What sort of pressure drop is considered acceptable when using an intercooler? Or is there just a temp drop change that the designer looks for?

Jeff

 

1psi or less is desirable. Over 2psi is excessive.

Although important for turbo applications... pressure drop is even more so with superchargers. To get say 7.5psi (0.5bar) in the intake manifold the supercharger must make 7.5psi PLUS the pressure drop of the entire system from the outlet through the throttle body. Each bend, transition, obstruction, etc adds to this drop. If the system (including the intercooler) adds 3psi total drop that means that the SC must pump 10psi. This requires more HP from the crankshaft and creates more heat to be dissapated. Since the heat removed is a ratio the intercooler (IC)outlet temps will usually go up as well. All this is known as parasitic loss. Not mentioned above but also important is unobstructed air flow into the SC itself.

Since the SC is directly connected to the crankshaft it must be geared to produce the desired boost WITH all losses considered. If you replace one IC with a less restrictive but equally thermally effective IC will result in MORE boost at the engine and less parasitic loss. If you want less boost you change the gearing and further reduce parasitic loss. With the higher flow IC (say 1psi of loss vs. 2psi) your supercharger will actually make less MEASURED boost before the IC but be flowing more air mass. This is due to the lower restriction of the more effective IC.

 

Assume that pressure drop and flow rates are equal, would an intecooler with poorly designed end tanks that make sharp 90 degree bends rather than smooth mandrel bent curve reduce power or would it not matter since the system under constant pressure? This is an argument I've had with the vendor so it would be interesting to hear from someone knowledgable in this area.

 

Aerodynamics are a key component to pressure loss. Restrictive flowing components before and after the actual core will increase the pressure loss relative to that core. The issue really becomes what is the weakest link? If the core is too small for the flow requirement, regardless of the plumbing efficiency... you will have a pressure loss. If the plumbing is too restictive for the flow requirement, you will have a pressure loss regardless of the core capacity.

So, the answer to your question is IF you have 2 different IC configurations (core and plumbing) and they BOTH create the SAME pressure loss and the SAME temperature drop at the SAME mass air flow rate (power)... then (other than aesthetics, packaging and weight) are for all intents and purposes... the SAME.

Now, in actuallity, this is highly unlikely. If end tanks are of poor design they will have to be of larger volume to flow the same as a better design. Using larger than required parts creates excess volume. Flow capacity may be acceptable but velocity goes down. Also, IC system volume effects the mass flow requirement of the compressor for any given boost level.

 

Some further thoughts spurred by my recent response to a private email. This deals with what happens when replacing an intercooler (IC) on a supercharged (SC) engine with a larger volume IC.

Without a pulley change the SC has a relatively fixed output. If you increase the volume between the engine and the SC this volume will act like a spring and you will reduce pressure and therefore available energy to fill the cylinders (boost drop). This boost drop will reduce power in some situations. Since the boost from the larger IC is probably cooler and therefore denser the boost drop is not as bad as just lowering boost with the stock IC but it can still cost power. On a SC application that will not have the boost increased (pulley change) volume should be minimized and efficiency (aerodynamics and core heat exchange rate and flow) should be maximized. If you increased the SC speed (pulley change) the larger IC should provide a greater margin of protection against detonation than the stock part. At some point you will either reach the capacity of the stock SC or IC. At some point the larger IC would produce more power than the stock.

On another note, if the inlet to the SC is restrictive you are loosing efficiency and boost. It is valuable to measure the pressure at the SC inlet to determine if a loss is occuring (almost always there will be some). Reducing this loss is "free" power.

Regarding ducting of air to and through the IC core... ducting increases airflow through the core. Ducting is important in all applications involving airflow.

Regarding thermal efficiency... generally, 75% or better is a good intercooler. 100% (or greater) never occurs unless using sub ambient medium such as ice water in an air/water IC.

IC thermal efficiency ratio is calculated by:
IC inlet temp minus IC outlet temp divided by IC inlet temp minus ambient temp.
Example: IC inlet = 200, IC outlet = 100, ambient = 80. 200-100=100, 200-80=120, 100/120=.833, this = an IC thermal efficiency of 83% for this example.

 

Are you refering to the actual a/c casting? Are you talking about porting the s/c inlet?

 

Actually, I am referring to ALL the plumbing from in front of the radiator to the throttle body. The aluminum intake may respond to porting... I have no data. The feed area should be as large as possible and of course only see unheated air. Any loss prior to the SC will be multiplied by the pressure ratio of the compressor. If the IC outlet has 9.8psi and there is 2psi lost through the IC core (guessing, I do not have test data) then the SC is making 11.8psi. Ambient is aprox 14.7 (sea level, altitude reduces this). If there is a 2in/vac drop prior to the SC inlet (apx 1psi)... it only have 13.7 to start with. Under this "guess" data... that would be a pressure ratio of 1.86 (13.7 + 11.8 / 13.7) Now if you eliminate that drop you should see that 1psi gained at the SC intake multiplied by the SC pressure ratio and gain nearly 2psi of boost. Understand these are hypothetical numbers and other real world variables apply but the basic concept is accurate.

To estimate the power gain divide your rwhp by the combined total of your boost + ambient and multiply by your estimated boost gain + ambient.

Example: 150rwhp / (14.7 + 12) = 5.62 x (14.7 + 14) = 161rwhp
(this is for a 2psi gain on the above example)

 

You know SC units don't really provide boost in PSI they provide boost in Volume per Rev. Does anyone know how much air the SLK230 SC unit pump per one Rev of the SC pulley? to increase boost you could I guess built a smaller intake manifold.

NP

 

I found this on Magnuson's website. The MP62 is Eaton's 4th generation compressor but still very similar to the 3rd generation device used in the SLK. From the graph, is seems that you'd have about 450 cfm @ 14,000 rpm. The M90 would give you about 520 cfm at the same speed.

 

Here's attempt #2 at having an effective intercooler. Unfortunately it's really pouring outside, many roads are flooded so I can't do much testing. This unit is made by Racetec. A lot of care was taken to minimize pressure loss. The core is made in Germany and is supposed to be one of the best flowing cores out there. Also the end tanks are even and smooth with only 2 welds on the sides vs. 4 welds on corners where pressure is greatest like other ICs. The tubing is mandrel bent. I'll be doing some more testing when it dries up outside.

 

That IC certainly looks the part. I look forward to your test results.

Would that IC fit into a 1999-2000 C230K?

Thanks,

Jeff

 

No, the 98-99 is different but if it works out maybe they'll make it for you. There is interest from the 98-99 SLKs. Do you know if the 98-99 SLK use the same factory intercooler?

 

How are things going Buell, did you get the chance to do any testing?