OHV vs. OHC

[Michael_S]

I hope you ladies and gentlemen don't mind, but I'd like to draw upon your knowledge to answer a question.

What advantages are there, if any, to making an Over Head Valve (aka pushrod) engine vs. an Over Head Cam engine?

The GM 3.4L V6, 3.8L V6, and 5.7L V8 all make much less power than SOHC or DOHC engines with the same displacement. The Dodge Viper needs 8.3 liters in its OHV V8 to get 500 horsepower.

So why did GM just put a 3.5L OHV V6 in the new Malibu, announce a next generation 3.9L OHV V6 for the other sedans, and announce a new 5.3L OHV V8 for the next Corvette and next year's SUVs?

I'm very confused by this.

[InSaiyan]

For starters, GM sucks. That 3.4 (3400) is an overbored 3.1, which is a stroked 2.8, explaining why they're so unreliable. btw, where have you seen a 3.4/3.8/5.7 ohc engine?
A pushrod engine doesnt nessessarily mean less power. The '87-93 5.0 H.O (pushrod) was rated at 225hp, the version that the Vics got was a mere 150hp,160hp with dual exhaust.
The '96 4.6 ohc GT engine was rated at 235/240hp, but the Vics only got around 200. It all depends on how the manufacturer builds and tunes the engine.
Look on a drag strip with American cars, chances are everything thats not a '96+ Mustang has a pushrod engine. Its cheaper and arguably more reliable to build an ohv engine (less parts). Also look at dyno charts, an ohv 302 will tend to put out more torque the hp, the ohc/dohc 4.6 is usually just the opposite. A 302 will the same hp as a 4.6 will put down more torque, torque wins races.
btw, the Viper doesnt "need" 8.3 liters, but the techies and engineers at Dodge didnt want to try I guess. I know a guy who's got a 1999 Trans Am (automatic with t-tops :) ) that runs mid-9's (yeah, in the 1/4, not no stinkin 1/8), he's got a stock bottom end minus the forged pistons, and an ATI Procharger, probably putting down over 500hp at the wheels. Its an ohv 5.7 (347 c.i., they havent been 350's since TPI). All that and it DID pass emissions.

[sailsmen]

Over head cam places the part that moves the valves in close proximity to the valve allowing for more accurate valve control, less friction and as a result higher RPM.

[TripleTransAm]

Horsepower is just a mathematical product of torque x RPM. High horsepower doesn't necessarily mean a torquey engine or good acceleration. So big hp numbers can be attained just by making the engine produce *some* useable torque at higher RPM. And the way to do that is to get the losses out of stuff like valvetrain, pistons, internal friction, airflow, etc... stuff that sucks up lots of power at higher RPM.

OHC valvetrains are much lighter and less complex than the OHV counterparts. Forget pushrods, lifters (in some cases), rocker arms (in some cases)... not to mention that 4 valves per cylinder are downright quasi-impossible to implement in an OHV configuration... and multi-cams (one intake, one exhaust) are totally unfeasible.

So this lightness of valvetrain along with easier intake/exhaust port design makes for more power up top.

But there's no reason why one can't design a modern efficient engine based on the older OHV style that delivers exceptional power. Again, I gotta point to the LS1... > 300 lb-ft. torque curve flatter than my ex-girlfriend's chest from just under 2000 until about 5700, and what great numbers too. Over 350 lb-ft of torque at the peak, and a solid 340 hp at the crank, for my "slow" '98 WS6. All with "ancient" OHV technology. Oh, I almost forgot the 33 mpg highway mileage I consistently get on long trips (verified once again in September).

I'd say the reason they went with OHV engines in the entry-level engines mentioned was to control costs... the OHV components are less expensive and less 'exotic', as the complexity rises with OHC designs.


About the GM V8 engine sizes, the LT1 is also supposed to be a true 350, but the LS1s (1997-present) are actually about 5.6l (346 to be exact).

[deerejoe]

"Ford Tool": (aka "beater") is the tool of choice for Ironworkers!!

Cut to size, beat to fit and paint to match...luv my Marauder.

[Michael_S]

Thank you for the info. I'm not doubting that OHV engines can work well, last a long time, or deliver reasonable gas mileage. Nobody claims the Corvette ZO6 or the Viper are slow.

I also understand the relationship between horsepower and torque, horsepower = (torque * RPM)/5250 RPMs. So 240 horsepower at 2000 RPMs means insane amounts of power, while 240 horsepower at 8500 RPMs is not nearly as impressive (e.g. Honda S2000, which from what I read doesn't behave like a Sports car until you're above 5500 RPMs.)

There's three things I am questioning here:
1. If the valvetrain is simpler and lighter for an OHC engine, then how come OHV engines are easier to produce?

2. If an OHC engine has a lighter valvetrain, I would think it would both be able to rev higher than an OHV engine and also make just as much torque in the lower RPM range.

3. If an OHC engine can rev higher than an OHV engine more easily, wouldn't it make sense to drop a smaller, lighter OHC engine in a performance car and then just jack up the final drive ratio to take advantage of the bigger RPM range? That way, even though there's less power at the crank I would think you've got the same power to the ground and you're also saving a lot of weight on the engine. - From what I understand, that's how the Honda S2000 could break a 6 second 0-60 with a measly 153 pound feet of torque available at 5500 RPM.

[derek77]

I'll throw this in the OHV vs. OHC fire.

DIAGRAM OHV 3-Valve Cylinder Heads (http://media.gm.com/division/2004_prodinfo/tech_displays/powertrain/OHV_3valve.pdf) DIAGRAM

By Bill Visnic

Ward's Auto World, Oct 1, 2003 Brought to you by:

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In one of the most significant redesigns in the 50-plus-year history of its “small-block” V-8 engine architecture, General Motors Corp. powertrain engineers are developing a new 3-valve-per-cylinder layout for GM's overhead-valve V-8 and V-6 engines.

The new 3-valve design is meant to enhance the performance of the OHV V-6 and V-8 engines — currently using 2-valve-per-cylinder layouts — that GM prefers to the overhead-camshaft engines often favored by Japanese and European auto makers. GM Powertrain builds millions of the current OHV V-6/V-8 2-valve engines.

That group includes the legendary small-block V-8, which GM installs in everything from Chevrolet Silverado to Cadillac CTS.

GM Powertrain sources will not say when the 3-valve engines will be in production. And although Alan Hayman, manager-advanced concept group in GM Powertrain's Advanced Powertrain unit, will not confirm a production-vehicle launch date for the new engine design, he says launch of the 3-valve architecture would come after the so-called “Gen IV” version of the small-block V-8 is introduced some time next year.

For V-6s, sources say the 3-valve architecture — which employs a unique “transfer pushrod and transfer arm” to activate the exhaust valve — could add 10% to 15% more power with no impact on fuel economy.

A new cylinder head design employs the transfer arm and pushrod to locate the exhaust valve across the head from the dual intake valves. The new layout also repositions the sparkplug to a central location between the intake and exhaust valves.

“We're doing it for performance,” Hayman says. “The customer is asking for more performance without a cost increase.”

Hayman says a V-6 generating 240 hp, for example, could produce 270 hp by using the 3-valve cylinder head — yet the cost increase would be marginal. “A lot cheaper than an overhead-cam 3-valve,” says Hayman, in apparent reference to Ford Motor Co.'s new-for-'04 3-valve design for its Triton V-8 engine family.

He says GM's new 3-valve design is perfectly suited to fulfill the low-cost priority. The layout “requires minimal tearup to the (engine) lower end,” a critical factor in curbing the need for costly new components or castings.

The cost saving in relation to the new performance netted by the 3-valve design is significant. Hayman says that for a 2-valve V-6, adopting the 3-valve cylinder head is about one-third the cost of going to an overhead-cam/4-valve-per-cylinder layout. “We're excited about that,” he says.

Equally important, the 3-valve design is fully compatible with Displacement on Demand, or DOD, the cylinder-deactivation technology GM Powertrain will begin this year to widely deploy to enhance the fuel economy of its overhead-valve V-8 and V-6 engines.

Hayman says because changes for the new engine design are limited almost exclusively to the cylinder head itself, 2-valve and 3-valve variants can run down the same production lines, and “there are only three or four machining differences.”

Hayman says the 3-valve variants likely would be “upgrade” engines, particularly in the case of the V-6s. The 3-valve engines also might be used to differentiate between brands, with the higher-performance 3-valve version of a V-6 going to Pontiac, for instance, underscoring that division's performance image.

Because of the “integral” design, there are no packaging constraints, he adds: The 3-valve V-6s and V-8s could be fitted in any vehicle that uses the 2-valve counterpart.

“I would think we'd always keep the 2-valve (design) as long as it meets the requirement of the market. We'll consider what the marketplace is asking for.”

Hayman agrees the move to a 3-valve layout is a significant step for the beloved small-block V-8 that dates to 1955. “It's as significant as going from Gen II to Gen III.”

But the 5-decade-old small-block's 2-valve design, he insists, still holds room for improvement.

“The 3-valve is a way in the future,” Hayman says. “There are some surprises in store for the 2-valve (small-block V-8). And we're not talking about 10 or 15 hp. Our goal is to remain the leader in truck powertrains.”

[Michael_S]

Thanks, derek77! Very cool.

[RCSignals]

GM has a good OHC design in the Northstar engines (except for the starter placement)

GM is stuck on Cam in block, probably because it can be built using existing tooling. They were working on a two cam in block design. Very odd. Their three valve cam in clock design describe above is typical GM "Rube Goldberg" stuff.

GM has done a lot of weird things in their cars, just look at brake systems of GM cars in the '50s, master cylinder placements, actuating devices etc. Weird stuff when Chrysler and Ford were using the simple systems basically still employed today (and finally adopted by GM)

[InSaiyan]

Does GM even have non-4-banger ohc engine? Besides Northstars and the "Twin Dual Cam 3.4"? < which put out a healthy 215hp in some early 90's Lumina Z34's. Quite a bit more power than any n/a 6 cylinder made before then.

btw, I didnt mean less parts in ohv, theres pushrods and lifters of course. Its just that it costs less for 16 pushrods, lifters, rockers, and one cam than it does for 16 rockers and two cams.
I love the sound of the 4.6 and 5.4, but that classic 289/302 sound cant be beat.

[RCSignals]

Originally posted by InSaiyan


btw, I didnt mean less parts in ohv, theres pushrods and lifters of course. Its just that it costs less for 16 pushrods, lifters, rockers, and one cam than it does for 16 rockers and two cams.

and that is why GM sticks with them. Cost for them to change.




Originally posted by InSaiyan
I love the sound of the 4.6 and 5.4, but that classic 289/302 sound cant be beat.

You think the "Windsors" sound good, listen to a Y-Block sometime, particularly one with duals and Smithys or Glas packs.

[JohnE]

There is another important advantage of an OHV vs OHC to car builders, Size. The overall package can be smaller with a OHV, especially the valve cover area.

OHV has less friction, thus more power at low rpms. It's only at high rpms that the valve trains swap with each other in performance.

Just wait until Electric Valve Technology actually comes into production.

[RCSignals]

OHV has less friction

I don't see how

[MapleLeafMerc]

I'm just gonna subscribe and watch the show here. Like home schooling for free:lol:

This one could have legs.

[Michael_S]

Originally posted by JohnE
There is another important advantage of an OHV vs OHC to car builders, Size. The overall package can be smaller with a OHV, especially the valve cover area.

OHV has less friction, thus more power at low rpms. It's only at high rpms that the valve trains swap with each other in performance.

Just wait until Electric Valve Technology actually comes into production.

I believe you that OHV engines take up less space all together. I think I even read somewhere that it's easier to fit GM's 5.7 liter Pushrod V8 into an engine bay than the 4.6 liter DOHC NorthStar engine.

If the OHV has less friction at lower RPMs, I can see how that's a huge advantage for street performance.

From what I understand, the current drawback to Electric Valve Technology is that it requires a lot more electrical juice than the standard car battery can provide. Many automotive manufacturers aren't that interested in it because it would require major (expensive) changes to the electrical system to support it.

So... maybe it would mix well with hybrid technology. If you already have a 270 volt battery in the car (the 2004 Toyota Prius has it), that should be more than enough power to open and close the valves in any timing you like.

[derek77]

What's the advantage of either one, I'm not sure. Some say OHV engines don't get the gas mileage and don't make the same hp/liter as the OHC engines. But then you look at the OHV engines from GM and even DiamlerChrysler. The GM V8s get good gas mileage and make great power. DC new 5.7 liter makes great power in their trucks and soon in their cars. Did you know that the new Magnum will be classified as a truck when it goes on sale, just as the PT Cruiser? Just had to let you know. Back to what I was talking about. The new 3.5 liter in the Malibu with a 4-speed gets better gas mileage than the Accord and Camry V6s that have 5-speeds.

Then some people say the OHCs don't have the torque that the OHVs have. Right, tell that to Ford. Look at the new 5.4 in the trucks. People have also said the OHC cost more than a OHV. Does anyone have any numbers on this? I would like to see for myself and compare. But I have never seen any numbers published.

So, which one is better, It's a matter of taste I guess.



Originally posted by Michael_S
From what I understand, the current drawback to Electric Valve Technology is that it requires a lot more electrical juice than the standard car battery can provide. Many automotive manufacturers aren't that interested in it because it would require major (expensive) changes to the electrical system to support it.

That's where the 42v system comes into play. You'll have electric A/C, steering, and braking to go along with the electric valves.

[Michael_S]

Originally posted by derek77
What's the advantage of either one, I'm not sure. Some say OHV engines don't get the gas mileage and don't make the same hp/liter as the OHC engines. But then you look at the OHV engines from GM and even DiamlerChrysler. The GM V8s get good gas mileage and make great power. DC new 5.7 liter makes great power in their trucks and soon in their cars. Did you know that the new Magnum will be classified as a truck when it goes on sale, just as the PT Cruiser? Just had to let you know. Back to what I was talking about. The new 3.5 liter in the Malibu with a 4-speed gets better gas mileage than the Accord and Camry V6s that have 5-speeds.

Then some people say the OHCs don't have the torque that the OHVs have. Right, tell that to Ford. Look at the new 5.4 in the trucks. People have also said the OHC cost more than a OHV. Does anyone have any numbers on this? I would like to see for myself and compare. But I have never seen any numbers published.

So, which one is better, It's a matter of taste I guess.




That's where the 42v system comes into play. You'll have electric A/C, steering, and braking to go along with the electric valves.

I understand that the 42v system has big advantages. I was just stating the reasons why it wasn't in production yet, from what I read. Still, it makes sense to implement it with a hybrid system since the electrical is already in place.


The new Malibu gets better mileage than the Accord V6? I would curious to see what their relative weights and accelerations were. The Accord does have 40 more horsepower, and I can easily see how that would offset the better gas mileage from a 5-speed auto transmission.

GM is also supposedly going to implement the DisplacementOnDemand, which shuts off 2 or 4 cylinders under light loads for an average 6% improved gas mileage. The initial reviews of the concept DoD engines were very good.

Oh, and as for cost - I would bet that the costs aren't that different, but GM doesn't like the cost to convert from one to another. I'm sure that's expensive, no matter which way you go.

[TripleTransAm]

The big thing behind the push for 42V systems (42? 48? I can't remember) is the ability to work more accessories while providing ignition power to the engine. It's all about power...

Some math:
Most know about the V=I x R (where I = electric current, R = resistance within the current path, and V = voltage across that resistance).
Many don't know about P = V x I (P = power (ie. watts, etc.), V = voltage, I = current).

So the more equipment / accessories / engine controls get lopped onto the car's electrical system, the greater the power demand. With a fixed 12V (approx.) system, that means the current has to shoot up to provide the demanded power. That means stressed out alternators, fried wiring, losses due to heat, etc.

So by upping the voltage, you can deliver the necessary power while keeping the current in check (this is the reason why some North American household appliances require 220 V instead of 110V).

[thedarkside]

There is another important advantage of an OHV vs OHC to car builders, Size. The overall package can be smaller with a OHV, especially the valve cover area.

No kidding, I have a 3.0 DOHC mitsu motor that takes up more space than my three fiddy.
And while ford may have a torque monster 5.4 ohc, generally speaking ohc motors have less low end power/flat torque curve compared to good old pushrod power. Lets not forget spinning one cam is easier than 2 or 4. There are lots of good things about ohc, but there a pita to work on so I'm just gonna ignore the good and :flamer: on

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[Michael_S]

I'm glad I asked the question. I've learned quite a bit. Thanks for contributing, everbody.