Here is a general recipe for turning your XR4Ti into a G Stock autocrosser.
Wheels and Tires
Since the factory supplied us with 5.5" inch wide wheels, we are limited by Stock class rules to nothing wider. If you own an early car, remember that it is within the rules to update to the larger 87-89 15 x 5.5" wheels.
I have seen racing tires as wide as 205, section width, on factory 15" wheels. I know Yokohama had 195/60 15's in the A008RS and A008RSII tires. However, this tire is now discontinued, replaced by the A032R (which doesn't come in the same size), so it is a bit difficult to track down a set. Don't think that bigger is always better. For tires, forcing a wide tire on a narrow rim, you are sacrificing sidewall stiffness. For wheels, the stock 15" wheel is going to be heavier than the stock 14" wheel.
A heavy wheel comes into play in three different functions. One, it adds general weight to the car. Two, it adds rotating mass to the drivetrain. I've heard that adding one pound of rotating mass to the driveline is like adding eight pounds to the car. And three, it adds unsprung weight, reducing the ability of the suspension to follow the road. Now the relationships between these three functions isn't so cut and dried, but this is the general outline.
If you aren't on such a tight budget, there are some 15 x 5.5" wheels available in the aftermarket. However, don't look too hard if you are willing to spend the money. Panasport will make a 15 x 5.5" wheel for you, and it'll be light, too. I recall the Panasports in the 15 x 5.5" size weighs somewhere around 14 lbs., which is an improvement over the smaller 14" wheels as well as the 15's. I know the Morgan guys run the same diameter and width as the stock XR, but I am sure the bolt pattern and hubcenter are different. Panasport usually supplies hub blanks that can be cut to fit. I believe Bill Cobb at Racing Wheel Services will do this.
Please note: Most racing tire manufactures will be very resistant to selling you 205 section width tires for the 5.5" wheels. I know BFG wouldn't sell me 205's in their Comp T/A R1 for my 5.5" wheels. However, note that this tire is now discontinued and their resistance to selling was based on the assymetric sidewall design of the first generation R1.
First off, make sure the engine management system has Code 11's (no problems). There is a large difference in performance between a stock engine with computer/sensor problems and a stock engine that is running properly.
Next step is the exhaust system. Get a good 3" cat-back system. We can't do anything to the cat or before the cat, but a well bent 3" system the rest of the way back will help things.
After the exhaust, replace the stock air filter with a drop-in K&N air filter. No one has proven that it actually makes more power, however, you won't see very many autocrossers in stock class without one. Also, you'll never have to replace it, and it's legal, so why not modify your car to the limit of the rules, eh.
Got an early XR (before mid-86)? You exhaust manifold might suck bad. The recommended exhaust manifold is called the E6 casting, in reference to the part number cast on the manifold. Updating an early car could be considered legal as the old ones love to crack and they aren't being made any more. This would fall under the clause of parts supply rules. Do your research.
Don't even try an intercooler unless you are the only one in the class, which isn't likely in any stock class.
This could easily be put into the engine section as a lot of items are interrelated, but I decided instead to dedicate a full section to the turbo. First of all, in stock class there is nothing you can do legally to the turbo, short of replace it with an identical unit. So why devote a section to it?
I believe that 14.7 psi is the maximum legal allowable boost. Generally, the SCCA allows whatever the shop manual specifies as the boost limit is the boost limit for a car prepared to stock rules.
Transmission and Drivetrain
Get synthetic fluids for the diff and gearbox. Keep in mind that leaks are more likely to occur when running synthetic fluids.
I don't think that a legal GS XR will kill a T9 quickly. If you shift smart, it'll be fine. I know a guy who ran the T9 in his Street Prepared XR that dynoed at 214 hp @ the rear wheels. He ran it for a while and was just prepared with parts to do a rebuild, just in case.
There isn't a whole bunch you can do with the suspension in G Stock. Most racers in GS get a performance alignment and install adjustable dampers (aka shocks and struts). However, there are a few other legal mods that you can do to an XR. Here is a general list:
Install Koni (or brand of your preference/budget) adjustable dampers. Shop carefully as some brands/models require the rear shocks to be removed for adjustment. If you go with Konis, I believe the stiffer of the two choices are the Koni yellows (oranges?). I think the reds are essentially the RS or RS500 dampers and require removal of the rear shocks for adjustment. This may not be your bag and isn't convienent, but our touring suspension can always use a push in the right direction.
Why adjustable dampers? Well, the dampers allow for additional suspension tuning control. We really can use as much help as we can get our hands on.
We are legally allowed to replace the front swaybar. However, a larger front bar will most likely serve to enhance the low-speed, pre-boost corner exit push the XR loves to exhibit. Stay stock here, unless you feel like the front end rolls to much. Before you do replace the bar, make sure you have considered replacing the front dampers (struts) with better/adjustable units. The swaybar is looked at as a necessary evil, as it is designed to create weight transfer from one side of the car to the other. Quick lesson; two equally (vertically) loaded tires (ie 600 lbs. left side, and 600 lbs. right side) provide better lateral grip than two unequally loaded tires (ie 300 lbs. left side, 900 lbs. right side), even if the load shared between the two sets is the same (ie, 1200 for the equal load, and 1200 for the unequal load). To repeat, the swaybar, by design, transfer load from one side to the other. Get better roll control with dampers first.
We are not legally allowed to replace the rear swaybar, which is fairly unfortunate considering how much help we need at the rear end of the car.
We are not legally allowed to replace the springs with anything but stock replacement. Since stock springs appear to be no longer available, there may be some room given to what is considered a stock spring. Both Rapido and BAT have springs that they list as "stock". This is worth some research.
Make sure everything is in solid working order, with no leaks, no worn components, and make sure everything is clean. Note that a troublesome and/or sticky caliper can cause all kinds of trouble. I've flatspotted a set of R1's with only 25 runs on them because I had a tempermental caliper that liked to lock early. This can greatly reduce how deep you can go into a corner. Braking performance is very poor like this because I'm limited to how early that caliper locks.
When rebuilding a caliper, note that a phenolic piston isn't a great idea with the temps involved in racing. I know that Eclipses have a tendency to set their calipers on fire with the stock setup. I am pretty sure this is related to the fact that the phenolic piston doesn't allow the heat to be transmitted away from pad area, and the victim is the rubber seal.
Been a while since you've changed the brake fluid? Replace it with Ford DOT 3. Seems like everybody uses this stuff. Best stuff short of racing synthetic (in racing brakes).
It is legal to change the pad and shoe compound, and it's not a bad idea. I am about to try out my fourth compound. Every one of them has behaved in a different manner. There are a few things to look at which will help you tune. Look for how responsive they are when cold, their initial bite when you apply the brakes, how well they absorb heat and continue to work, and overall grip.
Make sure the braking system is in good working order. The condition of the rear brakes has a significant effect on how well the front brakes work. Rebuilding the rear brakes is fairly inexpensive and the results are generally notable as soon as you drive the car.
Note about big brakes: Big brakes aren't likely to be useful. Just like a larger wheel, they add rotating and unsprung mass. The purpose of the big brakes is really two-fold. One, it increases thermal mass to absorb the heat generated to help reduce fade. I've only heard of brake fade in enduro-autocrosses, where they take runs for 24 hours. Two, it increases swept area, or the area that the brake pad comes in contact with, more or less, thereby increasing available braking power. This isn't such a big deal, since our stock brakes (with a good pad compound) can lock up R-compound tires without much fuss. If your brakes can't, they need some help.
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