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Upgrade Path for Stock J-Spec


Wez
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Hiya Guys,

 

Just wondering what would be the recommended upgrade path for a stock J-Spec, I am guessing induction and exhaust and losing one of the cats.

 

What items are good and how much are they (roughly), I have also read that the stock fuel pump isn`t all that on a J-Spec aswell so i guess replacing that will need doing aswell.

 

:stupid:

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Cheers Alex,

 

But i thought removing both cats on a J-Spec was a bit dodgy as the waistgate is not big enough to flow the extra air therefore giving you a boost creep problem at WOT.

 

Is this true?

 

What is the max safe psi for a stock J-Spec, i thought it was arround 18psi.

 

:stupid:

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Fuel Cut Controller (TRL VFCC)

 

De-cat both pipes (or first downpipe CAT as a minimum)

 

But i thought removing both cats on a J-Spec was a bit dodgy as the waistgate is not big enough to flow the extra air therefore giving you a boost creep problem at WOT

 

You may experience this. But generally it is above 11psi (stock) but below 18psi (modified), and.........

 

What is the max safe psi for a stock J-Spec, i thought it was arround 18psi.

 

yes 18psi is the safe maximum.

 

So most people remove both CATs and get an increased minimum boost level but as this is normally below 18psi they can safely control it.

 

Regards

Pete

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Thanks for the info Pete...

 

What is a good set of front pipes, I noticed that apexi do a downpipe set for the J-Spec, are they any good? As for the rest of the exhaust I am planning to go for the blitz nur spec.

 

For induction I was looking at getting an apexi cone.

 

Is the reason that 18psi is the max because of the map sensor not being able to accuratly read above 18psi?

 

I was looking at your fcd device as this is the one that everyone seems to be using :)

 

 

Regards

Wez

 

:stupid:

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Thanks for the info guys,

 

So you remove the cats and replace them with restricted downpipes, or have I got it wrong?

 

2.5" sounds small, i thought the apexi downpipes were larger than that, and the nur spec is deff larger than that.

 

I am guessing that is because of the tiny waistgate again, has anyone bored out there waistgate to get round this problem so they can use normal bore downpipes?

 

£300 pounds sound like a fair price, who can supply them?

 

 

:baa: :baa: :stupid: :baa: :baa:

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Well, the effect of fitting "bottlenecked" downpipes (like Chris makes) is to restrict the amount of exhaust gases coming out of the engine, and therefore the amount of exhaust gases which go through the turbo spinning it up. The result, less boost.

 

If you were to enlarge your wastegate or fit an external one, this would flow more exhaust around the turbos (as opposed to spinning them up). The result, less boost.

 

The downpipe method is cheaper and easier. I think Alex has a pair of Chris Wilson pipes for sale, making it cheaper still!

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The MAP sensor definitely does read up to 18psi.

Above that and you reach the upper limit of 4.95V output from the 5V MAP device.

So even if the sensor inside could read above 18psi the output stage is limited to a max of 4.95V

 

18psi is not the max because of the MAP, that just happens to be a coincidence.

It's the point at which safe fueling runs out. This is why you have to be aware of the dangers in hot weather with any turboed vehicle if you use more fuel as a power generator which is leaving you less fuel to cool the air charge down. Remember turbos are a big heater. As you compress a gas the molecules collide with one another in the turbo housing and collision causes energy release in the form of heat.

 

The Supra normally has enough excess fuel to handle 18psi of boost and still cool the air charge enough. But on v.hot days be careful. I only run 18psi with water injection as a backup.

 

As for pipes, Chris Wilson or Leon Green can supply their own versions. Chris' are 2/5" and won't cause any boost creep, Leons are 3" and you MAY find you get boost creep.

My minimum boost level increased to around 16.5psi from 11psi stock, but as I wanted to run 18psi with a boost controller I wasn't bothered.

 

Regards

Pete

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I think that you should go with the 2.5" pipes, as some j-specs have had near un-controllable boost with 3" pipes.

 

Although you could of course get the 3" pipes and get a 2.5" restrictor ring fitted.

 

I personally have Leon's 3" pipes, and boost is fine due to the car being UK-spec, which do not seem to 'overboost'. :)

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I have 3" pipework

A complete 3" Mongoose system actually

 

And no excessive boost creep and mines a Jap spec.

 

BUT!!!! I do have hybrid turbos and an enlarged wastegate etc.

 

I had 3" downpipes with my stock back box system and NO boost creep BUT on fitting the hybrids realised there was a breathing problem and this was eventually tracked down to a collapsed back box section. Hence the replacement Mongoose system.

 

(Mongoose system incidentally is £500 for the whole lot which is the same price for the fancy HKS or Blitz big bore back ends. I felt it was more ecomonical for me, plus quieter for the many motorway miles I do.)

 

Pete

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Just a quick note about the differences between restrictive pipes and wastegates. Fitting a more restrictive pipe will NOT have the same effect as a larger wastegate. Here's why.

 

The force which drives the turbine is related to the pressure difference between the inlet and outlet of the turbine housing. By increasing back pressure in the downpipe, the outlet pressure is raised, thus the difference is lowered and also the driving force.

 

If the dowpipe is increased in size then the back pressure post-turbo is reduced. Initially this means that there is more driving force, which allows the turbo to spool up more quickly. But then we hit a problem. In order to maintain the same driving force as before, the pressure pre-turbo needs to be reduced. This is the job of the wastegate, but as it is too small, it cannot lower the pressure enough. The result is boost creep.

 

If we now port the wastegate, such that it is able to flow enough gas to lower the pressure pre-turbo to get the same pressure difference again (and hence the same driving force as before, so same boost, no creep) then we find that the pressiure in the exhaust AND the exhaust manifold have been reduced. This means that there is less residual burnt gas in the cylinder at the end of the exhaust stroke, which in turn means that we can get more fresh charge into the cylinder, and if we have more fuel and air, we have more power.

 

So essentially, a small wastegate and a restrictive downpipe is EVIL. Not only does it reduce power output by hammering volumetric efficiency, it also places a higher thermal stress on the engine, as more hot gas remains in the cylinder (poor extraction). This in turn means that to make the same power as with a free flowing system you need more boost to cram the mixture in there. More pressure means more temperature rise in the intake charge due to adiabatic compression and compressor inefficiency. Which means that for a given intercooler, the charge going in is already hotter, never mind the extra residual gas at the end of the compression stroke.

 

People often have toruble understanding that boost on its own means nothing. I proved this point quite graphically on the dyno by getting more power than a chap that was running 50% more boost than I was; both cars with the same displacement, but one breathing well and the other an ill-matched setup that was superheating the intake.

 

Please do not be fooled into thinking that a restrictive downpipe is an engineered solution. It is a band-aid to counteract the effects of a mismatched turbo setup. The right method is to correct the cause of the problem (too small a wastegate), not to try to butcher the pipework to make the system work with it.

 

Cheers,

 

Pat.

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hmm, when given the choice of enlarging a wastegate which means a full turbo off job along with shipping the turbos off for a week, and just changing the downpipe at a fraction of the cost and time I will accept the downpipe every time.

 

Its all a question of acceptable cost when modifying a car. Nobody on here would accept the cost of enlarging the wastegate when the cost compared to increase in power is small compared to the increase when using a downpipe.

 

JB

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It's an interesting technical point, but I'm not sure if it applies in this context.

 

We have a stock system with a small wastegate and a restrictive exhaust. We add a less restrictive exhaust system and this now allows slightly too much exhaust through the turbo impeller rather than the wastegate. So we pinch the pipe a tad in some fashion to make the exhaust system slightly more restrictive than it is now, but still not as restrictive as the stock system. Ergo, a sensible level of boost is maintained.

 

For ultimate efficiency, as in getting as much exhaust gas out the door as fast as possible, an enlarged wastegate would seem ideal. But you would really need a different exhaust manifold as well, before that stage, as that's the biggie in the whole 'restrictive output' thang (by design, to accomodate the sequential twin setup). And when you are looking at different manifolds, you are looking at bigger turbos to match that performance. And they have their own wastegates...

 

IMOO. (Overinflated :) )

 

-Ian

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Originally posted by pat

Just a quick note about the differences between restrictive pipes and wastegates. Fitting a more restrictive pipe will NOT have the same effect as a larger wastegate. Here's why. . . .

 

 

Cheers,

 

Pat.

 

Well, yeah, I was gonna say all that but I didn't want to go over peoples heads with it :D

 

Seriously though, some good info in there - I sort of knew that's what was happening but I couldn't have written it down so well!

 

Like Ian and Branners have said, probably overkill on a largely stock system with many huge restrictions in it anyway.

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Branners,

 

Perhaps most people aren't quite as adventurous, but I have no issue in taking a tungsten carbide grinding burr to a turbo to port it. I've done turbos, exhaust pipes, manifolds, collectors, the list goes on and on. It's not difficult, in fact the hardest part of the job is getting the turbo off the car. Once off the actuator can be removed to allow the button to open and thus allow access to the wastegate orifice. About 15 minutes later you have a ported high flow wastegate. The only downsides are if a) yiou go too big and the hole ends up bigger than the button and b) the actuator cracking pressure will be reduced a little because the increased wastegate button area exposed to exhaust manifold pressure results in a greater force against the actuator trying to open the wastegate.

 

Of course this kind of thing should really only be done in conjunction with a good boost controller, or preferably a replacement mappable ECU.

 

Ian,

 

any back pressure behind the turbo is too much back pressure, pure and simple.... in an ideal world you'de want a vacuum there but this isn't an ideal world so the best you can hope to achieve is atmospheric pressure, and realistically a couple of PSI if you don't want to wake the dead as you're driving past the local grave yard.

 

The idea of deliberately introducing a restriction to control boost seems, to me at least, absurd. The whole point of the replacement downpipe is to reduce back pressure as far as possible. It would be a different issue entirely if the restriction was added to increase dynamic compression (and thus thermal efficiency) at high vacuum (basically EGR).

 

Thoughts on exhaust manifold design accepted, but that would be a major monetary investment, whereas porting the wastegate is a temporal investment (ie it's not a 5 minute job). I'de call it a weekend job. The cost of the grinding burr should be no more than UKP 20, so it's not going to break the bank.

 

Of course, in an ideal world, you would replace the exhaust manifold for a stainless steel tubular item, and then fit a large turbo with a variable A/R ratio exhaust scroll (this is NOT the same as the Aerocharger VATN or Garrett VNT) allowing good spool up with low back pressure at higher RPM / flow. But we're talking serious money here, while an exhaust system and ported wastegate will give good gains for relatively little investment.

 

Cheers,

 

Pat.

 

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The Turbo wastegate outlet on the jap spec is 2 3/4". I installed this size custom down pipe along with a 3" HKS midpipe and a 4" HKS Priest exhaust. I have not had any boost creep. Without the Profec B switched on I am able to hit max boost of 1 bar. :D

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Guest Martin F
Originally posted by randy

The Turbo wastegate outlet on the jap spec is 2 3/4".

 

I think you mean the exhaust outlet diameter, which is a completely different thing to the wastegate diameter.

 

If you had a wastegate diameter of 2 3/4" you'd go from full boost to no boost quicker than hitting a brick wall.... :eek:

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Originally posted by pat

Ian,

 

any back pressure behind the turbo is too much back pressure, pure and simple.... in an ideal world you'de want a vacuum there but this isn't an ideal world so the best you can hope to achieve is atmospheric pressure, and realistically a couple of PSI if you don't want to wake the dead as you're driving past the local grave yard.

 

The idea of deliberately introducing a restriction to control boost seems, to me at least, absurd. The whole point of the replacement downpipe is to reduce back pressure as far as possible.

 

I don't understand. The stock exhaust puts restriction X on the turbo, a very free-flowing one puts restiction Z on the turbo but causes too high a boost level, what's wrong with a slightly narrower exhaust causing restriction Y that limits the boost to an acceptable level?

 

Or does the enlarged wastegate allow a greater volume of exhaust gas over the same given time to escape the engine compared to a slightly restricted exhaust and a normal wastegate, therefore increasing the overall flow of gas through the engine at the same level of boost? Does this improve the engine performance/heat levels?

 

Just trying to learn :)

 

-Ian

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Ian,

 

My comments are directed at the scavenging capabilities of the engine and how they are compromised by high exhaust backpressures.

 

Imagine for a moment that you are shoving 1 bar into the intake maifold, then the pressure in the exhaust manifold will somewhat higher than 1 bar on stock turbos (you can only get the pressure in the exhaust to be lower than the intake with huge A/R ratio exhaust housings or variable geometry turbos).

 

When the exhaust valve is open and the piston is pushing out the burnt mixture, there will be a volume at the top of the stroke which remains and cannot be pushed out (this is the clearance volume). This volume is constant.

 

If we take the case of a restrictive exhaust system, then the pressure in the exhaust, and by the same token, in the exhaust manifold, will be greater. This means a greater mass of gas (same volume, roughly same temps, but more pressure) will be left in the clearance volume.

 

When the intake valve opens and the piston draws in fresh mixture from the intake manifold, there is always some burnt mixture left in the cylinder. Sometimes this is wanted (EGR) and sometimes it is not (big power). In the latter case, for the same boost pressure, and the same RPM, your piston will be inducting less fresh mixture than an engine with a less restrictive exhaust system. So with less mixture going in it will make less power at the same boost level.

 

Also, the burnt gas that is left in the cylinder will be quite hot, and this will heat up the incoming mixture. If the fresh mixture is starting off hotter, then the compression temperature due to adiabatic compression will also be higher, and somewhat closer to the autoignition temperature of the fuel. In other words, the system is more prone to detonation because of the residual heat in the cylinder. The only way to stop it from detting would be to lower the incoming charge temp more (fit a bigger intercooler) or to back off the ignition timing. The usual quick fix method is to back off the timing, indeed the ECU should so this for you automatically if it is working properly.

 

Timing can have a huge effect on power and torque (well, they inextricably linked). I would expect to lose about 10 to 15 lb/ft for every degree that you lose; at 5152 RPM that's up to 15 BHP per degree. It doesn't sound like a lot, but what if the ECU has to pull out 3 degrees ? (I'm not saying that it will, just that it could).

 

There are other ways of improving the situation. You could get some longer overlap cams so that the remaining gas is literally "blown out" by the incoming charge, but of course some of the fresh charge will join it on its way down the exhaust and that is obviously wasting fuel. Alternatively yoiu can just throw more fuel at it so that the energy consumed in turning the fuel into a gas reduces the temps again (old Cossie trick) or you can fit water injection (works best with a 50/50 water/methanol mix). But at the end of the day these are all "band aids"... sometimes these can be useful for extending the performance envelope where certain restrictions are applied to what you can and cannot change (eg formula racing, rallying etc) and also in the realm of street cars where one doesn't wish to sacrifice day to day driveability just to get every last possible BHP out of the engine.

 

Certainly a free flowing exhaust should not impact the car's driveability in traffic so in this instance that makes more sense than a band aid. But if one were to go to the length of dropping the compression down to (say) 7:1 then that would no longer be a useful street machine and perhaps a "band aid" would make more sense (unless it really is a pure race car).

 

Hope this helps,

 

Pat.

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