What kinda horse power increase do you think is reasonable to expect on the dyno when you go from stock to shorties, 3" y pipe, 3" cat, 3" call back...

 

This is just general info.....there may be some tricks to our I am not aware of, but anyhow.

I've dyno'd many cars in which cat backs alone did nothing to the hp, sometimes even harming hp.
You have to remember that the engine is just an air pump. You gain HP by increasing velocity/volume of air INTO the engine with the addition of more fuel.

A bigger exhaust is only needed when you are pulling more air in. On the flip side, some exhaust setups can help air flow OUT easier, and even aid in the "sucking" of air out of the cylinders instead of them fighting the air out. Kind of what x-pipes do.

On an older car, anything new might help the HP, since the 20 year old cat is more clogged up than a brand new one, but shouldn't expect to see more than what you'd have with a stock exhaust. I know I had my cat removed, and although I could be a noticeable small power gain, it couldn't have been more than 5-10hp. The only reason I seen the gain is because the cat was decreasing the hp due to falling apart and become restrictive.

That's just my thoughts. I took some power and performance classes and studied air flow, and air has very* similar properties to water in regards to how it acts in twists/turns restrictions and expansions. Neat stuff really

 

What motor? L98 may have more gains in power/tq compared to a LB9 or a TBI based motor.

There was a good thread here showing some dyno results from one guys L98 TPI (i think L98) based motor.....He dyno'd after every mod he made. I believe the shorty headers and catback picked up something like 15 hp? I cant seem to find the thread. But in general, all the bolt ons, from exhaust to intake parts on a 350 TPI motor should make another 30-40whp over stock, depending.


Exhaust, not all the much to be gained, 15-20hp sounds right. Motors arent that high performance oriented to begin with so they dont need alot of exhaust mods to "breathe". Its not like a late model 400hp LSx motor that can gain a good bit with bolt ons.

 

TBI is essentially the same as the LG4, which can pick up 40hp with headers, 3" cat, and a 3" cat-back compared to stock 2.25" exhaust. You will also pick up some TQ and will rev higher due to a big drop in back pressure.

 

40HP with just exhaust upgrade? Forgive me but that sounds overzealous. Dyno sheets or it doesn't seem likely.

 

I picked up over a second and over 7mph with a similar upgrade, just left the factory manifolds and went with a y-pipe... Back in 1990.

 

wow

 

Yes the LG4 (and TBI) exhaust systems are that restrictive on the 305 engines. Part of GM's attempt to make 2 carbed 305 power ratings - the 190hp HO and the 150hp LG4. The also lowered the timing, single snorkel air cleaner, clutch fan, and a smaller cam...

 

How restrictive? Check post #7 of the following link for comparison pics of the LG4 and L69 Y pipe:

https://www.thirdgen.org/forums/exha...-question.html

JamesC

 

You stated you picked up 40hp with just headers and a catback....

Yet you continue to state the difference between the two motors was timing/air cleaner/cooling fan clutch-nonclutch/and cam.

So which is it?

40hp with headers, catback, timing, and a bigger cam sounds ALOT more reasonable and realistic. Headers and exhaust alone.....not so much. Saying that headers and exhaust alone gave you 40hp would indicate a huge problem if our motors were really taking in that much air, using that much fuel, yet the exhaust was insanely restrictive to less 40hp off the motor.