My RB26 Cams Experience in RB25

[Sydneykid]

Oh man SK this is getting painful. You do everything else right by the sounds of it but why recommend putting mechanical cams in a hydraulic motor? There is a reason why the cams are different and you know it. You wouldn't put an oil tank below the height of a dry sump pump would you? So why do this?

At 0.25mm lift the RB20/25 hydraulic cams have about 230 degrees on the seat. (0.25mm is just for a comparison)

At 0.25mm lift the RB26 solid cams have 310 degrees on the seat.  

Thats 40 degrees a side per lobe!  

Consider that SAE quote 0.15mm running clearance for a hydraulic lifter the problem only gets worse (because at these heights the mechanical ramps have a much smaller gradient) While these sorts of lifts don't create much airflow they do lose the vital seal needed to give the motor some dynamic compression down low which it needs for a stable and smooth idle.

In some cases I would agree you could get away with running a mechanical profile hydraulic but the mechanical Nissan profiles have such large and aggressive ramps. It is a known fact that to get the GTR past emissions the clearances would be at their correct point (about 0.50mm) and then closed up to make them sound fat on idle while not losing top end performance. My understanding is that the factory clearances are about 0.30mm or so with about 0.05mm expansion from cold to hot - is this right?

Before we start, I need to clarify a couple of things. I have NEVER tried RB26 cams in an RB25. However I have used RB26 cams in 3 X RB20's. One was at high k's (170,000) and had slow to pump up hydraulic followers. This one definitely received R32GTR standard camshafts, they came out a of Production Race car engine and the camshafts had been scrutineered a few times and were definitely STANDARD. That engine had stronger than standard valve spring seat pressure. The RB26 cams that went into the other 2 X RB20's were not as certain to be STANDARD.

Ok Luke, let's examine the facts .......

The max follower (solid) to cam clearance in a GTR is 0.50mm (as per the R32GTR Nissan workshop manual)

The maximum slump on an RB20 hydraulic follower is 1.0mm (as per the R32GTST Nissan workshop manual). They don't quote the standard slump.

I have measured a low K's RB25 engine and the slump was 0.6mm, which seems reasonable.

What's "slump"? It is the amount a hydraulic lifter can be depressed when the engine isn't running, but measured immediatley afer it has been running for 10 minutes at ~4,000 rpm. It is a measure of how good the seals are in the hydrailic follower (ie; how much oil "leaks" past them). The amount of air in the oil is also a very minor factor, as the minute air bubbles can be compessed.

So, if I have an "average" RB20 with its hydraulic followers, it will have MORE clearance (0.10mm) between the camshaft and the follower than a GTR set at its recommended clearance of 0.50mm. That's when the engine isn't running, of course.

The real question here is how much slump does the hydrailic lifter have when the engine is idling? That's not an easy question to answer, as it depends on the seal of the followers, the valve spring tension, how much airation there is of the oil and finallly the engine's oil pressure. This is maybe the reaon why some times it works and others times it doesn't.

It would seem logical that an RB20 with poor slump (over 1mm), low oil pressure at idle and strong valve springs would have a good chance of using the RB26 cams successfully. Certainly a better chance than a low k's, minimal slump, good oil pump (low airation), standard valve springs RB25.

This is not a simple "yes it will" or "no it won't" discussion:cheers:

PS, you can't compare this with say a Chevy pushrod V8, as they have a rocker ratio which amplifies the slump (1.5 or 1.7 times). A Chevy with hydrailic lifters and a solid cam idles quite OK, but doesn't produce much power. This is because the max lift is lowered by the slump times the rocker ratio.

[lukevl]

Holy moly max slump 1.0mm!? Thats incredible. 0.6mm is also ludicrous. But then again when its running it shouldn't do that with good seals. I see what you're saying by the seal etc and I assumed that most Rb owners what have reasonably good quality lifters. If they do sag that much it would almost be bearable.

However with the RB vs pushrod arguement, the RBs would have less spring pressure by a small amount (depending on if they are standard or who sets them up obviously) and less inetrial weight and no ratio, but the pushrods do have a much larger (by area) hydraulic piston to push on and as a result after the ratio and inertial mass wouldn't far off each other.

Anyway, agree with someone's previous post that you should just put an RB26 head on to begin with. The weight difference between a hydraulic follower and the mechanical ones is huge as a percentage.

Also $350 is about the average price for grinding the GTR cams to a hydraulic version and something that I highly recommend. No headaches then and its not really that expensive for what you get.