[abbike18]

I've got a gen 1 (non-TU) B58 and I'm in Colorado. On a stage 1 tune the turbo is hitting 19psig.

Our atmospheric psia (absolute pressure) is about 12.2psi. So with the turbo psig of 19 that means about 31.2psia in the charge pipe.

If you look at sea level with a 14.7psia, that's the equivalent of the turbo pulling a 16.5psig boost.

So my point is that our turbos at high altitude have to work way harder to hit the torque requests.

Anyone have any insight in to the benefits of a bigger turbo to overcome the altitude? Like at altitude, does it make sense to just install a stage 1 pure turbo to make up for the higher output required by simple OTS tunes?

[chenry]

I would give Pro Tuning Freaks (BM3) a call and see what they have to say in that you are tuned by them. While I am not directly impacted by a similar situation (I live at or below sea level here in Cali) - like you I am curious - let us know what they have to say and please share.

You have raised an interesting question and I look forward to hearing what BM3 has to share with you

[abbike18]

Thanks, that's a good suggestion. I've got an inquiry in to PTF and X-PH to get their input on the question.

[abbike18]

So I've got inquiries out to a few turbo manufacturers as well as tuners on this question, but I've also been doing some of my own digging.

I was able to find a clean log of a 4th gear pull from someone in France with BM3 Stage 1. The important part here is they are at sea level:
https://www.bootmod3.net/log?id=61d0...0b435b30031199

Here is my BM3 log from ~5000k ft elevation:
https://www.bootmod3.net/log?id=6201...729b3a508701fd

I am on the 91 octane tune, and the French pull is 93. But the important thing here is that both pulls are targeting about 32psia (absolute) of pressure in the charge pipe. Both are at WOT.

Attached is a plot of the engine RPM vs the MAF rate. What is interesting here is that you see the pull at altitude start to struggle in the low 5k rpm range.

If you look at the high altitude log above, at 5300rpm the WGDC starts to go up - the same spot the high altitude log starts to struggle to deliver the air volumes the tune wants.

So there could be two reasons for this:
1.) The turbo has to work harder at altitude, which means you need better turbine flow to deliver that power to the compressor. I am seeing the effects of a constricted downpipe, which reduce the ability of the turbine to extract power due to back pressure.
2.) The compressor is actually at its limit for the high altitude pressure ratio and required flow. The turbine is spinning harder from the WG closing, but its never enough because the compressor has nothing left to give.

The solution to #1 is a Downpipe. The solution to #2 is a Downpipe and turbo. So the clear path for me is to put in a Downpipe and see what that does to the logs, then decide if a bigger turbo is needed to hit the OTS tunes at altitude.

[abbike18]

OK So I've gone a little nuts on this issue over the past few days.

First, I went skiing so naturally I did a 4th gear pull at around 9500ft. You can see the results of that in the first plot where I added that log to the previous plot. Basically the turbo really struggled catering in the 4000s to hit boost.

Then I realized I had been testing the limits of the turbo in relation to flow rate and pressure ratio, so if I mapped my 5k ft elevation and 9500ft elevation pull with a sea level pull for the stock turbo, I should be able to see the right hand drop off of the stock compressor map. Check out the second plot. You can clearly see where the stock compressor just drops off the efficiency map - I aded a squiggle to make it more clear if it wasn't already.

You can see the for the stock turbo a PR of 2 falls off in the low 40s lb/min of flow rate. 2.5 gets you to high 30s and a PR of 3 you drop off in the mid 30s.

So then I thought of what's the max I would want to push my car? Probably 22psi with a flow rate of 50 lb/min. So at Denver that's a PR of 2.79 and at 9500 ft that's a pressure ratio of 3.08. Sure, I'd love to have it rip at 11k ft over a pass but I'm not going to design the turbo around 0.1% of the mileage I do.

Then I searched the forums for logs of the Pure 800 and VTT GC turbos to see what their capability was. Check out the third and 4th plots to see that. The Pure 800 log is from a user who blew up their engine when they lost a cylinder and the DME freaked out and tried to hit the torque curve on 5 cylinders so it closed the WG and spiked the boost. RIP engine.

Both plots show that either turbo could hit a PR of 3 with a flow rate of > 50.

So the next question...spool. I still want a torque machine for fun around town driving. So I plotted the engine RPMs vs the boost pressure to see how the different turbos spool. See 5th plot. I tried to find logs where people floored it at as low RPM as possible, but it was hard. However you can really see that the stock turbo is the best, and the VTT gives almost stock spool. But the Pure 800 is a bit behind in spool.

So...my analysis shows the VTT GC to be a good option for CO Rockies drivers who are looking to not push the limits too far, but have a turbo that can hit typical stage 1/2 boost levels and retain near stock like spool.

[suby01]

This is cool info for a read. 👍

[F32Fleet]

Bigger turbo means more lag.

What is your end goal?

[abbike18]

Originally Posted by F32Fleet

Bigger turbo means more lag.

What is your end goal?

To reliably hit typical stage 1/2 tune boost requirements (total psia, not just psig boost) without tanking the turbo efficiency while at Colorado altitudes.