Cam/Duration selection

[Roy]

So i have decided to go for some 260/8.5mm Tomei ProCam for both inlet and exhaust.

But a thing i have never been able to get my head around is the whole differing inlet to exhaust cam duration on RB engines.

My understanding is that on the exhaust side you can normally get away by going for a slightly longer duration without affecting response a whole lot. On the inlet side though you have to be carfeul not to go too big adn subsequently affect cylinder fillign and response.

So the quesiton is can a cam / engine guru give me any advise on whether an otherwise std RB20 with a 420hp turbo make better use of 260/8.5mm lift in/ex or should i go 260/8.5mm lift inlet and 270/8.5mm exhaust.

Ive asked a few engine / performance places but im still hunting for a cam upgrade that will work nicely.

[KamikazeR33]

since rb20 doesnt have nvcs i would look at going for a setup which u usually see on rb26's.

most that i have seen have the same spec cams on both sides

but then again ive seen a few high hp ones with smaller inlet ones

SK i leave this to u (or someone else with a rb20)

btw roy for my rb30det ive put in tomei poncam inlet 256 9mm and hks 264 9mm exhaust cam

[Roy]

So no one is a cam guru?!? Can perhaps steer me in the right direction...pricing still hasnt come back so still havent ordered anything yet:(

[sky30]

Im pretty sure tomei also make the 260, and 270 in a 8.8mm that suit hydraulic lifters.

[discopotato03]

Roy I think the most success comes from extra lift rather than duration . Back in my Mini days I read David Vizards books which go into a lot of theory about heads and cam profiles . My interpretation is that generally the greatest port restriction is the valves themselves . Four valve heads breathe better than two valve ones because of the increased valve area . In standard form cams for four valve heads are pretty soft because they have extra valve area and tame profiles give good spread of torque over a wide rev range . With cam lift obviously a high lift cam has more average off the seat valve lift than a cam with the same timing but less lift . Full lift is only for a very short time so average lift probably dictates how well it all breathes . I see long period cams as a means of extending an engines rev range upwards , the higher it revs the less time is available to charge the cylinders on the induction stroke so holding the valves open longer means better filling . Ultimately long period cams have more overlap which leads to reversion at low revs and means rough running . The trapping efficiency is not there with longer period cam profiles . It is there with shorter duration profiles and the extra lift (average lift) from faster opening valves gives good breathing and good operating characteristics as well . There are people around like Ivan Tigh who can weld up some production cams and regrind them to suit custom needs , its an expensive process but may result in the ultimate short duration high lift RB cams .

Cheers A .

[Roy]

Butn want to stick with std valvespring and hydraulic setup so limits me to around the 8.5mm. Dont mind if i lose bottom end, dont have a lot anyway...but as long as i can make good gains over 4,000rpm then id be happy.

Perhaps i should stick with the 260s, though i was thinking that if i go a bit big in the cam dept as i have with turbo, then given time i would have some goodies that would work if i increased displacement

[discopotato03]

High rpm engines do really need higher rate springs to avoid valve bounce .

Were you thinking of doing the RB24 conversion ?

[Eek-A-Mouse]

i had 260* 8.8mm lift ProCams in my CA19DET and there was a little low end loss, but screamed from 4500rpm-8000rpm, i wanted to go with TOMEI ProCams 256* 8.5 IN and 260* 8.8 EX cams, TOMEI valve springs and Gears on the RB i am planning on building next.

i am hopeing to lower the lowend/midrange grunt a little and i have been told this is a safe turbo cam set up, with my 260* pairs i had slight overlap dialed in... similar setups are poplular wiht the HKS cam kits as you can get different lifts/Steps with the same duration... but i too wish to keep hyraulic lifters.... so we'll see...

[Roy]

Was going to sticj with a rev limit of approx 8,000rpm, so again looking at making do with the std valvesprings.

If it all dies, or doesnt work, then i would most liekly grab an RB25DE head and do a 25/26/30 bottom end...whatever is cost effective at the time

[Roy]

I should add im in no hurry to change engines. I hope to see this engien last me a long time, but i would rather buy bits that work better then std on the RB20, even if they are not optimised on the RB20, and shoudl disaster strike then allow me to re-use...i went thru engine dramas earlier this year, and should history repeat itself i wont be repeating the mistakes of past:)

[Sydneykid]

The old different cams question pops up again. I have generally found that when a subject keeps popping up, it is usually because there is no one RIGHT answer. I have tried angines with the same duration on inlet and exhaust and I have tried angines with different duration on inlet and exhaust. I can't say that one is better than the other every time. It depends on compression ratio, fuel, tuning, intercooler efficiency, exhaust design, turbo selection etc etc. There simply are too many variables to come up with a definitive answer.

With the hydraulic followers in RB20/25's I would be very careful with choosing a duration over 256/260. Particularly if you are not intending to change the valve springs. This becomes even more apparent when the boost exceeds 1.1 bar. The inlet valves tend not to follow the lobes on closing because the boost holds them open. This causes problems in the compression/combustion process (lowers the effective compression ratio). If you increase the duration, it just makes this problem worse.

My 20 cents worth:cheers:

[Ustasa]

Have to agree with the spiel about the stock springs Roy. It’s pointless going for baby cams due to little increase in power. Same story with big cams and stock springs, just doesn’t cut it when you are trying to pack in 20+psi boost.

Something like a RB20 needs 272’s all round with matched springs. Don’t **** around mixing and matching some custom combo when Tomei and HKS sell cam/springs ready to go. If you are going to modify it, taking half measures just won’t do when it comes to something as important as the head.

You have a good turbo kit so take advantage of that and the willingness of the rb20 to rev it’s nut off Get some biggish cams/springs and you won’t be disappointed.

[Roy]

Springs? That means taking the head off again:(

SH1T, i should head my own advice, if you want to go quick, buy a quick car to start with:(

[NickR33]

I bought some Tomei 256 Procams for my RB25 rebuild... but they were delayed getting here and are now sitting on my floor while I run my new motor in... so I can't offer any thoughts... but soon I will do the 1000km oil change, dyno tune and throw in the cams, then maybe I can speak from experience... BTW the build quality of the Tomei product seems very good in my humble opinion...

[Targa Tom]

I showed my engine mate who has shagged round with circuit cars for yonks the Tomei poncam specs.

He has seen my car on a dyno and immediatly pointed to the 260's (matched) "That's what you're thing needs!!"

As for the springs, I think I would put in as a "cna't hurt it" measure.

But as a Targa car, we are working within a lot of restrictive rules compared to improved production...

and for TT we need to chase 2500 - 3500 kw. Whoever says you don't drive off boost is kidding themselves... more mid range is an old bike trick. Helps carry corner speed and stops the buzzy agreesiveness from what should be suttle throttle resets.

Just a bit off, but we put all the gtr's behind one of our projects at Basky a couple of weeks ago. @ a mid 60, maybe only Whitey R34N1($150k+) and Greg Garwood, (X WalkerMC - NSW, Mines R33 Vspec - Cost = heaps) have gone faster...

So spend sensible dollars and you will go fast.

TT