35' Baja Outlaw with twin 496 HO's, with 3" straight exhaust tips with no flapper. I can not keep the rear of the boat clean. I run 87 octane fuel and after about 20 mins of running, the swim platform is covered in black. Thinking of getting 10 inch angled tips, wondering if this will help? Please advise, as I am new to power boating.

You didn't post year of boat. I believe Merc has an ECM flash up-date for the soot..

Longer exhaust tips will help some. The ECM reflash will also help a lot.

Get reflash, my buddy got it done for his 496HO from Dustin at Whipple, made a huge difference. I don' think the tips will change anything, the reflash is the better $ spent.

Might want to do a compression test and see if your engines are healthy. Same engines, same tips, same gas, same non tuned ECU's, only in a 30 OL. Ran all summer and had some, not a ton of exhaust soot under the swim platform. If fact we found the engines run cleaner on 87 vs 91 a couple years back regarding soot build up.