scottydl
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There's nothin' like an American V-8...
Posts: 7,373
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Post by scottydl on Sept 20, 2004 14:28:55 GMT -6
Okay, so I hear these terms all the time (and I know the Aurora is DOHC), what's really the difference? SOHC = ? DOHC = ? And there's another one with an 'I' ... ? Enlighten me, please.  |
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Post by kobalt on Sept 20, 2004 15:08:14 GMT -6
In laymens terms:
SOHC - single overhead camshaft driving intake and exhaust valves. Relatively simple design works great on larger engines that rely on displacement to make power. USUALLY 2 valves per cylinder.
DOHC - dual overhead camshats operate intake and exhaust valves independently. This design is more complex with more moving parts. Most DOHC engines sport 4 valves per cylinder which enables for more sophisticated timing. The end result: A DOHC engine USUALLY runs smoother, revs higher and makes more power per unit of displacement than a similar motor with SOHC.
Varaible Valve Timing - A complex camshaft(less) design which can allow for independent valve operation at different intervals at any given rpm. Design varies from manufacturer to manufacturer, but when implemented is USUALLY superior in both smoothness and power delivery than either SOHC or DOHC.
I capped USUALLY as there are (and alway will be) exceptions.
Examples of equal size motors:
2003 Ford Crown Victoria
4.6L SOHC - 239hp
2003 Cadillac DeVille
4.6L DOHC - 300hp
2003 BMW X5
4.6L VVT - 340hp
EDIT: Almost forgot, "I" usually stands for Inline. Inline engines have cylinders arranged in a row - or in a line. This design is almost perfectly balanced, as all of the engine's power travels in one direction. The reslut is improved smoothness and an inherent ability to tach much higher than eigher V6 or V8's. More rpm can equal more peak power. It is a practical design for 4-cylinder motors, as these usually lack displacement. The reason you see so little inline 6 or inline 8 engines is their size - an inline 6 can be just about as long as a V12 and an inline 8 would be comparable to a V16 in size - unpractical.
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Post by JimW on Sept 20, 2004 16:00:40 GMT -6
Damn good summary there Kobalt...thank you!
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Post by kobalt on Sept 20, 2004 16:13:58 GMT -6
No problem - glad I could explain!
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Post by erw38 on Sept 20, 2004 20:59:02 GMT -6
Damn, now my brains tired. Nap Time!
Seriously tho......great post..........I didn't know 98% of the stuff.
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Post by R4D30N on Sept 20, 2004 21:14:53 GMT -6
kobalt: the bmw x5 motor takes ALOT More air, the VVT is pretty much just a gas saving option (I.E Honda's V-Tec) which means variable valve timing and electronic lift control.
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Post by kobalt on Sept 20, 2004 22:22:20 GMT -6
While peak numbers do increase with the use of variable valve timing technologies, I only used it for simplicity's sake of the argument - hence "laymens terms". Where variable valve timing does it's real magic is in the entire rpm range, providing a siginficantly fatter torque curve throught - this simply translates into better throttle response and way more power on demand, especially in the lower rpm rage, where most day to day driving occurs.
While I have to agree that bmw breathes the best of the bunch, it has little to do with honda's vtec. While vtec uses two seperate electronically-selected cam profiles for eigher low or high rpm, bmw uses a camshaftless solenoid system for an almost infinite variablity.
As for fuel effeciency, it is a welcome side effect: More torque at any given rpm means you mash the pedal less and don't waste fuel.
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scottydl
Super Moderator
There's nothin' like an American V-8...
Posts: 7,373
Staff Member
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Post by scottydl on Sept 21, 2004 9:29:46 GMT -6
WOW, I got more than I bargained for with those detailed explanations! (And that's a good thing). Thanks kobalt!  |
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Post by R4D30N on Sept 21, 2004 13:23:20 GMT -6
While peak numbers do increase with the use of variable valve timing technologies, I only used it for simplicity's sake of the argument - hence "laymens terms". Where variable valve timing does it's real magic is in the entire rpm range, providing a siginficantly fatter torque curve throught - this simply translates into better throttle response and way more power on demand, especially in the lower rpm rage, where most day to day driving occurs. While I have to agree that bmw breathes the best of the bunch, it has little to do with honda's vtec. While vtec uses two seperate electronically-selected cam profiles for eigher low or high rpm, bmw uses a camshaftless solenoid system for an almost infinite variablity. As for fuel effeciency, it is a welcome side effect: More torque at any given rpm means you mash the pedal less and don't waste fuel. I like the explanation  |
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