AM cams?

[sprint32]

Hi guys,

would there be any perceivable advantage to running larger cams with a rb25 turbo on my rb20. I know that tomei poncams give another 30 degrees and more lift, would this give more rapid spoolup and possibly more top end power as i belive that the standard cams are choking even the stock power output in the top end no-matter what boost you run.

Thoughts.

[Merli]

Sydneykid has stated that stock RB26 cams are an excellent upgrade for RB20s, being longer duration and higher lift...

And I just happen to have a set for sale out of a low kms 1996 R33 GTR

PM me if you're interested. Will be a LOT cheaper than Tomei cams ($300 plus post)

[rb20-calais]

Merli,

Could be interested mate, anyone know the modifications and cost to get these fitted to an RB20?

Evan

[sprint32]

Whats the lift and duration of those?

[Merli]

--------------------------Inlet--------Exhaust

Timing--------------------240---------236

Lift-----------------------8.58--------8.58

Lobe Centre Angle---------113---------125

[Merli]

As for RB26 cams they fit RB20's, there is some benefit from adj pulleys as well. They do work beter if you also use the RB26 solid lifters to replace the RB20 Hydraulics. As for power increase, I have never done a bac kto back, but around 10 to 15 rwkw looks reasonable. That's max power increase but I think the average power increase would be more, maybe 20 to 25 rwkw.

[armbrusb]

Merli, do you know what the lobe centre angle is? I understand lift and duration but lobe centre angle?

[Merli]

Overlap is the portion of the engines rotation for which the exhaust valve is still open right at the end of the exhaust stroke, and the intake valve is also open to give a head start to the incoming charge. The exhaust gas is whistling out of the exhaust port at the same time the intake charge is starting to trickle past the intake valve. Lots of overlap works great at high rpm because more intake charge manages to cram itself into the cylinder, but lots of overlap will also make the engine run badly at low rpm, as exhaust gas manages to make its way back up the intake manifold, diluting the incoming air/fuel charge, and depositing soot on the intake runners, carburetor, etc. If you have more than about 40-50 degrees of overlap, you're looking at something more like a race cam than a street cam.

Lobe centre angle, on the other hand, is the number of degrees between the point where the intake valve is at maximum lift and the point where the exhaust valve is at maximum lift.

The two angles are related - for a given lobe design, as the LCA gets smaller, the overlap gets larger. Smaller degree LCA's usually give you lots of low rpm torque, but not as much high-rpm power. Larger-degree LCA's give you lots of high rpm power, but poor low-rpm torque.

[blind_elk]

Coincidentally, the very latest HPI (arrived in last night's mail) has a bit of a discussion on the various Nissan cam designs. The specs can be found on Tomei's website at www.tomei-p.co.jp/_2003web-catalogue/e090_camshaft-specs.html

A slight adjustment to Merli's explanation - the difference between the max lift of the inlet and exhaust lobes is often referred to as LSA (Lobe Separation Angle), because there is also another LCA (Lobe Centreline Angle), which is the point (in crankshaft degrees) of maximum lift of the lobe, and is generally the midpoint of the duration. LSA is half the difference in LCAs.

Julian Edgar's "21st Century Performance" has a good explanation of all the terms in camshaft design (despite him getting it wrong about the duration and Ex-close values for a RB26DETT).

[Warpspeed]

Some excellent explanations of cam terms chaps. The only thing I can add is to say that the amount of overlap a turbo engine can tolerate has more to do with how much total exhaust back pressure there is than anything else.

So if you run a stock turbo with small exhaust turbine and stock exhaust system, do not try to run too much valve overlap or exhaust reversion will kill power everywhere.

With a larger turbo you can run more aggressive cams. But it will be more peaky.

The worst thing you can do is overcam your engine. so big turbo big cam, little turbo little cam is the way to go.