OHC vs SOHC

[darkjedi]

What is the real difference in these head types? i am only talking about the 2 valve versions. SOHC seems to be preffered by everyone and the simpler version (OHC) was rarely used.
I want opinions from people who have knowlege of this things.

[Bishop]

ohc is direct acting on the valves, sohc uses rockers to actuate the valves. ohc is lower friction but iirc cant handle as large a cam and as high rpm.

[RobtheFiend]

Uploaded with ImageShack.us

Aftermarket cylinderhead for Volvo B23 engines. Uses OHC. 19mm valve lift. Can use 19.5 mm lift, before needing to be modified.
In Rally Cross these engines rev over 7500rpm and has over 100 hp/liter.

[Bishop]

Uploaded with ImageShack.us

Aftermarket cylinderhead for Volvo B23 engines. Uses OHC. 19mm valve lift. Can use 19.5 mm lift, before needing to be modified.
In Rally Cross these engines rev over 7500rpm and has over 100 hp/liter.

I stand corrected, that is a big ass cam.

[Daffyflyer]

The positioning of the valves (one next to the other) is slightly less than idea from a combustion chamber design point of view, and a little more difficult to design good flowing ports for. However the lack of valvetrain mass/play (with cam straight on bucket) is excellent, so they can usually cope with a few revs.

[darkjedi]

so it all has to do with the chamber's shape and placement of valves? i did not know that. thanks

[Daffyflyer]

so it all has to do with the chamber's shape and placement of valves? i did not know that. thanks

Well, all the reasons I can think of do at least.

[RobtheFiend]

Another reason for using OHC: you can have an idiotic valve overlap, more than 70-80 deg, and there is no risk that the valves will hit each other.

[darkjedi]

i was under the impression valve cross duration should be at a minimun as air can escape through the exhaust.

[RobtheFiend]

For a low emission daily driver, yes.

For a fire breathing performance engine, nope.

You need a certain overlap to get the engine to function on higher rpm:s.

[Moth]

If I remember correctly... the direct-acting overhead cam design has an issue with the valve stems flexing because of the lateral forces imparted on them from the cams. The rocker arms of the SOHC design take the lateral stress off of the valve stems.

[RobtheFiend]

If they do, you have a serious problem.
Because the cam acts on the lifter/bucket, not directly on the valvestem.
It's to small (8mm), on the Volvo Evo cyl.head in the picture i found info that the buckets was 44mm/40mm (in/ex) in diameter.

[darkjedi]

that`s the typical diameter of a valve stem, doesn't seem small.
if it flexes on the top side (at the buckets) it's not a problem, but at the valve side is.

[RobtheFiend]

What i mean is, the cam is not acting on the valve. It's acting on the bucket, so no lateral forces will act on the valve.

The type of engine that has that problem is pushrod or SOHC engines with high lift without a roller tip rocker.
The tip can move back and forth over a millimeter.

[darkjedi]

i've studiedthe difference a bit and I came to the following conclussion:
The OHC config flows poorly at high rpms because intake and exhaust ports are forced to take sharp turns before the valves and scavenging is limited, but at higher rpms will allow a bit more overlap as air cannot easily escape out the exhaust. this advantage aplies only to NA engines as a forced induction will cause the mixture to flow out the exhaust. on another note cams will only lift the valve to a 1:1 ratio. these are great for tuning at a specific rpm range, mostly low to mid range as they are good for economy aplications.

SOHC will allow greater lifts with milder cams, ports flow alot better and will aid power over the entire rev range. however valve overlap should be kept at a minimum as at high rpm the proximity of the portss will cause fast moving air to go out the exhaust. they allow tuning for the entire rev range and also allow for slightly better compression, but the heads are somewhat heavier and the larger mass of the valvetrain makes them suffer at high rpms. these systems allow for less effiency at lower rpm, but more at mid-high range.

Overall SOHC is slightly better than OHC in terms ofperformance/effiency, but are more complex and have larger and heavier heads.