The suspension is an obvious place to start, and it dramatically changes the way the car handles. However, as far as lap time reduction goes, weight reduction may be the best modification. I'll cover what I've done, and then list what's possible.
I began with removing the spare and jack before each event. This is free and easy, and drops about 40 lbs, even if it is off the end that's not hurting so badly. Running with less gas is another big step that's easy and free.
Some contend that weight should be left on the rear of the car, such as the jack/spare and fuel. Yes, the front end is over-loaded, but think about it this way: the front end has a given amount of work to do, based on the amout of force the car's weight puts on it in a corner. Reducing weight from the rear end will make the f/r split worse, but it will in no way make the car slower: the front end has the same amount of work to do, the rear has less, and you can accelerate and brake better with less mass! It is ALWAYS better to lose unneeded weight! The car might feel more balanced with more rear weight, but it will not be faster.
I neglected weight when dicussing suspension setups. I lost about 15lbs going to coilovers. A stock-replacement kit like the Koni or BAT kit will weight very close to the same as stock. 15 lbs isn't much, but some of it is unsprung, which helps the suspension work more effectively.
When chosing tires and wheels, weight is important! I run heavy (19lb) wheels and tires (21lb). 40 lb per corner isn't
really all that bad, but it's not that good either. Rotational mass kills your acceleration and braking by much more than non-rotational mass, so light wheels are great. Plus, since there's 4 wheels and tires, total weight loss can add up more quickly than you might think. I imagine you could get some 12lb wheels, and combined with 20 lb tires that's 32 lbs total weight loss - and it's all rotational! That's pretty serious. I'll cover wheels and tires later, after I finish some testing, and hit on some other aspects to the choice.
Next I pulled the battery out of the engine bay and put it in the trunk. Again, there are many differing viewpoints on this.
1) Get a small battery and leave it under the hood. Relocation cabling, distribution boxes, and hardware adds weight, so relocating is bad. Besides, putting it way back over the rear bumper hurts transitions, since you increase the polar moment of the car.
2) Put a heavy battery in the trunk. You need the weight back there.
3) Put a lightweight battery in the trunk. A gain of a couple lbs is worth getting 15 lbs (an average lightweight battery) moved back.
There is truth in 1 and 3. Obviously 2 isn't right, as I mentioned above. Depending on the car and application, either way might be better. I put the heavy battery in the trunk as a stepping stone to 3. I think for this platform it's not a bad idea. I added minimal weight by using 4 ga wire (it works fine, but make VERY good connections), and ripping out the heavy stock battery box. I'll take a 2-3 lb penalty to get 15 lbs moved back.
I chose this battery:
It's a Hawker Odyssey PC680, 16 amp-hours, 15 lbs. It may or may not work well in the winter - I'll see soon - but it will start just about anything well in anything but very cold weather. It was a Valentine's Day gift (yes, she's that great), and I'll put it in soon.
I also replaced the stock airbox with a K&N and custom box. This should be good for more power (engine installment coming sometime in the future to discuss this), but it also lost a lb or so. No biggie, but it's something.
So besides lightening things I needed, I began getting rid of things I didn't need. T/C is on it's way out, which is worth a few lbs right off the front. A/C is coming out too. A/C is not only heavy, but right up front, and should be a big help.
That's what I've done: what else is there?
You can put in any front seats as long as they are 15lbs. Read the rules for exact details.
Exhaust is VERY heavy. Going to a lighter setup can be very benificial, and possibly give power too. I know an S2000 driver with a 5 lb Ti exhaust!
You can also pull off exterior trim (knauberizing/debadging) and a few other things with small weight benifits.
I have a heavy car. I have a LOT of options. Looking at my (1995) sales brochure, here's what you could skip:
Power mirrors ("remote control" are standard - not power, I assumme)
Spoiler
Fog lamps
ABS
Traction Control
Keyless entry
A/C (STS has a rule always allowing A/C removal in part or in whole)
Bigger center console
Seat valance storage bins
driver's seat lumbar adjustment
Leather
light group
power windows
power locks
moonroof (this is BAD! it's a lot of weight, and it's very high.)
cruise control
rear window defroster
floor mats (you can, and should, remove these anyway when you run)
power driver's seat
Folding rear seats
casette player/premium sound
Obviously, some of these are negligible or things you might want despite a slight weight penalty. Also, if you remove any options, you have to be left with a car exactly as it could have been ordered: that is, since the SE and LX came with the folding rear seats, you can't put non-folding seats into one of them. The trouble, of course, is finding a GL V-6 so you can build this option-less car. But, the option is there...
There is one last area for weight reduction: the bodykit allowance. Bumper covers are heavy, and by building a carbon fiber or aluminum kit, you could not only get a LOT of weight off both ends, but you could possibly get some downforce on the front. I don't know of anyone that has done this to an ST* car yet, but I think it's coming. Civics might (I don't really know) have light bumper covers, but some other cars don't, and there is weight to be lost!
My car, with the weight loss I listed, should be 2800 lbs or a little under this year without gas or driver. That's a LOT of weight gone! But, guestimating what a perfectly built car could be, I cam up with 2600 lbs. The more I think about it, the more I think it's quite possible. Mid 2500's might be feasible, in fact, if you replace every legal part with a lighter variant. That is a really, really big deal.
Of course, the same amount of work on a Civic gets it down to around 1900 lbs. Oh well.
