Has anybody used these? They're supposed to be a direct replacement for the Gill manifolds and will bolt up to your existing tailpipes. The tubes are short but I would think they would still be better than the manifolds. My motors are HP 500s, might add 50 to 75 more hp this winter when they're freshend up. Would like to have the CMI's but don't want to move holes in the transom and also the cost.

http://www.hardin-marine.com/detail~ID~226.aspx

I have them on my 454 Mag carb engine. This is my second season with them. I was using the short captains call risers and had issues with my y-pipe. I had to cut my captains call diverters to make them fit. I eventually removed the y-pipe and ran straight pipes to full time mufflers because of leaks around the diverters. I did not notice any alignment or height issues on my 97 Checkmate 253. The quality of the header is excellent. My only gripe was the captains call not bolting up properly.

If you CI are remaining the same and you are staying NA, your HP goals may be optimistic with the "Gill style" exhaust. If you go to CMI E-tops you will likely make more torque in the 4500-5000 RPM range = more HP.
To gain 75 HP with Gill style exhaust you will likely need more RPM = less reliability.

I looked hard at the cyclone shorty's but in the end I went back with Gil manifolds. Douglas Marine Supply gave me a fantastic deal to replace the Gil manifolds. From all the info I could get, you will not see enough HP improvement with the cyclone's over the Gil's to notice in the boat. So, you spend an extra $800-900/set over Gil replacements to make it easier to change the spark plugs. My Gil manifolds lasted 8 years.

I upgraded my 500hps with cams, heads, intakes, carbs to 595 hp on the dyno. Probably 550 or so at the prop now. Gained 6 mph in the boat with gil exhausts at 5500 rpm. Put on nose cones and gained another 4 mph. No promises that the CMI's would do better than that either. Too iffy for the extra money involved with my setup, I would have to have custom tail pipes built which are fairly high $ on top of the CMI manifolds.

I used a set of Cyclone headers on my 1999 ZT28 Checkmat i also had to cut my tubes to make them fit. Had no problems with the Captains Call.

Hello I would like to just add a little to this conversation. First off the Gil manifold is an excellent manifold as a rotary telephone is a excellent phone. Mercury used the Gil manifolds for years on the HP500 and 540 Bulldog with great success it wasn't until it became that the market place "YOU GUYS" wanted not only a more attractive looking part but a better performing part. Honestly the engine builders we speak with only see about a 25 horsepower gain on the HP500 engines. The bigger differences escalte as the horsepower and engine out put grow. Those numbers climb past 60 horsepower as the engine climbs past 600 horsepower in a modified version. If cost is your #1 consern you cant beat buying aluminum parts. Stainless is far more expensive, look at props for an example. If appearance and the desire to build more than a HP500 type of power that is where our Cyclone really shines. And for clarification they are a direct replacment for the GIL manifold. For the gentle man with the silent choice dilemma he must have bought a whole system which all aftermarket silent choice risers require trimming of the silent choice unit even a GIL as the riser is identical. Happy boating.