Single vs. Dual cam engine design

I have a few questions for all you technically savvy guys out there about engine design and performance.

It seems to me that there are a fair number of dual overhead cam designs out there for high performance 4-stroke dirtbikes (maybe more than single cam/rocker arm designs?). I have noticed that KTM is going to be releasing its first dual cam design this year with a 250cc bike. I wonder... what could have been the reason for this design configuration over the previous (single cam) design? In all the reviews I have read concerning this engine it certainly packs quite a whallop for its size. Granted, I understand that there are a ton of factors that contribute to a powerful/reliable engine but I was wondering why the dual cam design over the previous? I have heard that a dual cam design means more moving parts, complexity etc. but will a dual cam design inheirently do its job better (open the valves) and/or more efficiently over a single cam/rocker arm design?

Husaberg has obviously been a die hard single cam proponent. Do you think that they will continue in that path for the future and refine thier design? Could the Husaberg engine benefit from a twin cam configuration? (I realize that this would be a HUGE design change) I was just thinking about this as I see KTM do a big refinement on thier 250cc race engine. I have only owned a Husaberg (which I absolutly love) and I was just hoping that their engineering dept. will continually be on the cutting edge of technology and development to stay at the top of the heap.



littleyellowbat
'99 FE501

LYB

the suzuki and yamaha machines are DOHC and most europeans have stuck to SOHC until now coz it's cheaper, easier to maintain and has been in the past just as good.

however the time is upon us to justify the extra weight, cost, complexity and height-height of engine. without it i don't think KTM can keep up on a production engine.

DOHC is better because it has less moving parts and less chance for things (valves) to float around and clout the piston. you can produce longer cam timing and thus more power.

true, a tuned SOHC will still by far superced anything presently on the market but you need a measure of safety in the design and i would think that this is close to being marginal.

DOHC is easier to tune and i 'taff timed' my WR400 on TT and produced a lot more torque using the VT camwheels from a company in clearwater called falicon crankshafts.

regards

Taffy

I agree with Taffy. The yamaha is a 5 valve design. I would see it difficult for anyone to produce a SOHC 5 valve design.

Dual cam would essentially eliminate the Rocker arms. Which is a source of failure, as we all have found at one point or another. The rocker is also another tolerance stack up between the cam and valve. Also, the rocker is basically a spring. Though quite stiff, it is still a spring between the cam and valve.

I think the weight would almost be a wash. If you remove the two rockers, shafts, endplugs, and bearings, and add one more cam and pulley, you would only add a bit more weight. Granted, the casting would be a little larger to account for the extra cam, but I would even think that the existing Husaberg head would almost fit a DOHC setup.

I have never seen the guts of the head of a DOHC yamaha, do they have followers? IF so, this would add weight. If anyone has a picture of a Yamaha head they could post, that would be cool.

Also, with industry making major advances in materials technology, I would imagine that a fiber belt with steel reinforcements attached to either carbon fiber or high strength plastic cam pulleys would also reduce a DOHC weight. Heat might pose an issue...

This is a great forum topic I feel we haven't quite discussed. Kudos. I am interested to hear more of others chiming in on this one.

Hi,

for me it´s not easy to explain these technical details
in english. What Taffy already said is right.

Let me explain it in easy words: An engine between 400 and 650 ccm
doesn´t need 2 cams for producing much power.
It is a fact that you need a little bit more cubics to get more power when comparing a single cam engine to a double cam engine.
So KTM had problems with their old 4 stroke 250 exc. The engine was too weak.
A double cam engine like the Japanese engines had a big power advantage to the KTM engine. A DOHC engine produces much more rpm´s than a SOHC.
Additional to that problems, KTM just used the 400/520 exc engine only with less stroke.
It wasn´t too much work for their engineers but the result wasn´t the best.
So they designed a new 250 ccm engine. An tell me what you want, but I´m convinced that Husaberg supported them with their "know how" and helped them to design this new power-engine.
Some details let me think that.

I´m sure Lineaweaver can tell you more about the difference between both engine types.

greetings

hribman

Parsko said:

I have never seen the guts of the head of a DOHC yamaha, do they have followers? IF so, this would add weight. If anyone has a picture of a Yamaha head they could post, that would be cool.

Click to expand...

The yamaha 5 valve motors are a shim under bucket design, with the cam lobe directly pushing the valve open(via the bucket of course).

Click on the following link, Click the "Click here to enter" button on the resulting page, then click engine.
5-valve Yamaha
This is the Quad motor, but is the same design as the WRF/YZF motors, with a few tuning differences(transmission gearing/cam timing) for the quad application.

My understanding of the advantage of the DOHC set up is because it does not use rocker arms the RPM ceiling is increased.This of course has the potential to produce more HP.Lighter valve train mass=more rpm.There are other ways to achieve this, the most popular is desmodromic valve operation(Ducati).I believe that KTM 250 sohc engine has no issues except peak HP,but since they(KTM) are keen to be competitive in MX they must make a more competitive motor.Question is if you are a woods rider is that the most important thing that you require?...nsman

I thought Yamaha had patents on the 5 valve DOHC design & that was what stopped others using it. The design sure does seem tidy. To look at the 5 valve head, there is certainly not much area that is not either valves or spark plug.

How do the sliding cam follower surfaces of the DOHC compare to the roller followers of the SOHC, both from friction & valve gear geometry? I would have thought more aggressive open & close ramps (eg '02 'berg cams) would be possible with roller followers but less so with flat sliders. Whether you can make valve gear with aggressive cam profiles hold together is another question!

Hi Gang,

My recollection of the basic issues is that the single cam is supposed to be lighter and that any weight high up in the motor saved will assist in the steering of the bike. Twin cams means more revs reliably and hence potentially more power. If all is equal, ie combustion chamber design valve size, bore and stroke etc and revs are not required then pushrods will still do the job. Its just a means of opening and closing the valves.

I believe that Audi has some 5 valve heads as well as Toyota. Toyota have been partnering with Yamaha for decades so that should be no surprise. Dunno whether Audi pays Yamaha or not.

Interestingly, it is strongly rumored that Yamaha has abandoned 5v heads in MotoGp but persists with them with production bikes due to marketing and potential loss of face issues.

Cheers
Steve

So it sounds as though power delivery would be the major performance difference in a single vs dual cam design. Seems to me that maybe the 450cc Husaberg engine revs quite well and that had it a dual cam config that it wouldnt make much difference....How bout on the 650? I'm sure that the SM guys would love to keep a little more of that awesome power curve in the stratosphere RPM range.

Thanks for all the feedback...it has been interesting discussion.

littleyellowbat
'99 FE501

Until we have electronically opened valves that a computer can vary the valve timing (increasing duration as the RPM goes up) one is stuck with a fixed compromise. In a quest for all around high performance, valve timing (duration) has generally been decreasing with more aggressive opening and closing of the valves, hence the current state of rocker arm problems. To help reduce valve train loads the valves are becoming lightweight titanium which also requires a lesser spring. The DOHC reduces the valve train weight by eliminating the rocker, but now rather than having the cam down in the head between the in. and ex. valves, the cams are up high above the valves. Extra engine height isn't an issue on a 250, but maximum HP is. Big bores would be difficult to ride in such a radical state of tune and the engine height is a concern, hence the trend toward rockers and lower RPM levels on open class bikes. The stroke on a big bore limits the RPM levels, even on road bikes where the power could be used. Lightweight 550-600 V Twin similar to the 450 Aprilla is working on, would fix that problem if the V angle was kept narrow enough to package the thing. Fire it with the big bang method (both cylinders firing close together) so the tire has time to catch up and bite, and it might just make a great motard engine. Sorry, got off track there. Us nuts with ME backgrounds tend to do that.
Dan

Hello Dan,

dsducati said:

... Big bores would be difficult to ride in such a radical state of tune and the engine height is a concern, hence the trend toward rockers and lower RPM levels on open class bikes. The stroke on a big bore limits the RPM levels, even on road bikes where the power could be used. ...

Click to expand...

You know the Husaberg spec. To obtain a good service life, what should the RPM limit be?

i've had my shot at this one once already so just as a ditty i will say that the modern DOHC is barely taller and barely heavier than SOHC.

wanna see what it looked like 50 years ago-frightening! the engines were 4-6" taller and lord knows how much heavier from messrs norton, morini and guzzi etc. and they were only two-valve heads!

i believe the BMW single is 5-valve and i think you'll find that there isn't a restrictive patent if one at all from yamaha on the 5-valve set-up.

the japs have a 'gentlemens agreement as i'm aware not to steel certain ideas that the other has. look at how all four came up with different rear suspension set-ups in the 80's and 90's!

only one company used to use the single sided swing arm yet mv agusta used it in the late 40's nullifying any patents. ducati have it though!!!!

fact is the higher revs come from more finite control of the valve to piston and not much to do with cam duration (although still important it's not thee critical factor here). todays cams are '*******' compared to some of the old stuff.

what with perfect carburation, ignition advance curves, exhaust lengths, combustion chamber shape, scavenging and very little reciprocating weight and friction; we now have a bike that pulls from zero and has more on top. and it wasn't the cam timing!

what i pesonally find amazing is how we said 30 years ago that the 2-stroke and 4-stroke were at the end of their development. how wrong we were! honda produced the NR500 in 1980 with oval pistons and 8-valves per cylinder. they thought it was the only answer.

i guess it produced at best 110-120bhp yet they could get that out of a production world supersport racer 20 years later!

i believe the latest world supersport 600's are on 155bhp and that's with strict rules and in the meantime a 125GP 2-stroke is on 70bhp.

now that's awesome!

regards

Taffy

The 03 650 has an 82 MM stroke and with a piston speed limited to 4500FPS would safely rev 8500. Used to limit piston speeds to 4000FPS, but with modern materials, now V Twin suberbikes see 5000FPS regularly. Engine life isn't too long at those speeds though. Rockers do fine at 8000.
A short 57mm stroke would allow 12000, making DOHC a big plus. Of course RPM limits on cam designs still also revolve around the acceleration levels placed on the valves through the opening and closing ramps. Since a valve flows more as it is opened more, the trend is to get it open quick in the name of performance and close it rather quickly as well. This helps driveability down at low RPM levels as well as helping with the dreaded flame outs. As much as I like the old C cam for valve train life, the new cams don't have the driveability issues. Now for a tangent again, formula one engines have pneumatically opened valves and the new crop of diesels will have electronically opened valves. Lets see, with the current trend towards electric start, our bikes now have batteries and decent charging so.....when they get the RPM levels up on the electronics and the things mass produced(probably due to some EPA mandates that make this necessary on road models) maybe we'll see this off road some day? I'll probably be too old to ride though.
Dan

Ok I did not read through all the post. But as you know I had a 2000 yz426. Ask me anything about that bike and I can tell you about it. The 5 valve is better for more power and a longer lasting valve train. The yamaha valves are industructable! At least when they went to a singal pieace valve they are. As for weight I don't think it ways any more then the singal cam crf set up. Main reason is it is also includeing rocker arms and everything to adjust them. Plus for the first few years honda has had major problems with there valve train on the CRF. But from what I can tell now they seem to have resolved the problems. But I would take the yamaha 5 valve over any other any day.

Just a thought, doesn't direct DOHC cup on valve impart a force perpendicular to the valve opening direction therefore wearing the valve guide ?

5 valves give more valve surface area than 4 valves allowing more flow.

DOHC with a central pinion driven via chain sounds good.

Time will tell :roll:

3 inlets doesn't flow much if any better than 2. the surface area which the air is slowed by, the 3rd guide, the third valve in the way all add up.

i have no idea-regarding the middle inlet; (and i respectfully suggest nobody here except dale may know) whether sending a mixture at a different angle down the outside wall of the cylinder is a bowl of cherries or not. i suspect not.

regarding the cam opening the bucket and shim 'off centre' it doesn't! the contact point of the cam is always in the middle of the shim/bucket creating downward pressure. the flanks of the cam (sides) are designed for this purpose.

going back through the thread. the need for DOHC is diminished with the larger displacement engines. quite simply, if it's going to happen, it'll happen from the smallest up to the biggest engine.

regards

Taffy

The "Bucket" on a DOHC engine is a thin wall piston of sorts that reside in a corresponding cylinder head bore. The side load is applied to this bucket, not the valve stem. A shim sets on a recessed area on the top of the bucket to adjust the valves, so actually the cam rides on the shim. But......float a valve and they sometimes spit the shim, so some ultra high RPM racing designs place the shim under the bucket. Major pain to adjust the valves as cams and buckets have to be removed to change a shim. Most often see Shim over Bucket.
Dan

Another Thing To Concider Is The Fact That Fuel Flows Faster Through More Valves,Potentially Creating A Better Mix.

dsducati said:

The "Bucket" on a DOHC engine is a thin wall piston of sorts that reside in a corresponding cylinder head bore. The side load is applied to this bucket, not the valve stem. A shim sets on a recessed area on the top of the bucket to adjust the valves, so actually the cam rides on the shim. But......float a valve and they sometimes spit the shim, so some ultra high RPM racing designs place the shim under the bucket. Major pain to adjust the valves as cams and buckets have to be removed to change a shim. Most often see Shim over Bucket.
Dan

Click to expand...

I went 5 years and never had to adjust once with my 426. They always stayed in perfect spec. Even after break-in. Thats with rideing hard every weekend.