I have an 86 LG4. I put flowtech (summit brand) shorties, and flowtech y-pipe on my car. I was going to keep the stock cat back for now but i found a really cheap one. It is only 2.5" cat-back though. Everyone says not to get 2.5in cause its too small, but for a basically stock LG4 i think it would be fine. Am I wrong? I also have a straight 2.5" pipe instead of a cat. Do i need a 3" system or will I be ok with a 2.5" one?

 

if you ever plan on upgrading the engine or swapping in a larger engine, than buying a 3" catback will prolly save you money long run(re-use cat back). But 2.5" will be better than stock so i'd say go for it if it is actually that much cheaper. But if it's only like 50 bucks cheaper than a 3" don't waste your time with the 2.5.

 

The exhaust on your car now is 2.5"... changing to another 2.5" system will get you nowhere. The upgrade to 3" is worth is, even on a stock 305. Just wait untill you can spend the money for a 3" system, you won't regret it.

 

last i remember stock lg4's had 2-1/4 stock.

 

my lb9 had 2/1/4 and now it has a 3" and it has helped alot i dont know about the lg4 but i think it would help you.

 

I'm gonna get the 2.5in anyway. Everything I have is 2.5in so i'm thinking that 3in won't be so much different. I read online somewhere that two guys with bascially the same camaro raced and were only 1/10 of a sec difference. One had a 3in and the other had a 2.5in. I'm not looking for huge performance. After looking at the stock catback i think that anything is better than it. Its only like an inch in diameter at the bends. Its kinda sickening. I also heard that the 2.5in dynomax is quiet, not much louder than stock, which is good cause i don't feel like drawing a lot of attention to myself. When i do swap motors it will be with an LS1 or something light. Ill redo all the exhaust at that point and that won't be for a long while. Thanks for the responses though.

 

i noticed you basically ended your topic but felt like posting neway
if its dual 2.5 then u should be fine either way just as long as u have everything thats supposed to be there not enough back pressure and u'll burn ur valves out
my 91 rs has a 305 and i've got a single exhaust 3" magnaflow cat and a 3" edelbrock cat back and it runs great been that way for a while

 

Completely disregard this comment, back pressure has absolutely nothing to do with "burning your valves" too SHORT of an exhaust could possibly warp your valves due to cool air getting on the HOT exhaust valves, but you would basically need to be running open shorties for that to happen, even open long tubes would be acceptable.

 

I was one of the first people to swap an LG4 2.5" exhaust for a 3" (aftermarket one) and there was no way in this universe an equivalent car would be within a tenth - lucky if it was a second. The swap is worth 35 to 40hp, even with stock manifolds.

 

35-40hp, huh?

I wonder what my LO3 would see if I traded my 2.25" stocker for a 3"??

 

the LG4 is a whole different animal with the addition of a performance exhaust system.

My buddy and I recently installed hooker 2055's and their 3" cat back on his 85 LG4. WOW. it actually has a little pull to it now. I was suprised how easily it'll pass 'ole granny on a 4-2 downshift. night and day difference, and I have a lot of stick time behind the LG4 variety. I wouldn't try to drag an LS1, but it does feel a lot more like a v8 should.

 

I should look for an LG4 setup to swap my TBI for then! I wonder hoh much work that would be.

 

35-40 Hp from exhaust on an LG4 (or L03) is complete bullshit. You might see 20 Hp at the flywheel from headers to tailpipe in 3" exhaust if you've got a good tune.

 

So going from 17.2 at 80mph to 15.2 at 92mph with the exhaust and a few other assorted bolt on goodies is more than 20hp (for the exhaust). I picked up 20hp from going to a 3.5" mufflex exhaust from a 3" banks on a 380hp car.

TBI cars (if tuned) will pick up similar gains to the LG4.


NO BS

 

What are "a few assorted goodies"? Stock there's no way in hell an LG4 could pick that much up from just headers and exhaust. I'm thinking, intake, timing advance, full tune up, smog removal, gears, lightening mods, headers 3" cat and 3" exhaust. Trapping 80 MPH in an LG4 would probably point toward it being out of tune in the first place. Also... do you have a dyno sheet to prove this or are you just using a calculator? You can't just run in here with guns ablazing making outlandish claims and not expect to be challenged.

 

80mph is about what they do stock - with an auto and 3.08 gears.

I'll tell you my mods to run the times. 92mph in a 3500lb car is 220 to 225hp:

Banks 3" cat back and 3" y-pipe
3" IROC cat
Dual electric fans
Crank underdrive pulley
Edelbrock performer EGR intake
Dual snorkel with K&N
Edelbrock micro plus timing computer

Considering this way back in 1991 and I did not tune the carb, it did indeed pick up 75hp total - approx 40hp from the exhaust.

My LG4 IROC ran the same times without the fans or pulley (all other mods the same) with the carb tuning in 2001.

Considering that I was the creator of the calculator way back in time... chassis dynos weren't even on the market back when I had my car in 305 trim.

You might want to check who wrote a good portion of the tech articles on this site ....


Me

 

None of your numbers add up... there's an unaccounted variable somewhere. No headers? No cam? I just don't buy it.

 

I did the math, size of gasoline molecules burnt and unburnt, heat expansion, etc.
It all works out to a sharply diminishing return when the exhaust flow is just under 3 times the maximum WOT intake flow in CFM.
For me, with a big block TBI at around 650 CFM, it works out to optimum at a single 4 inch, or true dual 3 inch. Anything bigger is overkill. ( you lose velocity quickly )

 

That variable would be knowledge....

You should hear people's mouths when I go to E-town pop the hood looking like a stock LG4 with headers then run fast enough to get booted off the track.