SOHC vs OHV

[ebbfolls]

what happens if we put in a turbo or supercharger in an all motor setup? will it match the turboed smaller engines?

That's a complex question to answer. But to keep it simple, adding a turbo or supercharger (they differ only in terms of where the driving energy is sourced) is merely helping the engine put more air/fuel mixture into the combustion chamber. With more air/fuel mixture, you get more bmep (brake mean effective pressure--the pressure that drives the piston down on the power stroke), hence you get more torque, ulitimately giving you more power output [Power = (Torque x RPM)/5252].

Be aware though that putting a turbo on an engine designed to be normally aspirated can create problems. A turbo setup is a complete package which includes re-tuning an engine to make it capable of handling intake boost. Some engines are however 'over-designed' in the sense that they are built stronger than they need to be. This is what makes a lot of Japanese engines popular. Honda B-series engines, for example, can handle mild turbocharging without breaking under the increased strain. For serious turbocharging pressures, however, you need to replace some of the internals to maintain engine reliability.

There are roughly 3 different approaches to creating power: a) Forced Induction, b) Variable-valve timing, and c) Displacement (No Replacement For Displacement, 'ika nga). One good example of the independence of each approach is found in the Ferrari-Porsche rivalry. Porsche favors using twin turbos while Ferrari relies on high rpm (via variable valve timing), and both are just as good in attaining their common goal.

[osama]

it would be wrong to sweepingly conclude that Japanese/German technology is superior to the Americans. The 4.6 liter & 5 liter OHV Ford Racing Engines being installed in the new continuation production of the SHELBY GT500E (Eleonor) and the Super Snake (with supercharger) has all the necessary power-sapping pollution control mechanisms. The Super Snake (Eleanor with Supercharger) has an HP rating 700ph and 600NM of torque notwithstanding all the pollution-control gadgets installed to comply with the stringent clean-air law of the U.S.

[Panzer]

In terms of economy po... ano po ang mas matipid sa dalawa... 8 SOHC or 8 OHV

[ebbfolls]

In terms of economy po... ano po ang mas matipid sa dalawa... 8 SOHC or 8 OHV

The economy of the engine is a complex subject but in general terms, it depends upon the efficiency of the engine in transforming energy into motion.

In simpler terms, it all depends partly on the tuning of the engine. By 'tuning', we mean the rpm range at which the engine is most efficient in transforming energy into motion. The engine 'tune' is set by the diameter and length of the intake runners/exhaust headers, AND the profile of the cams. It is the cams that determine the timing and the amount of opening of the valves, which in turn determine the amount of air-fuel mixture that enters AND remains in the combustion chamber for the next compression stroke. Altering the profile of the cams is a way to change the 'schedule' of the opening and closing of the valves. Since the air-fuel is a compressible fluid, it behaves differently at different speeds, and it is this characteristic that makes it impossible to design a cam profile that will be efficient at both high speed and low speed. In other words, a cam designer can make a cam that is efficient at low speed, but it will be inefficient at high speed, and vice-versa. (That's why VTEC/VVT- are such big deals because they actually have 2 cam profiles built into one.)

Because OHC is a result of a design effort to make the engine more capable of higher speeds, engines in this configuration are usually (not always) designed with cams that are more efficient at higher speeds. Likewise, since OHV is not ideal for higher engine speeds, the cams for engines of this kind are usually efficicient at lower rpm. This is just a generalization and there is nothing to prevent a designer to use a low-rpm-efficient cam on an OHC engine.

For obvious reasons, an engine running at lower rpm is going to be more 'economical' than one operating at high speed. It therefore follows that an engine 'tuned' to be efficient at low speed will save more gas than one that needs to be run hard to produce torque. But as I said earlier, an engine's efficiency is not simply all a matter of tuning. Aside from gas-flow tuning, there are other factors that determine the engine's efficiency such as, compression ratio, combustion efficiency, and internal friction--all of which have improved over the years. As such, an OHV engine of the 70's or 80's cannot be expected to be as economical and powerful as an OHC engine of the mid-90's and onward, inspite of the fact that it is better tuned to be efficient at lower rpm.

[notEworthy27]

well said ebbfolls now I completely understand

[Alpha_One]

OHV (pushrod) designs are still widely used because they're still very practical. Widely used in heavy-duty low RPM applications, they're actually more reliable than OHC designs because there isn't a timing belt/chain. As many have said here before, OHV tends to be more economical because they are tuned for low RPM. It's actually impossible to make them go very fast, the pushrod connection doesn't "react" fast enough at high speeds to make the engine efficient (and the long rods would actually break at high speeds). Thus they're tuned to rev low. The Toyota 7k Revo engine redlines at 5.5k I think.

While OHV designs are very old, "F" head or "side-valve" designs were actually used before the OHV design came into wide use. These had the valves inside the engine block, beside the piston. These were not very efficient since they couldn't be compact and had bad airflow properties (which is why they were phased out), but they were very cheap to produce since they were much simpler to make (which is why they were once widely used in the first place).

Actually most of today's "modern" engine technology (DOHC, variable valve timing, etc.) were old ideas. Much of the technology in a "modern" engine had been first thought out, or had been first used, in the 1930s or earlier, but they weren't very practical back then. IMO, the internal combustion engine is already very mature. Perhaps at it's peak or even post-mature already, not much can be done to improve it. But we have to live with it for the meantime, by finding ways to squeeze out the last possible bits of efficiency we can find in it, through technologies such as Commonrail Diesel Injection and Direct Fuel Injection (in gasoline engines). ICE's need to be replaced already, but we still have to live with them for quite a while until we can find another practical alternative energy source.

[woodfire]

i guess the Internal Combustion Engines survives to this date because as they say: "if it aint broken, no need to fix it."

the wave of the future will focus more on environmentally sound engines! In the US of A, the hybrid technology is gaining ground. In Europe, the Diesel platform is widely popular (btw they already use rapeseed-sourced biodiesel). In fact, BMW and their Stuttggart competitor are expanding their diesel model line-up in 2005!!!

BTW, OHC and DOHC technology were invented in the U.S. and were improved upon by the Japs and the Teutonians.

[ebbfolls]

...they're actually more reliable than OHC designs because there isn't a timing belt/chain.

You are correct about OHV engines needing less maintainance than OHC engines. However, OHV engines do use chains to connect the camshaft to the crankshaft. I think it's only in F1 car engines and Harley-Davidson motorcycles that gear drives are used to run the camshaft. Aside from F1, gear-driven cams have no longer been used since perhaps the 50's.

Regardless, the chains used in OHV car engines do not require as frequent a service interval as the belts used in OHC. Also, a few engines use chains to drive their OHC camshafts. This is not common, however, because of the chain's noisier operating characteristics. Nevertheless, timing chains in motorcyles do require more frequent replacement by reason of their naturally higher amount of revolutions (not to mention the higher operating rpms) compared to a car engine.

[notEworthy27]

very very informative. I now have an idea of how OHV and OHC differs na. hehe