sc2 engine intake bore (small) sc1 intake bore (large) why??

[fixityourselfer]

the holes going into the engine on the intake side seem like really big square holes.

the holes on the sc2 engine are really small. roundish and smaller then on the sc1 sohc engine.

whats up with that??

I got to look at my dohc intake holes when I took the manifold off, and I was surprised to see that the intake manifold gaskets that go on the sc1 sohc engines have way bigger holes for the air to go into the engine.

thats wierd, no?
bigger holes means MORE power, and yet the dohc sc2 engine has SMALLER HOLES!!

can anyone explain that?

[madpogue]

Are you talking about the intake PORTS between the intake manifold and cylinder head? You do realize there are TWO for each cylinder on the DOHC, and ONE on the SOHC, right?

[PlasticCarsRock]

With twice as many cams and valves, the head is much more complex with regards to cooling and lubricating. There isn't as much spare room in the DOHC head to allow for larger ports.

Even with the smaller ports, Saturn was able to achieve the extra power they wanted. Given the complete lack of any performance oriented re-designs throughout the lifespan of the S-Series, it's obvious that Saturn had no desire to increase the power beyond 124 hp.

The port design (size and sharp angle) is one of the biggest problems with the engine when it comes to trying to make any significant power (without boost).

Also, smaller ports actually help with low-end torque (because of the increased velocity of the air). It isn't until the higher rpms that the restriction becomes a problem. If you enlarge the ports much (such as gasket-matching), it is typical to notice a decrease in low end power, but the increase in the top end makes it worthwhile for most people.

Originally Posted by madpogue

Are you talking about the intake PORTS between the intake manifold and cylinder head? You do realize there are TWO for each cylinder on the DOHC, and ONE on the SOHC, right?

There's still only one port where the manifold meets the head--it splits into two right there, though. I've never seen a bare SOHC head, but I have seen the gasket for one, and I believe that the ports where the manifold meets the head are, in fact, much larger (and square). In other words the SOHC ports are larger than both DOHC ports combined. The SOHC ports obviously must get much smaller as they near the valves, though, and the change in port size is not such a good thing (better than smaller but even ports the whole way, perhaps, but not much better).

[td1238]

Part of the trick is fuel vaporization. Long, narrow intake runners also achieve this. They create high vacuum, which aids in fuel vaporization, much velocity, which stirs up the fuel, and a long runner for the fuel to vaporize in (assuming that the runner is hot). The last benefit doesn't apply so much to MPFI engines, where the fuel is injected right at the intake valve. It applies more to TBFI and carbureted engines.

The long and/or narrow intake runners of course restrict WOT airflow, and reduce power here, but they make for great low-end power and efficiency.

Some high-end designs call for variable length intake runners, or switching between runners of different shapes and sizes.