Pushrod vs. SOHC vs. DOHC

[ivymike1031]

Originally Posted by 454Casull

IM - that link says 60-deg V6 engines don't need balance shafts. True?

well, it's true that it says that.

In a typical 60deg V6, the crankpins for each pair of cylinders are offset by 60deg, with a "flying" web in between, such that the conrods appear to cross each other when viewed from the end. There is a complete balance of primary (shaking) forces, but counterweights are needed to balance the forward rotating moment. There is a leftover "reverse" rotating moment, which is typically not counteracted with balancer shafts.

For comparison:
In a typical 90deg V6, the crankpins for each pair are offset by 30deg instead of 60deg, again with flying webs. Shaking forces are completely balanced, and counterweights take care of a forward rotating moment. There are still unbalanced primary and secondary moments, but it is not usual to use balancer shafts.

[Steel]

60 degree V6? The hell tehy don't! the only types of engines *I* know that dont need balancing shafts are Boxter type engines of any cylinder number, I6's and V12's

*edit* well, i was thinking of any type of balancing system at all

[ivymike1031]

Originally Posted by Steel

60 degree V6? The hell tehy don't! the only types of engines *I* know that dont need balancing shafts are Boxter type engines of any cylinder number, I6's and V12's

*edit* well, i was thinking of any type of balancing system at all

well there certainly are lots of V6 engines that DO use balance shafts...

[beef_bourito]

Well, i really started this thread to see how the different setups would effect performance. I understand that xOHC give higher revs, but what are the differences of the two in terms of performance. I have, however, not seen one shared oppinion on the pros of the Pushrod engine. and i'm asking this in terms of racing engines, not necessarily for everyday driving.

Moreover, what was that hissyfit quaddriver threw all about lol.

And one more thing, would you want a muscle engine (V8, 500+ Hp) to rev high?

[Evil Result]

when i think of a "muscle car" i think big V-8's with big torque at lower rev's.

higher reving engines create more friction over time than lower reving engines, but that depends on design. Higher reving engines can also be geared down to produce the same ammount of torque at the wheels as a "muscle car" V-8.

A question i have although off topic is.... fuel effiency, what would have better fuel effiency based on equal horsepower outputs say 500hp, using the same mechanical design. This means same fuel delivery, ignition, cam system, induction, exhaust, ect, ect. Just a difference between revs and displacement, which would be more efficent, higher displacement lower reving engine, or a lower displacement higher reving engine?

[calgary_redneck]

haha dutifully answers questions? ya with his charming sarcasm and arrogances

[ivymike1031]

Originally Posted by Evil Result

A question i have although off topic is.... fuel effiency, what would have better fuel effiency based on equal horsepower outputs say 500hp, using the same mechanical design. This means same fuel delivery, ignition, cam system, induction, exhaust, ect, ect. Just a difference between revs and displacement, which would be more efficent, higher displacement lower reving engine, or a lower displacement higher reving engine?

I gather that what you're saying is that you want to know, for a given BMEP and power output, whether a low-rev engine or high-rev engine of the same general layout would be more efficient. I'd have to say that the low-rev engine would be, primarily due to the reduced friction losses (and probably reduced pumping losses too, but I'm a bit out on a limb here - no time to check).

[texan]

Originally Posted by calgary_redneck

haha dutifully answers questions? ya with his charming sarcasm and arrogances

Well gee.. let's break this down. You = 1 month board member. Me = 3 year board member. Now who is more qualified to comment on past behavior again?

This is what happens to all techs on all boards, given a long enough timeline. They get sick and tired of newbs walking up and performing the ceremonial pissing match routine, and eventually (after many rounds of this banter) begin to try and shut things down with a vengance when this type of thing occurs. Then said newb calls them out as being overconfident, less knowledgeable than they are, overbearing in their demeanor and generally just a complete know-it-all asshole. Which of course all the other newbs rally around, and all the people in the know (that being 3 year vets like myself) roll their eyes and speak up for a second... even though they are aware this board member doesn't really need any help.

And eventually, through actions of people like you, these techs stop posting constantly. At first they get lots of PM's asking about things, but eventually their presence is forgotten and they fall by the wayside. And it's all because of newbs pissing on the parade, asking questions they've answered 35 times in searchable terms, and generally making a nuissance of themselves. I speak from long experience here BTW, having shunned three different "once were mine" forums over the last 6 years. A word of advice... earn your position by showing your chops in helping others, not chopping the established folk down.

[SaabJohan]

Originally Posted by Evil Result

when i think of a "muscle car" i think big V-8's with big torque at lower rev's.

higher reving engines create more friction over time than lower reving engines, but that depends on design. Higher reving engines can also be geared down to produce the same ammount of torque at the wheels as a "muscle car" V-8.

A question i have although off topic is.... fuel effiency, what would have better fuel effiency based on equal horsepower outputs say 500hp, using the same mechanical design. This means same fuel delivery, ignition, cam system, induction, exhaust, ect, ect. Just a difference between revs and displacement, which would be more efficent, higher displacement lower reving engine, or a lower displacement higher reving engine?

Friction loss can in general be estimated from mean piston velocity, longer stroke and higher rpm = higher friction loss.

Part throttle efficiency tend to be higher for small displacement engines due to smaller pumping losses.

[ivymike1031]

Originally Posted by SaabJohan

Part throttle efficiency tend to be higher for small displacement engines due to smaller pumping losses.

Aren't the volumetric flow rates the same, though, based on the assumptions above (displ increased, rpm decreased, power constant)?

[88cryan]

Allright, I am going to add my two cents to this little debate. I personally think that neither engine style is better than the other, it just depends on the type of application it is going into.

Torque= The amount of work that gets done.
Horsepower= The speed at which the work gets done.

OHV PUSHROD

Characteristics:
1. Higher torque
2. Lower horsepower
3. More reciprocating mass, meaning less RPMs
4. Narrower and more compact
5. Cheaper to make
6. Slightly cheaper to maintain


OHC & DOHC

Characteristics:
1. Lower torque
2. Higher horsepower
3. Less reciprocating mass, meaning higher RPMs
4. Little more costly to maintain
5. Typically more efficient
6. More horsepower from less displacement


Q. Why do OHC/DOHC engines have more horsepower than a PUSHROD engine.
A. For several reasons: 1. They breathe better allowing more air/fuel to enter the combustion chamber, and allow the exhaust to leave faster. 2. They have less reciprocating mass allowing them to rev alot higher. 3. Efficiency equals power, variable valve timing, etc all make more power.

Q. Why do PUSHROD enignes have more torque than an OHC/DOHC?
A. The reciprocating mass that hurts their horsepower and their rev potential is the reason that they have great torque. The extra mass in the valvetrain allows the engine to "hit harder".

Q. So what applications usually have a PUSHROD engine?
A. Heavier cars, trucks, etc where the extra torque of the engine will be needed to do work ie: a 9,000 pound work truck wouldn't "shouldn't " have an OHC/DOHC enigne, to achieve the torque needed to tow, etc the engine would have to rev alot higher to do the same work that a pushrod engine could do at lower revs. The result would be a shorter engine life if it was an OHC/DOHC.

Q. What applications usually have OHC/DOHC?
A. Lightweight cars, small trucks, etc where the extra horsepower will be more beneficial than having more torque ie: If a 2,400 pound Honda Civic had a high torque pushrod engine, all that would happen when the driver took off would be a bunch of tire spin, and no go! Whereas their DOHC/OHC engines can rev to quickly to 9k RPMs without much wheelspin.

To: The other comments I've seen,

*Timing belts are supposed to be quieter, however I don't think they are.
*Timing belts should be changed at most every 100,000 miles. (Not only because they can break after that long, but they tend to stretch out causing poor performance.)
*The GM 3800 V6 is one of the best engines ever made. FACT by JD Power
*Timing chains are better (my opinion) even Nissian swtched all their cars to timing chains only! GO NISSIAN
*Torque is what throws you back in your seat.

To: beef_bourito, As far as racing goes I would think that the pushrod engine would be prefered, due to the cost, simplicity, durability, etc.

[88cryan]

Allright, I am going to add my two cents to this little debate. I personally think that neither engine style is better than the other, it just depends on the type of application it is going into.

Torque= The amount of work that gets done.
Horsepower= The speed at which the work gets done.

OHV PUSHROD

Characteristics:
1. Higher torque
2. Lower horsepower
3. More reciprocating mass, meaning less RPMs
4. Narrower and more compact
5. Cheaper to make
6. Slightly cheaper to maintain


OHC & DOHC

Characteristics:
1. Lower torque
2. Higher horsepower
3. Less reciprocating mass, meaning higher RPMs
4. Little more costly to maintain
5. Typically more efficient
6. More horsepower from less displacement


Q. Why do OHC/DOHC engines have more horsepower than a PUSHROD engine.
A. For several reasons: 1. They breathe better allowing more air/fuel to enter the combustion chamber, and allow the exhaust to leave faster. 2. They have less reciprocating mass allowing them to rev alot higher. 3. Efficiency equals power, variable valve timing, etc all make more power.

Q. Why do PUSHROD enignes have more torque than an OHC/DOHC?
A. The reciprocating mass that hurts their horsepower and their rev potential is the reason that they have great torque. The extra mass in the valvetrain allows the engine to "hit harder".

Q. So what applications usually have a PUSHROD engine?
A. Heavier cars, trucks, etc where the extra torque of the engine will be needed to do work ie: a 9,000 pound work truck wouldn't "shouldn't " have an OHC/DOHC enigne, to achieve the torque needed to tow, etc the engine would have to rev alot higher to do the same work that a pushrod engine could do at lower revs. The result would be a shorter engine life if it was an OHC/DOHC.

Q. What applications usually have OHC/DOHC?
A. Lightweight cars, small trucks, etc where the extra horsepower will be more beneficial than having more torque ie: If a 2,400 pound Honda Civic had a high torque pushrod engine, all that would happen when the driver took off would be a bunch of tire spin, and no go! Whereas their DOHC/OHC engines can rev to quickly to 9k RPMs without much wheelspin.

To: The other comments I've seen,

*Timing belts are supposed to be quieter, however I don't think they are.
*Timing belts should be changed at most every 100,000 miles. (Not only because they can break after that long, but they tend to stretch out causing poor performance.)
*The GM 3800 V6 is one of the best engines ever made. FACT by JD Power
*Timing chains are better (my opinion) even Nissian swtched all their cars to timing chains only! GO NISSIAN
*Torque is what throws you back in your seat.

To: beef_bourito, As far as racing goes I would think that the pushrod engine would be prefered, due to the cost, simplicity, durability, etc. The pushrod engine is easier to maintain (and cheaper). No timing belts to change, etc.

[curtis73]

OHV, or cam in block, or pushrod is a tried and true method. Its drawbacks are additional weight in the valvetrain which can limit its RPM potential, but its dead-nuts reliable with a short chain that rarely fails.

OHC or DOHC does simplify the valvetrain and make it lighter, but it relies on a long belt or chain that if it fails can cause a complete engine failure with damage to nearly all internal parts. The timing belt idea keeps rotating weight down, but its not very reliable and needs to be changed regularly. The chain is more reliable, but adds a good bit of friction and weight to the rotating assembly.

In most street applications, its rarely an issue. There are pushrod engines that easily rev to 10,000 rpm, and there are tons of DOHC engines that never go above 4000 rpm.

Its a design application, but in almost all cases, both can be designed to work in whatever application you want except the extremes. If you're designing a 16,000 rpm sport bike engine, you'd use OHC. If you're using a V-configured engine in a reliability application, it might be wiser to use pushrods for simplicity and keeping the car out of the warranty repair bay.

[slideways...]

well, since this thread seems to be continuing, what about future automotive applications using desmodromic valvetrains? i mean if the cost/complexity can be reduced, does anyone here see this happening? think of it, no such thing as valve float...tempting.

[curtis73]

Aw... CRIPES i fell for it.

Replied to an old thread. Dang it, 88Cryan, don't revive old threads.

Closed.