SBC, V8s, and OHV drivetrains

[mjoekingz28]

Hey, im just wondering who all still uses pushrod OHV engines. I see the new Chevy V8s are probably putting over 100hp more than my 6.0 and I just cant figure out where it comes from.


Also, are the Hemis still old-school?

I think pretty much all Fords are OHC now.

Why does it matter? Well for one reason, imo, the timing chain may outlive one from an OHC.....anymore benefits to OHV engines that you can think of?

 

[BobsArmory]

Sometimes high tech isn't the best answer to the problem. Some times it is.

 

[Red91]

I'm pretty sure it's just GM and Chrysler nowadays. Personally, I haven't seen proof that either valve train configuration is more reliable. Both OHV and OHC engines have their benefits and drawbacks.

 

[Charlie1935]

Cheaper to manufacture.

 

[simple_gifts]

OHC won't fit under a Corvette hood.

 

[michaelluscher]

I like the smaller and more compact nature of a OHC.

Alleged concessions to modernity have given us some really good engines like the 4.6 Mod motor, the GM high feature 3.6, and the Pentastar V6.

 

[OneEyeJack]

Originally Posted By: simple_gifts
OHC won't fit under a Corvette hood.


+1

 

[fdcg27]

A pushrod engine of any give displacement is more compact than an OHC one.
I wonder why OHC engines have completely displaced pushrod designs in smaller car engines?
A short chain is always more durable than a long one, or a timing belt.
A pushrod inline four would be a smaller engine, so would be easier to package.
The question should be why pushrod valvetrains aren't more common.
They aren't any older tech than OHC designs, either.
Either arrangement dates back to well before anyone here was born.

 

[sciphi]

Pushrods are more reciprocating mass that isn't involved in moving the car, as are the rocker arms the pushrods actuate. Likely the valve springs have to be stiffer to help force the pushrod back down to get ready for the next bump in the cam. So in theory there's a bit of power left on the table. Contrast that with a long belt or chain whizzing around and across the guides, and with today's materials and lubricants it's likely a wash in power.

Not sure why it's the preference for OHC over OHV.

 

[Cujet]

The Honda motorcycles CX500 and 650 used pushrod, 4 valve heads with great success. They were known to be very reliable and long lived. They also operated at a fairly high RPM all the time. It was not unusual for them to see 10,000 RPM.

It's also not true that a shorter chain will last longer or be more reliable. A longer chain's individual links will see fewer turns. The bottom line is that there is no difference in lifespan between long and short chains.

 

[bdcardinal]

Originally Posted By: OneEyeJack
Originally Posted By: simple_gifts
OHC won't fit under a Corvette hood.


+1


LT5 in the C4 Vette was DOHC.

It all comes down to manufacturer preference. Ford likes the cammer engines, and they have made huge improvements to the modular engine starting with the Town Car in 1991 all the way tot the Trinity 5.8 and Coyote 5.0 engines.

GM and Chrysler seem to both prefer pushrod V8s, and they have made them work well. Seems like Chrysler gave up on the OHC v8 and replaced it with their Hemi V8.

The new Hemi is not a true Hemi like the originals.

 

[SteveSRT8]

IMO the packaging issue is huge. There is a reason that the new Vette has the lowest center of gravity ever measured! It's that super compact OHV engine design. Yes, any design can be made to fit, but size is very important, and DOHC means giant heads and tons more pieces and parts.

Most any old hot rodder will tell you simple is good for reliability and many folks prefer simple over complicated. Roller bearings and light weight have made OHV valvetrains capable of near magic these days, running huge cam profiles quietly and reliably....

 

[Cujet]

My Honda S2000 has one of the highest "specific outputs" of any commonly available piston engines. At 125HP/L, absolutely amazing piston speed at nearly 5000 feet/m (second highest of any production car)

So what? It's a gutless, torque-less, annoyingly revvy engine that is incredibly impractical. I've added a turbo to make it more pleasant. It now makes 407HP at the rear wheels. It now weighs more than a Corvette engine, get's worse MPG and is still sometimes annoying to drive due to lack of torque. I'd love an S2000 with a 6.2L LS3.

We used 4 valve, cross flow, double overhead, forced induction engines in WW-II. They are nothing new. There is a place for each design type.

 

[DriveHard]

Originally Posted By: Cujet


It's also not true that a shorter chain will last longer or be more reliable. A longer chain's individual links will see fewer turns. The bottom line is that there is no difference in lifespan between long and short chains.


I would have to respectfully dis-agree with that statement.

On an OHV V8 engine, each link will only have to bend and un-bend twice per revolution, and the extent of the bend is quite small. Both are well under 90 degrees.

On a OHC engine, there is usually AT LEAST 4 bends and un-bends per revolution, and often they are of higher angle than that of an OHV engine. So, for each engine revolution, the OHC engine will cause more wear on each link.

Furthermore, the longer distance between the crank and the camshaft on an OHC engine will have more effects of chain wear than a shorter OHV cam chain. If each link wears 0.005", and there are only 7 chain links between the crank and the camshaft on an OHV engine compared to the 20 or so links between the crank and cam on an OHC engine, you can see how the wear per link will effect the cam/crankshaft relationship much more quickly on the OHC engine as compared to an OHV engine. (yes, I realize cam position sensors and cam phasers can help with this problem)

There are trade-offs for each design, and they both have their place. IMHO, OHV offers more positives than the OHC design for most applications.

 

[Cujet]

Funny how my F150's 4.6L timing chain is still intact at well north of 300,000 miles. And Bob's Chevy small block is on it's 2nd timing chain.

The long chain only revolves once for each 8 engine revolutions. Very low loading, and near zero heat buildup. And, it's easy to tension without adding stress.

 

[wirelessF]

Until you have to service one of these:
Audi:

GM 3.6L V6

Timing chains shouldn't have to be serviced unless the top end has to be removed but in these cases the timing chains seems to create more issues then being more reliable. IMO real timing chains are in OHV engines where they are more reliable.

 

[Eddie]

The thinking was that 1. OHC allowed higher RPM. 2. OHC allowed greater freedom in designing exhaust & intake ports & 3. fewer parts to break. Ed

 

[bdcardinal]

Originally Posted By: Eddie
The thinking was that 1. OHC allowed higher RPM. 2. OHC allowed greater freedom in designing exhaust & intake ports & 3. fewer parts to break. Ed


Both are true. Generally a DOHC engine will be higher revving than a OHV engine. The highest revving engines are DOHC, F1 V10s were pushing 21,000RPM in the glory days, of course they did not use metal valve springs.

Also port design is almost unlimited on a DOHC engine since there are no pushrods in the way of the intake ports. Check out the ports on a Ford DOHC 4.6/5.0/5.4/5.8, they are huge and flow like a toilet flushing.

The parts breaking thing is a wash. Both have their plusses and minuses, all comes down to the manufacturer's individual designs.

 

[fdcg27]

Originally Posted By: Cujet
Funny how my F150's 4.6L timing chain is still intact at well north of 300,000 miles. And Bob's Chevy small block is on it's 2nd timing chain.

The long chain only revolves once for each 8 engine revolutions. Very low loading, and near zero heat buildup. And, it's easy to tension without adding stress.

Also funny that two of three of the 4.6 CVPIs in our fleet at work required valve guide seal replacement at well under 200K and two of the three also required timing chain replacement at well under 200K.
IOW, YMMV.

 

[dailydriver]

Originally Posted By: Cujet

Another too cool swap!

If it fits in there, my guess is it will fit in an FT86/BRZ as well.