Avve

Hi,
I am about to buy a TII with the intention to do some road racing. Since the class I will attend is based on an effect/weight ratio, I can either race an S4 coupe or an S5 'vertible (most S5 convertibles here in Sweden have the turbo engine), without having (i.e. being allowed) to lighten either of them much. So, is a convertible (with a 6-point roll cage) noticeably "worse" to race than a fixed head??? Will the chassis flex to much to make a good track day car? The car will not be subjected to a lot of tuning parts, except from a new exhaust and intake (of course the suspension will be upgraded and the brakes race-prepared). The problem is that it seems to be easier to find a TII convertible than a coupe...
regards
Avve/Sweden

SLOASFK

iTrader: (5)

from what I've heard, the convertibles are almost as stiff as the hard tops.

an S4 Turbo weighs in at around 2850, an S5 vert non-turbo weighs in at a touch over 3,000. Add the turbo stuff, it's gonna be one heavy car..

Avve

Eh, another problem is the wankel engine itself: if I race an "untouched" wankel, my car will have the designated factory amount of horsepower, and the S4 TII then needs to have a race ready weight (including my 170lbs) of at least 2946. If less, I will be moved up one class. It really is useless doing any tuning at all (except cat-back exhaust and air filter change), as the car then will be moved to a class where at least 400 hp is needed, to be competitive...and that will really be expensive, considering the tuning costs, brake upgrades and so on. And yes, I realize that an S5 Convertible TII will need to be heavy, 3238 lbs...Of course I could go up a class, but the costs will probably rise more than the fun factor.
hi,
Avve

RX-Heven

iTrader: (1)

Sure you can.
http://www.awrracing.com/pages/proje...x/epconv3.html

The vert is stiffer than most other verts out there but not as stiff as the hardtops, especially when then top is down (or removed), which is required for the track.

However, The vert can be made to be just as stiff and even lighter than the hardtop.

Formerly known as jgrewe

We have one of each in our shop, non-turbos though. The 'vert weighs about 200 lbs more than the coupe but most of the weight is down low. The actual floor pan and rocker panels are made from steel that is easlity twice the thickness as the coupe. Both are prepped to E Production rules in SCCA. With a cage either would be stiff.

Avve

Thanx for the info! Really appreciate your answers! Now I can contact the seller of the '90 vert Turbo I've been watching for some time...We may race with the top up by the way.

sub9lulu

iTrader: (7)

if u throw a 10 or 12 point rollbar / cage in it the stiffness doesnt matter anymore
alot of weight on the convertible is the convertible motor

hey john what happen to your older account ?

Houpty GT

I autocross my convertible a few times a month and I also have a mechanical engineering degree so I can put some knowledge and experience behind what I tell you. The convertible can do alright and be fun but the coupe is always going to have the advantage. The coupe gains its stiffness from the distance between the floorboard and the roof much like the stiffness of corragated cardboard or a steel I-beam. Take a piece of corrugated cardboard and smash it down flat and see how much floppier it is. Well that is going to be the floor of your convertible between the doors. A few hundred pounds of reinforcement on the floor is not going to be a substitute for the roof. In addition the RX-7 was not designed as a convertible and is more of a modification than an initial design.

The problem with body flex is that as the body twist the suspension changes in the front and back and results in less ability to hold the road. This problem with the convertible is also addressed briefly in the Mazda RX-7 Performance Handbook. A roll cage could make significant improvements if it were welded to the windshield frame but I do not know if this is allowed and have never seen it done before. The roll cage is just going to add additional weight to the already heavy coupe.

I run with my top down and the targa panel removed. I am skeptical of any improvements in stiffness from running with the roof panel up. The hoop and fiberglass roof panel are hinged at multible points. The hoop has play in it that can be seen when it is opened and closed as does the roof panel that creaks going down the road during everyday driving. It would take a great deal of flex before the top would make any difference what so ever. The top also could only provide support in one direction unlike a coupe's roof which supports several types of strength. Driving with the top up also adds to the already bad problems of extra overal weight up high which is the worst place for it and shifting the weight of the hoop up high and towards the front of these happy to understeer cars. There were several bumps on our autocross course today and I could definitely tell there was some serious movement in the chassis. You can race the convertible and have a blast but there is a reason why people are going to want to buy up the coupes first. I would suggest you save the convertible for the street and hold out a little longer to find a coupe.
If you are worried about the coupe coming up under weight, can probally add a roll cage to the car to meet minimum weight requirements?

If anybody wants to argue with this, show me some numbers or proof to what you say.

toplessFC3Sman

iTrader: (1)

the 2nd gen 7 was designed as both a vert and a coupe from the beginning, thats why they have two different chassis codes. With the top up and in place, the 'vert is also 99% as stiff as the coupe. Icemark's page has more info on it. The top on our 'verts is much more structural than that of many other cars, which is why the targa panel is solid, and that there is essentially a roll-hoop built into the top (under the cloth bit)