Being an EngineLabs reader, you are probably more familiar than the average bear with how and why engines work. You probably also know most of the generalities associated with different engine types. One of those generalities is that pushrod engines aren’t meant to be RPM monsters. While the statement is mostly accurate, first, it’s relative, and second, it’s a generalization.
Jan 27, 2004 Pushrod vs. DOHC Engineering/Technical Car Forums. Generally speaking, overhead cam engines allow higher rev operation, and thus are better for peak power output. Both push rod and OHC engines need oil to the top of the motor, escecially with high performance engines when valve spring cooling is important.
However, Jason Fenske of Engineering Explained fired up his trusty video camera and dove deeper into the reasoning behind the statement that “Pushods can’t rev high.” Fenske starts with a cool overview of a disassembled single-cylinder engine, and breaks down how the rotating assembly and valvetrain work in unison, Barney-style. He then translates that into automotive V8 pushrod engines.
Uncontrolled Valve Motion
As Fenske explains it, there are three major drawbacks when trying to get a pushrod engine to perform at high-RPM. The first reason is uncontrolled valve events – also known as valve float.
“The entire [valvetrain] assembly has to reciprocate back and forth very fast. You have a lot of mass in the system that is trying to change direction very quickly as you get into higher RPM,” Fenske explains.
“The reciprocating mass can start to outrun the spring. There comes a point where parts of the system lose direct contact with one another, and the valve movement is no longer directly related to the camshaft’s movement.”
Using a disassembled single-cylinder engine as a demonstration tool, Fenske illustrates a typical in-block-cam, OHV valvetrain setup, with the lifter, pushrod, rocker arm, valve, spring, retainer, and lock. All those parts moving in unison make for a lot of reciprocating mass that have to be controlled at high-RPM.
Once you encounter valve float, it can be as minor as simply a loss of power, as the valves are no longer doing what they are supposed to do, when they are supposed to do it, or it can be so severely out of sync, that things start impacting one another.
“Stiffer springs will help you keep everything in contact in the higher RPM ranges,” says Fenske. “However, [the stiffer valvesprings] result in an efficiency loss, as it takes more system effort to compress that stiffer spring.” That increased parasitic loss can negate any benefit you might see by spinning the engine higher, unless you have a system that is specifically designed to operate in the higher RPM range.
Another factor in the equation is pushrod deflection in high-RPM applications. “The length does play somewhat of a role. If the pushrods are not strong enough, they can bend, meaning you won’t have proper valve opening and closing,” says Fenske. “I think this is an easier problem to address, by simply developing stronger pushrods.”
The main component eliminated in an overhead cam engine is the pushrod, and removing that variable is one of the key factors in allowing more control of the valve at higher rotational speeds for a variety of reasons.
“DOHC engines have four valves per cylinder rather than two, so this means the valves are smaller, each with their own spring,” says Fenske. “There’s also no pushrod, so you’ve eliminated a good amount of reciprocating mass.”
This is where the “generally-speaking” and “relatively” statements come into play. Afterall, we can list off several forms of racing where pushrod engines spin to the moon. Some of them are designed to live for 500 or so miles without being torn apart, while some are lucky to make it 10 miles before needing new components, but they do perform quite well. And as Fenske points out, “Then you see Formula 1 engines spinning to 20,000 rpm.”
To accomplish that feat, the dual overhead cam engines used in F1 have done away with coil valve springs altogether, instead using pneumatic springs.
“By using a pneumatic spring with air pressure instead of a steel coil spring, the valves are easier to control, and you don’t run into harmonic vibration issues,” says Fenske. “Generally speaking, dual overhead camshaft designs, especially using a pneumatic valve, will be able to rev higher than a traditional pushrod setup in the same application.”
Due to packaging constraints. most OHV engines are two-valve designs. While those typically work well in lower-RPM applications, a multi-valve-per-port design will inherently outflow a two-valve design at elevated RPM.
Airflow Restrictions
“At high RPM you need lots of air,” says Fenske. “While it is possible for a pushrod engine to use more than two valves per cylinder, it’s quite a complex design.” That design not only adds cost and complexity to the system, we’d have to imagine it would also add significantly more mass to the valvetrain, further exacerbating the previously discussed issue. “If you were to go to a 4-valve setup, you’d reduce the size of the valves so you would be partly improving the situation, but you’d be eliminating one of the key advantages of a pushrod engine which is its beautiful simplicity.”
“[A two-valve-per-cylinder] design generally limits airflow at high-RPM,” Fenske says. “At lower RPM, you can actually benefit from the two larger valves, as it will increase velocity of the intake charge and promote better fuel mixing. However, an engine revving at 10,000 rpm vs 5,000 rpm, ideally, will be pulling in about twice as much air.”
Here’s a model of a four-valve pushrod design. While you improve airflow potential, you also increase complexity and valvetrain weight by adding double the pushrods and rocker arms.
By increasing the number of valves in the chamber, even though they are smaller valves, a simple area calculation shows that your total valve area is increased. While there are chamber and camshaft designs out there to maximize the two-valve high-RPM flow, they are still outmatched in the upper-RPM range by a multiple-valve-per-port design. “Ultimately, a four-valve system will be able to flow more air through it at higher RPM,” says Fenske.
Valve Timing
Another interesting subject Fenske broaches in his video is adjustable valve timing and how it affects engine performance. While most domestic V8 race engines lock out any variable timing features, he raises the point that not being able to control each side of the equation (intake and exhaust timing) independently and dynamically is a hindrance on a pushrod engine.
“Having a single camshaft controlling everything limits your control. All that can be changed is when the valves are opening and closing in relation to camshaft rotation,” Fenske says. “While Variable Cam Timing is present in some OHV V8s, that only changes the timing of the system as a whole. You can’t change duration of the intake or exhaust independently of one another, adjust overlap, or lift.”
Using this awesome 3D-printed LS engine model, Fenske explains the variable camshaft timing limitations of an OHV cam-in-block engine design.
All of those variables can be adjusted independently on a dual overhead camshaft engine, and with modern controls and designs, adjusting them on the fly is reality, not fantasy. “You’re able to control the intake and exhaust valves independently of one another, as well as have multiple lobe profiles for lift and duration on each camshaft,” says Fenske. “That, along with the ability to adjust valve overlap allows for optimal airflow efficiency in any RPM range.”
So while it’s obvious that an overhead camshaft arrangement is advantageous in a high-RPM application, it’s bit of an oversimplification to simply say that pushrod engines can’t rev high. Sure they have some inherent challenges to overcome, but like anything in this world, with enough effort, knowledge, and money, anything is possible.
Overhead valves open vertically from the head into thecombustion chamber. Overhead valve engines use a system of lifters,push rods, and rocker arms to operate the valves. The cam on anoverhead valve system is adjacent to the crankshaft, and thelifters, push rods and rockers work together to operate thevalves.
Overhead cam engines position the camshaft directly above thevalves. In that position, there is much less weight of the valveoperation mechanism, resulting in a potentially higher engineoperating speed.
Mainly, the difference is that in an overhead cam engine, the camshaft is mounted on the head of the engine, and the valves are still overhead, in an overhead valve engine (sometimes called a 'pushrod motor'), the camshaft is in the crankcase. Read More
1990 is ka24e, single overhead cam, 1991 is ka24de, dual overhead cam. Read More
Exhaust cam, is the cam that control valve timing on a DOHC, (DUAL OVERHEAD CAM) motor. Read More
Under the valve cover, of an overhead valve engine. They push the rockers on the opposite side of the valve stems. Overhead Cam engines do not have push rods. Read More
The Z24 engine is an overhead valve engine and the Z34 is an overhead cam engine. The Z34's engine is pretty tight in it's engine bay so if you were thinking of doing a 'swap', Z34 engine will not fit in the Z24. Read More
A high lift cam lifts an engine valve by a higher amount than a low lift cam. Read More
The 4.7L is an overhead cam V8. The 4.0 is a push rod inline 6. Read More
DOHC - Dual Overhead Cam. This means that there is an intake and an exhaust cam, allowing the valves to move differently for the intake and exhaust. SOHC uses a single cam with more lobes Read More
The difference between the two engines is a Honda civic dx stock engine is 1.6L single overhead cam nonvetch motor and the Honda civi si stock engine is a 1.6L doul overhead cam vetch motor. I'm not positive about the exact number but the difference is about 70 horse power between the dx and si motors Read More
If this is the (S.O.C) single overhead cam, there is no adjustments. The cam is worn, or there is a stuck valve, or broken valve spring. Hope this helps, GOOD LUCK! Read More
I am not familiar with these terms. In 2008 three engines were available in the Outback. EJ253 is single overhead cam 2.5, EJ255 is double overhead cam turbo 2.5, and EZ30D is double overhead cam 3.0 six cylinder. According to Subaru no other engines were available. Read More
Well the 2300 is an overhead-can set up. Which means it doesent have push rods or lifters. The cam is what actually pushes the valves open and closed. The cam is set right under the valve cover. Read More
On an overhead cam engine, the lobes on the cam push the rocker arms which push the valves open. Read More
where is the pvc valve on a 2002 ford focus dual overhead cam ztec 2.0 Read More
Do you mean Dual Overhead Cam and Overhead Valve. Overhead Valve has the camshaft positioned just above the Crankshaft in the block of the engine and uses Pushrods to actuate the valves. Overhead Cam(Single Camshaft) and Dual Overhead Cam(Twin Camshafts) have the Camshaft positioned in the head directly over the valves. As a rule of thumb OHV(Over Head Valve) engines have smaller and shorter heads while SOHC(Single Over Head Camshaft) and especially DOHC(Dual Over Head… Read More
single overhead cam Not true, it is cam in block design and over head valve (OHV). Read More
Timing belts are usually only used for Overhead Cam Engines ( DOHC or SOHC). The Iron Duke is a overhead valve engine and not an over head cam engine. The Iron Duke has cam gears instead of a chain or a belt. Read More
The 4.2 L - V6 engine used in the 1997 Ford F-150 is an OHV ( overhead valve not an overhead cam ) if that is the engine you are referring to ? Read More
all the civcs have a single overhead cam motor(sohc) the civic si is the only one with a dual overhead cam motor Read More
Older engines, not overhead cam, it goes in from the front of the motor and you need to put in new bearings with the cam. Overhead cam is under the valve cover and is held in place with caps that bolt on. Take the timing belt pulley off the front, hold tension on it so that the timing doesn't jump, remove the caps and lift out the cam shaft. Read More
The lifter bucket, which is on top of the valve stem, and the spring in the Overhead Cam engine aides in adjusting the valve clearance. Read More
2.2L D-tech . Its a 16 valve, dual overhead cam engine, with 130hp. Read More
standard will have a single overhead cam the espresso will have a dual over head cam all the front hood will have a bulge on the passenger side of the hood that is one way to identify that this is a dohc. Read More
The difference between a GT and an LX is the GT is a 2 door , the LX is a 4 door. The GT has a 1.8 L dual overhead cam with 4 valves per cylinder. An LX has a 1.9 liter engine with a single overhead cam and 2 valves per cylinder. The GT has a tach. The LX doesn't. Read More
On a 1997 Ford Taurus : The 3.0 ' Vulcan ' V6 engine is an Over Head Valve / pushrod design engine The 3.0 ' Duratec ' V6 engine is a Dual Over Head Cam design engine Read More
Since an overhead cam engine is a little different in it's design, it doesn't, technically have lifters. There is, however, a pad between the camshaft and the valve stem. The pads are available in different thicknesses and when the machine shop sets up the head, the machinist sets up the clearance between the cam and the pad. To change the thickness of the pad, you must first remove the camshaft. Don't bother trying this if… Read More
The older ones, non ecotec are not overhead cam. they Use the cam in block with pushrods. Read More
No the valve seals are between the cam and the head, under the valve spring Read More
It's an over head valve engine ( OHV ) it isn't an over head cam engine Read More
No it does not. The only engine in the 1988 Cadillac De Ville was the 4.5 Digital fuel injected engine. The 4.5 engine is a 2 valve per cylinder overhead valve (not overhead cam) engine. It was a good engine but not in the power category as the Northstar. Read More
Jeep uses an orfice line, not a pcv system. They can only be purchased at the dealer. Read More
No , the 3.8 liter Ford V6 engine is an OHV ( over head valve ) design Read More
First of all if it is an sl1 it is not a Dual overhead cam It is a single overhead cam Either engine only has 1 t-stat Read More
An overhead valve (valve in head) engine has the valves in the head (not in the block) and they are in line with the motion of the piston, but are above it. Valves are usually operated by push rods or by overhead cam (driven by timing belt/chain from the crankshaft). The engine can be in any orientation : ie the cylinder motion could be horizontal or vertical. Overhead valve engines are high performance engines used… Read More
the standand 4 litre is a over head valve activated by push rod center cam the 4 litre sohc is a single over head cam the cam location is above the valves Read More
The 2006 Ford Taurus has the 3.0 liter ' Vulcan ' V6 ( or flex fuel version ) It is an over head valve / pushrod design engine Read More
A conventional cam has more moving parts as it is placed within the engine block and uses lifters, pushrods, and rocker arms that all must work together with the cam and the valves. By moving the cam to above the valve train, called an overhead cam, there are now less moving parts, the engine becomes more efficient (can mean more power), can reach higher revolutions per minute (revs), and generally is more reliable. Read More
SOHC--Single OverHead Cam DOHC -- Dual OverHead Cam Read More
There are engines with both. If the car is a twin cam then it's a double overhead cam. Single cam has less power but is more economical. Read More
It has a single cam. But the question is confusing. Usually it is asked if it is a single or dual overhead cam when it is overhead head cam. Your engine is a pushrod, which was only offered in single cam Read More
Its a single cam.. it says ' 2.0 Split Cam on the cover of the engine... A dual cam would say ' 16V Zetec' on the engine cover. Read More
First of all the Coupe has an iron block overhead valve (16 valve) 5.7L V8 with 240hp. The L98. The ZR-1 has a Lotus designed, Mercury Marine built, aluminum block dual overhead cam (32 valve) 5.7L V8.It's called the LT5. It has 375Hp. The suspension is more track tuned. Higher spring rates, bigger anti-roll bars. Things like that. The rear body work is wider to fit wider rear rubber. There is also center high mounted… Read More