Why is the pushrod 'obselete'?

**Wyldeone22** · 4-2-04, 22:30

I love torque.... the pushrod does that very well

chevy's 5.7l gets great gas mileage for the power...

why is everyone starting to flame pushrods? just curious...

I think the saleen s7 is even pushrod....

Just curious.... no flaming please lol.

**Trent** · 4-2-04, 22:46

that's a good question. a lot of guys in the import scene are quick to bash pushrod v8's. However, the old clunky big-blocks of yesteryear have tightened up quite a bit. Look at the LS1 corvette engine for example. That's a pretty sophisticated engine in and of itself. Not everything needs VTEC to make power.

**Arkaine** · 4-2-04, 22:50

what exactly is a pushrod? sorry for my ignorance

**RbluEMx6** · 4-2-04, 22:58

A pushrod has a hole in the center of it that flows oil into the heads as to not allow the valves to burn. The pushrod lifts the rocker arm which in turn lift the valves, it is located inside the engine.

I took a stab at explaining in a plain context. I am not sure if I am entirely correct. Ill research it some more.

I think drivitlikustolit would have a much better explaination than I would.

**Trent** · 4-2-04, 23:00

Here ya go Jay,

http://auto.howstuffworks.com/camshaft5.htm

This explains it with a nice little graphic showing it in action.

Trent

**Aussomea** · 4-2-04, 23:38

i love pushrod engines but there is no reason why the ammount of torque found in pushrod engines cant be found it OHC motors. im buying a pushrod engine powered pontiac GTP this weekend

i love my mx6 but it doesnt have near the amount of torque that the gtp does

being able to swap rockers to make more power and get better gas mileage is pretty sweet..

**Rat** · 4-3-04, 0:16

First off, for those who don't know, I'm a die-hard muscle car guy, so this question is right in my wheelhouse, and I'm gonna try to drive it out of the park....

What is a pushrod? A pushrod is a rod that goes between your lifter and your rocker arm. The camshaft pushes the lifter, the lifter pushes the pushrod, the pushrod pushes the rocker arm, the rocker arm "rocks" and pushes the valve stem, which opens the valve. Therefore, when people talk about pushrod engines, they are really talking about cam-in-block, valve-in-head engines, which are more commonly referred to "OHV" or overhead valve engines.

So, why is the pushrod obsolete? Well, clearly it isn't. However, it is inherently inferior to overhead cams. There are a couple reasons for that:

1. The more components there are in the valvetrain, the more mechanical losses due to heat and friction there will be. It's more reciprocating mass, which robs horsepower because the engine has to spend more energy on moving its own valvetrain.
2. The more mechanisms you put between your cam and your valve, the less control you have over the motion of the valve. Pushrods can flex and bend and so on, and even though that flexing is undetectable to the naked eye (unless something goes horribly wrong), it still takes away from the predictability of the valve's motion, which makes fine-tuning the engine more difficult.
3. When the cam is in the block, it is more difficult to have dual cams, which is one more loss in control over the valves' motion.

Obviously, I still love pushrod engines, and Wyldeone already made a great case for why they aren't "obsolete," but an engine with overhead cams should generally be superior to an exactly identical engine with the cam in the block.

I hope that helps some....

**rcoolb2002** · 4-3-04, 0:30

I think there is alot of flaming on the pushrod engines because the corvette has one, and they need to find SOMETHING wrong with it, since its so perfect

. To a ricer there has to be a fault, somewhere, somehow, in that terrible American sports car.

**Rick** · 4-3-04, 0:42

Yup. DOHC will always have more relative potential than pushrods. When revs combined with appropriate flow equal more power, and pushrods drastically limit the engine's maximum speed... GM proved ten years ago (okay, so it was really Lotus and Mercury Marine) that the DOHC lt5 could create as much power as the current Ls6, while using what are (though I can't verify it) undoubtedly lesser designs for engine control.

I wouldn't say that pushrods are obsolete, but they are archaic, and I look forward to the day that GM finally invests the time into making their own DOHC small block Corvette, to show the world just what's wrong with the Viper and Modena.

**Jon** · 4-3-04, 0:50

Personally, I love SBC's (Small Block Chevy's) even though they have their own quirks.

I wouldn't call pushrod obsolete, as obsolete is generally a definition for something that is no longer produced.

People generally frown on pushrod engines because:

They limit the number of valves a cylinder can have.
There are more moving parts then a modern Cam-on-Shim or Cam-On-Bucket design.
They can be a pain to work with. Changing out the lifters requires removing the intake manifold. Removing the cam usually requires completely dissambly of the front end on a rear wheel drive car, or pulling the motor on a front wheel drive car.
Pushrods don't allow any way to have any form of variable valve control.
Pushrods can flex at high-rpm.

**RbluEMx6** · 4-3-04, 2:00

Rat, Rick, and Flyswat, great info. I didnt mean to undermind anyone. Keep those informative post coming.

**drvITlîkUstôlIT** · 4-3-04, 12:43

Everyone did a great job explaining that. I don't have much else to add.

One thing to keep in mind is pushrod engines are cheap to make. The cam is positioned in the engine block close to the crankshaft so it only takes one sprocket on each and a short chain to spin the cam.

While it does add components that are needed inside the engine, there are less things to make room for outside the engine. A big benefit of stuffing a big V8 in a car.

Plus, because of the short distance, an actual metal chain can be used. You can't do that very well with OHC engines because there would be too much slack and noise in the chain. Obviously it's done, but not often.

The other great thing is the cam is oiled very well being where it is. AAMOF, the oiling system itself is great. Oil is pumped through galleys in the block, goes into the lifters through holes on the sides, then is pumped up into the pushrod from a plunger inside the lifter. It then comes out the top of the pushrod, runs across the rocker arm onto the valves and valve springs to help lubricate and cool, back down through the head into the lifter valley, falls over the camshaft and distributor gear as well as through the front onto the timing chain, then back into the oil pan.

People are always under the impression that pushrod engines can't rev as high because of the mass of components. That's not entirely true. A SBC, SBF, or even a Mopar can run very high (Easily to 9,000 RPM) with the right componenets. The problem lyes in the rotating mass of the crank, pistons and rods.

The primary purpose of an engine with more displacement in production cars is torque. That's what you feel in the SOTP meter.

The cam that is used is very tame so that more lower RPM torque is made. Because of that, cheaper heavier parts can be used everywhere else since the engine will rarely see more than 6,000 RPM. This again keeps production costs down.

Plus there's a huge advantage with rocker arms. Lift ratio. An OHC (with the exception of some I4's) doesn't use rocker arms, so when the lobe lift is .300", the valve moves off the seat .300" (minus any lash needed). With a 1.5:1 rocker arm ratio (common if SBC's of yester-year) the valve moves .450" off the seat which helps make up for using fewer valves. It also allows the cam to have a very slow, gradual accelerated rate of lift, yet be able to open the valve quite a bit. Very good for longevity. AAMOF, the LS1/LS6 uses a rocker arm ratio of either 1.7 or 1.8:1 (I can't recall exactly).

The reason a Corvette hovers around $50,000 is because it's cheap to make. Pushrod engines are cheap to make. The components are cheap to make. It's also really a very reliable system despite how cheap it is. That in turn keeps warranty claims down.

Sure GM could put an overhead cam engine in the Corvette, but by the time they did all that the cost would be enough to put the Corvette well above $75,000. That's not what they, nor the Corvette enthusiasts want. IIRC, the ZR1 (the Corvette with the LT5) cost about that in 1991 yet only produced 405 HP. That's what the ZO6 puts out 10 yrs later and $20,000 cheaper.

**TheBouleOfFools** · 4-3-04, 15:50

Ahh I was so happy because I knew the answer to the "pushrod" question, and then everyone beat me to the punch =[