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SOHC vs DOHC

john112deere

caffeine junkie
Staff member
10,807
405
central Vermont
What is the advantage of DOHC over SOHC? Manufacturers brag about it, but I think that's mostly tapping into the "more is better" mentality...

My understanding (which is limited at best, and quite possibly wrong) is that the primary advantage of OHC instead of pushrods is that it limits valve float because there's less mass to the valvetrain.

Can somebody explain (using small words, please) what the difference/advantages between the one or two overhead cams is?
 

SuperCab

Moderator
Staff member
10,068
547
Montana
The biggest difference I know of is that DOHC is a pain in the arse to work on, SOHC is half the pain.

I like the old pushrod engines myself.

What is the engineering reason to use 2 cams? I don't know. Someone smarter than us will reply and enlighten us though.
 

Fellro

Moderator
Staff member
You tend to find DOHC setups on 4 valve/cylinder heads, while SOHC is typically two valve/cylinder heads.
 

polarbear

just growing older not up
12,878
607
Boring, Oregon
Good question. The Corvette gets ragged on sometimes because it still has a pushrod engine. A 400+ HP pushroad that can propel the car to over 180 mph, and still turn 25-30 mpg in normal driving. Pizzes the multi-cam, block-made-from-unobtanium crowd off to no end (Porsche owners, specifically).

The Z06 really annoys them, with over 500 HP, while the ZR1 will probably send them over the edge.'hah'

Both designs are older than the hills. The OHC is supposed to be simpler and more friendly for high-rpm applications. The GM LS block proves, though, that a modern, thoroughly developed pushrod motor is still competitive with anything more exotic.
 
I can tell you that my 4.0 SOHC engine has at least 4 chains inside it. Does not sound like a good idea to me!

Yes...they go out and are very expensive to replace. Not sure what else they tear up when they go out.

I hate the modern technology though (if you did not already know that).

While I am not into corvettes, they do still have a push rod engine.
 

blackhat620

You Had to be There
1,687
150
Arizona
What is the advantage of DOHC over SOHC? Manufacturers brag about it, but I think that's mostly tapping into the "more is better" mentality...

My understanding (which is limited at best, and quite possibly wrong) is that the primary advantage of OHC instead of pushrods is that it limits valve float because there's less mass to the valvetrain.

Can somebody explain (using small words, please) what the difference/advantages between the one or two overhead cams is?

Ian,

The advantage of OHC engines compared to OHV(pushrod) is exactly what you stated. Less valve train mass with OHC engines results in more precise valve timing do to less valve float. Also it is much easier and more efficient to run multivalve (more than two per cylinder) engines with OHC setup. You can also run variable valve timing with an OHC engine. It is rare to find multiple valve OHV engines (low RPM diesel engines are the exception) as the RPM range is extremely limited in a multiple valve OHV engine.

A typical 2-valve per cylinder OHV engine can safely run to 11,000 RPM in race motors (with solid lifters), were as a multiple valve OHC engine can run to 20,000 RPM. So a smaller cubic inch engine can produce more power with an OHC than the same size OHV engine.
OHC engines have been around since about 1912 and were found in Bentley's Bugatti's, Porsche, Fiat, Sunbeam, Peugeot, Alfa Romeo etc.
(Note: The first high compression OHV engine was introduced in 1949 by Oldsmobile as the "Rocket V8" aka Rocket 88).

The advantage of OHV engines is that they are more compact, cam timing is simpler than in OHC engines and they are cheaper to build. GM is still the largest OHV engine manufacture in the world.


SOHC engines are lighter than DOHC engines so they produce more Hp & torque at lower RPM's than DOHC engines. Basically a SOHC engine produces more Hp & Torque at low RPMS while a DOHC engines produces more Hp & Torque at Peak RPM's and have a wider power band. DOHC engines are more efficient with multiple valve (more than two per cylinder) heads than SOHC engines and it is easier to have more precise cam grinds on a DOHC engine. Another advantage of a DOHC engine is that since the cams run down each side of the head the spark plug can be centered in the combustion chamber for the most efficient burn pattern. In a SOHC the spark plugs must be offset in the combustion chamber since the cam runs down the center of the head. Variable valve timing is easier to achieve on DOHC engines, than SOHC.
(Note: Chrysler introduced the first variable valve OHV engine and it is installed in the 2008 600Hp Dodge Viper)

Disadvantage of DOHC is that the cam drive & timing is more complex and the engine is wider than a SOHC.

Main reason most foreign cars are OHC is that many years ago many foreign countries passed laws penalizing larger displacement engines and imposing fines on the manufactures for making large displacement engines, do to the false belief that large displacement engines were less efficient. In order for the foreign automobile manfactures to produce sufficient Hp & Torque with small displacement engines it was necessary to design the engines to run at high RPM's and so OHC engines were mandatory to meet the RPM requirments.

Each type of engine OHV, SOHC & DOHC has its advantages and disadvantages, choice depends on the specific application needs.

The future technology is cam less engines, they are in existance but are much more expensive to manufacture than OHV or OHC engines. Cam less engines are supposed to be 10-20% more efficient than OHV or OHC engines. Rumor has it that Alfa Romeo may introduce a cam less engine in 2009.
(Note: The first cam less engine design was in the 1947 Tucker Torpedo but legal problems led to it never reaching the production stage).
 
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john112deere

caffeine junkie
Staff member
10,807
405
central Vermont
Cool...

Thanks.

I've heard a little about camless engines. I think Brad mentioned something about Caterpillar working on one. I'm kind of in Brian's camp as far as new technology like that, but it IS pretty cool.
 

blackhat620

You Had to be There
1,687
150
Arizona
I hate the modern technology though (if you did not already know that).

Actually OHC engines have been used in automobiles since about 1912, but OHV engines in automobiles were not in use until 1949. Early automobile engines were either flat head or OHC. Obviously the introduction of the Ford Flathead V8 in 1932 had a huge impact on the automobile market do to its simple design, easy modification and repair for its day. It still suffered from the poor flow characteristics of flathead engines however. Ford jumped on the OHV bandwagon in 1954 with the Y-block after Oldsmobile's success with their OHV V8 launched in 1949.

(Note: About 1947 Ardun had a OHV conversion the Ford Flathead, and in 1948 Joe Davis had a DOHC Ford Flathead V8).
www.streetrodderweb.com/hotnews/0804sr_ardun_heads/index.html
www.roadsters.com/flathead/

Yes OHC engines have longer timing chains/belts than OHV engines and do to the longer chain/belt and higher RPM they must be changed sooner than the timing chain on an OHV engine.
 
Last edited:
Ian,

The advantage of OHC engines compared to OHV(pushrod) is exactly what you stated. Less valve train mass with OHC engines results in more precise valve timing do to less valve float. Also it is much easier and more efficient to run multivalve (more than two per cylinder) engines with OHC setup. You can also run variable valve timing with an OHC engine. It is rare to find multiple valve OHV engines (low RPM diesel engines are the exception) as the RPM range is extremely limited in a multiple valve OHV engine.

A typical 2-valve per cylinder OHV engine can safely run to 11,000 RPM in race motors (with solid lifters), were as a multiple valve OHC engine can run to 20,000 RPM. So a smaller cubic inch engine can produce more power with an OHC than the same size OHV engine.
OHC engines have been around since about 1912 and were found in Bentley's Bugatti's, Porsche, Fiat, Sunbeam, Peugeot, Alfa Romeo etc.
(Note: The first high compression OHV engine was introduced in 1949 by Oldsmobile as the "Rocket V8" aka Rocket 88).

The advantage of OHV engines is that they are more compact, cam timing is simpler than in OHC engines and they are cheaper to build. GM is still the largest OHV engine manufacture in the world.


SOHC engines are lighter than DOHC engines so they produce more Hp & torque at lower RPM's than DOHC engines. Basically a SOHC engine produces more Hp & Torque at low RPMS while a DOHC engines produces more Hp & Torque at Peak RPM's and have a wider power band. DOHC engines are more efficient with multiple valve (more than two per cylinder) heads than SOHC engines and it is easier to have more precise cam grinds on a DOHC engine. Another advantage of a DOHC engine is that since the cams run down each side of the head the spark plug can be centered in the combustion chamber for the most efficient burn pattern. In a SOHC the spark plugs must be offset in the combustion chamber since the cam runs down the center of the head. Variable valve timing is easier to achieve on DOHC engines, than SOHC.
(Note: Chrysler introduced the first variable valve OHV engine and it is installed in the 2008 600Hp Dodge Viper)

Disadvantage of DOHC is that the cam drive & timing is more complex and the engine is wider than a SOHC.

Main reason most foreign cars are OHC is that many years ago many foreign countries passed laws penalizing larger displacement engines and imposing fines on the manufactures for making large displacement engines, do to the false belief that large displacement engines were less efficient. In order for the foreign automobile manfactures to produce sufficient Hp & Torque with small displacement engines it was necessary to design the engines to run at high RPM's and so OHC engines were mandatory to meet the RPM requirments.

Each type of engine OHV, SOHC & DOHC has its advantages and disadvantages, choice depends on the specific application needs.

The future technology is cam less engines, they are in existance but are much more expensive to manufacture than OHV or OHC engines. Cam less engines are supposed to be 10-20% more efficient than OHV or OHC engines. Rumor has it that Alfa Romeo may introduce a cam less engine in 2009.
(Note: The first cam less engine design was in the 1947 Tucker Torpedo but legal problems led to it never reaching the production stage).




Pretty sure GM had VVT on its pushrod 6.2 in '07 (Cad and Denali)


And I respectfully disagree that cam placement or number of cams has anything to do with powerband.

Bore, stroke, rod ratio, cam specs, valve sizes etc....dictate powerband.

100% agree that multi valve set ups are better served by multi cams (even though Honda has many 4 valve motors that use SOHC)

Another interesting point is that a lot of the current DOHC 4 valve motors (5.7 Toyota, all the Ford DOHC's) use followers or rocker arms to activate the valves which negates some of the uber-high RPM capability which is supposed to be the DOHC biggest advantage.
 

blackhat620

You Had to be There
1,687
150
Arizona
And I respectfully disagree that cam placement or number of cams has anything to do with powerband.

Generally speaking a DOHC non-rocker arm motor will have a wider powerband resulting in more overall power than a SOHC rocker arm motor do to less mechanical loss, more precise cam grinds and a higher achieveable RPM on the DOHC motor compared to a SOHC.

Pretty sure GM had VVT on its pushrod 6.2 in '07 (Cad and Denali)

Sorry for the confusion did not mean to imply that variable valve timing was not possible on a OHV engine, just more difficult. The GM 6.2 used in the 07 Escalade & Denali actually uses Variable Cam Timing to achieve Variable Valve Timing.
www.edmunds.com/insideline/do/Features/articleId=107589

I believe the 6.0 V* in the 2008 Tahoe/Yukon Hybrid uses VCT to achieve VVT as well.

Ford started using VCT to achieve VVT in the 2004 Triton 5.4L 3-valve and 4.6L 3-valve 2005 in the mustange, 2006 in the explorer
 
Generally speaking a DOHC non-rocker arm motor will have a wider powerband resulting in more overall power than a SOHC rocker arm motor do to less mechanical loss, more precise cam grinds and a higher achieveable RPM on the DOHC motor compared to a SOHC.

Power HIGHER in the powerband I'll agree (still depends on the parameters I listed and the tune))

But I still disagree on "wider" powerband just because it's a 4 valve.


I do think I see where you're going though:

In theory, a 4 valve motor should scream on top end just because of the air it can move at high RPM.

This of course would, in theory, make it a dog down low because port velocity is too slow.


But with VVT or controlled intake runners, you should, in theory, have the best of both worlds.


I also agree 100% with you that multi cams/valves are primarily to make up for displacement., but those LSx motors have shown the world that you can still have low end and high end with 2 valves and pushrods......and big cubes.
 

blackhat620

You Had to be There
1,687
150
Arizona
Power HIGHER in the powerband I'll agree (still depends on the parameters I listed and the tune))

But I still disagree on "wider" powerband just because it's a 4 valve.


I do think I see where you're going though:

In theory, a 4 valve motor should scream on top end just because of the air it can move at high RPM.

This of course would, in theory, make it a dog down low because port velocity is too slow.


But with VVT or controlled intake runners, you should, in theory, have the best of both worlds.


I also agree 100% with you that multi cams/valves are primarily to make up for displacement., but those LSx motors have shown the world that you can still have low end and high end with 2 valves and pushrods......and big cubes.


Basically we are on the same page. Remember my response was to the OP question of the advantage of DOHC to SOHC in simple terms.

Now considering your list, if you have two identical engines, a DOHC will have a wider & longer powerband and outperform a SOHC on the dyno. And yes a 4 valve will outperform a 2-valve all else being the same. Also generally speaking two identical engines a SOHC 2-valve will have more low end power than a DOHC 4-valve engine for the exact flow dynamics you stated.

Now if they can get the camless engine design figured out to be economical i think we will see some great advances. IMO i think we are a long way from seeing the death of big cube OHV engines. OHV engines are still my engine of choice both on and off the track.
 

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