SOHC vs DOHC Valvetrains: A Comparison
Plenty of people have been talking about how the Savvy only has an SOHC engine so it is inferior, and old technology. So I decided to post this entry so that people will be better informed on what SOHC or DOHC really is.
SOHC and DOHC are basically two different ways to configure the valvetrain. In the whole valve train setup there are camshafts, lifters, rockers, valves and springs. It is best that the valvetrain is light. Valves open at intervals of 25 times a second at 3000rpm. If the valvetrain is heavy, when the camshaft lifts it the valvetrain will just keep going up until the valve spring catches it. This is called valve float. You can overcome this with stronger (which means heavier) springs, but this extra weight increases the power requirements to open the valve.
The cams are driven by the crankshaft, using either a belt or chain called a timing belt or timing chain. If the timing belt/chain snaps and the camshaft stops spinning, the piston coming back up the combustion chamber might hit the open valves. This is very costly. That’s why you should always change your timing belt/chain at the interval specified by your car manufacturer.
SOHC refers to Single Overhead Camshaft. In the days before DOHC, it was known as OHC, with no need to differentiate between a single or double camshaft. In SOHC, the camshaft is situated in the cylinder head, above the valves. The valves are opened and closed either directly with a shim between the cam lobe and the valve stem, or via a rocker arm. SOHC engine valve configurations typically have 2 or 3 valves per cylinder. It is also possible to have 4 valves per cylinder using SOHC but this translates into a complicated combination of rocker arms and cam lobe shapes. An example would be the 4G92P in my car which is a SOHC but has 4 valves per cylinder.
DOHC refers to Double Overhead Camshaft. This arrangement uses two camshafts in each cylinder head. Two cams per cylinder head means that a DOHC V engine has 4 camshafts because it has 2 banks of cylinder heads. This allows the manufacturer to easily implement a 4 valve per cylinder setup. Most of the time it also allows the engine to rev higher. It also allows better placement of the valves in an optimized setup that gives you maximum performance. But the disadvantage of such a setup is more weight, more cost and more complexity. It takes more stuff to drive two camshafts. The main reason to use DOHC is to drive more valves per cylinder. If a SOHC setup can allow 4 valves per cylinder, having a DOHC engine will not bring that much benefits over SOHC and the additional weight becomes a burden instead. DOHC engines also allows the spark plug to be placed right in the middle of the combustion chamber. This promotes efficient combustion. With SOHC, the camshaft is usually in the middle of the head because it has to drive both the intake and exhaust valves, robbing the sparkplug of it’s optimal location.
In the end, a SOHC 16 valve engine would have better torque on the low end where the DOHC valvetrain’s weight results in lower torque. But at high engine speeds, the 16 valve DOHC engine’s peak torque and horsepower would be greater. That’s the trade-off. With the amount of valves being equal, SOHC has better low-end torque because the valvetrain package is lighter while DOHC has better top-end power.
Other benefits of DOHC would be making it easier to implement variable valve timing technologies (which I will cover in another blog post) and also you can tweak it better with adjustable cam pulleys. If you were to put high-profile cams in your DOHC engine, the cam lobe profile can also be more optimized than a SOHC engine because you can play around with the lobe shape easier with separate camshafts for the intake and exhaust valves.
Why more valves per cylinder? Why not just make 1 huge intake valve and 1 huge exhaust valve? Bigger valves weigh more than the smaller one, so controlling the extra weight as it gets flung open and close becomes difficult. The spring has to be stiffer. A stiffer spring means more energy has to be spent overcoming the valve pressure. This partially oversets the gains which a bigger valve has to offer. Another problem with a single big valve is at lower RPMs the intake velocity will be lower. I’m sure you guys know this… the same amount of air going through a big pipe will have lower pressure than the same air going through a smaller opening. Think of how you can control the water pressure of your garden hose by adjusting the opening size with your finger. Because of the velocity drop, low RPM torque and driveability will suffer. Although two smaller valves weigh the same as 1 big valve, and with the extra rocker arms and springs they can actually end up weighing more, this is offset by less mass to be overcome when opening and closing the valves.
So, are more valves per cylinder really that beneficial? Here’s comparing a Nissan VG30E to a VG30DE. Both are 3 liter electronic fuel injection engines, but one is a SOHC with 2 valves per cylinder and the other is DOHC with 4 valves per cylinder.
We can see that torque at low RPMs are about the same for both engines, but at higher speeds the 2-valve per cylinder engine has reached it’s peak and has to switch to the next gear while the twincam 4 valve per cylinder engine continues making more power at the top-end of the powerband. The powerband is also longer. (Note: I’ve added this graph here to show you the difference between 2-valve and 4-valves, not SOHC vs DOHC. Couldn’t find any graphs for SOHC vs DOHC. Sorry!)
So to sum it all up, SOHC has better low-end power, DOHC has better high-end power and overall maximum power. 4 valves per cylinder is much better than 2 valves per cylinder and it doesn’t matter whether 4-valves is achieved via SOHC or DOHC.
Next stop: Kuantan
June 22, 2005 @ 6:49 pm
Informative! I always knew what SOHC and DOHC were good at, but never known why really…this post explains it all!
June 22, 2005 @ 9:32 pm
IMPRESSIVE… Are you mechanical pro…
June 22, 2005 @ 9:46 pm
I’m seeing stars…
June 22, 2005 @ 10:41 pm
I still reckon a DOHC engine would do some justice to the already small 1.2L savvy. The reason is because you are going to have to push the car harder in each gear (this is where the high end torque would benefit) to get it to the required speed faster. allowing twin cams wold also allow easier profiling of the intake and exhaust valves for a slightly optimum perfomance. This would increase the price of the car though, if it was done.
June 22, 2005 @ 11:01 pm
waaah… barulah clear skit!!! hehehe good work Paul!!! i did my automotive subject in those univ day, but in arabic country in the middle east the abriviation always wind my head in pening kepala… hey, i thik you should post these article of yours to the univ or college that major in auto as a side reference, it will felp a lot to student to understand more easy!!! good work Paul!!!
June 22, 2005 @ 11:04 pm
thik = think
felp = help
hehe… sorryyy, speed typing, but not speed driving..
June 23, 2005 @ 10:05 am
Good job Paul.
Never did understand this till now, with more and more car going to DOHC, I always think DOHC is better then SOHC… but it become clear now.
June 23, 2005 @ 11:38 am
hmmm…just hit me…ur car running on a mivec? I know it’s 4G92 but i duno where ur P came from..ehhehe
June 23, 2005 @ 11:48 am
karheng: 4G92P… the P stands for Proton! LOL! It’s a normal single cam 1.6 lar… not Mivec. 4G92 engine code as far as I know could mean 3 engines… 4G92 SOHC, 4G92 DOHC and 4G92 DOHC Mivec
June 23, 2005 @ 3:00 pm
Very good explaination. It is interesting to know that Honda City and Jazz are using SOHC config to their engine, and nobody complains about it
June 23, 2005 @ 3:06 pm
macam tu sesuai ler savvy pakai SOHC emmmm …tak ler ketinggalan cam tu
June 23, 2005 @ 3:11 pm
Ezmil: the honda city and jazz are using SOHC, and only has 2 valves per cylinder for the i-DSI versions. they couldn’t fit more valves per cylinder because of the dual sparkplug design. takes up alot of space.
the VTEC SOHC 1.5 in the honda jazz vtec uses a 12V/16V design where there are 4 valves per cylinder but only 3 valves work at low RPM and all 4 valves work at high RPM.
June 24, 2005 @ 9:48 pm
Good job!!!!! We always have some idiots who would thrash Proton for using SOHC engine, but Honda is also using it but goes scot free without any bashing.
Anyway, if you are referring to the Savvy and MyVi, it wont be a fair comparisons because the SAvvy, eventhough has a lower hp rating, will still run circles around the MYVI since the MYVI is a class larger and heavier. I’ve tried both cars back to back.
August 28, 2005 @ 4:06 am
I feel there is something small to clarify and I’m sorry if I unintentionally offend anyone.
The main reason why there is a difference in torque between the 2 engines is not because of the no. of camshafts but the number of valves.
When you have 2 valves, the intake pressure at low revs is actually higher cos the intake port is smaller and that gives more pressure. Thats why at one time toyota 16v heads came with T-VIS. it shut one of the 2 intake ports per cylinder to increase intake speed at low revs. Higher pressure increases intake speed resulting in more air.
Nowadays, they put a medium sized port size to reduce this problem.
August 28, 2005 @ 10:39 am
sakaic: yup, that was what i was trying to say in my blog post. honda’s 1.5 vtec used in the jazz and city vtec actualy works like the T-VIS you mentioned.
September 11, 2005 @ 5:41 pm
I’m thinking of getting a Honda Accord 2.2 with auto transmission.. but I don’t know what to choose.. sohc vtec or dohc vtec.. the salesman told me that if I’m going for auto, then dohc will make no big difference over the sohc.. so sohc is a wise choise.. is that true?
October 7, 2005 @ 4:35 am
Let’s say a race a DOCH VTEC with my SOHC 16 valves, you think i will bet the DOCH VTEC? What is the difference what are my advantages.
November 6, 2005 @ 1:43 pm
Hi Paul,
I found this Blog on SOHC vs. DOHC to be very informative. I just have a question or two concening today’s technology and your information in relation to the new 2006 Honda Civic (Si). Obviously changes in technology are always changing things, and create new alternatives.
Now, if you compare two vehicles with the same amounts of valves, the new 2006 Honda Civic’s (Si) SOHCs with it’s systematic timing and control in valve configurations (i-vtec), to let’s say the Scion tC’s DOHC (possibly w/o a supercharger), same engine used on the Toyota Camry, Would these new configurations change the overviews (rules)?
According to your explanation and my limtied knowledge, the Civic (Si) would probably beat the tC in a 1/4 mile but not in the long-run. I cannot say I understand the Si’s engine configurations completely, but Would these rules still apply in low-end torque, high engine speeds, peak torque, and horsepower?
November 6, 2005 @ 2:20 pm
Just a note, I realized I placed the Si model in my questions to what may seem, as a SOHC. I know it is a DOHC, but I just wanted to know the comparisons results or theory between these 2 DOHC engines just as well as the SOHC vs. DOHC results.
November 12, 2005 @ 7:18 pm
These were better times in your blog Paul.. Less flaming, more information. Time to change all that.. You have to make the change
December 30, 2005 @ 4:04 pm
Wait a minute Paul, are you saying that the City 1.5 VTEC actually just shuts one intake valve at low rpm, and have all intake valve operating at higher rpm? What about valve timing and lift? Does that stays the same or does that varies too?
July 17, 2008 @ 7:40 pm
Between DOHC and SOHC which one is more fuel saver?
March 18, 2009 @ 3:07 am
Paul,
I may be stupid on all of this but can you just tell me which one would be best for normal driving on a 1991 Dodge Stealth RT 3.0 5 speed.
Susan
March 21, 2009 @ 11:29 pm
its great dude.. can u give more detail over the complication of having 4 valves in sohc and explain me the example 4G92P which you have used…
if possible post to my mail…please…..great work
April 7, 2009 @ 12:50 pm
You left out the biggest reason for 4 valves/cyl- valve area. The total area of the valves is much greater because the non-valve area is smaller with 4 valves. It’s like the jar of marbles problem- you can fit more in if the particles are smaller.
April 11, 2009 @ 4:27 pm
hey paul, i’ve had a mazda 323f 1.6L 16v dohc for ebout 250000kms, now i’ve bought the exact same car but in a 1.8L grsi 16v dohc version at around 175000kms,
now i’m looking to transplant some parts from my old one, but i can’t quite get my finger around that grsi thingy sticking on to the back end like ted kennedy on a hogie, what does it mean, and is it possible to somehow supercharge the engine (turbo)? as you have no doubt guessed i know jack about car engine’s but i know what i like so can anyone help, some tips on how to do this, and what grsi stands for?
cheers,
Y
May 22, 2009 @ 6:25 am
I just visited a Dodge dealer. I saw the Grand Caravan I like very much, but one of the main differences between its engine and the toyota minivan is the SOHC (toyota van has DOHC). I wonder if this makes the Toyota more reliable van, but after reading your article, I understand this is not the reason why Toyota is more reliable. Thank you!
Ken N.