Proper sized exhaust
 

O.k., the stars are aligning for December and I should be able to get the muffler(s) in. Are their any opinions as to what size exhaust I should run? I'm nor sure yet if I'm going to go single or dual.

I have done the following mods:
Stock 305, Dual intake/Holley500 cfm carb., changed rear gears to 3.92 - 3.23.

Thanks for your thoughts.

Re: Proper sized exhaust
 

How big is the outlet on the manifold, 2 1/2"? Whatever it is, I would use that dimension to the Y, then go up 1/2" after that to the back. If going with a dual exhaust, install a cross pipe and keep the diameter the same all the way. (My 2 cents worth of free advice).

Re: Proper sized exhaust
 

Agree with WDShaffer. Duals are nice. Put mufflers in rear. Better for flow and will not give you a noise headache in cab. If duals, get quiet ones. IMO.

This is commentary not aimed at anyone.
This Holley carb thing is over rated. By the math, 305 reaches equilibrium at about 300 CFM. You only have so much manifold vacuum. 500 CFM seems to work because we fiddle with them and get a fuel to air balance that the motor wants. Not more, not less. So you tune/jet your Holley to be a WW. The 305 was designed for torque not revs. The "extra power" may be imagined by us guys wanting to feel like we are supermodifiers. If you want a hot rod, a 305 ain't it. Most of our trucks have stump puller trannys and rear end ratios. But a really good motor. So why are we looking for higher RPM's? So we can blow up something? I will love my WW's and Rochesters. If a person does a Holley thing and they like it, it may not hurt anything as long as you have the money, don't over rev, don't experience low gas mileage, don't bog down on take off or don't constantly fiddle diddle. If you argue with scientific Carburetor math, have a good excuse that no one else thought of in 150 years.

Re: Proper sized exhaust
 

GMC V6's are not immune from the benefits of increased airflow at engine speeds greater than torque peak RPM.

While they may ultimately not respond (percentage wise) like some other engines, it seems most report gains with the 500 Holley and a dual exhaust. I tend to think that the restrictive factory carburetion was more for economy and a reduction in HP for longevity under a wide range of users than anything else. The factory single exhaust was the lowest cost solution and worked ok with the small factory carbs.

A less restrictive exhaust tends to lower EGT's at the exhaust valve, exhaust port and manifold too.

While calculations show 500 CFM to be overkill on a 305 running at factory designed engine speeds, as with many things, calculations do not represent real world results. More than likely, the same engine with a 500 CFM carb will make more horsepower than the same engine with a 350 CFM carb, or the factory carb.

And, as far as Holley carbs go, the 2 bbls and 4 bbls are rated using different pressure drops across the carb, so comparisons between the two isn't really an apples to apples comparison. And I have no idea what pressure drop either Carter or Rochester may have used to rate their carburetors, so comparison between the manufacturers is just about impossible.

Re: Proper sized exhaust
 

Speaking of Holley, the company that wants to sell you a carburetor and make money. Here is a quote from their Technical Section. I think they don't adhere to "calculations do not represent real world results. I think they think calculations are real world and "more than likely" is not real world.
"Use a carburetor with an airflow
rating equal to or slightly smaller
than the air requirement of your
engine."

Their chart shows a 305 Cu In motor with 4000 RPM needs a 297.5 CFM carb. Round up to 300. This calculation is at 85% VE which Holley states is a "High Performance VE".
My thought is not to convince anyone who wants to "carb up". My info is for new owners/members to have a second opinion backed by actual, real world info from Holley and others.
VE= Volumetric Efficiency
I am not comparing 2bbl to 4 bbl, I am talking about a Holly 500 CFM 2bbl on a 305 V6. Apples to apples. Putting a 4 bbl on one would be ridiculous.
Many of us, including me, can't get over our early days of 283's, 327's, 350's and putting on duals and a 4 bbl. We can't remember that in the 60's, fuel economy was not a real issue and V8 motors were build with some rev left in them up until 1972/73.
305 V6 is not that world. It is however a neat motor for reliability, nostalgia and correctness for 6066. For revs, no good, motor weighs 800 and all the rotating components are heavy except the aluminum pistons. That gives two obstacles for us speed freaks, rolling mass weight and rotating assembly weight.

Re: Proper sized exhaust
 

I have been very happy with my Holley conversion. The truck starts easier, is smoother through the RPM range and more responsive. Part that i really like is that the Holley is easily tune-able as far as changing jets, so one can dial it in.

Re: Proper sized exhaust
 

My 305 with WW starts in 1/2 revolution and does not need adjusting. Just to give both sides.

Re: Proper sized exhaust
 

Over the years, in my engine building and drag racing experience, I found many times that a carburetor with a larger flow rating than what was calculated made the car faster against the clocks.

Now, whether that was because of actual VE verses an arbitrary value of VE for calculation, I do not know. I, like you, did many 2 bbl to 4 bbl conversions along with dual exhaust and in all cases, saw a noticeable increase in power.

Even the lowly 6 cylinder Chevy reacted positively to an increase in breathing ability, just as a 305 V6 will.

Re: Proper sized exhaust
 

OK. Understand your position. Thanks Bill, good food for thought.
Ken

Re: Proper sized exhaust
 

Jake, here is a short video of my exhaust system. It's not very loud inside or out either (at least not to me). I wanted something rumbly but not droning . It has a nice burble to it; I think. Dual 2 1/4" head pipes to 2 1/2" with an X-Pipe/Hooker Aero-Chamber mufflers and routed all they way to the back bumper with some chrome tips. I was revving it up to about 2200 rpm's.

The exhaust guy did a pretty good job of keeping it tucked up and you can see how he turned the mufflers sideways to fit in the gap just ahead of the rear axle. The torsion bars are turned down a little and I have 2" drop blocks in back.

I have the Holley 500 as well. The loudest sound inside is the Carter electric fuel pump mounted underneath by the Fuel filter.
I shot this with a Sony HDR MV1 so it has two microphones in an XY pattern; I use it mostly for band videos and rehearsals.