How to double my character's movement speed whenever spacebar is pressed?

I am a Beginner to Unity and I am trying to code a very simple spaceship game for my first project and so far my ship moves great but I am hoping to make it have a boost effect whenever I push the space bar. I tired to create an if statement similar to one of the ones I found in a Unity tutorial, but nothing works when I push the space bar. My code that I am using for my character movement is below. The bit of code I am having trouble with is the if statement in the Void Update part. Anyone have any insights or pointers for me? Thanks in advance!

 using UnityEngine;
 using System.Collections;
 
 public class PlayerController : MonoBehaviour {
     
 
     public float speed;
 
     void FixedUpdate ()
     {
         var x = Input.GetAxis("Horizontal") * Time.deltaTime * 250.0f;
         var z = Input.GetAxis("Vertical") * Time.deltaTime * 8.0f;
         //Movement script
         transform.Rotate(0, x, 0);
         transform.Translate(0, 0, z);
     }
 
     void Update ()
     {
         if (Input.GetKey ("Space")) 
         {
             var z = Input.GetAxis ("Vertical") * Time.deltaTime * 16.0f; 
             // Spacebar is supposed to double movement speed when down, creating a boost effect
         }
 
         else 
         {
             //if spacebar is not pressed, speed should be regular.
             var z = Input.GetAxis("Vertical") * Time.deltaTime * 8.0f;    
         }
     }
 
     void OnTriggerEnter(Collider other) 
     {
         //This is my pickup logic
         if (other.gameObject.CompareTag ("Pick Up"))
         {
             other.gameObject.SetActive (false);
         }
     }
 }