View Full Version : 3" to 4" exhaust mod on 18'

brett h
06-09-2010, 09:57 PM
Hey guys, I have an 18 classic that has the 3" exhaust at the waterline and would like to get new 4" risers and tips and I would also be the one cutting new holes into the transom up higher like I see on the stock 4" 18's. Should it be straightforward or because it's 3" from the factory does the steering actuator and the wiring harness need to be moved or anything like that? I am also thinking of just leaving the 3" in the transom and capping them off, any problem with that? Thanks for all of your help.

06-09-2010, 10:07 PM
You have been a mamber since 03 and have only posted 3 times? Is this a new to you boat?

brett h
06-09-2010, 10:44 PM
No same boat, just ready to upgrade exhaust. thanks

06-10-2010, 07:49 AM
John on his 18 has S pipes made rather then having to redo the transom, going from 3 to 4 you will lose some bottom end and a tad of mid range but may gain a tiny bit up top. 3 inch is a good size for an unmodified SB, 4 is needed when you have enough mods in the block so it needs to breath.

brett h
06-10-2010, 05:14 PM
Thank you MOP! I bought this 18 back in '92 and the owner had put in a crate 350/310 hp chevy in it. The mod probably isn't worth it is it?

06-10-2010, 06:08 PM
Hey Brett, what year is your boat? I seem to remember reading somewhere that even the difference between 4" thru-hull and thru-hub (thru-prop) exhaust was negligible for stock-ish applications. I definitely don't think you'll see much difference between 3" and 4" exhaust. Probably not worth the cost and effort unless you're going to build something with monster HP. Post some pics of your boat :wink:

06-11-2010, 08:00 AM
Just did a 4 inch conversion on my 86 classic 18 went with glm aluminum manifolds n risers had custom 4inch stainless Z drop downs and 4 inch tips put in . I cut the 4 inch holes in the transom so that the extra inch goes higher. It fits just under the steering with no problems. It really looks aswome but did not see any hp gain. It cost me about $1600 and did all the work myself except the fabrication n welding of the drop downs they were $400 of the $1600

06-11-2010, 08:56 AM
I would agree with what has been said with "stock" stuff with the exception of going from a 3" stock cast iron setup to a newer 4" alum or hi-po riser setup. we did this on the cig 19 . we put on SS Marine risers and it let the stock 330hp motor breathe with loosing all the weight of the cast iron plumbing in the back and opening up the motor we picked up a few mph.

06-11-2010, 11:22 AM
Yea, you won't see any gains with the GLM exhaust, they don't flow any better than stock ones. Need to go with higher end manifolds that have a larger outlet on top flowing into a 4" riser. But, I agree there needs to be some major mods to justify the extra expense. If I remember right, my EMI system with custom risers was around $1800.