Cliff Manzke wrote:
Paul:
I've been reading all the turbo talk lately and finely got the chance to comment, so here goes:
I did some calculation based on a few assumptions for a 13 B, which are:
160 hp normally aspirated at sea level (29.92 inHg ambient pressure, 85°F)
BSFC of .55 lb/hp·hr
A/F ratio of 12.5 to 1, giving a BSAC (brake specific air consumption) of 6.875 lb/hp·hr
This gives an inlet airflow of 18.3 lb/min or 252.5 cfm at 85°F
Using these numbers and a few more assumptions, I figure 10.0 inHg of boost (giving 39.92 inHg MAP) will give you about 200 hp at sea level. I'm making some guesstimates here as I don't have a known accurate "hp vs. RPM vs. manifold pressure" map for a 13B. Please provide some better numbers if you have them, but remember that "desirable" numbers don't do much for turbo matching. Be honest!
So.....based on 200 hp at 39.9 inHg MAP, you'll get a corrected airflow of 22.0 lb/min and a pressure ratio of 1.33. Now, suppose you wish to maintain that power (200 hp) up to an ambient pressure of 20.0 inHg (just over 10,000 ft). This will result in a corrected airflow of 31.7 lb/min and a pressure ratio of 2.0, which puts you right in the middle of a TO4E-50 trim compressor map....And just about this time I read the post from Paul Yaw and he suggests the same thing. Thank you Paul for checking my sanity!
Now the other half of the puzzle is the turbine, and here Paul yaw suggests a "P" trim (74.2 mm dia wheel with a .76 trim) and a 1.00 A/R housing. I was going to suggest the same trim turbine with the next smaller housing (.81 A/R), but Paul may know something about 13B's that I don't.
The real point to all of this is that this is a pretty "big" turbocharger we're talking about, simply because the aircraft requirements are quite different than the automobile requirements. I can comfortably say that if the stock 13B turbocharger (a T3 of some sort?) was sized correctly for the street, it will be way too small for the air. Sure, it'll work, but the wastegate will likely be opened from the get-go (sea level), which only means more exhaust pressure and therefore more exhaust heat to get the desired boost. The desirable scenario is to have the wastegate shut (or better yet, no wastegate) at sea level and max power with the turbo producing the desired boost. This desired boost can fine tuned with selection of turbine housing, but only if the whole compressor/turbine assembly is in the right ball park to begin with. Given the low boost requirement at sea level (~10 inHg), the TO4E-50/P turbo efficiency will improve as altitude increases, hit a "sweet spot" s!
omewhere near 10,000 ft, and then begin to fall off again. By doing this, you MIGHT (big maybe here) be able to eliminate the wastegate altogether. This is a complicated issue and has a lot to do with how effective your intercooler is. With insufficient intercooler capacity, you will likely reach a detonation limit due to air charge temp long before you reach your flat rated altitude, in which case you need to 1) get a bigger intercooler, or 2) open the wastegate, or 3) close the throttle, or 4) use a bigger (less restrictive) turbine housing. Note that the last three options all produce the same end result, i.e., less boost and therefore less power. The first option is the best but not always practical.
As far as turbo overspeed goes, if you size the turbo such that it is optimum at something like 10,000 ft, then turbo overspeed is unlikely. The T4 turbines have a maximum speed of 135,000 rpm or so. In the case of the TO4E-50 this means a pressure ratio of at least 3.5, which equates to about 48 inHg MAP at 20,000 feet! On the other hand, high air charge temperature (and detonation) is likely to be a problem with unrestricted boost and a wastegate is a fool proof way to prevent that.
If a 13B will actually make more than 200 hp with only 10 inHg boost, then an even larger compressor might be in order. I recall 250 hp at 10 inHg coming up, in which case a TO4E-60 trim or even a TSO4 (84 mm wheel, .49 trim) might be a better choice. If anyone can provide some actual data I can make a better assessment of this.
As far as the T4 turbocharger hardware goes, the TO4E series is the latest (and most efficient), although all of it is pretty old stuff. The T4-P trim turbine map I have is dated 1968 and it hasn't changed since! The GT series (GT12, GT15, GT30, 32, 35, 37, 40, 42, 45, and maybe a few more in between) is newer and would be a better choice but unfortunately design information (compressor and turbine maps) are closely guarded secrets or Garrett/Allied Signal/Honeywell/General Electric, or whoever they are this year! If someone can get their hands on one, a GT35 or GT37 might be a good starting point for the 13B.
The bad news about Turbonetics is, ah, bad. Do get one of their catalogues if you can, as it is an excellent source for compressor maps and turbo parts. The folks at Turbonetics were always real helpful, so I hope the people (and knowledge) will carry on somewhere.
I hope this helps more than in confuses everyone. I'll be out of town for the next week and a half, so hold your questions. In the meantime, bug Jeff Spitzer for some TO4E parts to help "the cause"! Oh, and read the article "Turbo Power" in Kitplanes to get an idea on how not to do it.
Cliff Manzke
Cliff, we are fortunate indeed to have you and Jeff on here. You guys are the experts
in this field having got your airplanes up to 57,000 feet. Thanks a bunch.
I do indeed have the Turbonetics catalog with all the maps. I also have some power
curves for the stock 89-92 205 HP 13B Mazda with 5 psi boost and Paul Yaw's power curves
which I will upload when you get back if you remind me.
Paul Lamar
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