Pushrods vs DOHC

[Norm Peterson]

That is correct. But one more important thing is the ratio of bore/stroke.
An engine with a long stroke should have more torque than an engine with equal displacement but shorter stroke (and wider bore) because of the longer distance of the crankpin to the crankshaft centerline (leverage effect).

Be careful.

At equal displacement with (say) a half length stroke, the piston area will have to be double that of the long stroke engine's piston area. Theoretically, two times the area (double the force) multiplied by half the lever arm length sounds like a wash to me. It's other consequences that cause differences in specific torque (torque per unit volume). The finite flame speed of combustion comes to mind.


Norm

Sponsored

 

[BmacIL]

That is correct. But one more important thing is the ratio of bore/stroke.
An engine with a long stroke should have more torque than an engine with equal displacement but shorter stroke (and wider bore) because of the longer distance of the crankpin to the crankshaft centerline (leverage effect).
This is one reason why diesel engines usually have relatively high torque compared to gasoline engines. (Diesel engines are usually designed with a rel. long stroke.)

Actually one of the biggest reasons diesel engines make that much torque is the compression ratio being much higher than gasoline, spark-ignition engines as well as the nature of the combustion. The long stroke is effectively dictated because due to the high stresses, all the rotating components have to be quite strong and heavy, which means the engine speed can't be very high. Since engine speed can't be high, the way you get your desired displacement/compression ratio at the required power is through longer stroke.

 

[Shanghai Dan]

It is because it is impractical to have more than two valves per cylinder in a pushrod engine. All of the most efficient production engines use 4 or even 5 valves per cylinder. It's very difficult to do that without overhead cams. Two cams makes it easier to have independent valve timing. It's not just about make by power, its power combined with drive ability, emissions, and economy.
It can be done with pushrods, but you're at a disadvantage from the start.

Lots of diesels use 4 valves and pushrods. Most of the Ford, Chevy and Dodge diesels are that way. There is nothing stopping the combination and it's used for some of the biggest torque/HP motors out there (the 6+ liter diesel monsters).

 

[Conedodger]

One advantage that everyone is overlooking is that you can use variable valve timing on a DOHC engine. This one advantage is what allows the coyote to produce such a large flat torque curve as well as the potential for a very large power band.

 

[RevvdMedia]

One advantage that everyone is overlooking is that you can use variable valve timing on a DOHC engine. This one advantage is what allows the coyote to produce such a large flat torque curve as well as the potential for a very large power band.

Pulled from Wikipedia, although I have seen this in numerous other publications:

The new Corvette LT1 engine, the first of the Gen 5 family of Small Block engines, retains the push-rods acting on overhead valves design. It implements direct fuel injection, Active Fuel Management (cylinder deactivation), and continuously variable valve timing.

 

[cosmo]

Pulled from Wikipedia, although I have seen this in numerous other publications:

The new Corvette LT1 engine, the first of the Gen 5 family of Small Block engines, retains the push-rods acting on overhead valves design. It implements direct fuel injection, Active Fuel Management (cylinder deactivation), and continuously variable valve timing.

Coyote is Ti-VCT, allowing the intake and exhaust cams to operate independant of eachother. So you can open the intake valves late, and shut the exhaust valves early, and get a lot more EGR to reduce the combustion volume and get better gas mileage. You can also do the opposite, and increase power, more effectively than the LT1. The LT1's system is more similar to the 3V 4.6L.

 

[5.0GT]

The only knock from me about the coyote is the torque. I wish it had more. The LS/LT Gm motors have more torque and that's something I miss.

 

[Hack]

There are millions of details to compare and contrast, but a logical exercise can help to shortcut them. Look at the vehicles whose manufacturers spare no expense to provide the vehicles' owners with the best possible performance. Do they run pushrods or OHC technology? Do you think that those car companies have sufficient technical expertise to select the better of the two for that purpose?

 

[Trackaholic]

There are millions of details to compare and contrast, but a logical exercise can help to shortcut them. Look at the vehicles whose manufacturers spare no expense to provide the vehicles' owners with the best possible performance. Do they run pushrods or OHC technology? Do you think that those car companies have sufficient technical expertise to select the better of the two for that purpose?

This is tricky though, because many of those companies are from Europe where displacement taxes used to be common. Also, many like to leverage their racing heritage, where displacement is limited. Finally, they typically are more expensive, which allows greater freedom in parts cost. Those limits incentivise smaller displacement engines, which necessarily need to rev higher to make the same power as a larger displacement engine, forcing the use of a DOHC design.

Point being, there are many factors that impact engine choice, and if you compare an SBC to many world class engines from the likes of BMW and Porsche, the GM offering is pretty great, at least in the USA. Maybe won't be as culturally acceptable in Europe due to its large displacement, which has a stigma of being inefficient (even if the Vette gets pretty great fuel economy).

With that said, I do think the flexibility of independent camshaft timing for intake and exhaust, coupled with ever increasing emissions standards, will eventually force even GM to go with a DOHC motor (although maybe the V8 will be phased out before then, and they'll be left with TTV6's).

-T

 

[Strokerswild]

I'm amazed at how far GM has taken the pushrod architecture, but sooner or later they'll hit the wall with it in terms of balancing HP goals and emissions.

 

[mikeyjobu]

Be careful.

At equal displacement with (say) a half length stroke, the piston area will have to be double that of the long stroke engine's piston area. Theoretically, two times the area (double the force) multiplied by half the lever arm length sounds like a wash to me. It's other consequences that cause differences in specific torque (torque per unit volume). The finite flame speed of combustion comes to mind.


Norm

It may produce the same amount of torque -- but the power bands are going to look vastly different -- the stroker is going to make more down low, and the bored-out motor is going to make it's torque later in the rev range...

 

[Herr_Poopschitz]

It may produce the same amount of torque -- but the power bands are going to look vastly different -- the stroker is going to make more down low, and the bored-out motor is going to make it's torque later in the rev range...

This cannot be concluded based on the variables of bore and stroke alone.

 

[mikeyjobu]

This cannot be concluded based on the variables of bore and stroke alone.

Yes... but generally speaking, in like motors -- say same accessories and block and displacement from an engine builder, for example...

 

[arghx7]

Using current technology there are way more tricks available for head and piston design with DOHC than with a 2 valve pushrod counterflow wedge combustion chamber like GM uses.

You can use variable valve lift, either continuously variable (BMW Valvetronic) or 2 step (Honda VTEC, current GM 2.5L 4 cylinder).

You can use much milder cams to get the same amount of airflow. There's way more duration on a 2 valve pushrod engine than a DOHC, to make up for having less valve area.

Current pushrod engines (viper being the exception) don't have independent control of cam phasing. They are like the old 3 valve modular engines, where both intake and exhaust valves are retarded together "dual equal phasing."

For direct injection, with a pushrod you have pretty narrow options. It took GM a very long time to figure out how to design the heads and piston bowl to make it work - you need a high swirl combustion chamber, which hurts head flow even more. There aren't many places you can put the injector and spark plug. On a DOHC, you can have the injector in the center of the chamber or on the side, with variations of both.

All that being said, pushrod 2 valve counterflow heads are way more compact than DOHC and the single cam makes a much simpler chain drive. Hemi 2 valve heads are somewhere in the middle in terms of head size, as they are crossflow and have unequal length pushrods.

Lots of diesels use 4 valves and pushrods. Most of the Ford, Chevy and Dodge diesels are that way. There is nothing stopping the combination and it's used for some of the biggest torque/HP motors out there (the 6+ liter diesel monsters).

Yes. They don't rev very high though, even for diesels. So you don't have the same friction penalty. The combustion chamber geometry on a diesel is different from a gasoline engine. The focus is on a high swirl motion in the cylinder, which needs a helical intake port and piston bowl. The main concern with diesels are NOx and particulate emissions.

 

[Anthony@HTM]

As mentioned before the beauty of the LS motor is the compact size and all of the weight being down low. There's a reason why every RX7 and S13/14 puts a LS motor into their cars over a Coyote motor.

There's also cost, crate LS motors are cheap!

Sponsored