Why Does The Rgy/rgx Cam Have High Exhaust Lift?

[Razkin 0]

I have read several topics about cams for the RGY/RGX engine.

The DKZ cam is mentioned most and it has a good increase in max inlet lift, but the exhaust lift is less than the original turbo cam. I have not found a direct comparison including duration and I do not have the tools to do so myself. Tom Fenton has compared the 1.9/1.6gti cam to the turbo one and the duration on the exhaust was practically the same.

If the duration was shorter on the turbo cam I could imagine it would have to do something with cooling of the valve, other than that I can't think of anything that would explain it.

[petert 588]

DKZ inlet has same lift as D6B but less duration. The exhaust lobe is same as DFZ, thus even less duration. Overlap period is thus reduced making it ideal for turbo. A custom turbo grind would have a wider lobe centre line to minimise the overlap but still have decent lift/duration on the exhaust.

Edited March 8, 2012 by petert

[Razkin 0]

Yes, you don't need much overlap on turbo engines, so yeah it makes sense to use the DKZ cam as an upgrade. But why does the RGX/RGY cam have more exhaust than inlet valve lift? The theory as I understand it is that you need lesser combination of "time and lift"(in comparison to the inlet valve) that a valve is open on the exhaust stroke because the pressure in the combustion chamber is higer after combustion. That does'nt really change when comparing turbo and non turbo engines, I think.

It looks like PSA have uncessary given the turbo cam high exhaust lift, but that just sounds stupid on the otherhand the DKZ cam increases inlet mixture filling while having less time and lift to "exhuast" the gasses after combustion, but still makes more power.

[Tom Fenton 1,542]

Increased exhaust lift and/or duration, alongside a wider LCA, is common on turbo cams to help the flow on the exhaust side to partially counter the effects of the restrictive turbo, compared to a free flowing exhaust system. Wider LCA stops the turbo blowing fresh charge through and out of the exhaust on overlap.

[Razkin 0]

The original exhaust manifold is quite restrictive, so the high lift could be to aid flow. In other words it would also mean that when a DKZ cam is used the power increase in % is better when there is a less restrictive exhaust manifold, in theory at least. In reality there are more things you have to take in account.

[Tom Fenton 1,542]

When I said common on turbo cams I mean on a lot of turbo engines, regardless of exhaust manifold.

[Daviewonder 533 3 Cars]

Did Turbo Technics use the standard GTI cams or were they reground?

[wicked 103 3 Cars]

Where did you find that the turbo cam has more lift? afaik the turbo cam has less lift than the dkz cam.

[Tom Fenton 1,542]

I measured three cams, here is the graph.

[Razkin 0]

I have measured the RFX, RGX and DKZ cams with a slide gauge(total lobe hight - base circle).

DKZ max exhaust lift: 10,5mm

RGY/RGX : 10,9mm

 

The RFX has 10,8mm lift inlet and exhaust.

 

A slide gauge isn't the most accurate measurment, but it does show the difference between cams and most important for me wasn't the absolute lift, but the "combination" between inlet lift and exhaust lift on the cams.