Anyone?

So once you dial the cams in; what is the most common/recommended amount of degrees to be retarded or advanced for each cam to get best all round performance (Low and mid torque is my preference). Just like the settings I mentioned earlier for 272/272s where cars with those cams (all dialed in) and then set to either -4,-1 or -3,-3 responded in the same manner meaning they experienced Lots more torque down low and in mid range and very little loss up the top and running E316g.

Any correlation there?

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.

Thanks for taking the time to answer my question Dialcaliper. Truly appreciated.
This is great in depth info. I always say that it is good to either have a mental picture or a real animation of the engine in motion to fully comprehend what you are talking about.

Ps. Did you take away some fuel down low on your 264/272 set up or do you just let it run rich with this overlap at idle.
Also, how much base ignition timing are you running? I have mine set to 10 and I know it could take more without knock but I am not sure if I should without Dyno run? Any advice there?

Cheers

Hey BoostedAWD91 I think that the following thread might answer your question.

click

Quoting peter_pie:

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Also, how much base ignition timing are you running? I have mine set to 10 and I know it could take more without knock but I am not sure if I should without Dyno run? Any advice there?

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Honestly, I have never seen a GVR4/DSM on that can run that kind of base timing on a stock ECU. Our timing maps are fairly aggressive from the factory and in order to make the most of the available octane in a street car you'll probably need to run 5* (stock) or less. I realize you're in AUS so you might have different octane levels than we do in the U.S. but 10* base sounds like it'd even be a lot for race gas (116 octane).

John

Peter, don't forget that our 98octane is about the same as 93 - 94 in the states as we us RON which is a different measurement.
And I also think that 10deg base timing is too high for that octane rating. Bu if it is working for you then that's great

That doesn't make any sense. If you had the CAS off 180* then the timing wouldn't be off 20*. It sounds to me like you are misreading your timing marks or not putting the ECU into base timing mode. I can see the cams having a small effect on the amount of timing you can get away with but there is no way that you were running 20* base and only having a little knock. What are you basing these timing numbers off of? I'm assuming its off a timing light, right?

John

Quote:

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Later when I had the timing light I checked the timing (it was done the right way too) and there we actually 3 of us DSMers (so we call our selfs ) when this happened and we all were shocked at the result of around 20* base timing . So we quickly tried to retard the timing but the mark was going back and forth, nothing constant but ruffly between 15-25*

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You need to ground the timing-adjustment wire before you check base timing.
Otherwise, that is exactly what you will see. The ECU does not really run 5-degree timing at idle ( after the car is warmed-up etc ). The 5-degree is just the base timing, and the ECU adds timing on top of this base timing even at idle.