Dual exhaust??? Heres a new problem... I came to find that there are some parts that are not quite stock. The valve covers, and intake are obvious to me but I was hanging out with this random guy with a 72 and he made the statement "gee that sounds a little hotter than a 76 L48." The kid I got it from was the 2nd owner. He only had it two years but managed to abuse the hell out of it by just avoiding all regular maintenance. I claimed the vette as colatteral for an unpaid debt, and decided to keep it after getting it running.
So with that... How do I tell the difference between stock and true duals???

 

In 76 they had catalitic converters on the exhaust. they were not true duels. It came off the exhaust manifolds to one pipe into the converter and then split back into duels out the back. :chevy

 

Well, the first and most obvious indicator would be the presence of your catylitic converter. On stock '76's the exhaust should come together (thus, not dual) pass through the catylitic converter and then split again to your two mufflers.

Cheers

 

Look at the exhaust routing from under the hood or even better, under the car. True duals is just that, one pipe coming from each exhaust manifold exiting directly into an individual muffler and tailpipe. Some have an H or X pipe which improves flow, mounted under car. Factory system is an Y pipe where the pipes from the manifolds flow into one pipe, then into a cat. convertor, then separate again into two tailpipes.

Some of the posts from guys with stock L48 engines ('75 up) who make the switch to true duals (no cats) find a lot of extra horsepower even without any other mods!

 

68shark is right. If your car doesn't have true duals, get 'em. It's a worthwhile endeavor. Throttle response is noticealbly crisper, and it pulls better up to speed. Not to mention the great sound! :D

 

True duals with flowmasters=the way to go. :D They sound great, the performance is great, and did I mention the sound yet? :D :D :D :D :cheers: :cheers: :cheers:

 

"...Some of the posts from guys with stock L48 engines ('75 up) who make the switch to true duals (no cats) find a lot of extra horsepower even without any other mods!"

Woohooohoohoo... my nipples just got hard. :D
Anyone know the laws about this in MO? Ifin the law frowns on it, what about a high flow cat? ...or two if I split em?

 

You oughta check the law. In Texas, a 25 year old car is exempt. :p:

 

You know, come to think about it; when I was crawling around under the car I don't recall seeing any cats. I noticed the true dual exhaust, but no cats.


[Modified by Dwayne 76, 9:22 AM 6/10/2002]

 

Can anyone venture a guess as to how much more horsepower with a true dual set-up?

 

I had a stock system with (empty) converter and really beaten up pipes, large portion of them were crushed from ground contact.

Last winter I installed Dynomax headers into true dual 2 1/2 pipes and freeflow mufflers. A new crossmember allow straight connections from the collectors to the mufflers.

Big surprise: My subjective feeling is that the power has significantly gone down, especially mid range torque. (stock motor)

the sound is very nice compared to stock though...:-)

 

That's a bit of a surprise (lack of power)?? Did you go with 1 3/4" primaries or 1 5/8"??

 

Did you 're-tune' your engine/carb after? You will be running real lean if not.

 

Someone ran a calculation a while back showing that a 77 L48 will go from 180 net hp to 215 net by simply going to true duals. I think that is about 255-270 gross horsepower. A person can easily get 300 net hp with a few more mods, like cam, intake and headers.

 

I have 1 5/8 primaries. However, concerning the headers, I've seen in earlier threads people say headers might reduce power with a stock engine.

Running lean might be a good point. I have not done anything to the Qjet, rods and nozzles are stock.
But after the mods I mentioned I have installed O2 sensors in both collectors and sure they indicate a lean condition.

On that subject, my driver side is always leaner than the passenger, is that normal?

 

Ingar,
Every vehicle that I've put a less restrictive exhaust system on has required the mixture to be richened up to get the power increase available. In some cases not doing this can burn pistons (certainly with the old 70's & 80's 4 pot *** bikes - ever heard a loud one "fluffing" & "popping" on acceleration? 90% of the time it's due to a weak mixture from the loud exhaust &/or less restrictive air filter). Strongly suggest you richen the mixture if it's running lean.
:cheers:
p.s. I noticed a whopping increase in power (using the "seat of the pants" dyno) after fitting duals - but I've got a computer sorting the mixture out for me.

 

Thanks!
Isn't there a connection between lean/rich running and gas milage?
My thought was that since my milage is "normal" at least I'm not running so lean that the motor will be hurt.

 

Not neccesarily! I once changed my bike exhausts, which caused a mid-range lean spot. It ran OK, maybe a bit down on power, & mileage seemed normal. I think what was happening was that I was compensating for the lean spot by opening the throttle more to get the power I was used to. Not sure of the physics of it, but it was only when cruising for a long period in the range that the flat spot was in, that I noticed really good mileage figures (80mpg instead of the normal 60). No other hints of being lean, no pinging or "fluffing", although the exhaust note sounded slightly flat. Later that day when it started getting dark, I noticed that my Headers were glowing slightly red :eek: I couldn't touch the engine as it was so hot & the oil had a consistancy of water. The plugs were pure white & I'm sure that if I hadn't richened up the carbs I would have been stuck in Spain with a holed piston. Up until then I had been blissfully unaware of just how lean it was running.
It's definately worth checking. I've fixed engines that have holed pistons due to lean mixtures and/or advanced timing & all the metal that is blown out of the top of the piston usually has a sightseeing tour around the rest of the engine. It's not pretty & can get expensive :(
:cheers: