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Unread 10-07-2012, 07:33 AM   #1
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Default Even though it's a Honda magazine, what they're talking about applies to all engines.

Even though it's a Honda magazine, what they're talking about applies to all engines.

"...all things being equal, a single-overhead-cam engine will produce more torque than a twin-cam one. Fewer parts, which results in less valvetrain inertia, translates into more useable energy. Of course, all things are never equal, and the ability to adjust intake and exhaust valve phasing independent of one another without having to install a new cam to do so is what makes DOHC engines so hard to beat."

I've always been a fan of single cams, myself. I felt like my D16Z6 I had in my Del Sol had more bottom end than a B16 would have. And if you really look at Honda's (this could apply to other manufacturers too), the DOHC engines are almost always geared shorter than their SOHC platforms. Makes sense, considering they lack as much bottom end torque. A great example is the C-series Hondas versus the J-series.

The C32A (3.2L DOHC VTEC V6) from the NSX made 290HP at 7100 RPM, and peaked on torque at 5500 RPM, with 224 lb-feet. The J32A3 (SOHC VTEC V6) makes 270HP at 6300RPM, and 239 lb-feet of torque at 5000 RPM. It makes 15 lb-feet more, and peaks 500 RPM sooner.

Seems like a neat theory.

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Unread 10-07-2012, 08:50 AM   #2
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The torque an engine produces is not related to how many cams that engine has. What matters is:

Lobe lift
Cam Duration
Porting
Intake design
Exhaust design


If you wanted to you could take a B16 and turn it into a tractor engine. All you need is a pair of cams with incredibly mild duration, say 250 @ 0.050" lift, tons of lobe lift to allow a large volume into and out of the cylinder when the valve is open, and a long-tube header that scavenges best at low speed. Set one up like that and I wouldn't be surprised if it makes enough torque at around 2100RPM to shred factory driveshafts left right and center...as well as being so severely choked off at high speed it's unable to hit the ECU's rev limiter at all.



The main reason two-valve engines tend to make more torque is because they don't breathe very well at high speed anyway. As a result the engine designers put a more mild, lower-RPM cam in so the car has enough HP to roll down the road. This has a side effect of a slightly higher overall torque specification. Four valve engines are not limited in this way, so they tend to be higher revving with 'weaker' cam grinds...their HP comes from sheer RPM more than anything else. Like I said the secret's in the cam profile not how many cams the engine has.
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Unread 10-07-2012, 12:45 PM   #3
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Quote:
Fewer parts, which results in less valvetrain inertia, translates into more useable energy.
A smaller, but still important aspect to this; Internal friction rises with RPM (I believe it rises exponentially), and accessories become less efficient as well. The alternator and power steering pump together can put a 5-10hp load at peak drag on a typical passenger car engine, and spinning them slower eats less power. That's why underdrive pulleys free up a measurable amount of engine power.

High efficiency white paper from Delco - http://www.delcoremy.com/Documents/H...ite-Paper.aspx

Though the throttle plate is still the largest restriction in an engine. Going WOT and keeping the revs low greatly reduces pumping losses, producing more power. (I love diesels for this, some designs even had no throttle plate at all.) So an engine designed for more low end power with a low RPM cut off is inherently more efficient at turning fuel into power than one that makes high end power.

Forgive me if I'm going off on a tangent as I've been studying accessory drive design for the last few days.
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Last edited by S D; 10-07-2012 at 08:18 PM.

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