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Discussion and questions related to the course How to Degree a Cam
I would like some further advice on setting up adjustable sprockets on my Nissan 4 cyl.
The sprockets came with basic setup instructions to dial in, but nothing with regard to how many degrees advance or retard to setup each cam. And if any consideration is needed when using heavier valve springs/increased lift.
The sprockets do not have degree increment markings, they are relying on dial gauge to setup.
I am using a GTX35 series garrett turbo on a 4 cyl, need to benefit from earlier boost, as timing was maxed out at 24 degrees on dyno
Nissan FJ20ET, stock cams, heavier springs.
Adjustable cam gears are for when you want to shift the power band. If you want better high rpm power, you want to retard the intake cam and advance the exhaust cam. If you want better low rpm power, you want to advance the intake cam and retard the exhaust cam. Since these are fixed positions you are going to lose part of the powerband; it's basically for engines that are too old for continuous variable valve timing.
Yes, this is what i need to do, shift the power band lower. So referring back to my question, how do i know how many degrees to advance/retard each cam, and does heavier springs affect this?
What are the specs on the stock ones? I mean Opening and closing timings in crank degrees. if you dont know, measure that first.
Go in 10 degree steps on the dyno. I'm not sure at what lift those timings are, maybe 1mm. . No idea about safety of valvetrain to do this, not my area.
Try intake - exhaust
10 adv 0
20 adv 0
20 adv 10 ret
I'd drop that to 5 degrees, max', at a time, if you feel you must make adjustments - 2 degree steps would be preferable, IMO.
Start from the OEM marks - they may already be the optimum point.
Usually, it is beneficial to increase the lobe angle - reduce the overlap - by retarding the intake and/or advancing the exhaust - be careful, get them the wrong way round and you risk valve to piston issues.
If that doesn't give the results you're looking for, with a drop - and it should be very clear by ten degrees - you can try going the other way BUT, beware, that reduces the valve to piston clearance and if it is already marginal...
He wants to improve low end. So retarding intake isn't going to help. 5 degrees steps would be safer, just take longer.
Ah, you are thinking of the intake closing point and effective CR with a NA engine? I had in mind reducing the overlap and so increasing exhaust manifold pressure by reducing the pressure bleeding back into the cylinder, and even intake, resulting in the additional problem of exhaust dilution of the charge.
Might come down to swings and roundabouts what he finds is the best option for his particular setup - still consider 5 degrees to be a large swing, though, and suggest 2 degrees.
Hopefully the chap comes back with what worked for him.
Just to be clear, I was speaking in crank angle degrees, not cam degrees. Cam degrees of course are what he will directly alter, but the specifications of the valve events for the stock cams are in crank angle degrees.
A lot of the principles of valve timing still apply between n/a and turbo engines. In his case, with higher effective compression ratio at low rpm he will require less boost to make power due to higher trapped mass. Look at how a Fiat MultiAir engine works. At low rpm WOT it closes the intake valve near bottom dead center to achieve low end torque, and then closes later at higher rpm. In the case of OP, by advancing the intake cam for earlier intake valve opening, he will have more backflow and reversion into the intake manifold as you said (during overlap). That isn't a good thing, true, but once the intake pressure exceeds the exhaust pressure you get additional scavenging from overlap.
This is why on the most siphisticated of variable valve timing control systems for turbo engines (think BMW, GM, Toyota, Audi) they have a special variable valve timing transient capability for scavenging. When you tip in, it waits until boost is just starting to build before fully phasing the cams to get scavenging. This is the unfortunate reality of using such an old engine with something as simplistic as manually adjustable cam sprockets. You just have to pick a strategy and stick with it.
So advance intake and retard exhaust?
I will start with 2 degree increments, these are the size advancements on sprockets with markings on them anyway.
Yes but don't expect miracles. If you want better low end you might need a different turbo.
Correct, not expecting a magic fix, just wanting this larger turbo to spool up quicker.
I would say the other way round ;-) - but try Ray's first, by all means, but be aware of possible valve to piston interference, if the OEM is already a bit tight. You may be able to push down on the valves to check if they are getting close, if you have access and the spring seat pressure isn't too high.
You may end up doing both, but either way, please post your findings.
Sorry I'm late to this party. I think the general concepts have been covered by this stage. My own personal preference which is detailed in the course, is to make adjustments in 2 degree increments. This is coarse enough to register a change, but not so fine that you're going to spend hours on the dyno. You'll quickly see a trend forming as to what your engine specifically wants. As has been mentioned, the GT35 is a big turbo on an FJ20 so your bottom end is never going to be stellar.
Thanks Andre, the only issue outstanding for me is how to double check valves staying away from pistons once adjusting begins...
Sadly the only way to do this is with a tear down and a dummy assembly as covered in the course. Generally When I check the valve to piston clearance I'll also test to see what sort of range I can swing the cams through before I get too close and risk contact. There's no easy solution once the engine is built and running on a dyno except the potentially expensive method of trial and error. Not sure I can recommend that method myself sorry!