Quote:
Originally Posted by Muscle Master
I change the title
This is great I love you guys, I learning alot
..... keep it coming, more info
I wanna hear from you spike!
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lol I looked into this thread earlier... a lot of good info out there already.
one thing to keep in mind is rotating mass. the reason we can make gobs of torque with our "prehistoric" pushrod motors is that we have larger and more cyls/pistons which allow us to make more power down lower and per rpm. its not a necessity for a pushrod v-8 to have to spin to the moon just to get any power. now if you've got a dyno queen car, then spin to whatever the hell you want to to get your peak numbers. I like power under the curve. I like useable power. I LOVE TORQUE.
as ive said before, its nice to rev, its nicer to move.
that being said, there are v-10s, 12s, 14s, w-16s, etc out there making gobs of power (n/a or other wise) but take a v12 lamborghini making 500 hp.... compare that to a v-8 corvette making.... 500 hp. but look at the power bands and where their power is being made. v-12 is spinning to 7k before it starts making max power. v-8 is a little more than half that.
now, there's also v-8 OHC motors as well out there. like the ford 4.6 for example. as said before, with more cams, you've got more options for valve timing, but you also have more shit to break. I like the fact that the weakest part of my valvetrain is my pushrods. meaning that if something is going to break, a pushrod is going to be one of the first things to go. and thats fine, id rather break a pushrod, than snap one of my 30 foot timing chains (they arent really 30 ft long, just exaggerating my point) then ive got a valve sticking down into the chamber when the piston rises..... and boom. there goes my motor. pushrod bends/breaks..... car runs like ass till I get it shut down, then im out 40 buck to replace them.
one big bonus for pushrod motors (all things being equal, v-8 to v-8) is that it is more compact. If OHC were as compact as pushrod designs the pushrods would have been thrown out long ago. For in-line engines it's nice, cus you have plenty of room on either side and if it gets too tall, lay the engine over like the slant 6 from chrysler years ago. For most inline engines (either four-bangers or straight-sixes), OHC makes perfect sense. Smaller engines that make use of extremely high revs are also obvious applications. But if we look at passenger-car applications where the displacement is over 3 liters or so and revs rarely top 6K, then there is absolutely no reason to believe that a properly-designed pushrod engine cannot deliver power, refinement, and compactness comparable to an OHC engine. OHC engines allow higher rev operation, and thus are better for peak power output. They're found in many different exotic car and racing applications (Ferrari, IRL, etc) where high rpms are desirable. Using two cams instead of one (DOHC vs SOHC) can give you a bit more flexibility when designing lift profiles, as you'll have less contact stress, and you may be able to achieve higher speeds. DOHC is a logical choice if you have a variable cam phasing mechanism, as you can put your intake and exhaust lobes on separate shafts.
Both OHC and pushrod valvetrains can use timing chains or timing belts, but timing chains are more common on pushrod engines, and timing belts are more common on OHC engines. The advantage of a chain is that it lasts longer; belts are quieter, easier to package, and offer better high-rev performance if the drive load requirements aren't too high. OHC and pushrod configurations can both use hydraulic lash adjustment, which almost eliminates the need for periodic manual lash adjustments. Valvetrain friction can go either way. For reduced valvetrain friction, it is preferable to use a rolling follower, and it is preferable to use manual lash adjustment. Either of those can be done on a pushrod engine, but they seem to be more common on OHC engines. An OHC valvetrain will generally have less mass to control (hence the potential for higher operating speed), which translates to lower spring forces for a given operating speed, and lower friction.
they are both good for different things. however the old pushrod v-8 has beaten nearly every other type of motor in nearly every type of race out there. pushrod v-8s make (on average) 75% or more of their power from idle to redline where as most OHC engines dont make hardly any power till they are spinning high. for small econoboxes, for decent mpg (which also matters on weight of the vehicle), OHC is great. for power to move the whole family sedan loaded down with camping gear, for decent mpg at the same time you are towing a loaded down trailer, for MOVING when you hit the gas.... pushrod is FTW.