OHV vs. OHC

I'm not really knowledgeable about engines, but I have a handle on the differences between an Over Head Valve (aka Pushrod) engine and an Over Head Cam engine (usually referred to as SOHC or DOHC, single or dual over head cam).

I don't mean to pick on GM. I own a GM car. I just have noticed that GM uses a lot of 3.4 liter OHV V6s for 180 horsepower/205 torque, many 3.8 liter OHV V6s for 205 horsepower and 225 torque, and the 5.7 liter OHV V8s in the Corvette and new GTO with 350 horsepower and 365 torque.

Those are all respectable power figures, but you can get that much power from significantly smaller (and I'm guessing, lighter) engines with DOHC. The 3.3 liter DOHC V6 Toyota just released gets over 225 horsepower and torque. Nissan's 4.5 liter DOHC V8 in the M45 gets 335 + horsepower and torque. That's about 20% smaller than the Corvette engine for only 4% less power. The Dodge Viper uses an 8.3 liter V10 that gets 500 horsepower and 525 pound feet or torque. Those are impressive numbers, but not when you consider that most other exotic cars with naturally aspirated engines get nearly as much horsepower and torque with a quarter less displacement.

Heavier rotating/reciprocating accessories limit RPM. The higher the RPM, the higher the power, given the same torque throughout the RPM range.

BTW, OHV includes OHC. Just like how a square is always a rectangle, but not all rectangles are squares.

the high displacement pushrod engines make a lot of torque really low in the rev range. that is primarily why Gm and Dodge use them. plus they are cheaper to make and most backyard mechanics are familiar with them.

Heavier rotating/reciprocating accessories limit RPM. The higher the RPM, the higher the power, given the same torque throughout the RPM range.

BTW, OHV includes OHC. Just like how a square is always a rectangle, but not all rectangles are squares.

Thanks for the response.

I understand that the OHC engines can rev higher because of lighter accessories. That's what confused me. It seems like OHC engines are better than Pushrod engines in every way except price to manufacturer, and I wondered if price was the only reason they were still being produced.

Okay, so now I know OHV includes OHC. I guess I should have compared Pushrod vs. OHC.

the high displacement pushrod engines make a lot of torque really low in the rev range. that is primarily why Gm and Dodge use them. plus they are cheaper to make and most backyard mechanics are familiar with them.

Thanks for the reply.

So an equivalent displacement OHC engine would have less torque in the low RPMs? I can see why performance car drivers would want to use big pushrods, then. Thanks.

I suppose R&D of new technology always costs too much for the bean counters. Plus, if the engines currently in use deliver enough power/torque at an acceptable mileage, then there really isn't a problem. When I was younger, though, I did have a problem with the Viper's 8L engine making only 1 hp per cube...

I suppose R&D of new technology always costs too much for the bean counters. Plus, if the engines currently in use deliver enough power/torque at an acceptable mileage, then there really isn't a problem. When I was younger, though, I did have a problem with the Viper's 8L engine making only 1 hp per cube...

I understand that businesses sell product to make money, and I don't begrudge GM or any other automaker their profit.

I just wondered whether OHC engines are inherently superior to Pushrod engines across the board. As it turns out, each type has its own benefits and drawbacks, and that's fine. I thought OHC were superior, and GM continued to make pushrods just to save money.

I would think that any money spent developing a truly superior engine would be offset by the profit from increased sales it might bring.

What I really think GM should do is make a production version of the Cadillac 16 concept engine, the 13.6 liter V16 with 1000 horsepower and torque. I'll take two, please :biggrin:

an equivalent displacement OHC engine would have less torque in the low RPMs

I don't agree with that statement- if the valve lift profiles were the same, and the basic engine geometry was the same, then the torque curves should be very similar (if not identical).

GM seems to prefer timing chains over timing belts, and timing chains are hard to do with OHC engines. GM also seems to have a whole lot of customers who think that pushrod engines are somehow superior to OHC engines...

GM seems to prefer timing chains over timing belts, and timing chains are hard to do with OHC engines. GM also seems to have a whole lot of customers who think that pushrod engines are somehow superior to OHC engines...

Well, this particular car buyer just wants the most mileage efficient and powerful engine possible for any given price, regardless of the way valves are open and closed or whether it uses chains or belts. It's all theoretical anyway, since I intend to run my current car into the ground before replacing it.

I bought an Impala in 2001 right after my college graduation because, at the time, the only thing that mattered to me was a spacious interior, reliability, and good crash test scores. It's a decent car, don't get me wrong, but now I'm getting a lot more interested in performance.

I started wondering about the advantages and drawbacks of different engines when I compared the 2004 Impala SS with the 2004 Nissan Maxima. I realized that the 2004 Impala SS uses a supercharged 3.8 liter Pushrod V6 to get 280 torque and 240 horsepower while the Nissan DOHC VQ engine, a naturally aspirated V6, gets 265 horsepower and 260 torque out of 3.5 liters in the Maxima. The cars weigh about the same and are both FWD, but the Maxima is supposed to be significantly faster. On first glance, all that seems to indicate that DOHC is vastly superior to pushrod for generating power.

BMW and Saab are both using timing chains with overhead cams, I believe Ford are also using it on a few engines so this shouldn't cause any problems if the engine is designed for this.

It have at least been shown that a 4 valve head will give higher torque during the whole rpm range than a 2 valve head. A 4 valve head with pentroof type chambers will be difficult to make with pushrods if we want the intake and exhaust valve timing to be adjustable.

BMW and Saab are both using timing chains with overhead cams, I believe Ford are also using it on a few engines so this shouldn't cause any problems if the engine is designed for this.

It's more difficult & expensive to design it this way than to use a belt drive, at least in my experience. With a dohc v engine, chain dynamics get pretty hairy pretty fast - belts behave much better.

an equivalent displacement OHC engine would have less torque in the low RPMs

I don't agree with that statement- if the valve lift profiles were the same, and the basic engine geometry was the same, then the torque curves should be very similar (if not identical).

GM seems to prefer timing chains over timing belts, and timing chains are hard to do with OHC engines. GM also seems to have a whole lot of customers who think that pushrod engines are somehow superior to OHC engines...

After writting that I regreted it because i knew it was not complately true. I think it was the Corvette grand sport that chevrolet put DOHC on their 350 and it signifigantly increased power.

And as for the chain drive thing I think that belt drive is generally quiter and probably signifigantly cheaper.

After writting that I regreted it because i knew it was not complately true. I think it was the Corvette grand sport that chevrolet put DOHC on their 350 and it signifigantly increased power.

And as for the chain drive thing I think that belt drive is generally quiter and probably signifigantly cheaper.
Although not as reliable, I should think.

BMW and Saab are both using timing chains with overhead cams, I believe Ford are also using it on a few engines so this shouldn't cause any problems if the engine is designed for this.

It have at least been shown that a 4 valve head will give higher torque during the whole rpm range than a 2 valve head. A 4 valve head with pentroof type chambers will be difficult to make with pushrods if we want the intake and exhaust valve timing to be adjustable.
In the higher RPMs, definitely. But in the lower RPMs? With more valves (and hence more valve area) the intake charge velocity lowers, and cylinders don't fill as well (at low speeds).

Of course 4-valve heads have the potential to best 2-valve heads at all speeds, but IMO variable valve control would be needed.

advantages of a pushrod not mentioned-lower center of gravity due to lack of parts above engine block, torque and HP in everyday usage range (great for trucks etc. which need it to get a load moving). the c5r was one of the best GT cars in its class, it used a pushrod. keep in mind that a lighter rotating assembly will reduce the noticeable differences, but would be less capable of holding up to the heavier GM vehicles, exceptiong the use of space-age materials i.e. Ti alloys

THE END ALL BEAT ALL:
4+ second drag cars stil use over head valve push rod engines. So to say that the most powerfull cars in the world are still using bad technology is not necassarily right.
The overhead cam cars are less restricted from the factory, and also have far less potential power. The matter has been researched, and there is no comparison. There just is no replacement for displacement.
That being said, I agree about the newer 3.3L Toyota engines. My friend has an '4 Solara with a 3.3L and that thing rocks. I was very very impressed.
Jake

A 4 valve design head gives better torque over the whole rpm range when designed for similar power outputs. It also offers lower exhaust emissions and lower fuel consumption. But this at an increased cost. Today even heavy trucks use four valve technology but they often use SOHC with rockers or push rods, the later usually because it allows separate cylinder heads for fast repairs.

The power potential is far greater in a four valve DOHC than in an push rod two valve design. The only reason that two valve push rod engines still are used in dragracing such as Top Fuel is because everything else is banned. The same goes for NASCAR Nextel Cup and valve train specialists like Del West have infact said that it can be more difficult to make a NASCAR valvetrain than a DOHC system in an high speed engine.

There is a replacement for displacement, high engine speeds and turbochargers to mention two. Turbochargers usually tends to end up banned as they offer such an great advantage.

A DOHC engine isn't that much higher compared to a push rod engine, and when one counts in the increased power from the DOHC engine there isn't much to talk about. Most gains in lower center of gravity are achieved by a dry sump system and a smaller clutch.

Corvette C5-R competes in mostly endurance races where the engine is fitted with an restrictor. In that case you usually make the best results by reducing engine speed and make the engine large or even better turbocharged to reduce engine stress and increase driveability and reduce fuel consumption since you can't get more power out of the engine anyhow because of the restrictor.

A 4 valve design head gives better torque over the whole rpm range when designed for similar power outputs. It also offers lower exhaust emissions and lower fuel consumption. But this at an increased cost. Today even heavy trucks use four valve technology but they often use SOHC with rockers or push rods, the later usually because it allows separate cylinder heads for fast repairs.


I can't speak to repairs, but most heavy duty trucks use inline engines. For an inline engine, DOHC unequivocally is superior to pushrods. An inline pushrod engine adds additional reciprocating mass with no accompanying benefit.


The power potential is far greater in a four valve DOHC than in an push rod two valve design.


It's not 'far greater' until you are pretty high in the rev range.


The only reason that two valve push rod engines still are used in dragracing such as Top Fuel is because everything else is banned.
The same goes for NASCAR Nextel Cup and valve train specialists like Del West have infact said that it can be more difficult to make a NASCAR valvetrain than a DOHC system in an high speed engine.


Granted. But what works in a race car isn't necessarily appropriate or efficient for street use.


There is a replacement for displacement, high engine speeds and turbochargers to mention two. Turbochargers usually tends to end up banned as they offer such an great advantage.


As long as you design the heads on your pushrod for it, they can accept forced induction just as well as DOHC engines.


A DOHC engine isn't that much higher compared to a push rod engine, and when one counts in the increased power from the DOHC engine there isn't much to talk about. Most gains in lower center of gravity are achieved by a dry sump system and a smaller clutch.


This is where I disagree. A 4 valve DOHC V-configuration engine has 3 extra camshafts, 2 extra valves, and 2 extra springs per cylinder compared to a 2 valve V-configuration pushrod engine. That adds a lot of extra mass.

The easiest engines to use as benchmarks, naturally, are the LS-series from GM. The LS1 engine with an automatic weighs 495 pounds and is 5.7 liter pushrod V8 with horsepower 350 and torque 365. For comparison, the Cadillac NorthStar V8 in the XLR with an automatic weighs 490 pounds and is a 4.6 liter DOHC with horsepower 320 and torque 315. The NorthStar is superior by horsepower/liter but inferior by horsepower/pound of engine weight.

The Corvette C5 is larger than the XLR in every dimension while still being lighter and substantially faster.

The Mustang Mach1 DOHC V8 can't hold a candle to the LS1 in horsepower to liter or horsepower to weight, and while the supercharged form in the Cobra wins on horsepower to liter, it's still behind in horsepower to weight despite forced induction.


Corvette C5-R competes in mostly endurance races where the engine is fitted with an restrictor. In that case you usually make the best results by reducing engine speed and make the engine large or even better turbocharged to reduce engine stress and increase driveability and reduce fuel consumption since you can't get more power out of the engine anyhow because of the restrictor.

I don't know what the C5-R is, but the C5 itself is a phenomenal performance car for raw power, not just endurance. It won't out-handle an AWD Lancer Evolution or WRX STI, but in the hands of a skilled driver it will outrun any unmodified car you can get at or below its price range and beat all but a handful around curves.

The 2005 GTO may look boring as hell, but it's getting the LS2 engine. It's 0.3 liters bigger but 15 pounds lighter and just as fuel efficient as the LS1, with horsepower and torque peaking at a phenomenal 400. How many other cars can you buy with that kind of power or power to weight ratio for under $40,000? Zero.

The Japanese make phenomenal cars, don't get me wrong. But whether intentionally or not, their fans have adopted the perception that DOHC is always better. It isn't. In racing where money is no object and nobody cars about torque or horsepower below 5,000 RPM, DOHC is the way to go. But for a performance car that's drivable on a daily basis, pushrods are just fine.

An inline pushrod engine adds additional reciprocating mass with no accompanying benefit.
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This is where I disagree. A 4 valve DOHC V-configuration engine has 3 extra camshafts, 2 extra valves, and 2 extra springs per cylinder compared to a 2 valve V-configuration pushrod engine. That adds a lot of extra mass.



We dont mind you rebutting an argument, just pick one side and stay on it however ;-)

Saab is correct, most heavy truck engines (inline or V) using more than 2V heads use pushrods and rockers designed to activate more than one valve. This is not limited to pushrods, Honda has made 4V heads with SOHC for years and years.

there are 2 issues in the debate that people get confused.

1) 2v vs 4v

2) OHC vs OHV

considering the latter first, I state again 'Eds Maxim #475': valve actuation has no effect on power, and I add Eds corrolary #23: However OHC designs add a tremendous amount of friction and inertia to the motor which does have a net effect on power.

Now addressing the former: 4V designs will always make more power 'lift for lift' vs 2V designs - thats a no brainer. However, unless you use variable valve timing AND lift (which GM is working on) the power band is moved up the RPM scale. Good for racing motors. Bad for street/work motors. I submit most of us drive on the street and tow stuff a lot more than we race.

xOHC motors are in many cases a detriment to automotive designers. They severly limit the end result shape of the engine compartment. My fav example is this: the ford 4.6L or 5.4L SOHC Tritons are PHYSICALLY larger than the 7.3 powerstroke diesel. Honda and toyota have laid the xOHC motors at an angle to keep the hoodline reasonable, but in the end waste as much hood length as you would need for a V6 - a trade-off. Hence you see a lot of manufacturers push for xOHC V6 motors vs xOHC I4s - the motor is not as long (end seal to end seal) allowing a narrower track (or a longer travel for the control arms - take your pick), the motor is not as tall meaning you can get a good shape on the nose. The net cost is usually only 5 inches of extra hood length which seems to bother no one in the design department.

An inline pushrod engine adds additional reciprocating mass with no accompanying benefit.
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This is where I disagree. A 4 valve DOHC V-configuration engine has 3 extra camshafts, 2 extra valves, and 2 extra springs per cylinder compared to a 2 valve V-configuration pushrod engine. That adds a lot of extra mass.


We dont mind you rebutting an argument, just pick one side and stay on it however ;-)


Before I continue, you sound like you know more about this than I do. I'm not opposed to being corrected when I am wrong.

That said, let me attempt to clarify those two seemingly contradictory statements.

In an inline engine configuration, you have a row of cylinders, valves on the cylinders, and a camshaft (or two with DOHC). With a pushrod inline engine, the camshaft is not above the cylinder heads. Thus, it still requires rods and rocker arms. With an OHC engine, the camshaft is above the cylinder heads.

In that case, with one row of cylinders, there's no advantage to the pushrod because there's extra mass with no other benefit. There's a marginal beneft in that the pushrod inline engine is shorter. However, it's also wider.

In a V engine configuration, it's different. A pushrod V engine has one camshaft, while a SOHC or DOHC engine would have 2 or 4, respectively. The pushrod's camshaft is between the two rows of cylinders. That makes the overall engine shorter, which has advantages for fitting it in engine compartments and making it lighter.

Make sense?

It's not 'far greater' until you are pretty high in the rev range.

DOHC with four valve technology offers a more torque over the whole rpm range for two engines with similar power outputs as well as lower exhaust emissions, so it hasn't only high speed advantages. The advantages are great enough so that most car manufacturers accept the increased cost... and they usually hold on to their money.


Granted. But what works in a race car isn't necessarily appropriate or efficient for street use.

DOHC with 4V/cyl has benefits both for racing and street.


As long as you design the heads on your pushrod for it, they can accept forced induction just as well as DOHC engines.

But they will still have the disadvantages of the push rods.


This is where I disagree. A 4 valve DOHC V-configuration engine has 3 extra camshafts, 2 extra valves, and 2 extra springs per cylinder compared to a 2 valve V-configuration pushrod engine. That adds a lot of extra mass.

The DOHC engine has four smaller valves, and these actually has less reciprocating mass than of a two valve engine. This means that they are suitable for higher engine speeds or keep the speed the same and use lower spring rates which decrease friction. Furthermore the DOHC engine has no rockers or push rods and these are, and the rockers at least partially reciprocating weight. The extra cams, which usually only has a weight of a few kg each are only rotating weight and is mostly important to reduce as it reduce the weight of the engine, of this reason racing engines tend to use gun drilled camshafts


The easiest engines to use as benchmarks, naturally, are the LS-series from GM. The LS1 engine with an automatic weighs 495 pounds and is 5.7 liter pushrod V8 with horsepower 350 and torque 365. For comparison, the Cadillac NorthStar V8 in the XLR with an automatic weighs 490 pounds and is a 4.6 liter DOHC with horsepower 320 and torque 315. The NorthStar is superior by horsepower/liter but inferior by horsepower/pound of engine weight.

The Corvette C5 is larger than the XLR in every dimension while still being lighter and substantially faster.

The Mustang Mach1 DOHC V8 can't hold a candle to the LS1 in horsepower to liter or horsepower to weight, and while the supercharged form in the Cobra wins on horsepower to liter, it's still behind in horsepower to weight despite forced induction.

It's basicly impossible to try comparing the weight of two different engines. Let's say that I compare a Mopar 7 litre V8 engine with a Judd GV5 V10 which has a displacement of 5 litres. Both produce around 600 hp and are built for GT racing.

-- Mopar specs --
Length: 629 mm
Width: 597 mm
Height: 607 mm
Weight: 170 kgs.


-- Judd specs --
Length: 622.5 mm
Width: 569 mm
Height: 436 mm (excluding trumpets)
Weight: 140 kgs

As you can see the Judd engine is smaller, especially in height, and 30 kg lighter.

I don't know what the C5-R is, but the C5 itself is a phenomenal performance car for raw power, not just endurance. It won't out-handle an AWD Lancer Evolution or WRX STI, but in the hands of a skilled driver it will outrun any unmodified car you can get at or below its price range and beat all but a handful around curves.

The 2005 GTO may look boring as hell, but it's getting the LS2 engine. It's 0.3 liters bigger but 15 pounds lighter and just as fuel efficient as the LS1, with horsepower and torque peaking at a phenomenal 400. How many other cars can you buy with that kind of power or power to weight ratio for under $40,000? Zero.

Corvette C5-R

http://www.c5rmotorsports.com/new_images/webmain2.gif

It's a racing car built especially for races like Le Mans 24 hour race. It has an engine prepared by Katech engines.

Check out www.c5motorsports.com and www.katechengines.com

The Japanese make phenomenal cars, don't get me wrong. But whether intentionally or not, their fans have adopted the perception that DOHC is always better. It isn't. In racing where money is no object and nobody cars about torque or horsepower below 5,000 RPM, DOHC is the way to go. But for a performance car that's drivable on a daily basis, pushrods are just fine.

Actually pushrods aren't just fine, thats why japanese and european car manufacturers use 4v/cyl and over head cams, it's just that in europe and japan the demands on the engines have been higher than it have been in the US, the reasons for that is gas price, size of the roads, laws and so on. In the US it have earlier not been much of a reason to make costly small powerful engines when you instead could simply increase the displacement of the engine you have and get the power that you wanted, that was used in the pre war racing era in Europe too. Today that have changed and we can also see that on the engines from the american manufacturers which have become smaller and equipped with more costly solutions.

Another reasons for the smaller engines that I didn't mentioned earlier is probably that the racing engines in europe was smaller. The idea to make them smaller came after the second world war to ban the large airplane engine engines which was used earlier, and make the cars more suitable for trickier racetracks.