A Question For You High Boost People

[Kinkstaah]

Hi all

I got an answer to this in another thread, but because I am unsure I'm just going to ask again.

I recently toasted a turbo. Pretty bad.

I figured a nearly toasted turbo could explain my boost.

But now I have a new one, on a new engine, and very similar behaviour.

I'm hoping it's not just me.

In any case: For those of you who run a lot of boost (30psi region...)

How much do you make in 1st?

How about 2nd?

3rd?

4th?

5th?

6th if you have a R34 GTR and have security guards

Do you see an instant boost increase after you shift?

How much of this is load based? For people who make 30psi on the dyno in 4th, how much do you get while redlining 1st?

30? 17? 22? Varies?

I know that IN THEORY you get less resistance in 1st, so you will make less boost. You make none in neutral. You'd make more in 5th. Flow doesn't mean pressure. I get all of those concepts. You could say a super flowing head is going to max out a turbo at 20psi, while a stock head may max a turbo out at 35psi.

But I want to see numbers.

Anyone?

[r32-25t]

If tbe wastegate is doing it job properly you will see the same boost pressure in all gears

[Kinkstaah]

That's the thing, the wastegate in my application is absolutely sealed shut until the 1:1 gear. Freeboosting does NOT get the MAP pressure up to the point where the wastegate would open.

Hence me asking for high boost people. Obviously in a stock application when you're only building 5psi, it'll do so.

There's no boost leaks.

The wastegate is working properly.

The car makes power in the range you'd expect to be maxxing out the turbo.

I just wanted to know if I was the only one who noticed this kind of behaviour..

[GTSBoy]

That behaviour will depend very much on the front-rear match of the turbo and the match of the turbo to the engine. If the turbo is quite large then it is likely to be possible that the engine can't flow enough gas through the turbine to reach the required compressor speed to achieve max boost and hence open the wastegate. It would possibly sit somewhere less than peak boost. The relationship between the sizing of the turbine and compressor sides probably has a lot to say about how different that will be between various gears.

If the turbo was relatively smaller, then the engine could probably spin it up to make the boost limit with a smaller amount of gas flow, and this would show up as running with the wastegate open in more of the lower gears.

Going back to your first post, 30 psi target (say) with a big turbo is of course a very different thing to 30 psi with a smaller turbo. So there's nothing to say that having the large turbo running with closed wastegate at (say 26 psi) isn't making more power than the smaller one at 30 psi with the boost control working.

So while I don't have direct observational evidence for you, I can see exactly how it can work out the way you're seeing it happen.

[Kinkstaah]

I can see how that could work, GTSBoy, with a HUGE turbo, you may not actually spin it enough to make pressure.

However, mine isn't that scenario, in fact, it's the opposite.

I have a small turbo, on an engine which is (maybe?) happy to flow it quite a bit.

I'm actually concerned I'm maxxing the turbo out, so the engine is ingesting air so happily that the turbo can't flow enough to actually build up pressure with the wastegate completely shut, that the engine is happy to rev and redline without boost pressure 'backing up' so to speak. This effect is lesser with each gear as obviously it revs out slower.

Cause it certainly is flowing air.

I realise this scenario is ALSO 'possible', but don't know if anyone else has experienced it. Given my last turbo exploded in quick time I'd rather not repeat the scenario, if this is just dumb luck or something else, was curious to see if people see this kind of behaviour.

For reference, I make about 18psi in 1st gear from about 3500 rpm to 7000 rpm. If I shift, in the middle of this, 2nd will have boost at about 20psi. 3rd will have 22+ (wastegate is set to 20), but it will pin the wastegate open, whereas 2nd flutters with it briefly, and 1st never opens it.

In the past (at 30psi) nothing would open it, but you could see boost hit at 20ish, 25ish, and 29ish respectively. It was easy to see the boost pressure jump as soon as the shift happens. (dat auto life).

The only other example I can think of is something like a V8 (like the chequered tuning car) which is running 2x Hypergear turbos at 3.5 PSI in a LS1 V8.

Obviously 3psi in that car is not 3psi in a RB25. So if you have a wastegete set to 5psi, it'd NEVER open, but its maxxing the turbos out, and flowing air, and 'working correctly'. But it doesn't seem like its healthy long term to simply freeboost a turbo because the engine can ingest it. You'd think thatd be a sign of a wildly undersized turbo for the application and can't be a good setup.

[KiwiRS4T]

If you control boost with a Mac valve and a computer such as a Link you can have different settings for different gears.

[hypergear]

Well, you can drop in with your blown turbo I should be able to tell you why it blew

[Kinkstaah]

If you control boost with a Mac valve and a computer such as a Link you can have different settings for different gears.

Yes, I have this option with the Haltech too. The problem is, if I wanted 20psi in first, and second, I can't. I kind of want more boost. If I taper the boost down with a boost trim, the gate will open fine.

But what about guys running 50psi? Do they get all of that 50 in first as well? I am sure that if someone set their EWG to hold 100PSI they may only get .. 50 psi. Or if they put the same turbo on a 8.2L V10, they may only make 10psi, but the turbo is spinning at the same rate.

Just trying to find out if this 'is a thing'

The turbo wasn't blown. It was making power, even though the compressor wheel looked like this:

That's quite bad.

I'd rather that not happen again, lol.

[GH05T]

that looks like you threw a spoon at it O_O

[Kinkstaah]

The issues may not be connected, but.. was really trying to find out..

If max boost increases as you shift gears to a more 'loady' gear.

Specifically if you only make X boost in a setup where ordinarily, you'd be free-boosting.. where X increases (closer to gate opening) as you shift up into more loady gears.

[hypergear]

If the bearing's shattered the wear on the comp wheel will be mainly on the edges. If it exploded from surging or too much boost. you will have chunks of blade broken from the hub. In your case the "machine" work looks even, I suspect if there might be objects went through.

[Leroy Peterson]

im no turbo expert, but if you're concerned about compressor/turbine size, some calculations/assumptions can let you see where the motor sits on the turbo maps (roughly).

some car info would be good for those curious too...

firstly, your car is auto. secondly its a 2.8L stroker 25. thirdly, turbo size?

[Kinkstaah]

Well I was mainly talking to see if this is a common thing, if guys running 40PSI on a 7676 get boost in first gear.

Guys with -5's and 450RWKW and stuff.

But it's turned into Greg's weird setup.

For specifics on my setup, as above, it is a 2.8 BC kit, on a Neo with a (rather well, apparently) flowed head.

Currently it makes 380rwkw (407 when accounting for converter slip) at 21.5 PSI.

The turbo is a GTX3076R, with a .82 rear housing.

The boost CURVE is very similar in all gears. However, first gets about 17psi. Second just touches 20..21 (gate flutters oopen a tiny bit). 3rd and onward the gate is always open.

The actual TIME to rev out first and 2nd gear is over.. very quickly. Enough that "there is not enough load" to build the boost seems possible. MAP pressure will jump between gears, if you shift say, at 10psi, shifting to the next gear puts you instantly at 13psi, etc.

It behaves in the sense that you'd think it's a boost leak.

However power is on par with maxing out the turbo.

Hence me looking for more people with a lot of boost in a low gear, to see if it builds or not.

[jay-rod]

Put up a dyno sheet for Stao and Gtsboy

The gate not opening in second seems a little strange.

Does it have a low rev limit set?

[Kinkstaah]

Mind you, the gears in an auto are very long. 2nd revs out to 135ish kmh. (1st is about 68).

The gate will open.. if I set it to anything under 17psi in 1st, and anything UNDER 20 in 2nd. It will stay open at 22 in 3rd.

Previously, on the same turbo, and same gearbox, I would make about 18 in first.. ~25 in 2nd, and ~29 in 3rd. 3rd being the 1:1 gear.

New engine is clearly making more flow. Made the same power with 10psi less boost/restriction in the inlet manifold. But it has the side effect of the thing just not making boost.

It is going on the dyno next week. I'll get some pulls done in 1st, 2nd, 3rd etc and see if the behaviour happens. Given I am going back to try and run more than 22psi it may become more apparent as to what is going on.

I am assuming I'm maxxing out the turbo at this stage, because considering I have bought a brand new unit, and a brand new engine, and I have no boost leaks... it is either 'normal' or what else can it possibly be?

[GTSBoy]

If you want or need to crazy on diagnostics and try to work out what is really going on, then you need to add a couple of tapping points. You need to see if you're somehow pulling significant vacuum in front of the turbo. And you need to find out what the ex manifold pressure is. It would also be absolutely awesome if you found someone with a suitable tacho to measure the shaft speed.

[KiwiRS4T]

You didn't say at the outset that it was a 3076 - I think that's your explanation - its too small to make more boost with your stroker.

Boost is a measure of the resistance to flow. You are obviously flowing plenty to make the power you do but don't generate a big boost number to get there.

[AngryRB]

So if its flowing the same with 10psi less then that doesn't mean the shaft speed isn't at the limits does it? It could be ready to blow with more boost..

Edited April 9, 2015 by AngryRB

[discopotato03]

Interesting story . My take .

I don't think any RB25 was designed at Nissan to be a real power house in that engine family . They certainly didn't use it to go racing with .

A lot of things could be affecting your power and boosting characteristics and one I reckon could be the 25s inlet manifold design . What Nissan wanted was reasonable power output but more importantly good part throttle torque because that's what 90% of a road engines life is spent at . Its basically a hole on one side and six on the other so when you try to feed really high airflows through it isn't ideal . Nissan wouldn't have gone to the trouble they did with the 26s manifold if the far simpler 25s effort worked well enough .

Turbos . I think the GTX3076R is a bitzer because it has closer to GT3582Rs pumping capacity without the bigger turbos compressor housing or GT35 turbine . My gut feeling is it may work better on smaller higher revving engines like Hondas etc .

Often race engine turbos have quite big turbine sizes in relation to their compressors but they can also work over narrower rev ranges . You could also look at that as a small compressor for the turbine size because it means the same thing .

Personally I think for what you're trying to achieve an RB26 would have been a better basis mainly because of its head and inlet manifold system . Anyone who fabs exhaust manifolds can make them suit both stud patterns and some do .

I once went in a VL that had an RB26 and a GT3582R/1.06 AR - ext gate . It came on boost in the 3500-4000 range and actually needed a larger gate to make good power up high . Gas flow on that engine should have been pretty good both sides and I think it had a 4" exhaust off the turbine housing too .

A .

[GTSBoy]

Its basically a hole on one side and six on the other so when you try to feed really high airflows through it isn't ideal . Nissan wouldn't have gone to the trouble they did with the 26s manifold if the far simpler 25s effort worked well enough .

The only "effort" Nissan went to with the RB26 was to put 6 throttles in it. The only reason they did that was to get the throttle response associated with ITBs. It wouldn't have made sense to retain the plenum and runner design used on the other RB engines with the ITBs, and they also wanted a big FMIC, so a FFP made sense for a number of reasons. But one of those reasons isn't that the RB25 style plenum is inherently poor. They're actually pretty bloody good, considering the apparent ugliness of the design. They certainly do a very good job of giving decent air flow to each runner, and people run them to very high power levels without them causing much problem.