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Outline for tuning VTEC / VTC

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I am in the process of getting ready to tune my 300ZX myself. First car that I will have ever tuned. The only thing left that I don't have a clear strategy for is the dialing the in VTC system. Its very similar to VTEC and just allows an advance cam position.


At what point in the tuning process should I start dialing it in? I don't need a detailed explanation but rather a macro view point on where this variable should be accounted for.


This sort of cam control is very easy to tune. Simply perform a full dyno run with the cam control switched on for the entire run and another with it switched off for the entire run. You'll end up with two lines that cross and that's where you want the cam control to switch. In some instances you may find that the VCT wants to switch on at low rpm, off through the mid range and then on again at high rpm (or vice versa). Either way, the technique I described will point you in the right direction.

I usually do this before I get too involved with the tune - ie just get the tune to a point where the AFR is roughly right and don't worry about getting everything perfectly dialled in since the VE change from the cam switching can have a big impact on the fuel/ignition requirements.

Normally a simple rpm switch is sufficient, however make sure that you include a little hysteresis in the switch point (perhaps 50 rpm) and if there is a dramatic change in Ve across the switch point it may be worth adding additional rpm sites closely spaced around the switch over point for better control.

I will just elaborate a little on the above technique. Once you have the tune optimised it's always worth moving the cam changeover point up and down 300-500 rpm to see if there is any advantage to be had. Often once the rest of the tune is optimised you may find that your initial 'rough' changeover point may benefit from a slight adjustment.


Thanks for the response this makes perfect sense. When you say not to get too detailed in dialing in the A/F / Ignition....what does that mean?

With the way that I have watched you tune A/F in the videos (500 rpm increments and increasing RPM / Load) does that mean make bigger jumps say tune ever 1500 rpm and linearize in between at first for A/F?

What about Ignition? Do I tune for MBT in larger increments and get to what I feel is within 1-3 degrees of MBT and then play with VTC and after I see the intersection then look go back over both maps and get a lot more detailed?


Just pick a reasonable point to make the cam switchover.... and then start fine tuning your fuel and ignition numbers. And then go back and experiment with earlier and later vtec switchover points (readjusting AFR and ignition for best torque to suit). Its very simple.

Or you can do as i did (for various reasons that may not apply to most people) and make one table for "off cam" and one for "on cam".

Two separate fully tuned tables. This way you can change your crossover points at will without the need to retune anything.

Tables are switched when cams switch.

But tuning two tables obviously takes more time...

When I'm tuning this sort of cam control system I'll actually tend to break the rules a little and often do the full load tuning first so that I can find the correct cam changeover points. When I say just a rough tune what I mean is that we can allow the engine to be richer (or a little leaner) than ideal and we don't need the timing optimised at this stage. If it was a naturally aspirated engine I'd probably allow the AFR to be anywhere from 11.0:1-13.5:1 and set the timing at some point I know won't give any knock - Perhaps 20 degrees for example.

This let's you quickly dial in the cam changeover point before filling in the rest of the map. It can be a time saver since you aren't going to waste a lot of time building up the part throttle area of the map that's in the wrong cam timing for example. This is a slightly advanced technique though and if you aren't confident then I'd still follow my typical procedure of building up the map. Just understand that you may end up doubling up your work at some points.

It's also worth noting that if you want to get really fussy, the cam changeover point at light load will quite likely be different to what the engine wants to see at WOT. While most will be satisfied with a simple rpm switch, you can set up a window switch instead based on load and rpm. For example you may find that the perfect WOT changeover point is 5500 rpm for example, but if you are at 25% throttle and 5750 rpm you may see a small improvement in torque if you are still operating on the low cam for instance. This is getting quite fussy and isn't strictly necessary but it's worth mentioning for the sake of completeness.

Thanks! I at least have a game plan in place now and will start playing with this once I am on the dyno.

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