Whats The Low-down On Ohcs?

jro909 · March 3, 2011

i realised i know very little about overhead cams, and have had no experience with them at all. I know they can rev higher then pushrods and that they use belts instead of a chain for timing...but how long do the belts last? are they as reliable as pushrods?

Nytemare · March 3, 2011

i realised i know very little about overhead cams, and have had no experience with them at all. I know they can rev higher then pushrods and that they use belts instead of a chain for timing...but how long do the belts last? are they as reliable as pushrods?

Nope....Push rod engines can rev as high as some OHC engines...its still limited by the springs keepers and followers. Also the rod lenght and everything else that goes into a rev limit.

Some OHC engines....like ford and some dodges (Intrepid, and anything using that engine)have chains that drive the cams.

Belts last either 60K or 100K miles depending on the engine/year....chains are non maintenance items same as the chain on a push rod engine.

They are reliable as anything else...However a lot of OHC engines are interference type, where if the belt snaps valves can, and usually do contact the piston resulting in catastrophic failure.

Basically the cam pushes a follower which activates a valve ....kind of like a regular push rod engine minus the rods and upside down.

OHC is broken down into SOHC....one cam per bank actuating all the valves (intake/exhaust)

and DOHC...one cam for exhaust one cam for intake per bank.

The key difference in OHC and pushrod engines is Valve train losses due to friction and weight. Push rod valve trains weigh more and have more losses due to the friction of the rod, the lifters angles of everything etc...as well as oiling...the valve train on a push rod engine is generally oiled by oil flowing up through the rod. OHC engines get oiled directly from the oil pump. Any push rod engine with a shaft rocker system has this benefit too though.

There are befits to both and draw backs to both.

jro909 · March 3, 2011

thanks nytemare...this thread kind of came about when my buddy and i were talking about his honda civic..i said he probably should change the timing belts seeing at its at 100k and i then realised if some ohcs could have chains...now i know

also is the bad knocking from the OHC design? i notice alot of ford trucks knock bad even the new ones

Nytemare · March 3, 2011

thanks nytemare...this thread kind of came about when my buddy and i were talking about his honda civic..i said he probably should change the timing belts seeing at its at 100k and i then realised if some ohcs could have chains...now i know

also is the bad knocking from the OHC design? i notice alot of ford trucks knock bad even the new ones

That's piston slap man....your truck does it....you just don't hear it cause your inside when you start it up in the morning.

I even hear it in Mercedes, BMW, Toyota....seems everyone has a little slap nowadays.

Also notice a little puff of blue smoke at the first start of the day on most new vehicles.

Oh and yeah...100K is definitely time to replace the timing belt...Honda's have belts

SpeedyV · March 3, 2011

Oh and yeah...100K is definitely time to replace the timing belt...Honda's have belts

Not all Hondas, though...my Si had a chain ;)

jro909 · March 3, 2011

his is a dx i think...what happens if he lets them go?

on the ford subject...i'm talking knocking on acceleration and idle(warm) i see alot of fords come and go at work and notice the knocking while they get loaded..

music · March 4, 2011

...what happens if he lets them go?

Best case, motor no run. Worst case, bent valve(s)... and rebuild or replace head.

Nytemare · March 4, 2011

...what happens if he lets them go?

Best case, motor no run. Worst case, bent valve(s)... and rebuild or replace head.

And holes in the piston...seen that too...

All bad when things contact things that they're not supposed to.

Nytemare · March 4, 2011

Oh and yeah...100K is definitely time to replace the timing belt...Honda's have belts

Not all Hondas, though...my Si had a chain ;)

New one or an old one....I know they had chains back in the 80's and up to the 90's on some engines....but the newer ones should have belts. Maybe not though... I don't work on many Honda's.

You also see chains on most things with a V engine...

jro909 · March 4, 2011

its an 07...i'm not sure if its an I or a V configuration...

truckofwinandawesome · March 4, 2011

you are 120% correct Nytemare, BUT, Some OHCs CAN be Non-Interference Engines. 2 Valve SOHCs can be considered Non Interference, because there are only 2 valves to get in the way. 4 Valve SOHCs ARE Interference Engines, because the way the timing is. HOWEVER, There is a DOHC that Can be considered a Non Interference Engine, because of the 2 cams, BUT, there is Valve to Valve Interference.

Nytemare · March 4, 2011

you are 120% correct Nytemare, BUT, Some OHCs CAN be Non-Interference Engines. 2 Valve SOHCs can be considered Non Interference, because there are only 2 valves to get in the way. 4 Valve SOHCs ARE Interference Engines, because the way the timing is. HOWEVER, There is a DOHC that Can be considered a Non Interference Engine, because of the 2 cams, BUT, there is Valve to Valve Interference.

Yeah...its complicated most of the time....we just look at the book to see whats going on. I know my buddies DOHC Toyota Tercel was Non Int. But it did burn a valve or two when his belt snapped.

Nothing is interference as long as you change the belts at the recommended intervals.

And Jro, by V, I mean V6 and V8, etc....his is an I4

MikeNH · March 5, 2011

his is a dx i think...what happens if he lets them go?

on the ford subject...i'm talking knocking on acceleration and idle(warm) i see alot of fords come and go at work and notice the knocking while they get loaded..

I've seen on Super Dutys especially the exhaust manifolds rusting and making the engine tick in the situations you describe.

Nytemare · March 5, 2011

his is a dx i think...what happens if he lets them go?

on the ford subject...i'm talking knocking on acceleration and idle(warm) i see alot of fords come and go at work and notice the knocking while they get loaded..

I've seen on Super Dutys especially the exhaust manifolds rusting and making the engine tick in the situations you describe.

I don't know man...could be anything...they have cylinder deactivation, cooling jets, all kinds of crap put in there....I'm sure it don't make things quieter...