pushrod vs. ohc

The biggest advantage to chosing overhead cams is that is the only way ferraris come

Not that many years ago valve springs weren't all that good and limited the rpm a pushrod engine could spin and therefore the hp but that really hasn't been the case for 20-30 years now. now the drive to overhead cam is more about valve angle options to make better combustion chamber shapes or flow paths or ease of varying cam timing or lobe separation....both systems work pretty well though.

 

Oscillating masses waste energy -- so no pushrods nor rocker arms are best from this viewpoint. The downside for the overhead cam is the cam drive system is more complicated, and you need more cams if multi-bank and/or if you want a large angle between the valves and no rocker arms as mk e mentioned).

The single-cam-in-block-with-pushrods-and-rocker-arms advantage is compactness and lower cost.

 

Overhead cams give the designer more freedom for port shapes and positions, chamber shapes, valve quantity, positions and angles. That freedom is crucial to efficiency and emissions.

 

There are a bunch of features, here:

In block cams (IBC) have smaller packages, This means you can package a bigger (CID) engin in the same engine volume.

IBCs have lower friction. As one valve drops off the nose of the cam the springs energy is transfered back into the camshaft and into the next rising valve spring. In a V8 DOHC the closing exhaust valve has to transfer then energy out ot the cam belt and into another cam. The closing intake valve has to switch blocks to apply energy to the opening exhaust valve--2 belts, 2 rollers, 3 gears.

IBCs are easy to rollerize and this gets rid of massive amounts of friction (especially when a rocker arm is also rollerized). This is also why rollerized finger followers are popular in OHC designs.

The push rods from an IBC get in the way of the intake ports harming air-flow. Thus high RPMs and IBCs do not go together.

The IBC with pushrod are not as stiff and have much more mass than a direct actuating DOHC. This also hurts the ability to run at high RPMs. Once the pushrods have damaged the incomming air-flow, one can bend the headers around without constraining airflow any more than necessary. This reduces the volume of the bay housing the engine.

The drive mechanism in an IBC is 3 components (2 gears, a chain 9 journals, or 2 gears and 9 journals). The drive mechanism in the F355 is 3 gears, 6 pulleys, 2 belts, 2 tensioners, 4 bearings, 36 journals.

The center of gravity is lower since rocker arms are lighter than cam shafts for conventional V engines. Flat 6s and 8s are not at a disadvantage, here, nor are 120 degree V6s.

Advantages of OHC

Stiffer valve actuation (direct, finger followers or rocker arm followers)
lighter reciprocating mass
Better air-flow through better port shapes

Advantages of DOHC

Stiffest possible design
lightest possible design
highest RPM potential
Follows cam profile best
Best port shapes

The new Ford Coyote engine finally has (almost) vertical air-flow in the heads and is one reason it, at 5.0 litres --OK, 4.951; has almost as much power as the 6.3 litre Camaro engine 412 versus 425.

 

Of course, the holy grail over the last few years have been electronically actuated valves, rather than cam driven: to let the computer vary the valve timing with complete freedom.

The problems have been the speed and force available from the available solenoids. Neither are really up to the needs.

 

FIAT has developed an interesting twist - A SOHC engine with direct actuation of the exhaust valves, and hydraulic actuation of the intake valves. Through electronic control of the hydraulic system they can infinitely vary the intake valve behavior. Here's a link to one explanation: http://www.greencarcongress.com/2009/03/fiat-introduces.html

And a very pretty picture of the mechanism.

You can Google "Fiat Hydraulic Valve" and get lots of articles.
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Shhhhh.... don't tell that to the early Colombo motor guys...

Rick

 

Less oscillating mass = more RPM before valve train float = more normaly aspirated volumetic efficiency = more horsepower per liter

Of course we know that most of the pre-Fiat V12 Ferraris were SOHC with rockers.

 

An overhead cam lets you do cool stuff.

BMW is coming out with what they call their “Valvetronic” engine. In this application they have eliminated the throttle butterfly valve and they have replaced it with an electronic lever that controls valve lift instead.

This provides better mileage and near immediate throttle response according to BMW because “load control now takes place directly in the combustion chamber rather than in the throttle, eliminating the need to fill the intake manifold between the throttle butterfly and the combustion chamber”.

 

Pushrod deflection is another major issue.
People crap when they see the size of some of the pushrods I've installed in big inch, drag race engines. 9/16" to 7/16" taper with .188" wall thickness... they look like small baseball bats.
I know some of the blower motors now use 5/8" to 9/16 tapers now. All in the name of valve train stability.

 

It's a pretty amazing system. At the IAA in Frankfurt, I was so fascinated it took me over half an hour to look at it Strangely, Fiat/Alfa Romeo have not made any attempt to praise their revolutionary new technology, but hide it under the completely random name "Multi Air"...

 

Is this thread for real?

 

Technology changes everything this changed my view of push rod engines.

http://paceperformance.com/index.asp?PageAction=VIEWPROD&ProdID=248897

 

I've heard that the NASCAR guys not only go with fairly thick pushrods, but have gone to carbon fiber in order to prevent deflection, lower oscillating mass etc.

Which explains their fairly impressive feat of getting pushrod motors to run consistently between 9 and 10k RPMs. Which, granted, is 1/2 what F1 motors turn, but is still impressive for a pushrod engine.

 

wow i didnt know that. 9-10k rpms is way more than my dohc cars.

 

Pushrod motors at 9-10K rpms only have a lifespan of 25hours or less between refresh/rebuilds. That is way less then your dohc cars...

Rick

 

True - they swap motors out after about 800 miles. These aren't 'long lived' high revving motors.

(Interestingly 800 miles is about the same distance the F1 screamers run before being swapped)

 

I don't understand why. This is a 6.2 liter motor with a supercharger making 638 HP when Ferrari's 6.0 liter motor in the 599XX makes 700HP maturally asperated.

 

BMW has been using it in their non-M engines for over 6 years now.

 

I'm not an engineer by any stretch of the imagination, but I've always wondered why valves are operated by cams anyway?

has anybody ever used a rotary opening on top of the combustion chamber? so no valves just a spinning disk that lets in air/gas lets out exhaust? maybe there is no way to vary the openings with higher engine speed, but i would think that a pneumatic system or Hydraulic system would make this possible? just thought I would throw that out there?

 

A brand new gm motor is 23K. 23 K won't pay for a valve job on the 599XX both are impressive in their own right