Understand Correct Cam Choice

[Kaido_RR]

Firstly i would like to start out with this is not your typical "which cam is best for my car" topic, and YES I KNOW stock cams work well for my current power.

However i have come to the stage where it looks like i am have to take off my head to replace afew things and while the head is off i would like to change the cams and the supporting hardware. But before i go looking for which cam is best for my application i am wondering if anyone can give me a insight into what all the terms(angle, lift) mean and how they affect the power and torque.

Is it a good idea to go different angle/lift intake to exhaust?

What is the std spec for r33 rb26 cams?

Does anyone know the spec for Group A GT-R?

Also any personally experience from changing cams in RB26 and how it affect the car afterwards would be great also

Cheers

Edited March 13, 2011 by Kaido_RR

[Tony de Wonderful]

What is your application?

[Hanaldo]

My understanding is:

Lift = How far the valve is lifted off it's seat when the cam lobe is in it's highest position. Increases power without really changing the shape of the power curve, unless you increase duration (though this usually has to happen, depending on the type of lifters used? Correct me if I am wrong).

Duration = The angle that the valve stays off it's seat. So how long the valve is open. The higher the duration, the higher the rpm-range that the engine makes power.

Overlap = When both the intake and exhaust valves are open. The longer the overlap, the more top-end power you make at the expense of low-end and idle quality.

Timing = The degrees the valves open and close before TDC and BDC.

Not 100% sure if that is all right but that is how I understood it

[discopotato03]

If someone can find you a cam chart that shows the opening and closing positions of the valves and how the inlet and exhaust open periods compare . Note that with cam and therefore valve timing all numbers relate to crankshaft degrees except lobe center lines . Its done this way to relate valve timing to piston position ie when inlet valves start to open just before top dead center at the tail end of the exhaust stroke and going into the induction or inlet stroke .

Both sets of valves are generally always open for longer than the full inlet stroke or the full exhaust stroke and this makes it easy to work the full duration out from a cam chart or the advertised valve timing numbers .

Say for example the inlet numbers are 10-56 and the exhaust are 56-10 , this tells you that the inlet valves open 10 degrees before top dead center (BTDC) and close 56 degrees after bottom dead center (ABDC) . Now we know from top to bottom dead center has to be 180 crankshaft degrees so you add the BTDC number to the ABDC number and add 180 to get 246 degrees inlet valve duration .

For the exhaust its the same deal though its a rising rather than a falling piston stroke so they would be opening 56 degrees before bottom dead center (BBDC) and closing 10 degrees after top dead center (ATDC) .

The overlap is simply the two smaller numbers ie 10 + 10 added together because it represents the number of crankshaft degrees where the inlets are opening before top dead center (BTDC) and the exhausts are closing just after top dead center (ATDC) so in my example is 10 before plus 10 after to give 20 degrees overlap .

Don't be spooked by the TDC and BDC because all they mean is the piston at the very top of its stroke or the very bottom of its stroke . You can have a "B" or an "A" infront of either of the TDC or BDC and all that indicates is that the valve opening or closing events occur before (B) or after (A) top or bottom dead center positions .

The give away that cams are getting hotter is that overlap figure gets wider or numerically bigger and usually that's when the idle and low speed power suffers because the low speed trapping efficiency of the cylinders is compromised to make breathing improvements at the upper end of the rev range .

Now your call but I look at cams and turbos is a simalar way , go just that little bit too far and you lose the normal drive around power thats important on the street . My experience is that people who are conservative are disapointed less often than those who got a bit hungry and thought they could live with too much cam and or turbo .

Also its important to realise that multi throttle inlet systems like those on GTRs and GTiR's can use hotter cams and not get the reversion or choppy running at low revs as much , its because closed or nearly throttle plates physically block reversion waves that would ordinarily bounce around together in a single plenum and throttle type inlet manifold on an RB20/RB25/RB30E/ET .

A .

[R31Nismoid]

Stock cams don't work well, they work best.

That's the more accurate point i think.

You are running small turbos, putting in bigger cams will negate the bonus/advantage of having them.

Cams always look good on a dyno sheet. But if they don't help your lap time - what's the point

You are building a mild, yet fast track GTR,as many of us have said - don't try reinvent the wheel. Use what everyone else does!

the first 2-3 years you'll get more gains from simply driving. Only then, try development mod/testing as you won't be consistent enough.

[Kaido_RR]

If someone can find you a cam chart that shows the opening and closing positions of the valves and how the inlet and exhaust open periods compare . Note that with cam and therefore valve timing all numbers relate to crankshaft degrees except lobe center lines . Its done this way to relate valve timing to piston position ie when inlet valves start to open just before top dead center at the tail end of the exhaust stroke and going into the induction or inlet stroke .

Both sets of valves are generally always open for longer than the full inlet stroke or the full exhaust stroke and this makes it easy to work the full duration out from a cam chart or the advertised valve timing numbers .

Say for example the inlet numbers are 10-56 and the exhaust are 56-10 , this tells you that the inlet valves open 10 degrees before top dead center (BTDC) and close 56 degrees after bottom dead center (ABDC) . Now we know from top to bottom dead center has to be 180 crankshaft degrees so you add the BTDC number to the ABDC number and add 180 to get 246 degrees inlet valve duration .

For the exhaust its the same deal though its a rising rather than a falling piston stroke so they would be opening 56 degrees before bottom dead center (BBDC) and closing 10 degrees after top dead center (ATDC) .

The overlap is simply the two smaller numbers ie 10 + 10 added together because it represents the number of crankshaft degrees where the inlets are opening before top dead center (BTDC) and the exhausts are closing just after top dead center (ATDC) so in my example is 10 before plus 10 after to give 20 degrees overlap .

Don't be spooked by the TDC and BDC because all they mean is the piston at the very top of its stroke or the very bottom of its stroke . You can have a "B" or an "A" infront of either of the TDC or BDC and all that indicates is that the valve opening or closing events occur before (B) or after (A) top or bottom dead center positions .

The give away that cams are getting hotter is that overlap figure gets wider or numerically bigger and usually that's when the idle and low speed power suffers because the low speed trapping efficiency of the cylinders is compromised to make breathing improvements at the upper end of the rev range .

Now your call but I look at cams and turbos is a simalar way , go just that little bit too far and you lose the normal drive around power thats important on the street . My experience is that people who are conservative are disapointed less often than those who got a bit hungry and thought they could live with too much cam and or turbo .

Also its important to realise that multi throttle inlet systems like those on GTRs and GTiR's can use hotter cams and not get the reversion or choppy running at low revs as much , its because closed or nearly throttle plates physically block reversion waves that would ordinarily bounce around together in a single plenum and throttle type inlet manifold on an RB20/RB25/RB30E/ET .

A .

thanks discopotato what an insightful reply.

I agree that i don't want to go crazy big/huge cams, my turbos are relatively small (gt-ss) and so far the lag is not so bad, and the car pulls all the way to redline no issue. I was trying to work out if there is a possible way to possible shift the power band down alittle to help bring the turbos on earlier while still keeping the top end power. It does look like my head way need to come off therefore if i can do some head work with cams which will achieve the goals i want i think it will be a win win

[Beer Baron]

if you want to improve mid range and response with your GTSS turbos my advice is stick with stock cams but adjust the cam gears to get them coming on a little harder in the 3,500-5,500rpm range. should be able to gain 20kw or more in that range without losing any peak power.

[djr81]

Stock cam numbers are

Inlet

Opens 7 degrees BTDC

Closes 53 Degrees BBDC (Which I take to mean ABDC)

For a 240 degree duration

Exhaust

opens 63 degrees BBDC

closes 7 degrees BTDC.

for a 236 degree duration (or 250 degreee see below)

Yes it says BTDC because thats what the manual says. Which may either be wrong or explain why the exhaust cam gears help.

Anyway before getting carried away you need to work out at what point you need to have useable torque. No point having it at (say) 3500rpm if you never use the motor there or dont have any traction either.

[R31Nismoid]

I was trying to work out if there is a possible way to possible shift the power band down alittle to help bring the turbos on earlier while still keeping the top end power.

You can never have both.

As Richard said - stock cams + cam gears.

Why spend $1000-1500 on cams/fitting/tune etc for what might be nothing.

[discopotato03]

Yes I agree , I wouldn't be reaching for higher performance cams if you wanted to pull the lower mid range up .

If you really need the head off it probably wouldn't hurt to smooth the ports a little and blend everything in nicely around the valve seats . You could possibly smooth up the insides of the exhaust manifolds not to make them larger but to cut down any avoidable boundary layer flow losses .

I personally think an RB26 can use a bit more than 8.5 to 1 static CR so you could have the head decked but watch piston the valve clearances .

Good cam gears and a bit of playing around with the standard cams timing can probably gain you a bit easily enough .

What are you doing with computers and tuning >

A .

[NSNPWR]

A/M cams will give you more power for a given boost level compared to stockers...been proven time and time again.

I've put Tomei 260s in mine with -7s, still need to have a tune but as yet, no noticeable loss of drivability down low, in fact, considering I've turned boost down from 1bar to .7bar, I'd say it feels as strong as it did before, particularly from 4000 onwards. My 2c.

Edited March 15, 2011 by NSNPWR

[Kaido_RR]

Yes I agree , I wouldn't be reaching for higher performance cams if you wanted to pull the lower mid range up .

If you really need the head off it probably wouldn't hurt to smooth the ports a little and blend everything in nicely around the valve seats . You could possibly smooth up the insides of the exhaust manifolds not to make them larger but to cut down any avoidable boundary layer flow losses .

I personally think an RB26 can use a bit more than 8.5 to 1 static CR so you could have the head decked but watch piston the valve clearances .

Good cam gears and a bit of playing around with the standard cams timing can probably gain you a bit easily enough .

What are you doing with computers and tuning >

A .

running power fc D-Jetro with apexi map senors, currently sitting on 370rwhp @ 18psi with alittle some spike around the 5000rpm range to 20-22psi then back down to 18psi (remember off the top of my head as don't have dyno print out)

Was thinking i would run a thicker head-gasket if the head is coming off.

For the cams i was looking at some group a cams

Working angle Lift Valve timing

264°(66°) 9.7 105°

272°(68°) 10.1 105°

I have done alot of read regarding cams and there seems to be two side to each story one side is "stock cams + cam gears" other side is "a/m cams + cam gears = better than std"

Now i don't want to change cams more than once, my power range is 350-450hp i will never go more than that as i want a reliable track car. Now i now i can achieve that with stock cams (i have already) however will cams give me more torque through the mid range and more average power compare to std?

I know this is not a simple question to answer and their are alot of factors which play into it (like condition of engine etc), However there has to be a reason why some people get results from aftermarket cams and others don't?

Edited March 15, 2011 by Kaido_RR

[R31Nismoid]

however will cams give me more torque through the mid range and more average power compare to std?

As i said, go do your lap times and get consistent first. In 2-3 years, visit cams if you wanna spend money. Seriously.

On a dyno, loaded up - probably will see increases/gains.

However will that equate to faster lap times? Potentially not.

Circuit GTR's down here were faster with stock cams compared to larger ones (Poncams etc).

They had aftermarket cams in them (i can think of 5 GTRs instantly), put stock ones back in and all was the better for it.

Lap times are a FAR better judge of such things, and you will not be in a position to do that - for at least 2-3 years.

You need to stop parts racing, and start racing yourself first.

[discopotato03]

Nismoid would I be right in assuming that your message is to grow into a car rather than attempting to grow into a car that is a changing developing thing - in other words not a consistant known quantity ?

If it were me taking a production car club racing I would be concentrating on making the thing reliable in pretty std trim and getting to know its driving characteristics while improving my own skills .

I think this way people learn their own limits and more about where a car is std and what needs to change to make it a better club car .

I think there are times when the numbers are less important than just getting out and having a go , you can worry too much about the numbers and form atrifical limits based on if they please you or not .

The growing into the machine as it is should be the prioroty here ,IMO .

A .

[Joe_89]

This has been a good read, I understand cams a little more now so thatnks to the contributors

Dont mean to Thread-jack, but can somebody tell me, what do the guys that rev out to 9000rpm do in terms of cams?

[R31Nismoid]

Nismoid would I be right in assuming that your message is to grow into a car rather than attempting to grow into a car that is a changing developing thing - in other words not a consistant known quantity ?

If it were me taking a production car club racing I would be concentrating on making the thing reliable in pretty std trim and getting to know its driving characteristics while improving my own skills .

I think this way people learn their own limits and more about where a car is std and what needs to change to make it a better club car .

I think there are times when the numbers are less important than just getting out and having a go , you can worry too much about the numbers and form atrifical limits based on if they please you or not .

The growing into the machine as it is should be the prioroty here ,IMO .

A .

That is exactly my point! And the same point a number of others have had (Marlin, djr82 etc etc).

Over Kaido's threads (there is a massive one in MSport), it's just about him buying parts without any thought or experience.

He is building a track/circuit/club GTR. The best thing initially it get it reliable, which he has done with oil cooler/turbo swap and so on.

The next part now is to stop worrying about spending money and go and drive the car. Learn it, talk to others @ club days to see what makes their car fast. Workout what they have tried/have not tried.

I think that's the best advice - and best of all, it's free!!!!

I'm not having a go at him or anything like that, I just think it's very silly to be talking about spending thousands of dollars without even knowing how to drive the car fast at this stage. It's the same thing many have been saying for 6 months now.

I'm in a similar boat. The GTR I've purchased has very similar spec/proven formula to many other cars and at a recent hillclimb i was 8 seconds slower (over 60 sec lap).

It's not setup slow - its driver slow. It'll take me many sessions to knock off those 8 seconds. I will get there eventually but it won't be by spending $1,000's on parts i don't need. It's simply experience which costs nothing but holds the most value

[NSNPWR]

Also depends what you want to do with the car, not everyone tracks their GTR.

[R31Nismoid]

Indeed i agree. OP is building aclub level circuit car though, that much is certain. Have a read of his build thread in MSport section.

If it was drag then it would be a different situation etc.

[djr81]

The thing to remember is that seeing how the car is in WA the circuit in question is Wanneroo. It tends to favour top end hp rather than low - mid range torque. Understeer around there will kill you lap times too.

But the main point is to get out there & have a go. Doing so should tell you two things:

1. How much you need to improve by.

2. Hopefully where you need to improve.

The other thing is that it is more interesting modifying a car based on using it than building one without any feedback mechanisms other than from the internets.

Cams are a good example. If you get out there & have a steer you will quickly discover the motor doesnt drop below 4000rpm around the long circuit. So cams that work well at 3000rpm are wasted. On the other hand cams that work too high in the rev range are useless at events at the AHG centre.

Above all know that there is no one right solution. If you look at the quick cars around in WA you will see they make their lap speed in different ways. The very quick cars (of which there are very few) by definition do everything well. But they cost alot of money and have taken years of development.

[Beer Baron]

having owned more than 10 GTRs now with standard turbos, GTSS turbos, HKS 2530s, single turbos and N1 turbos, some with stock cams, one with HKS cams, one with midori cams, two with pon-cams I think I have a pretty good feel for them.

my advice again. for your car, with your turbos your best bang for buck option is getting some cam gears put in and tuned (along with any changes needed in the ecu). it will give you 20kw or so in the area I mentioned above which will help with punch out of corners and won't cost you any top end power (in fact you may even gain 10kw or so). that is about the best compromise you are going to get. also, you don't need the head off to change cams so honest it makes no difference if you do them now or a year from now. just do the cam gears get it tuned and see how that goes. bigger cams are going to rob you of power somewhere and give you some somewhere else. that is all they can do really.

my current GTR is a jun 2.7 with 2530s, tomei pon-cams and a motec etc. it's surprisingly torquey and very fat in the midrange but this could be down to the tune or even the stroker (even though it's only a small change in stroke). they suit the car quite well and I'm pretty happy with it's set-up, but for your current standard engine with GTSS I think starting with the cam gears is the way to go. on top of all that it's not wasted money. even if you do go cams in the future you can still use your cam gears with the new cams. there is basically no good reason NOT to do it this way.