A Question For You High Boost People

[zebra]

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 .

1 hole, into 8 holes

3000hp

[GTRPSI]

1 hole in the middle feeding 6 holes is more balanced than 1 big hole on 1 end feeding 6.

Turbo F1 era ran 1 hole in the middle feeding 4 with a log design intake manifold.

That was more a case of even air disributon and tuning the intake.

ITB were the best way of getting good throttle response.

Edited April 9, 2015 by GTRPSI

[jet_r31]

f**k knows. Its sounds like a boost leak

Thats the only time ive had the same problem.

Even my auto gt42 streeter would make fullboost in 1st

they should..autos applies lots and lots of load. Manuals are different again

Cheer

Darren

[jet_r31]

I have seen it happen due to to much turbine backpressure with internal gate not being big enough and because of load boost went higher every gear. But not like your describing..

Cheers

Darren

[AngryRB]

400kw's with a boost leak?? this thing has a shitload left in it then, hasn't even hit the GTX zone..

Edited April 9, 2015 by AngryRB

[Kinkstaah]

Glad to see it's not just confusing me .

Note: Before the engine was rebuilt the same spec turbo did this at 22-25-29 style PSI (per gear).

The guys doing the head stated the previous job was shit and theirs would flow way better.

Picked up a fair whack of response, and well, more powers for way less boost.

It may just be that they're right, and the thing is just too small/can't create resistance to flow in the head.

I imagine if you put a GTX3076 on a LS1, it would not create 30psi no matter what you do with it.

My main worry is what boost I should set the gate to, to avoid overspinning and frying the thing. I guess whatever boost level makes the power the turbo is rated at? (for lack of any other idea)

[GTSBoy]

Use the compressor map, the PR and the power level to get an idea of how fast it's spinning.

[jet_r31]

What does it do on a dyno

What boost does it run to make 380rwkw?

Same as on road, or is it going higher on the road due

To a lot more load?

Be better road tuning it and seeing how fuel it uses to determine

Boost pressure if its different on road.

Cheers

Darren

[Leroy Peterson]

Also decreased amount of boost required because increased comp ratio

[tridentt150v]

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

Swap for a 1.06.........and it will work better. The .82 is too small for what you are doing IMO.

[paulr33]

If it's a new garrett turbo I wouldn't imagine excessive shaft speed would be a large concern unless you are trying to run 95 psi through or some spastic number. The gate pressure is the target before it diverts excess pressure, ie "shaft speed control". As long as you have a target on the compressor map it should be within its engineered limits. The most likely failure would be oil contamination or dirt in the intake which gets into the bearings. IMHO these would more likely to kill a new garrett turbocharger.

Not being able to reach consistent boost in all gears sounds like something is mismatched. There's no reason you can only make 20 psi in gear 3 vs gear 4 for example. Think of psi as your turbo shaft speed. If the engine and setup is matched correctly, or not mismatched then your shaft speed should hit its target no problems. Of course if u are outside the engineering limits, ie expecting 80psi then no hope but at normal levels it should be consistent. My evo makes 19psi in every gear, why shouldn't it?

[YawRate]

Reading through your posts Kinkstaah, you mention the boost as 17, 20 and 22psi through the first 3 gears, what is it in 4th and 5th? Or did my reading comprehension let me down?

Without getting technical, I assume you're confident that the car isn't just traction limited in the lower gears? That would be the 100% simplest explanation but I didn't see it mentioned in your previous posts. Or slippage somewhere else in the driveline? Even if the mechanical engine load is absolutely identical through the gears (unlikely even with perfect traction, level road etc.), remember that the aerodynamic load will be higher in the higher gears

Depending on what you list the boost as in 4/5th gear, I don't think what your observing wouldn't necessarily be an "issue". For example, nobody has yet touched on the different locations that you are reading the intake pressure from vs where the wastegate pressure is being observed from, and I'd guess one is reading static pressure, the other total pressure (static + velocity pressure). What I'm getting at is its possible that the difference in airflow through the engine (what actually matters) in different gears might be hardly varying, yet producing noticeably different pressure readings.

Edited April 24, 2015 by YawRate

[scotty nm35]

Target boost won't ever be reached if there is a leak, which there was it seems. God only knows what the shaft speed reached to still supply 20psi...

[HarrisRacing]

I agree with boost leak or just something flat wrong with the exhaust manifold or design. What manifold is it?? How is the wastegate plumbed? Any pictures?

But yes boost leak starts to sound like a really good idea. And I agree that turbo is SCREAMING if it is doing what you say with a boost leak. Block off your BOV and go for a ride...could be something wrong with it and you just aren't seeing it because you are looking in the wrong place for the problem (that or the BOV is only leaking at high boost!).

That turbo on a stroker should make pretty much full boost straight off of a decent stall with an auto!

Edited April 24, 2015 by HarrisRacing

[paulr33]

thats a good point by scotty actually, if you've got a boost leak then yeah that might explain what you are seeing

suggest basic check for leaks

[AngryRB]

He's making the same power on lower boost, an RB30 with GT3076 makes 300kw's at 14psi?? an RB25 does it at 18psi usually on 98, that seems to be a fact, so seems logical that the RB30 breaths so much that its outflowing and running the GT30 into the limits , it just can't create that pressure when its being flowed out so efficiently???? ( hence shaft speed) its still making the same power though so there cant be much of a leak happening, must be just about maxing out surely...

Edited April 25, 2015 by AngryRB

[GTRPSI]

3.0L is like adding another 7th cylinder to a 2.6.....15% bigger.

[jet_r31]

When you get boost leaks with modern turbos, power honestly doesnt drop to

Much up top like people here think

so saying something is in ballpark power wise doesnt mean much

What you do lose is area under the curve. And i can tell you not many dyno

Operators pick it up on the dyno. Ive always found playing around on rd picks up that stuff. Dynoes do not load a car up like on road. Even on the harshest ramp rate. Experienced it many times myself..

My last build did same thing and was blowoff valve. Boost went from 21 to 27psi in midrange once fixed. Gate woudnt open in lower gears

power on dyno was within 10rwkw as what youd expect

its harder to pick up with autos due to load applied and the way loads applied...

Cheers

Darren

[paulr33]

could always look at a turbine shaft speed guage, gcg.com.au have them

[scotty nm35]

I pressure tested his intake for him yesterday, nothing leaking much. The gasket between the plenum halves was bubbling away as usual (although it was only a slow leak)

Back to the drawing board, seems it just can't supply enough air to make the target boost as Mark said above, or it's only leaking at WOT which makes it hard to pinpoint.