SVO coefficient of drag (Cx)

[yldouright]

I am considering an SVO purchase and am still researching. Has anyone ever put the SVOs in a wind tunnel for Cx numbers? Things like ride height and headlights can change this value significantly so numbers with details would be more valuable. I've seen numbers ranging from 0.35-0.38 but I'd like confirmation if I can get it. For comparison, a notchback Fox with complete air mods got 0.36.

With under body connectors and other handling tweaks, this car is supposed to be an excellent handler but can anyone here describe its character over 120mph? Does the rear spoiler on the car increase or decrease the drag? Is there any potential benefit treating the front air dam?

Update: The 1987-88 Fox body T-Bird had a 145mph top speed from 190bhp indicating a Cx of 0.345. If we plug in the 205bhp and 141mph top speed of the SVO, we extrapolate a Cx of 0.363 for the stock SVO. This assumes an identical rear and transmission in the same atmospheric conditions.

[gbeaird]

Welcome!

So if the Cd is 0.37 you're going with a notchback? Kind of an odd reason to select a SVO. Changing the ride height and adding a chin spoiler can cause the Cd to vary more than the .03 point range you list there. Just about any opportunity to restrict air under the car is a help. I believe a SN95 chin spoiler is a common addition.

We've never had out 86 more than about 80 MPH, though.

[yldouright]

Welcome!

So if the Cd is 0.37 you're going with a notchback? Kind of an odd reason to select a SVO. Changing the ride height and adding a chin spoiler can cause the Cd to vary more than the .03 point range you list there. Just about any opportunity to restrict air under the car is a help. I believe a SN95 chin spoiler is a common addition.

We've never had out 86 more than about 80 MPH, though.

Where did I say I was considering a notchback? An Audi 2.7L turbo, a WRX turbo or other good handling 4 seater with a minimum 12lbs/HP potential I would consider but I'd be fighting the imbalance with a notch on a track. I presented it only for purpose of comparison and because it was tested. As for handling at high speed, I was hoping someone here had some track time. I wasn't suggesting those speeds should be attained on public roads

The reason I'm asking about Cx is because it matters on the track and on fuel economy. If the SVO can't get under 0.36, tuning it to 300whp still won't make it competitive in its class.

[gbeaird]

I'm sure you weren't going to drive it at those speeds on public roads. We used to autocross our car. It was a dedicated autocross car for a while, so we never got over about 80, and even then, it was for only the blink of an eye.

What class are you trying to run it in? There are a couple of guys here who do regularly track their cars. I believe they are mostly DE type events, though, no class racing.

[GAboySVO]

I don't know the drag numbers, but I do track my car. To get the most out of an SVO on track you will need lowering springs and subframe connectors at a minimum. My SVO has a 92 GT hatch/spoiler and I'm not certain it does anything at speed. When the car first came out, they were very competitive with the Porche 944 Turbo cars on the track. You might try to find old Road & Track articles about the club racing then.

[yldouright]

What class are you trying to run it in? There are a couple of guys here who do regularly track their cars. I believe they are mostly DE type events, though, no class racing.

I was planning on running in SVRA events, historic stock and the like. I've been in cars and hot hatches at speed (>110mph) and you can feel even little wings and air effects settle a car down.

@GAboySVO
Did you see my post in the "Bolt on Performance" sticky thread? I'm curious to know if anyone agrees with my 'devil's advocate' analysis. Competitive with 944's? I suppose it depends on the Porsche and the driver

[Ken Potter]

Back in the day when I wasn't so smart I used to go a little too fast on the highways. I remember that once I got to about 100 I could see the wing starting to bow downwards. At 140 it was clearly being pushed down.

I also read somewhere many years ago that while the front of the SVO looks cool it's not the best design for high speed driving. Because of the massive opening in the lower bumper cover the engine compartment was pressurized to the point that air bled out onto the hood through the intercooler / scoop opening. This also reduced the cooling capability of the stock mount intercooler.

[Ken Potter]

I was planning on running in SVRA events, historic stock and the like. I've been in cars and hot hatches at speed (>110mph) and you can feel even little wings and air effects settle a car down.

@GAboySVO
Did you see my post in the "Bolt on Performance" sticky thread? I'm curious to know if anyone agrees with my 'devil's advocate' analysis. Competitive with 944's? I suppose it depends on the Porsche and the driver

Comparing the SVO and the 944 Turbo side by side the only thing the Porsche had the advantage on was braking. The SVO had slightly less HP but a fair amount more TQ. I drove both back in 86. I'll keep my SVOs.

[yldouright]

@Ken Potter
Great posts! If I understood you, the 2" opening above the bumper intended to force air through the radiator is actually causing that warmed up air to blow past the hood IC seal, counteracting it. It's possible the Fox Turbocoupes closed that gap for the aero gains but how did they route the air to the radiator? An SVO front air dam could be constructed to minimize this engine bay 'balloon effect' while rerouting air up into the radiator and a front protrusion can be mounted on the bumper to close that offensive air gap. Interestingly, the lower the car sits without these mods, the greater this engine bay balloon effect.

[Ken Potter]

@Ken Potter
Great posts! If I understood you, the 2" opening above the bumper intended to force air through the radiator is actually causing that warmed up air to blow past the hood IC seal, counteracting it. It's possible the Fox Turbocoupes closed that gap for the aero gains but how did they route the air to the radiator? An SVO front air dam could be constructed to minimize this engine bay 'balloon effect' while rerouting air up into the radiator and a front protrusion can be mounted on the bumper to close that offensive air gap. Interestingly, the lower the car sits without these mods, the greater this engine bay balloon effect.

Actually it was said to be the opening below the bumper. The air is being reversed through the intercooler and backwards out of the scoop. The seal is still in full contact with the intercooler.


Ideally the better design would be a front mount intercooler in the lower opening with simple louvers in the hood to allow the hot air to be released into the low pressure area at the base of the windshield. This would mean having to do away with the hood scoop all together.


Except that is one of the key design elements of the SVO. Functional or not I will keep mine.

[yldouright]

Actually it was said to be the opening below the bumper. The air is being reversed through the intercooler and backwards out of the scoop. The seal is still in full contact with the intercooler.


Ideally the better design would be a front mount intercooler in the lower opening with simple louvers in the hood to allow the hot air to be released into the low pressure area at the base of the windshield. This would mean having to do away with the hood scoop all together.


Except that is one of the key design elements of the SVO. Functional or not I will keep mine.

I agree, the hood scoop is what makes the distinctive SVO look but if the middle gap is not the problem there is a simple solution. I may not have understood the problem. To me, it seems plausible that the huge air pressure under the hood at speed lifts it to compromise the IC seal. Since the Turbocoupe deleted that gap, it seemed logical to assume that was the problem. At what speed does this balloon effect begin? Is there any way you could have misunderstood what you were told?

[Ken Potter]

I agree, the hood scoop is what makes the distinctive SVO look but if the middle gap is not the problem there is a simple solution. I may not have understood the problem. To me, it seems plausible that the huge air pressure under the hood at speed lifts it to compromise the IC seal. Since the Turbocoupe deleted that gap, it seemed logical to assume that was the problem. At what speed does this balloon effect begin? Is there any way you could have misunderstood what you were told?

I thought I was pretty clear about how the pressure changes in the engine compartment at higher speeds. But I'll go ahead and expand on that a little more.

First air flows through the intercooler core at the same rate whether it's coming in through the top via the hood scoop, or is being forced backwards from inside of the engine compartment due to the pressure change. The core itself doesn't "know" the difference. The seal is really there to keep water and leaves etc. from getting into the engine compartment and directly onto the turbo. While there are drains inside the hood they are towards the front of the scoop so any water accumulation will drain at very low speeds or while parked. At higher speeds water exits through the hood subframe and flows out near the hinges.

Second there are three routes for airflow into the engine compartment. First is the hood scoop, second are the openings directly above the bumper and then below the bumper. Both of these lower two open directly onto the radiator from the high pressure area created on the front of the car. There really is no difference other than the location as to airflow. However the lower opening is larger which allows for a greater "volume" of air to enter. That is why front mount intercoolers are mounted low in that area.

The hood does not lift.

The seal is sandwiched pretty tight so any movement isn't really possible. I have no idea why the Turbo Coupe is even relevant to the SVO. The hood openings and ducting are completely different between the two cars.

The only way to answer definitively at what speeds the air flow is reversed backwards out through the hood scoop opening onto the hood we would need a wind tunnel, smoke indicator wands, and barometric sensors. The sensors would need to be placed in all the areas affected at all speeds.

I do not have a wind tunnel. I do understand the design issues completely. This is the best answer I can give you and my fingers are tired.

Good luck.

[yldouright]

The seal is sandwiched pretty tight so any movement isn't really possible. I have no idea why the Turbo Coupe is even relevant to the SVO. The hood openings and ducting are completely different between the two cars.

I appreciate the effort you're making to answer my questions but I'm not completely satisfied with your explanation. I'm looking at the foxbody T-Coupes and I see a lot of similarity. The main visible difference is the gap over the bumper and since this problem is not common to both cars, it makes sense to suspect it. If there are treatments underneath that change what you describe in the SVO why can't we mimic them? Also, I believe my analysis is more plausible than yours. The IC seal is on the most flexible part of the hood and air pressure around the seal will pry its way under it if the flexing hood decompresses that seal. As I understand it, the air travels from the inter-cooler intake into the air filter before it gets into the engine. If there are no vacuum leaks, I don't understand how your explanation makes better sense than mine. As for the differences of passively in-taking air inside the bay as opposed to force ducting it in from outside, I think its obvious that the IC air will be cooler and faster even if it were slightly more restrictive. Furthermore, the shorter distance the air travels in the bay decreases the probability of it warming on its way to the engine. The IC is in the ideal spot if we can assure its seal. I do agree with your idea of hood venting and/or finding a way to decompress the air in the bay but I hope I've made you understand why my questions about your explanations persist.

[gbeaird]

The front end of the TC is a LOT different than the SVO. The TC has a lot better air flow control, probably spurred on by what was discovered with development of the SVO. There are a lot fewer openings in the front end of the TC, and it has a (deeper) chin spoiler, which will help the car pull air through the radiator, and probably maintain a little less pressure in the engine compartment at speed.

The amount of pressure, even at race speeds, are not enough to lift a steel SVO hood off the fenders. There are just too many air leaks around the edge of the hood, back of the engine compartment, AND through the intercooler scoop to allow that. It just doesn't happen. I'll be getting to test how a fiberglass hood does at highway speeds, but I suspect it, too, won't lift.

NO air goes from the hood scoop to the air filter, even if one has a cone-type filter mounted directly on the VAM. In the factory configuration, intake air comes from the area just outside of the passenger side inner fender.

The best way to 'decompress' the air pressure in the engine bay at speed would be to 1. reduce the open area under the front bumper, and 2. add a chin spoiler. Careful work to get the proper combination of the two would be necessary to keep the engine cool.

[Laredo]

As I understand it, the air travels from the inter-cooler intake into the air filter before it gets into the engine.[/COLOR]

Nope. That's not how it works. Have you ever looked under the hood of an SVO? If not, I would recommend start there (with the basics) before worrying about drag coefficients and wind tunnels. It ain't rocket surgery.