I would love to hear this debate from people who know more about these engines than I do. I want to discuss the advantages and disadvantages between pushrods and DOHC engines. It seems to me that yes VVT and all that is amazing but it lack the power output behind it. The main argument against pushrods is I would say that we need more liters to come up with the same power output. Let the battle begin

 

DOHC makes it easier to design a 4 valve head, but you also have to deal with the cam drive chains or belts. From what I have seen in the real world, the LS1 and later push rod engines give up nothing in ability to make power but are easier to build,maintain and tune than the DOHC designs out there.4.0 Ford v6 and the modular DOHC mustang engine are examples of a lot of effort for no gain.I just completed a timing chain tensioner and guide replacement on a v6 Ford, what an abortion of a design to work on. The tensioners all lose there plastic guides at about 150K.There are 5 different chains driving 2 cams,a jackshaft and a balancer and it requires a special set of timing tools to time the engine.I'll work on a pushrod engine anyday.The OHC engines have "hydraulic lifters" that operate as a pivot point for the rocker arm on the Ford design.

 

Less moving mass means higher rpm's before valve float. Removing the mass of the pushrods not to mention all the changes in direction of the force makes overhead cams a better choice. DOHC means a cam for exhaust valves and one for intakes (4 total-2 of each for engines with 2 heads). This usually menas multiple valves per cylinder (3 or more valves per cylinder). This allows for metter breathing but makes the cylinder heads complex.

 

OHV results in a smaller lighter overall motor. The LSX is an incredible motor that can make massive power in a small package.

Many cars swap LSXs and come out lighter (S14s, RX-7s, etc.)

DOHC will allow more air flow from a given head, but the motor/heads much be larger.

 

OHV = less parts to build, less rotating components, smaller, much more compact (emphasis on the last point)

OHC = ability to achieve (all things being equal) better VE due to more valve surface area (using more/smaller valves instead of less/bigger ones). more rotating parts, more parts period, more costly to build, physically larger/heavier.

hp/L was a big thing a few years back (and still is) but in reality, it's a worthess measurement. what really matters is engine output vs physical size/weight. cubic inches is irrelevant. if your engine has slightly better VE and 1000 more usable rpm at the expense of being 6" taller, 7" wider, and 120 lbs heavier, who gives a crap.

The LSx stuff is really a high point of engine design in simplicity, and size/weight per power output.

 

^ Well said... so many thing 5.7 V8 must be a "huge" motor, but they can't grasp the idea that specific output (hp/L) is meaningless and that HP/size&weight of the motor is what matters.

 

A very good point I argue. I love it when my friends with BMW's and *** cars sit and brag about how they have more valves and 4 cams. And I just laugh and say I make 2x the power, get 4mph better gas mileage, my engine weights 3/4th the weight, and can rev just as high. So what is the point unless you are going super duper and trying to rev to 10 grand.

 

And the 427 CI 7.0L version must be massive!

In reality, it's 21 lbs lighter, 4" narrower, 6" shorter (oil pan to intake) and probably has a CG 7" lower than the 3.0L DOHC Taurus SHO V6 in my SHO that I've tinkered with and track'd for the last decade.

 

You all are hitting on great ponts. I have heard the Internal combustion engine refered to as "An Elegant Balance", because you can make them so many different ways and with soo many different functions in mind. A Ship motor with a 3 meter bore has the same parts as the engine in your lawnmower, but they are completely diferent.
You will always be able to make more power from a DOHC engine than a OHV engine, all other things being equal, simply because you can spin it faster, but I agree that I don't want to spin my engine to 10k. I like driving down the highway at 1500 and getting 32 mpg
It is possible to use variable valve technology in OHV engines as well. GM uses quite a few cam phazers to advance and retard the timing of their valve trains.

 

All automobile valves are in the same place today-this was not always the case (look at the flathead Ford where the valves were next to the cylinder, not above). The difference is how they are opened. The cam can be in the engine (pushrods are used to push on rocker arms that open the valve) or the cam cam ne in the cylinder head where the cam lobe directly touches the valve stem (overhead cam). A OHC design has less moving parts and less moving mass that cam in engine designs. DOHC has 2 cams in the head (1 for intakes, the other for exhaust valves)-this is usually better when valves are very small and you have a lot of valves to control. The drive mechanisms to do overhead cam applications are usually LONG chains and elaborate chain tensioners vs. the old timing gear and short chain.

The type of use the engine will be put to should dictate what design to use. Most very high RPM engines (bike, Road race, etc) when the RPM's stay very high (8-16K rpm's) use OHC or DOHC designs to get around valve float. If you just stumping around on the street or ocassional 1/4 pops, the old pushrod design still works fine.

 

Yes because the C6Rs complete dominance of the ALMS and LeMans is sort of like "just stumping around on the street".

There are inherent advantages to an OHV design in racing as well.

 

Again, all cars are OVERHEAD VALVE today. OVERHEAD CAM and DOUBLE OVERHEAD CAM are other concepts.

 

Huh? What are you trying to say? All cars are not OHV today, go do some research and get back to me.

Exhibit 1: Look at the DOHC motor from a mustang cobra

There I win.

 

He is saying that the valve is above the combustion chamber which is true for the cobra engine. Both of the cams are there as well.

 

Yes... but that is not what distinguished OHC from OHV... the term for a traditional cam-in-block motor is OHV. The terms really describe the relationship between the cam and valve not the valve and combustion chamber.

These aren't flat heads.

 

They are both OHVs, as a basic or fundamental design category.
Pushrod vs. OHC is a sub-category as it merely relates to valve actuation.

I agree with you Cobra4B, pushrod design is far from dead as was "predicted" by some of the Eurotrash several years ago (witness the C6R and Viper SRT10 ACRs for example). They each have their place, depending on application.

 

To get make this argument a little philosophical, I think that each technology is great for different reasons in different applications. A Vette just wouldn't be a Vette with a Lexus-smooth 4.5 liter V8... and a Lexus wouldn't be a lexus with a 307 OHV V8 burbling through dual exhausts.

I love my LS1, just as I'm blown away by how smooth and fast the 5-valve per cylinder turbo engine in my Audi A4 revs. They're both amazing pieces of engineering.

Or put another way... sometimes you feel like getting dressed up, sometimes you feel like wearing jeans. In the right application, one is better than the other, but neither is inherently inferior or superior.

 

I have been impressed by the shear simplicity of the LSx design. When I look at the Ford engines, they are very well engineered motors but there is an additional level of complexity that I don't think is warranted given the outstanding output and reliability of the LSx platform. They are now even adapting the LS1 for use in light aircraft.

 

and heavier. That's why you don't see many larger displacemrnt DOHC designs for automobiles.

 

Used to be the case on OHC and OHV engines that the OHC engines "breath" better. Don't think that is the case anymore. OHV heads now flow VERY well (got to love technology). But for high revving engines (10k as mentioned earlier) You want to have OHC to reduce valve float. That is the only reason I see these days to have a OHC engine.

Heck, the C6 Z06 revs to 7k out of a OHV engine. Same as my DOHC ZR1.
Of course spinning a DOHC engine up is an awesome sound and a smooth idler.