What I know is that the OHV design is cheaper and more compact and gets the job done up to a point. An OHC design could make more HP due to being able to get more RPM but takes up more room and costs more. Just look at the new Ford 5.0 making more HP than the 5.7 Hemi.

 

Pushrods can match OHC designs RPM-for-RPM. Fewer parts, less expensive, more reliable, cheaper to repair and more compact. However, it takes mega money and has to meet internal (Chrysler) standards for durability and longevity, plus cost effectiveness. NASCAR engines push 10,500 RPMs for 3.5 hours at WOT. There are aftermarket companies building engines that can run 8500-9500 RPMs. Mucho f'in deniro kemosabe.

 

OHC is mostly an "empty" feature, unless you are designing an engine to go well beyond 6000 rpm. Below that, it offers nothing, even when actuating twice the valves. You only "need" to go well past 6000 rpm if you are trying to get more output out of a smaller engine. However, if you have the flexibility to go with larger displacement, then you can make that output w/o the heady rpm, and ohv is quite suited for that job. Leveraging displacement is a far more cost effective way to make power, as long as you have the room. Dodge and GM are the rare breed in these times to embrace the virtues of displacement.

The most important thing to realize is, while high rpm is nice, a large displacement ohv engine can give you all the power you need before you ever have to call on those high rpm's.

 

I actually like the fact that my muscle car still has a classic pushrod motor under the hood. Keeps me one step closer to the 60's than those over head cam motors

 

OHV motors are not as tall as OHC engines. That's why Corvette still uses a pushrod motor because it can fit under a low hood and has a lower center of gravity.

 

Yes, that is yet another good perk for ohv, and ironically another reason we may never see larger ohc engines with improved powerbands...they just won't fit in the same place a large ohv engine can go in a tight hoodline situation.

Here's another interesting pic to illustrate the point:

4.0L dohc v8 vs 5.7 L ohv v8

 

I am wondering why the hemi uses a pushrod engine versus OHC engine as seen in the "other" modern muscle cars.

Other modern muscle cars, plural?

Ford is the only modern muscle using OHC.

GM is still pushrod for their V8s (except for the C4 ZR-1 which is DOHC), so it would seem Ford, not Chrysler, is the odd man out.

DOHC also tend to be peakier, making more power up top, and taking more RPMs to get there, but having lower torque numbers down low.

 

This....... Plus DOHC engines have much more complex (expensive + reliability concerns)valve timing engineering than OHV engines.

 

Would be interesting to see the Toyota iForce v8 side-by-side with the Eagle Hemi (both being a handy 5.7 L). I looked around for some pics, but didn't see anything that would match up well in a single pic.

 

If memory serves me, the C4 dohc engine was built by Mercury Marine, although the design was by Lotus. It still holds the record for running 24 hours at 175+ mph.

 

Everyone always misses what really matters in the OHV/OHC debate. It's not so much as how the valves are actuated (it does matter) but the real advantage comes from the valves themselves.

OHV or pushrod engines are almost always 2 valves per cylinder. OHC are almost always 4 valves per cylinder. It's much easier to get 4 valves flowing more air than two bigger valves.

There are many advantages and disadvantages to both but airflow is one major advantage that cannot be denied.

 

...but when you go to 4-valves, each valve is then smaller and with less lift. So it may turn out that there is no flow advantage rpm-to-rpm, at all. It's just more suited to run to a higher rpm (and keep flowing, of course).

2 intake valves vs 1, does not automatically make for twice the flow. Otherwise, we would be having 3.6 L engines suddenly breathing (and developing likewise output) like 7.2 L engines, and we know that is just not the case, right? More valve area is only a benefit assuming a single valve is too small to keep up...but if it is sized bigger and with more lift, then there is no benefit at x rpm. The flow, that actually occurs, simply relies on engine displacement and engine vacuum, at that point.

...and contrary to your point, 2 valves flowing simultaneously is not more easy to get flow out of than 1 large valve. The flow dynamics are unavoidably more complex in the former, as the 2 valves will "fight" each other where they want to flow into the same region (i.e., you'll get less flow in practice than theory, given the physical valve window area). Getting 1 large valve to flow is definitely more easy, since you only have to optimize for 1 "source"...no fighting or contention with another nearby source. In the case of the Hemi design, a single flow source enters the combustion chamber at a perfect angle along the contour of the cylinder head surface, and that is why it is such a successful design for flow. A sohc Hemi design with only 1 intake valve/cyl would be similarly effective. That is what Porsche was doing in its earlier days. It is also what Ford did in a racing engine that quickly got outlawed.

 

I agree with a lot of the above but I'll throw out some more factors: OHC engines have lighter valve trains - no pushrods and often no rockers and lighter lifters or followers. With the lighter valve train, they require less valve spring tension. Because the engine is squeezing weaker valve springs, the mechanical resistance of the engine is less and this adds to gas mileage and power.

With an OHC engine, it's easier to engineer 3, 4 or more valve per cylinder configurations because it's easier to position the valves without involving complicated valve trains, especially with twin cams. An example if this is the various iterations of our own Chrysler Hemi. They have some pretty long rockers to reach the valves when they are at such a large angle to each other. On an OHV wedge motor with inline valves (like Chevy), this isn't such a big deal. But the heavy rockers and complicated valve train geometry on a Hemi take a stiffer valve spring to keep valve float under control.

Though a well designed 2 valve/cylinder engine might flow better than a poorly designed 4 valve/cylinder motor, all things being equal the one with more valves can have a broader power range and/or more high RPM power. It's just easier to move a bunch of small columns of air into the engine than fewer big ones. Air(and exhaust) has inertia, too, and smaller amounts are easier to stop and start. And of course, the simple,light valve train parts of an OHC engine can allow for a lot of RPM without valve float.

A big disadvantage to the OHC engines is the way the camshafts are run when they are that far from the crank. It takes long belts or long chains, either of which will generally stretch - a lot. This requires belt or chain tensioners, etc., adding to the complexity and cost of the motor, not to mention the cost of service. Many OHC engines with cam belts need to have them changed by 100K miles or so. OHV engines, which generally use short chains, can often go the life of the vehicle without any service in this area (I'm not going to touch the 5.7 Challenger timing chain controversy). When an OHC engine tosses a timing belt, at the least, the car quits, and at the most you have an engine full of bent valves and other damage. OHC engines with chains have problems, too. It's not unusual for the chains to stretch so much on high mileage engines that they wear through their housings. Talk about a serious oil leak! And if you have a V-Type engine, you have to have twice the number of camshafts and the parts to drive them. I'm really surprised that Ford made the move of making their V-8's OHC, as it costs a lot more per engine. Then again, Chrylser made the move with their new corporate V-6.

Just some things to think about. IMHO, the perfect motor would be OHC with 4 or 5 valves/cylinder, variable valve timing/lift and a bulletproof way of driving the cams - or maybe no cams at all if electronically activated valves get perfected.

Bill

 

I for one am very happy Chrysler and GM have stayed committed to the cam in block design. I have had two Mustang GT's, an 06 and a 12. The 5.0 was certainly a big imprvement, but it still does not have the low end feel I like, and though it screamed when I wound it out, I always felt it needed even shorter gearing.

On the other hand, the Hemi and the LS engines come in much larger displacement, and offer the possibility of even more displacement.

OHC engines are great, but I prefer cam in block engines in my cars.

 

...and the day that cam-in-cam technology can become cost feasible for everyday ohv engines, it'll be all over for ohc engines ( ok, exaggerating a bit, but you get my point). Generous displacement with variable, independent intake/exhaust timing on ohv will make for invincible powerbands. They won't be screaming well past 6000 rpm, but let's face it, if you get all the power you ever could use before 6000 rpm, nobody's gonna be complainin'. We don't "drive hard" just to ping the rpm. We drive hard because the power is accessible and making us go fast, regardless of the rpm. The only thing ohc can deliver, at that point, is screaming rpm and only a small audience of people really live and die by that requirement when all is said and done.

To an earlier point made about dohc having more simple valvetrain geometry...that's actually not true in a great majority of popular dohc makes running around these days. Very few are true "direct" actuation designs, which is where the simplicity argument would be valid. The common design these days still involves finger followers and hydraulic lash adjustments, so ultimately, they aren't "that" far removed from conventional ohv designs, as far as "extra parts" (then consider 2x the valves with associated hardware, and we are no longer talking about "less parts"). They just don't have an actual "pushrod", basically. Just look at the Pentastar setup...it has finger followers that essentially operate like a rocker arm as in an Eagle Hemi. It just pivots around a different point.

The only "true" direct-action dohc in relatively recent times that I know of (not that I keep up on every single engine under the sun to know) is Honda's VTEC. Maybe Nissan's VQ engine is, too...don't really know.

 

Randycat wrote: << The common design these days still involves finger followers and hydraulic lash adjustments, so ultimately, they aren't "that" far removed from conventional ohv designs, as far as "extra parts" (then consider 2x the valves with associated hardware, and we are no longer talking about "less parts"). They just don't have an actual "pushrod", basically.>>

True, but beyond the simplicity argument (and there have been a lot of motors out there that didn't use finger followers or hydraulic adjustors - some good, some very bad) there's the factor that an OHC hydraulic adjuster is not part of the reciprocating mass of the valve train. In an OHV engine, the lifter is going back and forth with the pushrod and has to be returned by the valve spring. In an OHC engine, the hydraulic adjusters are just fulcrums or pivot points for the finger followers or rockers. They aren't part of the valve train reciprocating mass so either lighter valve springs can be used or stiffer springs that will allow the engine to rev a lot higher without valve float.

Horsepower is a function of both torque and RPM. More RPM at the same torque will get you more horsepower. You are correct that variable valve timing can give an OHV engine a phenomenal power band, but that same engineering can be used with an OHC engine, giving that fat power band up to a much higher RPM.

Race engines built from scratch for F1 or Indy cars ditched OHV designs long ago. In fact, they're into some esoteric systems that I don't pretend to understand. But those guys don't have manufacturing costs to worry about.

Then there's the desmodromic engines with no valve springs. Instead they have another cam to close the valve, but that's a another topic for another day.

Bill

 

On the hydraulic lifter point, on ohc...sometimes it is a moving mass, sometimes not. Designs vary, and it is no guarantee "just because" it is an ohc design, because there are numerous ohc designs in use. Here's the real irony...if it is a direct-actuation design, then the hydraulic lash adjustment probably is a part of the moving mass. If it instead uses rocker arms, then the possibility is there that the lash adjustment is implemented in a non-moving mass area. So either way you go, you can't escape that there are "extra parts" involved in a modern ohc valvetrain. It's more "like" an ohv design than most people think, as far as parts count and moving parts. It's not as simple as the old days when ohc was more likely to be direct actuation, with the proviso that owner was responsible for regular manual lash adjustments.

 

Thanks for the information guys. It's great to read information (and opinions) that is otherwise difficult to come by. An interesting bit of reading is about the new corvette engine which still uses a small block V-8 with direct injection. A bit of "old" technology with some upgrades. I was particularly fascinated by the internal discussions about this very topic mentioned in the article: sticking with the OHV pushrod design or jumping over to the OVC design.

http://www.nytimes.com/2013/01/20/a...omobiles/talking-about-a-new-generation-for-the-corvette.html?pagewanted=1&_r=0

 

Here's the biggest irony to that thought...many people don't even realize that ohc is actually older than ohv. So the distinction of "old technology" between ohc and ohv is really a moot point. They're both old as dirt, but they both are still around because they are effective technology at doing what they do. I think inline engines were the first to appear, and they had ohc configurations since as far back as 1936.

 

To me the biggest advantage of DOHC is having VCT on intake and exhaust. Ford takes advantage of this and has an engine capable of a smooth idle and big torque curve topped with high RPM hp. A very versatile engine which I must confess to be a bit of a fan of (credit where it's due). The actual torque it makes pales in comparison to the Dodge & Chevy counterparts and this is due to its relatively small displacement (300 CI) so it compensates for this with big RPMs. It will never match the Hemi or any other bigger engine for that big torque feeling when you hit that loud pedal!