Pre/post intercooler boost pressures

[Mark Rinker]

Should there be any measureable difference in pre/post intercooler boost pressure readings? I am no thermodynamic engineer, but it would seem to me that the pressure drop should be negligable.

I am seeing as much as 25% variance between my dash mounted gauge (sender is post-intercooler, and lower) and MSD PC diagnostics software (sender is pre-intercooler, and higher).

Does the air charge actually cool, shrink, and lose that much of its pressure across the intercooler?

[More Power]

I haven't seen any good data, but some suggest a 2-3 psi drop across the intercooler, depending on boosted air temperature and boost pressures.... Yes, air density will increase and air volume will shrink when the boosted air is cooled across an intercooler.

You might try swapping senders to see if what you're seeing is due to gauge calibration or if some of it is due to density/pressure differences in/out. :)

Jim

[Mark Rinker]

Good idea.

I was thinking the simplest test would be to hook the MSD system over to the 'post intercooler' side. Both gauges should read exactly the same - any difference is calibration.

[More Power]

I was reading Corky Bell's book last night entitled "Maximum Boost". In the intercooler section, he talked about a goal of 1 psi of pressure drop across the intercooler for automotive applications. The lower the number, the more efficient the IC, regarding airflow. :) I suspect some of what you're seeing is due to gauge calibration....

[SoTxPollock]

Even if you were a thermodynamics whiz the real problem is that air is a mixture of various gases etc. Each gas has its own cubical volumetric coffecient of expansion that varies per degree of temperature change, so unless you were able to analyse the air composition, you'd be working with apples and oranges to get some other answer that would mean little. I can't imagine loosing 25 percent thru the intercooler for any reason and unless both gauges are identically calibrated and matched your are also measuring apples and oranges with two different gauges. I know that doesn't help, but its not so simple to accurately measure because of the changes that take place at the gas(air) is heated under pressure and cooled under pressure.
The fact is a similar action is taking place each time you drive and I'd venture a guess its the different gauges that are causing the large differential reading.

[Mark Rinker]

Agreed.

Adding to the complexity of the air charge mixture you mention - sometimes this particular air charge may or may not include propane!

This is based on a

1) the system being on or off, and;
2) a user defined boost threshold (1lbs to 99lbs) being monitored by the (pre intercooler) MSD boost pressure sender, and internal fuel map that controls propane flow, etc...

However, the discrepency is present in either case, and similar. It sure appears to be sender/calibration related at this point.

[dieseldummy]

I've been told that the 2-3 psi drop is usually the max number on a Dmax. Apparently they designed the intercoolers pretty efficiently so the pressure drop is minimal. I remember back when intercooling was a hot topic with the 6.5's and everyone said that the pressure loss was from cooling the air charge. Maybe the propane is cooling things down?

[JohnC]

I'm no expert here, but that never stopped me before...

Seems there are two sources of pressure drop in an intercoller. First is the drop due to temperature decrease. The ideal gas law gives a formula, but basically, the more the temperature drops, the more the pressure drops. So, more thermal efficiency results in more pressure drop. Second is the pressure drop due to flow resistance. Here more efficiency results in less drop. The best you could hope for is zero, so all the drop is due to temperature change.

If we assume constant volume, from the ideal gas law: T1/P1 = T2/P2

If we assume a 100*F temp drop and starting pressure of 30 psi (absolute) I think we come up with about 25 psi out, a 17% drop. So, the only way to get a 1 psi drop is to fiddle with the volume, in which case we can get just about any result we want, but that's cheating....

Did I come close?

[Mark Rinker]

Almost exactly the % drop that I am seeing, propane 'on' or 'off', doesn't seem to matter.