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.........--- GM Atlas 4200 Turbo Compressor Maps I ---

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On this page you will find some turbo compressor maps for turbos that I looked at for the GM Atlas 2.9 liter that I'm going to use in my lakester. A couple of these look like turbos that might work on the 4200 Atlas motor, the 4.2 liter that was used in the Trailblazer for some years.

The 2900 I have is the 4 cylinder version of it and they also made a 5 cylinder version first in a 3.5 then a 3.7 liter motor. The engines in the Atlas line are basically the same engine, just different number of cylinders. The 4 and 5 have balance shafts where as the 6 doesn't as it doesn't need them.

So some of these might be a possible turbos to use and some not. Please if you consider any of these you need to do "your" homework and don't make a decision based entirely on what you see below.

To create these maps I used Borg Warner's MatchBot web-base program that you can find ( HERE ).

Above is the input screen from MatchBot and I left all of the data as you see above but changed the rpm/boost points to represent different levels of boost. I also set the RPM points and the Intercooler Efficiency but left the rest at the MatchBot defaults. You will see the boost levels on each map and the corresponding peak HP for those levels.


To plot the points on the compressor maps I took the Pressure Ratios and Corrected Air Flow Rates (see the yellow arrows) and plotted them on the maps. Also note that the projected HP outputs, bottom yellow arrow, are also given. In this case 488 HP at the 10 lbs. of boost at 6300rpm.

A Pressure Ratio of 1 is atmospheric pressure at sea level and 2 would be twice that or about 15 lbs. of boost. Forget the boost just figure that at 15 lbs. you have twice atmospheric pressure so you are getting in twice the air in the motor so have the potential of creating twice the HP (minus other factors). This would be boost at the manifold and not at the compressor output and is the reason you see "corrected air flow rate". A PR of 3 would be 3 times the air vs. sea level. To illustrate some of this let's take a look at a map and compare how two different engines, one a 2 liter and the other a 4.2 liter would work with the same turbo.

The map above is for a HX-35 that a friend was interested in using on his 4200 (4.2 liter). The bottom two plots (black/orange) are for the 4200. The red/orange arrows show the points where the turbo just can't keep up with the air the 4200 needs which is at the 5000 rpm point for both levels of boost (8 lbs./10 lbs.). Past 5000 up to our 6300 rpm redline we move way off the map for our last two points. Even up to 5000 rpm the turbo is working in an inefficient area of the map where the air would be hot and causing potential problems.

I then left MatchBot's inputs alone and changed the engine size down to a 2 liter motor. With it I was able to get the boost up to ~ 29 lbs or a pressure ratio of 3 where the map extends the furtherest to the right and the compressor can develop up to 50 lb/min of air. There at 6300 rpm it made 382 HP but raising the rpm further to 7000 rpm and staying at 29-30 lbs. of boost now it made enough air to make 425 HP with the 2 liter motor.

So now without increasing the boost we raised the HP by more than 40 HP by raising the RPM to take advantage of the air the compressor is capable of producing. If we would of raised the boost we would not of been able to move as far to the right side of the map but would run into the speed line where the compressor wheel would over-speed and maybe fail. This is a case where more boost probably would not of made more HP as we were already at a point on the map of maximum air production by the compressor. This is an important point to understand.

You might say just run the 4200 at a higher boost, I hear this all the time. The problem is that at 29-30 lbs. of boost on the 4200 it will consume 3 times the amount of air that it would not under boost or 75 lbs. at 5000, 91 lbs. at 5800 and 98 lbs. of air at 6300. Looking at the map above the turbo's compressor is only capable of about 52 lbs. of air max so raising the boost is not going to work. We could raise the boost on the 2.0 liter since it consumes less air for each 2 revolutions of the motor.

NOTE: Think Air Consumption (the engine) and Air Production (the turbo) and not Boost to match a turbo to your situation.

The turbo above also runs out of the capability to produce the air the 4200 needs at higher rpm's.

The BW S300SX above does a little better up to about 560 HP and might work up to 500 HP with good intercooling but is still working mostly on the inefficient side of the map and running off the map at higher HP levels.


The BW K29 looks a lot better and would work well up to 600 HP and possibly near 700 HP and still looking very good down at 10 lbs. of boost this turbo looks like a nice combo except for the 1.0 A/R. You could mount a different turbine on it but that would probably hurt the output up top if that mattered. Also beware that the turbine exducer is not what you normally see and you will have to fab up a manifold/exhaust pipe to work with it.

At the time I write this I might put this turbo on the race car (lakester) for the first year as I'll be running lower boost numbers with the near stock engine. Then I could move it to the pickup with a 2900 in it and move up to a larger turbo for the lakester when the engine is built with good parts.

Next I looked at some Turbonetics' Turbos. I started with the T70 as it was too big for the 2900 and felt it might be a better fit on the 4200's 4.2 liter motor.

It looks like it would work and should spool well but at 6300 on all levels it was getting the choke side of the map on the right. I do feel though that I was on the conservative side using MatchBot to come up with this data so the plotted points might not be so close to the edge of the map in real life.

Since the T-70 was getting close I decided to try their T-72 (slightly larger) and feel this is the best match I've found to date for a 4200 in the way of a street/strip turbo with more emphasis on 'street'.

It is a Hurricane (P/N 11533) turbo from Turbonetics. One interesting thing about this turbo is that the ported shroud moves the surge line over to the left so it becomes more surge resistant while under lower boost levels than say the T-70 just above.

It uses the T-72 compressor along with a 68mm turbine with a very spool friendly .81 A/R. This should be a very responsive turbo on a 4200 and also has capabilities of over 600 HP and possibly over 700 HP if not with this turbine on it but a different one for one who later wants to move into the more 'race' category.

I talked to a tech at Turbonetics and he was the one that recommended it after I called about the T-72 and T70 and their HP76. The HP76 would be even more race friendly but not as responsive on the street where as the Hurricane 11533 should be a very responsive turbo for a 4200 on the street. It is available from Jegs and Summit Racing and others for $800 or less in the plain journal version or around $1200 in the ceramic ball bearing model that I don't think the majority of us need.

If you are looking for very high HP numbers with the 4200 and will give up lower rpm HP the above turbo at under $750 would be worth a look. It would be a little slow to spool with the 1.32 A/R on the turbine but a smaller turbine could be used on it and not give up too much on the top end.

Since I've been interested in a turbo mainly for my 2900 I'm using in the lakester and possibly one for a 3500/3700 if we buy a Colorado I haven't spent much time looking for a good match for the 4200. If you find one that interests you and can find the compressor map for it and e-mail me or PM me a link I'll run it through MatchBot, but the best thing is for you to start using the program yourself. There are some pretty good Youtube videos on how to use it at the bottom of the MatchBot Calculator page.

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