TheSamba.com :: View topic

Sun Aug 12, 2018 8:16 pm

I wasn't sure where to post this since its not vw specific, so figured I'd put it in here. So, I work with a Japanese fellow and we're both car guys and tend to talk cars on breaks and such. He looks down on domestic automakers for staying with OHV engines while the Japanese automakers have embraced DOHC engines, claiming they're more high technology.
my only defense is that OHV engines are more compact, but I know there is more to it than that, but don't know how to counter his argument. So, why do domestic manufacturers stick with OHV? And make massive power and good mpg.
The only DOHC engines I've driven were 4 cylinders and slugs at low rpm, but wake up at higher rpm, though annoying to drive in stop and go traffic.

Sun Aug 12, 2018 8:34 pm

Tell the little bastard that a DOHC engine is still a OHV engine, the only difference is that the C (camshaft) is also in the cylinder head and that (D) means that there are two of them. The flathead engine design was the one that was not considered OHV, the valves were in the block & the cylinder head was effectively there just to cap off the combustion chamber.

Sun Aug 12, 2018 8:40 pm

TDCTDI wrote: Tell the little bastard that a DOHC engine is still a OHV engine, the only difference is that the C (camshaft) is also in the cylinder head and that (D) means that there are two of them.
Ha, yeah ur right, maybe i should clarify...i mean pushrod engines. I just need some good arguments to let him know us Americans ain’t stupid

Sun Aug 12, 2018 9:26 pm

Basically, it boils down to where you need to get your power from. Japan & most European countries taxed & limited their vehicles on engine displacement. This meant smaller, lighter cars that relied on horsepower whereas the American cars were larger, heavier & relied more on torque.

Mon Aug 13, 2018 8:58 am

Onceler wrote: TDCTDI wrote: Tell the little bastard that a DOHC engine is still a OHV engine, the only difference is that the C (camshaft) is also in the cylinder head and that (D) means that there are two of them.
Ha, yeah ur right, maybe i should clarify...i mean pushrod engines. I just need some good arguments to let him know us Americans ain’t stupid

Each have advantages and disadvantages. But, he's still right ... Americans are stupid. :wink:

Mon Aug 13, 2018 10:32 am

Q-Dog wrote: Onceler wrote: TDCTDI wrote: Tell the little bastard that a DOHC engine is still a OHV engine, the only difference is that the C (camshaft) is also in the cylinder head and that (D) means that there are two of them.
Ha, yeah ur right, maybe i should clarify...i mean pushrod engines. I just need some good arguments to let him know us Americans ain’t stupid

Each have advantages and disadvantages. But, he's still right ... Americans are stupid. :wink:
Now don't go gettin' all political on us. :lol:

Mon Aug 13, 2018 10:45 am

well the missing low end probably has absoultly nothing to do with where the cam is located....just tell him it is what it is.and thats what it is.and yes americans also make plenty of OHC engines. and also DOHC engines.

Mon Aug 13, 2018 11:54 am

Turns out the premise of this post wasn't even correct. Let me explain. He drives a 2017 Dodge Charger AWD with a 3.6liter V6. He claimed it was a pushrod engine and I just took his word for it, but did see that it made almost 300hp so I was pretty impressed.
Well, after googling that engine, I find that it is actually a DOHC engine :roll:

So, I think it's safe to say that both of us are stupid

Mon Aug 13, 2018 3:40 pm

Can you imagine a Harley Davidson with with overhead cams? Fuggin ugly!!

Oh, waaaait . . .

Mon Aug 13, 2018 4:36 pm

OHV and SOHC cams use two valves per cylinder.
DOHC use two cams and use four valves per cylinder, also DOHC engines control their cam timing events depending on the RPM and load on the engine. This is what we call variable valve timing. By controlling intake cams and exhaust cams by retarding and advancing SEPARATLY we can get more torque out of the engine at lower rpm and by changing the events can get more HP at higher RPM. We also can tune the adjustable cams for better mileage. And four valves get more in and more out by volume and faster.

In the 60's some of the cam companies like Isky made mechanical advanced cams for V-8 engines, primarily for the SBC. This gave some low end torque and at high rpm it would advance the cam, problem was it advanced and retarded both the intake and exhaust events at the same time. DOHC is better because #1. 4 valves instead of two, and 2. you can control intake and exhaust valves separately for optimum HP and Torque.

Mon Aug 13, 2018 5:05 pm

Even though the premise of this thread turned out to be bs, why,for example, does GM stay with pushrods 2 valve technology, instead of DOHC 4 valve technology with their V8's? And doing quite well with it as far as power and mileage

Mon Aug 13, 2018 7:32 pm

Helfen wrote: OHV and SOHC cams use two valves per cylinder.
DOHC use two cams and use four valves per cylinder, also DOHC engines control their cam timing events depending on the RPM and load on the engine. This is what we call variable valve timing. By controlling intake cams and exhaust cams by retarding and advancing SEPARATLY we can get more torque out of the engine at lower rpm and by changing the events can get more HP at higher RPM. We also can tune the adjustable cams for better mileage. And four valves get more in and more out by volume and faster.

In the 60's some of the cam companies like Isky made mechanical advanced cams for V-8 engines, primarily for the SBC. This gave some low end torque and at high rpm it would advance the cam, problem was it advanced and retarded both the intake and exhaust events at the same time. DOHC is better because #1. 4 valves instead of two, and 2. you can control intake and exhaust valves separately for optimum HP and Torque.

And who came up with the variocam concept? Hmmm...

Tue Aug 14, 2018 7:35 am

Onceler wrote: Even though the premise of this thread turned out to be bs, why,for example, does GM stay with pushrods 2 valve technology, instead of DOHC 4 valve technology with their V8's? And doing quite well with it as far as power and mileage

Pushrod engines tend to be more compact.

Tue Aug 14, 2018 7:59 am

Helfen wrote: OHV and SOHC cams use two valves per cylinder.
DOHC use two cams and use four valves per cylinder, also DOHC engines control their cam timing events depending on the RPM and load on the engine.
There are always exceptions to the rules. :lol: VW had the "DOHC" VR6 that was 2 valves per cylinder but they got away with labeling it DOHC because there were two cams in the same cylinder head, One for the front bank & one for the rear bank.

VW also had the 20 valve engines which had 3 intake & 2 exhaust.

hotsam wrote:
Pushrod engines tend to be more compact.
Which brings us back to the VR6 which was a 15* V6 "DOHC" that took up the same space as a 4 cylinder.

Tue Aug 14, 2018 8:15 am

Onceler wrote: Even though the premise of this thread turned out to be bs, why,for example, does GM stay with pushrods 2 valve technology, instead of DOHC 4 valve technology with their V8's? And doing quite well with it as far as power and mileage

Cheaper, less complicated, easier to produce. Tradeoff, less efficiency=HP & Torque.

Tue Aug 14, 2018 8:29 am

hotsam wrote: Onceler wrote: Even though the premise of this thread turned out to be bs, why,for example, does GM stay with pushrods 2 valve technology, instead of DOHC 4 valve technology with their V8's? And doing quite well with it as far as power and mileage

Pushrod engines tend to be more compact.

GM built other engines like Pontiac's SOHC 6 from 1966-1969 or 1971-1977 Chevrolet 2300 SOHC 4.

When in January 1963 GM's self imposed ban on racing Pontiac had three OHC engines getting ready for production. The only engine that finally came out of advanced engineering was the 1966-69 OHC six.

If the ban hadn't gone into effect Chrysler's Hemi would have been fodder for the 421 DOHC 4 valve per cylinder Pontiac was going to produce for production. Even nearer to production was the 421 SOHC ( which used some of the same tooling and parts from the OHC Pontiac six) producing 650 HP on a four barrel carb. and this was 1963.

Tue Aug 14, 2018 8:48 am

Honestly I don't think one is really any better than the other. Just two different ways to get to the same result.

A lot probably has to do with the car design more than engine itself. Smaller imports couldn't cram a big American style engine and fit and would throw the weight bias off something terrible.

With todays cars all being computer controlled you can get the fuel efficiency pretty high on a V8 push rod engine through fuel management and you can get better low end torque on an import style engine through variable valve timing.

Either way smaller engines are turning much bigger HP numbers than they used to and its all through computer control and the evolution of fuel metering.

I think both of you are right in thinking that one engine is better than the other but only 50% of the time. Depends on your application, cost, weight, high end HP or low end torque, rotating mass, and other factors.

My prediction is that GM will have a DOHC engine in the mid-engine Corvette sooner than later. Cost will not really be an issue and using every bit of new technology to gain every advantage in the competitive super car market is paramount to the survival of the brand.

Maybe an automotive engineer can chime in and give us the pros and cons and why one design isn't universally used.

Tue Aug 14, 2018 9:31 am

I'm not an automotive engineer, but the reasons are something along the lines of:

DOHC:

Pro: More power from a given displacement
Con: Greater Cost/Larger Size/More Complex

Pushrod:

Pro: Less expensive/more power from smaller exterior dimensions/Less Complex
Con: Less power from a given displacement

Tue Aug 14, 2018 11:15 am

pondoras box wrote: Honestly I don't think one is really any better than the other. Just two different ways to get to the same result.

A lot probably has to do with the car design more than engine itself. Smaller imports couldn't cram a big American style engine and fit and would throw the weight bias off something terrible.

With todays cars all being computer controlled you can get the fuel efficiency pretty high on a V8 push rod engine through fuel management and you can get better low end torque on an import style engine through variable valve timing.

Either way smaller engines are turning much bigger HP numbers than they used to and its all through computer control and the evolution of fuel metering.

I think both of you are right in thinking that one engine is better than the other but only 50% of the time. Depends on your application, cost, weight, high end HP or low end torque, rotating mass, and other factors.

My prediction is that GM will have a DOHC engine in the mid-engine Corvette sooner than later. Cost will not really be an issue and using every bit of new technology to gain every advantage in the competitive super car market is paramount to the survival of the brand.

Maybe an automotive engineer can chime in and give us the pros and cons and why one design isn't universally used.

From a engineering point of view four valves per cylinder are more efficient. The more fuel mixture you can get in and exhaust out is just plain physics. Adjusting valve timing and separately ( VVT ) between intake cam and exhaust cam increase horsepower, torque, efficiency and lower emissions.

Engines with a single camshaft controlling intake and exhaust events are designed as a compromise between HP, Torque, emissions, efficiency.

Tue Aug 14, 2018 8:59 pm

Helfen wrote: pondoras box wrote: Honestly I don't think one is really any better than the other. Just two different ways to get to the same result.

A lot probably has to do with the car design more than engine itself. Smaller imports couldn't cram a big American style engine and fit and would throw the weight bias off something terrible.

With todays cars all being computer controlled you can get the fuel efficiency pretty high on a V8 push rod engine through fuel management and you can get better low end torque on an import style engine through variable valve timing.

Either way smaller engines are turning much bigger HP numbers than they used to and its all through computer control and the evolution of fuel metering.

I think both of you are right in thinking that one engine is better than the other but only 50% of the time. Depends on your application, cost, weight, high end HP or low end torque, rotating mass, and other factors.

My prediction is that GM will have a DOHC engine in the mid-engine Corvette sooner than later. Cost will not really be an issue and using every bit of new technology to gain every advantage in the competitive super car market is paramount to the survival of the brand.

Maybe an automotive engineer can chime in and give us the pros and cons and why one design isn't universally used.

From a engineering point of view four valves per cylinder are more efficient. The more fuel mixture you can get in and exhaust out is just plain physics. Adjusting valve timing and separately ( VVT ) between intake cam and exhaust cam increase horsepower, torque, efficiency and lower emissions.

Engines with a single camshaft controlling intake and exhaust events are designed as a compromise between HP, Torque, emissions, efficiency.
I think the long view win goes to America if you look at hp, torque, and mpg, with the least amount of cost and complexity. they had a good run when there wasn't any competition and the yen was cheap. Competition in the first world ain't so easy