dohc vs sohc

[gottaluvit]

What is the advantage of having a single overhead cam vs a souble overhead cam? The yfz has the dohc. Isn't there more power in a dohc? What reasoning did Honda have for only going with the sohc? Also, What is the difference in having four or five valves (yfz has five, trx has four). The only reason I am asking is that I often have to listen to some of my friends tell me how great their yfz is. They usually win, but are much better riders. You guys have any answers?

[beak7707]

The reason Honda has a single cam, is they found out out how to make good power with the unicam. Plus there are less parts to break or have to replace.

[asdf450r]

as far as i know the whole 4 vs 5 valve thing doesnt matter, i mean if having one extra intake valve was really THAT much more efficient then why havent any other companies adapted the concept? Keep in mind while 3 seems better than 2, why not just make two big (ger) intake valves vs three smaller intake valves and get the same effect with less moving mass (essentially).

Well obviously the yamaha engineers see some benefit for having the extra valve but imo i think its mostly just a marketing gimmick for people who dont know any better (from what it sounds like, your friends, trust me i have similar friends) and are reading the specs online thinking "wow yamaha's *insert vehicle here* has 5 valves instead of *insert competitive brand here's* 4 i think ill go with yamaha because its faster"


*shrug* makes sense to me

[440ex kid]

The reason for DOHC is because if you operate the valves directly you have less chance of floating a valve, and it also revs faster. On the 450r, the unicam is designed to do the same thing. They put the cam right over the intake valves which controls the floating of the valves, but it is alot less weight with one cam then two, which means fast revving leight weight motor that still makes alot of power.

[450 Racer R]

Originally posted by asdf450r
while 3 seems better than 2, why not just make two big (ger) intake valves vs three smaller intake valves

why not make one big intake valve and one big exhaust valve? I'm no expert, but I think there is a reason for the extra valve. it'd be like pinching a hose... it causes the air to flow faster. If you can't think about it and figure it out, just ask and I'll explain

[Ralph]

Originally posted by 450 Racer R
why not make one big intake valve and one big exhaust valve? I'm no expert, but I think there is a reason for the extra valve. it'd be like pinching a hose... it causes the air to flow faster. If you can't think about it and figure it out, just ask and I'll explain

Because if they wanted the 1 to be equal to the 2 it would have to be alot bigger diameter wise. meaning it could be harder to put in. If there were 2 then 2 smaller ones would fit easier. Since the engine is wider than it is tall, you can fit them in nicer instead of making it taller so you can fit 1 bigger area for a valve/port. same with the combustion chamber, i think it has more to do with how much room there is availiable and best fit. As for the yfz having 3, its prob just something do make all the kids go "Well i have more valves than you" like the other guys said.

I dont know if i explained my point all to well, i have the idea in my head and couldnt explain it to well. if nobody gets what im saying maybe ill draw a picture or something:p

[robman]

:confused: my head hurts after reading that,I think yamaha has a patent on the five valve design though.The honda motor will out last them all anyways so non of this matters.

[r450rr]

Originally posted by robman
:confused: my head hurts after reading that,I think yamaha has a patent on the five valve design though.The honda motor will out last them all anyways so non of this matters.



amen to that

[lrdyfz45021]

I read the honda only saved like 12 oz by using 1 cam instead of 2 like the yamaha so i dont see where there is that much of a weight saving in it

[K_Fulk]

Originally posted by lrdyfz45021
I read the honda only saved like 12 oz by using 1 cam instead of 2 like the yamaha so i dont see where there is that much of a weight saving in it

12 oz's of rotating mass in a small motor is a pretty big deal. Add 12 oz's to your flywheel and see how much slower it will accelerate.:confused: I dont know if that number is accurate though.

The way i look at it isthe less stuff thats in there, less stuff that can break.

[86atc250r]

Because if they wanted the 1 to be equal to the 2 it would have to be alot bigger diameter wise.

Actually, the opposite is true as far as diameter goes --- do the math, the area formula is an easy one.

One reason for multiple valves is low lift flow. Since circumference is more important at low lifts - multi-valves have an advantage.

Another reason is for mass -- smaller valves are lighter and easier to accelerate and control.

3 intake valves have a low lift flow advantage over two --- the trick is balancing that with having to use smaller exhaust valves to get everything to fit in the combustion chamber.

DOHC has two distinct advantages, first being direct acutation of the valves, the other is being able to individually tune the cam timing of the exhaust and intake events.

Honda's SOHC design is not the same as most associate SOHC with - it's an advanced design that that offers a lot of the direct valve acutation advantage that a DOHC engine does. It is also nice because you only need to replace one cam when going to an aftermarket grind.

Most people also associate SOHC with lower performance because that's usually the case in the automotive world. However, the SOHC is not the cause of the lower performance, it's just usually part of an all around lower performance package, because the automakers reserve their top performing combinations for their DOHC options (both for marketing purposes, and for the few advantages DOHC offers).

All in all both the TRX and YFZ have advantages and disadvantages. Both offer similar levels of performance. Buy the one that fits *your* needs best, not the one your friends like just because it happens to be the one they own.

[Ralph]

umm, no if the 1 had to be equal area wise to 2 smaller ones. it would have to be bigger.

a=pi * r^2

lets say the smallers ones are 1 inch in diameter.

then 3.14 * .5^2 = .785 in^2

that times 2(2 valves) is equal to 1.57 in^2

so if the single bigger one needed to have an area of 1.57 then

1.57=3.14 * r^2

divide both sides by 3.14

.5=r^2

square root of both sides

Radius is equal to .707 wich means the dia is 1.41

sorry for all the stupid math, i was bored...lol

I do totaly aggree with all the other points you had.

[86atc250r]

Let's do this the easy way...

Area:
1 x 2" diameter valve = 3.142 sq in
2 x 1.4" diameter valves = 3.08 sq in

Looks like one valve wins this one by a significant margin.

Circumference:
1 x 2" diameter valve = 6.283 in
2 x 1.4" diameter valves = 8.796 in

Looks like the two smaller valves win this contest - also by a significant margin.

[Ralph]

Originally posted by 86atc250r
Let's do this the easy way...

Area:
1 x 2" diameter valve = 3.142 sq in
2 x 1.4" diameter valves = 3.08 sq in

Looks like one valve wins this one by a significant margin.

Circumference:
1 x 2" diameter valve = 6.283 in
2 x 1.4" diameter valves = 8.796 in

Looks like the two smaller valves win this contest - also by a significant margin.

i thnink u are looking into it too deep. my point is that 1 valve with 1 sq inch of area would be "taller" than 2 valves with an added area of 1.

So instead of doing this [ O ]

its doing this [oo] its using up the space more efficiantly. I think we are debating over a misunderstanding between our 2 points.


lol * take 2 pizzas, if u were to make one pizza with 2 pizza's worth dough, then the pizza would be bigger:chinese: lol

[86atc250r]

Of course it will take a larger single valve to equal the area of two valves. It wouldn't make sense for one 1" valve to equal what two 1" valves do --- My point is that if you go to two valves, it takes much larger than what you'd at first expect to equal the area of a single --- i.e. it takes two valves, each larger than 1.4" to equal a single 2" valve.

However, if you look at circumference, the opposite is true.

So -- as I said before, multiple valves are better for low lift flow, but lower numbers of larger valves will produce more area more easily - which equates more to high lift flow.

It's a trade off that for the most part leans toward having multiple valves for a few different reasons.

[red2004 TRX450R]

But another important fact you are missing is air fuel velocity.
When running 2 smaller valves/ports the air is travelling at a greater speed then if there is only one valve/port witch equals more power! I could go on and on about this but ……

Also you have to consider valve shrouding (the effect of the valve opening being blocked by head, piston etc. ) effect on flow. If you can place several valves you could decrees the shrouding effect (or increase it) and effect power.

:D

witch has nothing to do with the topic dohc vs sohc.

[drgn_erth]

Originally posted by K_Fulk
12 oz's of rotating mass in a small motor is a pretty big deal. Add 12 oz's to your flywheel and see how much slower it will accelerate.:confused: I dont know if that number is accurate though.

The way i look at it isthe less stuff thats in there, less stuff that can break.

okay if honda saved 12 oz's why doesthe yfz accelerate faster?

[klz300]

Originally posted by drgn_erth
okay if honda saved 12 oz's why doesthe yfz accelerate faster?

because its a DOHC they rev faster plus its better :p

[K_Fulk]

Originally posted by drgn_erth
okay if honda saved 12 oz's why doesthe yfz accelerate faster?

Thats why I didnt know if those numbers are accurate, plus they could have saaved weight somewhere else too.