# Compressor Map Calculations (are frustrating)

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**1**Senior Member

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**Compressor Map Calculations (are frustrating)**

So this is my first post. Feel free to flame, since I realize that you guys have a hazing ritual and everything.

Anyway, I am currently doing research for a fun project. A turbo Miata has a lot going for it.

Right, so I've been reading various sites and I'm trying to calculate some workable numbers for turbos. I could post a question asking which turbo would be best for my specific setup (which I don't have yet--1.6 or 1.8) or I could try to learn some of the basic philosophy behind turbo charging. I followed a pretty nice article at Garret (Turbo Tech 103) and created a quickie excel spreadsheet, trying to fill in the numbers for the calculations as best as I can.

But, looking at these numbers, something seems off.

I've compared the numbers I've come up with to This Site and it seems to be pretty similar. Actually, I'll just use that site for now since we can all access it. Common ground and all. Here's what I put in:

200 HP (Crank I'm guessing, as that's what the Greddy site calculated)

1.6 Engine Size

12 Target AF (which seems quite rich, I would think 13 would be better, but as that was the default here and in the example at the Greddy site, I'll leave it)

.51 BSFC (using the help feature on the form, I see that .51 is for a Modern Stock Engine -- entire range is from .36 to .55--, but that is WAY different from the Greddy article. "General values of BSFC for turbocharged gasoline engines range from 0.50 to 0.60 and higher...Lower BSFC means that the engine requires less fuel to generate a given horsepower. Race fuels and aggressive tuning are required to reach the low end of the BSFC range described above."" This confuses me, these ranges are really different.

2.0 Max IC loss (Conservative estimate from the Greddy article)

6900 Redline

5500 Peak Power (this means peak torque right?)

'Max boost at' and 'min boost at', I left alone as I'm not really sure what these should be.

Ok, so here finally are my questions: are these numbers even close to correct for Miatas? What are good numbers to use for the 1.6 and the 1.8 engines? When the calculator says I need 19 psi, what the hell does that mean? It's got to be a function of the volume of air moved, not the actual pressure since each turbo configuration is different and this doesn't take into consideration any specifics of the turbos! Are there any t3/t4s in the drop down list? I found the t3s but I'm still trying to catch on to all of the different nomenclatures of the turbos.

I've come pretty far in my studies, but, I've gotten stuck and I'd love some clarification. If anyone can help, I'd greatly appreciate it.

Oilstain

(PS, don't flame without pics of cats)

Anyway, I am currently doing research for a fun project. A turbo Miata has a lot going for it.

Right, so I've been reading various sites and I'm trying to calculate some workable numbers for turbos. I could post a question asking which turbo would be best for my specific setup (which I don't have yet--1.6 or 1.8) or I could try to learn some of the basic philosophy behind turbo charging. I followed a pretty nice article at Garret (Turbo Tech 103) and created a quickie excel spreadsheet, trying to fill in the numbers for the calculations as best as I can.

But, looking at these numbers, something seems off.

I've compared the numbers I've come up with to This Site and it seems to be pretty similar. Actually, I'll just use that site for now since we can all access it. Common ground and all. Here's what I put in:

200 HP (Crank I'm guessing, as that's what the Greddy site calculated)

1.6 Engine Size

12 Target AF (which seems quite rich, I would think 13 would be better, but as that was the default here and in the example at the Greddy site, I'll leave it)

.51 BSFC (using the help feature on the form, I see that .51 is for a Modern Stock Engine -- entire range is from .36 to .55--, but that is WAY different from the Greddy article. "General values of BSFC for turbocharged gasoline engines range from 0.50 to 0.60 and higher...Lower BSFC means that the engine requires less fuel to generate a given horsepower. Race fuels and aggressive tuning are required to reach the low end of the BSFC range described above."" This confuses me, these ranges are really different.

2.0 Max IC loss (Conservative estimate from the Greddy article)

6900 Redline

5500 Peak Power (this means peak torque right?)

'Max boost at' and 'min boost at', I left alone as I'm not really sure what these should be.

Ok, so here finally are my questions: are these numbers even close to correct for Miatas? What are good numbers to use for the 1.6 and the 1.8 engines? When the calculator says I need 19 psi, what the hell does that mean? It's got to be a function of the volume of air moved, not the actual pressure since each turbo configuration is different and this doesn't take into consideration any specifics of the turbos! Are there any t3/t4s in the drop down list? I found the t3s but I'm still trying to catch on to all of the different nomenclatures of the turbos.

I've come pretty far in my studies, but, I've gotten stuck and I'd love some clarification. If anyone can help, I'd greatly appreciate it.

Oilstain

(PS, don't flame without pics of cats)

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**3****Boost Czar**

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calculations to determine the amount of boost you need is almost silly. I can make 200rwhp at 12psi or 240rwhp at 12psi. all dependant on the size of my exhaust.

the best thing that you can determine is if the compressor is large enough for your goals. And if so, will you be in the most efficient part of the turbo at that power level.

let us take the T3 for example. the super 60 can flow 35 lb/min. roughly converted, that equals 320BHP. [ 35 / (AFR) 12 / (BFSC/60) .00916 ]

so now you know that turbo is a good for 270-300rwhp or so at the max. Boost level isn't very important. but it should make 250 at 13-14psi or so, if you do it right on a 1.6L. power per psi will go up as the turbine a/r and stage increase, but spool will slow. and versly, the smaller the a/r, the faster the turbo spools, but the lower the hp you'll make up top. no matter what the psi.

psi is simply a measurement of restriction, you need to be more concerned about mass airflow volume. the more air you can push through the motor, the more power you'll make.

the best thing that you can determine is if the compressor is large enough for your goals. And if so, will you be in the most efficient part of the turbo at that power level.

let us take the T3 for example. the super 60 can flow 35 lb/min. roughly converted, that equals 320BHP. [ 35 / (AFR) 12 / (BFSC/60) .00916 ]

so now you know that turbo is a good for 270-300rwhp or so at the max. Boost level isn't very important. but it should make 250 at 13-14psi or so, if you do it right on a 1.6L. power per psi will go up as the turbine a/r and stage increase, but spool will slow. and versly, the smaller the a/r, the faster the turbo spools, but the lower the hp you'll make up top. no matter what the psi.

psi is simply a measurement of restriction, you need to be more concerned about mass airflow volume. the more air you can push through the motor, the more power you'll make.

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**4**Senior Member

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the best thing that you can determine is if the compressor is large enough for your goals. And if so, will you be in the most efficient part of the turbo at that power level.

let us take the T3 for example. the super 60 can flow 35 lb/min. roughly converted, that equals 320BHP. [ 35 / (AFR) 12 / (BFSC/60) .00916 ]

so now you know that turbo is a good for 270-300rwhp or so at the max. Boost level isn't very important. but it should make 250 at 13-14psi or so, if you do it right on a 1.6L. power per psi will go up as the turbine a/r and stage increase, but spool will slow. and versly, the smaller the a/r, the faster the turbo spools, but the lower the hp you'll make up top. no matter what the psi.

psi is simply a measurement of restriction, you need to be more concerned about mass airflow volume. the more air you can push through the motor, the more power you'll make.

Your comment about the super 60 being 35 lb/min to give 320 HP looks consistent with the real life examples I've read about. I'll post my work when I get home tonight, but I'm pretty sure the calculations I made pointed to significantly higher flow required for the hp. I'll double check what I've done to see if I screwed up the calculations.

I've learned enough to not be concerned with teh boozt psi yo's! It just seems difficult to know if the numbers I'm looking at are accurate at all.

Thanks for the help so far!

Edit: Forgot to ask, What is a realistic BSFC for a stock 1.6 and 1.8 Miata engine? This has a pretty significant impact

*Last edited by oilstain; 07-07-2008 at 03:30 PM.*

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Alright, here's my data (1.6) on top of a t3 super 60 map.

First and foremost concern: My little red line runs right into the surge territory. How critical is this? Huge freakin deal, or not?

I realize that using an overly large turbo on any application will keep the spool times long, and there could be some compressor surge, but really, where is the line between non-optimal performance, and why did it break, I was only running 3 psi?

I figure that line is there for a reason, so I'm looking for turbos that can hold all my data points.

First and foremost concern: My little red line runs right into the surge territory. How critical is this? Huge freakin deal, or not?

I realize that using an overly large turbo on any application will keep the spool times long, and there could be some compressor surge, but really, where is the line between non-optimal performance, and why did it break, I was only running 3 psi?

I figure that line is there for a reason, so I'm looking for turbos that can hold all my data points.

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Can anyone take a look at the map I posted and give me some feedback as to whether the plot points look realistic for a 1.6 at 250hp? I'm thinking they are a bit high, but I'm not sure where in my calculations I'm off.

What is a realistic BSFC to use?

From my Excel calculations:

6900 rpm

250 HP Flywheel

95 Volumetric Efficiency (VE) range in the 95%-99%

130 Intake Manifold Temperature (TM

12 A/F ratio

0.51 Brake Specific Fuel Consumption (BSFC)

1.6 91-93

32.1 Pounds per minute Airflow Capacity (Wa) corrected

Same numbers in the squirrel calculator:

33.4 lb/min @ 6900

25.5 lb/min @ 5500

Pretty close, but garbage in = garbage out.

What is a realistic BSFC to use?

What is a realistic BSFC to use?

From my Excel calculations:

6900 rpm

250 HP Flywheel

95 Volumetric Efficiency (VE) range in the 95%-99%

130 Intake Manifold Temperature (TM

12 A/F ratio

0.51 Brake Specific Fuel Consumption (BSFC)

1.6 91-93

32.1 Pounds per minute Airflow Capacity (Wa) corrected

Same numbers in the squirrel calculator:

33.4 lb/min @ 6900

25.5 lb/min @ 5500

Pretty close, but garbage in = garbage out.

What is a realistic BSFC to use?

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**9**Junior Member

Join Date: Aug 2007

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If you are looking to build a turbo miata, I would recommend you just look around here at other peoples setups. This will give you a real feel for what you can do with a given turbo.

Also check out BEGI. They have several turbo kits that are specially designed for miatas. I used to be confused about all this turbo nonsense too, but spend some time around here and you shall learn the ways of turbo miatas.

Also check out BEGI. They have several turbo kits that are specially designed for miatas. I used to be confused about all this turbo nonsense too, but spend some time around here and you shall learn the ways of turbo miatas.

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Your results are awesome, and it's a great set up to copy. You've sold me on your results! My point is that doing the number crunching, a combo like yours puts the turbo off the map and into surge territory. I want to know why. Are my numbers wrong? Is your setup surging, but surging just isn't that bad? What?

I've asked these questions before because I'm trying to learn more theory, then which setup to copy.

*Last edited by oilstain; 07-08-2008 at 12:49 PM.*

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**13**Senior Member

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For a forum that berates people who don't learn enough before asking questions, it sure seems like you'd rather not see people think for themselves. I was looking for knowledge that I can scale to other engines and other cars, not just what works best on one particular motor gunning for one particular HP rating.

New approach, did anyone run these numbers to figure out what theoretically works best, or did everyone use the copy/trial/error method to get to their set ups? In other words, does anyone else even give a crap about the compressor maps?

New approach, did anyone run these numbers to figure out what theoretically works best, or did everyone use the copy/trial/error method to get to their set ups? In other words, does anyone else even give a crap about the compressor maps?

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**14**Here are my calcs. The green line is a pressure ratio of 2.1.

The points (red, on my green line) from L to R are; 2.5k, 4k, 4.5k, 6.5k and 7.5k (don't ask why the odd numbers, no reason).

I don't think you'll see 15psi @ 2.5k, so that being on the left of the surge line isn't a big deal...

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**17**Here is a link to the math I used, but with numbers related to your car (CID, boost pressure, etc).

http://superhachi.com/theory/compmap/

What I did was calculate out the charge air density, then made a spreadsheet to do the calculations for mass airflow rate based on rpm (CID for a 1.6L is 97.638). I did every 500 rpm from 2.5k to 7.5k, but only plotted a few.

Then I made another excel column to calculate out the corrected (for temp) numbers.

If you want, I can email you my spreadsheet tomorrow.

PS - I can't vouch for the vailidity of that compressor map. You might want to check out Garret's site to double check that ****.

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