Why am I hitting a power wall with my stroked RB25?

[TurboTapin]

I was back on the dyno again yesterday after completely revamping my methanol injection system. I was making 440kw at 19psi. Great, this is more then I was making last summer with my old meth system at that pressure, and I figured it would be smooth sailing up to 30+psi. We continued increasing from there and by 25psi, I had only hit 455kw. Dyno sadly lost utility power at this point and I left. We seemed to be on course to hit the same power wall as last summer (477kw@27psi).

This is a built RB25 S1 stroked to a 2.8. Built and ported head with Kelford 272/262 cams. Precision 6466 Turbo. Tuner last year believed the power wall is a mix of RPM limitations (Hydro lifters/Limit set at 7600RPM) and fueling. With my new meth system, we're seeing zero knock now but it still won't go up. 

What else can I do at this point? Here's my train of thought. 

- It's clear from the dyno graph that I would easily make power to 8500+RPM but I'm limited to 7600RPM. I could risk increasing my limiter, but hydro lifters have me worried. What are my options here besides going NEO head or solid lifter conversion? I'm rocking a 1.05AR turbine on my 6466. Would dropping down to a .84 help shift that power down in anyway?

- We're only seeing roughly a 30kw difference at the top end with VCT on or off. Is this normal? I want to confirm VCT is working properly.

- Should I look into other cam options? 

- Should I just keep increasing boost and see if she wakes up later on? I would expect to hit 600rwkw around 35psi with my 6466. 

 

Edited 19 hours ago by TurboTapin

[Dose Pipe Sutututu]

A few suggestions:

• Do you have the option of retarding your exhaust cam gear to bring the power band over to the left?
• VCT would be turned off after you make peak torque, you see the power/torque will decay faster with it left on all the way to the limiter
  • Once VCT is turned off, you'll find the motor will take a bit more timing being ramped in
• Send its mum, more boost!
  • Of course, depends on your exhaust pressure.

[TurboTapin]

7 hours ago, Dose Pipe Sutututu said:

A few suggestions:

• Do you have the option of retarding your exhaust cam gear to bring the power band over to the left?
• VCT would be turned off after you make peak torque, you see the power/torque will decay faster with it left on all the way to the limiter
  • Once VCT is turned off, you'll find the motor will take a bit more timing being ramped in
• Send its mum, more boost!
  • Of course, depends on your exhaust pressure.

I do have an adjustable exhaust cam gear but I always presumed these only lead to minor changes. I'll play around with it next week and report back. Also wouldn't advancing, not retarding shift that power down lower? 

We played around with VCT last year and it made more power with it on until redline. I had read somewhere that this is often the case with short runner intakes like I'm running but I'll revisit this after adjusting my exhaust cam. 

I have a 4" downpipe and 3.5" line with two open dumps. I'm thinking back pressure would only start to be an issue at 35+ psi. I often hear people state things like "oh my turbo really only wakes up past 30psi" which goes against everything I know about centrifugal compressors. Is there any actual logic to this? 

Thanks for the help dose. 

 

[Duncan]

Well, its worth noting that 450+ is probably enough to get you to church on time each Sunday already.....

You mentioned cams, did you upgrade the valve springs as well? Factory ones will be well past their use case by now. And yes, the hydraulic lifters are likely to limit your usable rpms and AFAIK you need a (now rare and expensive) head swap to rb26 or even better rb25neo for that (to retain VCT)

I don't think 4"/3.5" should be a restriction. What about the intake, do you have an upsized intercooler and what is the throttle body?

[TurboTapin]

6 minutes ago, Duncan said:

Well, its worth noting that 450+ is probably enough to get you to church on time each Sunday already.....

You mentioned cams, did you upgrade the valve springs as well? Factory ones will be well past their use case by now. And yes, the hydraulic lifters are likely to limit your usable rpms and AFAIK you need a (now rare and expensive) head swap to rb26 or even better rb25neo for that (to retain VCT)

I don't think 4"/3.5" should be a restriction. What about the intake, do you have an upsized intercooler and what is the throttle body?

Head is completely built. I tossed the entire Ferrea catalogue at it. Head has also had a lot of porting work done to it. When I see someone making 600+kw with a hydro s1 head in the rb25 turbo thread here, I like to presume it's not a head/rpm limitation. Although I agree, it would definetely help. 

82mm bosch dbw, 4" core with 3" pipes. IAT's were ice cold on the dyno. It was 18c ambient and i'm shooting water/meth. 

[TurboTapin]

41 minutes ago, Duncan said:

Well, its worth noting that 450+ is probably enough to get you to church on time each Sunday already.....

You mentioned cams, did you upgrade the valve springs as well? Factory ones will be well past their use case by now. And yes, the hydraulic lifters are likely to limit your usable rpms and AFAIK you need a (now rare and expensive) head swap to rb26 or even better rb25neo for that (to retain VCT)

I don't think 4"/3.5" should be a restriction. What about the intake, do you have an upsized intercooler and what is the throttle body?

Looking into it more, I noticed my intercooler is rated for maximum 700hp. At the size and thickness of it, I would not have expected this. Are these rating generally only for temps or should I be worried about flow restrictions? 

[Duncan]

Yeah I was asking about intake for flow restrictions, not IAT (esp since you are on meth)....not sure of an easy way to tell if it is a restriction but consider that the size of the intake and exit welded to it do not guarantee how much air can flow through the core

[Dose Pipe Sutututu]

This is for the 6466 Gen 2, from my understanding you need to send its out of the stratosphere for the 6466 to boogie.

 

[GTSBoy]

10 hours ago, Dose Pipe Sutututu said:

Of course, depends on your exhaust pressure.

2 hours ago, TurboTapin said:

I have a 4" downpipe and 3.5" line with two open dumps. I'm thinking back pressure would only start to be an issue at

Back pressure is not really a function of the exhaust itself, but the turbine/housing. So the question/point remains valid. I would be fitting an EMAP tapping point and seeing what is going on in the exhaust manifold before committing to throwing boost at it.

2 hours ago, TurboTapin said:

Are these rating generally only for temps or should I be worried about flow restrictions?

Flow. No intercooler is actually rated for continuous operation at any power level wrt temperatures, as they are no actually capable of shifting the required amounts of heat on a continuous basis. They get hot during a pul, then dump some of that heat into the external air (and some back into the internal air) during off load moments/periods).

You probaby do need a bigger core, given the power level you're already at is somewhat above the "rating" of the cooler.

Put pressure tapping points either side of the core (which possibly you already have, if you have a boost source near the compressor outlet and tapping near the throttle for existing purposes) and report on delta P across the core.

 

I haven't done the mass flow calculation to see where you would be sitting on the compressor map at the ~450is rwkW level we're talking here, but if you look just at the PR you're running at, which is ~2.8-2.9, then you have a really really wide swath of the compressor map to run in, and so long as you were making between about 45-75 lb/min of flow, you should have really good comp efficiency, with the ideal being betweem 55 and 65. I can't see the need to shift drastically on the map to get better results.

[Dose Pipe Sutututu]

3 hours ago, TurboTapin said:

Also wouldn't advancing, not retarding shift that power down lower

Increasing overlap will bring the torque down earlier (and make your idle sound sick lol). Worth a shot (if you dyno time) to experience a bit (providing you are confident your valves won't kiss your pistons). Is to just take off 5 degrees across the boosted area of the map, and then retard the exhaust cam let's say, 3 degrees at a time and then overlay all the runs with VCT on all the way and VCT off as soon as you make positive pressure.

Ideally you would have a total of 6x overlays to looks at

1. 0 exhaust & VCT off once manifold pressure is +ve
2. 0 exhaust & VCT on all the way
3. -5 exhaust & VCT off once manifold pressure is +ve
4. -5 exhaust & VCT on all the way
5. -10 exhaust & VCT off once manifold pressure is +ve
6. -10 exhaust & VCT on all the way

Just leave your O2 wideband closed loop on for all of the boost area & RPM, and hopefully there isn't too much fuel correction needed and the Haltech is fast enough to compensate (I do this, then apply the correction to the main table).

Getting that overlay data should give you a good indication of what to aim for.

[TurboTapin]

2 hours ago, Dose Pipe Sutututu said:

This is for the 6466 Gen 2, from my understanding you need to send its out of the stratosphere for the 6466 to boogie.

 

You sir, big win tonight. Precision never supplied compressor maps in the past and Idk when they started, but it seems they only offer for a few of the older gen2's which is exactly what I have. I punched in some numbers and seem to have gone with the wrong turbo. 600-700whp isn't in the most efficient area and is close to the choke line. 800whp is well off the map.

Punching the numbers into a 68mm gen2 turbo map is bang on in the center. Here I was thinking going smaller would shift the power band down, but in reality it would just make it so much worst due to my rev limit. I would have never thought. 

Edited 7 hours ago by TurboTapin

[TurboTapin]

1 hour ago, GTSBoy said:

Back pressure is not really a function of the exhaust itself, but the turbine/housing. So the question/point remains valid. I would be fitting an EMAP tapping point and seeing what is going on in the exhaust manifold before committing to throwing boost at it.

Flow. No intercooler is actually rated for continuous operation at any power level wrt temperatures, as they are no actually capable of shifting the required amounts of heat on a continuous basis. They get hot during a pul, then dump some of that heat into the external air (and some back into the internal air) during off load moments/periods).

You probaby do need a bigger core, given the power level you're already at is somewhat above the "rating" of the cooler.

Put pressure tapping points either side of the core (which possibly you already have, if you have a boost source near the compressor outlet and tapping near the throttle for existing purposes) and report on delta P across the core.

 

I haven't done the mass flow calculation to see where you would be sitting on the compressor map at the ~450is rwkW level we're talking here, but if you look just at the PR you're running at, which is ~2.8-2.9, then you have a really really wide swath of the compressor map to run in, and so long as you were making between about 45-75 lb/min of flow, you should have really good comp efficiency, with the ideal being betweem 55 and 65. I can't see the need to shift drastically on the map to get better results.

I'm an idiot, my intercooler is rated for 1000hp. I had clicked on the wrong product. Knowing the delta P would be nice, but I'm doubtful I'll do it. Now as for an EMAP, that would be great and I'll get around to it eventually but from my findings in my last post, I'm considering a turbo swap now.