Inverse progressive boost

I've pondered over this for a little while. The faster an engine is revving, the faster the turbo needs to spin to keep up with the demand. With that in mind it stands to reason that a turbo producing 1.0bar at 7000rpm is working a LOT harder than a turbo that is producing 1.0bar at 4000rpm.

Currently we restrict our TT systems to 1.2bar, as that's seen as the safe limit of the turbo. However, the only time that limit is being approached is at the upper end of the RPM limit of the car. This means that at the lower RPM levels the turbos could actually produce more pressure safely.

So, what would it take to create inversely progressive boost? Perhaps 1.4bar at 4krpm, gradually dropping down to 1.2bar by 6krpm? Any potential for doing such a thing? I just feel that there would be gains to be had for acceleration and area under the curve. My TT system hit around 1.3bar at the transition point and then pulled back to 1.2bar due to the gain on my boost controller, it was ridiculous how hard the boost came in and really made for a kick in the pants when it took off.

That's twin-charger territory.

When I had a medium-sized single on standard cams it did something similar as the cams started to choke at higher RPM. I think a better approach would be to expand the area under the torque curve as much as possible with the given turbocharger's efficiency range and then see what boost you're left with at the end.

The way I see it, the performance of a compressor is a balance of max boost vs the largest area possible under the curve, so there's always going to be a compromise somewhere. The 1.2 BAR is what Supra tuners have found BPU to work best at over the years.

When I had a medium-sized single on standard cams it did something similar as the cams started to choke at higher RPM. I think a better approach would be to expand the area under the torque curve as much as possible with the given turbocharger's efficiency range and then see what boost you're left with at the end.

 

The way I see it, the performance of a compressor is a balance of max boost vs the largest area possible under the curve, so there's always going to be a compromise somewhere. The 1.2 BAR is what Supra tuners have found BPU to work best at over the years.

With regards to the Supra, 1.2bar IMU was seen to be the maximum safe limit. I doubt there was the option to have boost control so accurate when that figure was determined and it was also a basic upgrade without any push on using complex mapping to control everything.

I only have my own experience to pull from and my bum dyno, but I can honestly say that 0.1bar difference at the transition was night and day. I found it by accident when I was tweaking my boost controller and decided to run right on the ragged edge of 1.2bar territory. To that end I was left with 1.3bar at the transition, decreasing to 1.2bar over a few 100 RPM. If I had a way of extending that 1.3bar over say 2krpm then I would have.

In actual fact, if I had a way of experimenting I would happily go with 1.4 at the transition, lowering to 1.3 by 5krpm then down to 1.2 by 6krpm. Downfall is you may be risking the turbo's, but I honestly don't see there being more stress on the turbo at 1.4bar @ 4krpm than there is at 1.2bar @ 6krpm.

The compressor map would definitely show this though.

The same principle could definitely be pulled through with other turbos but I dare say when you're into that territory, it's more cam related than turbo efficiency.

Interesting read.

Although, surely its the pressure on the ceramic turbine blades, that's the biggest risk?

I was under the impression that anything above 1.2 bar no matter what the rpm is, is moving beyond the recomended safe limit of the materials used in the OEM exhaust turbine construction.

Obviously I am eluding to the JSPEC only.

Certainly interesting though, if your theory is correct.

My boost controller allows me to hit 1.35bar @ 4000rpm, and tails back down to 1.22-1.23 by 5500rpm. This happened purely by accident and has been running like it for a year with no issues. And there's nothing light about me, right foot included! Chris Bailey described my car as "savage" recently, as far as BPU'd cars go. I imagine this'd be why.

If they go pop, they go pop. I'll have an excuse to whack the single kit on!

Interesting read.

 

Although, surely its the pressure on the ceramic turbine blades, that's the biggest risk?

 

I was under the impression that anything above 1.2 bar no matter what the rpm is, is moving beyond the recomended safe limit of the materials used in the OEM exhaust turbine construction.

 

Obviously I am eluding to the JSPEC only.

 

Certainly interesting though, if your theory is correct.

I don't think the pressure has anything to do with it to be honest. A couple of bar of pressure isn't a lot when considering the metals and ceramics in play. I believe it's the speed of the turbine causes them to break up. I can't remember the RPM of the turbine at 1.2bar@7krpm.... but it's fast, very very fast. The centrifugal forces are absolutely massive on the outer tips and increasing the speed by another 10-20% is deadly to the turbines. As the speed increases the heat also increases, so to see a 10% pressure change you need to increase the speed by more than 10%. That's a whole other ballgame though.

I would never go past 1.2bar anywhere near the top end of the rev limit as you would simply be working the turbo far too hard. At the lower end of the revs the speed of the turbo will be greatly reduced from it's 7krpm speed so I don't see any reason for not bumping it a little closer to it's known max.

For example, if 1.2bar @ 7k RPM saw the turbine spin at 60k RPM. We could calculate that the speed at 5k rpm would be 43k RPM (This is an incredibly rough calculation). If 60k rpm = 1.2bar @ 7krpm then 60krpm @ 5k RPM would allow for 1.68bar. Now, again, this is an incredibly rough calculation not taking into account all manner of things, but it does the job of pointing out the relationship.

60K RPM @ 7K = 1.2Bar

60K RPM @ 6K = 1.4Bar

60K RPM @ 5K = 1.68Bar

60K RPM @ 4K = 2.1Bar

My boost controller allows me to hit 1.35bar @ 4000rpm, and tails back down to 1.22-1.23 by 5500rpm. This happened purely by accident and has been running like it for a year with no issues. And there's nothing light about me, right foot included! Chris Bailey described my car as "savage" recently, as far as BPU'd cars go. I imagine this'd be why.

 

If they go pop, they go pop. I'll have an excuse to whack the single kit on!

That was my thoughts and setup pretty much to a tee. It made some difference, although I was coming from an MBC.

Interesting stuff Scott,

Thanks for the reply, I can see alot of effort went into that explaination.

I always drive like a granny anyway, but certainly sounds really effective and interesting too.

Like your thinking on this, sounds interesting.

I think you may be forgetting that in order to produce the required pressure the turbo needs to spin at a speed that will produce that pressure. its all very well making rough calculations as to what RPM the turbo is spinning at, but its all down to gas speed, which is a by product of load, so the turbo could well be spinning at the required speed (say 60k to make 1.4bar) at high load at 3K rpm,

Which if your saying the std turbo is starting to get into its danger zone at 1.4bar at 60K rpm/7K rpm engine speed, its loadings are going to be the same at the high load/3K point as it will at 7K rpm, so what I'm saying is that the proposed reduction as the engine rpm and turbo rpm/pressure is not going to make the turbos work less.

Electronic wastegate control, as fitted to 99% of modern turbo engines, will allow more boost at a lower RPM than at a higher RPM, if so mapped, as will a variable vane turbo.

I think you may be forgetting that in order to produce the required pressure the turbo needs to spin at a speed that will produce that pressure. its all very well making rough calculations as to what RPM the turbo is spinning at, but its all down to gas speed, which is a by product of load, so the turbo could well be spinning at the required speed (say 60k to make 1.4bar) at high load at 3K rpm,

Which if your saying the std turbo is starting to get into its danger zone at 1.4bar at 60K rpm/7K rpm engine speed, its loadings are going to be the same at the high load/3K point as it will at 7K rpm, so what I'm saying is that the proposed reduction as the engine rpm and turbo rpm/pressure is not going to make the turbos work less.

I don't want the turbo to work less though, I want it to work more.

We know a few things about the BPU Supra just from driving them over the years. 1.2bar DEFINITELY isn't the maximum boost that the turbo will produce relative to the gas speed, that's the reason we need restrictor rings in place as they will go WAY beyond that quite easily. The trouble is that they go so far beyond their capacity that they end up exploding. We know trough experience that 1.2bar at the upper limit of the rev range is the maximum safe limit of the turbos (+/- a decimal here or there). From here there would be a lot more complicated mathematics required to figure out the safe limits at the various rev ranges but it can be said with a lot of certainty that 1.2bar at 4krpm isn't the limit, as it's nowhere near the same turbine speed as 1.2bar at 4krpm.

The options left are to either suck it and see, or do a lot of figuring out. Personally, if I was in the position to give it a go, I would have a spare set of tubbys on standby and I would be running 1.6 progressively lowering to 1.2 between 4krpm and 6krpm. If that ran for 6 months and didn't blow I would bump it up to 1.8, then 2.0. If I hit 2.0bar at 4krpm in a higher gear I think the torque would be immense and the area under the graph would be significantly increased. At that point I would know i was pushing my luck and I would stop. When they blow... they blow, that would be the fun for me.

I understand that the turbo would be working closer to it's safe limit for more of the time, and thus it would fail quicker than it would with a standard boost control..... what a way to go though

Electronic wastegate control, as fitted to 99% of modern turbo engines, will allow more boost at a lower RPM than at a higher RPM, if so mapped, as will a variable vane turbo.

This honestly did cross my mind as it does feel that way with a lot of turbo diesels lol. Although that's possibly in my head and something else to do with the way a TD engine works, it does make a lot of sense with the mid-range power of the hot hatches