take care of before it was good truck but it is a now reliable. The engine is smooth as silk and uses no oil. There are rumors it is a replacement junkyard engine and I have had odd parts mismatches when replacing things so I don’t know… All I know is that the engine is solid and reliable now that all the messed up sensors and such have been changed. I have no idea what oil was run in it before but it now gets synthetic and there are no leaks and no oil usage.
My other vehicle is my good gas mileage car… a 1994 Geo Metro 3 cyl 1.0L that gets 52-55mpg! I basically went through the entire thing and replaced all the common wear items after I got it. Yes, it is a timing belt… BUT a non-interference engine. It cost me $45 for a Gates kit with tensioner and timing seals included. I was able to do the job myself in about 2 hours in my driveway with nothing more than a basic metric socket set, a car jack, and a block of wood. I put a jack under the oil pan, placed a block of wood between the oil pan and jack (to protect the oil pan), and unbolted the passenger side engine mounts. I used the jack to raise and lower the engine to unbolt the various parts that needed to come off for the job.
I also replaced the water pump as it is used to mount the tensioner pulley for the timing belt and must come off for that replacement job. I messed up and was a tooth or two off the first time around. The engine simply wouldn’t start but no damage was done.
Yes, EPA, this engine is non-interference and gets between 52-55 MPG! Sure, it is a very basic car but is so simple to repair and gets great mileage rivaling or exceeding that of modern day hybrids using 25+ year old technology. That 3 cylinder Suzuki Swift engine is simple and no frills yet efficient and plenty powerful for someone who just wants to get around. The car is like an oversized go-kart and kinda fun to drive. Maintenance is simple and very forgiving in the case of a timing belt. Sure, it might have to be done several times during the life of a car, but it isn’t a big deal on this. Not too much needs removal and it is quite easy. Plus, if you mess up, you don’t destroy the engine.
On to the original topic… As for he mechanic stating that the head gasket looked ok on the offending car, I don’t know how to take this. The head HAD to come off for this repair. Did they re-use the head gasket???
a gear drive cam system may work on a pushrod motor with a block mounted cam. almost all new motors are ohc and are forced to use a belt or chain drive. every honda or toyota uses belts due to logistics, not some engineer preferring one design or another. why do so many chain driven ohc motors have issues with worn tensioners and failing chain guides or chain sprockets that wear thru? engineers think chains solve issues of rubber timing belts failing and they actually present an entire new series of wear issues.
Valve reliefs alone won’t do it. On some interference engines, the valves can hit each other.