Vct Vs Poncam

[CMS Drifter]

Hi guys,

I am looking to build one of these Rb30Det monsters with my RB25 VCT head.

I was just wondering if it was worth keeping the std cam and vct or upgrading to an aftermarket item.

Has anyone done any comparisons in power delivery with the std vct setup and aftermarket inlet cams????

[jarrod83]

why not poncams and vct?

[urtwhistle]

I also would love to see some 1 do a direct comparison.

the problem is any decent drop in cam means deleating the VCT. The stock cams are actually extremly good and longer duration cams just make it a dogg in low range.

The real downfall of the stock cams in my point of view is actually the amount of lift not the duration. I would love to see the comparison between standard and standard with the cams welded to suit an extre 1 or 2mm of lift as the only change.

Chris

[KiwiRS4T]

Enabling VCT requires just a little work. Poncams are getting on for a $1000.

If you have the money do yourself and us a favour.

Run your engine with VCT and the stock cams.

Then install the Poncams and see what difference it makes ( I an mostly interested in the mid range, not the power at max revs).

With the poncams in you can test with the VCT on or off. If it runs better with it off then you can disable it with not very much lost.

If the poncams don't do a lot then you can resell them.

[CMS Drifter]

I had considered fitting an RB26 intake cam and getting rid of the VCT but not sure how it will affect my midrange.

I'm not sure if the poncams are VCT capable.. (can somebody confirm?)

Failing all of the above, I was thinking of taking the std RB25 VCT cam and getting it re-ground for higher lift and slightly longer duration. I have some specs from Tighe that I was looking at trying to match.

If only I had free access to a dyno

[Craved]

rb25det poncams are VCT friendly

[TiTAN]

RB26 cams require solid lifters, do not run them on the hydraulic ones.

[CMS Drifter]

Thanks for clearing that up Craved

TitAN- I understand that the RB26 has solid lifters and the Rb25 ran with hydraulics, but what is the difference in cams to run either?

I know that the hydraulic lifters are a bit more forgiving and quieter but dont like too many revs or they can "pump up" -This is why the RB26 was fitted with solids to allow it to rev harder being a race-bred engine

But...

I still dont see or understand why there would be any difference in cam profiles for hydraulic or solid lifters...

Can somebody in the camshaft game shed any light on this for us??

At present I am using an HKS step 1 exhaust cam from an Rb26 in my RB25 powered R33 track car and doesnt seem to be causing any problems (been fitted and raced for 12 months)

If somebody in WA (Perth or south of) is willing to let me use their dyno for a few hours for a heavily reduced price, I will trial std Rb25 VCT cam and tune Vs Std Rb26 cam in the same engine in the same car...

[TiTAN]

Hydraulic lifters are subject to this thing called "hydraulic intensity" which limits how quickly the cam can open the valve without "collapsing" the lifter mechanism (which would be detrimental in terms of performance). With a solid lifter, the cam profile can be more aggressive and can accelerate the valve off the seat and open it fully more quickly, resulting in "more area under the curve" (more valve opening during the same period) than is possible with a hydraulic. Because of the difference in ramp rates on the camshafts, in general it's not wise to put a solid lifters cam on hydraulic lifters, and vice versa.

[zebra]

Solid cams + hydraulic lifters = chewed out lifters and cam + massive lifter pump up issues.

[urtwhistle]

soooo am I reading this correct. I could put a 26 exhaust cam in my neo head????

[CMS Drifter]

That makes sense TitAN- Thankyou for explaining further!!

urtwhistle- You can do that, but at your own risk. So far my cam is working well, but following zebra's comment damage to cam/ lifters is probable..

I will look at getting a std vct cam re-ground for greater lift, but try retain stock ramp rates

[KiwiRS4T]

I don't see how you can get more lift by removing material with grinding. I think better valve springs is the only head mod needed. What is your power goal? Probably better to spend money on the bottom end.

[TiTAN]

soooo am I reading this correct. I could put a 26 exhaust cam in my neo head????

NEO has solid lifters, so yes you could but i would check the specs of the cams before changing them (unless you mean aftermarket 26 ones)

That makes sense TitAN- Thankyou for explaining further!!

I will look at getting a std vct cam re-ground for greater lift, but try retain stock ramp rates

dont go more than 9mm lift

[CMS Drifter]

KiwiRS4T- When you grind a cam for more lift, you are removing material from the base circle, this increase the difference from base cicrle to tip of lobe (increasing lift)

By grinding the base circle down and flattening the top of the lobe, you can gain a small amount of lift and longer duration.

Better springs will help with higher lift cams to prevent the coils binding. May also help with higher revving engines. As I dont plan on revving harder than 7000rpm with the 30 I doubt I'll need upgraded springs.

The bottom end of the 30 is quite strong as is, so I'll save my money maybe just for ARP bolts right through.

For me to get a cam ground is cheap as I am good friends with company owners that do cam grinding

dont go more than 9mm lift

Why not more than 9mm?

I am currently running the HKS Rb26 exhaust cam (8.7mm lift) without any issues.. I do understand cam geometry could be an issue with hydraulic intensity as you mentioned before especially with grinding the standard cam down to acquire that amount of lift..

[urtwhistle]

That makes sense TitAN- Thankyou for explaining further!!

urtwhistle- You can do that, but at your own risk. So far my cam is working well, but following zebra's comment damage to cam/ lifters is probable..

I will look at getting a std vct cam re-ground for greater lift, but try retain stock ramp rates

After thinking about this thread I called "southside engine centre" who do some very nice engines and discussed machining the base circles to suit more lift.

The main problem that I never thought of was that due to the NEO and 26 heads having solid lifters would mean reshimming the head with much bigger shims. Which runs a higher risk of spitting a shim out and smashing the cam. They strongly suggested not going this route and fitting an after market cam that didn't require using big shims. That might not be such an issue in a hydraulic head though.

The link below shows that a NEO ex cam would probably be better then a R34 ex cam. It has more lift but only slightly less duration.

http://www.tomei-p.co.jp/_2003web-catalogue/e090_camshaft-specs.html

[urtwhistle]

and I ment to say NEO being better than rb26 cams

[KiwiRS4T]

KiwiRS4T- When you grind a cam for more lift, you are removing material from the base circle, this increase the difference from base cicrle to tip of lobe (increasing lift)

By grinding the base circle down and flattening the top of the lobe, you can gain a small amount of lift and longer duration.

..

Thanks for that... geddit now. Regarding bottom end I guess it depends on your power goal. Mine is not forged either - just rings bearings new conrod bolts and an N1 (!!) oil pump. According to RIPS good for mega kw but I will be happy to crack 300 and ecstatic if it will make 330.