Racecar100

Hi All
I am setting up my S2k with a HKS supercharger kit. Using an AEM series 2 Plug in ECU. The car is a dedicated competition vehicle and only used on the road to drive it to the track. My question is how should the Vtec be controlled. The AEM allows both RPM and throttle position control of engagement. I think the factory system uses RPM and manifold absolute pressure for control. What I am struggling to understand is why there is a load axis (TPS or MAP) control. I would have thought that the VE change that happens with camshaft profile change would be solely dependent on RPM. Is the load axis control just a fuel economy or emissions consideration. The reason that this is of interest is that the AEM treatment of fuel changes for Vtec is pretty rough. Rather than have a completely separate map for when Vtec is engaged it only offers a set % of enrichment when the engine is in vtec. I have read posts where it looks like this leads to drivability problems and I did notice that throttle response was not great when heal and toe was used with the car in NA trim. My idea is that if there is no advantage other than emissions or fuel economy I can set the TPS conditions so that they are never met and the engagement is solely based on RPM which will give me full control of fueling. Any thoughts?

jflies

Sorry nobody's answered you yet. I don't know the answer to your question but I can think of a few issues with making VTEC only RPM dependent:

- If you lower VTEC engagement RPM to something like 4000, then you would be cruising on the freeway in vtec. Noise and low MPG might get annoying.
- Unnecessary VTEC engagement will put more wear on the valvetrain. I'd imagine the springs, retainers, cams, etc., not lasting as long as you'd expect for a Honda.
- The valvetrain losses will be higher in VTEC, and the longer duration may allow the engine to pull more air past the butterfly, further increasing these losses when decelerating. With extra engine braking on the rear wheels, the car might be more susceptible to lift-off oversteer.

Hopefully someone more knowledgeable chimes in. Good luck.

RolanTHUNDER

Stock ECU takes into account throttle % as well as RPM and MAP readings. It is load dependent.
The AEM offers the same control/parameters. My car is a dream to heel/toe with no issues in terms of response and I'm still NA (AP1, bolt ons and AEM Series 2 ECU tuned).

I believe my VTEC TPS is set to on above 60% and off below 40% with RPM allowing VTEC on above 5000 rpm and off below 4500 rpm. I want to change this by reducing both figures by 500 rpm respectively and see how that feels. Corner exit is more Rock 'n Roll now thanks to such a strong midrange

RolanTHUNDER

The valve train and cams are designed to be stronger than required for VTEC all day, every day. Thanks to TPS having a say in whether VTEC is on or off in addition to engagement RPM, when you're cruising the chances are high that even if you're at or above the set engagement RPM you will not be in VTEC since you are most probably around 20 to 30% throttle in a typical cruise scenario. The benefit will be that as soon as u give it more than the set TPS % it will engage VTEC making for quick overtaking or just that pure joy the system gives you when u feel it kick in.

Tuning is about fine control over the machine. Mastery over the hardware without limitations.

Chuck S

Old thread but @RolanTHUNDER has valuable corrections to incorrect assumptions.

My VTEC is set to 3600 rpm and I can cruise on the highway at 79mph * at about 4000+ rpm on the economy cam, not the VTEC. VTEC engages with pressure on the accelerator pedal, no downshift needed for much highway passing.

-- Chuck

* I never exceed 79mph since Virginia thinks 80mph is a felony and radar detectors are illegal.

RolanTHUNDER

That's it Chuck. Glad to see someone else has first hand experience with this as well.

Slowcrash_101

Is VTEC oil pressure dependent? I would imagine so as engaging VTEC lowers oil pressure by a certain amount. I believe Honda put's VTEC where it does because it doesn't want additional wear to the engine. At 6,000rpm you have a much higher chance of having hydrodynamic oil film vs a lower rpm. VTEC engagement will produce slightly more spark knock combined with less oil pressure. Older engines with sticky oil jet check valves would be more at risk from lowering VTEC, as they'd increase the tendency for LSPI. To the extent SN oils protect from these things anyway.

Chuck S

Oil pressure spec at 3000 rpm is 85psi and is the only pressure spec near where VTEC will apply. VTEC will not engage unless the engine is "warm" but I've not seen any specs nor know exactly how it works.

I'm personally convinced (with no inside knowledge) Honda set VTEC at 6000rpm strictly for the gimmick VTEC Yo! effect -- sudden burst of power. A sudden 25 or 30 HP hit is noticeable as the power jumps from the low power curve (looking at a dyno graph) to the high power curve. There is no VTEC Yo with my 3600rpm engagement under WOT other than change in exhaust note and the power continuing to smoothly and quickly climb. Power just slides seamlessly from the low power curve to the high power curve and builds quickly. So quickly in lower gears I fitted a red line alarm beeper as my eyes are too busy outside the car to watch the tach.

And, as a reminder, the car does not run on the VTEC cam at much less than WOT -- pedal to the floor. It's a both-and. Both rpm and engine load.

-- Chuck

RolanTHUNDER

High oil pressure required yes, but not dependent as such. It will engage if RPM and TPS load match given values. Bear in mind that our cars had catalytic converters as stock. Those heavy things forced the engine to lose power if VTEC is engaged below 6000 rpm and Honda could not release the car with lower than expected power or weird power loss during VTEC. Oil pressure climbs very quickly with rpm until it hits a maximum. I'm not sure what the figures are between 1000 to 2000 to 3000 rpm for example but i imagine by 3000 rpm there is more than enough oil pressure for safe VTEC engagement. If not, then Chuck is playing with fire since his VTEC engages at just 3600 rpm lol.

Someone can chime in on exact oil pressure per 1000 rpms but another thing to consider is oil viscosity variances. We all use different oils and oil thickness influences oil pressure too.

Chuck S

Never seem a graph plotting S2000 oil pressure vs rpm. Only specs available are 36psi at idle and 85psi at 3000rpm. Would be interesting.

Nor have I seen evidence VTEC engagement does more than a short, temporary change in oil pressure/oil flow all the solenoid does is divert a small bit of oil to the VTEC cam and hold it there with oil pressure. Using a water hose model turning on the valve with the other end on the hose sealed only permits a small flow until the hose is filled and pressure returns to whatever it was to start with. We're talking about no more than a couple ounces of oil not what's in 100 feet of garden hose. Higher flow means less pressure but there's no flow within the VTEC cylinder -- or my capped garden hose -- once it's full. Again using home water to illustrate; When a contractor severed one of my underground lawn irrigation PVC pipes the water just flowed out without the restriction of the sprinkler heads and pressure in the home dropped to the point my faucets just trickled and my washing machine would not run. As soon as the irrigation timed zone shut off pressure was back. This irrigation zone only ran for a few minutes a few times a week just before dawn and I didn't notice it for weeks other than a persistent wet spot in my lawn.

I don't think there's any oil flow change hence no pressure change, just diversion to the dead end VTEC cylinder. Oil pressure to the engine should not be affected regardless of VTEC or not.

I await more knowledgeable information.

-- Chuck