Potentially Stupid Question - Overhead Cam Engines

Potentially Stupid Question - Overhead Cam Engines

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Discussion

budfox

Original Poster:

1,510 posts

128 months

Saturday 25th July 2015
quotequote all
This could be an embarrassment but I'm going to ask anyway.

Why don't all engines use overhead camshafts? What is gained by the complexity of a camshaft/pushrod/rocker system? Surely OHC is simpler, cheaper, more efficient, more reliable etc...

Might have been asked a thousand times before, but there you go.

iva cosworth

44,044 posts

162 months

Saturday 25th July 2015
quotequote all
Sidevalve and overhead valve engines are just old tech.

The OHC became mainstream from the 1970s onwards.

calibrax

4,788 posts

210 months

Saturday 25th July 2015
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Most engines are OHC based nowadays. Can't think of a current car that has a pushrod engine... there probably are some, but they will be niche.

eldar

21,614 posts

195 months

Saturday 25th July 2015
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Now days, really a no brainer, except for cheap or special duty engines. (lawnmowers or bulldozers)

Previously, OHC was fairly expensive engineering and those costs didn't bring enough benefits in performance/economy terms given the standards of ignition and carburettion.

The advent of the microchip meant the various technologies all worked together better.

tumble dryer

1,999 posts

126 months

Saturday 25th July 2015
quotequote all
iva cosworth said:
Sidevalve and overhead valve engines are just old tech.

The OHC became mainstream from the 1970s onwards.
As above.

Nobody actually thought about another way around the problem, cos it wasn't one at the time.


Which makes me think, what else has seen such an engineering step-change of thought? wankels... Others??

buggalugs

9,243 posts

236 months

Saturday 25th July 2015
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Engine is much more compact especially the heads. No long timing belt or chain. Less oil required to the heads. Cheaper overall. Probably missed a few.

Ozzie Osmond

21,189 posts

245 months

Saturday 25th July 2015
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budfox said:
Surely OHC is simpler.
No, it's a lot more complex. Trickier to build and lots more to go wrong. But that has become less important as reliability of everything has improved over the decades.

Some American engines use cam in block and pushrods. Not because they are old-tech but because it has advantages. Most particularly it makes a very light and compact engine which leads to low CofG and low bonnet line for sportscars.

budfox

Original Poster:

1,510 posts

128 months

Saturday 25th July 2015
quotequote all
Thanks all, glad it wasn't a stupid question.

PanzerCommander

5,026 posts

217 months

Saturday 25th July 2015
quotequote all
Ozzie Osmond said:
No, it's a lot more complex. Trickier to build and lots more to go wrong. But that has become less important as reliability of everything has improved over the decades.

Some American engines use cam in block and pushrods. Not because they are old-tech but because it has advantages. Most particularly it makes a very light and compact engine which leads to low CofG and low bonnet line for sportscars.
This.

budfox said:
Thanks all, glad it wasn't a stupid question.
Definitely not a stupid question.

My 2006 Mustang GT has a 4.6L SOHC engine (3 valves per cylinder), a friend has a 2002 Camaro with a 5.7L LS1 pushrod engine (2 valves per cylinder). The 5.7 is noticeably smaller than the 4.6. Though many of the newer engines are going OHV now.

Quite a lot of racing applications (especially drag racing) use pushrod engines because they are much quicker to strip down and re-build; for instance you can pull the heads off without having to worry about re-setting the cam timing (unless you have to remove the crank/camshaft), if it were an OHV engine with chains/belts it would take much longer.

feef

5,206 posts

182 months

Saturday 25th July 2015
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Pushrod engines tend not to rev as high as an OHC engine, however, so are more suited to 'lazy' torquey engines

AIUI, NASCAR still requires all engines to be pushrod

apotts

254 posts

206 months

Sunday 26th July 2015
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3 litre IL4 sidevalve engine. Gasses have to go in and out the same side. Very simple engine, cheap to make, reliable and very compact.



1.6 litre IL4 OHV engine. Gasses now flow across the chamber, but poor valve position and angle, rpm limited by pushrods. Engine taller and more complex.



3 litre IL4 OHC engine. Lot of gubbins now in the head, but valve angles and position much better (DOHC), higher rpm. Complex, and not compact.


apotts

254 posts

206 months

Sunday 26th July 2015
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So to answer, OHC is more expensive than OHV. Cheap cars need cheap engines. H Ford had it right - he would go to great lengths to reduce the component count by just 1.

HustleRussell

24,602 posts

159 months

Sunday 26th July 2015
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Yeah but as IVA Cosworth said, OHV has all but died out in Western Europe at least. OHC is quieter and more efficient. Ford were probably among the last to ditch OHV, using them in the KA until about 10 years ago (?)

TheAngryDog

12,394 posts

208 months

Sunday 26th July 2015
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The ls7 engine is push Rod as well. 1 cam is cheaper than 4 that's for sure

Mr2Mike

20,143 posts

254 months

Sunday 26th July 2015
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HustleRussell said:
Yeah but as IVA Cosworth said, OHV has all but died out in Western Europe at least. OHC is quieter and more efficient
OHC is not inherently more efficient, but it does permit the design of more efficient combustion chamber shapes that would be difficult for an OHV configuration.

kambites

67,462 posts

220 months

Sunday 26th July 2015
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apotts said:
3 litre IL4 sidevalve engine. Gasses have to go in and out the same side. Very simple engine, cheap to make, reliable and very compact.



1.6 litre IL4 OHV engine. Gasses now flow across the chamber, but poor valve position and angle, rpm limited by pushrods. Engine taller and more complex.



3 litre IL4 OHC engine. Lot of gubbins now in the head, but valve angles and position much better (DOHC), higher rpm. Complex, and not compact.

Good summery but perhaps worth pointing out that for a long time OHV engines were generally not cross-flow.

Mr2Mike

20,143 posts

254 months

Sunday 26th July 2015
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kambites said:
Good summery but perhaps worth pointing out that for a long time OHV engines were generally not cross-flow.
yes

Sidevalve, OHV and OHC engines have all been made in crossflow and non-crossflow designs. One does not imply the other even if certain combinations are more popular.


M4cruiser

3,550 posts

149 months

Sunday 26th July 2015
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Maintenance is another reason.

Adjusting the valve clearances was dead easy and cost nothing on most OHV, I did the Vauxhall Viva's rocker nuts many times, even re-used the same rocker cover gasket, redface

Whereas altering the shims on an early OHC took ages, i.e. the camshaft had to come off.

I'm guessing the improvements in design, material quality and oil quality have reduced the need for valve clearance adjustment, most OHC will go without adjustment until the head gasket or cambelt break. furious




Mr2Mike

20,143 posts

254 months

Sunday 26th July 2015
quotequote all
M4cruiser said:
Maintenance is another reason.

Adjusting the valve clearances was dead easy and cost nothing on most OHV, I did the Vauxhall Viva's rocker nuts many times, even re-used the same rocker cover gasket, redface

Whereas altering the shims on an early OHC took ages, i.e. the camshaft had to come off.

I'm guessing the improvements in design, material quality and oil quality have reduced the need for valve clearance adjustment, most OHC will go without adjustment until the head gasket or cambelt break. furious
Again, the method of valve clearance adjustment is a separate design decision. Plenty of OHC engines use rockers with screw and locknut adjustment, including very high revving designs such as the infamous Honda VTEC engines.

anonymous-user

53 months

Sunday 26th July 2015
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Mr2Mike said:
OHC is not inherently more efficient
It kinda is though. Because the reciprocating part of the drive is more compact it is fundamentally stiffer, and hence you can reach significantly higher valve accelerations than with a push rod system. (OHV has a short rotary path, and a long reciprocating one, the opposite is true for PHC). As such, you can successfully use valve event profiles that increase either efficiency or power, or both, profiles that would not be realisable for an OHV push rod architecture.


TBH, in Europe, the biggest driver to OHC was the requirement to individually phase intake and exhaust events, something that although possible with 'single cam' OHV, is significantly more complex.