Turbo 3400, F23 build, 1985 GT (Page 26/26)
1985 Fiero GT NOV 19, 09:18 AM

quote
Originally posted by pmbrunelle:

Besides the full power points which you have plotted on the graph, you should also estimate how little power you might generate for pressure ratios of 2, 3, etc.

You want to make sure that in low-RPM, high-boost situations, the compressor is not operating in the surge region to the left of the efficiency islands.



Ok, looking into it further, I found THIS thread with a wheel to flywheel calculation based off THIS thread with a dyno chart. Based off that, at 2500 rpm, a 2.8 is 65hp, times that by 1.17 for flywheel HP is 76hp. 76x2 for 2 bar of boost is 152. On the compressor map for the 2871 turbo, 150hp at a pressure ratio of 2 is right on the line for 68% efficiency, very close to the surge region, but not there yet. So that would mean I would have full boost (200kpa for now) at 2500 rpm or slightly below? That's significantly better than it was at 3200 or above. Do I need a still smaller turbo than that?

[This message has been edited by 1985 Fiero GT (edited 11-19-2025).]

pmbrunelle NOV 19, 04:58 PM

quote
Originally posted by 1985 Fiero GT:
On the compressor map for the 2871 turbo, 150hp at a pressure ratio of 2 is right on the line for 68% efficiency, very close to the surge region, but not there yet.



If you want the turbo to operate there with the engine at 2500 RPM, then it doesn't look like a good choice. A smaller compressor, such as 2860R, looks like it would fit more with your goals of having more boost down low.


quote
Originally posted by 1985 Fiero GT:
So that would mean I would have full boost (200kpa for now) at 2500 rpm or slightly below?



No, plotting that point on the compressor map does not mean you will have full boost at that RPM. It's just saying that if the engine happens to run there, then you will be at/near the surge limit.

Whether the engine/turbo combination will actually make "full boost" there depends on whether there is enough exhaust gas to spool up the turbo. In a trial/error process, you can try turbine housings with different A/Rs to find the balance between boost threshold and top-end power. I am not sure how to predict a boost threshold on paper; perhaps try a turbine housing with a small A/R to begin?

[This message has been edited by pmbrunelle (edited 11-19-2025).]

1985 Fiero GT NOV 19, 05:36 PM

quote
Originally posted by pmbrunelle:


If you want the turbo to operate there with the engine at 2500 RPM, then it doesn't look like a good choice. A smaller compressor, such as 2860R, looks like it would fit more with your goals of having more boost down low.
No, plotting that point on the compressor map does not mean you will have full boost at that RPM. It's just saying that if the engine happens to run there, then you will be at/near the surge limit.

Whether the engine/turbo combination will actually make "full boost" there depends on whether there is enough exhaust gas to spool up the turbo. In a trial/error process, you can try turbine housings with different A/Rs to find the balance between boost threshold and top-end power. I am not sure how to predict a boost threshold on paper; perhaps try a turbine housing with a small A/R to begin?



The 2867 is the next step down, assuming 50hp na at 1000 rpm (I think that's what that blurry dyno picture says), that would be 117 @ the flywheel at 200kpa, which is within the map still. So that would put every engine operating point on the map somewhere, down low is still near the surge limit, but considering what I'm using currently is, it'll be much different. I'm not specifically looking for full boost right from idle, but I would like to lower the rpm needed a little bit. As a comparison, the closest map I could find for my eBay Turbo (just some map for a t04e compressor) puts the surge line at 16 lb/min@200kpa, the 2867 is at like 11 lb/min@200kpa, so the compressor can't do anything until the engine can take in and put out 160vs110hp. The dyno chart had 80hp @ 3000 rpm, which is around the time I would have just started building boost, so there's a bit of a coincidental correlation there, 55hp was somewhere between 1000 and 2500 rpm on the chart, that would likely be fine for me, while keeping peak power in the most efficient area, and leaving room in that peak efficiency island for upgrades and more boost.
Looking at the 2860 map, it definitely puts my current peak power/boost nicely in the middle of the peak efficiency, and excels more at the lower power/rpms. It doesn't cut to much off of the potential high end, certainly nothing I'd miss right now, and unlikely anything I'd ever get to: 400hp@325kpa for 2867 vs 375hp@300kpa for 2860, both unlikely numbers.