I have a 1975 L82 convertible, Auto, A/C with two tops. I have owned this car since 1983 and all the numbers match. I have true dual exhausts and no cats. The engine has 77,000 miles on it and the engine is tired. I have convinced myself that this car is worth keeping a numbers matching car and i want to rebuild the engine so that I get around 300HP. This car is a street car and I don't want a lope in the idle. What parts (cam, Timing chain, rockers, pistons, etc) would you suggest I use to accomplish my goal of 300HP and still have a matching numbers car.

Help

Thanks

 

Assuming that you want to keep the factory heads, a lot of that horsepower can be unlocked with some very good machinework and blueprinting. I realize that your block can not be milled because you do not want to lose your numbers but having the mains align honed and the cylinders bored and torqueplate honed is a good start.

If the money is there, buy a set of low buck I-beam 6" rods or you can have the existing rods reconditioned. The longer rod will allow for a shorter and lighter piston and it is worth a couple HP and a small amount of torque. The stock crank can be polished or ground; whichever is needed.

Run the KB Hypereutectic pistons such as the KB106 for the stock rod or the KB112 if you go with the longer rod. There is a 97 gram difference in weight between the two, the 6" rod piston being lighter. Both are set up for the 1/16" compression rings which will reduce some drag over the factory 5/64 rings.

Always have the rotating assembly balanced. Most people think that operation is a race only thing but it is not.

If you are OK with it, have some minor porting done on the heads. Aftermarket heads are leaps and bounds better than the factory stuff but you will lose the external OEM look. I would recommend some bowl blending and short turn radius work. Do not bother with match porting as it does very little and it is a lot easier to spot that work was done if the intake is pulled. That may or may not be an issue for you but I figured I would mention it. A good rule of thumb is the easiest areas to port generally make the least difference so it is best to leave it to a pro.

I would run a cam with a 112-114 lobe seperation so you do not have that real lopey idle and I would keep the lift under .500 so you can stay with a stock diameter spring such as the Z28 spring. You also do not need to change to screw-in studs at that point either. The duration at .050" should be in the low 220 range to still make some power and still sound like it is somewhat factory.

Use a good performance aftermarket chain like a Cloyes or anything along that line. The OEM chains have some retard built into them so they are not the way to go. Have the cam degreed to make sure what is in the engine matches the cam card.

For the rockers, run a full roller and not just a roller tip. There is some HP there and I would recommend a steel rocker over aluminum just for the longevity.

The rest of the parts should be all OEM replacement. Do not run a high volume pump as it is a HP eater and it completely not necessary.

There are other areas to get some horsepower such as some porting on the factory intake, some fuel and ignition mods. It is attainable for you but you have to go about it with a little more thought then someone that is not worried about an OEM look.

If you have any questions, please feel free to PM me.

 

If you want to keep it numbers matching, there is not a whole lot that you can do that you haven't done already. The biggest thing was putting the dual exhaust on. Typically, with the 76cc heads, I would tell you to put some flat top pistons in it, but I believe you already have close to flat tops in there with the L82. I also think the L82 pistons are forged.

You could always go with a slightly larger cam, but you are about at your limit with the auto transmission and stock torque converter. In addition, the L82 cam is a fairly good cam.