Rb26 Heads And Cams

[discopotato03]

I started this one to not distract the "Will this do 350Kw 26/30" thread .

SK mentioned the Tomei 264 high lift cams for RB 26 and someone said something about 260 and 9.15 lift . Are these the same thing as I'm interested in short timed (duration ) high lift cams . David Vizard always said that high lift and short timing was the way to fill the cylinders and not lose bottom end power , sort of a good way to make grunt without extreme revs or in my case revs or boost .

With the heads I understand some minor fettling is required so the cam lobes don't hit the casting near the bucket bores ? Also are the cam bearing caps held down with studs/nuts on the 26 heads , seem to remember some people having dramas with broken bolts (?) in the RB20 with 26 cams posts .

Cheers A .

[Amaru]

The 260/9.15mm are poncams designed to bolt straight into an RB26 (although myself and some others have had to replace the shims to get a perfect fit). The 264 is the entry level procam, and runs much more lift than the 260's.

Its a 30 minute job to cut back the head with a die grinder for cam clearance, probably one of the easiest parts of the installation.

I run 260/9.15mm camshafts in my car (2530s) which is mainly a street car/weekend warrior. They are awesome cams and really woke the car up, and it keeps pulling hard to redline now whereas it used to run out of puff at 7500rpm. Id definately reccomend them, but then again, I havnt had experience with the larger duration/lift cams so cant really comment on them

[proengines]

Big lift and short duration is normally better than low lift and big duration, thats why so many pushrod engines now run roller cams, they can lift more for the same degree of camshaft rotation and the area under the valve (if you graphed it) is much greater for the same duration, meaning more gas can flow through for the same duration. This works very well for a n/a engine but with turbos it is a little different.

An RB can run a fairly agressive lobe due to the large bucket diameter and can lift fairly quickly. I think it is worth running a fair bit of duration on the exhaust cam as you increase the time that exhaust gas is flowing out of the port and I'd say it provides smoother exhaust flow to the turbo, especially with twin turbos where you only have 3 cylinders feeding each turbo which means only 3 pulses every 2 rpm. Extra duration on the intake cam usually means more overlap (depending what sort of lobe centres the cams are set on in relation to each other). A little extra overlap is a good thing (apart from at very low rpm) with a turbo engine as it provides good scavenging. The overlap is when both valves are open at the same time, at the end of the exhaust stroke and beginning of the intake stroke and with a turbo engine there is air being forced into the cylinder which means that more of the burnt gasses can be purged from the chamber allowing fresh air and fuel to fill the chamber and a better charge to burn on the next stroke. This is called scavenging and is a good thing for power. A lot of 2 stoke diesels run superchargers, turbos or both for just this reason. You also lose a little of the fresh charge out the exhaust which will burn in the exhaust and expand, keeping the turbo spooling.

That's a really long winded way of saying I think it is worth running some extra duration, for a street/strip/track combo engine, I like the HKS 264/8.7mm lift cams, they make a lot of low rpm power and spool the turbo quickly. The last one we did was running a 35/40 single with a fairly small exhaust housing and was making 18psi under 4000rpm. Peak torque was under 5000 and was almost a flat line from there onwards. It made an easy 320rwKw at 7000 and would have kept rising but the ignition was breaking down (coils).

I just thought I'd add, the 8.7mm lift is the actual valve lift, the lift on the cam is 9.1ish but once you subtract the .4mm or so valve clearance it comes out at 8.7, I'm pretty sure the Tomei 9.1 lift works out the same way.

By the way, I love that word "fettling", reminds me of the old pommy educational movies they used to make us watch as a fitting and machining aprentice. I can just see an old guy with a shirt and tie and a dust coat using a bearing scraper to "fettle" an RB head.

Edited October 6, 2005 by proengines

[discopotato03]

Yes fettling , I saw some of those films when I did my F/M trade course .

Also got to work on some early 645 series EMD (GM) Diesels with Roots Blowers , the later ones have a turbocharger (with free wheeling clutch) thats allows the turbo to run faster than the gear train as revs rise - to scavenge .

I'll have to look into Tomi profiles but my interest is in high lift - sort of open the valves quickly , a long way but not for too long . My main concern is this valve guide cracking business , is it a high rev problem ie "valve crash" or bounce and shock loads ? I thought the whole idea of the inverted bucket was to take all side loads off the valve stem and guide .

I should mention at this point that the engine will be an RB26/31 with a 1.06ARR GT3540R so exhaust restriction (bad for scavenging - reversion) should not be a problem . I would like this engine to do what WilliamsF1's RB26 does but with more capacity and the single turbo .

Cam cap fasteners , stud/nut on 26 and bolts on 20/25 ?

Cheers A .

[Abo Bob]

Re: low down power - I would have thought that the capacity of the engine would make up for the overlap of a more conventionally agressive set of cams, at least in relation to an RB26.

The overlap is part of what gives the extra power though because the incoming air is forcing a more complete evacuation of the exahust gases from the cylinder. At least that's how I understand it.

[Abo Bob]

Of course, now that I re-read proengines post I am just repeating what he said.

Sorry - just ignore me.