ZeroBanger

a search has not given me exactly what I need.

What is the CFM requirement that an intercooler would need to meet to be able to run 14-15 psi on the stock twins or a small single?

more to the point, is there some sort of formula to figure this out?

ZeroBanger

NM, I'm guessing 450 CFM is enough.

BicuspiD

I know this article states its for cats, and evens refers to a future (non existent) explanation on rotary flow numbers, but I distinctly remember selling 650 and 750cfm carbs at racing beat for NA motors... I think your guess is low. Obviously you dont want to take the same bigger is better apporoach that they suggest with a cat, but looking at their numbers it shows considerably higher than 450



Taken from http://rotarynews.com/?q=node/view/164




"The first question should be, "How much flow should a new cat be capable of?" The obvious answer is, "If it flows as much as the engine, you're set." Figuring out how much air the engine flows, is another article. Really. Stay tuned, and we'll have an upcoming article on rotary flow numbers. Explaining the how and why of both the NA and turbo flow numbers would pull us in another direction. Although related, this article is about cats. Just use these numbers. An NA 13b, with 95% volumetric efficiency, at 8000 RPM, flows 352 CFM. Got boost? Using the same 13b, with 95% VE, at 8000 RPM, and 5 pounds of boost flows 472 CFM. 7.5 pounds of boost flows 531 CFM, 10 pounds of boost flows 591 CFM, 12.5 pounds of boost flows 651, CFM and 15 pounds of boost flows 710 CFM. These numbers are not meant to indicate that a 13b runs at 95% VE, or that a particular turbo produces a certain amount of boost at 8000 RPM. They are simply ballpark numbers. A word of caution about estimating flow and cat sizes; It's better to err on the high side. This is because if a converter is undersized, aside from being an exhaust impediment, it will swell up, turn purple, and should fail the visual portion of any smog test. "

ZeroBanger

whoa...Dude...I'm gonna have to go with 750-800. Thanks alot!!

turbojeff

iTrader: (30)

Look at it this way. 1300 cc engine right, 80 CID? Well that is only 2 chambers. The rotors spin at 1/3 crankshaft speed and there are two of them. 14.7psi would double the amount of atmospheric "volume" of air in the combustion chamber.

The engine can spin 8,000 rpm.

Convert cubic inches into cubic ft and you have your answer.

Do you remember how to do word problems?

BicuspiD

CID X RPM X Pressure? (14.7 for NA, +1 per psi of boost?) / 1728 (cfi to cfm) ???

ZeroBanger

I find it hard to believe that the puny stock IC from the rx7 can flow anywhere near the 1700 cfm. it looks like it could barely flow 250 CFM...

but I dont know thats why im asking


instead of getting rid of my intercooler completely Im considering to get a water to air intercooler. A very small one.

PVerdieck

Is it for limited dragging action? That is about all I have heard those are good for. Put some icewater in, and you are good for a few passes.

KevinK2

roughly ...

15 psi on stocks is about 400 eng hp (350 at wheels)

400/.7 = 570 cold cfm into turbo

spearco tables 'rate' ICs based on HOT flow at 10 psi, and 1.5 psi drop across IC. can fine tune with their charts.

for HOT flow at 15 psi: 570 cfm x .65 = 370 cfm at bottom of spearco charts. better to go for 1 psi drop or less.

Kevin T. Wyum

The important part, as KevinK2 touched on above, is that there is no specific CFM number that any intercooler is good for. The question is how much restriction (pressure drop) do you want to live with. As he mentioned Spearco charts CFM flowed at a specific pressure drop, that means you can flow significantly more CFM than they list but your pressure drop is going to continue to increase.

Kevin T. Wyum

ZeroBanger

thanks everyone. I just have one more confusion...I thought the small intercoolers such as blitz or greddy will not flow enough air for a large turbo so in that case the flow rate is important..am I wrong?

KevinK2

on the charge flow end, large gross core area and short tube flow length will trend toward low pressure drop.

the blitz and greddy smics have 4" thk cores, and shaped end tanks. both have same or more gross core flow area vs M2/ASP, so pressure drop would likely be similar. they just will not cool as well. The current pfs ic has a very good core, but some tight pipe bends.


intercooler dimensions/info:

http://www.scuderiaciriani.com/rx7/i...olers.html#ICP

ZeroBanger

wow..thats pretty awesome. That means I can use a single turbo with my blitz since I have water injection...I didn't realize that it flowed that well.

Thats awesome news, thanks alot!

SPOautos

If you take the rwhp you want to make then mulitply it by 1.9 that seems to be pretty close rule of thumb.

If you want to make it good for 350rwhp then that would be about 665cfm.

Also, I did exhaustive research on liquid/air designs for use in a street car and if you'd like to discuss or ask me any questions feel free to PM me.

STEPHEN

airborne

all i have to say is a coolcharge2 IC is NOT big enough for 15psi. i just know.

KevinK2

" ..... pressure drop would likely be similar" is my logical opinion, but not a fact.

I think you can easily measure the current system drop with a large hand held press/vac gauge. 1st measure boost at wot in 3rd/6500 at a point on the intake elbo (aws port?) then tee into the supply line to the pressure chamber, before the check valve. this line comes from a port at the back of the y pipe. the difference in readings will be the IC and piping pressure drop.

Best way is to use a differntial gauge and isolate the IC. I got mine from mcmaster carr for about $60, 3.6 psi max range. On another car, drilled and tapped inlet and outlet IC nozzles for 1/8" od threaded alum tube that i made, epoxy'd them in.

-------------------

from SPO: "If you want to make it good for 350rwhp then that would be about 665cfm"

likely a good general rule for selecting a spearco core. But that cfm would be ambient flow into turbo, and not directly compared to the spearco IC ratings, which I believe is hot pressurized cfm at 1.5" drop across the core with 10 psi at the inlet. the spearco table 'rating' is less than a related compressor inlet flow.

SPOautos

Actually that info wasnt IC spacific. Its a rule of thumb to use when back tracking to see how much rwhp a turbo will make based on the flow according to compressor maps. You convert the lbs/min to cfm then divide by 1.9 to get the rwhp it should create on a rotary. I got that info from Chris Anderson.

Basically, that should be the approx amout of cfm needed to make x rwhp. If you have the rwhp number you can multiply by 1.9 to work back into the needed cfm

If I'm not misstaken compressor maps are turbo outlet numbers so it should would work fairly well for IC sizing based on hp number...within a margin anyway.

Anyway, like I said...its a rule of thumb that from all the comparisons I've made seems to be a fairly decent figure

STEPHEN

KevinK2

Still likely good rule for IC's, but the normal use is for compressor sizing, where maps are based on cold "corrected" flow into the compressor inlet.

Usual factor I use for compresssor maps is .7 x cfm = eng hp based on garret info, so to go from rwhp to cold, ambient cfm into compressor, factor would be 1/.7/.85=1.7. But rotary makes more wasted heat with some of the cfm, so .62 could be closer than garret's .7, getting your 1.9.