OHC vs OHV

alfadriver wrote:

Curmudgeon wrote: I was always (and still am) a fan of belt driven OHC. Quiet, inexpensive compared to chain drive and pretty damn reliable. I have seen lots of belts with a recommended 105k interval still in place at 175k or even higher. It's also cheaper to repair or replace (unless it's an interference engine and it busts).

Honestly, you COULD hold your breath on that one. Recent developments have produced wet belts (oil bath), which are durable. The minuses of belts are being fixed pretty quickly so that their pluses will put them back ahead of chains. And that does go for both under head cams and over head cams for drive.

Major curiosity bump: what are the drawbacks to belts from a powertrain engineer's perspective?

mad_machine wrote: that is pretty slick.... are SBC interference engines?

depends on what they are or what the build is.....I don't think most are, but some with longer stroke and higher lift cams most likely are interference.

Javelin wrote:

Curmudgeon wrote: I was always (and still am) a fan of belt driven OHC. Quiet, inexpensive compared to chain drive and pretty damn reliable. I have seen lots of belts with a recommended 105k interval still in place at 175k or even higher. It's also cheaper to repair or replace (unless it's an interference engine and it busts).

You don't need OHC to enjoy all of the positives of a belt drive! They make timing belt kits for a *ton* of V8's. Pretty slick.

I remember when those first came out, thought it was so cool that you could put a cam belt on a SBC.

I'll just leave this here ...

(January '73 HRM)

Needle roller bearings!

Now we're getting serious! (Cue the tiny violin for Babbitt bearings...)

Curmudgeon wrote: Major curiosity bump: what are the drawbacks to belts from a powertrain engineer's perspective?

None, really. Durability is pretty much it. But it's pretty darned important.

Cam accuracy is the same- the belts don't stretch. Quieter, lower rotating mass, less friction, etc.

But that durability issue is oh so #1.

slantvaliant wrote: I'll just leave this here ... (January '73 HRM)

Say WHAT? OHC slant 6?

alfadriver wrote:

Curmudgeon wrote: Major curiosity bump: what are the drawbacks to belts from a powertrain engineer's perspective?

None, really. Durability is pretty much it. But it's pretty darned important. Cam accuracy is the same- the belts don't stretch. Quieter, lower rotating mass, less friction, etc. But that durability issue is oh so #1.

Yeah, I can see that. People coming through here still piss and moan about 60k timing belt intervals.

alfadriver wrote:

Curmudgeon wrote: Major curiosity bump: what are the drawbacks to belts from a powertrain engineer's perspective?

None, really. Durability is pretty much it. But it's pretty darned important. Cam accuracy is the same- the belts don't stretch. Quieter, lower rotating mass, less friction, etc. But that durability issue is oh so #1.

yes. Durability is the problem. A good belt can be replaced in a short amount of time.. at least half the time of a chain.. but needs to get replaced twice as often ( or more) than a chain

mad_machine wrote:

Javelin wrote:

Zomby Woof wrote:

And to be honest, it is a more reliable and rebust design.

I disagree. With less moving parts, the OHC setup will be more reliable, and most OHC top ends that I know of are far more robust than OHV.

And I disagree with you. OHC setups really only lose 1 part (the pushrod). They still have to have a rocker, lifter, spring, retainer, etc. And you're adding in an extra cam (or 3), higher pressures, and can't have the advantageous ratios. Not to mention a much larger timing train (look at Ford Mod and Audi V8 chain failures). In a V-engine, OHV is about the best setup you can do from an engineering standpoint.

not necessarily true. Many OHC engines have the cams actuating directly on the lifters.. no rockers are involved at all. That does discard quite a few parts

tHEN you get the Honda's with 3 cam lobes and extra rockers and cam followers etc. My old SBC is MUCH simpler than a new DOHC VTEC Honda engine in regards to valvetrain.

mad_machine wrote:

alfadriver wrote:

Curmudgeon wrote: Major curiosity bump: what are the drawbacks to belts from a powertrain engineer's perspective?

None, really. Durability is pretty much it. But it's pretty darned important. Cam accuracy is the same- the belts don't stretch. Quieter, lower rotating mass, less friction, etc. But that durability issue is oh so #1.

yes. Durability is the problem. A good belt can be replaced in a short amount of time.. at least half the time of a chain.. but needs to get replaced twice as often ( or more) than a chain

Again, depends on the vehicle. Starting to see a lot of the Honda K-series needing chains and guides at 120k, which is a mere 15k miles past the timing belt replacement. Considering it takes twice as long and costs 3 times as much, not really seeing the benefit there.

At least it's not like VWAG chain engines, starting with the VR6 and then on to the 2nd (3rd?) generation V8s and the transverse I5s...

...where the chain is on the back of the engine, presumably for NVH reasons. Kind of a slick idea, instead of a bulky chain case on the front, use the bellhousing to hold in the noise (see Quad 4 for what happens when you don't have bulk to hide the noise) but DAMN servicing is expensive.

But then, VWAG products seem to be "throw away after 60k" products nowadays.

In reply to Alan Cesar:

A lot of engines in that era had atmospheric intake valves. My 1928 McCormick Deering 1.5 hp uses one. Light isn't a word I would use to describe it, at 458 pounds. The exhaust push rod probably weighs as much as all the push rods, lifters and valves in a LSX. Then again it's 84 years old and still runs so maybe heavy isn't all bad.

For the interested, a video link that shows a close up of the valves.

http://www.youtube.com/watch?v=AmUAoTo-gQ4

To everyone in general:

I will say this about OHV vs OHC. I've never had to change a cam chain on a OHV engine. From a durability standpoint, I prefer them. I've put well over 300K miles on a few of them. I don't own a vehicle with a belt drive cam anymore, you guys can keep the belts. at least until my ADD kicks in again.

From a packaging standpoint, I prefer the OHV as well. If Chevy hadn't stuck with that old push rod engine, the front of the C6 would be as tall as the Mustang and I don't see anyone trying to stuff a 5.4 Triton in a Miata, MG, or a RX-7.

Toyman01 wrote: I will say this about OHV vs OHC. I've never had to change a cam chain on a OHV engine.

I've done quite a few of them, but it was usually a "while we're in there" while repairing a leaking timing cover gasket. If it had play, it got replaced, and if it was over 100k, it had play.

I understand that chain failures were common when plastic timing gears were used. Fortunately, those are super-rare nowadays, although I did reseal a low-mile Cad 500 that still had one, and had a couple missing teeth.

I don't see anyone trying to stuff a 5.4 Triton in a Miata, MG, or a RX-7.

I've seen a 460 in an RX-7, and the 460 is about the same size as a 4.6. Mind you, the 4.6 is pretty much maxed-out displacement-wise, maaaybe get another half-liter, while the 460 was built with lots of room to grow with a 514ci version planned, and has been stroked-n-poked to over 600ci with the stock deck height...

I drove a 302-engined MG today. "Tight fit" is a good way to describe it. No way in hell an OHC V8 is going to fit in that engine bay. It required fenderwell headers that the steering shaft snakes through (need to get a pic of this!) and the oil filter is just gone - one hole is plugged and the other has a 90deg AN fitting sticking out with a 180 threaded onto that.

I am more referring to inline 4s with chains.. they should get replaced around 150,000 the SBC has a very short chain with.. but even it can stretch.

As for price for replacing a belt.. don;t know.. I have always done my own. Even my old Tiburon's belt, that was jammed against the side of the engine bay.. and I replaced a cam with a more aggressive one.. only took a couple of hours.

A fiat spider I can do in less than an hour.. probably closer to half an hour

Curmudgeon wrote:

alfadriver wrote:

Curmudgeon wrote: Major curiosity bump: what are the drawbacks to belts from a powertrain engineer's perspective?

None, really. Durability is pretty much it. But it's pretty darned important. Cam accuracy is the same- the belts don't stretch. Quieter, lower rotating mass, less friction, etc. But that durability issue is oh so #1.

Yeah, I can see that. People coming through here still piss and moan about 60k timing belt intervals.

The goal, really, is no replacement. Which is one reason why the wet belt is being developed. Will it happen? Dunno. Hope so- a percent here or there means something.

It's worth discussing cost, too. Price the cost of upgrading the cam, springs, retainers, etc. in a pushrod 5.0 vs. the same upgrade in a 4.6 DOHC.

mad_machine wrote: I am more referring to inline 4s with chains.. they should get replaced around 150,000 the SBC has a very short chain with.. but even it can stretch. As for price for replacing a belt.. don;t know.. I have always done my own. Even my old Tiburon's belt, that was jammed against the side of the engine bay.. and I replaced a cam with a more aggressive one.. only took a couple of hours. A fiat spider I can do in less than an hour.. probably closer to half an hour

OK, lets look at costs. Chain type OHC Honda engine is $400 in parts plus 6.5 hours of labor every 150k miles compared to the $140 For a timing belt and tensioner every 105k miles.

Chains are not very "cost effective" in this manner. Add in the extra noise and drag how is a timing chained OHC cam engine a "smart move"?

In reply to Knurled:

Its V.A.G. for a reason......because when maintence comes up its the only money reducing agent know to man that's greater than a woman......

In reply to Bobzilla:

Am I the only one here who has to deal with a timing belt and timing chain on the same engine? LoL

Knurled wrote: I've seen a 460 in an RX-7,

That's a lot of weight on the front end. I bet it would do the quarter in a hurry though.

Toyman01 wrote:

Knurled wrote: I've seen a 460 in an RX-7,

That's a lot of weight on the front end. I bet it would do the quarter in a hurry though.

Not that much different than the iron 5.3/6.0 Chevy engines that people keep talking about, if you go with an aluminum intake and heads.

As I recall, he had issues getting it to clear the steering box.

It wasn't all that fast, as it was a Pro Street car.

Did someone ask for a multivalve DOHC belt drive V engine?

You want gnarly induction? Look up sleeve valves. I'd like to see this make a comeback with today's synthetic oils to cut down friction.

Bristol Hurcules

Head may explode.

Knurled wrote:

Toyman01 wrote:

Knurled wrote: I've seen a 460 in an RX-7,

That's a lot of weight on the front end. I bet it would do the quarter in a hurry though.

Not that much different than the iron 5.3/6.0 Chevy engines that people keep talking about, if you go with an aluminum intake and heads. As I recall, he had issues getting it to clear the steering box. It wasn't all that fast, as it was a Pro Street car.

http://innerwitness.org/automotive/rx7liter/rx7l_photos.html