Advantages and disadvantages of valvetrain desing

[JasonY]

You realise the SOHC Mivec heads blow our heads out of the water flow wise too right?

Jason

[3startuna]

do you want to have a logical conversation or just have an arguement to compare e-dick sizes?

Notice I was comparing 6G72s of similar years.

Technology has improved over the last 12 years that my engine debuted.

But still the principle is still the same. Im sure if mitsu realeased another DOHC 6G with MIVEC it would obliterate the SOHC.

More valves = More VE
More VE = More Power you cant beat it man.

if SOHC is so good and mitsu loves the tech so much why dont evos run it?

Evos are of course the top of mistu's market when it comes to performance?

SOHC is cheaper to make, compared to DOHC, but when it comes to performance you cant beat the twin cams

[3startuna]

also people take the emotion out of your arguements please. Theirs no reason for anyone to get pissed off were having a difference of opinion not a war. No need for anyone to get upset.

[inperfectdarkness]

what jason said ^ ditto. SOHC has evolved to entirely supplant any advantages that DOHC might have had. i knew with the advent of V-tech (read: ANY adjustable valve timing) that it was only a matter of time. ferrari's cams that move linearly to adjust lobes--is probably the best implementation of it yet.

i still think the best valve type is what is sometimes referred to as a "rotating cylinder valve". the combustion chamber has no valves, only a sleeve which moves to vent exhaust & allow intake. i doubt you'll ever see it exercised in an automotive application.

more valves does NOT = better. it's actually like pistons on a brake caliper. more does NOT = better. STRONGER = better. imagine you have an engine with 10 intake valves, and 10 exhaust valves. (i know 3startuna is about to shoot a wad on his keyboard...but let's continue). you have to sacrifice overall size of the valve to fit more in. and you also have to figure out a way to make 1 injector evenly distribute the intake charge amongst 10 valves.

meanwhile, an identical engine with only 1 intake & 1 exhaust valve can make HUGE valves. this makes it much easier to flow into--because you have a single, much larger opening. it's like pouring sand through a colander vs. pouring sand through a pvc pipe. it's also infinately easier to position the cylinder, valve, injector, etc to facilitate the proper flow of the intake charge.

now in the modern I.C. engine, due to the spark being at the top center of the combustion chamber, it usually makes sense to have 4 valves per cylinder--it just physically fits well. but you DON'T NEED TWO OVERHEAD CAMS TO DRIVE THEM. one is sufficient.

i used to think as you. i really did. "if mitsubishi can do all that with an SOHC, then the DOHC will blow it away". it's not happening. the 6g75 engine is a SOHC 24 valve v-6. when they started making SOHC with 24 valves...it's over. DOHC is cooked. no-one wants extra drivetrain loss & rotating weight in an engine; when you can operate the same # of cams with less.

you can't increase power by adding more valves. i'm not entirely convinced of the effectiveness of 5 or 6 valve engines--unless they posess a radically different cylinder head that greatly facilitates it. even then, you still have to engineer it so the charge will swirl properly.

perhaps if you could make the valves rise OUT, away from the combustion chamber...rather than pushing IN, towards the chamber--you could create a better way to inject the next intake charge. but i doubt that would happen, since you would actually be magnifying the pressure in the intake before releasing it--instead of vice-versa. the miller-cycle engine is a close relative of this idea...pressurizing the intake against the incoming flow from the supercharger.

[3startuna]

Originally Posted by inperfectdarkness

what jason said ^ ditto. SOHC has evolved to entirely supplant any advantages that DOHC might have had. i knew with the advent of V-tech (read: ANY adjustable valve timing) that it was only a matter of time. ferrari's cams that move linearly to adjust lobes--is probably the best implementation of it yet.

i still think the best valve type is what is sometimes referred to as a "rotating cylinder valve". the combustion chamber has no valves, only a sleeve which moves to vent exhaust & allow intake. i doubt you'll ever see it exercised in an automotive application.

They are coming out with something similar in F1. Ill see if I can find more info about it, but I saw it a few months back

[JasonY]

Oh and they blew DR stage 3 heads out of the water flow wise to boot, for what its worth.

I like what works in the simplest manner possible for its given application.

Edited to provide info on my claim:
a 6G75 present for you (ras463us)lol

Jason

[inperfectdarkness]

jason said it all. if there was an easy way to drop a 6g75 into a 3/s, keep it AWD, and run td-05's...i'd have my credit card out already.

[metalman]

I think the greatest advantage of overhead cams and multiple valve set ups is the reduced valve train weight. This gives the engine the ability to turn much higher rpm's, which in turn gives the designer/tuner the ability to make power at those rpm's. You also have much better valve timing control with a lighter overhead cam design. At high rpm's pushrods begin to become subject to harmonics (they bend), and precise valve timing necessary for high rpm power is lost. Not to mention the heavier valves are subject to float (lose contact with the rest of the valve train). Both engines designs have their place, but if you want a high rpm screamer, light valve trains are the ticket.

[inperfectdarkness]

agreed. however diesels show us that you don't need a 5 digit rpm band to make gobs of power. consequently, if your rpm band is narrow, your powerband is narrow. this equates to an easier time adjusting your powercurve. the right gearing and transmission will effectively negate any disadvantages of not having higher rpm bands.

though i still prefer less valvetrain weight.