Rat,
True, it should be able to give you more power in a given RPM range, but no matter where the cam is, the power band is pretty much going to be limited to about a 3200-3500 RPM range. Where that range is depends on cam specs, type of intake/exhaust etc. It doesn't automatically mean that because an engine is OHC that it will be more of a high winder. One other advantage that OHC gives you is the ease of instituting a variable valve timing system. This can extend the effective operating range of an engine to a much wider range than the typical 3200 RPM range. The new 3V 5.4 has about a 4000 RPM range where it makes a good amount of power. Obviously there are going to be exceptions to these generalizations, but it is true that the OHC design gives you more potential and easier flexibility.

 

This is one very awesome point you pointed out! Variable valve timing with an OHV engine is possible but does not work nearly as well as an OHC engine. This is the one thing that is going to dramatically improve engines in the present and future. I actually think they are going go to camless engines eventually and the valves will be operated by air or electro-magnets but for the next ten years or so VVT will be the hot ticket. They actually made a ford ranger with a 4cyl that had electromagnetic valves but it was very noisy and didn't work quite as well as it should have.

 

One big advantage of OHC is you don't have to make the intake runners go AROUND the pushrods. The plugs go in the top of the head too, which makes the spark plug angle different.

These changes alter emissions... the spark plug angle helps the mix burn more completely and evenly. And not having to shift the intake runner around the pushrod helps to some degree. At least they are seperate and spaced evenly helping the mix temp be consistent in each cylinder.

Just be thankful Ford had enough brains to use a CHAIN instead of a belt.

 

I thank Ford everyday for that.

 

So far I see rally no reason for the OHC in the trucks, as the 5.4 in my truck seems no more powerful to me than a 5.8 in an older one. The 5.4 definatly wind higher though, and seems pretty sluggish in my truck untill it is above 3000 rpm. The numbers say that it makes great low end, but it just doesn't seem that way in my truck. It definately pulls harder higher in the rpm. I would personally prefer an updated version of the 5.8 with iron block and iron heads and at least an aluminum intake manifold, not this plastic crap. Don't get me wrong, I like my new truck, but quality is nowhere near as good as my 92, and it uses cheasier matierials to boot.

 

Multi valve ohc's on a long stroke, small bore engine will never make much sense to me. I'm with you 92. An updated Windsor would do the job better and not take up as much space as a big block. When somebody gives me a valid reason why the 3 and 4 valve only make a whopping 40 hp over the 2 valve, I'll stop thinking of the 4.6/5.4 as an abortion.

 

92f150, I don't see what it has to do with OHC. Your comparing one entire engine to another, not OHC vs. OHV. If you don't like the 5.4L you can't say it's because it's an OHC engine, it's a different engine all together than the 5.8L. I still don't see how OHV can have more low end torgue, or more HP, or anything that differs from an OHV.

 

The longer stroke smaller bore was for emissions reasons. You can get a cleaner burn. Still though, the stroke length on the 5.4 is very close to a 454 Chevy. No one has any problem with getting them to wind out. BTW, the 4V can and does make more than 40 more hp than the 2V.

 

A plastic intake makes sense. It is lighter and it doesn't conduct heat as well as Al or iron. And because the intake is cooler, the incoming air is cooler. That reduces pinging and increases power. Why would a metal intake be better?

 

The 3v gets 15 more ft-lbs, you may say big deal, but it starts just as low and holds it much better. Ford could have easily had more HP with the 5.4, but it would require more revs. The main problem that I see with the new 5.4 is 5000 RPM HP peak, and 5000 RPM Redline. Hmm, what were they thinking with that. A good example of the 4v 5.4 is the Cobra R a couple years ago. 385 HP, that's more than the 2v Supercharged Lightning, although not as much torque.

 

Multi-valve (4 valves / cylinder), and variable valve timing is the way to go, and these technologies are made possible by OHC designs. A good example of this is the Honda S2000, the 2.0L
N/A engine cranks out 240HP, due to their V-tech (variable valve timing) design.....

 

I don't think OHC makes 4-valves per cylinder "possible". It makes it easier, but there is no reason you can't do a 4-valve design with pushrods (been done, Chevy? Forked rocker-arm)

Also, VVT doesn't make high HP possible, it broadens the torque curve to make it driveable on the street. Peak HP has nothing to do with VVT. 240HP is easy, just rev it to hell...

HP=Torque*RPM/5252

Go above 5252RPM, and it's real easy to get high HP without a lot of torque.

You need two cams to seperate intake and exhaust variability. Of course, that is very easy to do with OHC (DOHC really) but not entirely necessary. Actually, I CAN think of a way it wouldn't need 2 cams...

The MAJOR advantage of OHC over pushrods is the weight/friction savings. Less valve-spring is required because you don't have to push the pushrod/lifter back. Less friction because smaller valve-springs. Smaller rocker arms because less valve-spring. Less inertia overall because you don't have a big lifter/pushrod/valve-spring/rocker to push around. Cams are smaller, because they don't need to take the load of a heavy valve spring.

Oh, and stop saying OHC vs. OHV - OHV is overhead-valve. We all have THOSE, right? Unless you're running a flat-head, that is...

 

While I agree with krewat, I'm still not convinced. Wouldn't every engine made be ohc if they are that much better than pushrods?

 

"Wouldn't every engine made be ohc if they are that much better than pushrods?"

That argument holds exactly NO water. I think everyone would agree that 4 wheel independent suspension is much better than a solid axle, in almost all applications (besides drag racing), yet you don't see that on every vehicle. A lot more manufacturers are coming around on a lot more of their vehicles, but it's not everywhere yet. We've all know for many years the advantages of 4 wheel independent suspension, yet it still wasn't on every vehicle and was actually on very few until recently.

 

Actually, I'm all FOR over-head cams...

Does Ford produce any more pushrod motors for auto or truck use? And a diesel is not a Ford anyway, so don't even bring it up...

Chevy does, Dodge does.

Besides them, there are really no other pushrod motors left. Like I said, the big advantage of OHC is less friction and inertia. Pushing those pushrods, heavy rockers and heavy valve-springs back and forth takes a toll in overall power.

Valve-angle and spark plug angle can be almost anything, which also makes a big difference in emissions...

So, I see two advantages.

First, friction and inertial power losses of heavier valve-train. You could probably make a 4-valve with less overall valve-train powerloss than a 2-valve pushrod motor.

Second, emissions. Valve-angle and sparkplug angles are very important considerations in designing heads with less emissions.