Supra 2JZ-GTE cams and their optimisation

 

Overlap allows boost to leak straight to the exhaust, right?

Wrong! That's what illiterate backstreet 'mechanics' believe, and several tuning books reinforce.

This is only true in supercharged engines - turbochargers create such a restriction in the exhaust that most of the time we face exactly the opposite: reversion

 

Backpressure and turbochargers

There are several locations where backpressure can be measured in the exhaust path:

Before the turbine: this what the engine sees. It is the max value but hard to measure in real life

After the turbine: this indicates the flow restriction of the whole exhaust. It is the easiest to measure in real life.

After the cat: useful only really for troubleshooting if the cat is suspected of being blocked. Sometimes it's easy if there is already a second O2 sensor bung.

Before the silencer: only useful for checking up on the flow restriction of the silencer

Obviously the figure is higher the closer we are to the exhaust valves, and lower towards the exhaust tip (diffuser) where it reaches zero, as it is atmospheric pressure there at last!

Backpressure also varies with revs (increases as revs go up and it is not linear). It is also a function of boost: more boost, more exh backpressure --- or the higher the intake manifold pressure, the higher the exhaust manifold pressure.

It is not just the exh backpressure that is important to us: even more important is the pressure ratio at any one time, the Intake/Exh pressure ratio. Why? I hear you ask.

Glad you asked. It is a fundamental and simple thing, yet most people don't get it, including many so-called 'tuners'.

Suppose we have a pipe with two open endings, let's name them A and B. The pipe just sits there on the table. Ending A is under 1bar absolute pressure (atmospheric) and so is ending B. Do we register any flow within the pipe? Of course not, there is no pressure differential across the endings, as we said the pipe just sits there. Simple you'd think eh?

Now suppose we have a cylinder with valves on top, that can open or close, or both be open at the same time (much like an engine during overlap). Let's call the sides intake and exhaust. If both sides are under the same pressure, will there be any airflow? Nope, there is no pressure differential so there is no reason for the gases to move either way. This is oversimplifying the workings of an engine at 6K rpm, but the concept is the same at a macro-level.

Now suppose that this engine registers 1 bar boost at the intake and 3 bar backpressure at the exhaust side. Which side will the gases flow during overlap?
Doesn't take a rocket scientist to figure this one out, does it?
This is the case with most factory turbocharged engines: from the point of max torque upwards, exh pressure far exceeds intake pressure, at high revs even exceeding 2.5:1

 

Backpressure and the SupraTT

If you're doing 1.5bar boost on a stock turbo, that could well mean 4.5bar backpressure at top revs. In that scenario would you like to be running cams with any overlap? I didn't think so..
Toyota engineers didn't think so either, so they endowed the 2JZGTE with minimal overlap, almost zero in fact. On paper it is 7degrees (end of exh 4 ATDC + start of intake 3BTDC) but with stock valve lifts this is practically zero (the 'area under the curve' is miniscule. If you had the same degrees overlap with wilder cams, then you'd have more overlap despite the degrees being the same)

The designers went out of their way to eliminate overlap and there is a good reason why: they have also gone out of their way to increase backpressure all over the rev range, so they were trying to keep reversion (exh gases backing into the chambers) to a minimum.

In the case here, turbocharger backpressure is an indication of how much the exhaust gas stream is being squeezed for power. The tighter the gap, the more energy can be extracted from the exh gases, and the stronger the forces that will drive the compressor (giving us boost). Unfortunately this tight squeeze also suffocates the engine and creates evil reversion that poisons the intake and preheats the chambers, sending EGTs skyhigh. So a delicate balance has to be found. Cam timing is part of this balance.

The supraTT has two small turbos attached to the engine, as we know. Small, as in each one can only produce 150-200bhp with ease. At low revs (until 4K) only one of the turbos is seen by the engine, so all cylinders are looking at a single tiny turbine. That is the reason exh backpressure is abnormally high in this engine. The upshot is that it builds good boost from 2000rpm upwards. At around 3Krpm the backpressure becomes very high and at 4Krpm painful, but that is OK because by that time the second turbo comes online and the exhaust 'size' that the engine sees doubles in size. So backpressure eases down at 4-5Krpm and then hits highs again at top revs. The turbos are being squeezed dry throughout the whole rev range.

This is also evident when people perform the 'True Twin' mod, where both turbos are wired to be online all the time. One of the 'advantages' they claim is a lot more exhaust noise at low revs. That is because exh backpressure at low revs is very low with this configuration. Too low perhaps, that's why the engine cannot make any boost at that point. You see, another byproduct of exh backpressure 'strangulation' is exhaust silencing. The energy goes into spinning the compressor, not into waking up the neighbours.

 

 

	JSpec	UK/US spec
Intake lift (mm)	7.80	8.25
Intake duration (deg)	224	233
Intake Open BTDC (deg)	3	3
Intake Close ABDC (deg)	41	50
Intake Centre ATDC (deg)	109	113
Exh lift (mm)	8.4	8.4
Exh duration (deg)	236	236
Exh Open BBDC (deg)	52	52
Exh Close ATDC (deg)	4	4
Exh Centre (deg)	114	114
Overlap (deg)	7	7

 

Here is a good basic writeup on the cylinder head of this engine Cylinder head 101 by Lance Wolrab. (local copy here)

One intersting observation might be that he doesn't like the exhaust port design, in his own words "...the exhausts are diabolical. Japanese engine theory bases exhaust port design on thermodynamics, not airflow..."

However, the Japanese may be correct in basing their designs on thermodynamics. The temperatures involved are considerable, as we know EGTs can exceed 900C under full throttle. Therefore static flowbench figures done under room temperature are bound to be flawed.

On to Gotchas & Tips...

 

Various tech

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

RB25 DET

There is one inlet cam lobe per valve, with a duration of 240 degrees, and this is advanced and retarded over a range of 20 degrees. The advanced timing is inlet opens 20 BTDC, closes 40 ABDC. The retarded timing is, inlet opens at 0 TDC, closes 60 ABDC.

The cam is retarded at idle for just about zero overlap, it is fully advanced at around 1,500 RPM, and again fully retarded above around 4,200 RPM.

These timings give a very worthwhile low RPM torque boost of around 5% on the RB25DET. The same valve timing on the RB26DETT does stuff all.

Both the RB25DET and RB26DETT both have 240 degree duration (seat to seat) inlet cams, although the lift profiles are quite different. The factory RB26DETT inlet timing is: open 4 BTDC, close at 56ABDC.

 

VG30 DETT

NVCS (Nissan Valve Timing Control System) was used to vary the timing of the inlet cam by 20 degrees:

	Inlet Valve Opens	Inlet Valve Closes
NVCS on	19 degrees BTDC	49 degrees ABDC
NVCS off	-1 degree BTDC	69 degrees ABDC

The timing of the exhaust valves was fixed, with opening occurring at 59 degrees BBDC and closing at 9 degrees ATDC.

At idle and low loads the NVCS is off, while at medium to high level loads at less than 6100 rpm the NVCS is on. At all loads above 6100 rpm, NVCS is off. The action of NVCS makes a substantial difference at engine speeds below 6100 rpm, adding as much as 30ft-lbs to the torque output.

The 28 degrees of overlap when NVCS is on is high compared with what would be used without having variable valve timing. The similar era Nissan RB30ET SOHC turbo in-line six (as fitted to the Holden VL Turbo) can be compared - it has a cam overlap of only 18 degrees. Note that the graph shows that more torque is obtained everywhere under 6100 rpm with the NVCS 'on' valve timing; however the 8 degree overlap that occurs with the NVCS off helps give a very smooth idle.

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Ask him if he's measured them himself. If he has (or knows someone who has) maybe he knows how they really compare to the stock ones.
From what I've found through others, the real differences are smaller than advertised.
As a rule, I'd always go with the 'mildest' I can get away with, turbos love that.
I'd avoid fancy lifts too, if 9mm were safe, Toyota would have done it first. The whole valvetrain suffers then, shims need changing, springs break, lobes wear out quickly.
And how do you know that the lobe profile is as safe as the stock one? You don't.
You only find out the hard way. A few extra Gs and the valves start to crack.
Longer durations mean that the exh valves have less time to sit on the seats, to dissipate their heat.
So they start to burn out.
It's swings and roundabouts you see, once you move far away from stock

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Do you happen to know the exhaust-side A/R of your turbo?
It might be imprinted on the outside of the turbine. It's important.

At the moment I'm on the stock j-spec cams, which I think have slightly less duration than the UK ones.

The inlet one, yes.
it's got 7 degrees less duration and almost half a mm less lift.

I've bought 264 cams second hand from MTTE. They trade - but rarely tout for business - on here. He was kind of pushing the cam gears, but I'm not going to buy them.

Some times you're better off with adjustable cam gears, it's not cut-and-shut.

The 256/264 combination (for HKS) seems popular,

popular with people who sell them?
Those who buy them might be clueless and blame the turbo/the weather/the fuel/etc, or might be smart enough to keep their mouth shut so they can sell them later on.
You never know.

but I was figuring that as these are fairly mild changes anyway, I'd be OK with 264/264.

You may be interested to know that aftermarket cams are made to horrifyingly wide standards. Even the angles can be way off, or the lobes misadjusted for a few cylinders. Or break apart for no reason.
You get the idea, they don't have to adhere to any standards, like OEM parts have to.

As a smallish turbo,it spools up pretty quick anyway, so I thought I could sacrifice a little low end power for a bigger hit further up the range.

Changing the cam timing (swapping cams or playing with adjustable gears) can have a dramatic effect on how the turbo comes on.

A cheap way forward is to leave the exh cam alone and swap for a UK-spec inlet cam (dumped by someone who thinks Toyota don't know what they're doing)

If you still feel itchy, you could go up to an exh 256 max and and a 256 inlet as well.
As I said, I'm not even sure how the '256' figure from these people compares to Toyota's figures. It might be very different.

Your EGTs comment concerns me a little though. Are the high EGTs just the result of mating aggressive cams with a restrictive manifold.

yes, in many ways 'aggressive' cams and restrictive manifolds/turbos lead to excessive EGTs, through horrific reversion.
This you can dial out with adjustable cams though, reducing your effective compression ratio at the same time (two birds with one stone)