Stock 305, thoughts on 3” tubing?

[Jim Rockford]

Very true, Yes the OEM put alot of crappy exhaust , but people forget that they spent millions engineering the exhaust to be cheap, efficient for the most part and emissions. same with the air inlet on newer cars, the engineers spend millions designing and engineering and testing the intake, Yes they have to worry about sound, but at the same time they designed it to flaw as much as possible to make peak power and efficiency and people go and replace them with some pipe from home depot. if it was that simple the OEMs would have made it that way. they dont want to spend any more than they have to per unit , penny pincers have ruined alot of good things . Main reason I installed my exhaust was due to the fact Im going to be installing a 350 and Im looking to get all the power and mileage I can out of it , and the headers where cheap. I was planning on using the tubular small block factory manifolds, but the new cost was too prohibitive.

 

[Jim Rockford]

 

[08Malibu]

Very true, Yes the OEM put alot of crappy exhaust , but people forget that they spent millions engineering the exhaust to be cheap, efficient for the most part and emissions. same with the air inlet on newer cars, the engineers spend millions designing and engineering and testing the intake, Yes they have to worry about sound, but at the same time they designed it to flaw as much as possible to make peak power and efficiency and people go and replace them with some pipe from home depot. if it was that simple the OEMs would have made it that way. they dont want to spend any more than they have to per unit , penny pincers have ruined alot of good things . Main reason I installed my exhaust was due to the fact Im going to be installing a 350 and Im looking to get all the power and mileage I can out of it , and the headers where cheap. I was planning on using the tubular small block factory manifolds, but the new cost was too prohibitive.

If you thing the original oem exhaust has the optimal flow, I have a bridge to sell you.

 

[Clone TIE Pilot]

If you thing the original oem exhaust has the optimal flow, I have a bridge to sell you.

Optimal sizing for the RPM band and displacement, but not flow. However, they sure did cut corners to save costs, especially with the exhaust manifolds. OEMs will somewhat choke engines to lower peak torque RPM to a targeted RPM. Also they choke HP for product class positioning such as having the HP of a base model and performance model not to close together by choking the base model.

 

[08Malibu]

Optimal sizing for the RPM band and displacement, but not flow. However, they sure did cut corners to save costs, especially with the exhaust manifolds. OEMs will somewhat choke engines to lower peak torque RPM to a targeted RPM. Also they choke HP for product class positioning such as having the HP of a base model and performance model not to close together by choking the base model.

There are other factors in play as well. Noise level, ground clearance, heat traveling back into the cabin, cost, and emissions. So the stock exhaust is are big compromise.

 

[Jim Rockford]

its a compromise of alot of things. Its not max flow but just enough to get the job done. I can tell you from experience a single 3 1/2 isnt as good as a 3 inch single for a checy pickup . with a vortec 350. a bigger exhaust without changing the amount of air going through the exhaust, is counter productive and a waste of money.
The cam, valves, compression ratio all determine the amount of exhaust you truly need. gm gave these cars just enough to get the job done stock.

 

[Clone TIE Pilot]

There are other factors in play as well. Noise level, ground clearance, heat traveling back into the cabin, cost, and emissions. So the stock exhaust is are big compromise.

Yep, the engineers have much more to think about than max HP or sounding cool.

 

[Ugly1]

Unless you are running massive hp your engine needs some back pressure. If you want more noise/sound, 2.25” to 2.5” with free flowing mufflers is the way to go

 

[Ugly1]

every application will be slightly different but on average , more is not better for low end Tq. maybe loudness but not TQ. if u r going more than 10% over manufacturer size on a stock engine u r making things worse, especially w/o a tune

Area of a circle is Pie x R squared
so a 3 inch single system is 3.14 x [1.5x1.5]=7.065[overkill on stock 80s 305]
a dual 2.25 system equals 3.14 x [1.125x1.125]x2pipes=7.94[way overkill]
Unless you put a blower on that dusty old 150 hp 305 i think, no i know, dual 2.25s is overkill, dual 1.75 would be sufficient and not lose any scavenging effect and low end tq.
this is exactly what my cutlass 305 had on it, no cats, just dual 1.75 flowing thru glasspaks

one shitty thing about stock old 305s i noticed in my teardown was that warmup valve in passenger manifold, that even when open had to be blocking a very signifcant amt of exhaust flow, should be removed if you value power over faster warmup.


2 smaller duals will always give you a higher pitch, more poppy raspy sound.
I put my 300 hp LS on a single 3 inch just for the burbly , low pitch Big Block sound, and it does sound like a BBc and im not sure i wouldve saved any lowend tq going with single 2.75 inch since my exhaust tube is so short, only 3 feet.
if you have a stock 305 and want bassy burbly Big block sound , go with a single 2.5 inch system and maybe a much larger tip, like 3-4 inches.

ive done this exact trick on my civic, i cut the muffler off entirely and just welded on a 14" long 4 inch tip and it made the 1.8L 4 cylinder sound like a 3 L v6
im sure all my neighbors hated it when i got up at 630 am to goto work , cuz for that first ten sec at startup it was extra loud.
not as loud as my LS with single chamber muffler and 3 inch system but now i live more rural so less neighbors

removing cats and resonators also makes a big difference on sound
some of the best sounding cars have longtube headers, no cats, no resonators and straight thru mufflers.

If done right you get some good flames out them when the pipes are as straight and glass packs 🤣. Got that on a buddies S10 build with a 305. No one would ride his butt after seeing that!

 

[Clone TIE Pilot]

Unless you are running massive hp your engine needs some back pressure. If you want more noise/sound, 2.25” to 2.5” with free flowing mufflers is the way to go

Backpressure kills power, low or high. A pipe diameter ( Cross sectional area), must be matched to the pressure drop (RPM) for the highest flow. Keeping RPM constant, if we decrease the pipe diameter we can increase flow to a point. Too small of a pipe can choke flow while too large will lack velocity. Therefore, exhaust pipe diameter can be used as a tuning tool to help move peak torque up or down the RPM band. Now intake runner and exhaust header length can be used to rock the power around the peak torque RPM. Then gearing to keep the engine in the desired powerband.