[bazza]

Sorry Harvey, completely missed this thread.

Originally Posted by oab_au

Yes mate I have done a bit on retarding the inlet timing, and yes it does affect the rpm that the torque is produced.
This was on a Nissan 3lt, moving the inlet 20*.

Nice. That's exactly what I'd expect with 20 degrees change and is consistent with what I saw. With the AVCS engines I was playing with, I was advancing and retarding up to 45 degrees with massive changes in bottom end response and power. One of the very handy things was being able to "un-restrict" the engine while boosting to remove compressor surge. Basically a situation where the engine is not able to flow the air the compressor is supplying and starts stalling the blade and pushing back out past the blades. Basically you'd open the engine up (retard the cam back to 0) and the surge would go away.

As you can see it picked up some top end torque, but the loss at the bottom end cost in the "off the start" acceleration.

When you are working out what type of cam timing to use, you have to look at where you need the torque to be in the rpm range that you are going to use. If you are using the standard inlet manifold, and still running the higher diff ratio, you need the torque that it provides between 2200 and 4000, then the 240* timing has to be the same to allow the Inertia system to work. The only change that will benefit is more lift. Both cams can go to 8.75mm, the inlets giving the best increase.

If it is to be a high rpm engine, with the torque from 4000 to 6000, or higher, running a low diff ratio, then you will have to do away with the low end torque from the Inertia system, and just tune to work in the higher rpms. To do this, either the standard inlet runners will have to be shortened, or 6 tuned length throttle bodies will be needed, to allow. The inlet resonance to suite the higher torque peak.

There are two cam functions that control the engines breathing. Valve lift will allow more air to enter the cylinder at all engine speeds, great for higher engine speeds, but will degrade the inlet velocity, that will hurt combustion at lower engine rpms. If there is less turbulence in the chamber then all the mixture will not burn and power will drop.

The other function is Duration, which is really the end result, of setting the valve timing points that the engine needs, to allow the torque to peak to arrive at the higher rpm. The air does not accelerate as fast as the piston, lagging behind as the rpm rises, and the inlet valve has to close when the cylinder pressure is at the highest.
At the 4800 rpm that the torque is at now, the inlet valve closes at 54* After Bottom Dead Center. As the engine speed rises the maximum cylinder pressure arrives letter when the piston is further up the cylinder. So we have to close the inlet valve about 10* latter to trap the highest pressure in the cylinder.

The exhaust valve timing, besides allowing the cylinder pressure time to drop, has to start the pressure wave that will start the inlet flow. Advancing the exhaust opening point from the 55* Before Bottom Dead Center, to about 65* BBDC to drop the pressure.

Adding boost from a supercharger will not change things greatly. A bit more exhaust opening advance may be needed, depending on how much boost is used. A Turbo does loose the exhaust action to start the inlet flow, as there is no resonance wave to help. This engine needs the inlet valve opened earlier to keep up with the engine speed, but if too much overlap is used, the exhaust gas pressure can interfere with the inlet phase.

Harvey.

This is some pretty awesome information.

With turbocharging things change a little. You obviously force the air into the piston and when that valve opens you've got X psi of air forcing itself into a vaccum. So the issue with the air not accelerating as the piston moves down is very much reduced. You then have the exhaust gas powering the turbo, then you have turbulent airflow past the intercooler so you prolly tend not to see the fuel mix issues you would with an NA motor.

One major concern I have with the stock cams is that due to running hydraulic buckets and the associated ramp effects, the known durations of 230-240 (or whatever they are, sorry cannot recall off top of head) they really become more like 200-220 degrees or thereabouts if I'm not mistaken. Most likely why during testing as boost was increased peak power kept moving to the left which is very consistent with the other results showing the cams out of puff. Of course this is not fact and I also wonder if the valve seat pressure and or bounce possible contributed - however then again the RPM was very low - only 5500 rpm etc.

Also one other thing is the resonance system. Once on boost I don't think it makes any difference - the exhaust gas is powering the compressor and the compressor is forcing the engine to do some work etc etc. However from testing I've noticed under 0 psi (vacuum) and coming up onto full boost it does make a huge difference to the response and feel. Very hard to produce good data though and since the engine is boosting, quite tricky to figure out how to control the vacuum driven iris, hmmm. Could put a wastegate actuator there instead I guess?!

Anyway a friend of mine is in the process of flowing his EG33 heads, measuring everything and getting some cams made up. The cam shop has requested the current cam configuration and cams so they can measure the stock setup and go from there. Our plan is to use solid buckets and later model valves (most likely supertech). I'd be very keen to hear any advice you have regarding this. I am thinking of following something along the lines of 256 duration with 9.5 mm lift, aiming for peak power at 7000 rpm.