What happens when you run MHD OTS Map on 17T's

[RollinE92]

Genuinely curious and want to learn from others who know this stuff better than I, so please correct me if my understanding is incorrect.

Plenty of threads with people asking if you can run OTS maps on 17T's, the general consensus has always been that you absolutely need a custom tune because OTS were designed for stock turbos, but no one really explains why. Just repeat what everyone else is saying. But what actually happens when you run an OTS map on upgraded turbos?

I see guys talking about bigger turbos produce more volume of air at 18psi vs. stock turbo at 18psi, which makes sense since bigger turbo = higher cfm, however N54 does not have a MAF sensor, but instead has a MAP & TMAP to measure pressure which in turn tells DME what to expect in terms of fueling. After combustion we have short band and wide band o2 sensors that measure the amount of oxygen in the exhaust and DME will adjust (add or remove) fuel accordingly.

So what would one expect when running an OTS map on larger than stock turbos exactly?

 

[carabuser]

The WDGC base table is set up for stock turbos. It's a starting point lookup based on target airflow and target boost pressure. So the DME knows it wants to target 300g/s and 15PSI it will use that 3D lookup to get a base value to send to the WG. On a healthy car that will be 95% correct and the rest will be covered by the boost control PID that corrects based on the error value of the boost target.

With 17Ts that base table will be too high, and you end up over boosting. As a result the DME will reduce the WG as much as allowed by the PID (which is only a small amount) and still be above boost target, so it will then close the throttle plate as an emergency measure to reduce the boost.

Having the throttle plate constantly modulating at WOT trying to control the boost is bad, you'll frequently lean out in areas where the throttle control is too slow to react. In my experience you'll also see extra timing corrections which are probably caused by the poor airflow distribution as a result of the throttle being used to hold back the charge pressure.

You're also not going to be getting any benefit from the larger turbos as it will be capped by the stock turbo boost targets.

 

[pbondar]

The WDGC base table is set up for stock turbos. It's a starting point lookup based on target airflow and target boost pressure. So the DME knows it wants to target 300g/s and 15PSI it will use that 3D lookup to get a base value to send to the WG. On a healthy car that will be 95% correct and the rest will be covered by the boost control PID that corrects based on the error value of the boost target.

With 17Ts that base table will be too high, and you end up over boosting. As a result the DME will reduce the WG as much as allowed by the PID (which is only a small amount) and still be above boost target, so it will then close the throttle plate as an emergency measure to reduce the boost.

Having the throttle plate constantly modulating at WOT trying to control the boost is bad, you'll frequently lean out in areas where the throttle control is too slow to react. In my experience you'll also see extra timing corrections which are probably caused by the poor airflow distribution as a result of the throttle being used to hold back the charge pressure.

You're also not going to be getting any benefit from the larger turbos as it will be capped by the stock turbo boost targets.

Likewise heard cited that shall not..but this is the first time I’ve heard a cogent response…

 

[IQraceworks]

I've seen several people claim that they are running 17t's on the MHD maps with no issues at all.........but I really find that hard to believe (because of the very good response posted above). A good custom tune is not that expensive, if you can't afford the tune.....stay with stock sized turbos. Just my opinion.....

 

[marv85]

I realize you are asking about the MHD OTS maps but I've been running 1KM0S stock map with my hydra HP650 turbos and it's been great. Is it optimized? No, it's not. It would definitely benefit from custom tuning, which I'm planning to go for soon.

Boost control is different on 1KM0S though. I've taken the car to a track day running the 1M map and the car performed great. Obviously, the 1M map isn't pushing a lot of power relative to most MHD OTS maps.

Here's a third gear log.
https://datazap.me/u/marv85/log-1714341204?log=0&data=3-4-20

 

[carabuser]

I realize you are asking about the MHD OTS maps but I've been running 1KM0S stock map with my hydra HP650 turbos and it's been great. Is it optimized? No, it's not. It would definitely benefit from custom tuning, which I'm planning to go for soon.

Boost control is different on 1KM0S though. I've taken the car to a track day running the 1M map and the car performed great. Obviously, the 1M map isn't pushing a lot of power relative to most MHD OTS maps.

Here's a third gear log.
https://datazap.me/u/marv85/log-1714341204?log=0&data=3-4-20

I don't think the turbos are working particularly well there. You're failing the hit the 11psi boost target and the PID had timed out. HP650 turbos should be boosting more than 10PSI at 52% WGDC in the midrange. Maybe one of them is missing the vacuum line?

It's like the worst of both worlds, not spooling until 3k and not even hitting stock turbo targets.

 

[wheela]

I realize you are asking about the MHD OTS maps but I've been running 1KM0S stock map with my hydra HP650 turbos and it's been great. Is it optimized? No, it's not. It would definitely benefit from custom tuning, which I'm planning to go for soon.

Boost control is different on 1KM0S though. I've taken the car to a track day running the 1M map and the car performed great. Obviously, the 1M map isn't pushing a lot of power relative to most MHD OTS maps.

Here's a third gear log.
https://datazap.me/u/marv85/log-1714341204?log=0&data=3-4-20

But with those bigger turbos, it now looks to be taking about 2 seconds to get up to the boost target. You're only hitting about 75% wgdc during spool, so by running this tune I think you're also leaving a lot of response on the table in addition to not having higher load targets that these bigger turbos can support. Boost is also still oscillating a bit before settling in near the boost target. I know you're already aware of this, so not trying to knit pick or anything, just pointing some details out for OP since he's looking for specifics

I am curious though, did you use this 1KMOS tune on your stock turbos before upgrading to the hp650's? If so, I'm curious how it felt driving before/after the upgrade. Could you tell a loss in response and gain in top end just from swapping turbos?

 

[marv85]

But with those bigger turbos, it now looks to be taking about 2 seconds to get up to the boost target. You're only hitting about 75% wgdc during spool, so by running this tune I think you're also leaving a lot of response on the table in addition to not having higher load targets that these bigger turbos can support. Boost is also still oscillating a bit before settling in near the boost target. I know you're already aware of this, so not trying to knit pick or anything, just pointing some details out for OP since he's looking for specifics

I am curious though, did you use this 1KMOS tune on your stock turbos before upgrading to the hp650's? If so, I'm curious how it felt driving before/after the upgrade. Could you tell a loss in response and gain in top end just from swapping turbos?

you're not nitpicking at all, I appreciate your feedback. The only reason I'm running 1KM0s is because I haven't had enough time to custom tune ahead of a track day and the stock tune, despite it being far from optimized, was good enough for me not to postpone my track day.

I did use 1KM0S on stock turbos but mine were tired. How does that car feel now? it feels a lot laggier, but once boost builds, it's definitely faster. But there should be a lot more room on those turbos. I've been in contact with hydra sharing logs and such because I've personally felt that WGDC is a little high for the boost, which is what @carabuser was eluding to in his previous post. I'm having Hydra tune my car though, so we'll see how that goes. If there are installation issues, the logs will show them. I guess my old turbos were so bad that the car feels better (minus the lag).

 

[RollinE92]

The WDGC base table is set up for stock turbos. It's a starting point lookup based on target airflow and target boost pressure. So the DME knows it wants to target 300g/s and 15PSI it will use that 3D lookup to get a base value to send to the WG. On a healthy car that will be 95% correct and the rest will be covered by the boost control PID that corrects based on the error value of the boost target.

With 17Ts that base table will be too high, and you end up over boosting. As a result the DME will reduce the WG as much as allowed by the PID (which is only a small amount) and still be above boost target, so it will then close the throttle plate as an emergency measure to reduce the boost.

Having the throttle plate constantly modulating at WOT trying to control the boost is bad, you'll frequently lean out in areas where the throttle control is too slow to react. In my experience you'll also see extra timing corrections which are probably caused by the poor airflow distribution as a result of the throttle being used to hold back the charge pressure.

You're also not going to be getting any benefit from the larger turbos as it will be capped by the stock turbo boost targets.

Got it, makes sense. I was actually watching 0-60 on YouTube and he pretty much explained it as such. It comes down to WGDC and throttle closure from going over target boost. At the moment, I am not looking to push WOT anytime soon so custom tune will have to wait unfortunately. Unless there's a way I can adjust WGDC or generate and upload my own custom PID

Idk what I was thinking when I got the 17Ts, got new index 12 injectors, silicone inlets, downpipes, stage 2 LPFP, colder spark plugs, new coils, walnut blast etc. Car is pretty much ready maintenance wise. Only got 113k miles on the ride.