My friend and I was arguing about which setup is better. Is a twin cam better or single cam better for all-purpose from drag and track setup. He said single cam allows the motor to rev faster and makes more power. Can you guys give me some ammo to knock his theories down. I assume they better because Honda put dual on all their cars that demand the most power available.

 

Is your friend suggesting that a single cam, dual valve engine is better than twin cam, multi-valve engine? He's right that less rotational mass helps a little, but if it is at the expense of the ability of the engine to breathe, then it will hurt more than it will help.

Another thing to consider: You need some pretty aggressive cam lobes on a single cam engine (unless you have something like a VTEC setup) to get good aspiration. If he has an aggressive cam, his car is going to idle like a boat.

 

He said a built D16 is better then a built B16.

 

Are you suggesting that a rough idle makes a pushrod 2-valve engine inferior to that with an OHC multi-valve top end?

A pushrod engine can and will breathe just as easily as an OHC engine. You said so yourself. Yes, it will have a nice catucka catucka catucka to its idle, but it doesn't stall and idles at a reasonable 700rpm, so what's the big deal? The only legitimate comparison, with the idle quality as your criterion, is that the pushrod engine may not sound as good as the OHC. But that's purely subjective.

The lope at idle is a result of a fat, tight cam; the bigger the cam, the more pronounced the lope. It has a bigger cam because it has less valves. Two large valves in a given bore will have a lesser total valve face area than four small valves in the same bore. For equal airflow per volume (rather than velocity), the valves need to open more, and for a longer period of time. It's part of the design.

HPE, one of the leading LS1 tuners, offers their version of the GM LS6 cylinder head. It's a GM casting, but they've reworked it to flow 350+ at 8000. You don't need the extra valves.

 

It is true that a single cam setup has less rotational mass. A DOHC setup simply allows the engineer (and tuner) to control the timing of the intake and exhaust valves independently of each other. First, there's the obvious, you have an intake and exhaust cam seperate from each other. And each will have it's own valve duration and lift.

And if you want to change the characteristics of the exhaust side only (or intake side for that matter), you can swap that cam out without having to touch the intake cam. This is not possible on a single cam setup; the entire cam must be taken out and replaced.

But the key ingredient is valve overlap. With a dual cam setup, valve overlap can be controlled (with the use of cam gears, etc). At higher RPM's the amount of time that the valves stay open is so short, that you need to have both of them open at the same time (for a little while anyways) in order to achieve a "scavenging" effect of the cylinder. Basically, when the intake charge is being drawn in (intake valves open) the exhaust valves are still open. By having the exhaust valves open, the negative pressure will actually suck out as much of the exhaust remaining in the cylinder into the exhaust manifold, allowing more intake charge to come in and fill that cylinder. Guess what? This is one of the primary principles of VTEC. And it works well at high RPM. It does not work well at low RPM, which is why you have the guys in here talking about the lopey idle. VTEC accomodates this also...but now I'm rambling (if you understand VTEC, then any further explanation will be useless).

Getting a single cam setup to perform all of this is doable, but at what cost? That's why Honda threw two cams in and was done with it!

Hope all this helps!

 

We have dual profile cams.


Wrong. On my '69 Camaro, I run a more aggresive tune for 118-octane. For 93-octane, I retard the cam 3

 

You are correct alexf20c, I misspoke when referring to have to remove the entire cam when trying to change the characteristics of the intake or exhaust side independently of each other.

 

Your argument should've been that most multi-valve OHC engines are available with variable cam timing and/or lift/duration, unlike all pushrod 2-valve engines. I have, however, been working on my own variable cam timing...

It's so simple, I don't know why nobody has done it yet. I fabricated some bracketry to mount a pair of electronic solenoids on the front of one of my sbc 350's. I've tried a selecter shaft and stepper motor, but was not very successful. The solenoid actuates a rod (like a hydraulic ram) which is attached to the timing chain tensioner. By extending or retracting the rod, you push/pull the timing belt, thus advancing or retarding the entire cam by a few degrees. Of course, there's an additional chain tensioner (spring loaded) on the opposite side to take up the slack created by the push/pull of the first tensioner.

The hard part is writing the code which adapts the timing to the engine performance parameters.

 

Also, the setup requires a very heavy-duty solenoid. So far, the only piece which has not failed under the pressure of the timing chain is the solenoid off the starter from a Detroit diesel.




But back to topic, the real advantage of OHC vs. OHV is the packaging and cooling efficiency. On a more anal level, you could argue that the pushrod engine has more drivetrain friction... But, the aftermarket is incredibly efficient these days. There's anything from full-bearing roller rocker shafts, all the way to needle cam bearings.

 

But I guess, E36Ap1, you were trying to argue the other side?