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.

Re: Proper sized exhaust
 

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.

[/QUOTE]

Turbobill. I fully agree with you on that:upyes: Years ago I tested all 3 of these carbs on my 68 305E and the Holley 500 was still by far the top performer, it's all about the real world results:thumbsup:

Re: Proper sized exhaust
 

Back to JB's question. Funky's video spells it out in the opening screen: 2 1/4" opening up to 2 1/2", with an X-pipe and dual exhaust. You can change the sound with different mufflers and pipes after the mufflers. Thanks Funky, I enjoyed every note. The PO of my truck put the mufflers under the cab--with no pipes after. Talk about loud with a "Wheels of Tragedy" scenario.

Re: Proper sized exhaust
 

My exhaust manifolds measured 2.25 so I went with 2.25 inch pipes to the muffler and exited the muffler with 2.5 inch tail pipes. True duals. No cross over. :ok:

https://www.youtube.com/watch?v=RCfanx30JO8

Re: Proper sized exhaust
 

I always thought those 40's would sound too loud ,
but they seem tame enough on you video TJ.
How are they on the highway?

Re: Proper sized exhaust
 

I am a young buck so loud may have a different meaning to me. haha

Personally I like a little bit of Loud so it may be harder for me to say. The 40's were actually Quieter than I expected. Cruising down the highway at 2400 RPM they are very quiet. At 2000 RPM they Can get a little annoying if you don't mind some cab noise. But floor that sucker and rev it out and at 2500-3500 that tone will bounce off of fences and canyons and put a big smile on your face. lol I will be doing a wot run/driving video soon.

Re: Proper sized exhaust
 

[/QUOTE]Turbobill. I fully agree with you on that:upyes: Years ago I tested all 3 of these carbs on my 68 305E and the Holley 500 was still by far the top performer, it's all about the real world results:thumbsup:[/QUOTE]

Exactly!!!! Math results aren't Everything.

According to Math the small 2 stroke chainsaw engine is not supposed to even run. BUT! They run very well and very efficiently. Real world is what matters.

Math would show that my 600 edelbrock 4 barrel would drown my 305 out, yet it performs rather well! :thumbsup:

Re: Proper sized exhaust
 

Here is a video of a 305 with flowmaster 50's. Bout the same, but at RPM they are supposed to be tamer than the 40's.

https://www.youtube.com/watch?v=FkpTLxOj-rI

Re: Proper sized exhaust
 

Ok thanks Guys, I'll go the dual exhaust route with 21/2 pipes and a cross over.

Jake

Re: Proper sized exhaust
 

Be sure to post a video!

Re: Proper sized exhaust
 

I'm pretty happy with 2 1/2 inch duals and 50 series mufflers on my 68 478M, of course that's the same system when I had the 305E, In the future I may switch over to 3 inch pipes and 40 series mufflers:teehee:

Re: Proper sized exhaust
 

That 478 will love 3 inch. haha There's actually a guy who's done it. Not sure if you've seen the vid. https://www.youtube.com/watch?v=liKIlN9Z9Jg

I've been wanting to go back to the exhaust guy and try some other mufflers for sound reference.

Re: Proper sized exhaust
 

So if the stock Stromberg is in the 300 CFM range, math shows that it is too small. The normal old go to "(CID X RPM X VE)/3456 shows that we only need a 285 CFM carb at 3800 RPM with 85% VE. But, that is not the whole story. Do some looking around, and you will find that 4 barrel carb CFM ratings are at 1.5 inches of mercury while 2 barrel carb ratings are at 3 inches of mercury. At wide open throttle on an engine aimed at performance, we don't want 3 or even 1.5 inches of pressure drop. Eight to nine tenths of an inch is much better so we have to adjust our 285 CFM.

To do so, we need to divide the 285 CFM by the square root of the quotient of our desired vacuum (.8) over the rated vacuum (3). This shows us that we need a two barrel carburetor "rated" at 552 CFM.

Do the math with a 4 barrel carb and we would need one "rated" at 390 CFM - I think Holley even happens to make a 390 CFM 4 barrel...

So yes, you should see a large performance increase from your stock 300-ish CFM carb to a two barrel "rated" at 500 CFM.

Re: Proper sized exhaust
 

Are you sure you are a Red neck farm hand? I would guess a math professor.

Re: Proper sized exhaust
 

Least this is a man who actually Knows his math! :welldone: