Motorsports Village

coverting 44" to 28"
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Author:  zums [ Wed Aug 22, 2012 4:43 pm ]
Post subject:  coverting 44" to 28"

teting the exhaust, when converting from a higher test pressure, im using 44" in this case, what cfm loss when coverted back to 28" would you cosider maximum safe without hp loss

Author:  rick360 [ Wed Aug 22, 2012 9:40 pm ]
Post subject:  Re: coverting 44" to 28"

When converting cfm numbers from a higher test pressure to a lower pressure to see any velocity related problems I normally see them within a few cfm. It depends some on how you are doing it and how your bench ranges compare. You can test at low lifts and see how much difference there since the velocity over the turn will be low, if they are close there they should stay close at any lift if there is no problem with the port.

One thing to remember when testing at different depressions/pressures is the flow range of the internal orifice changes as the pressure changes. So a flow range that is 300cfm at 28" on exhaust will become 303.9cfm to get actual cfm and then do the cfm conversion (cfm x sqrt(28/44)) after that. Not sure many people use the different range, but it is only a correct range at the intended calibration pressure/depression. On intake the range gets smaller as depression goes up.

The SF manual has a chart to show the range correction for various test pressures. The SF300 is calibrated to 25"wc and the SF600 ranges are calibrated at 28"wc.


Author:  zums [ Thu Aug 23, 2012 6:34 am ]
Post subject:  Re: coverting 44" to 28"

rick, where did the 303.9 come from, at .1 the flow is within 2cfm above that it ranges from 6-8cfm, im testing at 44 using the incline as high as a percent that you can for that range then using the conversion factor to get the 28 #s

Author:  rick360 [ Thu Aug 23, 2012 10:12 am ]
Post subject:  Re: coverting 44" to 28"

If your bench ranges are calibrated at 28"wc and you want to test at 44"wc the range would need multiplied by 1.013 to get the correct range for that test pressure so your actual cfm will be correct before converting back to 28" numbers. To get the 1.013 multiplier I extrapolated to 44" using the SF manual chart I mentioned.

If your 44" reading converted back to 28" is lower (normally is) than the measured 28" cfm numbers by 6-8 and you correct your range by 1.013 you will be within a couple cfm and should be fine.

Are you testing with a pipe?

I am not sure how many exhaust ports have a flow separation problem that can be seen at 44"wc since the exhaust ports have a much different relative valve angle than the intake. I think you will find very few exhaust ports that show that problem. If you could flow at 200psi you might learn something. :-k

On the intake it is very common to see flow separation, especially if someone has a low-port head and is porting/testing at 10-15"wc. I have seen significant power improvements on a head that lost flow when testing at 40" vs 28". I fixed it and picked the car up at least a tenth and the flow at 28" was unchanged or down at mid-lift a little.

On the exhaust, get your throat area and exit area right for the combo and make it flow as much and as smooth as possible. Concentrate on the intake, you will see more gains there.


Author:  zums [ Thu Aug 23, 2012 3:39 pm ]
Post subject:  Re: coverting 44" to 28"

rick, first id like to say thanks for a good technical response, i appreciate that, just so i follow what you are saying ill put my #s through what i think your saying , ex flow @ 28 no pipe 204 as measured, ex flow @44 247 as measured, converted from formula 197, are you saying to multiply this # by 1.013 at this point or after the 28" then convert, because that would make the spread worse, i tested with and without the pipe and the spread curve was very close, do you have a formula for accurate exit and throat area, again thanks- tom

Author:  rick360 [ Thu Aug 23, 2012 7:37 pm ]
Post subject:  Re: coverting 44" to 28"

That is the idea and the math will work out the same but I do it like this. I'll use your numbers with an assumed exhaust range of 250cfm at 28".

Flow at 28" measured = (81.6% X 250) = 204cfm actual flow at 28"wc

With the same flow range testing at 44" changes the 250cfm range => 250 x 1.013 = 253.25 cfm

Flow at 44" measured = (98.8% x 253.25) = 250.2 actual flow at 44"wc

Convert flow at 44" to 28" => 250.2 x (sqrt(28/44))= 199.6cfm @ 28"(converted)

I do it this way because the actual range changes at a 100% scale reading when the test pressure is not the same as the original calibration.

I don't know the formula right off for the change in range for pressure changes. I use the chart in a SF300 manual and extrapolate/interpolate from those. On the EX side it is roughly a multiplier of 1 + (.000818181 per 1"wc increase in test pressure). It really wouldn't be linear, but in the small range of pressure used it is close enough. A formula would use absolute pressure of the test to get precise range of an orifice.


Author:  rick360 [ Thu Aug 23, 2012 9:08 pm ]
Post subject:  Re: coverting 44" to 28"

zums wrote:
do you have a formula for accurate exit and throat area, again thanks- tom

Tom, I use Engine Pro mostly to help me size the throat but I also use PipeMax for some things. Then the exit area depends on the type of port (hi vs low) and the type engine. For higher rpm more all-out type engines I'd make the exit about 1.333 x throat area and bigger than that for a lo port and/or lower rpm milder type engine, around 1.5 x throat. Just guidelines, not absolutes.


Author:  zums [ Fri Aug 24, 2012 7:33 am ]
Post subject:  Re: coverting 44" to 28"

a simple formula can make your life alot easier, thanks again rick for sharing that info , it helps alot- tom

Author:  rick360 [ Sat Aug 25, 2012 10:32 am ]
Post subject:  Re: coverting 44" to 28"

I did a little reading of the Engine Pro book to see better how the math is done in the program. Then I did some experimenting with some engine files I have saved from Engine Pro to see what factors are driving the ex mcsa recommendations.

If you take the bore area x .111 it will get you very close to what Engine Pro predicts in most cases. On very large engines and/or very high output engines (>2.4 hp/ci ) the throat gets larger than this.

Using a bore of 4.030"
bore area= 3.1416 x (4.030/2)^2 = 12.75557303 in²
ex throat area = 12.75557303 x .111 = 1.416 in²

That's a simple version of what Engine Pro recommends and EP has worked well for me.


Author:  zums [ Sat Aug 25, 2012 8:10 pm ]
Post subject:  Re: coverting 44" to 28"

is that with the stem subtracted or just the raw data, i tried it with my combo, its prety neat, how can it get close w/out cid and rpm factored in

Author:  rick360 [ Sun Aug 26, 2012 12:37 am ]
Post subject:  Re: coverting 44" to 28"

The area with the stem subtracted out.

Engine Pro uses a lot of other variables and when the power goes up and/or the engine size and/or rpm goes up it will use a value from one of the other formulas. There are several formulas calculating the exhaust throat area based on HP rpm, max allowed velocity, intake port size etc and if the others don't need it to be bigger than the 11.1% of bore area that is the smallest that Engine Pro allows as a results. If you are building typical bracket engines with 23º heads then it uses that 11.1% minimum because the amount of exhaust gas won't be enough to need it larger.

If the stroke is longer the rpm will go down which will end up having more time to exhaust the cylinder offsetting the bigger volume of gas in the cylinder.


Author:  zums [ Sun Aug 26, 2012 10:13 am ]
Post subject:  Re: coverting 44" to 28"

its good of you to show the math, thanks again rick- tom

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