Rb26 Cam Choice

[JH32]

I liked a question that Beer Baron posed in the RB26 Turbo Choice thread, and would like to know the answers myself, so I thought I'd quote it here for discussion:

I've got a question for R32-GTS and Gav about cams.

What did you guys think of the HKS 272s? Also what maching was neccesary to fit them? did you need upgraded valve springs? did you need any other hardware?

Gav, I've heard from a number of tuners that it's possible to 'over-cam' the inlet on RB26s so i have been thinking about going for a similar set-up to you (264in 272ex). What's it like? what are the advantages of this over say the 272/272 set-up? mmm i love a lumpy idle, and a nicely cammed motor feels good too

[Fitzpatrick Speed Works]

Its not possible to overcam the inlet so thats garbage, the inlet side of a rb26 head flows 40% more air than the exhaust side so in reality you can run a much larger cam on the inlet. The true reason you see the bigger exhaust side is to get it flowing without the overlap issues assciated with big cams, its to try and discuise overlap.... You want the exhaust to flow as much as possible as they are so crap from standard, but you dont want to have a lumpy idle or excessive loss of down low torque you run with the smaller inlet bigger exhaust as a trade off. If you just dont care and want big all round you run with the same. Hope this helps and for futer reference on the flow bench the rb26 inlet runs 145cfm as opposed to the exhaust running only 95cfm, the exhaust side of an rb26 sux big time!!!!!

[Gav]

Daniel gave a better explanation than I'm able to offer.

The long and the short of my choice was based on my tuner's recommendation at the time as he had seen good results from step 2 cams (11.?? mm lift) and the 264IN/272EX.

The idle isn't too lumpy at all, although it does hunt around a little unless idle is set to around 1000 rpm.

In terms of hardware, I added valve springs to suit as well as new retainers and guides - all HKS. Best to buy a matched kit along with the cams at the same time.

[MattSR]

"Hope this helps and for futer reference on the flow bench the rb26 inlet runs 145cfm as opposed to the exhaust running only 95cfm, the exhaust side of an rb26 sux big time!!!!!"

This is usually the case with ANY engine though... Those readings are taken with the same pressure across them, but in reality, the pressure in the cylinder on the exhaust stroke is much, much higher than whats in the inlet manifold..

Thats not to say that you are wrong or anything, just saying that its not that unusual for a production engine.

Cheers,

Matt

[Beer Baron]

thanks Gav and daniel. I was curious as it's something i've seen quite a lot. i don't mind the lumpy idle but i do like low down torque. perhaps a 264/272 combo like your gav may be the way to go...

[blk180]

I want to run 272 in/out 10.25 lift would like striaght tho.also would be adding springs and new retainers at ther same time.I already have bronze valve guides from when the head was rebuilt. Gav are you runing soild lifters for over 11.x lift

[Gav]

Gav are you runing soild lifters for over 11.x lift

<{POST_SNAPBACK}>

I thought RB26 lifters were solid anyway??

[blk180]

sorry most likey they are i just seen that tomei make value lifters for cams running over 11.x

[R32-GTS]

BLK180 your thinking of buckets. Need tomei buckets with anything over 11mm lift. I run HKS 272 272 step II with 10.25mm lift and can use stsandard buckets and retainers.

Very lumpy idle but i love it sounds so nice .

[SLY33]

this is good. any chance of a sound file of your idle r32-gts?

love to hear it as im choosing cams at the moment..

can someone explain the "function" of 1) increasing the duration, and 2) increasing the lift??

i know that the higher duration means the valve is open longer, and that higher lift is opening the valve more, but what does each of these achive as far as peak power gained vs low down power lost, torque etc?? i guess im asking what does it do to the shape of the power curve??

as you can get smallish duration with biggish lift eg 260 10.8, and bigger duration and smaller lift.. thats what confuses me. which does what and which suits what?!

cheers

Edited November 24, 2005 by SLY33

[R32-GTS]

Yeh i have a sound file . Want me to email it to you ?

Edited November 24, 2005 by R32-GTS

[Fitzpatrick Speed Works]

MattSR

Yes the exhaust usually flows less on most engines but here are some figures for you.

The exhaust on an rb26 flows 65% of the inlet.

on a fj20 it is 85%, on a sr20 it is 88%. On a lexus v8 it it 90%, and the best factory headed production car is the evo 7 lancer with the exhaust flowing 96% of the inlet. So in respect to these figures you are correct but the rb26 is dropping the ball big time and coul be alot better, and alot of the design has to do with port angle due to probably packaging restrictions due to twin turbos.

[discopotato03]

My understanding is that long duration cams help to extend an engines rev range upwards . As revs rise there is less and less time to charge and vent the cylinders so opening the valves earlier and closing them later in the cycle gives more time to breathe and vent .

Trouble is you get cranky characteristics because the valves are open too long in the scheme of things at lower revs and this reduces trapping efficiency . The fuel air charge is difficult to compress in the chamber if the valve/s are not seated . Until the right air speed is reached you get reversals in both ports and that "cammy" effect with long period profiles . When the right speed is reached air, which has mass , flows in the right direction ie in inlet and out exhaust valves without the reversion .

Higher lift cams obviously open or lift the valves further off their seats to let the charge in . When a cams profile is changed so that there is more lift for the same duration the lift curve obviously changes . For the same amount of camshaft rotation in degrees (from when the valve just starts to open) the higher lift version will have the valve further off its seat righ through its lift phase ie not just at full lift . A valve further open gives greater flow potential to fill the cylinder , I'm told generally the valve is the greatest restriction to port flow .

I believe high lift is the way to go but the valve train has to be able to support it . Standard springs are likely to coil bind with high lift profiles so cost and effort to fit better ones is involved . Direct cam lobe on bucket valve trains are good at handling the loads up to a point but extremes go beyond the material limitations of standard valves/springs/collets/buckets eventually . RB heads I believe need some fettling around the sides of the bucket or lifter bores to clear the extended noses of high lift cams . This is hardly a bolt on type mod so goes against what the Japs can do and afford given that labor charges over there are huge .

It seems the long period cams are used with less than ultimate lift so that they can be fitted to a standard head and valve train - bolt on/in mod . These types of cams will increase breathing potential but move the usable rev range upwards - provided they don't valve bounce and loose the valves sealing ability .

I like the idea of shorter duration high lift cams as the breathing ability is there without the reversion dramas , meaning high flowing port but with good trapping efficiency through limited valve open timing . I doubt anyone is gonna make me a 256 deg 11mm lift cam so the Tomei Pro Cam at I think 260 and ~ 11mm will have to do . Hard on the valve train but easy on the bottom end I suppose . Good oils should make it reliable I hope .

Cheers A , open to ideas/opinions .

Always check piston to valve clearance .

Edited November 24, 2005 by discopotato03

[pnblight]

something you might find of interest is the cams tomei recomend for there street turbo kits (nothing over 260deg duration just different lift profiles)

http://www.tomei-p.co.jp/_2003web-catalogu...28_ARMSkit.html

Recently after porting my rb26 head i did find the exhaust port in need of quiet a bit of work and the inlet was pretty much fine just needed a good smoothing of casting marks etc. I would think there would be conciderable gains to be had on the exhaust side through porting which should help to even up the mismatch from inlet to exhaust stated above, but i would also think the the flow rates would not need to be the same as the combustion pressure driving the exhaust gas would be far greater than the boost pressure driving the inlet. And all in all the rb26 head is a work of art as its a 2.6litre motor that can flow 600bhp through the std head which is simply awesome.

pete

[gtr jason]

r32 gts can you put the 272 with 10.25 lift straight in with out any mods to the head

[R32-GTS]

No you need to machine the head to fit.

[Mik]

r32 gts can you put the 272 with 10.25 lift straight in with out any mods to the head

<{POST_SNAPBACK}>

It depends on the brand, some yes, others no

[Gav]

something you might find of interest is the cams tomei recomend for there street turbo kits (nothing over 260deg duration just different lift profiles)

http://www.tomei-p.co.jp/_2003web-catalogu...28_ARMSkit.html

<{POST_SNAPBACK}>

Slightly off topic and I aplogise, but the Tomei URL that Pete quoted is interesting.

The specs on the ARMS B9046 GTR kit are interesting - the turbine and compressor wheels sizes are huge compared to other "bolt on" kits!

Is this the highest power bolt on kit available?

[Beer Baron]

they seem to think so!

The biggest bolt on turbine ever. Actuator management achieved violently high output with quick response character.

haha violently high output. i love it. not too bad a price either. about $6K for the kit.

[pnblight]

Slightly off topic and I aplogise, but the Tomei URL that Pete quoted is interesting.

The specs on the ARMS B9046 GTR kit are interesting - the turbine and compressor wheels sizes are huge compared to other "bolt on" kits! 

Is this the highest power bolt on kit available?

<{POST_SNAPBACK}>

they do look like an interesting bolt on don't they and to run the cams surgested would make one nasty rb26