ENGINE TYPES

[LilBoiBlu]

SUP, i was juss wonderin wuts the difference between an sohc and a dohc engine?

[jonnaro]

single overhead cam

vs

dual overhead cam

[dp]

lil power vs. more power.

[SiN-6]

SOHC have lower rev limit with more torque at the bottom.
DOHC has high rev limit and more horsepower at the top

[SE-Rage]

Cams are short for cam shafts. Cam shafts are a part of a system called a valve train. A valve train consists of the cam shafts and of course valves. There are two valves to each cylinder. One valve opens and closes the entrance to the intake port and the other controls the valve to the exhaust port (with the help of rocker arms and springs). These ports lead to the cylinder. Anyway, back to cams; the cam shafts have lobes (these lobes are actually called cams or cam lobes) on it to control whether the valve is closed or open. Most engines nowadays have what's called an "overhead" camshaft, which is just that; the cams are overhead or above the valves.

SOHC or Single Overhead Cams have only two valves per cylinder, which is the "normal" setup. This is why an engine with 4 cylinders that is SOHC has 8 valves.

DOHC or Dual Overhead Cams have 4 valves per cylinder for higher performance. 2 exhaust valves and 2 intake valves to allow for more passage into and out of the combustion chamber. Of course, a 4 cylinder DOHC engine has 16 valves. (Easy math, no?) This arrangement of 4 valves per cylinder requires 2 camshafts per cylinder. 2 camshafts, thus "dual" overhead cams.

The camshafts are linked to the timing belt/chain so that the valve openings/closings coincide with the piston movement.

Now, this VTEC thingie that these Honduh owners keep talking about but don't know what it does is actually an ingenious thing. VTEC or VVTL is Variable Valve Timing and Lift. There is a direct relationship between the shape of the lobes of the camshaft and how the engine performs at different speeds (RPM's). The faster the engine spins or the piston pumps, the slower you want the intake ports to close so you can let in more of the air/fuel mixture. This is called valve lift. You want the valves to close slower because the faster the engine spins, the faster the air/fuel mixture is entering the cylinder (thus not getting enough air/fuel). I believe (correct me if I'm wrong) a VTEC setup has an extra intake cam/lobe with its own rocker. The profile on this lobe keeps the valve open a little longer than the other lobe profile. At a slower engine speed, this rocker is not connected to any valves, but as the engine hits a certain speed (about 5500 RPM?) the extra rocker locks to the two other rockers controlling the two intake valves, so the valves now follow the enhanced lobe profile which controls the valves to close slower.

This is why aftermarket cams are rough at idle, because the exhaust valve openings/closings and the intake valve openings/closings overlap. But through the ingenuity of VTEC or more correctly VVTL, we can now have lobes designed for slow speeds saving gas and then have lobes designed for higher performance as the engine speeds up.

Don't go off on how Honduh is great yet, people are working on Infinite Variable Valve Timing. Oh yeah, *drools*. I believe this would work by a solenoid connected to a computer to control the timing rather than relying on the camshaft. This means maximum performance/efficiency at every RPM!

[Edited by SE-Rage on 03-01-2001 at 12:02 AM]

[klenboy]

Uhm, there are sohc engines that have 16 valves for instance
the 1.6 vtec engines have 16 valves. Also there are dohc engines that have 8 valves! For those of you proud 1st gen celica owners you know what I'm talkin' about.

[SE-Rage]

Hmm, yes, there are SOHC engines that have 16 valves. But I'm not quite sure how a cam shaft can contain enough lobes to operate that many valves. If someone out there knows, please explain. I'm assuming that it is possible because of the extra lobe and rocker due to the VTEC system.

As for a DOHC engine with only 8 valves, I don't see the point. The whole point of having two camshafts per cylinder would be to have the ability to operate more valves. If you're going to do the "standard" 2 valves per cylinder, why add another camshaft? I'm not denying that it exists, I'm just questioning its usage.

In general, for inline 4 cylinder engines, a single cam design will have 8 valves and a dual cam design will have 16.

[SE-Rage]

Well, I guess it is possible to have enough lobes to control 16 valves from a single cam setup.

[klenboy]

In the sohc vtec engines the vtec lobes are only for the intake valves as for the dohc vtec engines there are vtec lobes for both intake and exhaust. I can't answer exactly how those camshafts can fit all those lobes. The best way is to look at them and see how they design it.

As for the DOHC 8-valve engine I have one good example that totally kicks ass. Toyota's 18rg. Even though there's only 2 valves per cylinder the valves are huge!!!! And back then there was an option from the factory to even have "bigger valves." It's the valve size that makes this engine have so much bottom-end torque (18rg is a 2.0 liter with 140hp and around 140 + ft/lbs of torque at around 3,200 rpm or something like that). What I'm trying to say is that to compensate for the lacking # of valves they instead increased the valve sizes instead. I've heard of hp #'s going up to 320 naturally aspirated when built and tuned up right. This engine was used mostly in rally racing so they need more bottom and midrange, which was where the power band of this engine is.

If you notice most cars with 4 valves/cylinder have mostly midrange to top end power bands (w/ exceptions of course) and the muscle-car V8's with just 2 valves/cylinder have power mostly concentrated at the bottom end and midrange (again w/ exceptions).

[SE-Rage]

Okay, I understand the desire for bigger valves, but why would you need two cams to control it? Why can't a single camshaft control larger valves if there are still only 2 valves per cylinder?

Not flaming, bagging, arguing. Just curious and hoping to learn more stuff.

[NA95IntegraLSR]

Originally posted by SE-Rage





Now, this VTEC thingie that these Honduh owners keep talking about but don't know what it does is actually an ingenious thing. VTEC or VVTL is Variable Valve Timing and Lift. There is a direct relationship between the shape of the lobes of the camshaft and how the engine performs at different speeds (RPM's). The faster the engine spins or the piston pumps, the slower you want the intake ports to close so you can let in more of the air/fuel mixture. This is called valve lift. You want the valves to close slower because the faster the engine spins, the faster the air/fuel mixture is entering the cylinder (thus not getting enough air/fuel). I believe (correct me if I'm wrong) a VTEC setup has an extra intake cam/lobe with its own rocker. The profile on this lobe keeps the valve open a little longer than the other lobe profile. At a slower engine speed, this rocker is not connected to any valves, but as the engine hits a certain speed (about 5500 RPM?) the extra rocker locks to the two other rockers controlling the two intake valves, so the valves now follow the enhanced lobe profile which controls the valves to close slower.



[Edited by SE-Rage on 03-01-2001 at 12:02 AM]


VTEC

Valve Ticks Engine Cracks.... LOL

[Nick]

Great site TypeIII!! Thanks for the recomendation!

Peace:D

[SE-Rage]

Originally posted by TypeIII

One of the BEST sites I've been to... It talks about VVT/L, mult-valve technology, forced induction... Actually they talk about everything that's related to a car. EXCELLENT reading material for us car freaks. I strong recommend you spending a couple of hours going through the site. *and they even had a section about the VR6!!!!* :D

Seriously sweet site. Oh, and I'm down with the VR6, the way they engineered it to fit was ingenious.

[Chris]

Originally posted by SE-Rage
Hmm, yes, there are SOHC engines that have 16 valves. But I'm not quite sure how a cam shaft can contain enough lobes to operate that many valves. If someone out there knows, please explain. I'm assuming that it is possible because of the extra lobe and rocker due to the VTEC system.

As for a DOHC engine with only 8 valves, I don't see the point. The whole point of having two camshafts per cylinder would be to have the ability to operate more valves. If you're going to do the "standard" 2 valves per cylinder, why add another camshaft? I'm not denying that it exists, I'm just questioning its usage.

In general, for inline 4 cylinder engines, a single cam design will have 8 valves and a dual cam design will have 16.


Its not the cam, its the rocker. The rocker splits into two.

[killazx]

what about the pushrods and lifters?