FORUMula1.com - F1 Forum

[robinna]

How much better mpg would my '78 Grand Prix get without a cat? If I removed the cat on my '78 Grand Prix, how much better gas mileage (and power) would I get? I understand that the early catalytic converters were big and clunky and robbed a lot of power. Right now I get about 21 mpg highway and 105 hp with a 3.8l 231cc V6. I live in Wyoming so smog tests are not a big concern.
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[texasmr2]

Your gains will not be noticeable although you could install a high flow cat so you still comply with emissions.

[What's Burning?]

Removing the cat alone can actually hurt your performance. The car is tuned for it, so you'd have to work with someone that can tune it, and IMO you won't get any real benefit unless you also change to a free flow exhaust.

BTW 105 HP seems very low for a 3.8 liter 6 cylinder engine, you sure?

[texasmr2]

Thats about right but I actually found some info that said 110HP. Remember it was CAFE chocking engines with all the emissions crap back then. Heck even the two optional V8's were only 135hp and 150hp !!

[What's Burning?]

Thats about right but I actually found some info that said 110HP. Remember it was CAFE chocking engines with all the emissions crap back then. Heck even the two optional V8's were only 135hp and 150hp !!

I was getting 132 out of my 1985 GTI with a very tuned 1.8l 4 banger and getting over 30 mpg while at it!

[texasmr2]

Two very different era's in automotive history.

[What's Burning?]

Two very different era's in automotive history.

Yeah... and one of those eras netted us both the Gremlin AND the Pacer!

[texasmr2]

Two very different era's in automotive history.

Yeah... and one of those eras netted us both the Gremlin AND the Pacer!

That should explain everything .

[Fred_C_Dobbs]

I used to get 28 mpg at a steady 80 mph in a 280 bhp 2.5L 4-banger Porsche 951.

The late 70s were a horrible era for performance in US-built cars. When the "energy crisis" arrived, Detroit were completely unprepared for the clammoring for fuel-efficient cars and their response essentially was to simply make everything run too lean. But too lean a fuel mixture promotes compression ignition (especially with "cheap" petrol) so they also lowered the compression ratio to reduce pre-ignition. Between the two restrictions, the "lean burn" engines were quite anemic. The 1978 350 c.i. Chevrolet Corvette engine turned a whopping 185 bhp ...on paper anyway.

Is it fuel injected or carburetted? The first step is enriching the fuel mixture, but that can only go so far because of the low compression. And of course enriching the fuel mixture will reduce fuel economy.

[madbrad]

As has been said, reducing the backpressure can net a reduction in peak bhp which you could document by dynoing the car before and after. There are so many other factors that contribute to what happens when you reduce the backpressure alone. I don't pretend to know much about it but I can say in general that while the concept of freer breathing is a good one, it only works if you also make changes to other parameters, like mixture and timing, as well as the needed corresponding reduction in intake resrictions. As it sits now, the engine is set up to optimally run with the exhaust it came with.