redoldsman

I did something yesterday I never in my life thought I would do. I put twin high flow catalytic converters on my 75 Hurst Olds. My wife commented it really seemed to be putting out some fumes when it was sitting in the driveway running. When I got the car in October it had true duals on it and was pretty loud. I changed the mufflers to short Dynomax Super Turbos and had an H pipe put in. It sounded perfect. The twin cats quietened it down a little more to where it is almost too quiet but the odor is gone. We didn't get any fumes in the car when we were driving before. I swear it seemed to run better coming home but I know that is imagination. I am curious if anybody else has done this. The cats were from Walker (Dynomax). My muffler shop guy said there were the first he has ever put on and he has been doing this for a good many years. I bought these from Amazon for $75 each. At Summit they were well over $100 each. That is the same scenario I got on the mufflers and free shipping with Amazon Prime. Now I have to go out and hug some trees (just kidding).

RetroRanger

i guess to paraphrase the old chinese food joke (you dont feed cat, cat feeds you)

you dont put cat on you take cat off !!!!

Whats the condition of the carb, could it just need a tune ?

redoldsman

I just rebuilt the carb about 3 weeks ago. Surprisingly these are fairly common. They are high flow and I had plenty of room for them. Heck it is probably cleaner than it was in 1975 and it doesn't have one of those old pellet converters on it with single exhaust.

69CSHC

Welcome to the club, I have true duals with H pipe equivalent high flow cats and performance mufflers.

How great is it to be able to stand behind your car while it's running and not feel like every second that passes is taking the life out of you.

Track tested multiple times 0 loss of performance on my stock engine. (If anything car seems quicker...)

I even went so far as to have the cats put on, taken off, and put on again, 1/4 mile tests before during and after. And yes my car runs considerably smoother and quieter with them than without.

First pair of cats were Flowmaster universal high flow 50 each from either Summit or Jegs. (I highly recommend them !)

Second and current pair are Magnaflow high flow universal. Much more expensive than the Flowmaster equivalent. And car performed the same.

I believe if they do hurt performance it will be on the extreme end. Not with cars like ours that are in the 200-250 HP range ...

If your engine is healthy as I'm figuring it is. Your car never ran better than it does now. You basically added 40 HP from your minor exhaust mods.

69CSHC

In case you're interested your exact model was tested when new by Car Craft Magazine August 1975. The 455 H/O ran 16.6 @ 83 MPH. Essentially as good as Olds would ever be RWD from then on. 10 years later Car & Driver 1985 posted the exact same performance figures with a then new G-Body 442.

Some may say wow, how, with the 75 being so much heavier and only having 10 more horsepower ... It's the enormous torque hit at wot that equalized everything. Rendered the 85 442s 3.73 rear and 4 speed gearing irrelevant.

Now with your ultra basic simple mods your looking at 15.5 @ 90 and matching an original 1964 442s performance.

P.S. even if you're not very into performance. It may interest you to see how other Olds faired after 1975. I have a list of road tests with sources put together, from 1964-1988. I will dedicate a thread to it in the general section.

redoldsman

Thanks Phil. That is some very interesting info. Although the horsepower was down in 75, the 455 still kicked out 350 foot pounds of torque. As you mentioned in the comparison with the 85 that is where the difference comes in. I am not going to hot rod this car and the only other change I might consider is a Holley Sniper fuel injection unit. I bought this car in October and am really enjoying it.