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Camshaft stages Vs. Different altitudes

Engine Building Fundamentals

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Hi guys,

I'm building an forged 2.3 ford ecoboost engine and I have a question about the camshaft stage I should run because of something I've heard. The only thing I haven't bought are the camshafts, but I already ported all the head, double springs, bigger valves, in fact all you can do for this head.

The question is I'm at 2,400 meters altitude, so this guy told me that the highest camshaft stage doesn't work at this altitude, in fact there is not the gain it supposed to give at sea level and I may get better results with a middle stage. I guess is because of the air density, bigger valve lift and duration will get more air but with less oxygen which isn't effective, am I right? I just would like to understand it a little bit more.

Thanks in advance.

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Who exactly did you talk to and what is this person's qualifications? The short answer is, bigger cams are going to work the turbo harder in non high rpm areas (any rpm that the cam grind and VVT angle are not at optimum breathing). High altitude amplifies that effect. You need to match it with a higher flowing compressor wheel and you need to re tune your intake VVT.

From a perspective of engine performance, the most important number (assuming a fixed VVT position) when picking cams is the intake valve closing timing, in degrees after bottom dead center. A higher number means the piston is at a higher position in the compression stroke when the valve is closed. I circled that in red on your screenshot.

Probably the second most important is the exhaust opening timing in degrees before bottom dead center. A higher number means the exhaust valve opens earlier in the expansion stroke, which reduces backpressure at higher rpm and boost (but is worse for fuel economy and low end torque). I circled that in blue in your screenshot. You can see that each camshaft has a later intake valve closing timing (longer duration) and an earlier exhaust valve opening timing (longer duration). At higher altitude, the pressure ratio for the turbo (Y axis of compressor map, see GTX30 map attached) increases and the turbo speed increases. When the intake valve timing is earlier or later than optimum for that rpm, the turbo has to work harder.

The bigger grind of the cam is going to make it harder to get the right VVT position for lower rpm. You will be running Miller cycle with low effective compression ratio, similar to what a Fiat Multi Air engine does or one of the old Mazda Miller cycle engines does. If you look at MAF.png I attached, it shows how volumetric efficiency % changes with cam duration and intake VVT at 2400rpm on a non boosted engine. Too big of duration will hurt your breathing, requiring more boost to make power.

On the exhaust side, the bigger cam will help with higher altitude. This is because the turbo needs to work harder at high rpm, and earlier exhaust valve opening relieves the backpressure better. Again though, the VVT needs to be tuned.

TL;DR You can use the big cams, but you need a bigger turbo and retuned VVT maps.

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