How to decide whether to remove rear spoiler to decrease drag, improve mileage

[Sven7]

I'm going to wager it's increasing drag. The air wouldn't be separating from the roof surface that early, and the spoiler actually ramps up instead of tapering down.

Find someone who owns one without the spoiler, do a tuft test, and see if the air is separating before or after the place where the spoiler is. If it's afterward, the spoiler's probably hurting your FE.

[ERTW]

Originally Posted by MetroMPG

That's what I meant to say: coastdown test, coastdown test, coastdown test.

you mean wind tunnel. that's how oem's decide on the shape of the spoiler - depending on whether they want to decrease drag, and/or lift. aero is very counterintuitive. that's why oem's spend hundreds of hours shaping bodies...after hundreds of computer hours running CFD.

as well intentioned as a person may be, coast down tests need to follow a rigorous and disciplined procedures. properly done, they have very good correlation to wind tunnels. a handful of casual coast down tests are a waste of time.

[aerohead]

Originally Posted by Wonderjava

so in other words your opinion is no it should stay where it is?

If you have the on-board computer,with instantaneous mpg display,and if the wing is not a major pain to remove and install,then you could do back-to-back runs and look for a change.
While you're out there,if you slipped, momentarily,in behind a semi-trailer rig,you might get a sense of what the car is capable of if completely streamlined.
It's a dangerous thing to do,and the trucker won't like you for doing it,so I never recommended it.

[Wonderjava]

i actually tried the rig scenario on my way back from somewhere here a couple weeks ago, and i did notice a major change in mpg...it went from whatever it had been (it has been very poor) to about 39 mpg when i was in behind the rig. but i dont have an onboard computer, in fact i dont have any mods at all like that, ive seen suggestions about scangauge, but ive also seen that is terribly expensive.

[aerohead]

Originally Posted by Wonderjava

i actually tried the rig scenario on my way back from somewhere here a couple weeks ago, and i did notice a major change in mpg...it went from whatever it had been (it has been very poor) to about 39 mpg when i was in behind the rig. but i dont have an onboard computer, in fact i dont have any mods at all like that, ive seen suggestions about scangauge, but ive also seen that is terribly expensive.

With a one time investment,the gauge would easily pay for itself in time saved acquiring data.And you could install it in your future vehicles after you move on from the PT.
They're extremely accurate and you'd know almost instantly if mock up was performing and worth proper fabrication.

[Wonderjava]

ya the problem is it costs too much right now, and i dont know how to install it

[2000mc]

Originally Posted by Frank Lee

If it was my car, I'd try it without and try to see if a trend shows up in the gaslog.

+1

[Piwoslaw]

Originally Posted by MetroMPG

I'd try driving it backwards, without any lips or spoilers.

Looking at how the PT is low&narrow in the front and tall&wide in the rear, I suspect that whoever designed it had mirror-vision.

[Wonderjava]

haha, you really think im gonna roll down the street backwards? ya id get stopped by the cops in like seconds...not happening.

[radioranger]

The back of my 98 escort 4 dr has a very round shape, will adding a tiny lip spoiler as on a 98 taurus [ built in to the light ) make a difference?