Lifting rear of roof, what are the benefits Options

[retrotech]

I remember a thread a long time ago. Can some one tell me the benefits of raising the rear of the btoom. Do you think it would have any effect on air flow over engine lid, in a positive way?
Opinions?

[Aaron Cox]

search this subject with john rogers......

he did this on his race car

[retrotech]

Can't find it. Do you know the answer?

[Aaron Cox]

QUOTE (retrotech @ Jan 1 2006, 08:37 PM)

Can't find it. Do you know the answer?

something about airflow....

not sure. PM him and advise him of this thread? (IMG:http://www.914world.com/bbs2/html/emoticons/confused24.gif)

[john rogers]

I just sent a long PM explaining why I ( and some others ) have dome it.

[Aaron Cox]

QUOTE (john rogers @ Jan 1 2006, 09:12 PM)

I just sent a long PM explaining why I ( and some others ) have dome it.

mind posting it here john (IMG:http://www.914world.com/bbs2/html/emoticons/tongue.gif)

[john rogers]

Okay, here is what I sent in the PM. Has this been backed by scientific data, not yet as NASA and DEI have ignored my requests to use their wind tunnels but it seems to work.

Hi Stephen, I am the one that mentioned that. One of the reasons a 914 gets squirrely at high speed is the rear tends to lift, so the IMSA and POC and PCA cars run a big rear wing to eliminate the lift. The factory did not do that and with racing the vintage 2L series now days, a wing is not allowed either. So how do you cancel the lift, which is a large part due to the vacuum created behind the rear window, over the engine cover. At 7K RPM the fan is sucking a huge amount of air down through that opening being blown out the bottom and the air needs to come from some place, so many racers just cut the roof off and eliminated the rear window. I have seen several cars from the 70's that kept the roof but had it raised at the back edge and when I tried it I noted a large increase in the wind in my face coming in the side window opening, so it has to be going out the roof opeing at the top. I also noted the car was really stable at full throttle when taking turn 1 and 2 at the CA Speedway at full throttle so I am thinking the rear has less lift now as it was pretty twitchy before. Since my 914, like most of the cars have the rear of the targa top bolted down, raising it was just a matter of making some spacers and using longer bolts. Is the rear lift being canceled, I think so do to the increased stability of the car at high speed as the 911s I race with float all over the track at full speed, but they all have 4 to 6 holes in the lexan rear windows to try to eliminate the rear lift some? Hope this helps some.

[North Bay 914]

So does that explain the vortex generators on Wayne Baker's 2.0 4 cyl. car?

[john rogers]

Hummm, most people don't ever notice things like that. I have also seen those on many new PCA GT-3 race cars too. I think his roof is lifted some too?

[lincoln]

this is actually interesting- seems like years ago i read something about race drivers modifying there own cars with a slight bump , or ramp , amost like a TA spoiler on the roofs , to assist in the airflow over the car and while i cant recall where i read it i seem to recall that it also did just this same trick

[North Bay 914]

Quick definition of Vortex generator at this link...here is the rest of the car, it decimated the 2.0 liter field that day with Waynye behind the wheel. He told me he would let the 2.0 six cars by so he could re-pass them.

http://www.aerospaceweb.org/question/aerod...ics/q0009.shtml

[North Bay 914]

Another...

[alpha434]

have you seen the ferrari racers? The ones with the exotic panelwork use little aluminum tabs to disperse the air randomly infront of those ugly panels. It seems that having the air broken is more predictable than having to calculate the downforce on those tabs.

What is the proper term for the really long flare things that stick out and look cool anyway. That one california company makes a kit for the 914 to make it like that.

Anyway, some ricer; the WRX or a toyota, uses dispersion tabs on the roof too. They are very uniform up down nubs. Like one side of a zipper. Only like 9 tabs on the roof though. so they're big. The article i read on it paid reference to audi doing something similar in the old days........

[john rogers]

Wayne is a pretty fast driver, IMSA and Sebring champion remember. At the Palm Springs Revival race in November he was holding off Tommy Thompson's 2.8L RS based 911 with that same 4 cylinder car!!!

[alpha434]

that link was very informative. I bookmarked it. (IMG:http://www.914world.com/bbs2/html/emoticons/clap.gif)

[Chris Pincetich]

I've seen vortex generators marketed/used to increase vehicle fuel efficiency - there was an ad for a goofy looking twin turbine looking wing in an electric car newlsetter I got a while back. I checked thier website, can't remember the site now, and it claimed that the vortexes created by their wing actually pull the car forward. Some physics about how the vortexes created behind the wing generate force forward. The experimental mini-van had a 15% gain in fuel efficiency.

I'd be interested in learning more about the 914 vortex generator on the race car-like was it custom? Who makes such a thing?
(IMG:http://www.914world.com/bbs2/html/emoticons/beerchug.gif)

[North Bay 914]

Wayne is also a very nice guy. It was a joy to speak with him. That roof panel was 100% custom. It was also very thin. Definitaly not just added to an original roof. You can see it has DZUS fasteners on it.

[alpha434]

chris- a fuel effifiency increase of 15% indicates an incredible reduction in drag!!! Please track down that hyper link!!!

And i think the proper term for ferrari fins is "canards"

[TimT]

Try this site they sell stick on vortex generators

[brant]

We did this...
I can't really tell you if there is a reduction in lift.
I wouldn't doubt it, but its just not one of those things I can absolutely confirm that I'm feeling.

The reason we did this was to reduce drag.
most of the race cars around here do.
if fact most of the GT type car either run no windshield and no rear window, or else a slopped windshiedl and only half of a rear window

here is our rear window.
we figure all the air coming in the side windows needs to go somewhere or else the cockpit is a huge sail: