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VCT vs. fuel economy

Variable Cam Control Tuning

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Hello folks

I have a CA20DET where I've adapted the RB25DE VCT on a custom made camshaft. So far its working as it should.

My question is:

How does 30° inlet cam advance affect fuel efficiency?

Because as far as I know the VCT on the RB25 is just engaged from approx. 1300 rpm up to 5500 rpm, but it doesn't consider engine load. So even in very light cruise it will be ON in that rpm range and giving that 30° inlet cam advance. I could imagine that the large In-EX overlap isn't beneficial for economy.

My cam specs are:

IN: 248° duration, set at 120° center lobe (90° when VCT is engaged)

EX: 248° duration, set at 119° center lobe

So far I am happy with these cam settings, but it hasn't been tuned in on the dyno yet.

generally on std cams the best ecomomy will be to eave it off until you reach 0 vacuum so i would normally use three conditions >1200 rpm 95kpa or 20% tps either works well

Regards Ross

Thanks Ross for your answer.

That's what I've imagined.

But strange that it's not controlled like that on the original ECU of the RB25. Or maybe my info on that is wrong and it does consider load as well.

Not so familiar with the engine, but can you check if "on" actually advances or retards the camshaft? Some manufacturers use one, some the other.

Something to consider, though, is that while it's advancing the opening point, it's also advancing the closing point, which will reduce the reversion and increase the dynamic compression ratio. In the end, the trade-off may be well worth it.

Is there any exhaust camshaft variation, as that also will reduce overlap at the cost of some loss of cylinder expansion on the piston towards the bottom of the stroke - might, again, be worth the trade-off as the lower rpm may mean most of the useful work has already been extracted from the fuel?

Anyone else?

It's definitely like that: switching ON does advance the IN cam by 30°.

The EX cam is fixed. Well I have a vernier pulley on it, but no variability during operation.

I might need to daily drive the car from fill up to fill up with VCT switched ON and then VCT switched OFF. That way I could see how it affects fuel economy as the engine would for 99% be in the rev range where VCT is usually switched ON (approx. 1300-5500rpm).

I've done the test. VCT permanently activated increased fuel consumption just by very little. Around as little as +3% what is almost the measuring tolerance of that measuring method. I am surprised. I've thought it would be much less efficient with such an overlap.