I was crunching some numbers this morning and remembered why I gave up on this stuff for my intake. I can figure cross section area, I can know or guess at temperature, but I have no idea what velocity air will move thru a pipe.

I figured out that to get 105 SCFM of air thru a 2" pipe V would have to be 11MPH (1000 ft/min). But I have no idea if that is a realistic number and since I had no way to measure for actual air velocity I had to drop the whole idea.

I sorry, I won't be able to provide an example like I thought. frown

 

Looking up stuff about this instead of cleaning the house, I came across This web page

I did the math and estimated exhaust flow rate at 436 acfm and V=4133 ft/min thru a 2.25" pipe. (this assumes a backpressure of 1 in/hg (14 in/h20)or less)

I came across this web page and punched in all the data and came up with a air intake requirement of 169 SCFM at 100% VE @ 4800 rpm. (the reason 4800 rpm is that is the peak torque, the general accepted point for calculating maximum VE of an engine). 85% VE (the best you can expect from an NA tune) is 144 SCFM.

This good, we have at least theoretical numbers to work with to plug into the formulas. Skierd, you with me so far?

[ January 11, 2003, 08:25 AM: Message edited by: blue2000 ]

 

I'm starting to feel like a post-whore so this is the last one. A link to some tech articles about exhaust (2 parts).

Go here

 

Yeah, i've just been away all weekend.

Using this site i got an exhuast flow CFM of 278.98 assuming max load is at 3800rpms and VE is 85%.

2" exhaust gives a exhaust gas velocity of 12,680.9 ft/min.

2.25" exhuast gives 10107.97 ft/min.

2.5" exhuast gives 8205.29 ft/min.

Granted, this really means nothing untill i have something to compare it too... So I went ahead and calculated a 2.25" exhuast on a K20A engine. 2.25" seems to be a good size because thats what DC Sports uses for their RSX exhaust, and I just happen to have a back issue of SCC that has a dynochart for the RSX. Assuming 90% VE and using a torque peak of 4000rpm, i get an exhaust CFM of 365.28.

A 2.25" pipe gives... 13234.62 ft/min. A 2.5" give 10743.395 ft/min.

Looks like i'm going for a 2" pipe... something still isnt sitting right with me though. Anyone wanna send me some money to buy the HVE catback and the KORE catback and dyno time to test to see which is better.

 

skierd, something is wrong with our math. I don't have time to go over it in detail at the moment, I'll check it this afternoon. maybe that is what is bothering you?

 

Thats prolly got something to with it. I was thinking not having all the information to do the right math...

 

I agree, we are missing something. It's just not turning out correctly. We may just have to wait for you to get into your thermodynamic class next quarter to figure this one.

Sorry I couldn't be more help. I wonder if you could write to some technical type person at MBRP or KORE and get the total backpressure vs. a stock system. Presumably they did testing to find out which diameter exhaust works best. But then again, maybe they didn't. I've not been too impressed by the scientific backup for the import tuning "upgrades" so far.