Depends on horsepower and performance goals. Most street big blocks with 500 or less HP don't need 3" exhaust IMO.
I got 3" because my M/T repro headers are 3.5" at the collector and Torque Tech sold a kit that fits for not much more than 2.5" so I did. I don't feel like the 3" is holding me back and the last time I raced I didn't bother to even uncap it, just swapped on the slicks and ran it. Without accounting for weather it ran within a 1/10 of uncapped.
When my poston's 1 7/8 tube headers finally couldnt be fix anymore......was so thin I couldn't weld the cracks without burning through instantly. We bought ta''s 2 1/8 tube headers. They actually fit better and all the bolts were easier to get to besides the middle one and rear one. But we picked up a tenth from just them, motor sounded so much different. I find it hard the see how even in 500 hp how the 3 inch would cost power over the 2.5 inch.
Oh ya it does! My friends comment that my car sounds wicked when I leave and get on it a bit. It sounded good with the 2.5 also, but the 3 inch is awesome.
As heavy as our cars ours what is 8 or 10 pounds. The 3 inch does get clearance issues sometimes around parts or ground. A 2.5 inch pipe has area of 4.9 Sq in. A 3 in pipe has a little over 7 Sq in area. Almost a 45% increase
So with me running manifolds is it worth it to go 3"? I have the old gsca 2.5" setup but the tailpipes are all bent up(a whole nother story). I was thinking of just getting the three inch tail pipes and moving forward from there using the 2.5" mid section. Silly? Looks like I can get the 3" tailpipes for about $170 shipped.
With running the manifolds I doubt the 3 in tails will help any. The manifolds and existing 2.5 inch pipes will already flow less than the rear pipes.........so don't look for the 3 inch tails to gain any power over what the 2.5 did have b4 the pipes got bent. In general when deal with exhaust, or basically anything with inside daimeter....hoses, fuel line etc. Once you drop to a smaller size in the middle going larger later won't flow any more really than the smaller front size. It just doesn't continue the restriction. Now the game changes some with pressure like fuel or boosted air. But the smallest id will always be were the biggest choke point is....and once the flow at a given pressure is max out.........nothing will really change that. I the example in the video, the headers has 2 inch pipe per cylinder, so each cylinder has 3.14 Sq inch to flow through at the highest pressure area.....so a total of 12.56 Sq in feeding into a 3.5 in collector for 9.61 sq in then to 3 in pipe for 7.07 sq in......or 4.91 sq in for the 2.5. Its easy to see how the 2.5 pipe really bottle necks it down. If they would run a 3.5 inch exhaust I bet they could get same numbers as open headers. Now what will 10hp or so loss equate to on the track.........not much Ever notice top fuel doest run a collector......cause it's a restriction
With a big balance tube or a 180 system I bet there wouldn't be much if any gain with the bigger pipes. Merge collectors help too. Most of the critical flow is ahead of the mufflers/exhaust system IMO.
The biggest pipe needs to be closest to the motor. As it leaves the motor, it immediately begins to cool. As it cools, it has less volume and therefore needs less pipe to maintain the same capacity. Additionally, each cylinder only needs access to the collector every other revolution so a smaller collector isn't a huge handicap. A 1 7/8" header pipe has an area of 2.76"sq. A 3.5" collector has an area of 9.62"sq. That's a ratio of about 3.5:1 so it is well sized to handle 4 intermittent pulses. Having said that, 3" tailpipes aren't even required on 3" header back systems. By the time the exhaust leaves the muffler, one size smaller pipe is sufficient. 3" looks meaner but isn't a requirement.
I run 3" mandrel bent exhaust out to the rear bumper. I used to have it dump at the axle. Can't compare E.T.s with the previous 'dumping at the axle' pipes it had because we made a couple 'other minor changes at Finishline and Eric (Horsepower Innovations) and I are swapping on an E85 carb. I changed to a full system because it is just a slowass street car and I wanted to actually carry on a conversation while driving. And wife drives it occasionally.
Well I can say for certain, on more than once I have forgotten the generator in my trunk. It's where I put it on long trips since I have an open trailer and my trunk locks and is big enough to fit it. It's not a big generator, but it's moreally than 10 pounds......I would guess close to 75. My car doesn't slow down at all. In fact sometimes on slick tracks I put it back in to help put a touch more weight over the tires. My car is 3850 with me in it 10 pounds doesn't mean squat on my car....that's like 1.5 gals of gas.......and I don't race heads up, so the .005 it might really make is close enough to 0 to not worry about........but the extra 20hp it might make on a motor that needed it would make some time up.......... So if I had to carry 10 pounds to make 20 extra hp, that's a no brainer, I'm not a pro stk car, but I bet even they would car 10 extra pounds evenly distributed to get 20 more hp.....and ever .001 means something to them
A first hand test was done 20 years ago on my sons mid 12 sec. street car. He had SP2 cam with poston 1 7/8 headers and 3.73 gear. With 3" exhaust vs open header on the same day, 2 each runs back to back, the full exhaust only slowed down less than .05 seconds in the quarter.
I know different engines react differently, but I really like this scientific & in-depth explanation of the effect of different sized piping. https://motordyneengineering.com/exhaust-backpressure-and-scavenging/
