i would like to know how to test them for the rocket car i'm making.
Commercial model rocket engines for bottom and middle stages are designed to "burn through" and send hot gases upwards when they burn up the last of their fuel. Taping the engines together sometimes can keep the 2nd stage from releasing unless very little tape is used.
If you want long burning engines for a rocket car, you can buy larger engines. A D-12 engine burns for only something like 4 seconds, as the number refers to the impulse power rather than the burn time. I believe there are E and F engines with much longer burns.
Model engines don't really get very hot, which is why model rockets are almost always made of paper. The spent engine casing surface will get hot enough to possibly burn your skin, but not hot enough to ignite anything.
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Would this be a small rocket car model-type thing, or a life-size real car mod?
What do you want to get out of the test? Do you want to measure thrust? Do you want to see how big the flame is? Do you want to know how hot it gets around the engine? AOTA?
If you can give us some specs on the project, we can probably help more effectively.
We used to stack them up, with one layer of masking tape to hold each rocket to the next. For a rocket car, you probably don't want any delay between rockets.