i appreciate your reply.....so correct me if i'm wrong but isnt it true that a header is better than a manifold when it comes to performance engines ? hence i believe that a cmi header would be better than a simple gil manifold.....i can understand the fact that the engine is not gonna gain any hp coz exhaust is not gonna gain anything in this case.....but at least the engine would be working more freely and more pleasently with the headers....coz hey if the manifolds are better than a header.....than why do high performance engines always come with headers and not manifolds ?? just my 2cents....

I went from gil's to cmi's and had no gains. But they look awesome....

There is a thread here somewhere i read the 500HP dynoed 50 hp more with Sport Tubes over the stock Gills . I think it may have been Ray at Raylar at the time .

May gain peak 50 HP but loose low end and mid range w 500ci motor or smaller....once you pass the larger 540ci motors w good heads start to benefit from the increase header flow.

Some years back, Stainless Marine were testing their manifolds up at Chief. The SM's both standard and Gen 3's blew away the headers.

If I remember correctly, the Gil's have individual runners within the casting. The E-Tops are short runner headers, so the actual individual runner length might not increase all that much. Not 100% sure on the Gils ... I could be wrong about them having individual runners inside that casting but I'm pretty sure they are individual runners. When the CMI's start to leak, your speed and hp will go to zero, though.

this is fact . on my previous 598 i put brand new sportubes on it from the start . motor seemed very sluggish going on plane . after two noise tickets i purchased a pair of cmi sound elimination muffs . that motor came alive in the lower rpms . backpressure was the key . my boy always wanted me to take them off on ocation . they never came off . the performance was huge down low with the added backpressure . true i was amazed .

Gils start choking at about 600hp on an NA engine. I think you will gain a little on the top end and the lows and midrange should stay pretty constant.
I switched from stock Gils to CMI's on a 525SC and did not gain anything.

yeahhh i dont think i am gonna gain any hp on the motor.......but i believe that the exhaust is key to an engine's performance.......you see i think that an engine with 600hp+ may find some restriction when breathing through a GIL exhaust......YES back pressure is always a key.....but shouldnt the engine breathe more smoothly through a CMI header ? i dont know just dont wanna throw cash down the drain.....loooool