Camshafts for the EG33.

[oab_au]

This topic came up in the High Compression thread, so to keep them apart we can talk about it here.

Bazza

That's right mate, standard cams at this point in time. They turn rather asthmatic around 550-600 bhp which from what I recalling Ecutune Mike saying it's mainly the exhaust cam.

Bazza

Does anyone have any testing behind this with the EG33 NA motor and the results before and after dialing in the cams? Reason I ask is I don't think it makes any difference on our motors and if anything actually gains top end.

Reason why is I've done a fair bit successful AVCS tuning on the intake cam (02-07 STI heads). As most know it effectively advances the intake cam during low end RPM and you wind it back to zero as revs and air flow climbs.

The result is a massive improvement in low end torque. 0 advance gives the best top end (no bottom end) so theoretically a couple more degrees of retard will help with more top end. Also the exhaust AVCS works by retarding the cam as the rpm and airflow climb - retarding makes more top end power.

So in english, decking the head in an EG33 and the resulting timing belt effects which cause overall retardation of all cams should if anything make the car more powerful up top due.

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*.



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.