Previous in Forum: Shear Strength Nylon Screw   Next in Forum: Air Spring Replacement
Close
Close
Close
12 comments
Active Contributor

Join Date: Jul 2007
Posts: 18

Cam Shaft Questions

08/20/2007 8:32 PM

hello

i want to ask about the difference between the engines with cam shaft that is located directly at cylinder head valve & engines that have cam shaft not direct to valves.

wanna ur help

Register to Reply
Pathfinder Tags: engines
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".
Anonymous Poster
#1

Re: Cam Shaft Questions

08/21/2007 2:49 AM

Hi, OHC arrangements evolved mainly from a desire to reduce manufacturing costs. There are some advantages derived from reduced mass and linkage friction by using OHC techniques.

Register to Reply
Anonymous Poster
#2

Re: Cam Shaft Questions

08/21/2007 3:32 AM

Send a private message to 'Sidevalveguru'.

Register to Reply
4
Guru

Join Date: Jul 2007
Location: UK, Midlands
Posts: 515
Good Answers: 2
#3

Re: Cam Shaft Questions

08/21/2007 9:36 AM

In years gone by dozens or even hunderds of descriptive books were written. Now probably the preserve of car-boot sales, and the automobilia stands at shows.

OHC -overhead camshaft: This is not one design or arrangement but a general statement that the camshaft is positioned above the combustion chamber. More is implied by the terms Double Overhead Camshaft (DOHC) and Single Overhead Camshaft (SOHC). The cam lobes might or might not operate the valves through rockers. A DOHC arrangement usually has the camshafts acting through 'bucket' tappets (followers), a reliable and robust arrangement. Older examples of this type had two valves per cylinder and hemispherical combustion chambers, today the 'pent-roof' combustion chamber with 4 valves (or more) is favoured. SOHC arrangements are many and varied. If the combustion chamber is a relatively crude flat-topped affair, the cam lobes can all act directly on the valves. More usually rockers are interposed enabling a greater freedom in combustion chamber design and valve lift/dwell.

OHV - Overhead Valve, usually with camshaft in block or crankcase operating through pushrods and rockers. Some variants have managed hemispherical combustion chambers at the expense of either package space or cost/complication. The extra mass of the valvetrain generally meant a lower revving engine. There have been some very successful twin-camshaft pushrod designs in conjuntion with 'hemi heads'.

Side-valve - Valves situated in the cylinder block beside the cylinders and operated directly by a low-mounted camshaft. Remained popular for very many years among American manufacturers. The gas flow and thermodynamics are probably the worst of the three arrangements. Some effective designs came about by combining side- and overhead-valve into "inlet-over-exhaust.

I was brought up to believe that 'ordinary' engines remained pushrod for so long for ease of maintenance. When regular 'decoking' was necessary it was better if it could be accomplished without disturbing the valve timing.

__________________
Wish I was here more often.
Register to Reply Good Answer (Score 4)
Guru

Join Date: Jul 2006
Location: Silicon Valley
Posts: 5356
Good Answers: 49
#5
In reply to #3

Re: Cam Shaft Questions

08/22/2007 11:16 PM

In your side-valve description, I noticed you didn't mention that most unholy of mechanical names... "The Flathead!"

God, I hate those things!!!

__________________
"Perplexity is the beginning of dementia" - Professor Coriolus
Register to Reply
Guru

Join Date: Jul 2007
Location: UK, Midlands
Posts: 515
Good Answers: 2
#8
In reply to #5

Re: Cam Shaft Questions

08/24/2007 5:54 AM

Rate 'em along with Flat-Earthers?? Sorry for substandard terminology.

__________________
Wish I was here more often.
Register to Reply
Power-User

Join Date: Jan 2007
Location: The Capital City, Cow Hampshire, USA
Posts: 477
Good Answers: 3
#12
In reply to #5

Re: Cam Shaft Questions

05/15/2008 5:57 AM

Now, now; no name calling!

"flatheads" were introduced as an economy move.

My assertion is that a 'high-tech' sidevalve will beat a cheapy ohc setup.

It was as much the $5 carby, The 1930 metallurgy, & lubrication as the basic design.

__________________
If you always do what you've always done, You'll always get what you've always had!
Register to Reply
Anonymous Poster
#4

Re: Cam Shaft Questions

08/22/2007 2:26 AM

Mr A.Grahame Bell has written two very detailed and easy to read books on engine design. Either of these books: 'four stroke performance tuning' OR 'performance tuning' are published by haynes books in england. they are available at quite reasonable prices through Amazon ( they were the cheapest when I bought my second edition recently) and are probably available at your engineering library.

This authors books give great detail as to the ins and outs of overhead camshafts and various other valve issues.

Register to Reply
Guru
Popular Science - Weaponology - New Member Safety - ESD - New Member Hobbies - Fishing - New Member

Join Date: Sep 2006
Location: Near Frankfurt am Main, Germany. 50.390866N, 8.884827E
Posts: 17996
Good Answers: 200
#6

Re: Cam Shaft Questions

08/23/2007 2:32 AM

Have all your questions been answered? Your question was very wide ranging and maybe not so easy to answer, so if it has not been achieved yet, let us know exactly why.

__________________
"What others say about you reveals more about them, than it does you." Anon.
Register to Reply
Guru

Join Date: Jul 2006
Location: Silicon Valley
Posts: 5356
Good Answers: 49
#7

Re: Cam Shaft Questions

08/24/2007 12:50 AM

Here's one thing that hasn't been answered - If you intend to spend a lot of time around red-line, you're more likely to "float a valve" with inside cams, push rods, lifters, and rockers than you are with overhead cams.

"Floating a valve" is when a valve can't shut fast enough not to be hit by the rising piston head. BAM!!! I think you can imagine the result.

__________________
"Perplexity is the beginning of dementia" - Professor Coriolus
Register to Reply
Guru

Join Date: Jul 2007
Location: UK, Midlands
Posts: 515
Good Answers: 2
#9
In reply to #7

Re: Cam Shaft Questions

08/24/2007 6:06 AM

The two reasons, I've always thought, that the DOHC layout became the norm for sports engines was the ability run faster without problems like valve bounce and the distinct imporvement in gas flow and combustion chamber shape that inclined valves offered. The 'pent roof' shape appeared practically at the dawn of time, well shortly after breakfast in a 'day in the life' of the internal combustion engine; take a look at the output of the likes of Sunbeam circa 1914-1916. For some reason the 'hemi' design became the accepted optimum for a long time (maybe Ricardo?) before being pushed out again by the likes of Keith Duckworth.

Just rambling on amusing myself.

__________________
Wish I was here more often.
Register to Reply
Guru

Join Date: Jul 2007
Location: UK, Midlands
Posts: 515
Good Answers: 2
#10

Re: Cam Shaft Questions

08/24/2007 6:31 AM

OK perhaps a summary of the differences rather that re-amble thro history:

Side-valve with flat head: Simple and cheap to manufacture and maintain. Rather poor gas-flow (cylinder filling) although the valves needn't be vertical. In the 'old days' when it was current there was probably too much heat lost into the large cylinder head surface, having inlet and exhaust valves and ports side-by-side isn't a good idea etc. Still prevails in lawn mowers and the like where efficiency doesn't hold sway.

OHV: Originally cheaper to implement and easier to maintain than SOHC/DOHC. The advent of toothed belts changed this quite dramatically. Look at older bevel driven OHC drives. Expensive compared with low-mounted camshaft and pushrods, Adoption of 'Reynolds' chain helped but it was still a pain when cylinder heads had to come off regularly for 'decoking' and repair. Some OHV designs turned out to be quite efficient (e.g. BMC 'A' series) but not on a par with DOHC.

SOHC: somewhat pointless half-way-house. These days probably more expensive than DOHC for an inclined valve/high revving application. With vertical valves it's just an expensive alternative to OHV which might be coaxed to rev a bit more, never the less existed for quite a long period (Ford 'Pinto'.

So DOHC is the layout of choice today! Reliability and life between overhauls of well developed designs/fuels/lubricant combined with the cheapness of toothed-belt drive and it's abillity to make more power and less emssions make it so.

__________________
Wish I was here more often.
Register to Reply
Guru
Popular Science - Weaponology - New Member Safety - ESD - New Member Hobbies - Fishing - New Member

Join Date: Sep 2006
Location: Near Frankfurt am Main, Germany. 50.390866N, 8.884827E
Posts: 17996
Good Answers: 200
#11
In reply to #10

Re: Cam Shaft Questions

08/24/2007 7:47 AM

I liked what you wrote, it was a good description in the main, though with regard to OHV, you could mention also that many of the OHV engines (push rod) were actually older side valve engines that were converted....even I believe your beloved "A" series engine was originally side valve if I remember correctly!!

It was a cheaper way to go than directly to OHC (and a lot of people were taken in by this!!), but the reliability of OHC was questionable, except in some exotic designs generally.

The Japanese were instrumental to my mind in bringing OHC into the everyday world in the late 60's and early 70's (Mazda for one) as up till then, OHC was mainly for the average person, far too expensive and had had sensitive reliability (MGA 1622 DOHC for example).

The first reliable OHC car engine to my mind from a British manufacturer were the Hillman Imp engine(s), but they still had valve shimming for adjustment purposes.....it took ages to get it just right, especially if you had to go and buy the different shims each time!!! Not that it was needed that often though!!

They Japanese were excellent and made a good job of a) internal head design with the inlet and exhaust on either side of the engine, driven by one camshaft using camshaft followers that also allowed valve clearances to be easily adjusted, without using shims.

It was to my mind, an excellent compromise for everyday usage.

I have to admit though that over the last 26 years or so, my design interests have centered more with regard to Diesel than petrol engines.....I like Torque and economy!

__________________
"What others say about you reveals more about them, than it does you." Anon.
Register to Reply
Register to Reply 12 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".
Copy to Clipboard

Users who posted comments:

Andy Germany (2); Anonymous Poster (3); sidevalveguru (1); vermin (2); Wrenched (4)

Previous in Forum: Shear Strength Nylon Screw   Next in Forum: Air Spring Replacement
You might be interested in: Gas Valves, Industrial Valves, Solenoid Valves

Advertisement