Alright I know a good bit about cars but I am not familiar with the difference in SOHC and DOHC. I know that DOHC has two cams but what does this mean technically and performance wise. Also, I have noticed most performance cars are DOHC why isnt the types?

 

While admittedly not an engineer, DOHC has always been associated with more of a performance flavored engine. At least initially, but it seems as though Honda has gotten by just fine with SOHC engines.

With a DOHC design you can devote a cam to a single row of valves (don't know the benefits) whereas in a SOHC engine the one cam has to operate both banks of valves.

I guess by going with a SOHC design you can argue that there is lighter weight (one cam per head versus two) and less complexity, but I don't know the performance ramifications.

I'm also interested. Good Thread.

 

http://www.howstuffworks.com/camshaft2.htm

 

I think there may be a benefit in having DOHC when tuning the car, but other than that I can't think of anything. I asked this question a long time ago and got limited response, so maybe this time around someone can help out.

 

Ask RAdams. He seems very knowledgeable about engines in general...

 

Thanks for the ringing endorsement! As luck would have it, I typed a 5 paragraph response that easily topped 400 words and IE crashed as I hit the "Submit Reply" button

The link to the howstuffworks website above does a good job in explaining the difference between the two configurations.

When I'm bored at work tomorrow I'll try to recreate what I typed just a minute ago.

 

That sucks you lost what you were going to post.
It is not the difference, as I understand the fundamental difference; it is more about what is the advantage of DOHC compared to SOHC.

 

I think the main fundamental advantage is in tunability... both from the factory and in the aftermarket.

In our cars, since they're SOHC, there is only room on the camshaft for VTEC lobes for the intake valves. Lack of VTEC on the exhaust valves is an inherent "strike" right off the bat. Also, the VTEC lobe on the intake side can't be made too "wild" without the exhaust cam lobe's ability to get all that air out of the combustion chamber.

As far as the aftermarket is concerned, you cannot alter valve overlap with a SOHC engine through the use of an adjustable cam sprocket as the valve overlap is "set in stone" due, again, to the fact that the lobes for both intake and exhaust valves are on one common camshaft. You're pretty much forced to stick with the factory tuning (which Honda is, luckily, very good at). However, when it's all said and done it does limit the ultimate tunability of the car (with the stock cams anyway) were you to go wild with engine mods (forced induction, for example, generally prefers less valve overlap in which case you'd have to have a custom cam ground rather than being able to just adjust each camshaft individually)

On a side note, an adjustable cam sprocket would be hard to implement on the TL in the first place as the ECU uses marks on the cam sprockets as cues for the ignition system.

Lastly, and perhaps the most overlooked item, is packaging. A DOHC head is significantly larger than a SOHC head as it has to accomodate the width of two camshafts (and the accompanying sprockets that drive them).

 

Thanks for the explanation.

 

Keeping in mind of course that most DOHC engines don't have another set of lobes...

 

as i know...both kind of engines has their both upsides and downsides...

traditional DOHC engines are better at high rpm and more potential for high HP...but they don't have much low-end torque as the SOHC engines have...and they cost more for production

SOHC engines have better low-end torque but not enough power at high end...but with better fuel economy and emission.

now it seems doesn't reallie matter with both designs...
since they are all using those valve control systems...

thanks to VTEC, otherwise our SOHC engines would be the goat...haha...and screams to 9000rpm with DOHC engines like S2000
thanks to VVT-i, otherwise DOHC engines wouldn't have so much torque at low end and better fuel economy..

 

I think the one advantage of DOHC (besides mod-ability) is the fact that you can add more valves per cylinder, and vary the timing of the intake and exhaust valves independently.

I doubt that the performance gain of DOHC in VTEC engines will be worth the extra weight, space and cost requirements, especially on a V6.

If the SOHC setup provides sufficient intake/exhaust port area, why go with DOHC? Sure, it's nice to vary intake and exhaust valve timing, but most of the gain is realized by varying the intake timing and lift anyway.

 

See the link that copland007 posted it pretty much covers the explanation.

 

Adding additional valves per cylinder isn't really a factor. Our engines are SOHC and we have 4 valves per cylinder. Many Audis, for example, have 5 valves per cylinder. However, since we have VTEC on the intake side we already DO have 3 intake valve lobes for each cylinder on the camshaft, so it's not really a lack of space that's preventing that.

This may or not be the case. Witness the NSX engine: It's a 3.2L also, but pumps out 290hp. I realize that it's a bit shy on torque and that it needs to be revved to extract maximum horsepower, so it may be more accurate to say that the DOHC VTEC implementation may not be suitable to a TL (given the driving style/demographics of the average TL owner) rather than saying it's not worth it period.

Intake/exhaust port sizing/volume isn't really affected by cam configuration, so I'm not sure what you mean by that. As far as varying the valve timing and lift -- you're right that altering the intake valves makes the most power but know there is additional power to be had, sometimes a significant amount, when timing and lift on both intake and exhaust valves is variable. It's clearly evident across a wide range of manufacturers from Honda (compare a SOHC VTEC and DOHC VTEC engine of similar displacement) to BMW (Double VANOS versus VANOS).

With that being said, I'm excited to think about the possibilities of a DOHC i-VTEC engine. Peak power would be impressive, but so would the midrange punch afforded to the engine by being able to alter BOTH the intake and exhaust cam phasing in real-time. I wonder when we'll see that...

 

How many valves does the TL-S have? 24 or 12?

 

All Acuras have 4 per cylinder, hence all TLs have 24.

 

While we're talking at forms of VVT, the BMW Vanos system is cam phasing where as VTEC is cam changing correct?

 

I also thought Vanos and VVT-i affected lift and timing throughout the power band where VTEC only does it at a certain RPM. However the new i-VTEC will alter lift and timing through the whole powerband, correct?

 

VTEC simply employs a more aggressive cam lobe and additional rocker arm for each pair of valves. At a factory-set RPM, oil is diverted into passages in the head which engage a little pin in the sides of the rocker arms that lock the three together. Now, rather than "freewheeling", the center rocker is locked to the one on either side of it and all three follow the aggressive VTEC cam lobe. The VTEC cam lobe features altered valve timing and greater valve lift (compared to the milder lobes that the car runs on when VTEC is off) The transition to VTEC is quite noticable audibly as well as through the seat of the pants (in some applications).

i-VTEC takes VTEC one step further and features a multi-piece cam sprocket that allows the car's ECU to adjust intake valve timing by altering the camshaft sprocket position in relation to the crankshaft pulley. Now we have a seamless transition that occurs over a wide range of adjustment. You cannot hear or feel this system (called VTC [for Variable Timing Control] by itself).

So, some quick "math":

The VTC component of i-VTEC is analagous to Toyota's VVTi.
VTC + VTEC = i-VTEC which is analagous to Toyota's VVTLi.

As far as I know, Toyota never had an engine (in the U.S. anyway) that employed ONLY a camshaft with dual cam profiles (like VTEC) so there would be no VTEC (by itself) counterpart from Toyota.

 

RAdams: I remember a long time ago that DOHC was touted to allow 4 or even 5 valves per cylinder, plus independent timing of the intake and exhaust ports; that's why I mentioned more valves per cylinder as one of its advantages.

You're right though, you can cram 4 valves per cylinder with a single cam.

The intake/exhaust port sizing is related to the number of valves: the more valves you have, the bigger the port area you can carve out of the cylinder head without needing enormous valves.

While cam phasing and lift changes are exciting, I'm really keeping my eyes on solenoid-controlled valves. You will see a quantum leap in engine performance and efficiency with that system, while reducing the part count and size of the engine.

 

I guess the port sizing issue is half and half... the port size could be kept the same, however you'd have additional valve stems protruding into the port area so there would be less available space for air down in there, but it's easily made up for by multi-angle valve jobs, swirl-polished valves, etc.

Solenoid-controlled valves would be great in that valve timing and lift could be altered across an unlimited range and in real-time, although I think cost is going to be an issue until it hits the mainstream. Until then, we're making tremendous progress using plain old low-tech camshatfs

 

Don't wonder anymore.

The RSX is:

2.0-liter, DOHC i-VTEC, 16-valve

Horsepower

RSX - 160 hp @ 6500 rpm
RSX Type S - 200 hp @ 7400 rpm

If you were just refering to the TL, then I wondering too.

 

One of the ironies of this particular technology -- just the cam phasing, is that the primary driver for a number of the companies is the ability to adjust the overlap to dilute the intake mix for emissions control issues (lower peak combustion temps for NOx control). There are a number of racing people who dump the vary-timing-only-systems for racing (due to the tight ratio gearboxes available and the need to save weight). If it was that great (as the super technology) they would keep it in. The variable lift is nice, but the variable timing largely "smooths" out the torque as opposed to giving it some brutal kick in the rear

As you were mentioning, the big payoff is with the totally variable lift and timing -- the ability to control lift *and* duration will become more prevalent as the mass of the components can be lowered to accommodate higher revving engines. That variable lift when combined with phasing AND low mass are the keys to allowing a tuner to get some kick-ass power at high rpms AND low rpms (A few looks at some valve port to valve lift charts will clue a person to why all the variable resonance and variable induction stuff is so important to make all of this work.)

AS a side note, there are a number of ways that a SOHC can mimic the operation of a DOHC (not it terms of intake vs. exhaust valve phasing), but low-mass titanium rocker arms can be used to drive off a center shaft if the object is to allow a streamlined airflow into the cylinder chambers. The geometry of a DOHC can be approximated by a SOHC (with some reduction is maximum valve speed).

The multiple valve issues you mentioned is also right on, and while there are flow issues involved, there are also heat conduction issues that come into play. A couple of big valves get a lot hotter than two smaller valves that flow the same amount of air. Even with sodium filled and titanium valves, there is a point where the valves just can't transfer enough heat away to the seat (even with intake charge cooling) and that was another motivation to move to multiple valves... The other issue concerns the ability to control the nasty spring harmonics that come with the higher masses of the bigger valves -- there is a point where there are more parasitic losses from trying to drive a really nasty set of valve springs (with damper spring); the engine loses power that is never recovered. (I'm ignoring pneumatic valve systems [F1] here)

Finally, one of the items that most people don't know about -- why the Acuras/Hondas are such great power makers while making getting good gas mileage! At low speeds, the 2-intake valves are generally opened at slightly different times to create a swirl that allows for a "lean burn" condition to be created; this allows for a very low fuel-to-air mix to economize on gas during non-VTEC operation (grandma mode). When the VTEC is engaged, the 3rd lobe operates both valves at the same rate and the object them becomes, "MAKE ME SOME BIG TIME POWER"!


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