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Re: How do you read compressor maps?
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Posted by Beaudreau (more from Beaudreau) on Sun, 15 Jul 2001 09:19:49 Share Post by Email
In Reply to: How do you read compressor maps?, Cutic, Fri, 13 Jul 2001 22:53:54
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I'll try, I think that I have a pretty good idea of what they are about. The truth is they are useless unless you have a lot info about your engine. So, I am going to hope that you know how much air your engine will move.


The x axis is how much air the engine is pumping in lbs/min. Be aware that the engines VE will change through out the RPM range. The highest VE will be at the engines torque peak.

The y axis is the pressure ratio of the compressor wheel, the higher the ratio, the higher the boost.

The stuff in the center is the actual map for that particular compressor wheel.

1. At the surge limit, the compressor wheel is totally inefficient, either it's unable to pump because it's not spinning fast enough, or what it does pump gains so much heat that it's like not pumping at all.

2. The main island in the map, is the compressor's peak efficiency zone, lines outside of this area are less efficient.

3. The RPM lines are how fast the compresor wheel is turning. This can be changed with a different ratio, A/R, turbine housing. Turbine trim, clipping, will also effect this. You have to know how much exhaust gas is pushing the turbine to come up with this number.

When selecting a compressor that is the best match for your engine, select a flow map where the point the maximum boost is first achieved is close to the surge line, this will ensure maximum compressor efficiency through the engines maximum torque RPM.

You need to know a lot stuff about an engine to use the maps. There are formulas to figure all that stuff out, but you are still plugging in numbers that are only best guesses.

For a stock engine you can work backwards from the stock published HP/torque curve. The info from an N/A engine may really give you more accurate info on the engine's VE, even though they use different cams.

Someone that really wanted to do this right would dyno their built turbo engine in N/A form, then work backwards to get good numbers to work with.

Here's a link that may help you, there are some neat calculators, but you still have to know the numbers to plug in. You may have to use MSIE instead of Netscape to view the site.

I hope this was useful and that I'm not too far off on my descriptions. If you can get someone that can give you good numbers to plug-in, I'd like to know them also.

Ray Hall Turbocharging

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