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Old 10-11-2018, 09:44 PM   #1
parb
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2.5" vs. 2" exhaust

Hi all,

i'm in the final stretch of restoring my 72 GMC Jimmy. I have gotten from a member here a set of nice ram horns for the engine.
My plan is to drop them down into a 2.5" exhaust system with borla mufflers (i have borla's on another car and i like the sound, plus i like the guy behind borla -chatted to him and he is a real nice guy).

I'm not the best mechanic so i am having a mechanic help me out. he told me earlier today that going for a 2.5" exhaust may not be the best, and that those old 350 chevy v8's needed a bit of back pressure to run right. He suggested 2" as the right size for my exhaust.

Everything i've read on this forum suggest that 2.5" is a good choice for this engine.

What do you all think? Am i making a mistake going with 2.5" inch exhaust from the ram horns all the way to the exhaust tips and i should really make that into a 2" exhaust?

I still have some time to change my mind if that is the right choice, but i would love your inputs. especially those who have put on a 2.5" system.

My engine otherwise is bone stock.

thanks!
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Old 10-11-2018, 11:06 PM   #2
truckster
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Re: 2.5" vs. 2" exhaust

"Engines need back pressure" is a myth.

https://www.youtube.com/watch?v=jjPeP_Nn2B4

Go with the 2.5. If you go with the 2 inch you'll always think, "I could have gone bigger." I've never heard anyone say, "I wish I had smaller exhaust."
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Old 10-11-2018, 11:21 PM   #3
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Re: 2.5" vs. 2" exhaust

Go 2-1/2"
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Old 10-12-2018, 12:47 AM   #4
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Re: 2.5" vs. 2" exhaust

thanks!!
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Old 10-12-2018, 06:28 AM   #5
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Re: 2.5" vs. 2" exhaust

A 350ci with a 2" or 2.25" exhaust will develop more low end torque than with a 2.5" exhaust, a 2.5" exhaust helps develop more horsepower in upper ranges than a 2 or 2.25 exhaust. Most trucks benefit more from torque than horsepower in daily use.
We generally use 2 or 2.25 on stock to mild 350ci, if it's being built to race and heavily modified we then step up to the 2.5" exhaust.
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Last edited by GMCPaul; 10-12-2018 at 06:47 AM.
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Old 10-17-2018, 02:53 PM   #6
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Re: 2.5" vs. 2" exhaust

If you do decide to go with a 2.5" exhaust, I suggest replacing your stock 2" ram horn exhaust manifolds with a set of 2.5" ram horn manifolds to take full advantage of the bigger diameter tubing. These are reproduced by Dorman and are a direct bolt on.
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Old 10-17-2018, 07:17 PM   #7
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Re: 2.5" vs. 2" exhaust

I've always done my 350 dual exhaust in 2 1/4" unless it was a true beast of an engine
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Old 10-18-2018, 01:54 PM   #8
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Re: 2.5" vs. 2" exhaust

Quote:
Originally Posted by cleszkie View Post
If you do decide to go with a 2.5" exhaust, I suggest replacing your stock 2" ram horn exhaust manifolds with a set of 2.5" ram horn manifolds to take full advantage of the bigger diameter tubing. These are reproduced by Dorman and are a direct bolt on.
I sold him my Dorman 2.5" corvette style manifolds. He's all set.
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Old 10-18-2018, 04:05 PM   #9
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Re: 2.5" vs. 2" exhaust

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I sold him my Dorman 2.5" corvette style manifolds. He's all set.
I think he's going to be very happy with that solution.
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Old 10-18-2018, 06:24 PM   #10
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Re: 2.5" vs. 2" exhaust

That whole backpressure thing is a myth. An engine is an air pump and putting a cork in the backside is of no assistance.

Go with the 2 1/2". With a compression bend exhaust like a local muffler shop will make, it's really equivalent to a 2 1/4" mandrel bent setup.

The 2 1/2" rams horns and 2 1/2" exhaust is what I did on my Jimmy with some Dynomax Hemi Turbo mufflers (long 20" case) and I have been super happy with that setup and the "butt-o-meter" suggests no power fall off compared to the Heddman headers and 2 1/4" exhaust it replaced. We are doing exactly the same on my kid's Jimmy project except running some more teenager centric Magnaflow mufflers that came off a parts truck I had.
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Old 10-20-2018, 10:14 PM   #11
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Re: 2.5" vs. 2" exhaust

Quote:
Originally Posted by Mike C View Post
That whole backpressure thing is a myth. An engine is an air pump and putting a cork in the backside is of no assistance.

Go with the 2 1/2". With a compression bend exhaust like a local muffler shop will make, it's really equivalent to a 2 1/4" mandrel bent setup.

The 2 1/2" rams horns and 2 1/2" exhaust is what I did on my Jimmy with some Dynomax Hemi Turbo mufflers (long 20" case) and I have been super happy with that setup and the "butt-o-meter" suggests no power fall off compared to the Heddman headers and 2 1/4" exhaust it replaced. We are doing exactly the same on my kid's Jimmy project except running some more teenager centric Magnaflow mufflers that came off a parts truck I had.
I'm an engineer by training so I read up on the science of it. The backpressure stuff is a myth except in a narrow band in two stroke engines. What is important in a manifold is the design so that airflow creates pressure drops where the ports meet to scavenge the gas outwards into the exhaust. But you need to reduce pressure in the exhaust as you push air through it and wider diameter system does that really well.

Putting backpressure behind a big carb and a scavenging manifold is like inhaling through a giant hose and exhaling through a tiny straw, it simply reduces performance.

There is an excellent video posted by the hot rod network where they change exhaust from 2" to 3" to open pipes whith the same engine sitting in an engine dynoamometer.

Open pipes creates better performance in all aspects except a narrow rpm band below 2000 rpm where they theorized that the manifold of the 2" system scavenges really efficiently. But add 500 rpm and overall performance increases by 30% making that narrow band of efficiency pointless. The 3" system lost what looked like 5% vs open pipes. The 2" system lost 15%. A fairly siszeable loss for just a few lbs of more metal in the exhaust.

Either way, I enjoyed reading up on the science of it. 2.5" pipes is what I'm going with. I'm using borla mufflers at 2.5" in and out. I have borlas on my Dodge viper and I have been really happy with them.

Last edited by parb; 10-20-2018 at 10:23 PM.
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Old 10-20-2018, 11:16 PM   #12
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Re: 2.5" vs. 2" exhaust

I'm not convinced that a 2" or 2-1/4" down pipe bolted to an exhaust manifold makes more torque at any RPM than a 2.5" pipe. Even the best cast iron manifolds have so much restriction (therefore velocity), that a large pipe bolted to the manifold flange is nothing but a relief!

Now if we are talking about header primary tube size, that's a very different story because those pipes are bolted directly to the exhaust ports. But even with headers, once you get past the primary tubes, the bigger the better.

Someone above knows his stuff when he mentioned exhaust back-pressure being critical in a high performance two-stoke engine. It's the only way to keep the intake charge from getting sucked out the exhaust port at low-mid RPMs. Modern two-stokes use slide valves that seal off the top 1/3 or so of the exhaust port until RPMs build.
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Old 10-21-2018, 09:47 AM   #13
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Re: 2.5" vs. 2" exhaust

Is there an exhaust manufacturer that makes an complete 2.5" header back exhaust system for a 69-72 blazer?
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Old 10-21-2018, 10:17 AM   #14
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Re: 2.5" vs. 2" exhaust

header back - not that I have found. Manifold (center dump) back = yes (but in 2.25")

I have used the Heartthrob Exhaust kit on several of my blazers with no issue.

Install Pix - http://www.heartthrobexhaust.com/4159M.html

Install Instructions Link

They have both steel and stainless - Heartthrob Exhaust Page

Last time I bought one it was ~$550 shipped or so a few years back.
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Old 10-21-2018, 10:27 AM   #15
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Re: 2.5" vs. 2" exhaust

You might want to try HeartThrob Exhaust. I ordered their kit for my 82 C10 through one of their distributors for a very low price, a little over $200 shipped as I recall. Pipes are 2-1/4" mandrel bent with a choice of tailpipe exit -- sides or straight back. I specified their stock replacement mufflers, which sounded weird, so ended up replacing them with Walker Dynomax Super Turbos.

http://www.heartthrobexhaust.com/

Call them. They are very helpful. I actually spoke with the guy who said he'd be the person bending the pipes for my kit.

Please know that a header back kit will probably need some mods, because all headers are different.

Edit: I think the dealer I bought from was Auto Accessories Garage. They had a new customer discount and free shipping, although the kit shipped directly from HeartThrob.
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Old 10-21-2018, 11:29 AM   #16
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Re: 2.5" vs. 2" exhaust

I am going to address this to parb since he started this in hopes it won't be a hijack! Did you see any research that addressed bends and restriction? I have to play with water velocities and flow rates with plumbing and I know that bigger pipes (2 12"-3" in my applications flow much more efficiently than 1 1/2") but I also have to be aware of bends. In my case, each 90 equates to ten feet of pipe. I am just wondering if the hard bend (for example) coming off of the exhaust manifold (be it header or stock manifold) creates any concern on the engine or what occurs after that point.

Interesting conversation, with many smarter guys on here than me!
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Old 10-21-2018, 01:29 PM   #17
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Re: 2.5" vs. 2" exhaust

The larger the bend radius the better. But if you don't have a lot of room for a large, gradual bend at least use a mandrel bent section for the down pipe instead of a muffler shop "crush bend". I would bet that a 2-1/4" system with mandrel bends throughout will outflow a 2-1/2" system with crush bends.

On another note, I think it was David Vizard who did some flow measurement tests using mufflers that were one size larger than the exhaust pipes. Even if the mufflers are the straight-through type, going to a larger size noticeably increased flow. (Of course you'll need adapters.) And don't even use a stock-type muffler that necks down internally. The 2-1/4" Heartthrob mufflers I mentioned above necked down to approximately 1-7/8" internally and I have seen that with other cheap OE-replacement mufflers. I would have been better off with a single 2-1/2" system!
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Old 10-22-2018, 04:05 AM   #18
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Re: 2.5" vs. 2" exhaust

This is right. Again, not being an expert but I do a fair amount of work in mathematics in my day job so I read a couple of papers on fluid dynamics to educate myself over the weekend. The science says that when you reduce the pipe dimension you need to increase the velocity to exhaust the same volume of gas. Pretty intuitive for most. There is more effort involved to increase the velocity so the engine has to work harder to exhaust the gas. There is no back pressure per se, just increases in velocity which gets more sensitive to the transitions from laminar to turbulent flows which simplified is what causes the effort to move gas through a pipe to be increased. Or even more fundamentally, is friction in the gas and across surfaces in the pipe that converts motion to heat and energy losses in the gas velocity. https://en.m.wikipedia.org/wiki/Reynolds_number

The bends are particularly interesting to me. In the outer radius of a bend the gas has to travel farther. Kind of like the outer track in an oval race track, is a longer distance. But gas is occupying the entire cross section of the tube in the bend, and so the gas has different friction between the molecules on the inside track vs the outside track. This creates turbulence and that means that we will lose velocity and have to increase energy to move the gas to compensate for the losses.

The bends creates Eddie's, little waves of turbulence in the gas stream. This also takes more effort to push gas through due to friction (velocity losses due to friction converting motion to heat). In addition thee shorter the radius the more the shearing friction between the different velocities of the gas in motion leading different pressure gradients across the section of pipe. But this pressure affects the overall effort for gas to go through the entire pipe, thus bends, and especially tight bends increase overall effort of pushing gas through a pipe.

To sum it up. Big diameter pipes are better. Long radius in bends are better. Fewer bends are better. It's just about as simple as that. Ok. I simplified it a lot, gas from an engine is hot and compresses, it also cools across the exhaust system. This is the math of Navier-Stokes and its a whole field until itself that I claim no expertise in... https://en.m.wikipedia.org/wiki/Deri...okes_equations

I'm pretty sure that the doorman horn type exhaust manifolds work well because they create eddies that scavenge the manifold opening on neighboring cylinders when the engine pulses gas into the manifold.

Who would have known that a simple exhaust has this much science to it?

Last edited by parb; 10-22-2018 at 04:16 AM.
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Old 10-22-2018, 10:18 AM   #19
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Re: 2.5" vs. 2" exhaust

Interesting stuff!
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Old 10-22-2018, 12:15 PM   #20
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Re: 2.5" vs. 2" exhaust

Very cool and well explained for the layman!
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Old 10-22-2018, 01:14 PM   #21
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Re: 2.5" vs. 2" exhaust

Quote:
Originally Posted by parb View Post

To sum it up. Big diameter pipes are better. Long radius in bends are better. Fewer bends are better. It's just about as simple as that.
That's 100% true. I'm no expert either, but here's what I earned selling industrial valves and flow meters for 5 years: Piping bends mess with a flow meter so badly that you can't get an accurate flow measurement unless the meter is installed in a straight section of pipe. And the meter must be installed several "pipe diameters" (as many as 10) distant from piping bends.

Same is true of control valves. They need straight sections of pipe before and after to work properly, reduce noise, and event prevent damage.

TMI?
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