So what happens if your intercooler is too big?

[daveismissing]

Well? 

[Steve Wood]

compressor surge, lack of throttle response, slower boost ramp up, less air flow thru the radiator if a front mount

[earlbrown]

On the upside it might get you a bench racing win.

[daveismissing]

In "Black Air" the engineers implied they were physically limited and would have gone bigger if it had fit...

[Steve Wood]

Now that is strange.  First thing we did was cut off the inlets and outlets and replaced them with larger sizes to reduce backpressure and the second thing was to cut one in half and add it to the bottom of the original along with new side tanks....fit with no problem.

But that has nothing to do with an intercooler being too big.

[Scoobum]

Higher engine temperature and less throttle response has always kept me away from a FMIC. My other thot has been that the alky will cool the intake charge.

[Steve Wood]

alky is a great equalizer.  A slic with good flow and alky will work really well even if the ic is on the small side from a cooling stance

[motorhead]

I can post datalogs of my auto-x runs that indicate my car has zero issues with throttle response or spool time.  Cooling issues are easily overcome as well.  However, there is no such thing as a free lunch once you move away from stock.

As far as the engineers being physically limited vs. big neck/stretched intercoolers goes, you have to appreciate that these were regular production line vehicles (on an existing chassis not designed to fit all the new fancy EFI turbo crap) and the added headaches of fitting a larger intercooler was probably not as cut and dry as we'd make it out to be once engine meets chassis meets body meets road.

[daveismissing]

What would be the metric for defining "size" ?
square inches of surface? Airflow at a given pressure? Something else?

[Steve Wood]

volume vs spool time vs compressor surge....or simply the time it takes the turbo to move enuf air to turn the semi-dead air inside the intercooler into useful flow?

[motorhead]

volume vs spool time vs compressor surge....or simply the time it takes the turbo to move enuf air to turn the semi-dead air inside the intercooler into useful flow?

I would believe that pressure differential/restriction on the inlet and outlet sides, and size of the charge piping, also would have to be considered.  With a nod to backpressure on the exhaust side (pre- and post-turbine) too.

I know I got damn lucky using an unproven FMIC out of a diesel semi in my Buick.  I am considering moving up to a 3" outlet charge pipe this season for giggles; currently 2.5" on both sides.

[Scoobum]

I'm stickin' with my GBody stretched SLIC.

[motorhead]

I'm stickin' with my GBody stretched SLIC.

I am not suggesting otherwise, as I am certain it is an awesome piece of kit.

In reality, I should be using an SLIC for better weight distribution.

[Scoobum]

volume vs spool time vs compressor surge....or simply the time it takes the turbo to move enuf air to turn the semi-dead air inside the intercooler into useful flow?

Correct me if I'm wrong. The shorter distance the air has to travel...the quicker the throttle response.

[earlbrown]

Keep in mind every cubic foot of air you add (compressor spool to intake valves) is air that has to be compressed before it gets to the cylinder.   

Lets say you have 1 cubic foot of airspace and good throttle response and you're running 14.7 pounds of boost. With that setup you have to stuff 2 cubic feet of air into that space to have have your 14.7#s.....

If you get a big ass front mount with huge pipes and add another cubic foot of interior airspace you now have to stuff 4 cubic feet of air in there to see the same boost at the intake valves.  (I understand these are simplistic numbers based on 100% efficiency). To make things worse, the pressure increase will come in the form of a compression wave. Meaning when the turbo gets on the clock, the first thing to get pressurized is the intercooler inlet pipe, then the core, then the outlet tank, then the TB's pressure drop, then the plenum THEN the backside of the intake valve. When you're measuring response in milliseconds, that stuff can add up.

Big ass piping can add unnecessary volume fairly easily. Since the area increases at a the swuare X Pi, it can get ugly quickly.

for example:  2" pipe contains .0218 cubic feet per linear foot
                      3" pipe contains .0480 cubic feet per linear foot
                      4" pipe contains .0873 cubic feet per linear foot

Note the difference between 2" and 3" is over double, the difference between 2" and 4" is over 4 times as big.

Front mounts have, what, 9 feet of plumbing?