If the car has a 3.73 in it then you probably don't want to go to a lower gear. Changing the gear will not increase power output of the engine. Going to a lower (numerically higher) rear gear will give the engine mechanical advantage and will improve acceleration but you don't want to go much steeper than 3.73 for the street.
If you don't want to change the engine or transmission and you are unhappy with the performance of the car, then you need to optimize what you have so you can maximize the performance you get out of it. We can help but you have to be able to tell us in detail what you have. We need to know:
- What engine is it? 305 or 350? - casting numbers on the block will help identify it
- What heads are on it? - casting numbers will help again, assuming they are production heads. Are they iron or aluminum?
- What type of carburetor is on it? Quadrajet? Holley? Edelbrock?
- What type of exhaust system is on it? Stock? Duals? Headers?
- What type of ignition system does it have? Stock HEI?
- What type of work, if any was done to the transmission? Shift kit? Torque converter with higher stall speed?
- What is the rear gear in the car right now? Is it a 3.73?
- What do you want from the car? How will you use the car? Street only?
This information would get us a good start on identifying what you have.
If you can't tell us, in detail, what you have, then it is impossible for us to make a good recommendation on how to make the car better. I saw in the thread above you asking for where to find the casting numbers on the engine. Don't be afraid to do your own research. A simple Google Search "Small Block Chevy Identification" quickly gives you the picture below. I'm not trying to come off the wrong way here. We like to help members here but many will not respond to your posts if they feel like you are not willing to do some of the work yourself and just want someone else to give you all the answers. Another good Google search that will likely help you is "Small Block Chevy Head Identification".
There are some very knowledgeable folks on here that can make these cars run very fast. I encourage you to answer the questions above the best you can. If you do, I'm sure you will get some very good advice.
Pictures will help a lot as well. Give us some good pictures of under the hood and under the car. I see you posted a pic of the transmission earlier in the thread so it looks like posting them is not a problem. Hope this helps.
Jared
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