Interference Engines--Why?


#1

a naive question, but why, if a failed $23 serpentine belt will fry a $3000+ interference engine, would anyone build an interference engine? Is an interference engine so much more efficient, cheap, easier to repair or what?


#2

The answer is a lengthy one, but briefly, the best breathing, most efficient and frugal engines have combustion chambers that often dictate an interference design.

Of course the next question is, why don’t all engines have the more reliable and less maintenance-intensive timing chains? We have 2 cars, a Nissan and a Toyota, and both have timing chains. The only car I’ve ever owned with a timing belt(s) was a 1977 Dodge (Mitsubishi) Colt. It had many other problems than changing the belts.

It’s interesting that in some years, Toyota would put a timing belt on a car engine, but use a chain on that same engine when used in a truck.


#3

The design allows for a higher compression ratio, which means more power out of the same engine. If the timing belt is changed on schedule, there’s very little risk of a problem.


#4

The increased efficiency to meet fuel mileage standards is part of the answer. I would prefer a non-interference engine with slightly lower gasoline mileage. As an analogy, I remember back in the 1950’s when the overhead valve engine replaced the L-head (flat head) engine. The overhead valve engine was more complicated, but was much more efficient for a given engine displacement. The 1952 Dodge L-head 6 my parents had back in the 1950’s was certainly much easier for do-it-yourself repairs than the overhead valve engines.

Many push type lawnmowers have gone from side valve engines in the block to overhead valve engines. When lawnmowers go to overhead cam interference engines with timing belts, I’ll buy a goat to keep my yard mowed.


#5

I can’t believe that the dimples in the top of a piston would make a significant difference in performance.


#6

The cost of a new timing belt every 100k is insignificant compared to the total cost of owning and driving a car.


#7

It’s all part of a big conspiracy to permanently remove older, high-emissions cars from the road and force consumers to buy new cars instead of trying to repair their old ones…

As many posts on this board have demonstrated, today’s car owner is seldom able to open the hood and check the engine oil level. Hoping that he /she is aware of the need to replace a belt at a given mileage is asking a lot. Car makers are well aware of these things…

In the end, belts are used instead of chains as a matter of COST. It’s the cheapest way to build an engine. Every part on a car, every nut, bolt and screw has gone through a cost analysis. If they can make it cheaper and still meet quality goals, they will.

With consumers demanding longer, better warranties, timing belts are quickly disappearing from the automotive scene…


#8

Only if you replace the belt, or are even aware of the belt…For many drivers, it’s an unseen death-trap for their cars…For most car owners today, $600-$900 maintenance bills are NOT “insignificant” …


#9

The serpentine belt drives your alternator, PS pump, AC, etc… If this breaks, it won’t damage the pistons or valves.

The timing belt is separate, it turns the camshaft(s) (which opens and closes the valves.)


#10

Interference engines are more efficient. They allow us to get more miles per gallon, which lessens our dependence on oil. Lessening our demand for oil keeps the price of fuel low.

Like all engine designs, interference engines depend on the owner properly maintaining the vehicle. As long as you do that, this should never become an issue.

Replacing the timing belt (and the belt tensioner pulley) can cost as little as $0.003 per mile*, which is less than what most people spend on oil changes.

*This calculation is based on keeping a Honda Civic for 270,000 miles, hiring someone to change the timing belt and timing belt tensioner at 90,000 miles, and hiring someone to change the timing belt, tensioner, and water pump at 180,000 miles.


#11

This thread is not about serpentine belts…It’s about Timing Belts…


#12

The OP wrote “Serpentine Belt.” Never wrote timing belt. For his/her benefit I was merely pointing out the difference. Is this not ok to do?


#13

“*This calculation is based on keeping a Honda Civic for 270,000 miles, hiring someone to change the timing belt and timing belt tensioner at 90,000 miles, and hiring someone to change the timing belt, tensioner, and water pump at 180,000 miles.”

But how much does it cost them if they NEGLECT to perform this maintenance, as they are VERY prone to do. Junkyards are FULL of Honda Civics with broken belts and bent valves…The cost per mile suddenly goes WAY up…


#14

If you are spending “$600-$900” for a simple timing belt job, you should shop around for a better price. I had my timing belt job done at 90,000 miles at a Honda dealership for about $350. However, oil changes at the same dealership would have cost $600 over the course of the same 90,000 miles! (My owner’s manual recommends changing the oil every 3,750 miles, in case you would like to check my math.)

Most people who properly maintain their cars spend less on timing belt jobs than they do on oil changes, yet I don’t hear you complaining about having to change your oil. Why not?


#15

[i]The cost of neglect is always high.[/i] This is an issue whether your car has a timing belt or a timing chain.

I am sure the junkyards are full of cars that have broken transmissions because the owners never changed the transmission fluid. I am sure junkyards are full of cars that have blown engines because the oil was never changed. I am sure junkyards are full of cars that have blown engines because the coolant was never changed. Need I go on?


#16

But they are NOT full of cars with non-interference engines or cars equipped with timing CHAINS…

This thread is about a $30 belt sending a $5000(?) car to the scrap heap…


#17

Yes they are. Are you telling me you don’t have to change the fluids in a car with a timing chain?

[i]No, this is a tread about owner neglect sending a $5,000 car to the scrap heap.[/i] A timing belt is an inanimate object. It can’t send your car anywhere, and a timing chain can’t force people to perform proper maintenance on other areas of their vehicles.


#18

What should be done is to have an intake valve relief in the top of each piston (the exhaust valves won’t hit in most cases).
In my opinion anyway, a single valve relief will prevent engine damage and there isn’t a single person in the world who would ever notice one iota of difference between an engine with reliefs as compared to one without; and that includes performance, fuel mileage, or any other criteria.

I’d be willing to bet that one would not even see a difference if both engines were run up on a dyno.


#19

there isn’t a single person in the world who would ever notice one iota of difference between an engine with reliefs as compared to one without; and that includes performance, fuel mileage, or any other criteria.

Unfortunately, there is at least one; the EPA man. Those reliefs contribute to crevice volume and will impact emissions.

I just thought of another potential stakeholder that might be concerned; the bean counters at the car manufacturer. It’s likely to cost slightly more for an added feature in the piston dome…


#20

Yea, lack of maintenance is the most expensive mistake you can make on a car. If they get a timing chain, they will just not bother checking their oil level or never change the transmission fluid. There is a large group of drivers who manage to find creative ways of killing a car.