75 Catalytic Converter
Thread Starter
Heel & Toe
Joined: Apr 2008
Posts: 16
Likes: 0
From: Mundelein IL
75 Catalytic Converter
Hello everyone, I had a few questions about emissions. I've got a 1975 Corvette, and was wondering how noticable of a power increase there would be by removing the cat. The car has a 650cfm carb, which I'm mentioning because I know it doesnt really add anything by itself. I'm thinking maybe with the carb and the removal of the cat I'll get at least maybe a couple more hp? Also another question, not really tech, but how hard is it/what is the procedure for making the car emissions exempt? Thanks!
Instructor
Joined: Apr 2006
Posts: 121
Likes: 1
I removed the cat. from my 75 and replaced the complete exhaust with true duels. The car came stock with a 3:08 rear and I changed that for a 3:55 rear. Also changed the intake and cam. The A.I.R. pump and all vacuum switches have been removed. The car runs great. All of the factory parts are put away in a safe place in case I ever want my 75 to go back to original.
Melting Slicks
Joined: Mar 2007
Posts: 2,100
Likes: 12
From: Frederick Maryland
A lot depends on the condition the cat is in. If it's nearly clogged, removing it will definitely make a difference, but so will replacing it with a new , unrestricted one.
Replacing the cat and adding true dual exhaust will make more of a difference...maybe not on a dyno but with the "seat of the pants dyno". It will likely run smoother and stronger, but probably not as much as you might think against an original, new exhaust...but definitely against a restricted exhaust.
On the '78 L82 I once owned, adding true duals and dumping the cat did make a nice difference in driveability and responsiveness. Maybe not on a dyno as I said, but I definitely liked the difference.
Replacing the cat and adding true dual exhaust will make more of a difference...maybe not on a dyno but with the "seat of the pants dyno". It will likely run smoother and stronger, but probably not as much as you might think against an original, new exhaust...but definitely against a restricted exhaust.
On the '78 L82 I once owned, adding true duals and dumping the cat did make a nice difference in driveability and responsiveness. Maybe not on a dyno as I said, but I definitely liked the difference.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
If it's a '75 base engine car, you can remove the CAT, change the carb and you will still be disappointed with performance. You'd do better to keep the carb, change the heads and exhaust manifolds, put a larger CAT and a larger diameter [3" ??] exhaust on it. At least, you would be able to pass inspection, anyway.
Drifting
Joined: Apr 2008
Posts: 1,540
Likes: 51
From: Lexington Park Maryland
A car as old as yours I don't think has to go through emmissions testing anymore..., at leats here in Maryland anyway. I would replace the exhaust with a stock one from a '74 which is a true duel. The '75 is really a Y pipe to a single exhaust ending with two tail pipes. I know, I had a '75 years ago. Keep your exhaust and CAT in your basemet in order to re-install if you ever want to sell the car and it needs to pass an inspection (which is different than emissions).
Race Director
Joined: Sep 2001
Posts: 10,560
Likes: 439
From: Wellington, FL
St. Jude Donor '11 thru '25
A car as old as yours I don't think has to go through emmissions testing anymore..., at leats here in Maryland anyway. I would replace the exhaust with a stock one from a '74 which is a true duel. The '75 is really a Y pipe to a single exhaust ending with two tail pipes. I know, I had a '75 years ago. Keep your exhaust and CAT in your basemet in order to re-install if you ever want to sell the car and it needs to pass an inspection (which is different than emissions).
Good advice in to hanging on to the old parts. 
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Safety Car
Joined: Feb 2006
Posts: 3,557
Likes: 0
From: Ridgeway VA
When I first got my '75 I replaced the cat because of age - with a new one your performance will be better and with a tune-up even better. Most important is put the car on a lift and check the exhaust for leaks - I had a 1/4" hole in the exhaust just before the cat.
Heel & Toe
Joined: Apr 2008
Posts: 19
Likes: 0
OK so I have a 75 vette and I have the same size carb but I put in hooker headers and a 2 1/2 true duel.... I also did a couple other things that helped me out as well but i do reccomend doing that it wont give u much HP but it does drasticly increase the sound. The 75 doesnt have much HP to start off with but its still a corvette... I do recomend taking tha CAT out and replacing with true duels. http://www.cardomain.com/ride/3069265
