Air-to-air split intercooler sizing

[Zach34]

I've been researching intercooler sizing, and I think I have a viable plan but I'm hoping to bounce it off of some of you more experienced turbo guys.

My planned setup:

-EJ25 (2005 STI motor)
-Garrett GTX3071R turbo with external wastegate
-Target HP = 280-320
-818R intended for track-only use.

The turbo is large for my power goals, but my engine builder recommended it as an option and the LB/MIN flow rates I've calculated would put me exactly in the middle of its optimal efficiency range, which should mean a cooler intake charge at sustained WOT on the track and lower turbo RPM for less stress on the turbo and oil system, among other benefits.

I want to use the side vents/scoops for the intercooler(s). Since the side vent openings are very small (I plan to enlarge them like Hobby Racer did), I'm considering splitting the airflow from the turbo outlet to two identical intercoolers, one per side vent. If my calculations are correct (and that's a BIG IF), my turbo's CFM output will be very comparable to the stock turbo's max output - mid-400's CFM, 460 to be exact. That makes sense because the stock engine/turbo produces about 300 hp, and I've seen someone report from an unknown source that the stock turbo produces 435 cfm. Therefore, I'm thinking the total volume/flow capacity of my dual-intercooler setup should be roughly the same as the OEM TMIC.

So here's a comparison between the OEM IC and some custom-size cores I found from Bell Intercoolers:

The OEM STI intercooler's core measures 18.5 x 6.625 x 3" for a total volume of 368ci. I have found no flow data on it.

The Bell A300051120 (https://bellintercoolers.com/i-30005...ficiency.html#!) measures 12 x 5.1 x 3" for a volume of 183.6ci. Multiply that by 2 since I'm using 2 of them for a total volume of 367.2ci. Bell claims it flows 261 CFM (again, 2x = 522 CFM total).

My volume calculations do not account for internal construction differences between the OEM and Bell IC's that might mean more or less air capacity in the IC core, but hopefully that is minimal.

Will this work? Am I out to lunch on this idea?

Any suggestions on Intake pipe sizing?

[frankc5r]

What kind of track car? autocross where you are doing 30 sec runs at slow speeds or real tracks where races can last a hour.
You only need about 30 mph to get full flow to an air ic. Hard to do in autocross.
But air-air is best for long track events. Not sure I would split things up. An old formula from my factory racing days as applied to radiators was you need about 1/5 to 1/7 the area of the
radiator for air inlet size. If you opened drv side inlet to max size, that would appear to be enough air and simplify plumbing.
Been engineer on 24 hr LeMans cars and if u look at them with way more hp than your target, they have radiator on one side and air-air on other side.

Get a copy of Corkey Bells book, Maximum Boost. He is a good friend a very sharp. He has math formulas u can use for air flow and tubing sizes. You dont want sonic speeds.
I used the formulas for calculating losses over length of pipe and response times. Not as much as u would think

[Zach34]

Road courses almost exclusively. 20+ minutes on track or more at a time.

I did come across some excerpts from Maximum Boost in my online research. Going to pick up a copy. You know the author? That's really cool!

I think it would be difficult to open up one of the side scoops sufficiently to accomodate something the size of the stock TMIC. 12" is about the max height. Hobby Racer widened his to what looks like about 4.5 inches. The IC could be angled to expose the full width to that narrow opening, but I think a small fan or two might be needed to pull air through the IC regardless. I don't think much airflow gets pulled into those side vents.

I agree, I'm not sure if splitting the intake charge between 2 IC's is going to cost me more fabrication and flow problems than it's worth. Here's a list of what I know will need cool air:

-Engine air intake
-Intercooler
-Oil cooler
-Transmission cooler

As it happens, we have 2 side scoops and two large scoops on the engine cover. Maybe this is a better setup:

-Engine air intake - engine cover hump #1
-Intercooler - engine cover hump #2
-Oil cooler - side scoop #1
-Transmission cooler - side scoop #2

That would package the air intake and IC in the same vicinity and potentially makes plumbing them much simpler. The downside is the fabrication project for the underside of the engine cover to accomodate all of that, and potential heat soak with a TMIC setup.

[frankc5r]

Tests have shown that typical electric fans used on typical oil coolers , ic heat exchangers are good for about 12 mph.
Not what u would get from side scoop or roof vents.
Good luck.

[RM1SepEx]

I got in a 40% larger than stock AAIC on one side and used a scoop, seems to work fine on the street and autocross, no road race experience. I rotated the turbo 135 degrees and used 2 1/2 IC piping. 12 inch fan on the IC don't need to use it very often.

[Zach34]

That's good to hear positive results from a side scoop position.

I admit I've started to become highly skeptical of the AAIC as an overall better solution to the AWIC in our cars. Mid/rear-engine cars with AAIC's seem to always have ram-air aerodynamically engineered into the body in some fashion. The 911 turbo's side scoops sit proud of the body panels in front of them on much more pronounced rear fender flares. The Lotus Exige has a dedicated scoop on the hood that no doubt gets clean air.

Looking at the wind tunnel videos of the 818R, the front hood and windscreen on the R completely spoils any air coming into the engine cover humps - plus the passenger seat would have to be removed to get a clear path to begin with. The airflow seems to mostly skip over the side scoops.

Although I would love the simplicity of plumbing a top-mount AAIC using the engine cover humps as the cooling air source, it would still be a big fabrication project to get all the ducting right without significantly hindering access to the engine, and I haven't found any info suggesting it would significantly outperform and AWIC aside from the weight penalty of the water and additional radiator. I want consistency and reliability on the track above all else, and the AWIC probably provides that with the least installation hassle assuming I'm diligent about avoiding leaks in the plumbing.

Probably going AWIC....

[phil1734]

Most of your interaction along the side of the car is driven by the wheel wake. As the smoke-wand tunnel tests were done with a stationary car at 40 or 45mph, it's impossible to interpret what is going on there.

A properly designed end fence on the front splitter would probably direct whatever air flow you wanted directly into the scoop (particularly the enlarged ones,) but since FFR didn't do it and I doubt any single individual is going to pony up and run their own rolling-road tunnel test, you'd have to spend copious amounts of time driving around with yarn tufts or similar to figure out what works. Probably not the most efficient use of time and effort.

And as always, remember that getting air to your heat exchanger of choice is only half the battle. You also need to give it a low pressure zone to go to after it has passed through.

[frankc5r]

Good comment Phil. On my awic heat exchanger ( in back),
I am going to duct air from roof to top and
put fan on bottom where it works better pulling.

[Mitch Wright]

As a side note I am at 285whp using a STi TMIC taking air from the right hump on my track only car. Intake temps are at or a few degrees above ambient temps taken from the IC close to the throttlebody so it does work. front finished 818.jpg 818 duct underside.jpg The fabrication isn't as tough as you might think, I have more photo's if you decide to use the hump.

[Zach34]

Thanks Mitch. I'd love to see more photos! The ones above were pretty small. That's great to hear that it works!

I'd like to have a passenger seat installed occasionally, which would block most of the airflow.

[Mitch Wright]

A passengers seat will be block the majority of the opening, I have my fuel cell in the passengers area so a seat was not part of my planning. TMIC Duct 1.jpgTMIC Duct 2.jpgTMIC Duct 3.jpg818 duct underside 2.jpg818 duct underside 3.jpgTMIC underside 4.jpg
PM me your e-mail and I will send you the photos that are larger.