cobra25!

For a 97 cobra what would be a good size turbo on a fully forged motorfor somewhere between 12-20 psi.

ZW99GT

Hmm. It all depends on what you want to do with it. HP? 1/8 mile car, 1/4 mile car, fun street car?

A good starting point would be a ~74mm.

Blownsvt

agreed, if you don't intend to go over 20psi a 72-74mm should be good. i personaly have a 76mm witha .96 ar housing.

r.barn

I love these.

Let's actually do the math instead of guessing.

Generally you want to use as small a turbo as possible. It will spool quicker
reach operating temp faster, cool down quicker and take up less room.

(289ci X 3,000rpm X 80%Ve X .5) / 1728 = 200cfm ....... rpm at which full boost is in.
(289ci X 6,500rpm X 80%Ve X .5) / 1728 = 430cfm ....... max rpm
(14.7 + 20psi)/14.7 = 2.36 pressure ratio.


200 X 2.36 = 472cfm
430 X 2.36 = 1015cfm

1 cf = 13lbs

472 / 13 = 36 lbs/min
1015 / 13 = 78 lbs/min.

Now we have our PR of 2.36 and lbs/min of 36 at 3k and 78 at 6.5k

Plotted out on a T-72 compressor map
Looks like a T-72 would work nice.

You want the blue lines as close to the center as you can
and if the red line crosses the "surge limit", thats not a good thing.

skatebaker

^ really dude? Did you seriously just do all that figuring lol? You have to much time on your hands haha

r.barn

Well I know the formula by heart from figuring out my custom system
and it took all of 60secs to punch those numbers into a calculator and cut paste the map into Adobe

Blownsvt

Funny how we got the same answer. but your calculations are a bit off because his motor is a 281 CI not 289CI.

r.barn

hmmmm, had always thought the 4.6's were a 289ci engine ....... learn something new everyday.

USMCrebel

lol nope the 4.6 is 281 the 289 is a 4.7

R could you explain you math 4 me

r.barn

It'd take too long, just get this book.
http://www.amazon.com/Maximum-Boost-.../dp/0837601606

Chopped full of good information on designing and understanding turbo systems