I sure thought they were.. I've read it countless times.
What i've heard is this:RB26dett cams are a great upgrade for the RB20det head. They have similar duration, but increased lift, which allows for more airflow. They may not help much at low rpm's, but really help at high rpm's. They are especially helpful if you have adjustable cam gears!
That's the myth...
I have a set of adj. cam gears already, and picked up a set of GTR cams from a friend (thanks Karl!).
Since I've learned that it's always best to do a lot of research before trying anything out, I poured myself into a few NICO threads.. .which eventually directed me to a Skylines Australia thread.. http://www.skylinesaustralia.c...+rb20
Started reading, and heard some great things!
But alas.. after digging into a the latter pages, turns out this mod doesn't work so well.
Here's the problem. The RB26dett has solid lifters. The RB20det has hydrolic lifters. The angle of the lobes (hence timing) is slightly different on the 26.
Here's the post that explains everything incredibly well.. and is worth the read even though it's long.
So in summary. RB26 cams don't work well unless you swap the RB20 over to solid lifters. Which still aren't quite right for the RB20 valve-train. Ultimately, the best option is a set of lash type Tomei Pro-cams (designed spec. for hydrolic lifters). You can get them from NICO sponsor RHDJapan for a bit over $600 including shipping.Warpspeed from SAU wrote:Well, I have never tried to fit an RB26 cam to an RB20, but I have a pretty fair idea of why you guys are having so many problems, so here goes:
First thing is that there will be a production variation between the sizes of the cam bearings. Not much, but then the bearing clearance is not much either, maybe a couple of thou. You cannot just swap cams without checking the bearing clearances. Well you can, but it is NOT a very good idea. You will never notice a sloppy bearing, but a tight one is going to be a real drama. So buy some plastigauge from Repco and measure the clearance of each cam journal with each cap properly torqued down.
The extra lift is so small it is hardly likely to bind the coils, but check it anyway. Turn the cam so each lobe is at maximum lift, and see how much further you can push down the follower. You should be able to get a 2mm gap at least before the valve spring goes solid. If not, you risk serious engine damage.
The cam drive gears ARE different between Skylines, so you cannot just bolt any old gears onto your cam and expect to get the exact factory timing. If in doubt get some adjustable cam gears, and take the time to set them up to Sydneykids figures.
The next problem is what has really defeated you guys and why your cars will not idle.
A hydraulic cam has no tappet clearance ramps. The base circle of the cam is flat, then the valve opens instantly at the specified opening point. Likewise it closes pretty suddenly back down to the base circle. Because the hydraulic lifters take up all the clearance, there will be no tappet noises.
A solid lifter cam is VERY different. From the flat base circle, there is a very shallow clearance ramp that begins very early and SLOWLY closes up the tappet clearance. Once the clearance has been taken up, the valve really starts to lift quickly. Likewise the valve drops fairly fast with the tappet clearance still all taken up, and there is an exit ramp that slowly opens up the tappet clearance.
These starting and ending ramps are very long, but only amount to perhaps half a millimeter of cam lobe lift.
If you fit a solid lifter cam like the GTR cam to a hydraulic lifter, like in an RB20, the valve timing at the beginning and ending is changed radically.
The hydraulic lifter goes solid, and the valve starts lifting off it's seat very early, much earlier than it should. The initial valve lift is extremely low, but the valve will still leak. Likewise during valve closing the valve is held just off its seat for an extended time.
What this means is that during engine idle, both valves will be open together for quite an extended time, but only just open. At 7,000 Rpm there is not enough time for any serious leakage. But at idle Rpm the valves will leak badly, because of the very large valve overlap this leak creates. It will sound fairly lumpy at idle, assuming you can actually get it to idle. A lumpy idle may sound really cool to some people.
Now the problem is, that at idle there is a fairly high vacuum in the inlet plenum. If both the inlet and exhaust valve are open together, exhaust will be sucked a fair way back up each intake runner. This exhaust will be drawn back into the engine causing the lumpy idle. The air fuel ratios will need some fairly drastic changes too from what they originally were. So don't expect it to work too well with a standard ECU. Even with a Power FC it will probably be quite difficult, if not impossible to get a smooth idle. I have never done this modification to an RB20, so I don't know how difficult it would actually be.
Your best bet might be to either go back to a proper hydraulic aftermarket cam, or fit the GTR solid lifters. I don't even know if that is possible on an RB20 head.
It is quite possible that a few guys have done this modification, and really like the rough idle, and say it makes the car sound mean. It probably also goes better at higher Rpm. Myself I prefer to have things working properly, and am not surprised some of you cannot get the engine to idle at all.
I wish you guys luck, but really what you are doing is not a terribly good idea unless the whole thing is approached properly.
Hope this thread helps clear up a common misconception.
