Welcome to the IMSA History unofficial website

This site is aimed at bringing you memories from the glory days of sportscar racing in the US.



Rusty Bond and Ren Tilton were regular entrants in the GTU class in the 70s.

The GTU were production based small bore cars.

The bulk of the entry lists was made of Porsche 911s and Porsche 914/6s, BMW 2002s, Ford Escorts and Alfa Romeo GTVs. Later Datsun Zs and Mazda RX7s, Pontiac Fieros and Dodge Daytonas would fill up the fields.


Proprietary engine block must be used; may be machined so long as camshaft location is not altered.

Bore and stroke may be altered not to exceed calculated 3.0L class limit.

Cylinder head must have same number and location of valves, ports and spark plugs. Cars with less than 2000cc may use any 4-valve head.

Induction is free, except turbocharging is only permitted on FIA recognized models.

Turbocharged engine displacement is 1.4 X actual displacement. Rotary engine displacement is 1.8 X actual displacement.

Drive Train:

Gearbox or transaxle are free but must remain in standard location.

Rear axle must remain live or independent as appropriate.


The standard body tub must be retained along with standard wheelbase. Tube frame extensions are permitted.

All suspension components may be modified or replaced so long as wheelbase remains standard.

Brakes and operating system are free but components must remain in the standard location.

Axle locating devices may not pass into the driver compartment; however, the rear seat well may be covered with sheet metal to satisfy this requirement.


The original external shape and material must be maintained except that the floor pan may be replaced by a continuous flat .032" steel or .040" alloy sheet. The firewall may be replaced with a similar metal sheet in the standard location.

Material of engine and luggage compartment covers, doors and fenders is free.

Fender extensions are allowed to cover the legal wheels and tires but should retain the standard opening shape as viewed from the side.

Any additional bodywork must not confuse the make and model identity of the car.

Bumpers and external decorative trim may be removed. Any substitute bumpers must have standard dimensions and shapes.

Wheels and Tires:

Wheel and tire section width (maximum width at widest point of tire) may not exceed 13".

All four wheels must have the same diameter.

Track dimension is limited by inner tire clearance and the permitted maximum car width.

Aerodynamic devices:

An optional rear spoiler may be fitted to the rearmost part of the body without protruding beyond the perimeter contour as viewed from above.

Maximum height 6" above the standard bodywork. May not be adjustable from within car. No air may pass between spoiler and body.

Any front device must be located below the centerline of the hubs and within the perimeter of the body when viewed from above.

Official weight, measured without fuel & driver, all tolerances included:

A. 4 cylinder pushrod 2-valve engines: 0.7 lbs/cc

B. Other 2-valve conventional engines: 0.8 lbs/cc

C. 2-valve turbocharged engines up to 2.0L: 0.85 lbs/cc

D. Rotary engines (carbureted only) : 0.9 lbs/cc

E. 4-valve conventional engines: 1.0 lbs/cc

F. Minimum weight of any car: 1600 lbs

G. Full tube frame cars: add 100#