Camshaft Changes

[Jay95R33]

Originally posted by Sil8ty

Have been advised to use a set of verniered cam gears on the std cam shafts will see, set up correctly around a 10-15 kW gain.. Guaranteed!!!!

I think a few blokes on here have the variable cam gears set up with pretty good results. Apparently they can only be put on the exhaust side due to the VCT on the inlet side.

I might have to get one to reach the 200rwkw mark I'm crossing my fingers for........ I'll see how I go. Knowing my luck my fuel pump will crap itself - or I might spin the wheel off my turbo

J

[Warpspeed]

Sil8ty has raised a few points.

A standard GTR with standard cams does not make best power between 8500RPM and 9000RPM. The factory rev limiter is set to 8000RPM or just above. The Nissan engineers designed the GTR to be a docile machine with a lot of power. The standard GTR cams are very mild in both lift, duration, and valve action. This is to enable the stock valve springs to last the life of the car. Max rated factory power is at 6800RPM by the way.

New aftermarket or reground cams are expensive. Lots of GTR owners replace their cams, so there are a lot of secondhand stock GTR cams available really cheaply, typically $250-$300 per pair. For this reason only, I suggested it might be worth a try.

Anyone who thinks that Nissan really screwed up with the valve timing, will probably get ten percent more power by being smarter than the Nissan engineers. For the rest of us, there may be a few Kw to be had right at the very top end in a modified engine, or with a stretched timing belt. This is not going to be cheap power though, unless you already have the adjustables, or can get them for nothing.

Standard GTR cam lift is about 8.2mm as I remember, the Japanese cam grinders offer cams with lifts up to 9.7mm as a drop in replacement. Beyond 10.5mm, head machining and special valve springs are required. I have seen cams quoted with 11.3mm lift.

Mechanical lifter cams have long ramps at the beginning and end of the lift curve. The opening ramp ensures the first bit of lift slowly and gently closes up the tappet clearance before the valve is rapidly accelerated off its seat. The closing ramp is to stop the valve being violently dropped onto the seat as the cam lobe pulls away from the tappet. The ramps reduce both noise and mechanical shock in the valvetrain.

Hydraulic cams do not have these ramps. The whole valvetrain is kept solid by oil pressure in the hydraulic lifter. The cam lobe is in full control of the valve motion at all times, so acceleration away from, and onto the seat can be quite rapid, and without shock.

You can fit a mechanical cam into an engine with hydraulic lifters without any problems. Putting a hydraulic cam ino a solid lifter engine is a very bad idea. So yes, GTR cams can be used in an RB25.

[meggala]

Ph is right Ihave confirmation from option mag in japan and what he says makes sense the sr 20 has one of 240, same with the rb25 and I assume the the rb20 is similar the N/A version cams are 248 ithis tallys witht eh 254/256 after makret cams offer by tomei apxi etc in fact tomei has tuned cars witht eh 256 cams to aruand 600 hp for endurance racing.

warpspeed I belive you when you say you measured them @ 250 but they might have been after market we aren't sure 100%.

I have look long and hard at doing this N/A cams swap but I believe that it wont be that benifital in the increase in power to jsut ify the cost and the addition wear to the motor but sil8ty has hit the nail on the head with the cam gear adustment these do give good cost benifits.

I have heard of a single cam in an rb25 engine being changed personally I wouldn't do it.

256 is the minimum I would change the cams for and probably I would go for 260's its very interestign but could be an expensive exercise If you ahd to pay some one for the work

meggala

[Warpspeed]

Hi Meggala.

The cams I measured came from the same source as a RB26 short block I purchased, which is fitted with the R33 oil pump, so I assumed the cams were from the same engine.

Measuring cams, particularly mechanical cams is a bit tricky. The clearance ramps are very gentle, there is only small lift for a lot of rotation. These ramps blend into the main lift curve, so it is impossible to tell exactly where the ramp ends and main lift begins.

There is no standard. Wade Cams in Melbourne have something like 4500 grinds available. I have listings of some of these. The quoted ramps vary from .006"to .017" on different grinds. The valve timing point is at the end of the ramp.

If you do not know where the ramp is supposed to end you cannot tell what the official duration is.

Suppose you measure 260 degrees at .008" lift and 245 degrees at .018 lift. What cam is this ? ? ? As the ramps are extremely long and gentle this would not be unreasonable.

On these GTR cams I have, I measured very close to 240 degrees duration for the inlet cam, and 250 degrees for the exhaust cam assuming a .012" measuring point. This seemed reasonable at the time.

It is quite possible though that the official Nissan duration figures are a few degrees different because they measure at a differnt lift point. So 246, or 253, or something could actually be correct.

In contrast hydraulic cams have absolutely no lift, then it suddenly takes off at one particular point. Standard measuring points for these are .005"or .007" lift, in practice the durations are very easy to measure and are quite repeatable.

I would dearly love to know whatever cam information you have, simply because junkyard cams are a very low cost way to experiment. If you know a certain version RB20 had this cam or RB25 that cam, it would be very handy indeed to know.

My computer simulation program Dyno 2000 allows testing of any camshaft you can possibly imagine with in n/a, turbo, or supercharged engines. It works like majic and gives a very good indication of what is best.

[Gradenko]

Originally posted by Warpspeed

If you fit a slightly longer duration cam that has both the overlap and the late inlet closing features together with a bit more lift, and no VCT, then you will get more torque everywhere

Warpspeed, I've been looking into VCT on RB25s and what you said is very interesting. I'm not too familiar with affects of duration and overlap, so can you explain how it works out better to have overlap? Is having overlap present at high rpm a detriment to making power?

BTW, what you said about VCT is spot on. Not active at idle, VCT activated just off idle (as measured by TPS), deactivated at around 5000rpm.

Originally posted by meggala

I have look long and hard at doing this N/A cams swap but I believe that it wont be that benifital in the increase in power to jsut ify the cost and the addition wear to the motor but sil8ty has hit the nail on the head with the cam gear adustment these do give good cost benifits.

meggala, what kind of extra wear would you expect the motor to experience from a cam swap? (Apart from wear caused by the driver testing out new-found power at every chance )

Warpspeed, I have a scan of the Option mag (I think) meggala is referring to. Here are the RB engine cams it lists. Any idea what cam your Dyno testing program would reconmend for a stock turbo'd RB25DET, putting out, say, 320hp at the fly?

RB25DET

IN EX

240 240

7.8 7.8

120 117

RB26DETT (R32, R33)

IN EX

240 236

8.58 8.58

113 125

RB26DETT (R34)

IN EX

240 236

8.58 8.58

117 121

Option mag scan is too big to attach, download from http://www.iinet.net.au/~bsisko/cams.jpg

[Warpspeed]

Hi Gradenko, thank you very much for the cam information, I have been looking for this for a very long time.

Valve overlap can be both a good thing and a bad thing, but is mostly bad. If you increase the cam duration you will get to a point where both valves are open together around TDC, as both valves are both at the ends of the lift curves they may not actually be very far off the seat, but both WILL be open together.

What happens at TDC overlap depends entirely on the relative pressures in the exhaust manifold and inlet manifold. Understanding this is the key.

If the exhaust manifold pressure is lower than inlet manifold pressure, then the exhaust will suck fresh mixture across the combustion chamber, clearing out all the hot exhaust remnants.

This can happen at full throttle in a highly tuned normally aspirated engine with open exhaust, in a supercharged engine, or a turbo RACING ENGINE where boost pressure is higher than turbine inlet pressure.

If exhaust pressure is greater than inlet manifold pressure, exhaust will flow backwards into the inlet runner, then be sucked into the cylinder during the induction stroke. This is very bad.

It will always occur at less than full throttle on any engine, so for a road car, excessive valve overlap is not a good idea. If the engine is only ever driven flat out, as in drag, or circuit racing it is less of a problem though, and the longer valve duration will give more top end power.

The original manufacturers these days are concernrd with emmissions as well as making a smooth running engine. The standard valve timing is almost guaranteed to have minimal or no valve overlap.

If all you care about is power at full throttle, and do not care about part throttle operation, you can run a bit more overlap. How much depends on the relative exhaust back pressure and boost pressure. This is very important to understand.

A stock turbo is going to have a fairly small exhaust turbine and housing, this will give a low boost threshold and less lag. It will also probably have a fairly restrictive exhaust system. Flat out at full throttle, the exhaust manifold pressure may be TWICE boost pressure, or even more ! If you wind up the boost higher it gets a lot worse. Putting a large overlap cam into such an engine will definitely lose you power.

On the other hand a monster turbo with large turbine may only produce usable boost high in the rev range, but the exhaust back pressure will be quite low, perhaps the same as boost pressure, or only slightly above. Long duration cams with a lot of overlap are going to really help airflow and peak power.

How much cam duration and overlap you can run depends on the relative exhaust and boost pressures. You either need to experiment yourself, or seek expert advice. So there is no single answer, it all depends on how radical you want to go.

[Gradenko]

Thanks for the explanation. The scan is actually from meggala ( I think), so thank him

From what I gather, another benefit of overlap for engines with low exhaust manifold pressure (large turbo's) is to would get boost out the exhaust valves and spool the turbo faster. Since turbo size and camshaft selection is chosen to complement each other, it may be best for me to leave stock camshafts in place with the stock turbo.

What I'm really interested in, is whether replacing VTC for a fixed timing cam would really improve everyday driveability (for a properly chosen non-stock cam/turbo combination). You mentioned that a cam with a little overlap, late inlet closing, a bit more lift and no VCT will get more torque everywhere. What about the same cam with VCT, couldn't that provide a little more overlap in low revs to spool the turbo up and still have all the benfits at the top end? Note, I don't know much about this - I'm just wildly guessing

[meggala]

yeah warp speed it is mine and my copy of the magazine has been lost so lucky some one sought to keep it it was very hard to verify the size of them and if anyone would know it would be tomei. if you note tomei use the only 256 cams but with 11.5 mm of lift some very interesting stuff floats around the hard part is decifering the japanese

meggala

[Warpspeed]

Once again thanks Meggala for the information.

I have been going through this and crunching some numbers to find out what the actual seat to seat valve timings are for the various engines.

They are all over the place. Most have either no overlap at all, or only one to three degrees which is really zero. The R34 RB25DET neo engine has twenty three degrees of overlap, and the latest SR20 DET has twenty one degrees of overlap.

Also lobe centres for both inlet and exhaust are also all over the place as well. I am sure Nissan have been trying very hard to stay within legal emmissions rather than seeking optimum power.

I am sure a lot can be done with valve timing on a stock engine with fairly mild cams. Figures for a non VVT early RB25DE are not given. At a guess it may have a pair of 248 cams, and these might actually go rather well in a stock RB25DET, who knows ?