Overhead cam question

I imagine many will think this to be a lame question, but I would like to know why an overhead cam arrangement is superior to a valve en bloc layout? I can imagine that OHC would trump OHV, eliminating all those moving parts, but fail to see why it would be superior to a simple valve-in-block. Thank you!

Valve in block, or side valve engines have limited breathing ability due to restrictive inlet/exhaust passages and non-optimal combustion chamber shape. OHV and OHC engines allow freer breathing and ideal combustion chamber shape. Gas flow is improved, in fact, the fuel/air charge can flow in one side, get sparked, do work and efficiently leave by the back door. The OHC engine has the added benefit of fewer moving parts, i.e., no pushrods, rocker arms, etc. all of which take HP to accelerate/decelerate and change direction.

Valve in block or sidevalve engines had a lot to offer in a period when fuel octane and compression ratios were low. At that time an OHV engine had no advantage in power or efficiency over a good flathead and the flathead had advantages of simplicity, silence, light weight low friction valve train and a broken valve spring did not wreck your engine.

However, when compression ratios got over 7 or 7.5:1 the flathead had to chose between low compression or cutting off breathing. The new OHV engines did not have this problem and soon proved to be more powerful and efficient.

Now the OHC has the advantages of OHV engines plus light weight low friction valve train. By eliminating pushrods the valve action becomes as light and direct as a flathead.

Also, there are no pushrods in the way of locating the intake and exhaust ports which allows more efficient breathing.

It was the toothed rubber belt that made the OHC layout practical. Light, cheap, and silent it is used on most OHC engines. There are a few chain drives but mostly on more expensive cars like Mercedes.

Back in the heyday of the flathead, meaning the twenties to the late forties, there were a few OHC engines by Bentley, Stutz, Duesenberg and a few others. They did have the advantage of better breathing and more power but the advantage was small compared to the expense and complexity. This meant such engines were mostly confined to the race tracks where it was worth spending a lot of money and work to get a small advantage in speed if it was enough to win the race.

I was surprised to find out, from the memoirs of a Bentley insider, that the celebrated 3 liter OHC Bentley of the twenties had about the same power and performance as a 1954 Morris Minor or Austin A30 with a 1 liter engine. Top speed 72 to 74 MPH. There were lots of flatheads that would do as well or better such as the Chrysler six in the twenties.

Thanks Bill and Rusty; it seems that your info pretty well closed the subject!

Actually the real advantage of the OHC is that it looks cool.

They are also not as prone to "valve float" at high RPM's. My DOHC Cosworth really winds up high

prs519,

Overhead cam engines are cool. Think Jaguar.

But if you want efficiency you need a "desmo" engine.

Look up DESMODROMIC VALVES. Think Ducati............

Mike in Colorado

Speaking of OHC vs valve in block. The celebrated Bentley 3 liter's main rival was the Vauxhall 30/98which had a sidevalve engine. It was an older design but was rather faster than the Bentley, but it was a 4.5 liter engine. Of course unless you are racing, engine capacity means little or nothing. You may well be better off with a larger, less sophisticated but less fussy engine of larger displacement which was something American car makers proved for years.

Valve in block, or side valve engines have limited breathing ability due to restrictive inlet/exhaust passages and non-optimal combustion chamber shape. OHV and OHC engines allow freer breathing and ideal combustion chamber shape. Gas flow is improved, in fact, the fuel/air charge can flow in one side, get sparked, do work and efficiently leave by the back door. The OHC engine has the added benefit of fewer moving parts, i.e., no pushrods, rocker arms, etc. all of which take HP to accelerate/decelerate and change direction.

There are many ( more with than without ) OHC engines that use rocker arms in particular SOHC engines. This example has the hydraulic lifter gallery directly on the SOHC. This particular V-6 engine which started life in the 1980's and lasted into the 2,000's is a two valve Hemi and is driven by a fiberglass reinforced cam timing belt ( John DeLorean patent while he was director of advanced engineering at Pontiac to be used on the Pontiac SOHC 6 cylinder 230-250 Cu. in engines from 1966 to 1969 which also used rocker arms pivots )

Nissan VG seres V-6 valve train;

http://i.ytimg.com/vi/AQdFcVY3948/maxresdefault.jpg

The Pontiac OHC 6 valve train;

http://www.overheadcammerschapter.150m.com/six_img_7.jpg

Here is a image of the older Nissan/datsun L series OHC ( which is a close copy of Mercedes engines and uses the same valve adjusting tool. This engine was used in 4 cylinder Datsun 510, 610, 710, and in six cylinder 810/Maxima, 240,260,280Z and in trucks from 1300cc to 2,000cc. The head like the Pontiac OHC is not a cross flow head or the intake port and the exhaust port are on the same side of the head ( like flatheads are ). Notice like the Pontiac OHC the rocker arm pivot sits on a adjustable ball and two coiled springs hold the rocker to the adjuster ball and also keep it aligned with the cam.

http://i140.photobucket.com/albums/r13/Dragonfly240/P2150004.jpg

The 3 L Bentley was stated by the factory as good for 95 mph. A short wheelbase 100 MPH model was also offered. A 3 L Bentley set a record in 1925 of 95mph in 24 hours . They were fast and very reliable. Overhead cam gives bigger valves, 4 per hole, and less moving parts, the Bentley tack runs out of needle space at 3500 rpm. 4 cylinder Bentleys utilized only gears and shafts no silly belts and chains, which can break easier.

Maybe a special racing model but not a standard one as sold to an ordinary customer. My information comes from the memoirs of Bentley's head of sales.

The Vauxhall 30/98 after 1922 has overhead valves. It was the earlier versions that were side valve. http://en.wikipedia.org/wiki/Vauxhall_30-98 I have ridden in one and they do go rather well. Very fast by the standards of 1920s cars. Many Bentleys had heavy bodies from new which limited performance but most survivors have had lighter bodies fitted.

Isotta-Fraschini had an ohc engine was early as 1908 in their light car, which probably influenced Ettore Bugatti when he designed his Type 13 only a few years later. This not only had ohc (with a non-detachable head) but also four valves per cylinder in the later versions (and all in a bore size of only about 69mm).

It's the 4 valves per cylinder, or at least the hemi head design that allows larger valves in an OHC engine.

An OHV with valves all in a row will actually have smaller valves than a sidevalve. If you see one with the head off, or if you see the head gasket, notice that the valve chamber is wider than the cylinder. This is impossible with a conventional OHV. In the days of the long stroke, small bore motor the difference was significant. One reason the pushrod OHV had little or no advantage over the sidevalve.

prs519,

Overhead cam engines are cool. Think Jaguar.

But if you want efficiency you need a "desmo" engine.

Look up DESMODROMIC VALVES. Think Ducati............

Mike in Colorado

Since no one else answered...desmodromic valves are mechanically opened and mechanically closed, no springs, thus no float at high rpm. There were also "valve in valve" arrangements, which had to be mechanical nightmares.

From my limited understanding the OHC provided a valve configuration that allowed the valves to be somewhat "unshrouded" in relation to the combustion chamber,therefore

it seemed to give more power but because the "breathing" was a bit better ?

Take a Hemi 265 for example with tripple webers,how much power did they make in comparison to say an XR6 turbo?,of which the later had all the good OHC,efi,24valves,VCT etc?

are these the same as ones very early inventors called atmospheric valves (which is almost self-explanatory)?

Again from my limited understanding the "atmospheric valve" engine worked upon the principle of "self induction" to open/close the valve based upon atmospheric pressure within the confines of combustion procedures?

The OHC engine generally had a combustion chamber shape that would allow the intake valve to be un-shrouded which would allow "better breathing" simply because the head of the valve did not "see" a cylinder wall when the intake charge was on the way in?

Nevertheless,i still like my Briggs & Stratton side valve engine on my lawn mower despite the so called inadequate combustion process

But having said that ,the port shape on 2 valve engines should not be under estimated,i think the E49 Dodge Charger over here made about 2-4 bhp less than an XR6t and it only had 2 valves/cyl. ...tripple carbs of course but the Xr6 had some very good new technology.

The engineers at the time sort of knew a little bit ?

just my opinion that's all...

Actually the real advantage of the OHC is that it looks cool.

This!

Again from my limited understanding the "atmospheric valve" engine worked upon the principle of "self induction" to open/close the valve based upon atmospheric pressure within the confines of combustion procedures?

The OHC engine generally had a combustion chamber shape that would allow the intake valve to be un-shrouded which would allow "better breathing" simply because the head of the valve did not "see" a cylinder wall when the intake charge was on the way in?

Nevertheless,i still like my Briggs & Stratton side valve engine on my lawn mower despite the so called inadequate combustion process

But having said that ,the port shape on 2 valve engines should not be under estimated,i think the E49 Dodge Charger over here made about 2-4 bhp less than an XR6t and it only had 2 valves/cyl. ...tripple carbs of course but the Xr6 had some very good new technology.

The engineers at the time sort of knew a little bit ?

just my opinion that's all...

The Pontiac V-8 from 1955 to 1967 used a bathtub combustion chamber and the valves were somewhat shrouded, however from 1967 to the end of a real Pontiac V-8 in 1981 the valve angle was changed and a open chamber combustion chamber was use. You will also know the 348-409 Chevy V-8 cylinder head had no combustion chamber in the head at all and that means the valves were not shrouded. There are many examples of un shrouded valves in a wedge combustion chamber in pushrod engines. Pontiac "D" port head with bathtub chamber;

Pontiac "D" port with open chamber;

http://www.crankshaftcoalition.com/wiki/images/thumb/3/39/Open_chamber_head_68_up.jpg/300px-Open_chamber_head_68_up.jpg

la al al la la trying to get my message to stick it says its too short.

The Pontiac V-8 from 1955 to 1967 used a bathtub combustion chamber and the valves were somewhat shrouded, however from 1967 to the end of a real Pontiac V-8 in 1981 the valve angle was changed and a open chamber combustion chamber was use. You will also know the 348-409 Chevy V-8 cylinder head had no combustion chamber in the head at all and that means the valves were not shrouded. There are many examples of un shrouded valves in a wedge combustion chamber in pushrod engines. Pontiac "D" port head with bathtub chamber; message too short?

Helfen, thanks and I was interested in finding that the 55 Pontiac (of which, I have one) was valved in this manner. It begs another question for me. Apparently the 278? Pontiac engines were inferior to the 283-350 family of engines GM offered, as one seldom hears of builders using them for their projects, nor are they offered as "crate" to my knowledge. I wonder if the valve arrangement was a contributor in making these less desirable engines? Sorry to sidetrack my own subject, but who should care, really? Thanks to those contributing....keep em coming. There were upteen numbers of 55 Pontiacs produced, so one would think that these engine (maybe 287s?) would be all over in the hotrods.

Pontiac "D" port with open chamber;

http://www.crankshaftcoalition.com/wiki/images/thumb/3/39/Open_chamber_head_68_up.jpg/300px-Open_chamber_head_68_up.jpg

From my limited understanding the OHC provided a valve configuration that allowed the valves to be somewhat "unshrouded" in relation to the combustion chamber,therefore

it seemed to give more power but because the "breathing" was a bit better ?

Take a Hemi 265 for example with tripple webers,how much power did they make in comparison to say an XR6 turbo?,of which the later had all the good OHC,efi,24valves,VCT etc?

This is true if it is a hemi head. Not true if the valves are all in a row.

I was speaking specifically of the long stroke, small bore design that was common to flathead and OHV engines in the classic era.

The pushrod OHV engines of the fifties and up, are a big bore, short stroke design. They do not suffer from valve shrouding or small valves. They have a large, unshrouded combustion chamber compared to their displacement.

la al al la la trying to get my message to stick it says its too short.

The Pontiac V-8 engine from 1955-1979 all use the same basic architecture. There is no small block or big block Pontiac. The 287 to the 455 all have the same external dimension and surprisingly all use the same dimension connecting rodand cylinder bore spacing. In those same years Cadillac had three major revisions to their engines, Buick had two, Olds had two and came away with a tall and short deck version, and Chevrolet besides the small block had a "W" big block and a Mark 4 Big Block. So how could the Pontiac engine be inferior??? Hot rods get Chevrolet small blocks because they fit into hot rods better and parts are cheaper. BTW the valve train Chevy uses in the ( independent ball rocker arm was a Pontiac invention in 1948 which Ed Cole of Chevrolet decided to beg the corporation to force Pontiac to let him use for his engine) Chevy small and big block use Pontiac's valve train. Is a SBC better than a Pontiac? just check the line bore and the cylinder ridge of the two after 100K and come back to me with your own answer.

To build a engine from 287 to 455 and all the engine sizes that Pontiac had in-between, which engine of the five divisions do you think was the best thought out that didn't have major revisions or new generations of engines added???? Only Pontiac.