Boost

[Boogieman98]

I'm curious as to why our cars produce less boost in the lower gears than 4th. I know in access tuner race it has a target boost for each gear. At least that is what I think I had seen. I just don't quite understand why it doesn't do max boost in every gear. Also why do most tuners don't really tune for boost for 5th and 6th. Both local tuners I've used over the years don't do it so I don't think it is an isolated thing. ( My tuner of choice is HOLYCRAPITSFAST)


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[IGOTASTi]

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[Batmobile_Engage]

I think it has to do with the load on the engine---I hope this makes sense---and is correct.

4th gear is 1.000:1 ratio, which is why 4th is chosen on the dyno for the majority of tuning. 1st - 3rd ratios > 1.000:1, putting even less load on the engine than 4th gear at the same rpm. 5th and 6th, of course, are overdrive gears. At WOT in an overdrive gear, more load is placed on the engine, requiring more fuel, which produces a more powerful exhaust flow, thus driving the turbo even harder.

Reminds me of how the Ecoboost F-150 guys say, "the turbos really come alive and get on peak boost in a heartbeat when I'm towing the boat". Well, yeah...more load on engine = more exhaust flow = more immediate turbo response + potential for overboost.

I think @HolyCrapItsFast wrote something similar one time about the effects of "lugging" the engine, i.e. WOT at low rpm in high gear. There's also some discussions about this in the Sticky, "good info for first time turbo car owners" or something like that.

My current tune hits about 19 psi in 1st - 4th and about 21 psi in 5th - 6th. For this reason, I try not to go WOT in 5th or 6th unless I'm already doing +120 mph.

 

[Boogieman98]

Could you not set it to apply more boost at less load and not to exceed that even when more load is applied?

For me Tuning 5th and 6th for max boost is a must. I can't tell you how many times I run at of gear.


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[Batmobile_Engage]

Could you not set it to apply more boost at less load and not to exceed that even when more load is applied?

For me Tuning 5th and 6th for max boost is a must. I can't tell you how many times I run at of gear.

I think you can, but that requires some fancy ecu and/or electronic boost control solutions, beyond that of the factory equipment. But once again, not 100% sure.

 

[HolyCrapItsFast]

Yes it is a load based device before anything so that is the one thing that effects boost the most. You can control how the boost responds with a variety of different tables available. You are correct that Cobb has per-gear tables but that wasn't always the case. Most tuners don't spend the time to tune all of the relevant tables and parameters because it just takes to much time to do in allot of cases. Subaru dynamic boost logic is quite extensive.

The 5th and 6th gear boost can be a little tricky in that if you do it wrong you can damage the engine. You certainly do not want the same WG duty in the lower RPM range when in 5th and 6th gear because, as Batmobile mentioned, you are in a lugging scenario. I have blown motors while lugging under boost in 5th and 6th. So i will set 5th and 6th gear to take away WG duty below 5000 rpm, but above either leave it the same as 4th or decrease it to a lesser degree. If you run out of steam beyond that it is simply because your turbo just can't produce it.

I have been spending allot of time with the per-gear boost tables and if you want we can revisit your tune to better tailor your boost.

 

[Boogieman98]

Your tunes as always don't seem to have these issues as you and I always address them. It is just funny how these issues are never addressed with any of the others tuners I have worked with even though they are fully aware I track the car and on many occasions be going flat out in 5th and 6th gear. I thought I would bring them up here on the forum so that others can address these issues either with their local tuner or simply have you tune them since you are already familiar with doing so.

 

[HolyCrapItsFast]

Well, as mentioned, it is just to much time for the average tuner to investigate some of these issues and I don't fault them for it at all. A typical dyno session should realistically only take you a few hours if you are doing it from scratch and tailoring Subaru boost control could add hours more in some cases. It is just simply not cost effective, from a business stand point, to put that much time into it.

That said, a good tuner will already have done this work and have some base maps prepared. We usually use our down time for base maps and to investigate better strategies when it comes to tailoring special parameters. For instance, with the GTR we have defined a number of valve timing tables for different setups but we did that over the course of a year. When you are talking about Subaru boost logic, it is unnecessarily complicated and you could have so many different map strategies for so many different setups you will never have enough time in a lifetime to cover them all. Your per-gear strategy is going to be different from drag racing to circuit racing to street racing to daily driving. And that is just per-gear tables. Getting the boost curve right for one gear alone can be a challenge in itself for some turbos. In most cases and for most people, just making the numbers and producing a decent boost curve is enough.

It has been understood for many years that high load gears will produce more boost so nothing has really changed there. It is just that there are more tables now to help minimize that effect. But you get what you get with a waste gate controlled turbo. I wish Subaru would start adopting Variable Vain Turbos. Then we wouldn't have these issues.

 

[Batmobile_Engage]

I wish Subaru would start adopting Variable Vain Turbos. Then we wouldn't have these issues.

I was under the impression that this has been tried before, but the operating life of VVT/VGT turbos in gasoline applications is terrible at best. Diesels have lower EGTs where a VGT can survive for a reasonable period of time. The only VGT setups I have seen that worked exceptionally well on a gasoline engine, had a ton of water+meth introduced to suppress high EGTs and a fairly rich EFR as well.

 

[HolyCrapItsFast]

The technology has improved greatly over the years. Porsche has been using them for years with good success.