Wetshak

How much horsepower would I gain if I removed the air pump and went to a true dual exhaust without the cat converter? I was thinking of adding headers as well. I am trying to get more HP without getting into the motor. I have a stock 81 vet . I posted this in the general forum and after I did I realized this should have been posted here! (DAH). Thanks to the members who answered in the other forum. Randy

Mike Ward

2 on a good day, 1 if you've got a normal car. Look elsewhere for more power.

NHRalph

Not sure how much you will gain but you will feel a big difference. I took off my cat, put on headers and side pipes ( My 77 didnt have an air pump) and it felt like a new engine. ( Of course I don't live in California )

SpyderD

There was a detailed post years ago - dyno tests, etc.

You will gain A HELL OF ALOT of HP by removing the cats and going to dual exhaust. 50, 60 horsepower? Around that range.

Open up the exhaust for sure - Look there first for more horsepower.

Wetshak

WOW!!! Just letting it breathe better will increase that much? I live in Canada and the car no longer has to pass emissions. That stuff is coming off!

yel76low

Headers... headers... headers...

And, I was going to ask everyone, so this seems as good a thread as any, because it will help the original poster. What size true duals should be used? Is 2" okay? Will there be a big difference between 2" and 2.5" for a near stock / or reasonably modified engine?

SanDiegoPaul

Headers + Cat Removal + Good state of tune = 15HP and some real seat of the pants feel too.

ClockworkOrange

Thought I heard that our stock ramshorn exhaust manifolds were close to header performance. Is this not true?

fauxrs2

Depends on what kind of catalytic convertor you are talking about...if its the original pancake (pellet style) one then removing that can give you some good gains. if its a modern hi-flow honeycomb style the effect will be significantly less. A modern convertor can rob as little as 3-4 hp. The air pump isnt costing you any power you can detect short of a dyno so removing that is just gonna clean up your engine bay.

Dual exhaust and headers is always an improvement over the factory 2-1-2 exhaust. Headers are also a benefit. You will not see full gains unless you then tune the engine to properly take advantage of the increased airflow, this could mean an increase in jet size in the carb, certainly changing from factory timing to optimised timing.

50-60 sounds high to me possibly with dyno tuning...

stingr69

The GM cat is real bad. The AIR pump is not that bad.

The cast iron manifolds are not bad if you are running a full exhaust and a fairly tame engine. Headers make more sense if you have an more open exhaust and are trying to take advantage of it with your combination. You kind of have to make the decision to run manifolds or headers at the front end. They are more of a limiting factor if you have a more radical setup.

-Mark.

NHRalph

Headers will make a big difference over stock manifolds. The stock system has 4 ports going into one at a fairly early stage, headers will allow the gases to flow out faster. The engine will start to breathe. Look at the improvements on just a cat-back system, imagine what it is with a full performance exhaust. The L-48 and L-82's suffered from breathing.
I don't have a dyno ( just seat of pants) but I think 10 to 15 horse increase with headers alone.

stock76

I read that somewhere too. I'll have to see if I can find the reference. I seem to recall that there was not much gain in going to headers on a stock engine. The BIG gain was going to 2.5" duals, no cats and straight through mufflers. I think the estimate was around 35 - 40 hp increase provided you had the carb adjusted properly and the timing set up right.