What size exhaust?

[z 28 jari]

These are exhaust systems what I have consider,both of them need to be modified from x-pipe to collector.

Holley 2x2.5": https://www.holley.com/products/exhaust ... 01426-RHKR

speed engineering 2x3": https://www.speed-engineering.com/shop- ... -exit.html

[enigma57]

Hi,
I need to upgrade my exhaust after headers at my -85 Camaro.It has 385sbc,and it has gone 12.25/114mph with current exhaust(3700lbs with driver).It has now 1 3/4" long tube headers,2,5"y-pipe and rest after y-pipe is single 3" pipe with dynomax muffler.It is weekend/fun toy.I probably put dual system to it,but pipe size is question.What do you recommended?

2.5" duals are a real hassle to install under a 3rd gen F-body whilst maintaining sufficient ground clearance. My son has a street driven '92 Camaro with mildly modified 350 engine and SLP exhaust with same header and exhaust sizing and type muffler as yours. Puts out an honest 360 HP at flywheel. Single 3" system from Y-pipe back is really all you need for your 385 HP 350 crate engine. If you have plans to upgrade cam at some point in future and increase power beyond 400 HP, you can always run a single 3.5" system from Y-pipe back. Muffler choices will be more limited though and it will be louder unless you split pipe before muffler inlets and run a pair of transverse mounted 2.5" mufflers. DynoMax Super Turbo mufflers will be quieter than most whilst equaling or exceeding flow of Flowmasters and a great many others.

Hope this helps,

Harry

[midnightbluS10]

Hi,
I need to upgrade my exhaust after headers at my -85 Camaro.It has 385sbc,and it has gone 12.25/114mph with current exhaust(3700lbs with driver).It has now 1 3/4" long tube headers,2,5"y-pipe and rest after y-pipe is single 3" pipe with dynomax muffler.It is weekend/fun toy.I probably put dual system to it,but pipe size is question.What do you recommended?

2.5" duals are a real hassle to install under a 3rd gen F-body whilst maintaining sufficient ground clearance. My son has a street driven '92 Camaro with mildly modified 350 engine and SLP exhaust with same header and exhaust sizing and type muffler as yours. Puts out an honest 360 HP at flywheel. Single 3" system from Y-pipe back is really all you need for your 385 HP 350 crate engine. If you have plans to upgrade cam at some point in future and increase power beyond 400 HP, you can always run a single 3.5" system from Y-pipe back. Muffler choices will be more limited though and it will be louder unless you split pipe before muffler inlets and run a pair of transverse mounted 2.5" mufflers. DynoMax Super Turbo mufflers will be quieter than most whilst equaling or exceeding flow of Flowmasters and a great many others.

Hope this helps,

Harry

Where'd you see anything about a 385hp, 350ci crate engine? He said it was a 385 sbc with no stated power level(quoted in bold above) <shrug> and by my calculations, he's past 400 fwhp at this point already.



Dual 2.5" will support around 500hp
Single 4" will support around 650hp
Dual 3" will support around 800hp


12.2's @ 3700 lbs through a few online calculators averages around 400 rwhp. If that's the case, I'd go for at least single 3" in order to give yourself room to grow. Dual 2.5's are going to be right at the verge of being to small, imo.

[hoffman900]

A single 3” will support almost 650hp. Need a different rule of thumb.

[cjperformance]

A single 3” will support almost 650hp. Need a different rule of thumb.

Well that depends greatly on the quality of the entire exhaust system!

[digger]

A single 3” will support almost 650hp. Need a different rule of thumb.

Well that depends greatly on the quality of the entire exhaust system!

And the CR and the lobe profile and system length

[cjperformance]

A single 3” will support almost 650hp. Need a different rule of thumb.

Well that depends greatly on the quality of the entire exhaust system!

And the CR and the lobe profile and system length

Totally!

[enigma57]

Hi,
I need to upgrade my exhaust after headers at my -85 Camaro.It has 385sbc,and it has gone 12.25/114mph with current exhaust(3700lbs with driver).It has now 1 3/4" long tube headers,2,5"y-pipe and rest after y-pipe is single 3" pipe with dynomax muffler.It is weekend/fun toy.I probably put dual system to it,but pipe size is question.What do you recommended?

2.5" duals are a real hassle to install under a 3rd gen F-body whilst maintaining sufficient ground clearance. My son has a street driven '92 Camaro with mildly modified 350 engine and SLP exhaust with same header and exhaust sizing and type muffler as yours. Puts out an honest 360 HP at flywheel. Single 3" system from Y-pipe back is really all you need for your 385 HP 350 crate engine. If you have plans to upgrade cam at some point in future and increase power beyond 400 HP, you can always run a single 3.5" system from Y-pipe back. Muffler choices will be more limited though and it will be louder unless you split pipe before muffler inlets and run a pair of transverse mounted 2.5" mufflers. DynoMax Super Turbo mufflers will be quieter than most whilst equaling or exceeding flow of Flowmasters and a great many others.

Hope this helps,

Harry

Where'd you see anything about a 385hp, 350ci crate engine? He said it was a 385 sbc with no stated power level(quoted in bold above) <shrug> and by my calculations, he's past 400 fwhp at this point already.


Dual 2.5" will support around 500hp
Single 4" will support around 650hp
Dual 3" will support around 800hp


12.2's @ 3700 lbs through a few online calculators averages around 400 rwhp. If that's the case, I'd go for at least single 3" in order to give yourself room to grow. Dual 2.5's are going to be right at the verge of being to small, imo.

Around these parts, a '385 sbc' usually refers to the 350 crate engine GMPP rates at 385 HP......

https://paceperformance.com/i-6255259-1 ... ngine.html

https://www.summitracing.com/parts/nal-19201331

This 385 crate engine is the basis for the sealed '604' circle track crate engine as well......

https://paceperformance.com/i-13561403- ... ngine.html

If I was incorrect in assuming the OP meant that, I reckon he will post here before long and straighten me out on that.

Your projected HP levels supported by pipe sizing posted above will depend upon a great many variables. In my experience, a full length muffled exhaust system would require larger pipe sizes to support those power levels, though. Exhaust system sizing charts based upon rough rules of thumb have been posted by several sources, some more conservative than others.

Here is one published by Flowmaster which in my opinion is on the conservative side......

https://www.hotrod.com/articles/what-ex ... work-best/

And here is one that works out pretty close to what the math supports......



But really, rough rules of thumb are just that...... Rough rules of thumb. A more accurate way of sizing the system downstream of collectors would be to work out pipe sizing based upon flow requirements at redline and factor in variables such as friction losses due to bends in system, flow capacity of the mufflers, etc.

David Vizard who posts here has done quite a bit of work in this area, some of which is touched on in this article......

https://www.musclecardiy.com/performanc ... ail-pipes/

A section of straight pipe the length of a typical muffler, rated at the same test pressure as a carb (1-1⁄2 inches of mercury), flows about 115 cfm per square inch. Given this flow rating, we see about 560 cfm from a 2-1⁄2-inch pipe. If we have a 2-1⁄2–inch muffler that flows 400 cfm, the engine reacts to this just the same as it does to a piece of straight pipe flowing 400 cfm. At 115 cfm per square inch, that’s the equivalent of a pipe only 2.1 inches in diameter.

The goal being 2.2cfm of flow per HP generated for a zero sum loss system.

400 hp X 2.2 cfm = 880 cfm divided by 115 cfm/sq. in. = 3.12" round cross section or ID (inside diameter) of exhaust tubing.

Nominal pipe sizing is measured by ID, but tube sizing as used in exhaust systems is measured by true OD (outside diameter).

16 gauge exhaust tubing has a wall thickness of 1/16" or 0.0625". So we add (2) wall thicknesses (1/8" or 0.125") to 3.12" and we have 3.245" OD tubing. So we round up to the next commercially available tubing size (3.50") and that should support 400 HP (single, straight pipe)...... So long as we minimize friction losses by making as few bends as possible to fit pipe under car and when we must make a bend, to use mandrel bends having as large a radius as possible.

880 cfm divided by 2 = 440 cfm divided by 118cfm/sq. in. = 2.20" round cross section (ID) = 0.125" for (2) wall thicknesses = 2.325" OD. So we round up to the next commercially available exhaust tubing size (2.50") for dual pipes.

Next order of business will be to find a couple mufflers that will silence the exhaust sufficiently whilst flowing a minimum of 2.2 cfm per HP generated. Meaning each muffler (dual exhaust system) must flow a minimum of 440 cfm to support 400 HP. In actual practice, I have found that this may require stepping up to the next larger internal core tube sizing (run mufflers having 3.00" internal core tubing using pipe increasers to fit them to 2.50" dia. exhaust tubing). And yes, running the larger dia. inner core will make the mufflers louder than running a 2.50" core. But that can amount to 20 HP or more at redline. So on a road car, a choice must be made.

Anyway...... As to pipe sizing downstream of collectors...... 400 HP requires either a single 3.50" OD tubing system or dual 2.50" OD tubing system with mufflers having 440 cfm min. flow capacity (each) as near as I can figure it. Will a 3.50" single or a 2.50" dual exhaust system support more than 400 HP? Of course. But as HP (and flow volume) increase, back pressure in the system will begin increasing as well and at power levels exceeding 400 HP there will be some HP losses incurred if pipe sizing and/or flow cpacity of muffler(s) are not increased to match the increased flow rate.

Hope this makes sense. As an old retired pipefitter, that's the best I can explain it. If you need a more detailed explanation, I am sure there are lots of guys here who went to college and are better at math than I.

A very Happy Thanksgiving to all,

Harry

[z 28 jari]

I'm not sure about current power level,but previous version of this engine was 355cid(engine dyno sheet of that engine was 413hp/439torque),and it trapped 12.7/107mph (3700lbs).Current version has more cam (230/236), ported heads+intake(total flow 262cfm),+30cid disaplement,compression ratio went from 10.2 to 10.6 and these headers:
https://www.ebay.com/itm/Maximizer-Head ... :rk:1:pf:0
Is there any difference at power is transition at collector(3" -> 2.5")was smoother?

Chassis and transmission are same as previous engine.

[enigma57]

Jari, I was incorrect in assuming that you were referring to the GMPP 385 crate engine. I believe midnightbluS10 is right about your engine exceeding 400 HP. My apologies.

Yes, any change in cross sectional area or transition in shape is better done as gradually as possible. However, it sounds as if you are likely in the 450 HP range. In that case, I would suggest either a dual 3.00" exhaust system or an exhaust system having 3.00" from collectors to Y-pipe and a single 4.00" system downstream of your Y-pipe if you want to get all the power you can from a full length muffled exhaust system.

Best regards,

Harry

[gmc406]

My current experience. I have an 89 Camaro with full interior, suprising it to weighs the same with me in it. I run a mild 420sbc with a 700r-4. In 2500 feet of air it runs 11.91 at 116ish. I run 3 inch pipes off the headers into a single 3 inch to the Flowmaster muffler with dual 2.5 tail pipes. The exhaust is all manderal bent. I pick up a tenth by uncorking the exhaust. I blame the “Blowmaster” muffler for the increase.

Ground clearance sux, I have to watch where I drive, but there isn’t a whole bunch you can do as far as exhaust options on these cars.