kablammyman

I was trying to figure out how to select a turbo size for my car by using the sticky at the top of this page. I then decided to make a quick and dirty C++ program to test a bunch of different turbos at once. Before I could do that, I just tried to make my program get the same results that were I the sticky. Everything was okay, until I had to use the formula:
T2 = T1 (P2 ÷ P1)0.283

With the numbers: T2 = 535 (24.7 ÷ 14.7)0.283
The guide got the answer of 620 °R. On the other hand, my program got 254.402! I then broke down the math...same answer. I soon used my Ti86, and got the same answer. I then tried searching on this site to see if the error was found, but nothing relevant came up.

This 1 wrong answer makes everything else that follows incorrect. Maybe, I'm tripping, but have anyone actually tried the math? What answers did you get? Maybe I’m doing something wrong. Also, I would like to point out that later on in the guide, the 620 he got is magically changed into a 656!

Where Outlet R is supposed to be T2 we found earlier... I’m really confused, so maybe I'm doing something wrong, or I missed something. Let me know what’s going on.
Thanx

rexsk8er

Tout = Tin + Tin x [-1+(Pout/Pin)0.263]
efficiency



Another webpages formula on temp rise. Not sure if that helps but notice the .263 in tht one is close to the .283 and may be the error.

Try what you have now but subistute .263 maybe thats the problem



Sean

kablammyman

Using the new formula gives me -597.15 and using the .263 conversion factor in the original formula gives me 263.42.
Maybe the formula from that other webpage may use different numbers in their variables, so if I could get that link, I can try their method. I’m gonna say forget the sticky on this site because even if I did get the 620 °R, he changes or uses some other unexplained temp value of 656°R, later in the article.

rexsk8er

well honestly theres so many variables in the equations that theres no solid way to caluclate it all.

thats just how it seems to me anyways, theres a gillion different variables you have to fill in even before you start any of the math, and if any one factor is slightly different he whole thing will be different, but a few facotrs like temp rise arnt quite as big of a deal as the compressor working with the motor and not gettin compressor surge.



Sean

kablammyman

I see what you are saying...so in your opinion, should I forget about the "math" and look for general turbo sizes? For example, I want a lite kit with mild boost, so I should just think about a smaller turbo in general? I know that turboing a car isn’t something you can just do, so, I want to do it right.

rexsk8er

well nah i wouldnt go hit or miss, id just fill in all the veriables with a best guess, if its not perfect so what itll be close. thats what im sayin. and since the ambient tempature is gonna be different on any given day theres no point in worring about the ideal temp rise since a intercooler is a good idea anyways.

like i said, get your motors flow at half throttle (~40% ve) and full throttle and find a compressor thatll fit w/o compressor surge. and go with it.




Sean