Cams and valve timing are actually affecting your effective compression and expansion ratios (which are quite different from the static compression ratio of your pistons). In a "theoretical engine" your cam would have a tiny 180 degree duration and instantaneous ramp rates (the valve opens to its full height immediately), and the intake and exhaust valves would open and close at the top and bottom of the stroke.

But because the air has inertia, and you can't open the valve immediately with a cam, you need to open the valves earlier so that the air starts moving before the piston starts down, and close them later so that you have the valve open while the air is still moving. You can also overlap the valves so that the air moving out of the exhaust "sucks" air in from the intake to get it started moving faster (aka scavenging)

All of those events mean that the actual compression ratio is never as high as the "static CR" of the pistons. This is why longer duration cams allow you to run more timing advance (because you've actually dropped your compression ratio even more). Alternately, you can bump the static CR ratio up higher to get back some of the actual CR that you lost. The longer duration the cam, the higher the static CR you need to get the same "actual" compression. (This is why people get away with running CR's of 9.0-10.0 while using large cams, but the same CR with shorter/stock cams are more likely to have detonation problems).

The general effects (broken down) are as follows:

Advance centerline (both cams): powerband moves to lower RPM
Retard centerline (both cams): powerband moves to higher RPM

Increase overlap (retard exhaust, advance intake): Sharper (higher) power/torque peaks, narrower powerband
Decrease overlap: (advance exhaust, retard intake intake): Flatter power/torque peaks, broader powerband
You are basically balancing valve opening and closing events to trade between stroke efficiency, effective compression (and expansion) ratios and cylinder filling efficiency.

Since the intake valve closes after the bottom of the intake stroke, closing the intake valve earlier gives you a longer compression stroke, and higher peak cylinder pressure.

But at higher RPMs the valves are open for a much shorter period - with the later intake opening there is not enough time for the cylinder to fill so the peak pressure actually drops off.

Likewise, the exhaust valve opens before the bottom of the exhaust stroke, so opening the exhaust valve later gives you a longer more efficient expansion stroke.

But at low RPM the exhaust valve closing later (while the intake valve is open) causes some of your intake charge to pass through the engine, and get vented into the exhaust manifold.


When you just move one gear, you're making two different adjustments, which can be good for specific situations. Advancing or retarding the intake gear only is an effective way to adjust engine behavior, without really messing everything up.

Advancing the intake gear simultaneously moves your powerband lower by moving the advancing the centerline (towards the RPM range of your turbo spoolup) and increases overlap, increasing the airflow at that point - you allow more air to pass out of the exhaust valve, helping the turbo spool and giving you a stronger, but peakier midrange. Obviously this ends giving you a up costing you on the top end because the earlier intake close reduces cylinder filling.

Retarding the intake gear simultaneously both moves your powerband higher (keeps the intake valve open longer) but also increases your compression and peak pressure, making the powerband broader and more drivable. You lose a little potential power due to lost overlap, but give the engine better manners in the mid to top end.

As you might guess, there's a lot of stuff going on, and a lot of compromises being made. The only way to really find the best setting is to decide what you're trying to do, and try adjusting a couple degrees at a time until you find what you want. There's no ideal setting for every part of the powerband (This is why variable valve timing is such a brilliant idea - you can, with certain limitations, find a more ideal setting for every part of the powerband)

Smaller turbos like advanced, shorter duration cams because they run out of breath up high, while big turbos are better with retarded, long duration cams because they spool up much later, and produce more power at the top end.

To make it even more complex, the cam settings on a turbo engine will vary based on where your turbo hits its sweet spot, and what kind of characteristics you're trying to get out of it. OEMs tend to put shorter cams with a peakier low end with small turbos, because you can make up for the loss at the top end by simply adding boost and throwing in taller gears. They do the same thing for auto transmissions. N/A MT cars tend to run higher RPMs, longer duration cams for more power, and shorter gearing. Ironically, cars with bigger turbos get tuned more like N/A engines.

Another sidenote is that a in a stroker engine, displacement is increased, but the valve events are happening at the same RPM as a normal stroke, you're just trying to pull more air in. As a result, the stroker will like a bit more cam duration and earlier valve opening due to the piston speed, but less overlap is required. This in turn gives you the characteristic of a broader torque band at lower RPMs (in addition to the increased torque from crank length). Likewise, it has a harder time breathing up top.

I guess I didn't really answer the question, because there's no "perfect" setting, but if you understand a bit about what's going on, you can experiment and find one you like with any type of cam.

The 3D maps are great for details but sometimes simplifying things can make them more readable by just plotting the load path your car takes through the map. The low end RPM range will be different depending on how laggy your turbo is but the mid and upper RPM range tends to be very similar at 200%-320% load no matter what turbo you have. Here are the maps in a simple table format for a 2000-8000 rpm WOT pull. I grouped the RPM's into 3 phases: Low, Mid, and High RPM's.

Hiboost Stock Turbo Mivec
---------Intake----Exhaust----Overlap
2000 --- 30 -------- -20 --------- 50
2500 --- 30 -------- -20 --------- 50
3000 --- 30 -------- -15 --------- 45
3500 --- 30 -------- -10 --------- 40

4000 --- 25 --------- -5 ---------- 30
4500 --- 25 ---------- 0 ---------- 25
5000 --- 25 ---------- 0 ---------- 25
5500 --- 25 ---------- 0 ---------- 25
6000 --- 20 ---------- 0 ---------- 20

6500 --- 15 ---------- 0 ---------- 15
7000 --- 10 ---------- 0 ---------- 10
7500 --- 10 ---------- 0 ---------- 10
8000 --- 10 ---------- 0 ---------- 10

Overlap is just a way to check in Mivec degrees how much relative overlap there is. The JDM maps had 45 tapering to 30 in the low RPM's and 25-30 in the mid and upper RPM's as reference.

Here are the general RPM ranges these effects would apply:

2000 Increase overlap (retard exhaust, advance intake): Sharper (higher) power/torque peaks, narrower powerband, Higher EGT, Great for spoolup
2500
3000
3500 Advance centerline (both cams): powerband moves to lower RPM

4000
4500
5000 Decrease overlap: (advance exhaust, retard intake intake): Flatter power/torque peaks, broader powerband, Lower EGT
5500
6000

6500 Retard centerline (both cams): powerband moves to higher RPM
7000
7500
8000

Here is the map that gained me some power on a Larger than stock turbo:

Hiboost GT30 Turbo Mivec
---------Intake----Exhaust----Overlap
2000 --- 30 -------- -20 --------- 50
2500 --- 30 -------- -20 --------- 50
3000 --- 30 -------- -15 --------- 45
3500 --- 30 -------- -10 --------- 40

4000 --- 25 --------- -5 ---------- 30
4500 --- 25 ---------- 0 ---------- 25
5000 --- 25 ---------- 0 ---------- 25
5500 --- 25 ---------- 0 ---------- 25
6000 --- 20 --------- -5 ---------- 25

6500 --- 15 -------- -10 --------- 25
7000 --- 10 -------- -10 --------- 20
7500 --- 10 -------- -10 --------- 20
8000 --- 10 -------- -10 --------- 20

This map gained me some power from 6000-8000 over my other map. Adding an extra -5 exhaust retard from 5500-8000 lost power again so it seems to be about in the sweet spot +- a few degrees. I also tested reducing intake advance by 5 degrees starting at 4500 and it also lost power.

My next test will be adding 5 degrees of intake advance from 3000-7000 to see if any RPM points have any gains. This map will be closer to what "BigT" was testing so we'll see if the larger turbo gains any HP from it.

Hiboost GT30 Turbo Mivec - Testing Advanced Intake
---------Intake----Exhaust----Overlap
2000 --- 30 -------- -20 --------- 50
2500 --- 30 -------- -20 --------- 50
3000 --- 35 -------- -15 --------- 50
3500 --- 35 -------- -10 --------- 45

4000 --- 30 --------- -5 ---------- 35
4500 --- 30 ---------- 0 ---------- 30
5000 --- 30 ---------- 0 ---------- 30
5500 --- 30 ---------- 0 ---------- 30
6000 --- 25 --------- -5 ---------- 30

6500 --- 20 -------- -10 --------- 30
7000 --- 15 -------- -10 --------- 25
7500 --- 10 -------- -10 --------- 20
8000 --- 10 -------- -10 --------- 20

If anyone wants to test this last map out on a FP Red/Dom2/GT30 turbo I would be interested in your results as well.