Hey! This site is reader-supported and we earn commissions if you purchase products from retailers after clicking on a link from our site.

by dan_t
(BC, Canada)

Hi all … I swapped out a 4B247 with a 2Z499 – what effect on CFM? I’m somewhat new to the world of air compressors so I did what research I could and selected a compressor off of Craigslist that had a reasonable CFM rating.

Speedaire 4B247 pump
Speedaire 4B247 pump

Only when I got home I realized that it looks like both the motor and the compressor have been replaced at some point in their lives.

The compressor is marked up as a Speedaire compressor 4B228B.

Its listed as having a Speedaire compressor 4B247 head but the reality is that it has a Speedaire compressor 2Z499.

The motor is 240v 15amp listed at 2.96kw (4hp).

The original specs for this compressor was 10.8 SCFM @ 90PSI – I’m trying to figure out the effect on this having had the compressor head changed.

Any information greatly appreciated.


Air compressor horsepower
by: pcbeachrat

First your true running horsepower on that motor is just barely over 3 horsepower. I will see if this link will paste on how to calculate TRUE HORSEPOWER: www.onlineconversion.com/motor_horsepower.htm . On this calculator keep the efficiency rating at 65% for this is about the most that air compressors run at full load.

Next I will post an article about scfm which is an imaginary scenario compressor manufacturers use now days which is basically an imaginary calculation to use. SCFM has nothing to do with true CFM,and can only calculate at certain altitudes,certain ambient temperatures,certain regular temperatures and many other factors. No One can calculate scfm by using these factors in different regions,so scfm is totally useless and means nothing. Let me look for the proper article on this…

First here is a very important article, again on true horsepower..please read this to understand true horsepower better. www.kevinsbrady.net/motors.pdf

Here is a quick skit on SCFM. en.wikipedia.org/wiki/SCFM

About the 6th paragraph on the following link tells you more about scfm being calculated,and that it means total different calculations depending on the certain conditions,temps etcetera in the area the compressor is operating from.  personalpages.tds.net/~thompson_wp/misc/cfm%20mystery.htm

Now I have shown you how to get true horsepower, but the scfm thing is about as real as Rumpelstiltskin, and is something that cannot be truthfully answered . For this, I cannot answer, but this will at least explain the truth about scfm. personalpages.tds.net/~thompson_wp/misc/cfm%20mystery.htm

Basically you can search the net and come up with many many descriptions of how scfm came about or figures out. CFM can be figured a lot closer to true regardless of altitude, temperature, ambient temperature, humidity etc . SCFM is closer to just simply conning the purchaser to believing they have something they don’t.


A lot of good content here PCBEACHRAT. Thanks!



by: pcbeachrat

I could not find the actual cfm info you asked for the motor that is supposed to be sized up with the pump model you have should only have a max of a 1 1/2 horsepower motor on it.

The motor you have is rated at 3.1 horsepower and will literally burn that pump up after a little use.

All pumps have a minimum and maximum speed at which they should turn,and they pair them up with a motor and/or the proper pulley size*(is very important) for the speed of the pump .On reciprocating compressors the lower speed you can run the pump the better and longer wear you will get from it,but it cannot be ran lower or higher than recommended.SPEED KILLS….Take an automobile for example,it can go 120 plus miles per hour,but if you keep it at that speed constantly it will wear out or blow up a lot quicker than if you slow the car down and run it at 55 miles per hours during it’s life span….You get a lot more life and better performance on the car .

My 2 stage pump is rated at 1150 rpm’s per minute,but I got the pump set(by putting right size of pulley on the motor) to running 850 rpms, which is it’s suggested minimum,and a good pump run low rpm’s you can get 30 years usage out of if maintained properly…


If you need help with this I would be happy to help ,but your going to have to get a different motor or pump to make that compressor last long..as far as the cfm….Yours with that 1.5 horsepower motor on it will be very low,probably around 3 to 5 [email protected] 90 psi,I would think.That rating is not a known fact just the estimate that came into my head.I do build compressors and sandblasters as well as repair them here…E mail is [email protected] if you need some additional info,will be glad to help out…Your motor is too big for that pump so something needs changed…HERE’S A HINT…I have had no luck with the Tractor Supply century motors…I have had 4 fry and one within 6 hours work on it…I go with Grainger and AO smith motors now….


Quick reply
by: Dan

I don’t see that having more HP on the motor should be any issue. It’s rated at the same RPM as the stock motor it just produces more torque at that same RPM.

I’m now not so sure that this has been modified from its original specifications. Both motor and pump are Drayco units and I believe they are the original manufactures.

I did a simple test of filling 50 gallons from 90-125psi and recording the amount of time it took. From that test I got an average of 8.2CFM at 107.5psi.

I’ve only had time to quickly scan your posts so far but I’ll make sure I’ll check it over again later if I get time.

Many Thanks 🙂


Additional CFM reading:

What size air compressor: