Specs for sizing a custom IC core

SCrazy

SCCoA Member
I'm looking to specifiy some requirements for a custom IC core and I'm having a difficult time figuring out how much CFM is required for given HP levels.

With current technology it looks like 500rwhp street SCs running I assume 20psi of boost will be a possibility so lets use that number for now.

One CFM rule of thumb I've seen is approximately 1.5 CFM per crank horsepower, I assume this is at some "nominal" temperature and pressure that was not given. So with 25% drivetrain losses and 50Hp to drive the blower I've got. (500 * 1.25) + 50 = 675 Hp or 675 * 1.5 = 1012 CFM.

I assume that number would be the CFM required and the intake to the blower.

Another way to look at CFM is the swept volume of the motor. The motor is 232 cubic inches and displaces that much charge air every two revolutions so at lets say 6500 rpm the motor moves: (6500 * 232)/3456 = 436 cfm.
The 3456 number is a conversion from ci to cf plus the adjustment for moving air every 2 revolutions.
 
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(Darn thing wouldn't let me edit the orginal post so here's the rest of my thought)

So hopefully now you can see my problem one method yeilds 436 cfm the other yeilds 1012 cfm, our blowers are around 700 cfm at 20psi and I have no idea what the Autorotors are pushing.

How the heck do you estimate CFM and pressure at the IC????
 
I just ordered a custom one from Bell Intercoolers. They told me for 400 RWHP that the IC that I bought would work. The core flows a little over 500 CFM
 
Flow rate vs. pressure drop

Something that might be of importance when looking at this issue is specifying a max. allowable pressure drop at the given flow rate at a given inlet pressure...

When dealing with compressed air flow through any device, there is actually a maximum achievable flow rate regardless of how high the inlet pressure gets. This flow is referred to as the critical flow rate ... this will also provide the maximum pressure drop accross the "device"


Perhaps the proper way of looking at this has to do with maximizing the effeciency of the intercooler in terms of pressure drop (how hard does the blower have to work / how many psi of boost will I lose) and the number of BTUs removed (heat transfer) from the air stream as it moves through the intercooler.


Also, from a "system's perspective" will there be any additional demands on the car? ie... air/air might involve the use of fans and placement of the IC may block air flow to the radiator ...air/water will have a pump plus additional weight and possibly some fans
 
From Bell Web Site

This is how Bell specs their flow rate

Air flow for each core, shown in cubic feet per minute (CFM, based on 1 psig pressure loss across the intercooler core and a charge-air pressure of 10 psig), is listed along with the core dimensions and core weight.
 
I was going to work with Bell as well but I wanted them to give me data on the cores (flow and pressure drop) which is specific to my application. I'm thinking 700 cfm @ 20psi but I want to know the "Science" behind the numbers which is the piece I've not been able to get my hands on.

Also when a company like Bell quotes numbers for core I wonder what they use for end tanks and inlet/outlet pipe geometery as that would have a huge effect on flow and pressure ratings.
 
compressed flow ... Darcy's Equation

I left engineering for marketing a few years back so forgive me if I get some assumptions wrong here...In my opinion, the engineering problem has to do with the fact that Bell presents their core flow rates using an inlet condition of 10 psig (P1 = 10 psig) while our "compressor" provides an inlet condition of 12 psig (89 to 93) and 15 psig (94/95). Keep in mind that the boost gauge is reading pressure on the outlet side of the IC

The bottom line ... you need to determine what the deltaP is going to be when the inlet pressure is >10psig.

My initial thought at this point is to solve for K (reference attached dwg). K will then be the "restriction factor" for the IC core, YdT&S will all be constants for the scenenario you are working on,deltaP will be 1 and P'1 will be 10.

Once you solve for K, rearrange the equation so you can solve for deltaP, then go back and let P'1 be 12 to 15 to 20 to"x", K will be the K you solved for above, Yd,T, and S will all still be constants.

Regarding the end caps, I agree a poor design will significantly affect performance, when using the BELL tables, you must assume they will design the end caps such that they will not adversly affect flow rates ...see the link I posted in the non-tech section today for more info on IC design

I hope this helps
 

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Mathematical analysis of aircraft intercooler design

For the die-hard do it yourself geeks....

"A mathematical analysis has been made to show the method of obtaining the dimensions of the intercooler that will use the least total power for a given set of design conditions. The results of this analysis have been used in a sample calculation and, on the basis of this calculation, a new inter cooler arrangement is suggested. Because the length of the two air passages of the new arrangement is short in comparison with the third dimension, the height of the intercooler, this intercooler arrangement has unusual dimensions. These dimensions give the proposed intercooler arrangement an advantage over one of usual dimensions because less total power will be consumed by the intercooler, the weight and volume of the intercooler will be smaller, and the pressure drop of both the engine air and the cooling air in passing through the intercooler will be lower"

http://naca.larc.nasa.gov/reports/1940/naca-tn-781/naca-tn-781.pdf
 

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Another good link discussing IC design issues ...

http://www.gmhightechperformance.com/tech/0104_htp_cooler/

"Thermal Efficiency

To calculate an intercooler's thermal efficiency, you take the temperature in from the turbo minus the temp out of the intercooler, divided by the temp in from the turbo minus the ambient (or outside) temperature. Take that number, multiply it by 100, and you have the cooler's thermal efficiency. Here is the formula:

Temp In-Temp Out
------------------------ X 100 = Thermal Efficiency
Temp In-Temp Ambient

Our stock, 157,000-mile, heat-cycled Garrett intercooler was tested in 69.3-degree-ambient temps. The air entering the intercooler from the turbo was at 267.4 degrees, and was cooled by the intercooler to 150.3 degrees.

267.4 - 150.3 = 117.1
----------------------------- X 100 = 59% Efficiency
267.4 - 69.3 = 198.1

Fifty-nine percent thermal efficiency is not good in anybody's book.
MPE's enormous super stock-location intercooler fared much better. During testing, the ambient air was at 80.7 degrees. From a 269.5-degree intercooler inlet temp, the Mease unit cooled the charge air to a chilly-by-comparison 102.8 degrees.

269.5 - 102.8 = 166.7
---------------------------- X 100 = 88% Efficiency
269.5 - 80.7 = 188.8

Eighty-eight percent--29 percent better than the stocker. Numbers like that justify the 900-horse core rating that MPE gives this cooler!

Pressure Efficiency

Finding pressure efficiency is easy: simply divide the pressure on the outlet side of the intercooler from the pressure on the inlet side.
Pressure Out
------------------- X 100 = Pressure Efficiency
Pressure In

Unfortunately, the scale that read pressure differences on the SuperFlow was 0 to 100psi, which meant that the recorded pressures weren't as precise as we would have needed to accurately test both intercoolers.

However, let me share my findings: during dyno testing, I was running 24 pounds of boost. When I removed the stock cooler and installed the MPE unit, I had to turn the boost rod out (or down) five full turns to get back to 24 pounds. Just installing the new intercooler would have bumped the boost from 24 to 29psi, which indicates a much lower pressure drop through the MPE core.

I've heard that a heat-cycled stock turbo Buick intercooler loses between 3-5psi through the core during high-boost operation. So let's use that worst-case-scenario 5psi drop in this formula, while running a hypothetical 24psi of boost:

24psi Out
-------------- X 100 = 82% Efficient
29psi In

The turbo has to push much more air to make up for that pressure loss, and more boost equals more heat. In this case, the stock intercooler would only be 82 percent efficient.

We don't know what the exact pressure drop is in the MPE stock-location intercooler, but let's draw on past aftermarket intercooler research and assume that it is exactly 1psi. So, at 24psi boost:

24psi Out
------------- X 100 = 96% Efficient
25psi In

The turbocharger doesn't have to work as hard with a 96 percent pressure efficiency, which means that cooler air will be entering the intercooler. The way the MPE intercooler performed for us so well, our guesstimate of 1psi pressure loss could be excessive"
 
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