I was just reading horsepower changed slightly sometime during the model year. How can I find out when mine was built? I can't read any of the info on the body tags. Last digits of the vin are G5904774.

 

Do you have the "conforming statement" sticker on the driver's door? At the top left corner there is a date printed. That's the month of production.

There were 35 thousand 1986 cars made and of those, 7315 were convertibles like yours. The VIN sequence for the 'verts were 900001 thru 907315. Convertibles were introduced in the last half of the model year.

The 5 HP increase was due to the introduction of aluminum cylinder heads. All 'verts got those heads.

If you really want some good history about the car, go to the National Corvette Museum website and order the build sheet and the window sticker. The build sheet is what followed the car down the assembly line so the workers could install the various components and options. The window sticker will have the list of standard equipment and the base price, options ordered and their price, and will also have the name of the dealer that took delivery of the car.

 

A quick easy way is to look at the valve covers, if they have bolts around the perimeter they are early cast iron heads. If they have the bolts in the center of the valve cover they are aluminum heads.

Note it doesn't make much difference, what diff gears you have will make the biggest difference.
Look at the build label, some have it in the rear storage compartment and some under the console lid.

Google the numbers to see what diff ratio you have, the performance option3.07 is a great ratio. The standard 2.59 ratio is not.

That is where the main difference in performance is found in standard corvettes, nothing wrong with the early iron heads.

Go for some long tube headers, adjustable fuel pressure for some real performance increase into low 13 sec quarters. From the mid 14 sec she would have done stack standard, 45-48 psi fuel pressure is the sweet spot.

Before i went to a blower i scared the pants of ZR1 drivers, with just bolt on changes and tuning.

In fact i still run the 85 iron heads on my blown 383 engine, only minor port matching combustion chamber work and valve seat angle changes make them flow quite well.

I couldn't justify spending up big on AFR heads, if she was naturally aspirated that would be a different story. However then the nice looking TPI manifold, would need to be changed to a mini ram.

 

Your car is a 235 HP aluminum head engine, it has the "extra" 5 HP.

 

my late year 86 runs, stock, the 1/4 at 14.037. It has the 307 rear end. The 259 runs mid 14s. Geradvg is correct, add a few add ons..an easy 13 second car.