the official turbo math thread

[MUDKICKR]

ok, i know we all would like to know what size turbos are the best size and will work the best. im going to start this thread in hopes that this math will help on this matter. if this gets made into a sticky that will be good. this will be the first of many math problems i will be adding


ok first set of math i will use the ford 302 as a sample.

302x5500x0.5x0.85 and divide it by 1728. it should equal 408 cfm

302 cubic inches/ 5500 rpm/ 0.5 due to the fact that a four-stroke-engine fills its cylinders only on one-half the revolutions/ and .85 is volumetric efficiency. the 1728 converts cubic inches to cubic feet.

i have plenty more info that i will be posting up but for now im out of time.

 

[typ4]

MAth, the only subject I sucked at in school. Although that was 30 years ago maybe it changed.


Also doesn't compression ratio have something to do with this?

 

[DeepRoots]

geez, do I need to break out my marine engineering texts?

 

[82F100SWB]

I'll save you LOTS of typing: http://www.oilburners.net/forums/showthread.php?t=18059&page=2

 

[Hyde]

Just wait ford Fordguy, his math confuses me but gets the job done

 

[h2odrx]

Just wait ford Fordguy, his math confuses me but gets the job done

 

[FordGuy100]

Just wait ford Fordguy, his math confuses me but gets the job done

...I'm pulling an A out of my **** in Calc this year, though I'm sure some people on here are at or above me. I'll look into this and try to help out as much as possible.

So we have the following forumla's to find the CFM of a 6.9 and 7.3

420 x 3500 x .50 x .85 (the volumetric effeciency seems a little low to me, I think I've read that is actually a little higher for turbo diesel's) /1728= 361.5

(444 x 3500 x .50 x .85 ) /1728 = 382.2 cfm's

Both seem a little low, but I guess we are turning fairly low RPM's. If these were gassers then it would probably be way more CFM movement (due to more rpm's).

So now we need to look at compressor maps. To move 361.5cfm's and 382.2 cfm's respectivly. Finding a turbo that will move that much would equal no boost, its going to have to put out more CFM's than what the engine will be taking in for it to create boost, keep that in mind

 

[Diesel JD]

Seems low cause he was using a 302 SB Ford as his example. Both y'alls math is over my head. If I can pass precalc and survey of calc, I will be jumping for joy.

 

[ghunt]

Actually, doesn't that need to be lbs/min? All compressor maps are done in lbs/min of airflow.

I was doing some figuring the other day...assuming a couple things (I used .85 for VE)....For a 7.3, running 15 psi at 3000 RPM, I got 39.5 lb/min.

15 psi at 2000 RPM was I believe around 25 lb/min.

12 psi was just a little lower, at the same RPM's.

I'm no math ****, just going off a formula a guy had posted online.

 

[seawalkersee]

Or....OR....you could look in the Ford book fer the 6.9 and see that it flows 380 cfms at 4000 (I think the RPM my be 4200 and can look if ya need me to).

SWS

 

[troupp]

So comparing a 302 to a 7.3........I should be able to take my 10 lb vortech setup off my 5.0 plumb it to my 7.3 and end up with 10-11 lbs boost? Just asking. I'm sure it's still more fun on the car it's already on.

 

[seawalkersee]

Its actaully different than that, but you are close. The M90 off of my 3.8 fed my SC. The book says the SC flows just 20 cfm less than the 6.9 but its at 2500 (or so) RPM higher. It would probably look like a HP to TQ graph on one engine with one climbing much faster but falling off sooner than the other.

SWS