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popular question, answers less evident; some thoughts 1 Saabers Like This Post!
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Posted by Stealthbomber (more from Stealthbomber) on Mon, 9 Sep 2002 19:05:38 Share Post by Email
In Reply to: airbox upgrade, uk turbo, Mon, 9 Sep 2002 17:30:43
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Greetings. I've been wondering about this as well... enhanced airflow into inlet and enhanced exhaust flow paths out of engine compartment (bonnet??) have been on my mind for a while. There have been some posts over time on the issue, but I haven't seen too much of a consensus. Yet it seems appropriate for discussion on the performance BBS, for we are trying to wring far more out of these platforms than the general driver.

An inlet situated very low or beneath the motor raises for some (including me) the possibility of high water problems. I've plowed into deeper than anticipated water a few times over the years and it certainly could happen again. I KNOW what it feels like to have all four wheels lurch sideways a few inches as the water pressure hits. It's not an experience I intend to repeat, but it could have been worse: having the engine stall out when you REALLY need to get across before you're swept away seems a really bad consequence! Others have worried that the engine vacuum would draw water up into the motor with consequences more fatal for the motor than the motorist, and not all roads are well-drained... in some places, they ARE the drainage. (On the other hand, some people claim have implemented low-mounted intakes with good results... so far.) Since I believe that either of these very bad consequences is theoretically possible, it seems an inlet site should be at or near foglight height at absolute lowest, and higher if possible. A front air dam would be very helpful. Absent an aftermarket front air dam to help gather air, alternative placements for the intake merit consideration.

A few people have run the ducting through the firewall into the fender and placed the air filter there. Todd C did this very neatly, and others also claimed good results. I don't know how much better the airflow is there. The concept seems reasonably good, however, and may merit serious thought. It might require additional inlet ducting to maximize the benefits. (A relevant concern is pressure drop... some have posted information suggesting that the pressure drops of the stock airbox and inlet snorkel, even without a filter installed, are not insignificant... adding to this with additional kinks and bends will not help. Others with open filters, such as I have, may have traded pressure restrictions for increased inlet temperatures. This post hopes to propose a partial solution for both sets of probems.)

MY personal thoughts - - entirely unvalidated by any scientific measurement at this point - - are that the optimum configuration is to have supplemental airways INTO the engine compartment at the upper, leading edge of the front hood AND supplemental exhaust airways OUT of the rear of the engine compartment. Let me explain.

The inlets could be a type I have heard described as "NACA-style", which are triangular openings in the sheetmetal, apex forward, base to the rear, with appropriate ducting beneath the sheet metal to route flow. (If properly installed, these supposedly flow very significant volumes of air at speed, without the "gimme a ticket" appearance of a raised hoodscoop.) With appropriate ducting, an alternative placement should be possible on the side fender immediately behind parking light/turn signal assembly.

There is a design question whether the only use of this air should be for cold air intake or whether it would be beneficial to help cool the engine compartment. This is discussed below.

The exhaust venting should be placed as high as practically and stylistically possible without undue negative consequences, for the idea is to employ the "hot air rises" principle to help cool the underhood/underbonnet temperatures. Particularly in hot climates, the underhood temperatures in a classic 900 are brutal, and the only outlet for these naturally rising currents is downward. Thus, stop and go or low-speed motoring would seem designed for baking, not for a performance motor. There are some aesthetic limits, however, on the exhaust gas outlet location; to me, it seems undesirable to place them where the engine heat and aroma would waft directly into the passenger compartment when windows/sunroof were open, or where oil particles and engine dirt could hit the windshield. These factors suggest a high side outlet location similar to the BMW Z-3, with ducting angled to channel flow through the inner firewall to the outer sheetmetal. Although not at optimum height, these still would be far better than the current system. An alternative might be to direct the outflow to the joint between the classic 900 hood and body, at the rearward edge, and to open this edge slightly to allow a rectangular exhaust flow.

If I am right (a longshot, but theoretically possible), the cumulative effect of increasing airflow into and out of the engine compartment would include:

1. reducing underhood temperatures to a measurable degree at speed and dramatically in stop and go traffic;

2. correspondingly increasing efficiency of intercooler operations and radiator cooling due to #1;

3. decreasing to some slight degree the ambient air intake temps if implemented without a designated cold air intake, and dramatically if one of the intakes were dedicated to a cold air intake source.

From these, I postulate the following hypothesis: A "good" arrangement would include: two NACA-style air inlets, located very far forward on the upper surface of the hood, one directing airflow past the exhaust manifold side of the motor and the other directing airflow past the intake, with two exhaust vents located on the rear sides of the engine compartment, either just below the hood or in the bottom edge of the hood, with ducting to direct exhaust flow out of the engine compartment and away from the car. (Some shielding/trap mechanism might be necessary to divert particulates/rainwater but this is readily possible)

Given the right fabrication, the end result would be subtle in appearance and dramatic in function... which is my definition of a perfect performance modification.

Unfortunately, my sheet metals skills are even worse than my automotive design engineering. It will remain up to others to suggest whether, or how, any of these ideas could be implemented.

I look forward to any thoughts in response.

Best regards to all,

Jim Watts
The Stealthbomber
87 SPG, Modestly Modified

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