I've attached an image hyperlink for a different car I just did. The red writing is what I added, don't copy line 0000E8CO on screenshot cause it will give you loads of hp (this is the turbo code, not what you're after.) Line 0000E970 is the exhaust code is where the C8 D8 is on the particular car I did. You should be able to work it out for any car using the screenshot.
Note that everyone's saves are slightly different as are cars depending on their type so you have to work out where your code needs to go.
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Click the link not the thumbnail.