Why is it that the a dohc setup will produce more hp than a sohc setup?

 

The main reason being you can operate more valves with a DOHC engine, meaning the engine will be more efficient, smoother, and run cooler. Greater airflow generally means more power. (There are a few 4-valve SOHC's(honda for one), but they tend to have more wear on the cam lobes and aren't as efficient.

 

First of all, DOHC stands for dual overhead cams, and SOHC single. Basic engine physics, these cams are driven usually by a belt or chain, which is driven by the crank. The OHC actuates valves with lobes that push down on valves that are pressed upwards by springs. When pushed down by the lobes on the cams, the valves open, and when pushed up by the springs on the low points of the cams, the valves close. This is a revolutionary change in cam actuation over pushrods which run parallel to valve movement, pushed up by lobes on the crank, to move a lever, better known as rockers, which then operate the valves. Thus as the lobe on the crank pushes the push rod up, the other end of the lever a.k.a. the rocker, is pushed down, opening the valves, and vice versa.

Now for the key part. SOHC's are practically limited to two valves per cylinder, much like pushrods, though still much more efficient. They are limited by space and wear. By having duals, you can have four. By having more valves, the efficiency of the burn, induction, and scavenging are increased, primarily due to swirl/turbulence as well as greater oppurtunity for variable valve timing. Just imagine, now you can have one intake valve open up fully, with the other just slightly so as to cause high swirl/turbulence to increase the amount of fuel burned for efficiency, or scavenge higher to increase flow rate at higher rpm's.

That's about it. If I'm incorrect anywhere, please fill in.

 

Pushrod engines actuate the valves by the cam moving the pushrod, not the crank. The crank is connected to your rods/pistons.

 

My bad, forgot that one step

 

Where does it say that a DOHC will make more HP than an SOHC? I would have to say the number of valves will have a greater effect. a well designed OHV or SOHC can make just as much or more HP than a DOHC.

 

To keep it simple you can pump more air in and out of a DOHC than a SOHC.

 

yea but on equal footing the dohc will prevail. poor dohc vs good sohc, sohc can be equal or better, good dohc vs good sohc, dohc would come out on top. i.e. J32A2(sohc) vs C32A(dohc), F22(sohc) vs H22(dohc)

 

actually there are plenty of SOHC motors that are 4 valves per cylinder.
With DOHC you can play around more with intake/exhaust valve timing, different cam profiles for intake or exhaust to yield the best power/torque and emmissions. when you only have 1 cam driving intake and exhaust valves, your more limited to what you can do.

 

If I'm not mistaken some Benzes are SOHC. More complicated is not always better--obviously the Corvette is a good example of where EFI and pushrods with OHV get the job done. Complicated sometimes is a bit ridiculous, for example, Honda lawn mowers that have a timing belt. WTF? You still push the thing, but to have the Honda name you pay triple when a $199 Murray is highly recommended by Consumer Reports and lasts 10 years anyway? I'm a su*** too buying a German VDO tire pressure gauge for $XX.XX when a $8 pencil one is just as accurate!

 

The main difference between pushrod, sohc and dohc is the amount of mechanism between the cam lobe and the valve - pushrod has the most, dohc has the least. Less is better. When the valve angles are optimized things get complicated for pushrod and sohc. Dohc gives the most freedom for variable timing too.

 

What wdave said, and...

...since DOHC engines have the lightest valvetrain mass, they are capable of revving the highest. SOHC engines seem to be limited to about 7000rpm, and pushrod with the heaviest valvetrain mass is limited to somewhere around 6000rpm, unless it's built.


All other things being equal, because a DOHC engine has the highest rev-capabilities, put the right cams and intake manifold on it and it will ultimately be capable of making more power than an identically sized SOHC, out of the mere fact that it can rev higher. However, in the production mass-produced engine world with 6000-7000rpm redlines, a SOHC 4-valver is just as capable of making every bit of the power that a DOHC engine is....

Nissan VQ35DE DOHC: 240-255HP, 246-265TQ
Honda J35A4 SOHC: 260HP, 250TQ

 

Right, the fact that an engine has dual cams instead of a single is not the key to the higher performance. 4 valves per cylinder is 4 valves per cylinder, no matter how they are acutated.

The main advantage of DOHC over SOHC is that you can independently adjust the timing of the exhaust and intake cams. You cannot do this with SOHC.

 

 

OHV isn't exactly simple. the long push rods, and rocker arms, thats a lot of extra stuff to move. SOHC, only has the rocker arms. DOHC has no rocker arms or pushrods. u make the method of driving the cam itself more complex, but the overall system loses parts.

 

Thats SOHC VTEC. VTEC is a science in itself practically. SOHC without VTEC is like the Acura TL/CL type regular or the RL, which produces 225 hp. So, DOHC may be even with SOHC, but it's SOHC with added complexity. Also, if your compare the 290 hp NSX, a DOHC VTEC V6 to the 287 hp 350Z's VQ, a DOHC V6, the 350Z being 3.5 liters vs the NSX's 3.2, and the 350Z's less complicated DOHC head vs the NSX mucho sophisticato DOHC VTEC head, I think you would come out better with the 350z becuase the power is less costly to produce, and it's less complicated. So, DOHC is simply better. I guess we should wait and see what the next NSX will be to have a more fair comparison than the newer car and the older one we have now. It's going to be interesting with that new NSX.

Another thing, someone mentioned about Benzes. I think it's also interesting that Benz has quietly gone to SOHC 3 valve per cylinder heads while most everyone else is still using four valves in DOHC, and Audi/Volkswagen is going overkill with their 5 valve heads. Mercedes Benz is the smartest one. Still making hi power motors with the least, or lesser amount of complexity. Case in point, the 97 Mercedes Benz E420 became the 98 E430. Both had the same exact power outputs, 275 hp 295 TQ, but Benz changed from a 4 valve DOHC head (32 valves total) on the E420 to a 3 valve SOHC head (24 total valves) in the E430. Of course the E430 upped the diplacement by 100cc. The less complicated, but still as powerful, car is the better one.

DW

 

The reason for the "reversion" to SOHC and 3-valve heads on most of the modern Benz engines was to clear out space in the head so that they could put in TWO spark plugs per cylinder. Benz engines are tuned more for insane low/mid-range power rather than all-out peak-HP anyways, and they don't really rev that high (don't need DOHC) so I guess this accomplished their goals better than DOHC/4-valve/single-spark.

It's tough to compare SOHC directly to DOHC these days because pretty much everybody is running variable valve timing, variable intakes, variable whatever....

You can argue that the SOHC I compared is a VTEC.
But the DOHC I compared had CVCTS.

 

OK. Fair enough. I forgot about the dual plugs in the Benzes and I didn't realize that the VTEC honda/Acuras don't have variable valve timing. Sheesh, such a dizzying array of engine technology.



DW

 

its a matter of efficiency. measure of efficiency is hp/L. all the hp/L monsters are DOHC with some sort of VTEC type system.

 

In the case of VTEC, high HP/L means you just have a very high revving engine. To rev high, you need DOHC. It doesn't mean the engine is any more "efficient" than another. I used to think like that, but saw the light.

 

Actually:

1) To rev high, you need a well balanced engine and enough gas. My stock 1600cc Datsun L16 (2 valve sohc) could rev to 7,000 rpm easy on dual SU carbs. Slap dual 40 dcoe Mikuni siddrafts and it would keep reving until the engine blew up. But you could rev it to 7500-8000 easy.
2) VTEC engines aren't really technically more efficent. It's just that they can feature two different cam profiles in one engine. I don't know how efficent each cam profile is by themselves though. But in gen, Honda heads flow well.

 

Yeah there are efficient SOHC engines. SOHC actually runs smoother than dual due to less moving parts. Honda's Vtec has made SOHC engines run with the best of ease for almost 10 years now. And I have never heard of SOHC having more engine wear than DOHC.

 

Yeah, SOHC may sound smoother, but you can't beat a DOHC sound, especially when it's running HARD! Sometimes I just floor it to hear all that valvetrain noise in the heads

DW

 

I think high HP/L = a ticking time bomb of wear and tear.

2 cases-in-point are Celica and Integra. What, they push like 200 hp out of a 2-liter motor yet their torque is that of a nine-stone weakling. If you popped that motor into a car the size of an Accord, Camry, Taurus, what would happen? That thing could not get the job done. Load up with 4 or 5 passengers and such a car would strain to get up the mildest of inclines. It's not the way that a normal motor would be designed, it's a very specific application that that's suited for.

 

SOHC valvetrain is much more complex than DOHC because of all the rockers, which DOHC doesn't have. This is why DOHC engines can rev higher - lighter valvetrain mass.

And I think overall engine smoothness has more to do with internal geometries, balancing, and noise dampening than it does with valvetrain arrangement.

 

 

well it still makes more power out of the same size, i guess efficiency is a bad word for it. but then that just proves that dohc is 'better.' only way to make lots of horsepower out of a certain displacement is to rev really high, dohc does it best.

 

yea and those giant GM motors last forever.
Low torque, have shorter gears, problem solved, but with that a new problem emerges, going through gears too fast, make engine rev higher, problem solved. Low torque and high horspower just means narrow powerband.

 

Gearing for what, 0-60 with 1 passenger? So shorter gearing will allow a 200 hp Integra motor to successfully propel a Camry-sized car with 4 passengers going up a hill? C'mon now.

Any idea how much HP a modern tractor-trailer has? Try in the 300 range. You think you can pop a 350Z motor into a tractor trailer and haul the load? Oh, I forgot, it's the gearing.

 

To pull the same load as a tractor trailer, you'll need to keep the Z's engine tached out at 6000rpm. On the otherhand, the giant turbo-diesel under the hood of a big rig is making 300HP at just above idle, along with over 1000 lb-ft of torque typically.

 

yea so u think a 1000ftlbs of torque is what it puts to the ground? i think not. it still is gearing doing the work. so why not use low torque engines for big cars or things that need to tow? size limitations, u can only fit so large a gear into a transmission housing and still have it be rationally sized. the 200hp teg motor puts out 130ftlbs of torque at the crank, using only gear and fd ratios, it puts out closer to 1800ftlbs on the other end of the transmission, make that FD a 5.5 for instance, and that number gets closer to 2300ftlbs, make the FD 6 and it puts out about as much torque as a maxima does after gear ratio and fd. now the only problem u have is the gear winding out way to fast, this is where increasing redline comes in, u increase redline until the gear winds out to a more usable speed. Size is limited, so having the engine make more torque to begin with is the reason they favor high torque engines over using gearing to make the output torque as high as it needs to be.

Ever wonder why the NSX runs similar times to the Supra, stock vs stock, even tho the NSX has almost a 100ftlbs less torque. Supra gets about 11.4lbs/ftlbs while the NSX gets 12.6lbs/ftlbs, yet they run similar times. Instead of using engine torque, and use teh torque number after transmission multipliers and dividing the weight of car by that new torque number, the new weight to powers are closer to 1.09lbs/ftlbs for supra and 1.03lbs/ftlbs for NSX. the race is quite a bit closer than the engine numbers would indicate.

 

 

well i made a mistake, the nsx is faster than supra, but its not only cuz its lighter. using cartest, not perfectly accurate, the supra runs 13.9@101, using the nsx engine numbers and weight with supra gearing it runs 13.8@106. The nsx runs 13.4@107.

 

It's not just gearing.

The fact that you'd have to keep a small cube Teg motor constantly wound up to 8000rpm just to go anywhere would make it much less reliable, and you'd be overhauling much more often, and it would probably be breaking down all the time. Instead of 300,000 mile overhauls, you'd be doing 50,000 mile overhauls. How long do you think a Teg engine is gonna last at WOT all the time and at 8000rpm just to keep a big rig moving?


So instead they just use a massive engine that has all the torque it needs right at idle, and doesn't even have to spin that fast to get going. It's more reliable, cheaper maintenance, and has much greater driveability than a theoretical B18 Teg engine pulling a big rig

 

Well, at least those ricers out there can put a VTEC sticker on their tractor

DW

 

 

its not just the engine either

 

Both the TL and CL do have SOHC Vtec. They do not share the same engine/design with the RL's engine just because the HP is the same.

The NSX engine is really no more complex than the 350z's one. Both are DOHC, have valve timing, and multistage intake. The reason the NSX costs so much is because of the materials used to make it. From the all aluminum body and suspension. To the titanium piston arms and lightweight alloy racing rims. Also the fact that the NSX is hand built would make the car that expensive due to the fact that it takes a long time to make just one. Rumor (yeah it's only rumor) has it that Honda is trying to fit some sort of Honda built V8 into it. A smaller rumor was that the V8 was supposed to be released as one of the 2 engines when the NSX was redesigned for the 2002 model year. I guess they couldn't meet the deadline for the V8 engine so they are just going to take there time now and finish it for maybe a 2004 release. Just think of what a DOHC I-Vtec V8 can do .

 

Well, The NSX is a great car, but at it's price point, and now with the 350Z nearly matching it in power and performance, but costing way less, the NSX is a 'dead car walking.'

DW

 

Even if gearing sizes/practicality wasn't an issue, big rigs would still be running what they are now - big diesels.