Stock 305, thoughts on 3” tubing?

[ck1984]

It'll be a combination of pipe size, bends, exhaust design & mufflers. 3" on a 305 is over kill & make it perform worse due to the lack of back pressure. Shop the mufflers & go from there.

isnt this back pressure thing a fallacy? its more like a small 2" pipe with the same amount of exhaust flowing thru as a 3" it will have a higher velocity of that exhausting air and that hot air has mass and it creates a suction effect/scavenging effect as it flows down that smaller pipe.
same amount of exhaust flowing in a 3 inch pipe has way less velocity and less scavenging effect, so instead of pulses of exhaust in a tight tube pulling each other along , you have pulses in a bigger tube kinda running into each other and having to be pushed out.
back pressure is just an assbackwards way of trying to say the same thing

 

[ck1984]

My opinion, 305 is not a performer, so do it! Doesn't make much power, can't sacrifice much power. You'll certainly have the thunder with 3"! If that's what you're after.

TQ is what moves heavy objects like cars from zero to 30 mph. you may gain some top end power for higher speeds but will take u longer to get there with less Tq and a huge 3 " system on stock 150 hp 305 is gonna sacrifice its low end tq
so if sound is more important than acceleration, go for the 3inch
but single 2.5 inch would sound almost as good but not scarifice as much lowend tq

 

[08Malibu]

I have long tubes, 2.5 true duals with an x pipe, and pypes street pro mufflers. The engine is a stock 86 Firebird 305. People tell me the car sounds good, but I feel like it sounds weak. Power is what I would expect from the weak engine.

 

[84dragcutlass]

If you watch a few of the engine masters shows by motortrend they prove that the legendary "back pressure" myth is a load of crap. They did quite a few comparisons of different pipe size, the difference was minimal, so i wouldnt really worry about it much. Its when you get up in the higher horsepower range that you have to take the exhaust size into big time consideration.

 

[motorheadmike]

If you watch a few of the engine masters shows by motortrend they prove that the legendary "back pressure" myth is a load of crap. They did quite a few comparisons of different pipe size, the difference was minimal, so i wouldnt really worry about it much. Its when you get up in the higher horsepower range that you have to take the exhaust size into big time consideration.

Thanks for bringing that up, I didn't have time to find it. Having a coffee break now.

The pipe size equation only accounts for the displacement of volume from the pump in a static or unchallenged state.

A 5L engine moving 100hp of air/fuel mass, needs far less exhaust flow potential than 200, 400, 800, 1000+ hp of air/fuel mass. And that still doesn't account for the thermal and velocity properties of the gas moving through the tube where more volume may be required to compensate for the demands. Eg. Larger piping up stream with hot gasses that have more distance between molecules, than downstream where the gasses have cooled and the volume required is lower.

In so many words, just go as big as you can and negate any potential restrictions. But, only so large that it is designed to flow efficiencently and effectively. Eg. Don't "T" pipes into eachother.

 

[ck1984]

Thanks for bringing that up, I didn't have time to find it. Having a coffee break now.

The pipe size equation only accounts for the displacement of volume from the pump in a static or unchallenged state.

A 5L engine moving 100hp of air/fuel mass, needs far less exhaust flow potential than a 200, 400, 800, 1000+ hp air/fuel mass. And that still doesn't account for the thermal and speed properties of the gas moving through the tube where more volume may be required to compensate for the demands. Eg. Larger piping up stream with hot gasses that have more distance between molecules, than downstream where the gasses have cooled and the volume required is lower.

In so many words, just go as big as you can and negate any potential restrictions. But, only so large that it is designed to flow efficiencently and effectively. Eg. Don't "T" pipes into eachother.

So reading between the lines, can a guy with too big of exhaust fix it somewhat by installing header wrap on headers and ypipe right up to the muffler to keep the heat energy in there and maybe keep the velocity a lil higher in the too big pipe?
header rap is fairly cheap, i did my ypipe right up to single 3" muffler for $35cdn
i did it to just not have that heat soaking into my transmission oilpan and cab since i had to delete the AC system to get Ls motor mounts on. [ also to hide my birdshit welding job ,lol]

 

[motorheadmike]

So reading between the lines, can a guy with too big of exhaust fix it somewhat by installing header wrap on headers and ypipe right up to the muffler to keep the heat energy in there and maybe keep the velocity a lil higher in the too big pipe?
header rap is fairly cheap, i did my ypipe right up to single 3" muffler for $35cdn
i did it to just not have that heat soaking into my transmission oilpan and cab since i had to delete the AC system to get Ls motor mounts on. [ also to hide my birdshit welding job ,lol]

No, you are overthinking it for your application. But, yes - shield critical components from heat as necessary.

 

[fauxfour2]

I have long tubes, 2.5 true duals with an x pipe, and pypes street pro mufflers. The engine is a stock 86 Firebird 305. People tell me the car sounds good, but I feel like it sounds weak. Power is what I would expect from the weak engine.

I have the same set up on my 403. Engine only has an upgraded cam but it actually sounds kinda buzzy and maybe even a little restricted at full throttle. At idle it sounds pretty good though. Weird.

 

[Clone TIE Pilot]

If you watch a few of the engine masters shows by motortrend they prove that the legendary "back pressure" myth is a load of crap. They did quite a few comparisons of different pipe size, the difference was minimal, so i wouldnt really worry about it much. Its when you get up in the higher horsepower range that you have to take the exhaust size into big time consideration.

Back pressure hurts torque. Its sizing the exhaust and intake to the RPM band where you want power.

 

[oldmansmonte]

TQ is what moves heavy objects like cars from zero to 30 mph. you may gain some top end power for higher speeds but will take u longer to get there with less Tq and a huge 3 " system on stock 150 hp 305 is gonna sacrifice its low end tq
so if sound is more important than acceleration, go for the 3inch
but single 2.5 inch would sound almost as good but not scarifice as much lowend tq

I have a 350 with 2.5 inch piping, high flow cats and flowmaster super 44’s and it’s not that loud. I ran no cats one summer and it was as loud as I wanted but I couldn’t stand the stench so I put them back on.

All I’m saying is it’s a 305. It’s a dog. You’re splitting hairs with the science. It’s probably got a open diff with 2:73 gears. The thunder might be all the fun he gets out of that thing.