304 Exhaust Size?

Caver Dave

Caver Dave

Just holdin' it down here in BFV
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I've already acquired the major bits (still waiting on a few small items) needed for transplanting a GM TBI on top of the 304... and already started the dismantle of the old bits (carb, ignition, etc.). My engine was a reported rebuild some 5 years ago w/ rebuilt heads (NO port work), OEM ex. manifolds, decent oil pressure & compression. Overall, it ran pretty dang good with the 2100. The TB will sit on a homemade 3/4" AL adapter on the stock 2bbl intake and be fired with a "de-mechanically advanced" DuraSpark dizzy thru the GM ignition module & TFI coil.

In an attempt to kill 2 birds, the exhaust needs work! Since my booty fab 2.25" duals (clearance issues constantly causing sealing issues) are toast *AND* I'll need at least one O2 bung in the pass. side... was exploring alternatives.

LoMod has offered up a recent exhaust (minor surface rust, but very good shape) from his project EV Commando, which should be bolt-in. It's 2" from each ex. manifold into a single 2" back to a generic "Flowmaster looking" muffler with tailpipe over the rear axle that exits the rear. The only potential things I can see doing... reworking the "Y" junction (pass. side looks simply butted into the drivers side tube) and depending on clearance/fit, cut the rear to exit out the side/corner... and of course O2 bung(s). Appears it should tuck up into the frame nicely = less chance of getting mashed/ripped by terrain than the current setup.

From what little I can find online (various sources/calcs), the 2" exhaust is optimal for a stock 304 operating under 4000RPM (much more... the valves float :lol: ).

My main concern is negating the increased air flow from the TB (approx. 500ish CFM) compared to the 1.08" 2100 (approx. 350CFM).

Looking for thoughts/opinions WITH data/reasoning to backup your response...
 
I'd go 2.5" single exhaust (with a high flow muffler) on a 304...I'm pretty sure 304s left the factory with 2.5" single exhaust.
 
IMO, 2" head pipes into a 2.5" exhaust would be a good bet; it'll be great for torque on that small engine. Are you gonna be running a catalytic convertor?
 
bowtieman55 said:
IMO, 2" head pipes into a 2.5" exhaust would be a good bet; it'll be great for torque on that small engine. Are you gonna be running a catalytic convertor?
Click to expand...

Sounds like the "new" plan is to get it running, 1/2 way tuned, and then have everything done from the Y back!

No CAT... '72 puts it outside the "35 years from date of manufacture" NC inspection range. Hadn't been inspected in several years prior to that anyway :D

:beer:
 
make sure you've got that o2 sensor installed before "tuning":beer:
 
My personal experiences were best with two 2 1/4" pipes from the manifolds into a 2.5" collector with 2.5" single out the back. Mine was a 360 TBI, but was basically what you have.

Plenty of flow and backpressure. No worries with clearance either.
 
Dave I am running 2" from passenger into 2 1/2" from drivers side into a flow master. No issues from the pane. Also mine is a 360. I have no problem floating valves at all.
 
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