What is better 2.5'' or 3'' for y-pipe? 
What is better 2.5'' or 3'' for y-pipe?
2.5" will obviously be better for a lowered car (takes up less space, more clearance) but as far as which makes more power, just too many factors (engine size, NA or FI, stock internals etc?) to consider there.
A properly designed 2.5" into 3" (or 3.5") can produce better torque on stock cammed cars because the narrower diameter helps keep the velocity of the exhaust gasses high. On cammed cars shifting 6500 and higher, I would go with 3" into 3.5"
A poorly designed merge will kill a Y-pipe using 2.5" sides
A poorly designed merge will kill a Y-pipe using 2.5" sides
So on a car that plans to have heads and cam in the future, is a 3" y-pipe (like qtp with flowmaster merge) going into a 3" I-pipe with a cutout going to make the most power next to true duals? How much difference is there between a 3" and a 2.5" y-pipe as far as power?
Originally Posted by Ragtop 99
A properly designed 2.5" into 3" (or 3.5") can produce better torque on stock cammed cars because the narrower diameter helps keep the velocity of the exhaust gasses high.
How much power do you lose going through high flow cats compared to an ORY? Because I have heard that without cats, there is rasp with most aftermarket catbacks. And rasp is something that will bug me, so if the power loss through cats is negligable, then I'll just use them.
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Originally Posted by AdamSS
NO WAY !!! - More like 2-3 rwhp.... It's a sad myth that high-flow cats take away more than that...
And if they rob any less power than that then they probably aren't really 'functioning' cats, more like small bullet mufflers with open screens in them at that point.
Well, I wouldn't really be keeping the cats to be "functioning" as far as emissions. We don't even have emissions testing in Missouri. I just don't want rasp from my exhaust. And I have heard a lot of stories of how bad rasp is without cats. Does the metallic substrate cats make any difference?
