CFM question, so difficult...

I just want to do a simple thing. See what CFM of air flows through a 58mm and 60mm throttle body. cant seem to find a formula or calculator anywhere.

Want to see if its worth boring a TB.

One thing I have found, the "How big of a TB do I need" calculators seem to be around 54mm for the 4.0's specs. Thinking a bored TB will do nothing for real power. I imagine if they coulda made more power with a bigger tb, it would have came with one.

CFM=max rpm x displ in cu in x .080
. __________.
3,456
The .080 is the volumetric flow of the engine,which could be higher with mods.

Also "search" for bored TB you will find its really not much help in HP but the butt dyno/brain thinks it does.

Wayne

You will get more bang-for-your-buck freeing up the exhaust--free flowing cat and muffler, larger diameter exhaust pipe cat-back.

To realize real gains from a larger TB you would need to upgrade the intake system: cam, valves, roller rockers, polish and port the head and intake, then boost the fuel system.

Not worth the bucks for a stock 4.0, IMHO.

Wayne Sihler said:

CFM=max rpm x displ in cu in x .080
. __________.
3,456
The .080 is the volumetric flow of the engine,which could be higher with mods.

Also "search" for bored TB you will find its really not much help in HP but the butt dyno/brain thinks it does.

Wayne

Click to expand...

I don't get it.

5200x242x.080 = 100672
100672/3456 ~ 29.129CFM?

I don't get it what is this telling me am I doing it right where does the 3456 come from?

summitlt said:

I just want to do a simple thing. See what CFM of air flows through a 58mm and 60mm throttle body. cant seem to find a formula or calculator anywhere.

Want to see if its worth boring a TB.

One thing I have found, the "How big of a TB do I need" calculators seem to be around 54mm for the 4.0's specs. Thinking a bored TB will do nothing for real power. I imagine if they coulda made more power with a bigger tb, it would have came with one.

Click to expand...

i think 54 is stock size. ive got a bored out one in the 62/63mm size. HUGE difference in throttle response, perhaps a slight performance gain. i also have a larger filter/intake tube, and a aftermarket exhaust. it will be noticeable with just upping the TB size...but not as much as if you did the other mods as well.

If you want to know the flow potential of a throttle body (I presume that's what you're looking for...) it's best to have the thing flowed on a bench.

NB: A bored-out throttle body isn't a performance adder on its own. What it will do for a stocker (and most strokers) is improve part-throttle response - but the limiting factor is still going to be present after the throttle body swap (read: "everything else.")

Frankly, if you're doing a performance build, a throttle body is just about the last thing you should think about as a "pure performance" part. You've got other work to do first (cam, head, valve ports, exhaust, ...)

I got the formula from the book "Jeep 4x4 Performance Handbook" 2nd edition by Jim Allen.
The 3,456 is a constant in the formula.Example was a 360@4800 rpm x.080/3456= 400 cfm

Same book lists the stock TB at 450 cfm
Example listed in book=stock 4.0 100% VE @ 5,500 only needs 385cfm

Wayne

Thats the answer I wanted, what the stock TB flowed and what a bored one would. I was just looking at taking the taper out of a JY throttle body, i realize now it isnt worth the time and effort. Didnt cost me anything.

Wayne Sihler said:

I got the formula from the book "Jeep 4x4 Performance Handbook" 2nd edition by Jim Allen.
The 3,456 is a constant in the formula.Example was a 360@4800 rpm x.080/3456= 400 cfm

Same book lists the stock TB at 450 cfm
Example listed in book=stock 4.0 100% VE @ 5,500 only needs 385cfm

Wayne

Click to expand...

This only makes sense if the 8% is replaced with 80% (.08 with .8)
I'll just go ahead and assume that this is really how the formula is supposed to be, since all these calculations end up being correct when said change is made.

3456 is the convertion factor used to change the cubic inches into cubic feet for a 4 cycle engine

The actual fomula (non condensed) looks like:
CFM = (CI/1728) x (RPM/2) x VE

http://www.stealth316.com/2-air-fuel-flow.htm
http://www.donaldson.com/en/exhaust/support/datalibrary/053704.pdf