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@IROCZman15 ’s thread got me thinking as I’m doing a similar setup for the exhaust behind my 383. I have Hooker 2210s to which I’ll be adding 3” collector pipes into a Flowmaster 4” y then into a Mufflex 4” system with a Hooker Max Flow muffler. I’m going to try to add a Flowmaster Outlaw muffler into the I-pipe if I can get it to fit since I already have it.
I say all that to say that I’m planning on having a local exhaust shop fab up the y-pipe using press bends as opposed to taking it to Hawk’s for a stainless mandrel bent y-pipe. I haven’t seen any definitive dyno data to show a hp gain going from a smooth press bent system to a mandrel bent system. It logically makes sense that a mandrel bent system would make more power, but is that only the case when the bend breaks the 2.2 cfm/hp rule?
I understand the why from a fluid dynamics standpoint, but I figured there would be good data somewhere outside of three graphs from Bob’s Muffler Shop.
Seems like I remember years ago where MagnaFlow claimed that crush bends reduced the internal area of the pipe approximately 20%. I see where Bob says that the crush pipe reduces a 3" pipe to an approximate 2.5" pipe. If you do the math, those numbers comes close. Looking at the flow numbers of a straight 3" pipe at 878cfm and a 2.5" pipe at 610cfm, that's about a 30% reduction in flow, which is also close to what Bob's website claims.
So, assuming all that is accurate, it looks like if you crush bend a 3" pipe you'll end up having about the flow equivalent of a 2.5" mandrel bent pipe. If you're not looking to make over 500HP, I'd seem to think that crush bent 3" pipes (as long as the bends aren't tight) should be "OK" for that power level. That said, you might as well connect those to a single 3.5" mandrel bent pipe as it would would flow about the same as the two crush bent 3" pipes.
On short runs like a y pipe, im not sure id worry too much about it. As said, it reduces area around the bend but you’d have to measure to get an idea of how much. I think the radius of the bend will matter more, if you can keep it smoother in the direction of flow it will help reduce loss from say tight 90 deg turns. If you look at pics of mufflex y pipe it doesnt have much bend to it which is good so the small area reduction may not hurt as much. For most 383 in the 450-500 hp range there probably isnt much to be had there. I would guess 10 hp or so maybe…but its hard to say. Most cars i seen make 10-20 hp with a open y pipe cutout vs full exhaust. So minor change in area in the ypipe region likely wont have much left to gain.
As a follow up to this, I happened to stumble across Engine Masters S6 E4 where they tested this. On a 593hp LS motor, they lost 1.7 hp and 3-4 lb-ft at some points through the curve (peak torque was down 2-3 lb-ft) when comparing a dual 3” press bent over-axle system vs. the same system with mandrel bends.
The press bent system had smooth bends like the local shop here uses so I feel safe saying I’m losing fewer than 2 hp if any.
As a follow up to this, I happened to stumble across Engine Masters S6 E4 where they tested this.
I watched that episode today. Interesting results. My predictions were not as dire as Freiburger's, but I was way off none the less. Always learn interesting things on that show. I wish they had time to explore more of their theories.
I watched that episode today. Interesting results. My predictions were not as dire as Freiburger's, but I was way off none the less. Always learn interesting things on that show. I wish they had time to explore more of their theories.
I was pretty shocked by the results as well, and I agree. I love being able to watch people test things back to back like this and the air filter test they did.
I watched that episode today. Interesting results. My predictions were not as dire as Freiburger's, but I was way off none the less. Always learn interesting things on that show. I wish they had time to explore more of their theories.
I wish they had tested the 2.5 vs 3" systems they tested on the big block with a crossover. The dual 2.5" system was only down a little power compared to a dual 3". Personally feel a good crossover would have brought the 2.5" system up to the dual 3" performance.
If my van had not come with dual 3" from the factory, I would have used dual 2.5" on my ~500 hp 383.
I wish they had tested the 2.5 vs 3" systems they tested on the big block with a crossover. The dual 2.5" system was only down a little power compared to a dual 3". Personally feel a good crossover would have brought the 2.5" system up to the dual 3" performance.
If my van had not come with dual 3" from the factory, I would have used dual 2.5" on my ~500 hp 383.
what kind of van came from the factory with dual 3" exhaust?
Post header install. We swapped the tubing into the high flow cats left and right and staggered the cats for better clearence.
Leads back to a dual 3" in, single 4" out walker ultra quiet and matching 4" tail pipe for a 2001-2006 3500 HD that I just swapped on it. Still need to take it and get it welded up.
It is very quiet for the power it makes and the system flows extremely well.
https://youtu.be/zJBiw3xkiYY
