SOHC vs DOHC

[boofeng]

2. Would it be "electronically" actuated on a bike, such as a Super 4 -- or does the camshaft move mechanically to "switch" profiles.

 

oh didn't see your question clearly... definitely the final stage must be mechanical. electronics might be involved along the way (e.g. sensors for triggering, feedback signals), but the real actuation must be mechanical, right? i mean, even electrical motors must ultimately run mechanical gear drives...?

 

btw for CB400 and VFR800, does it switch profiles? or change from 2-valve-per-cylinder to 4-valve-per-cylinder operation? i remember reading that it is the latter. which is part of the reason honda's bike VTEC is weak. the 4-valve operation might be good, but 2-valve operation on a 4-valve head isn't going to beat 2-valve-operation on cylinder heads designed for 2 valves!

 

just my opinion.... which might be wrong!

[redbeacon]

btw for CB400 and VFR800, does it switch profiles? or change from 2-valve-per-cylinder to 4-valve-per-cylinder operation? i remember reading that it is the latter. which is part of the reason honda's bike VTEC is weak. the 4-valve operation might be good, but 2-valve operation on a 4-valve head isn't going to beat 2-valve-operation on cylinder heads designed for 2 valves!

 

just my opinion.... which might be wrong!

 

Nowadays, I do hear the hype in bike factories/laboratories in which the engineers talk a lot about creating bikes with engines that -- put it simply -- operate economically, until the rider so demands power out from his riding experience, the engine changes mode right there and then blah blah blah...

 

Henceforth, your theory of "use 2 valves for fuel economy" might be the implementation on them CB400s.

 

Any CB riders wanna comment? I know it's a lil off-topic but 4-stroke bike engines do function somewhat the same way as cars do, i guess?

[boofeng]

yeah they're very similar, bike and car engines.

 

i think most of the tuning is less for economy than for broader useable power. e.g. if i'm not wrong most of the jap sportsbikes feature an exhaust valve in the midpipe (like yamaha's EXUP) which helps restrict the pipe for better low-end torque, but opens up for free-flowing high-end as well.

 

the CB400 doesn't seem to do much fuel-saving before VTEC kicks in... consumption is still in the region of 20km/l give or take a couple of km/l, nor does it consume a lot more when VTEC is activated. in comparison with non-VTEC 400cc inline 4s, e.g. bandit and gsr? or even when comparing with the 600cc inline 4s, which can get around 20km/l when ridden gently. seems like it's more tuned for good spread of power than good economy?

 

only in the case of the TDM 900 does it seem to be tuned for economy. apparently the airbox opens up more above 4k rpms, and riding below 4k rpms returns on average a low 20+ km/l !! quite impressive for a 900cc twin. (or perhaps it's just the parallel twin layout... it seems to be a very economical though low-powered design, c.f. kawasaki ER6 series, triumph 900s, bmw F800 series etc)

[boofeng]

in terms of performance, dohc has better torque when accelerating.

 

......and better performance usually leads to more wear & tear

 

it probably has better peak torque, but needs to rev higher to get it? and since it spins up faster it might seem to accelerate faster too.

 

and yes, although better performance usually leads to more wear and tear... it might not be a big difference, given how well engines are built nowadays. good materials and good manufacturing methods and tolerances help a lot. i believe most engines are designed to be used for a decent amount of time/mileage, and if you don't abuse it, chances are whether it's DOHC or SOHC won't matter. engines are very lasting (and car engines don't really see high mileage in singapore). in short, IMHO, maintenance isn't a big issue in choosing between SOHC or DOHC. both are quite trouble-free. the factors to consider would be the differences in performance, fuel consumption, and direct cost.

[paiseh]

add in another, Subaru's engine.

[Kan]

If I'm not wrong, SOHC is cheaper to produce.

[joajas]

Consumes more petrol compared to a standard piston engine.

But able to output much more power compared to its equivalent in displacement as well as physical dimension & weight.

Rotary engines are like 2-stroke engines for cars. No crankshafts, no conrods, no valves, no moving parts except the rotor itself. Also much lighter.

 

http://en.wikipedia.org/wiki/Wankel_engine

 

My fren ever wondered why a 1.3L Mazda RX8 can outgun his 1.5 modified Lancer (before he siao-siao installed a TD-04 unit)....

 

 

Hi, saw this thread and thought I can provide a line about the 1.3L Mazda RX8.

 

That RX8 1.3L produces 228 Bhp in stock condition. Which other 1.3l can do this?

[reddy]

the beauty of rotary engine... well the new golf gt is 1.4 l but output hp is 170bhp cos of the dual air charger(supercharge and turbocharge)

[Phang]

the amount of fuel a 1.3L rotary need to burn in order to generate this 228hp is equivalent (I suspect greater) to a similar output reciprocal piston engine e.g. K20A in DC5 (Honda Integra Type R)

[NormalAspirated]

..........

[DatsunKing]

rx-7 1.3 is a drinker, consumption higher than 2.0litre cars...

[STi]

We always use these layman terms to decribe SOHC and DOHC.

 

SOHC - one leg

 

DOHC - 2 leg

 

In other words no matter how u beefed up a one legger he can only hop compared to a 2 legger that can run.

 

Simple?

[Terrible One]

here's a good read..

 

http://paultan.org/archives/2005/06/22/sohc-vs-dohc-valvetrains/