SW20 MR2 Information to help you Drag Race better.
This will be an evolving database of info.
Thanks to Josue Echavarria @nos51 for the pic
My personal car. Wrinkling 225 drag radials and fronts off the ground.
I want to first preface the information you see below. This info is not written in stone and there are a myriad of combinations and, as yet, untested means to improve results. This is, however, a path in the right direction and use this info as you will. The last couple of years has brought about the quickest and fastest MR2s in history whether all-motor or Turbo. We have seen consistent 60 foot times in the 1.2s on boosted cars with the wizardry of modern electronics like what Fuel Tech brings to the table. All motor MR2s are seeing 1.4s on just weight reduction and manual clutch slipping . I personally gone 1.4s with my all motor build on a tiny 225 radial.
Tires: What better place to start than here. If you are reading this and serious about straight line acceleration, I will already assume you have an all motor car with a 225 width or better drag radial, or a boosted car with a minimum 275/50 Drag radial or 26-10 slick. All on 15" rims.
A drag radial will take more effort to make work as they are less tolerant of the sudden application of power. I highly advise adopting some means of clutch control, notes to be found below. The MR2 never likes wheel speed (tire slip) and even less on a radial. You will want to dead hook a radial and this why some means of controlling clutch slip on launch becomes necessary. Josue Echavarria has been 1.3 60s on a radial on my personal car has been 1.4s on just a 225/50. Many ask about tire pressure with a radial, start at 17psi and work your way down. The more pressure you can run the better for stability and mph.
Slicks are far more tolerant of wheel speed and really absorb the hit as the soft side wall construction really soaks up a brutal amount of drivetrain shock. Although this is the "easy" button for 1.4 to 1.2 60fts, you will be losing down track stability. That being said, any of these tires can be overpowered and only you and your tuner can determine how much power you can apply at any point on the track.
Notes on slicks: I would not even suggest you attempt full slicks on an SW20 unless your goal was a record time for your combination. The rear weight heavy nature of the MR2 makes for a very uneasy feeling at anything less than "loaded" or acceleration conditions. The minimum amount of coasting felt between a clutched shift is enough to feel like the car is rear steering.
Struts and Springs: There is so much to be unpacked here and times when one combo will work and when it won't. The factory suspension can work surprisingly well with its soft spring rates and typically worn dampers. This is because the SW20 suspension has very low antisquat values and relies on weight transfer to get "on the back tire" and stay there.
Without throwing a power number or time slip at the issue. I will say that aftermarket coilovers tend to be a step backwards in the traction department. The lack of front travel, excessive spring rates and stiff damping (even on the lowest settings) only aggravate the issue.
Spring rates: Rear, I recommend 200lb/in as a good starting point for a 2500 pound car. I go even softer as car weight decreases. Front spring weights are typically softer for the SW20 for balance but I have ran anywhere from 250lb/in to 150lb/in.
Strut travel: This is only an issue at the fronts. Aftermarket coilovers have significantly less travel than factory units. The issue really is with the droop travel (when front rises and shock extends). Limiting this travel adds the entire front suspension weight to the rising mass of the front end when travel limits are reached. This can be o.k if you are attempting to control a wheelie condition (think limiter straps) or it can be disadvantages as now it unloads the rear suspension and reduces traction. In conditions where you are not lifting the fronts (like on the street) a long travel OEM front strut works much better to get the car on the back tire and hook.
Sway bars: I absolutely recommend you run a rear sway bar, Even if you have removed the front sway bar. The stiffer the rear bar the better. Traction from a dig as well as down track stability will improve. Think of it as tying the left and right side ride height together like a straight axle.
Control Arms: As stated earlier, the OEM stuff works surprisingly well if maintained. Swapping to control arms with spherical bearings will not necessarily improve your 60ft times but have been proven to correct the tendency to rear steer from bushing compliance. They have straightened many a launch. Understand I am not talking about any Megan product as their rear joint design is inviting mayhem.
Geometry Changes: To this point the only effective geometry change we have seen is the correction in antisquat. By altering the mounting height of the tension rod we can improve from the OEM antisquat value of 18% to upwards of 42%. You can see this geometry change at work on my personal car in the above pic. I ran a soft #150 rear spring with antisquat in the %24 setting and lifted fronts with no noticeable rear compression.
Damper Settings: The settings here will be no different than your standard rwd front engine combo like a mustang. You want soft front extension and soft rear compression. The goal is to get the car on the tire and to stay there. Many dampers have a single adjustment that affects both compression and extension. You will find you can still benefit from setting rear soft.
Clutch control: You will absolutely need to figure out a way to modulate clutch application to save the driveline and insure maximum traction. There are two current ways to control clutch application on launch. The first is manually with steady pedal release. This method gets increasingly difficult to control as 60ft times improve and anything better than a 1.5 feels like being hit with a truck. The second is with some device to limit the rate at which the clutch pedal is released. This can be a hydraulic slipper valve that installs in the hydraulic line to the clutch or a mechanical device on the actual clutch pedal linkage to slow pedal travel. I run the mechanical type.
Alignment: My recommendations are not that far off from factory specs and really fairly simple. No glaring secrets. The biggest thing to note is factory specs allow for front toe out. Toe out in the front, while great for autocross and quick initial turn in, is awful for high speed stability with low offset wheels. I also list toe in total measurement difference from behind wheels versus in front of wheels (using toe plates).
Front: Toe In 1/8", you can add a 1/16 to see if stability increases.
Camber , 1degree negative. You do not want to go zero as camber will go positive with suspension extension/separation.
Rear: Toe In 1/8" to 3/16" , I have seen lower values with spherical bearing suspension.
Camber, 1 degree negative. Once again, you do not want to go zero as camber will go positive with suspension extension/separation.