Advantages and disadvantages of valvetrain desing

[bert0mac]

this isnt exactly 3s specific. But i think it would be very interesting to the real motorheads out there.

what are some advantages and disadvantages of an overhead cam design vs a pushrod style motor?

to start off. pushrod motors have the benefit of ratio rockers. so cams dont have to have large lobes to move the valves good distance.

[3startuna]

Overhead cam is the way to go. We can rev higher than pushrods.

OHC we can have more valves per cylinder. more valves = More torque and more power.

I also dont like pushrods. Cam in the engine block thats gay

[3startuna]

Also I think a DOHC engine is MUCH easier to work on than a pushrod motor.

For example, you dont need to take apart the entire valve train to change a head gasket. On a OHC engine, just take off the valve cover, release the tnesion in the timing belt, then lossen the headblots. and boom done.

Also, when it comes to mods. To run aftermarket cams its easy on a OHC motor. A pushrod engine you have to pull the engine, tear it down and basically rebuild it to get to the cam.

Another big advantage is that a OHC motor you can have more valves per cylinder. More valves = higher VE= more torque = more power

Pushrods, arent bad just older tech.

[bert0mac]

actualy. theres a lot of motors out there with 4 valves per cylinder and pushrods. ive seen pushrod chrysler 4valve motors rev past 10k. and they make some insane power.

theres also the issue that since the engine has to work to spin 4 cams as opposed to 1, that more hp is being lost at the crank.

also, with enough lift, more valves arent always a good thing. when lift = .25 times the diameter of the valve, the skirt area will equal the diameter of the valve. so once lift is more than .25d of the valve. life becomes a greater improvement than valve size.

another thing i read in a great article is about swirl pattern of incoming air charge as it enters the cylinder. designing heads with the valves in slightly different locations has been proven to change output numbers.

good input. anyone else?

[3startuna]

a big advantage to pushrods is costs. well not really, you save as your only have 1 cam. But in a high rev motor, you have to spend money on heavy duty rocker arms and stronger pushrods

ever held a rocker arm. that thing is HEAVY

[bert0mac]

lightweight ones arent

seriously though. im looking more into the flow design. not nessicarilly cfm. but swirl patterns, fuel atomization, peak hp vs average hp.
legit differences in design.

the pushrod design cant be that bad. the quickes cars on the planet use pushrods(top fuel dragsters)

[3startuna]

Originally Posted by bert0mac

lightweight ones arent

seriously though. im looking more into the flow design. not nessicarilly cfm. but swirl patterns, fuel atomization, peak hp vs average hp.
legit differences in design.

the pushrod design cant be that bad. the quickes cars on the planet use pushrods(top fuel dragsters)

yeah but consider their displacement. thats no joke. 15:1 on a 440 c.i engine is ALOT! Then you strap a blower on top of that and run ungodly high octane.

but yeah pushrods arent bad, its a different way of doing things. Ferrari ran pushrods in their 2003 F1 car so yeah its not a "bad" setup.

I prefer DOHC cause thats what i grew up on and I find they are easier to work with and simpler. Many people think otherwise.

Also as far as flow is concerned, I dont think pushrod or OHC is as important as the port shape.

I think push rod VS OHC determine how fast the valves are opening, and how high you can rev.

[donniekak]

Pushrod engines are smaller than ohc. By being smaller they can have a higher displacement than a ohc engine of the same weight. An ls1 weighs about the same as a rotary mazda. By being smaller they can be placed for optimum weight distribution. Ohc engines on the other hand can rev high without exotic materials, this allows them to have higher horsepower than a lower reving engine with the same amount of torque. Great for economy and performance. To each their own, no one ever lost a race and said "damn if i would/wouldn't have had ohc i would/wouldn't have won that race".

[blackrussian]

Shim-under-bucket setup is the weapon of choice on high RPM OHC applications. Chain drive valve train. Look into bike engines.

[3startuna]

Originally Posted by magnafsb

Shim-under-bucket setup is the weapon of choice on high RPM OHC applications. Chain drive valve train. Look into bike engines.

yup. its the same setup used in honda's B and K series and in toyota's 4age 20V
also in Nissan's RB, and VG. and In yahmaha's YZF-R6

I think the SR uses the hydralic lifters and rocker arm setup we do. and the 4G uses the same setup as our engines.

IMHO our setup can be made to rev as well I mean you have 4Gs reving to 10K. I think what we need is a good set of lifters. Springs and vales we already have