SOHC vs DOHC in economy cars

[rationull]

It seems like most econo-car engines, even those in the B-segment (Yaris, Accent, Versa for instance) tend to be DOHC these days. When manufacturers like Honda and Mitsubishi can get by with SOHC designs with 4 valves per cylinder (and still make comparable or better power than the competition), why is this the case? Given SOHC heads are smaller, lighter and (I assume) cheaper, why go DOHC for smaller, relatively low power engines?

The only reason I can think of is ease of implementation for variable valve timing. Honda and Mitsubishi, of course, use VTEC and MIVEC to get variable valve timing by way of multiple cam profiles in a SOHC design (and other manufacturers have similar tech as well). But Ford's 3V 4.6 V8 in the Mustang GT shows that you can even use cam phasers in a SOHC design. Is the ability to continuously vary intake vs exhaust overlap important enough to drive the decision to DOHC? Are there other major factors at play?

 

[StevieC]

Interesting... Want to see the answers. Good post bud!

 

[chrome]

It would seem from a historical standpoint, variable cam timing seems biased to achieving improved fuel economy, pollution management and torque spread. Variable valve lift tends to favor improving high rpm breathing, hence maximum absolute horsepower.

SOHC designs have limited variable cam phasing capability, as there is only ONE camshaft driving both intake and exhaust valves. DOHC designs overcome this problem, so the intake and exhaust valve timing can be varied independently, which leads to improved tuning flexibility.

 

[Shannow]

DOHC systems can (often) increase the side loading on the valves as the cam "wipes" across the things attached to the stem of the valve...SOHC systems, the (general) operation is more axially with the valve stem...potentially less guide wear.

As to VVT, some can be achieved by advancing/retarding the cam/s...it's possible to vary the timing of a single cam, using the principal that intake closing is the single most important cam timing event, but SOH with rockers can have variable lifts and ratios that would allow more timing events to be controlled.

 

[hone eagle]

Marketing department likes DOHC badging?

"we need those s badges!"

 

[Camu Mahubah]

Hey OP! Wanna really blow your mind? Ponder the desmodratic setup of a Ducati!

 

[Eddie]

Interesting topic. Are we assuming the all SOHC engines are 4 valver per cyl? Some are not.

 

[Shannow]

Originally Posted By: Camu Mahubah
Hey OP! Wanna really blow your mind? Ponder the desmodratic setup of a Ducati!

desmo stops you doing all sorts of variable stuff without a squillion dollars of tooling and engineering.

 

[Camu Mahubah]

I should have said Desmodromic instead of dratic...ah you can tell it's late!

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

Sorry to go a bit off topic but the thing is I wanted to get it across that there are other ways of doing things even if the big car manufacturers are choosing not to. Seems to me a DOHC design is one way to squeeze alot of power from a small displacement engine without getting all complicated. Sometimes the SOHC setups aren't as simple as one would think. THis is why they do it this way. If I'm wrong someone tell me.

 

[artificialist]

Honda used SOHC 4V in ordinary Civics and DOHC 4V in high performance Civics. The reason I read was that it gave the manufacturer more room to create wide angle cylinder heads which enable large ports and valves.

Some Dodge Neons were built this way as well.

Small ports and valves are great for low RPM engines that need more low end torque, large ports and valves are needed for high RPM engines that give up low end torque for high HP/Liter. That in mind, many manufacturers (Like Toyota) have a DOHC head with small ports and a DOHC head with large ports.

Also, VVT design varies. Some engines use a cam phaser to gain VVT, others (Like Honda) have camshafts with multiple profiles, enabling individual exhaust and intake timing even with SOHC.

SOHC 4V engines need rocker arms, DOHC 4V engines can be built with bucket lifters which might offset the price.

 

[IndyIan]

In the case of the Neon, in the second generation they kept the SOHC version instead of the DOHC. No variable valve timing but a variable intake runner system helped boost power a bit. The car got heavier and the SOHC was torquier at lower rpms which made it a better match especially mated to a 3 spd auto...

DOHC also allows the engine to turn more rpm without having problems in the valve train. SOHC tends to have more mass to move with rockers for some or all of the valves and that can lead to valve float. Also with less valve train mass a DOHC can run more aggresive cams for more high rpm hp. Good for selling cars!

 

[Papa Bear]

I think a flathead would be the best bet ...

 

[StevieC]

Originally Posted By: Papa Bear
I think a flathead would be the best bet ...

+1

 

[daves87rs]

Originally Posted By: Papa Bear
I think a flathead would be the best bet ...

+2

 

[JTK]

Originally Posted By: IndyIan
DOHC also allows the engine to turn more rpm without having problems in the valve train. SOHC tends to have more mass to move with rockers for some or all of the valves and that can lead to valve float.


Good point. Lots of parts on a multi-valve SOHC versus a DOHC with a simple direct acting configuration with shim/bucket lash adjustment. That would be the most simple & inexpensive you'd think? Dunno about performance/economy differences though.

Joel

 

[Steve S]

Originally Posted By: daves87rs
Originally Posted By: Papa Bear
I think a flathead would be the best bet ...

+2
A push rod engine would be better! flathead engines are compact yet would not pass any emmission standards and are not as efficient [breathing wise] The OHC engine is bulky and IMO sucky but the advantages is no pushrods and lighter valve springs and cheaper to manufacture.

 

[StevieC]

Meh, add another Catalytic converter and call it a day.

 

[Steve S]

Originally Posted By: JTK
Originally Posted By: IndyIan
DOHC also allows the engine to turn more rpm without having problems in the valve train. SOHC tends to have more mass to move with rockers for some or all of the valves and that can lead to valve float.


Good point. Lots of parts on a multi-valve SOHC versus a DOHC with a simple direct acting configuration with shim/bucket lash adjustment. That would be the most simple & inexpensive you'd think? Dunno about performance/economy differences though.

Joel
And faster lift rates can be achieved with the same lift and duration etc so more performance from the engine.

 

[Gary Allan]

Originally Posted By: JTK
Originally Posted By: IndyIan
DOHC also allows the engine to turn more rpm without having problems in the valve train. SOHC tends to have more mass to move with rockers for some or all of the valves and that can lead to valve float.


Good point. Lots of parts on a multi-valve SOHC versus a DOHC with a simple direct acting configuration with shim/bucket lash adjustment. That would be the most simple & inexpensive you'd think? Dunno about performance/economy differences though.

Joel


I would imagine that there's longevity advantages to the use of cam followers and HLA's over the shim bucket method. That doesn't speak to the performance/utility advantages of the setup, but in my mind set of having any maintenance issue engineered out of the mechanism, it's a consideration. Much depends on the proposed life span of the machine.

 

[tom slick]

Maybe DOHC allows more freedom in valve placement and angles? it certainly reduces parts which may reduce cost to the point that DOHC and SOHC cost the same to produce.