CrackHeadMel

iTrader: (2)

Alright, i picked up Maximum boost few days ago, read it cover to cover and have been re-reading sections whenever i have time. i thought i understood how to read a compressor map however i guess not, or maby im just confused

In Maximum boost, they mention a 60-1 compressor wheel, and show a map for it and the cfm is listed going almost to 900, however looking here at this compressor map online http://64.225.76.178/catalog/compmaps/fig9.html it tops off at maby 65cfm? or is it a diffirent unit of measure?

-Jacob

CypherNinja

Yes, its a different unit. The online map is using pounds of air per minute, which is a newer way of measuring. Maximum Boost is slightly outdated in this regard.

CrackHeadMel

iTrader: (2)

Lbs/Min = 0.0756 x CFM ?

Evil Aviator

You can just multiply your cfm by 0.069 to get a good ballpark figure for lbs/min. This isn't exactly correct, but I'm not sure if you want to deal with the "real" equations.

Two other things to note in the Maximum Boost book:

1) The Airflow Rate equation is for 4-stroke piston engines only. These engines are not very good (as we know, lol) and only fire half of their rated displacement per crankshaft revolution. Therefore, a "0.5" multiplier is in the equation so it calculates the fact that only half of the cylinders are actually firing per rpm. Rotary engines and 2-stroke piston engines fire ALL of their rated displacement per output shaft revolution because they fire ALL of their cylinders/rotors rather than half. Therefore, you need to either remove the 0.5 multiplier from the equation, double the rotary engine's displacement, or double your result in order to yield the correct cfm.

2) 1 Bar = about 14.5 psi, not 14.7 psi. This will not affect anything unless you are trying to convert your numbers to compare with engines outside the US. The other 14.7 numbers in the book are correct, since 14.7 is the international standard for sea level pressure, although this technically varies with altitude and ambient conditions (aka the high and low pressure areas shown on your local news TV station).

Also, just for my curiosity, do you now understand boost better? For example, do you understand that a turbocharger multiplies pressure rather than adding it? I'm just curious how regular people view that book.

Evil Aviator

Sometimes, lol.

That's air density at International Standard Atmosphere (ISA) conditions of standard sea level pressure 29.92inHg and 59degF, not corrected for humidity.
http://www.aeromech.usyd.edu.au/aero/atmos/atmos.html

CrackHeadMel

iTrader: (2)

Hmmm, you make the 'real' equation sound a little difficult to understand but never the less if its the correct way to go about I should probably learn it

Do you have a url or another book that would have this equation?, or maby its in the forum here already somewhere, ill search i spose

Thanks for the info on the 0.5 multiplyer in the cfm forumla. I knew that the rotary had double volume but i wasnt 100% on if i could just remove the 0.5 or not.

Yeah i really understand alot more about boost, i want to beable to understand exactly why im choosing what i choose for my car as i build it. There was alot of additional information that i didnt expect to see in this book. The only problem im still having is understanding a/r completly. Looking at Fig. 3.11 on pg 32 it shows how A6 over R6 is equal to A5 over R5 etc etc. However to me all i see the Area of a givin location in the scroll is equal to the radius of the turbin housing to the center of the scroll? I dont understand how a distance can be equal to an area i guess. I never had any highlevel math class's in Highschool so if this is something basic that im missing feel free to point it out Other than that i feel it was deffinetly worth the $30 something i paid for it

-Jacob

Evil Aviator

http://wahiduddin.net/calc/density_altitude.htm

You may also like this site. There are some math errors, but it gives you the basic idea.
http://www.gnttype.org/techarea/turbo/turboflow.html

The Area and Radius are not equal, their RATIO is equal. So given an AR of .96, for example, the Area is always .96 of the Radius, regardless of where you measure it on the scroll.

Example for .96 A/R ratio:

A1 = 1.92 sq in
R1 = 2 in
A1 / R1 = .96 A/R

A2 = 1.44 sq in
R2 = 1.5 in
A2 / R2 = .96 A/R

A3 = .96 sq in
R3 = 1 in
A3 / R3 = .96 A/R

I also think it's worth the money. Some parts are a bit basic (for me), there are a few mistakes, and it's annoying that Corky Bell does not discuss the rotary engine despite his experience with it, but I think it's a good book overall. His Supercharged book is even better, and uses lbs/min compressor maps, but other than the supercharger vs. turbocharger issue it has a lot of the same information as the Maximum Boost book.

CrackHeadMel

iTrader: (2)

Thanks for the urls. Ive only quickly scanned them as of now but the 2nd one seemed easy enough to understand for me

As for the A/R thing, Now im completely understand what they were getting at. A pretty obviouse mistake made but atleast its answered

When I was digging threw barns & nobles shelves looking for Maximum Boost i ran across his supercharger book and almost bought it, im sort of regreting not buying it now, but theres always next week.

Do you know of more advanced books covering roughly the same topics?

-Jacob

Evil Aviator

About half of the book has the same information as Maximum Boost (although slightly updated), but the supercharger portion is really good. Technically, a turbocharger is really a type of supercharger, so it all does relate.

"Turbochargers" by Hugh MacInnes is good, but it's horribly outdated. Still, Hugh MacInnes is "The Man", so it's still a good book if you want to learn about turbos via the old school method.

KevinK2

http://www.gnttype.org/techarea/turbo/turboflow.html

like evil said, some math errors (exponent is .286, not .268), and says 17 psi boost and then writes 19 in formula. But he has it al the steps right, but not smoothly presented. I have an excel spreadsheet to do all the math to generate the corrected mass flow and PR for garrett, mitsu, kkk, and holset 'test conditions'. Could e-mail it if u have access to excel.

Moisture is less than a 1% densitity error, and not worth messing with imho. only used in dyno correction factor to get a 'zero humidity' common basis.

Evil Aviator

Hey, he asked for it.

None of my aircraft performance charts has ever bothered with the humidity factor. It's more of an engineering issue rather than a user issue. However, it does affect the detonation threashold because it acts like natural water injection.

CrackHeadMel

iTrader: (2)

HA

I just burst out laughing when i read that

Thanks for pointing out the errors, and yes i would love to have a copy of any of the excel sheets if you dont mind sending them my way

my email address is crack_head_mel@Hotmail.com

This has to be one of the most helpful thread's i have ever participated in within this forum, hell any forum at all, thanks alot

-Jacob

KevinK2

done.

Turbo 3

iTrader: (3)

I'd be interested in the spreadsheet as well please. I made up my own for use the a T-76 on my project as well. Just checking to see if I did it correctly.

The thing I still don't understand is how you calculate what turbine A/R to run vs. what RPM you will start seeing boost at. If you take a look at my graph, again, I think it's pretty close but would like other opinions. Go about 1/3 down the page.

https://www.rx7club.com/showthread.p...hreadid=243960

Turbo 3

iTrader: (3)

Oops, forgot; my email is kerryb23@excite.com

KevinK2

The map may show flow capacity at 2k rpms and 1 bar for an FD, but it's more likely to occur with a 5.0 diesel at 1 bar and 1K revs. I think a/r selection is more by experience vs calculation.

Sent u my excel ... quick check has my flows much lower.

Evil Aviator

I am not aware of any user-friendly calculators for determining the optimal A/R for a given situation. This is an extremely difficult physics problem that is usually best solved by experience.

Compressor maps are just that... compressor maps. This means that they only cover the compressor wheel, not the turbine wheel, shaft, scroll, etc. The compressor map will show you the theoretical (assuming a perfectly matched turbine) boost rise if you follow the surge line up to your boost setting line on the compressor map.

Turbo 3

iTrader: (3)

Thanks Evil but that was the problem I was trying to deal with, avoiding the experience factor and finding something objective. Unfortunately, as you said, I haven't been able to find anything either that will be more definitive for my question. Add on top of that the total amount of BS I've dealt with on the 20B project and the wide range of answers, inexperience, etc, have made it just that much more fun! *sarcasm* At least the 2 rotor guys have a large amount of others' personal experiences to help them.

Evil Aviator

What did you expect? The 20B is even rare in Japan! When I started my 20B FC project, Chito was the only person I knew of in the US who had completed one. Geez, my turbo guy had to make the exhaust flange out of two J-tech 13B flanges, not even the Motec M800 could run split timing on it, and there certainly wasn't any such thing as K2RD or FC3S.org pre-made mounts. The only people who had a lot of 20B experience in the US were Rick Engman and Roger Mandeville, lol. You have it easy!

If you would like, I can post the Rick Engman method of choosing a turbo.

Mark'sMazda

please post it, myself and many other noOb'ish people would benifit.

Evil Aviator

The Rick Engman method of turbocharger selection is... ask the turbocharger representative.

No, I am not joking. I was very shocked to hear this at first, but then I began to realize that even the best rotary engine tuners will defer to experts in certain areas. This really changed the way I look at things.

FYI the other impression made on me by Downing Atlanta is that Jim Downing helps his crew members push his car back in the paddock area. I would have thought he would let them do all the work, but obviously this is what it takes to be truly great.

Hopefully the other noOb'ish people will benifit from this information as much as I did.