No at most the BB on the HP stage as that what gets the whole thing started. But the atmosphere no reason. I know they just redid the whole Garrett site and now it only lists the gtx version of the 5518 and that comp wheel that is the same wheel etc as the GTX4718. If you compare the two maps you can see how even though they have the same size wheels ( Inducer: 87.80 Exducer: 117.60) the design of the gtx wheel more significantly more air and does so more efficiently. As far as the same efficiency % islands they are about the same width etc. The break down of the GTX wheel is more detailed hence why they look more narrow. If you compare same % size island you can see they tend to match up with the GTX being better. Even though it moves more air it looses nothing on the bottom end. Either one IMO will meet what you need but one can cost way more.
Only way I would go with a GTX is if you some how got a killer deal on one. Otherwise a JB gt5518 or a R if again you get a great price on one. IMO a JB is what I would go for and look for a pull off as you did with your current turbo. Send it off to be upgraded to a GT55 center section's internals and the larger turbine wheel. That for find a cheap 5533 as was on the net a month ago and try it as is and if its too big or wrong comp perf have the comp swapped for the 118 wheel. You could likely sell the 133 wheel and cover some of your costs for the swap.
Since you are going for a big fuel only 700HP+ might as well take what you can get if you get the deal. So below is the maps so we can break down off what you are looking at needing and where/how it will fall in the turbos map ranges.
This is just a guesstimate on my part based on some computing and what people have made for HP from various turbos I could find flow data on.
700HP from a 7.3 from what I can gather from other HP goals will take about 120lb/min of flow. 75lb/min 500hp, 90lb/min 600, 120lb/min 700-725
To get PR break down for total and for each turbo:
Basic Formula for Pressure Ratio (boost +14.7)/14.7=PR (Not accounting for altitude changes, filter restriction, intercooler, piping)
Formula for Pressure Ratio for each Compressor in a Compound Setup: In compounds the turbos individual PRs are multiplied together to get total PR. So if you have the total you divide it by either one comp individual PRs. The Atmosphere Compressor is the easy one to compute along with the total combined PR. So The total PR divided by Atmosphere gives use what the Manifold Comp PR is. With that we can use the standard PR formula to see what it would be making if inlet was standard atmosphere pressure. This is helpful IMO as most people are use to looking at compressor perf by boost as opposed to PR.
Looking at the maps first we have to disregard any of the lb/min on the HP/manifold turbo as its going to be way off as its already dealing with a compressed charge. But what still matters is the PR and its efficiency as that still relevant.
Going with the gt4718 wheel map.
Looking at 120lb/min and then looking the middle more efficient islands it looks like in the 2.4-3.1 range. It will still keep things in the 73-76% range which is still decent. So split it and call it 2.7pr or 25psig.
Looking at the 38R the best range getting the most lb/min for PR is about 2.4-2.6 ratio so 2.5 which is about like it at a single in the 22psig range. When you look at B
ratios this is a good spot on a 38R.
As you can see they are actually quite balanced as far as PR and how hard they are working. This also puts both of them in the real meaty section of their maps. This is where they are getting the most lb/min@ the best PR.
As PR is what multiples to get you the total manifold PR/Boost why compounding is so effective.
Atmosphere: 2.7pr/25psig
High Pressure: 2.5pr/22psig
Total Manifold: 6.75pr/85psig
Likely have to drop off 3psi for filter, piping inefficiencies, and the IC. May have to tweak numbers up a bit for that.
Obviously this can be played with once you get real world numbers to find what works best for responsiveness, temp, etc.. Go up with the second stage and down with first etc.. I could not find a detailed 38R map with the islands as that would be of more use.
On the turbine side you have first hand experience with how having a turbine that can move more volume more efficiently can increase HP. Doing so even with a comp wheel that technically moves less air per rev as some making less max power (volvo gt4594 vs gt38r). So knowing that in this combo the 47 turbine is the bottle neck going to the larger and more effective 55 wheel should work in a similar manner and effectiveness. The 47 turbine @2.5pr move about 55lb/min @ 68% efficiency vs the 55 turbine moving closer to 70lb/min @ 82% efficiency. Moves more air at less waste about 20% better.
Regardless of everything what I would do if I were you is give Jose @ Forced Inductions a call (256)658-5798. Talk to him about gt5518.gt4718 and what is the cheapest path to get you to the gt55 turbine wheel and the 4718 comp wheel. He does a good number of conversion builds to this combo from what I understand. I am nothing but a "armchair" compound turbo student. I try to learn all I can but its still just academic on my part. Best to have a BTDT countless times giving you the final info on what will/will not be best.
