DavidNJ

I always thought VTEC had two intake lobes and one exhaust lobe. The ECU switching up around 5600rpm or based on throttle position and load and switching down at a lower speed after a slight period of less throttle, lower load.

However, when I look at cam specs I'm seeing three complete sets of numbers. Marked VTEC, Pri, and Sec, one example shows 238/240 on VTEC, 214/214 on Pri, and 198/198 on Sec. Lifts were .316/.304, .250/.243, and .186/.186.

It sounds like three different lobes for the intake and exhaust. I am very confused. What is going on?

Thanks,

David

P.S.
That cam is only an example. A B18 cam from the Comp catalog.

SC_Highlander

Yes, each cam has 3 lobes -- 2 that directly 'drive' the valves at low rpm, and one that 'drives' a common follower, which is connected to the valves only while 'in VTEC'. Here's a good description: http://auto.howstuffworks.com/framed.htm?p...vtec/index.html

DavidNJ

Ok, let me see if I get it.

Both the intake and the exhaust have VTEC lobes. The VTEC lobes are the same for both intakes, and the exhaust is the same for both exhausts.

The non-VTEC lobes are different for each valve, especially the intakes. So one valve is labeled primary and the other secondary. They operate together. The slightly different timing separates the flow cones past the valves so they don't interfer with each other.

Is that correct?

Now the next question. While the catalogs have a lot of B18 and H22 camshafts, there are next to none for the S2k (F20/F22, correct?). Who, preferably US camshaft grinder, makes cams for the S2k?

Thanks,

David

Greenlight

The non-VTEC lobes (Pri & Sec) open with slightly different valve timing to promote swirl in the cylinder at low RPMs.

A 4 valve engine typically has good in cylinder tumble, but poor swirl ( a two valve engine has good swirl, but poor tumble). The swirl/tumble is necessary to increase the turbulence in the cylinder. This is VERY critical at low RPM.

At low RPMs the velocity of the mixture in the cylinder in not sufficient to promote complete combustion, so the manufacturers try to increase the in cylinder velocity/turbulence using a variety of methods.

In order for the Sk2 to make the 240 hp the cylinder head port has to be relatively large. The trade off is that with such a large port the velocity in the port at low rpm is too low. Because of this and the relatively low piston velocity the velocity/turbulence in the cylinder is poor at low RPMs. Honda chose to have staggered valve timing to increase the turbulence in the cylinder.

At high RPMs the increased piston velocity causes the in cylinder mixture to be very turbulent, so there is no need to have two separate VTEC lobes with staggered valve timing. The port size/shape allows good port velocity (~250-275 ft/sec) at 8300 RPM.

Regarding US Cam Manufacturers, the only one that I know that is making cams for the S2k is Cam Motion, in Baton Rouge, LA (ask for Kip).

DavidNJ

Are there any cams on the market that work better than stock? With VTEC, it seems like you don't have the normal low speed/high speed trade-off.

Greenlight

You are correct, there are virtually no trade offs to make with VTEC.

That's why Honda was able to get it right the first time. There are no emission requirements at WOT, so Honda was able to extract the maximum power/durability for this combination.

However, when you change the OEM combination (i.e. improved intake/exhaust flow, increased compression ratio, etc.) a camshaft change will be required for maximum performance.