I have a 461 cu in with EMI manifolds with the standard cast risers and the Corsa cut out system. The last time on the dyno, this engine pulled 440HP on shop headers. The shop headers of course are dry, and a far cry from the EMI and Corsa system I described. I am currently running a Q-Jet, which was discussed in another post, but I will be changing to a 800 CFM Holley. Now, I am willing to change the exhaust risers to the SS that Eddie offers if there are worthwhile gains to be made. I will also reluctantly eliminate the Corsa System if it is causing a restriction problem. Has anyone experienced a comparison between these risers, or does any one have any input on results?

I'm running the same exhaust with mercsilent choice....454's making 505 hp through dynoed headers I dont think you will gain unless you willing to go to full tubulars like CMI or lightnings. The people at emi told me that the cast riseris fine up to 500 hp so I think your safe.The tubulars are good for about 20hp over cast.They are much better in the midrange,at the time I was building my engines I didnt have the extra $400 per set to go with the Lightnings but now wish I had. But at the time I spent the money on intakes and carbs....my .02

Dyno,
Thanks for the response. Yeah, I made the same mistake with the manifold/headers. Although I have been a boater for 26 years, this is the first attempt at building a performance engine. And, I have found out a lot of things the hard way, it's much different than getting performance out of a auto application. Some of my choices have been much to conservative. As far as the standard EMI riser, the actual exhaust opening on the discharge side of the riser looks like the same size as the ole stock 330 Merc riser. It's less than 3 inch's as I recall. So, I got these 049 large oval heads with the larger valves installed, a stepped up cam and valve train, with peak HP at about 5200 on the dyno, and I'm pushing the exhaust through an opening no better than stock. That is my concern!

The CMI's are tuned (maybe not equal length) headers. Most auto headers are 1 5/8" (small cubes) to 2" (large cube) into a 3-3 1/2" collector. Exhaust pipe sizes of 2"-3" for duel pipe street systems. Cast exhaust riser sysems have plenty of size but cappy scavenging. Adding water into these systems does not help. Its all about how much money you have to spend in boat exhaust.

I guess I would find out the measurment of the exit on a stainless one before I bought a set....call Snider Marine in Machesney park Ill. ask for Steve he is on the board some, and is a Emi dealer and alot of help!

That would be an interesting test to see if someone took a boat (around 400-500hp) and started with factory center rise with cast risers and thru-hulls on it and worked up to 4-tube headers showing each upgrades gain or loss in hp and tq I have heard all diff things on exhaust and would really like to see what works best for the money in an engine in the hp as stated above,.....Fred

All it takes is dyno time and a few turns of the wrench Fred! Maybe when I blow up these motors we can try it with the next ones!!!!

Lets do it, would be a neat test,....Fred

we need a set of stock manifolds, then we can tru my emi's and then we can run Wades Lightnings and then some dry headers could be a cool comparison. If we asked Tom Earhart I bet he could tell you the gains within a couple HP.....he's the best!

One of the tests needs to be the diff. between cast risers and tube S/S risers....Fred