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Cam Gear Adjustments

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Hey, Im interested in playing around with some Cam gear adjustments on my RB25DET just have a few questions...

What sort of procedure do you follow when making adjustments? ie the engine is custom tuned for the current setup with the stock cam dial in... which following method would you use.


Advance the intake Cam 2 degree's and leave the exhaust where it is, Check results... then retard the exhaust cam 2 degrees and check results and continue in increments


reverse of #1 and retard the exhaust cam first?


advance intake and retard exhaust at the same time?


Only touch the intake or the exhaust and not even touch the other...

Also what effects would advancing both the intake and exhaust cam have? can it be beneficial?

Am a little worried about valve to piston clearance, not sure what the clearance is standard. I know I can turn the motor over by hand first to check clearance but a little worried about the VCT system... not exactly sure how it works... pretty sure its only on the inlet cam but does it advance the cam from where it sits with engine off or retard?

How much effect does it have on fuel tuning? I imagine there would be more effect on spark timing? does advancing the inlet cam and closing the LSA usually require more or less advance?

Why do you think that advancing intake cam and retarding exhaust cam is what you need? This will increase the overlap, which is not loved by turbocharged engines.

First you should work out an objective, that should be achieved by rotating cams. E.g "I want to move the torque rack earlier/later"

Of course there are different ways to find the right cam timing.

First, a change in cam timing changes the volumetric efficiency. That means the AFR's and knock treshould does move also. Intake cam timing has a much more influence to the torque curve than exhaust cam timing.

So usually I do the follow procedere:

- Tune the engine roughly and get a feel where the knock treshould is. If possible set the AFR into the middle of the window where the engine makes most power. For turbo engines thats usually between 11.5 and 12.5. This way the engine should not change power with slightly different AFR values after changing cam timing Still, I would always pay attention that your boost sits exactly at the same level after a cam timing change. If necessary make also small adjustments to the Fuel map to hold AFR in the same range through out the tests.

- retard the tune to a quite safe state, get sure that your tune is a few degree back from the knock treshould and the turbo doesn't run at the edge. (If you would leave your turbo and ignition timing at the edge and your VE get with a cam timing change higher, the engine will start to knock and the turbo will fall out of his efficiency rage. We want find sloly the impact from camtimig to the torque band and don't want get fault results by other variables like turbo efficiency.)

- Play with inlet cam. Advance inlet cam spools the turbo quicker, more midrange, but makes less top end power. Retard for more top end at the cost of later and more aprupt spool up. At the begining I would do big steps of adjustment to clearly see what happens with the torque band. I usually adjust it in 5°KW steps

- If you found a suitable setup, start to play with exhaust cams.

- if you feel that it gets better with exhaust cam timing adjustments, you could move around both cams together. But in my experience this hasn't too much effect.

- on any changes, I would have always an eye on boost and AFR values. If necessary make slight adjustments to bring the values back to the previous level.

That's how I do it on engine equipped with adjustable cams on inlet and exhaust side.

In your case with VTC you can only adjust the exhaust cam stepless. The inlet cam has only two positions. So you just have to search the right shift point for every exhaust cam setup. That's an easy one.

- After an exhaust cam timing change, make a run with VTC ON over whole rev range and one with VTC OFF. Overlay both power curves, the point where they overlap each other is your optimal VTC shift point.

I have to sratch every time my had, which changes has which effect.

Does anybody have a nice overview card or something else?

Thanks Guys, How can I be sure that the valves will not hit the pistons when the VCT activates/de-activates?

Ie I can set the cam gears to xx degree's and turn the motor by hand and make sure everything clears... does anyone know how many degree's the VCT on a RB25DET S2 changes the timing?

Hey Andre, Was hoping you could give your insights?

Hey Clint, I'll add my 2c to the mix. For the most part I'd say Adrian has given a reasonable overview of how to approach the tuning and there is no 'right' or 'wrong' way of doing it.

It's important to understand what the likely effect of any cam timing changes will be. As a general guide advancing the cam centreline will improve low rpm torque/power, while retarding it will improve high rpm performance so there is some compromise. Also it's really important to understand that you are likely to be starting the process with no real idea what your current cam centreline is (unless you've actually dialled the cams in using a degree wheel and a dial gauge), so you may be starting from an overly advanced or retarded position. It's also worth pointing out that in most cases the engine will be more sensitive to changes in intake cam timing rather than exhaust.

I'll start by laying down a base run with the cams at your zero or baseline point. Get the tune close at this point but there is not point being overly fussy as the VE and hence fuel/ignition will change as the cam timing is altered. I won't start changing the cam timing until I have a repeatable baseline to work from - Often the changes we will see are small so we need to be able to overlay two runs back to back before trying to change the cam timing. At this point I also note the IAT and ECT I'm running the car at as it's important to replicate these parameters for consistency.

I'll start by moving the intake cam only by 4-5 degrees and then perform another run to see the effect. If you've gone the right way you will see the AFR move leaner which means we have improved VE. This should show an improvement in power in the area you're interested in maximising performance. Often we may have started by going the wrong way so if everything went backwards, you can simply go the opposite direction by the same amount and try again.

Basically I'll keep going until I see no further improvements in power. Once you get close you can drop your timing changes to perhaps 2 degrees to start fine tuning the timing. Once the intake cam is close to optimal I'll follow the same process on the exhaust. As I mentioned though often this will show minimal improvement.

As you move the intake cam relative to the exhaust or vice versa, we are also affecting the overlap and this can be as important as the actual cam timing. How far you wish to go will depend on how fussy you want to be and how much dyno time you want to burn through. Once you have got to the point above where you've finished adjusting the exhaust cam, you may see some benefit from revisiting the intake cam timing. How much more improvement you can expect to see will depend a little on how much you needed to move the exhaust cam. If for example you ended up advancing the exhaust cam 2 degrees then the chances are you're not going to see much improvement by optimising the intake cam again. If however you ended up moving the exhaust cam by perhaps 6-8 degrees then there may be more to be had by revisiting the intake cam timing.

Each time you make a change to the cam timing it will affect the engine's VE so your fuel table and ignition timing may need to be optimised. Understandably this can become a reasonably long and slow process to get everything absolutely perfect. It's best to take your time, make small adjustments and then assess the effect before deciding what to do next.

One last tip - Because you will inevitably end up with the engine cooling while you make these cam timing changes, I always do two runs after a cam timing change to eliminate heat soak and ensure consistency.

I trust that helps :)

Thanks Andre, how about avoiding valves from hitting pistons... I know I can make a change then turn the motor over by hand... but the engine may be a little cool and at higher rpm if it was close it may hit?? , I guess I could keep making adjustments and turning over by hand until it does actually hit and then make sure I stay back from that... How many degree's would you call safe. and then there is the issue of the VVT on the RB25DET, im not sure if when the soleniod is on it advances or retards the timing and by how much..

Hi, I have an rb25, and when my car was tuned we tried adjusting the cams a little, so i can share my limited experience of the topic. I have a vipec ecu, so i used the aux output for the vct solenoid. We found the best power gains through the rev range by having the inlet cam (solenoid on) in the advanced position between 1500 and 5200rpm. It really was a huge difference in the low and midrange compared to having it in the retarded position. I have read that the difference between advanced and retarded is 20 degrees, although i cant verify that number.

I also got a small gain through most of the rev range by retarding the exhaust cam 2 degrees. Sadly we didnt have time on the dyno to try adjusting the ex cam any more.

I found these explanations and illustrations on the Tomei site quite useful when adjusting my cams. Its basically a simplified version of what Andre explained in his post further up in this thread.


@Viper you really need some idea of what amount of movement you can safely get away with. This is something that needs to be done during the engine assembly as simply adjusting the cam until the valves touch is not a great option. You need a reasonable amount of valve to piston clearance to allow for rod stretch and potential valve float at high rpm. During engine assembly I like to keep a minimum of 0.060" and preferably more like 0.080" clearance and I check this with playdoh during assembly. You can then note down the extremes of movement that are safe on your vernier cam gear.

To be fair though your chances of valve to piston contact are much more limited with a stock or very mild cam and a stock piston. In these situations you can usually move the cam further than you need to without risk of contact - That is still not a rock solid guarantee though!

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